Patent Publication Number: US-9402610-B2

Title: Rib-protecting devices for thoracoscopic surgery, and related methods

Description:
PRIORITY AND RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application No. 61/573,583, entitled “METHODS AND DEVICES FOR REDUCING TISSUE TRAUMA DURING THORACOSCOPIC SURGERY,” filed on Sep. 8, 2011, which is incorporated herein by reference in its entirety. 
     The present application claims priority to U.S. Provisional Patent Application No. 61/687,585, entitled “METHODS AND DEVICES TO REDUCE TISSUE TRAUMA DURING THORACOSCOPIC SURGERY,” filed on May 23, 2012, which is incorporated herein by reference in its entirety. 
     The present application is also a continuation-in-part application of, and claims priority to, U.S. patent application Ser. No. 13/111,762, entitled “METHODS AND DEVICES TO DECREASE TISSUE TRAUMA DURING SURGERY,” filed on May 19, 2011, which is incorporated herein by reference in its entirety, which is a continuation-in-part application of U.S. patent application Ser. No. 12/422,584, entitled “METHODS AND DEVICES TO DECREASE TISSUE TRAUMA DURING SURGERY,” filed on Apr. 13, 2009, which is incorporated herein by reference in its entirety, and which claims priority to U.S. Provisional Patent Application No. 61/395,915, entitled “METHODS AND DEVICES TO DECREASE TISSUE TRAUMA DURING SURGERY,” filed on May 19, 2010, which is incorporated herein in its entirety. 
     The present application is also a continuation-in-part application of U.S. patent application Ser. No. 12/422,584, entitled “METHODS AND DEVICES TO DECREASE TISSUE TRAUMA DURING SURGERY,” filed on Apr. 13, 2009, which is incorporated herein by reference in its entirety, which claims priority to:
         a. U.S. patent application Ser. No. 12/422,584 claims priority to U.S. Provisional Patent Application No. 61/123,806, entitled “OSCILLATING LOADING TO MINIMIZE TISSUE TRAUMA DURING SURGICAL PROCEDURES,” filed on Apr. 11, 2008, which is incorporated herein by reference in its entirety, and   b. U.S. patent application Ser. No. 12/422,584 claims priority to U.S. Provisional Patent Application No. 61/044,154, entitled “METHODS FOR DETECTING TISSUE TRAUMA DURING SURGICAL RETRACTION,” filed on Apr. 11, 2008, which is incorporated herein by reference in its entirety, and   c. U.S. patent application Ser. No. 12/422,584 claims priority to U.S. Provisional Patent Application No. 61/127,575, entitled “SURGICAL RETRACTOR ARMS FOR REDUCED TISSUE TRAUMA,” filed on May 14, 2008, which is incorporated herein by reference in its entirety, and   d. U.S. patent application Ser. No. 12/422,584 claims priority to U.S. Provisional Patent Application No. 61/127,491, entitled “APPARATUS AND METHODS FOR REDUCING MECHANICAL LOADING AND TISSUE DAMAGE DURING MEDICAL PROCEDURES,” filed on May 14, 2008, which is incorporated herein by reference in its entirety, and   e. U.S. patent application Ser. No. 12/422,584 claims priority to U.S. Provisional Patent Application No. 61/131,752, entitled “APPARATUS AND METHODS FOR ENGAGING HARD TISSUES TO AVOID SOFT TISSUE DAMAGE DURING MEDICAL PROCEDURES,” filed on Jun. 12, 2008, which is incorporated herein by reference in its entirety.       

    
    
     FIELD OF THE DISCLOSURE 
     The field of the disclosure relates to surgical retractors and ports for thoracoscopic surgery. 
     BACKGROUND 
     Deformation of tissues is commonly performed during surgery or other medical procedures either to achieve surgical access or to specifically alter the dimensions of one part of the anatomy. Examples of deformations of tissue for surgical access include spreading ribs during a thoracotomy, spreading a bisected sternum during a sternotomy, and separating the vertebrae of the spine for surgery on the intervertebral disk. Examples of deformation of tissues to alter the dimensions of the tissue include angioplasty to open blocked arteries, valvuloplasty to enlarge heart valves, and distraction to adjust the position of vertebrae. Such deformations are collectively referred to herein as “retraction”. 
     Spreaders, retractors, distractors, and even angioplasty balloons (collectively called “retractors” here) can impose significant forces on the surrounding tissues during retraction. The resulting strain on these tissues, and on associated tissues such as the ligaments attaching ribs to vertebrae, can be large, leading to damage of these tissues, including the fracture of ribs and the rupture or irreversible deformation of ligaments and other fibrous tissues. 
     Retraction occurs in two different phases—deforming the tissue (referred to herein as the first phase or retraction) and holding the tissue at that deformation (referred to herein as the second phase of retraction). Usually both are done with the same instrument. For example, a rib spreader is used both to force the ribs apart during a thoracotomy and to hold the ribs apart during the surgical procedure. Sometimes two different instruments are used, especially if the deformation is to be permanent. For example, an angioplasty balloon is used to force open an atherosclerotic plaque, and then a stent is used to hold the artery open; or a distractor is used to separate vertebrae, and a metal plate is used to secure the vertebrae in that position. An example of two different instruments being used when the deformation is not permanent is disclosed in U.S. Pat. No. 5,201,325 by McEwen (McEwen, Auchinleck et al. 1993), therein a surgeon manually retract an incision with a disclosed retractor blade, and an automated mechanism is then used to hold the incision open. In the medical literature, both phases are frequently referred to as retraction. 
     Both phases of retraction traumatize tissue. Trauma from the first phase of retraction can include the rending and tearing of tissues—bones bend and break; muscles stretch beyond normal limits; ligaments and other connective tissues stretch and tear; or nerves are stretched. Trauma from the second phase of retraction can include ischemia of the tissue due to elevated tissue pressure, for example, under a retractor blade; blockage of nerves; and blockage of blood vessels causing ischemia in tissues distant from retraction. 
     Tissue trauma and ensuing complications resulting from retraction can be greater than the trauma resulting from the medical procedure that required the retraction. For example, thoracotomies are extremely traumatic, and can result in post-surgical pain and respiratory complications that exceed that of the thoracic procedure, such as a lung segmentectomy. 
     There is, therefore, need for improved methods and devices to perform one or both phases of retraction. 
     SUMMARY OF THE DETAILED DESCRIPTION 
     The embodiments disclosed herein provide devices adapted to create a surgical opening in an incision between adjacent ribs and then to protect the tissues surrounding the tissue, especially the ribs, from trauma arising from impingement by surgical instruments as the surgeon operates. In one non-limiting embodiment, a device comprises a rib-protecting clip for thoracic surgical access through an intercostal incision. The clip comprises at least one rib-engaging channel. The at least one rib engaging channel comprises a first end of the at least one rib-engaging channel, a second end of the at least one rib-engaging channel, and at least one raised member. The at least one raised member is disposed at or between the first end of the at least one rib-engaging channel and the second end of the at least one rib-engaging channel. The at least one raised member is further elevated from the at least one rib-engaging channel toward the rib. The at least one raised member is further disposed along a portion of the at least one rib-engaging channel such that at least one rib-contacting surface on the at least one raised member contacts the rib, and the rest of the at least one rib-engaging channel does not contact the rib. In this manner, the rib-engaging channel thus prevents impingement by surgical instruments onto the rib. 
     In another non-limiting embodiment, a rib-protecting port for thoracic surgical access through an intercostal incision is provided. The rib-protecting port comprises a chassis configured to engage adjacent cranial and caudal ribs and define a thoracic surgical access opening between the cranial and caudal ribs for access to the pleural cavity. The chassis comprises a first barrier member disposed on a first side of the chassis, the first barrier member configured to engage the cranial rib generally along a longitudinal axis of the cranial rib. The first barrier member further comprises a rib-facing surface configured to be apposed to the cranial rib, and an incision-facing surface configured to delimit one side of the thoracic surgical access opening. The chassis also comprises a second barrier member disposed on a second side of the chassis opposite the first side. The second barrier member is configured to engage the caudal rib generally along a longitudinal axis of the caudal rib. The second barrier member further comprises a rib-facing surface apposed to the caudal rib, and an incision-facing surface delimiting one side of the thoracic surgical access opening. The chassis further comprises a first end member disposed between first barrier member and second barrier member being at one end of the chassis, and a second end member disposed between first barrier member and second barrier member being at the other end of the chassis opposite the first end member. The chassis further comprises at least one first rib-engaging channel configured to engage the cranial rib, the at least one first rib-engaging channel formed by at least one rib-engaging hook being disposed on the bottom portion of the first side member; the rib-facing surface of the first barrier member, and at least one wing member being disposed on the top portion of the first barrier member. The at least one rib-engaging hook is configured in operative relation with the rib-facing surface of the first barrier member and the at least one wing member to surround a caudal margin of the cranial rib and position the first barrier member between the cranial rib and the thoracic surgical opening to protect the cranial rib from impingements in the thoracic surgical opening. The chassis further comprises at least one second rib-engaging channel configured to engage the caudal rib, the at least one second rib-engaging channel formed by at least one rib-engaging hook being disposed on the bottom portion of the second side member, the rib-facing surface of the second barrier member, and at least one wing member being disposed on the top portion of the second barrier member. The at least one rib-engaging hook is configured in operative relation with the rib-facing surface of the second barrier member and the at least one wing member to surround a cranial margin of the caudal rib and position the second barrier member between the caudal rib and the thoracic surgical opening to protect the caudal rib from impingements in the thoracic surgical opening. 
     In other embodiments, methods and devices are disclosed that are adapted to retract tissue in a broader variety of surgeries. In one embodiment, such a device comprises at least one pair of opposed retraction members, with each retraction member being able to operably engage the tissue to be retracted. A drive mechanism is operably engaged with at least one of the retraction members in each of the at least one pair of retraction members. The drive mechanism is adapted to provide a continuous, smooth deformation of the tissue, following, for example, a parabolic distance/time curve during retraction. 
     In another embodiment, retraction devices that are adapted to provide a constant force during retraction of tissue. The retraction devices comprise at least one retraction member, with the at least one retraction member being able to operably engage the tissue to be retracted. A drive mechanism is operably engaged with the at least one retraction member. 
     In another embodiment, a retraction device includes automated control while detecting imminent fracture. In this manner, the automated control comprises measuring the retraction force and monitoring for transients in the force signal, such as a negative-going spike or an increased variance in the force signal. 
     In another embodiment, a retraction device includes at least one pad in contact with the margins of an incision. The pad is adapted to cool the tissue at and surrounding the margin of the incision to reduce inflammation and minimize temporary ischemia of the tissue. 
     In another embodiment, a retraction device includes at least one pad in contact with the margins of an incision. The pad is adapted to elute pharmacologically active compounds into the tissues at the margin of the incision to achieve beneficial outcomes, such as hemostasis or reduced inflammation. 
     In another embodiment, a retraction device is provided to retract tissue. In this manner, multiple tissue engagers that automatically self-balance force comprise at least one retraction member, with the at least one retraction member being able to operably engage the tissue to be retracted. A drive mechanism is operably engaged with the at least one retraction member. 
     In another embodiment, a retraction device is provided to retract tissue with forces aligned with the retraction. The retraction device comprises at least one pair of opposed retraction members, with each retraction member being able to operably engage the tissue to be retracted. A drive mechanism is operably engaged with at least one of the retraction members in each of the at least one pair of retraction members. At least one of the retraction members comprises an arm that can rotate around an axis perpendicular to the drive axis connecting the two retraction members, permitting the retraction members to align with respect to the retraction. 
     In another embodiment, a retraction device is provided to retract tissue. In this manner, the retraction member comprises a retractor arm fitted with tissue engagers that engage hard tissues while minimizing deformation of soft tissues. 
     In another embodiment, a retraction device to retract tissues is disclosed wherein the creep of the tissues is accommodated. The retraction device comprises at least one pair of opposed retraction members, with each retraction member being able to operably engage the tissue to be retracted. A servo-drive mechanism is operably engaged with at least one of the retraction members in each of the at least one pair of retraction members such that the retraction members are driven apart. At least one retraction member comprises a retractor arm fitted with a force measuring device that measures force on the at least one retraction member. This measured force is used to determine the deformation of the retraction member, and the servo-drive mechanism adjusts the separation of the retraction members to accommodate for creep of the tissues. 
     In another embodiment a thoracic retractor for performing a thoracotomy is disclosed, comprising a linear drive element having at least two arms, with at least one of them moveable along the linear drive element, and at least one self-balancing tissue engager associated with each arm. The self-balancing tissue engager comprises a first rotary joint between the arm and a first balance bar, which has additional rotary joints on each of its two ends to which second balance bars join, and each balance bar has rotary joints on each of its two ends to which join descender posts that engage a rib on one side of the incision. Opposing arms and associated tissue engagers thus engage opposing ribs on each side of an incision and retraction in opposing directions along the linear drive element spreads the ribs apart to retract the thoracic tissue. 
     In another embodiment, two opposing arms are each associated with a doubletree balance bar, each doubletree balance bar having two ends to which rotatably join swingletree balance bars, one swingletree balance bar on each end of the doubletree balance bar. Each swingletree balance bar has two ends to which rotatably mount a descender post each of which engages a rib. There are thus four descender posts, with forces automatically balancing through the doubletree and swingletree balance bars, that engage a rib. 
     In another embodiment, a tissue engager for thoracic retraction is disclosed, comprising a balancing assembly having at least one descender post descending from at least one balance bar. The descender post comprises an elongate member with a first rib-forcing surface and a hook element with a second rib-forcing surface such that a gap region is formed between the first and second rib-forcing surfaces, the gap region being concave and extending far enough in the direction of retraction to place the second rib-forcing surface away from the neurovascular bundle. 
     In another embodiment, the concave shape of the gap region of the descender post is further defined as a tapered hollow defined by a taper point. 
     In another embodiment, the taper of the descender post forms an acute angle. 
     In another embodiment, the tissue engager has a plurality of descender posts. 
     In another embodiment, the tissue engager has a plurality of balanced descender posts. 
     In another embodiment, a tissue engager for thoracic retraction is disclosed, comprising at least one retraction bar capable of moving along a direction of retraction and having at least one descender post descending into the incision. The descender post has an elongate member, having two ends with a first rib-forcing surface adjacent to the second end and the first end joining a balance bar at a rotatable mount, and a hook element adjoining the second end of the second end of the elongate member. The hook element has a second rib-forcing surface adjacent the hook element&#39;s second end. The first end of the slender element joins the balance bar via a rotatable joint having a rotational axis that is vertical with respect to a plane of a patient&#39;s skin allowing the descender post to rotate to extend the hook element under a rib. 
     In another embodiment, the descender post is substantially curved, and the first rib-forcing surface projects a distance radially out from the vertical axis, thereby defining a moment arm reaching out from the vertical axis to the first rib-forcing surface. Thus when a rib bears on the first rib-forcing surface, the descender post has a moment that would make the descender post rotate about the vertical axis. 
     In another embodiment, the curved descender post can rotate around the vertical axis by 90 degrees. 
     In another embodiment, a tissue engager possessing a balancing assembly has an elastic element that provides elastic recoil to return the balancing assembly to its original configuration after deformation by a load. 
     In another embodiment, the elastic element is associated with the first joint axis. 
     In another embodiment, the elastic element is associated with the second joint element. 
     In another embodiment, the elastic element is associated with the third joint element. 
     In another embodiment, all components of a self-balancing tissue engager are substantially encompassed by an elastic sheath. 
     In another embodiment, the elastic element of the tissue engager has a Young&#39;s modulus between 0.1 MPa and 60 MPa to permit retaining a preferred positioning of the elements of a self-balancing tissue engager and recovering that preferred positioning of the elements. 
     In another embodiment an elastic element is spatially associated with the gap region and provides a padded surface to the patient&#39;s tissues. 
     A method for retracting thoracic tissue, comprising retracting thoracic tissue in the direction of a linear drive element by moving along the linear drive element at least one of two arms that are oriented substantially perpendicular to the direction of retraction while self-balancing tissue engagers automatically balance the forces amongst four descending posts that are pushing on a rib. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate two tests of a biological sample demonstrating the biomechanical phenomena of force relaxation and creep, respectively; 
         FIG. 2  is a diagram of a prior art retraction device; 
         FIG. 3  is a diagram of one embodiment of a device for separating anatomical elements using a spring that exerts substantially constant force on the anatomical elements; 
         FIG. 4  is a diagram of another embodiment of a device for separating anatomical elements using a spring attached to a moveable drive block that exerts substantially constant force on the anatomical elements; 
         FIG. 5  is a diagram of another embodiment of a device for separating anatomical elements using a spring attached to a moveable drive block that exerts substantially constant force on the anatomical elements, wherein the device also includes an adjustable indicator that is used to adjust the force exerted by the spring and a mechanical stop to limit the range of motion of a retraction element; 
         FIG. 6  is a diagram of another embodiment of a device for separating anatomical elements using a pneumatic cylinder to exert a substantially constant force, where in a pressure reservoir can be used to keep the pressure in the pneumatic cylinder substantially constant as the cylinder expands, and wherein a pump can be used to keep the pressure in the reservoir nearly constant; 
         FIG. 7  is a diagram of another embodiment a device for separating anatomical elements using a motor to exert a substantially constant force, wherein the electrical current driving the motor is kept substantially constant to keep the force substantially constant; 
         FIG. 8  is a diagram of another embodiment of a device for separating anatomical elements using a motor to exert a substantially constant force, wherein a force measuring device is used to determine the force, and a feedback loop is used to control the motor such that the force is substantially constant; 
         FIG. 9  is a diagram of another embodiment of a device for separating anatomical elements using a motor to exert a substantially constant force which includes an alternate means for mechanical coupling of the motor; 
         FIG. 10  is a diagram of another embodiment of a device for separating anatomical elements with an alternate configuration using more than one retraction element and also using a visual indicator of the motion of the retraction elements; 
         FIGS. 11A and 11B  illustrate exemplary time/displacement trajectories for retraction with oscillating loading; 
         FIG. 12  illustrates a prototype motorized retractor utilizing a bi-directional lead screw and that measures force on both retractor blades and separation of the blades; 
         FIG. 13  illustrates a force/time trace for a thoracotomy performed with the prototype motorized retractor of  FIG. 12 ; 
         FIGS. 14A and 14B  show acceleration of force relaxation during bouts of oscillating loading; 
         FIG. 15  shows a retraction for a thoracotomy in which oscillating loading is periodically applied; 
         FIG. 16  shows an example of a Finochietto thoracic retractor in the prior art; 
         FIG. 17  shows a thoracic retractor of the prior art with a standard hand-cranked rack-and-pinion and a thoracic retractor in which the hand crank is replaced with a motor; 
         FIG. 18  shows a thoracic retractor driven by a computer controlled motor on a ball screw; 
         FIG. 19  shows a thoracic retractor driven by a hydraulic cylinder; 
         FIG. 20  shows a thoracic retractor having two actuators, a first hand-driven actuator drives apart the arms of the retractor, and a second motorized retractor that drives oscillating motion; 
         FIG. 21  illustrates a thoracic retractor having a first hand-driven actuator that drives apart the arms of the retractor and a second hydraulically driven pressure pad used to drive oscillating motion; 
         FIG. 22  shows a thoracic retractor having a first hand-driven actuator that drives apart the arms of the retractor and a second voice coil actuator used to drive oscillating motion; 
         FIG. 23  shows a retractor having multiple arms and actuators that apply oscillating loads; 
         FIG. 24  shows a retractor having multiple pairs of arms and actuators, wherein one actuator separates the pairs of arms, while actuators on each arm drive oscillating motion; 
         FIG. 25  shows an angioplasty system for dilating tissues with an oscillating motion; 
         FIG. 26  shows another angioplasty system with an oscillating motion; 
         FIGS. 27A through 27C  illustrates an angioplasty system with two compartments to generate oscillating motions having higher frequencies; 
         FIGS. 28A and 28B  show another angioplasty system with two compartments to generate oscillating motions having higher frequencies; 
         FIGS. 29A and 29B  show another angioplasty system in which all components are contained in a single compartment to permit oscillating motions having higher frequencies; 
         FIGS. 30A and 30B  show another angioplasty system in which oscillating motions are driven by a thermally expanded bubble; 
         FIG. 31  illustrates how the time constant can be determined for force relaxation; 
         FIG. 32  illustrates how effective stiffness can be compared for sequential cycles of an oscillating loading; 
         FIG. 33  depicts an example retractors in the prior art; 
         FIG. 34  illustrates the retractor of  FIG. 33  fitted with a set of calipers for measuring the separation of retraction elements and with strain gauges for measuring the forces on each of the blades of the retraction elements; 
         FIG. 35  shows a prototype retractor having a motorized drive, a linear potentiometer for measuring the separation of the retraction elements, and strain gauges for measuring the forces on each of the blades of the retraction elements; 
         FIG. 36  shows force and displacement with respect to time for a retraction during an experimental thoracotomy on a pig carcass; 
         FIG. 37  shows force and displacement with respect to time for a retraction during an experiment thoracotomy on a pig using a Finochietto-style retractor; 
         FIGS. 38A through 38C  shows force and displacement with respect to time for a second retraction during another experimental thoracotomy on a pig carcass, wherein a larger break and two smaller breaks occurred during this retraction and two force events and a slope event preceded the larger break; 
         FIG. 39  shows the force and the slope of the force in an expanded view of the retraction in  FIG. 38 . This shows how examination of the slope provides a clearer signal of the slope and force events; 
         FIGS. 40A and 40B  illustrate the force and the slope of the force over time for a third retraction during another experimental thoracotomy on a pig carcass; 
         FIGS. 41A and 41B  illustrate the force and a second time derivative of the force (d2F/dt2) for two experimental retractions; 
         FIG. 42  shows an algorithm for detecting an imminent tissue fracture and pausing retraction in response; 
         FIG. 43  shows how acoustic events during retraction can occur over time during a retraction and how they can be used as predictors of tissue fracture; 
         FIG. 44  shows an example of a Finochietto thoracic retractor in the prior art; 
         FIGS. 45A and 45B  show an experimental thoracic retractor in the prior art; 
         FIG. 46  show the orientations and motions of a swingletree and the forces on the swingletree; 
         FIG. 47  shows the orientations and motions of a doubletree and the forces on the doubletree; 
         FIGS. 48A and 48B  shows an example of the prior art in which a derrick-like arm suspends a swingletree over an incision; 
         FIGS. 49A, 49B . 1  and  49 B. 2  show the use of a balancing assembly in a thoracic retractor having multiple retractor blades; 
         FIGS. 50A through 50C  show how a balancing assembly can be adjusted to provide any ratio of forces on multiple retractor blades; 
         FIG. 51  shows a balancing assembly having multiple tiers of balance bars; 
         FIG. 52  shows a balancing assembly having a number of blades that is not a multiple of 2; 
         FIG. 53  shows a retractor having a cable that permits a balance bar to rotate; 
         FIG. 54  shows a balancing assembly having multiple tiers, with each tier free to rotate; 
         FIGS. 55A through 55C  show top, side, and front views, respectively, of a balance assembly used for retracting a rib, wherein hooks that descend from the balance bars engage the rib; 
         FIGS. 56A through 56C  shows top, side, and front views, respectively, of a balance assembly used for retracting a rib, wherein hooks that descend from the balance bars engage the rib and an articulation in the balance bar permits the hooks to orient to the curvature of the rib; 
         FIG. 57  shows a thoracic retractor with a balancing assembly, wherein the arms of the retractor has articulations; 
         FIG. 58  shows a balancing assembly in which balance bars overlap; 
         FIGS. 59A through 59E  shows another embodiment of a retractor having articulations in the arms, retraction hooks to engage the tissues, and cables to provide automatic force balancing on the hooks; 
         FIGS. 60A and 60B  show the embodiment depicted in  FIGS. 59A through 59E , but as part of a retractor driven on a dual-thrust lead screw; 
         FIG. 61  shows another embodiment of a retractor using hydraulic cylinders to provide automatic force balancing on multiple retractor blades; 
         FIG. 62  shows another embodiment in which hydraulic cylinders provide automatic force balancing for multiple retraction hooks; 
         FIGS. 63A through 63E  show another embodiment in which fenestrated bars on a fulcrum provide automatic force balancing for multiple retraction hooks; 
         FIG. 64  show another embodiment in which pivots are used to provide adjustable pivot points for swingletrees; 
         FIG. 65A through 65C  show side views of the assembly in  FIG. 64 . 
         FIGS. 66A and 66B  show another embodiment in which pivots are used to provide adjustable pivot points for swingletrees; 
         FIG. 67  shows an example of a thoracic retractor in the prior art used in a thoracotomy; 
         FIGS. 68A and 68B  show examples of retractors in the prior art; 
         FIG. 69  diagrams the forces thought to act on the opposing blades of a retractor; 
         FIGS. 70A through 70D  show additional examples of the prior art in which curved blades or swiveling joints permit accommodation to forces of retraction; 
         FIG. 71  shows a more complete accounting of the forces and torques on a retractor; 
         FIG. 72  shows data from an experimental thoracotomy showing that the force is not equal on two opposing blades of a retractor; 
         FIG. 73  shows an embodiment in which the retraction units are driven by a dual-thrust lead screw; 
         FIG. 74  shows another embodiment in which the retraction units are driven by a dual-thrust lead screw, demonstrating rotational freedom of the arms; 
         FIG. 75  shows side and end views, respectively, of an embodiment of a retractor drive mechanism comprising a roller drive with a shaft having rectangular cross-section; 
         FIG. 76  shows how torques on the arms of a retractor increases forces on the drive rollers of a roller drive; 
         FIG. 77  shows a roller drive with a circular shaft and how alignment of the rollers with respect to the circular shaft drives rotation and translation of the shaft; 
         FIG. 78  illustrates multiple views of a roller drive with a circular shaft depicting how varying the alignment of the rollers with respect to the circular shaft provides variable control of shaft rotation and translation; 
         FIG. 79  shows another embodiment of a retractor using a roller drive with a circular shaft; 
         FIG. 80  shows a another n embodiment of a retractor having dovetail joints to permit additional motions of the retractor arms; 
         FIG. 81  shows another embodiment of a retractor arm having two dovetail joints to permit additional motions of the retractor arms; 
         FIGS. 82A and 82B  shows another embodiment of a retractor having two dual-thrust lead screws permitting an additional degree of freedom of motion; 
         FIG. 83  shows another embodiment of a retractor for thoracotomy comprising retractor blades pulled by straps attached to a patient; 
         FIG. 84  shows another embodiment of a retractor for sternotomy comprising retractor blades pulled by two ends of a strap that wraps around a patient; 
         FIG. 85  shows another embodiment of a retractor for sternotomy comprising retractor blades pulled by the two ends of a strap that wraps around a patient and inflatable balloons for generating tension; 
         FIG. 86  shows an example of a Weitland retractor in the prior art for retracting skin inserted into an incision in the skin; 
         FIG. 87  shows another embodiment of a retractor comprising retractor blades pulled by straps that wrap around the patient&#39;s wrist and having pull tabs for generating retraction forces; 
         FIGS. 88A and 88B  shows the anatomy of a chest wall around an incision for a thoracotomy; 
         FIG. 89  shows the deformation of the tissues of the chest wall by retractor blades during a thoracotomy; 
         FIG. 90  shows pinch points generated by retractor blades on the ribs and neurovascular bundle during a thoracotomy; 
         FIG. 91  shows regions of potential damage to tissues caused by elevated tissue pressure during thoracotomy; 
         FIGS. 92A through 92C  show an embodiment of a tissue engaging element comprising posts placed into holes drilled into adjacent ribs; 
         FIG. 93  shows another embodiment of a retractor comprising posts that engage the arms of the retractor; 
         FIGS. 94A and 94B  show another embodiment of a retractor comprising clips that grasp the ribs and attach to the arms of a retractor; 
         FIGS. 95A and 95B  show embodiments of retractor clips having one or two spikes, respectively, for engaging the ribs; 
         FIG. 96  shows a top view of another embodiment of a retractor comprising two retractor arms and multiple clips for engaging the ribs for a thoracotomy; 
         FIGS. 97A through 97D  show the top and side views of another embodiment of a retractor comprising two retractor arms having descender posts for engaging ribs, and two side views of a descender posts having hooks and rotatable mounts; 
         FIGS. 98A and 98B  show another embodiment of a descender post comprising a hook engaged with the retractor arm via a rotatable mount; 
         FIG. 99  shows a 3D model of a retractor having descender posts with hooks rotatably mounted on retractor arms; 
         FIG. 100  shows another embodiment of a retractor arm comprising an arm and a plurality of descender posts for engaging a rib; 
         FIGS. 101A through 101E  shows another embodiment of a descender post comprising a projection that projects laterally from the descender post and terminates in a tip having one of several configurations; 
         FIG. 102  shows another embodiment of a retractor comprising two retractor arms each having a first and a second descender post for engaging a rib; 
         FIG. 103  shows another embodiment of a retractor comprising multiple arms, each having descender posts and configured to engage multiple ribs on each side of a thoracotomy incision; 
         FIGS. 104A through 104C  shows another embodiment of a descender post comprising a post with clips on one end, wherein the clips close on a rib when pushed against the rib; 
         FIGS. 105A and 105B  show the deformations under loading of a thoracic retractor in the prior art; 
         FIG. 106  shows an embodiment comprising a retractor having two opposing blades, a servo-motor, a servo-controller, and linear potentiometer; 
         FIG. 107  shows an algorithm in which force on the retractor is used to determine and correct for deformation of the retractor when loaded; 
         FIG. 108  shows an embodiment of a device for compensating for changes in retractor deformation comprising a force measuring device, a force-to-deformation translator, a servo-controller and a servo-motor; 
         FIG. 109  shows an embodiment of a device for compensating for changes in retractor deformation comprising a force measuring device, a force-to-deformation translator, a servo-controller and a servo-motor in which all components fit onto one arm of the retractor; 
         FIG. 110  shows an embodiment of a thoracic retractor; 
         FIG. 111  shows an embodiment of a retraction driver; 
         FIG. 112  shows an embodiment of a retractor arm assembly for a thoracotomy; 
         FIG. 113  shows an enlarged view of a rotatable mount on a thoracic retractor; 
         FIG. 114  shows the design of the balance arms of a retractor arm assembly; 
         FIG. 115  shows an example of a user interface built into a thoracic retractor; 
         FIGS. 116A and 116B  show force and displacement for two automated thoracotomy retractions performed with a prototype thoracic retractor; and 
         FIGS. 117A through 117C  show the force on a left arm and an Event Detection Signal for the automated retractions shown in  FIGS. 116A and 116B . 
         FIG. 118 —A rib cage; inserted plane defines “profile” view. 
         FIGS. 119A through 119C —Profile view from a rib cage. 
         FIG. 120A —A thin Tissue Retraction Element in profile view. 
       FIG.  120 B 1 —A Tissue Retraction Element with a square shape of the prior art. 
       FIG.  120 B 2 —A Tissue Retraction Element with a polished surface. 
         FIG. 120C —A low-friction Tissue Retraction Element; and 
         FIG. 120D —a very low-friction Tissue Retraction Element. 
         FIG. 120E —A bagged, lubricated Tissue Retraction Element. 
         FIG. 120F —A Tissue Retraction Element with lubricant applied in the operating room. 
         FIG. 120G —A Tissue Retraction Element with water-activated lubricant coating. 
         FIG. 120H —A Tissue Retraction Element with an elastic sheath. 
         FIG. 120I —A Tissue Retraction Element with elastic joints. 
         FIG. 121 —A surgeon&#39;s finger, in section view. 
         FIG. 122 —An Articulated Safety Finger, in section view. 
         FIG. 123 —An Articulated Safety Finger, straight for insertion. 
         FIG. 124A —An Articulated Safety Finger, pulling on cable. 
         FIG. 124B —An Articulated Safety Finger, flexed. 
         FIG. 125 —Articulated Safety Finger, ready for retraction. 
         FIGS. 126A through 126C —Surgeon hand-actuating the ASF&#39;s Finger Flexing Lever. 
         FIG. 127 —Detail of the ASF&#39;s Finger Flexing Lever. 
         FIG. 128 —Detail showing the action of the ASF&#39;s Finger Flexing Lever. 
         FIG. 129 —Oblique view of Swinging Safety Finger TRE. 
         FIG. 130A —Profile view of Swinging Safety Finger closed and thin for insertion. 
         FIG. 130B —Profile view of Swinging Safety Finger opened and retracting tissue. 
         FIG. 131 —Swinging Safety Finger showing off-center deep area creating moment. 
         FIGS. 132  A through  132 C—Deployment sequence of the Swinging Safety Finger. 
         FIGS. 133A through 133F —Time-stepped views showing gradually changing profile of SSF showing three steps viewed from the side and same three steps viewed from above. 
         FIG. 134  shows how the gap accommodates many rib rotations and sizes to always protect the neurovascular bundle. 
         FIG. 135  shows a swinging safety finger with a rib resting onit. 
         FIGS. 136A through 136C  shows a helical retraction element that self-engages on insertion. 
         FIG. 137  shows a prototype automated retractor for thoracotomy. 
         FIG. 138  shows a retractor with tissue engagers having balancing beams and descender posts to retract ribs for thoracotomy. 
         FIG. 139  shows a self-balancing tissue engager for retracting ribs. 
         FIG. 140  shows a descender post engaging a rib for retraction. 
         FIG. 141  shows a photograph from a thoracotomy on a pig showing the gap. 
         FIG. 142A  shows a thoracoscopic port of the prior art. 
         FIG. 142B  shows another thoracoscopic port of the prior art. 
         FIG. 142C  shows a threaded thoracoscopic port of the prior art. 
         FIG. 143  shows a rib engager for retracting ribs. 
         FIG. 144A  shows a top view of a rib retractor for use in a thoracoscopic surgery. 
         FIG. 144B  shows a second top view of the above rib retractor as it retracts. 
         FIG. 144C  shows a third top view of the above rib retractor fully retracted. 
         FIG. 144D  shows a side view of the above rib retractor. 
         FIG. 144E  shows a second side view of the above retractor showing a deformation under load. 
         FIG. 145A  shows a top view of another rib retractor for use in a thoracoscopic surgery. 
         FIG. 145B  shows a second top view of the above retractor showing how the rib engagers re-orient. 
         FIG. 146A  shows a top view of another rib retractor for use in thoracoscopic surgery. 
         FIG. 146B  shows a second top view of the above retractor showing how the rib engagers re-orient. 
         FIG. 146C  shows a side view of the above retractor. 
         FIG. 147  shows a side view of another rib retractor for thoracoscopic surgery illustrating loading forces on components of the retractor. 
         FIG. 148A  shows a side view of another rib retractor for thoracoscopic surgery. 
         FIG. 148B  shows a top view of the above rib retractor. 
         FIG. 149A  shows a top view illustrating how an instrument mount can be added to a rib retractor for thoracoscopic surgery. 
         FIG. 149B  shows a side view of the above rib retractor. 
         FIG. 150A  shows a top view of another rib retractor with an instrument mount. 
         FIG. 150B  shows a side view of the above retractor. 
         FIG. 151  shows an oblique view of another version of the above retractor. 
         FIG. 152  shows the anatomy of a thoracic incision. 
         FIG. 153A  shows a device for retracting tissue around a thoracic incision. 
         FIG. 153B  shows an oblique view of the above device. 
         FIG. 153C  shows a side view of the above device. 
         FIG. 153D  shows an oblique view of a modified version of the above device. 
         FIG. 153E  shows a side view of the above device. 
         FIG. 154A  shows an oblique view of a clip for retracting thoracic tissue. 
         FIG. 154B  shows a sequential view of the above clip in side view. 
         FIG. 154C  shows a sequential view of the above clip being attached to a rib. 
         FIG. 154D  shows an oblique view of another clip for retracting thoracic tissue. 
         FIG. 155A  shows an oblique view of a rib-protecting port for thoracoscopy. 
         FIG. 155B  shows an end view of the above rib-protecting port. 
         FIG. 155C  shows a top view of the above rib-protecting port. 
         FIG. 155D  shows a side view of the above rib-protecting port. 
         FIG. 155E  shows a side view of one component of the above rib-protecting port. 
         FIG. 155F  shows a side view of another component of the above rib-protecting port. 
         FIG. 155G  shows an end view of the above rib-protecting port with a surgical instrument. 
         FIG. 155H  shows a sequential top view of the above rib-protecting port being positioned in an intercostal incision. 
         FIG. 156A  shows multiple views of another rib-protecting port. 
         FIG. 156B  shows an end view with exploded and assembled diagrams of the above rib-protecting port. 
         FIG. 156C  shows a sequential end view showing the above rib-protecting port being positioned in an intercostal incision. 
         FIG. 156D  shows an end view of the above rib-protecting port with three surgical instruments. 
         FIG. 157A  shows a sequential end view of the collapsed and deployed configurations of a rib-protecting port having a scissors-action. 
         FIG. 157B  shows an end view of the above rib-protecting port deployed between two ribs. 
         FIG. 158A  shows how a slanted component affects forces acting on a rib-protecting port. 
         FIG. 158B  further shows how a slanted component affects forces acting on the above rib-protecting port. 
         FIG. 159  shows the factors governing the size of the aperture of a rib-protecting port. 
         FIG. 160A  shows a rib-protecting port having straps. 
         FIG. 160B  shows the above rib-protecting port with straps in a thoracic incision. 
         FIG. 161  shows a rib-protecting port having an attached instrument mount for holding surgical instruments. 
     
    
    
     DETAILED DESCRIPTION 
     A. Constant Force (Creep) 
     Many biological materials are viscoelastic, so they exhibit time-dependent mechanical properties (Wainwright et al., 1976, Mechanical Design in Organisms, John Wiley &amp; Sons; Woo et al., 1999, Animal Models in Orthopaedic Research, CRC Press. pp. 175-96; Provenzano et al., 2001, Ann. Biomed. Eng. 29:908; Vanderby and Provenzano, 2003, J. Biomech. 10:1523; Yin and Elliott, 2004, J. Biomech. 37:907). To simplify this discussion, consider the force (the stress) required to stretch a sample of biological material. Consider  FIG. 1A  which illustrates test on a sample A 200  of a biological material whereby the sample A 200  is stretched by an instrument has a stationary unit A 202  that measures force and a moving unit A 204  that measures displacement while stretching a sample. The sample A 200  is initially stretched by having the moving unit A 204  move away from the stationary unit A 204 . The stationary unit A 204  then remains at a fixed position, holding the sample A 200  at constant deformation. Over time, the measured force decreases. This is an example of “force relaxation” or “stress relaxation”.  FIG. 1B  shows a similar test on sample A 200 . Initially, the sample A 200  is stretched; however, now moving unit A 204  moves such that a constant force is applied to sample A 200 , and now the sample A 200  stretches longer over time, a phenomenon known as “creep”. Force relaxation and creep occur when retracting an incision. The deformations of the tissues around the incision are more complex than the simple stretch shown in  FIGS. 1A and 1B , but the tissues, nevertheless, exhibit force relaxation and creep. 
     Standard practice during a sternotomy or thoracotomy is to spread the ribs slowly to a few centimeters, hold for a minute or so (allowing stress relaxation), and then slowly open over several minutes (allowing continued viscous deformation/stress relaxation) to the final opening. 
     The time dependent behavior of biological tissues has been specifically considered in the design of some retraction devices. U.S. Pat. No. 4,899,761 to Brown and Holmes discloses a distractor for separating vertebrae to measure spinal instability. The distractor of Brown and Holmes uses constant velocity deformation to standardize measurements of the mechanical properties of the motion segment of a spine to diagnose whether surgical intervention is necessary. Additionally, US Patent Application Publication No. 2006/0025656 to Buckner and Bolotin discloses stress relaxation as a means of reducing force during retraction. 
     Creep has not been considered in the design of retraction devices. However, application of a constant force ensures that (a) an unexpectedly or inappropriately large force is applied as would be the case for manual or motor driven retraction devices, and (b) viscoelastic deformation is allowed to proceed, thereby reducing the loads on anatomical elements that might rupture. 
     Different embodiments are disclosed, with reference to the figures, of assemblies and devices that apply a substantially constant force to one or more anatomical elements to move the anatomical elements. Not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure satisfies applicable legal requirements. 
       FIG. 2  illustrates a retraction device in the prior art. Retractor A 2  is a mechanical device utilizing two opposed retraction elements A 6 , A 8 . Each retraction element A 6 , A 8  has a blade A 4  that is inserted into an incision, each blade A 4  engaging one side of an incision. One retraction element A 8  is moveable with respect to the other retraction element A 6 , with motion being driven by a rack-and-pinion drive A 10  that is manually driven with a drive handle A 12 . The retraction elements A 6 , A 8  exert a force on the anatomical elements on either side of the incision to separate the anatomical elements, thereby opening the incision. A limitation of this device is that the force can vary dramatically with small displacements, thus an operator might exert an inappropriate force while attempting to move the retraction elements A 6 , A 8  only a small distance. 
       FIG. 3  illustrates an embodiment of the present invention which is designed to apply a constant force. It is a mechanical device utilizing two opposed retraction elements A 16  and A 18 . Each retraction element A 16 , A 18  has a blade A 14  (similar to blade A 4 ) that is inserted into an incision, each blade A 14  engaging one side of the incision. One retraction element A 16  is moveable with respect to the other retraction element A 18  and is mounted on a sliding carriage A 20 . The sliding carriage A 20  is driven by a spring A 26  that exerts a substantially constant force over the range of motion such that the force exerted by the opposing retraction elements A 16 , A 18  on the anatomical elements is substantially constant. The spring A 26  is connected to a moveable anchor block A 22  that allows an operator to adjust the stretch of the spring A 26  and, thereby, the force exerted by the spring A 26  on the anatomical element via the sliding carriage A 20 . The moveable anchor block A 22  has a lock screw A 24  to secure the position of moveable anchor block A 22  after adjustment. Thus, the spring A 26  serves to exert a substantially constant force, and this force cannot be accidentally exceeded by, for example, attempting to move the retraction elements A 16  and A 18  a small distance. Furthermore, if the spring A 26  does not have a large spring constant, then the distance from the sliding carriage A 20  to the moveable anchor block A 22  can be sufficiently large that small errors in adjustment of this distance do not introduce large errors in the force. 
       FIG. 4  illustrates another embodiment of the present invention which is designed to apply a substantially constant force that is larger than that depicted in  FIG. 3 . It is a mechanical device utilizing two opposed retraction elements A 30 , A 32 . Each retraction element A 30 , A 32  has a blade A 28  (similar to blade A 4 ) that is inserted into an incision, each blade A 28  engaging one side of the incision. One retraction element A 30  is moveable with respect to the other A 32 , being mounted on a sliding carriage A 34 . The sliding carriage A 34  is driven by a spring A 39  that exerts a substantially constant force over the range of motion such that the force exerted by the opposing retraction elements A 30  and A 32  on the anatomical elements is substantially constant. The spring A 39  is connected to a driven anchor block A 36  that allows an operator to adjust the stretch of the spring A 39  and, thereby, the force exerted by the spring A 39  on the anatomical element via the sliding carriage A 34 . The driven anchor block A 36  has a manual drive mechanism, such as a ratchet or a rack-and-pinion, driven by a handle A 38  for manual drive and, optionally, a lock screw to secure the position of the driven anchor block A 36  after adjustment. 
       FIG. 5  illustrates another embodiment of the present invention that provides an indicator of the force exerted on the tissue. It is a mechanical device utilizing two opposed retraction elements A 42 , A 44 . Each retraction element A 42 , A 44  has a blade A 40  (similar to blade A 4 ) that is inserted into an incision, each blade A 40  engaging one side of the incision. One retraction element A 42  is moveable with respect to the other retraction element A 44 , and is mounted on a sliding carriage A 46 . The sliding carriage A 46  is driven by a spring A 54  that exerts a substantially constant force over the range of motion such that the force exerted by the opposing retraction elements A 42 , A 44  on the anatomical elements is substantially constant. The spring A 54  is connected to a driven anchor block A 48  that allows an operator to adjust the stretch of the spring A 54  and, thereby, the force exerted by the spring A 54  on the anatomical element via the sliding carriage A 46 . The driven anchor block A 48  has a manual drive mechanism, such as a ratchet or a rack-and-pinion drive, driven by a handle A 50  for manual drive and, optionally, a lock screw to secure the position of the driven anchor block A 48  after adjustment. There is also a force indicator A 60  that is a graduated rod, with graduations indicating force exerted by the spring A 54  for the indicated stretch, that is used to indicate where to place the driven anchor block A 48  or whether the driven anchor block A 48  should be moved to maintain appropriate stretch of the spring A 54  to maintain a substantially constant force on the moveable retraction element A 42 . The position of the force indicator A 60  is secured by an indicator set screw A 52 . There is also a mechanical stop A 56  with its position secured by the stop set screw A 58  such that the motion of the sliding carriage A 46  cannot exceed a predetermined motion. 
       FIG. 6  illustrates another embodiment of the present invention that uses a pneumatic piston to exert a substantially constant force. It is a mechanical device utilizing two opposed retraction elements A 64 , A 66 . Each retraction element A 64 , A 66  has a blade A 62  (similar to blade A 4 ) that is inserted into an incision, each blade A 62  engaging one side of the incision. One retraction element A 64  is moveable with respect to the other retraction element A 66 , and is mounted on a sliding carriage A 68 . The sliding carriage A 68  is driven by a pneumatic piston A 74  that exerts a substantially constant force over the range of motion such that the force exerted by the opposing retraction elements A 64 , A 66  on the anatomical elements is substantially constant. The piston A 74  is connected to a pressure reservoir A 80  by a pressure hose A 76 . The pressure reservoir A 80  has sufficient volume of gas such that changes in the volume of the piston A 74  as the piston A 74  moves do not introduce large changes in the pressure. The pressure reservoir A 80  can be fitted with a pressure gage A 78  that allows an operator to observe the pressure. The pressure reservoir A 80  can be connected to a pressure pump A 86  that permits an operator to increase the pressure in the reservoir both to initiate the force at the piston A 74  or to prevent pressure from dropping in the pressure reservoir A 80  should the motion of the piston A 74  be too large, causing a drop in pressure, or to allow the piston A 74  to change from a first substantially constant force to a second substantially constant force. The pressure reservoir A 80  also can have a bleed valve A 82  that allows an operator to reduce the pressure, to release the pressure, or to move from a first substantially constant force to a second substantially constant force. Additionally, there can be a mechanical stop A 70  with its position secured by a stop set screw A 72  such that the motion of the sliding carriage A 68  cannot exceed a predetermined motion. 
       FIG. 7  illustrates another embodiment of the present invention which utilizes a motorized drive to exert a substantially constant force. It is a mechanical device utilizing two opposed retraction elements A 90 , A 92 . Each retraction element A 90 , A 92  has a blade A 88  (similar to blade A 4 ) that is inserted into an incision, each blade A 88  engaging one side of the incision. One retraction element A 90  is moveable with respect to the other retraction element A 92 , and is mounted on a sliding carriage A 94 . The sliding carriage A 94  is driven by a motor A 96  that exerts a substantially constant force over the range of motion such that the force exerted by the opposing retraction elements A 90 , A 92  on the anatomical elements is substantially constant. The motor A 96  is connected by an electrical cable A 98  to a motor controller (not shown). The motor controller ensures that the torque generated by the motor A 96  is substantially constant such that the force exerted by the opposing retraction elements A 90 , A 92  on the anatomical elements is substantially constant. 
       FIG. 8  illustrates another embodiment of the present invention which utilizes a feedback system to exert a substantially constant force. It is a mechanical device utilizing two opposed retraction elements A 102 , A 104 . Each retraction element A 102 , A 104  has a blade A 100  (similar to blade A 4 ) that is inserted into an incision, each blade A 100  engaging one side of the incision. One retraction element A 102  is moveable with respect to the other A 104  and is mounted on a sliding carriage A 106 . The sliding carriage A 106  is driven by a motor A 111  that exerts a substantially constant force over the range of motion such that the force exerted by the opposing retraction elements A 102 , A 104  on the anatomical elements is substantially constant. The motor A 111  is connected by an electrical cable A 108  to a motor controller A 114 . A force measuring device A 110  is attached to the retraction element A 104  (or optionally to retraction element A 102 ) such that the force measuring device A 110  determines the force exerted by the retraction element A 104  on the anatomical element. The force measuring device A 110  is connected to the motor controller A 114  via a signal cable A 112  such that the force is transmitted as a signal to the motor controller A 114 . The motor controller A 114  implements a feedback loop such that the force measured by the force measuring device A 110  is substantially constant such that the force exerted by the opposing retraction elements A 102 , A 104  on the anatomical elements is substantially constant. Motor controller A 114  can, optionally, be connected to another device (not shown) by cable A 116  to, for example, provide additional processing abilities or to provide a display of force. 
       FIG. 9  illustrates another embodiment of the present invention that is similar to that disclosed in  FIG. 7  but in which a motor A 124  is mounted differently. Motor A 124  is directly attached to retractor element A 120  by mount A 126 , and retractor element A 122  is directly attached to the linear drive shaft A 127 . This permits use of a differently configured motor, possibly with integrated motor controller (not shown) or connected by a cable A 128 . 
       FIG. 10  illustrates another embodiment of the present invention which is a mechanical device utilizing multiple moveable retraction elements A 132 , A 134  that are mounted on a frame A 130 . Each moveable retraction element A 132 , A 134  can be independently moved. The various mechanisms described in  FIGS. 3 through 9  for exerting a substantially constant force via the blades A 138  (examples of blades include curved or bent blades that extend into the incision) of retraction elements can be implemented for each of these moveable retraction elements A 132 , A 134 . The mechanisms can be implemented such that the force exerted by each individual moveable retraction element A 132 , A 134  is independent of the force exerted by any other moveable retraction element A 132 , A 134 . Optionally, position measuring devices (not shown, examples include linear potentiometers, LVDTs, optical encoders) can be placed on each moveable retraction element A 132  and A 134  such that an independent measure of position is determined and displayed on a visual position indicator A 136  on the frame A 130 . Alternatively, the force exerted by blades A 138  of retraction elements A 132 , A 134  on their respective anatomical elements can be measured by a force measuring device (not shown, examples include appropriately placed strain gauges), and the forces displayed on indicators (not shown) also on the frame A 130 . An electrical cable A 137  can be used to provide power to electrical devices on the frame A 130  and to convey electrical signals from electrical devices on the frame A 130  or elsewhere on the retractor to a separate motor controller (not shown) or computer (not shown). 
     B. Oscillating Loading 
     Deformation of biological materials during the first phase of retraction is usually done one-directionally—the deformation pushes anatomical elements apart (e.g., thoracotomy) or stretches arteries open (e.g., angioplasty). The direction of motion during deformation is rarely reversed and then only to correct for errors, such as to reposition a rib retractor that has slipped or to free a blood vessel that has accidentally been captured under a retractor blade. 
     Trauma to the displaced tissue is a common consequence of these deformations. Ribs fracture during thoracotomy, costosternal joints dislocate during sternotomy, muscles tear during retraction, and blood vessel walls rip during angioplasty. Even for those deformations used to change anatomical position or shape, damage to the tissue can be larger than desired; for example, a fibrous capsule might tear when stretching is preferred. 
     Many biological materials are viscoelastic, so they exhibit time-dependent mechanical properties (Wainwright, Biggs et al. 1976; Woo, Manson et al. 1999; Provenzano, Lakes et al. 2001; Vanderby and Provenzano 2003; Yin and Elliott 2004; Erdogan, Erdogan et al. 2005). 
     One behavior of biological complex materials that has not been considered in the design of retractors is “work” or “stress” softening. Work softening is evident during cyclic loading/unloading and is characterized as a reduction in the force at a given deformation during successive cycles, relative to the initial loading. Viscoelastic materials exhibit stress softening, but the initial stiffness recovers with rest for most non-biological viscoelastic materials (e.g. filled rubbers). For many biological materials, initial stiffness is not recovered, reflecting changes in the non-viscous components of the material, thought to arise from the irreversible dislocation of components (such as the unentanglement of tangled polymers), from plastic deformation of polymeric components, or from failure of microscopic components (such as the fracture of single molecules). The underlying phenomenology of stress softening is not well understood (Horgan, Ogden et al. 2004), especially for biological materials (Vincent 1975; Weisman, Pope et al. 1980; Fleck and Eifler 2003; Kirton, Taberner et al. 2004; Kirton, Taberner et al. 2004; Speich, Borgsmiller et al. 2005; Chaudhuri, Parekh et al. 2007; Dorfmann, Trimmer et al. 2007). Nevertheless, the generally observed phenomenon of work softening, or any change in material property when subjected to oscillating loading, can be exploited. 
     Thus, an alternate means of deforming tissue, relative to traditional unidirectional loading, is to cyclically load the tissue. For example, the blades of a retractor move forward and backward, or an angioplasty balloon cyclically inflates and deflates. 
     Oscillatory motion provides at least three benefits. First, it can “work soften” the material, decreasing the forces required to achieve a deformation. Second, oscillatory motion can be used to measure the viscoelastic parameters of the material (elastic and viscous moduli), and the results of these measurements can optionally be used to guide additional manipulations of the tissue. Third, a large number of small deformations in series can lead to small scale failure of components thus avoiding catastrophic failure of the entire structure—similar to the release of energy at a geologic fault line by many small tremors as opposed to one large earthquake. 
     Note that oscillation can be at different frequencies. A frequency sweep can be used to identify a harmonic. “White noise” can be used in dynamic analysis to determine multiple resonant frequencies that can arise from the composite nature of biological materials. Oscillation can be conducted at two different frequencies, either one following the other or with both frequencies superimposed, to act upon different components of the composite material comprising the tissue. For example, a lower frequency can be used to work soften a ligament and a higher frequency can be used to work soften a polymer by vibrating the molecules in the polymer. These frequencies can be fractions of a Hertz to a megaHertz. Thus, oscillations can include mechanical vibrations, acoustic vibrations, ultrasound, and any other reciprocating motion. 
     B.1 Reduction of the Force of Retraction &amp; Reducing Catastrophic Failure 
     B.1.1 Tissue Spreaders and Retractors 
     Oscillatory motion of a spreader or retractor can be generated in many different ways, depending on the necessary frequency and amplitude of actuation, which when coupled with the force of retraction and the mass of the oscillating system (retractor blade and tissue) determine the power requirements for the motor or other actuator. 
     For the following discussion, two motions are defined:
         1. the retraction motion, which is the overall, or average, motion during the first phase of retraction that is used to achieve the final deformation of the tissue; and   2. the oscillation motion, which is a motion that is superimposed on the retraction motion.       

     As shown in  FIG. 11A , the two motions can be performed separately in time, with a retraction motion B 4  proceeding to a given separation and then pausing, followed by an oscillation motion B 2 . Alternatively, as shown in  FIG. 11B , the retraction motion B 4  and the oscillation motion B 2  can be superimposed in time, thus the retraction motion B 6  would be a near-zero frequency component of the motion, and the oscillation motion would be the higher frequency component. 
     B.1.1.1 Experimental Results from Oscillating Loading of Tissues 
     B.1.1.1.1 An Example of a Retractor for Oscillating Motion 
     A retractor is shown in  FIG. 12  that uses a bi-directional ball screw B 10  (i.e., having two bearings that travel in opposite directions) that is driven by a stepper motor B 8 , which here is an MDrive 23Plus made by Intelligent Motion Systems, Inc. The bi-directional ball screw B 10  is mounted to a rail B 14  with two linear translation stages B 12 , which here are LWHG 25 made by IKO, such that each translation stage B 12  attaches to one of the bearings of the bi-directional ball screw B 10 , thus when the bi-directional ball screw B 10  is rotated by the motor B 8 , the translation stages B 12  travel in opposite directions. A retractor arm B 18 , fabricated by hand from mild steel angle iron that was cut/bent/welded into shape, is mounted to each translation stage B 12 . Each retractor arm B 18  has a retractor blade B 20  that is fabricated by hand with mild steel. 
     A linear potentiometer B 16 , which in this case is a 5 kOhm 100 mm made by Schaevitz, is used to measure separation of the retractor blades B 20 . The static mount of the potentiometer B 16  is affixed to the rail B 14 , and the piston of potentiometer B 16  is affixed to one of the translation stages B 12 . Note that any means of measuring displacement can be used here, such as optical encoders, contact and non-contact proximity sensors, digital calipers, and the like. 
     The retractor blades B 20  are instrumented with a full-bridge strain gauge assembly which includes two (2) gauges, which in this case are model CEA-06-125UN-350 made by Vishay Micro-Measurements, on each side of each retractor blade B 20 . The signal from the strain gauges is then amplified by a signal conditioner (not shown) which in this case is a Model OM-2 from 1-800-LoadCells. Note that force can be measured by any of several means, such as drive current on the motor (and other means of measuring torque on the drive mechanism), fiber optic strain gauges, optical sensors of deformation, and the like. 
     All signals from the potentiometer B 16  and the signal conditioners/strain gauges are read by a Windows-based computer using a data acquisition card, which in this case is a National Instruments Model USB-6211 and software, such as LabVIEW made by National Instruments, with software prepared by Katya Prince of Prince Consulting. 
     The stepper motor B 8  is controlled with IMS Terminal software, made by made by Intelligent Motion Systems, Inc. Note that a servo-motor can also be used. 
     The strain gauges were calibrated by hanging known weights from each blade B 20  of the retractor. The linear potentiometer B 16  was calibrated with a metric ruler. 
     B. 1.1.1.2 Experiments 
     A series of experiments were conducted with the retractor presented in  FIG. 12  using parts from pig cadavers. The parts were a “front quarter” purchased from Nahunta Pork Center (Pikeville, N.C.). A front quarter is basically a whole pig cut at the waist (forming a front half) and split down the vertebrae (forming left and right quarters); thus, each quarter had an intact rib cage (one side), spine (bisected), and shoulder. All parts had been refrigerated after slaughter, used within 24 hours of slaughter, and warmed by immersion in warm water (while wrapped in a plastic bag to prevent soaking of the tissue) to near body temperature (31° C. to 37° C.). The quarters ranged in size from 8 to 12 kg. 
     Thoracotomies were performed between three (3) to four (4) rib pairs on each quarter, almost always performing an incision between ribs five (5) and six (6), seven (7) and eight (8), nine (9) and ten (10), and eleven (11) and twelve (12). Thoracotomies were performed by:
         cutting the skin with a scalpel over the range of the thoracotomy;   bisecting the muscles overlying the ribs with a scalpel;   cutting through the intercostal tissues with a scalpel;   pushing a finger between the ribs to make a small opening;   inserting the closed blades of the retractor into the opening;   positioning the retractor such that the blades sat just dorsal of the midline and its axis of opening were parallel with the midline; and   initiating opening according to a specified algorithm via computer control of the stepper motor.       

     Incisions were typically 110 mm to 130 mm long, with longer incisions being performed on larger quarters. 
     Experimental retractions with the retractor shown in  FIG. 13  are the first simultaneous measurements of force and displacement during thoracic retraction. 
       FIG. 13  shows the displacement B 21  of the blades B 20  (i.e., the distance between the blades B 20 ), and force B 22  on one blade B 20  with respect to time for a “standard retraction”, similar to that defined by Bolotin et al. (Bolotin, Buckner et al. 2007), which proceeds as follows:
         a first move, opening to 40 mm in one (1) minute (⅔ of final opening),   pause 2 minutes for force relaxation,   a second move, opening to 60 mm in three (3) minutes (to the final opening).       

     Thus, a total opening of 60 mm is reached in 6 minutes in this example. Retraction was of a fully automated—the computer controlled the motor B 8 , and the motor drove the blades B 20  apart. Each of the two moves is constant velocity (40 mm/min for the first and 6.8 mm/min for the second). This somewhat matches the pace described by thoracic surgeons, but there is no standard clinical practice. Surgeons use a procedure defined by their training, personal experience, patient condition, and estimates of force applied at the handle of a hand-cranked retractor. Force relaxation, as described by Buckner and Bolotin (Buckner and Bolotin 2006; Bolotin, Buckner et al. 2007) is evident during a two-minute pause B 23 —the force required to maintain the 40 mm opening decreases with time. The points on the force B 22  marked with arrows B 24  mark significant tissue breaks, as evidenced by the change in the force/time slope and by audible “snaps” during the retraction. 
       FIG. 14A  shows a retraction in which the displacement B 21  is shown by the upper trace, and force B 22  on one blade is shown by the lower trace. There are four small retractions B 25  (2 mm each over 10 seconds, velocity=0.2 mm/s) of which the second two were followed by pauses B 26  of approximately 50 seconds and the first two were followed by pauses B 27  of 50 seconds interrupted by oscillation motions B 28 . The oscillation motions B 28  were 11 Hz with one (1) mm amplitude, 400 cycles, and given the high frequency of oscillation, they appear on the displacement trace as thickened regions of the trace. Force relaxation was seen for each of the four pauses B 26 , B 27 , as evidenced by the decrease in force that follows the onset of each pause. During each oscillation motion B 28 , the force oscillated with each cycle of opening/closing. Importantly, when the force minima are examined over successive cycles, the force dropped rapidly. 
       FIG. 14B  shows what the force/time curve looks like when only the force minima are considered—there is an accelerated force relaxation (AFR) B 30 , during the oscillation motions B 28 , as illustrated by the grey regions in  FIG. 14B . Thus, while the force declined during normal force relaxation (NFR) B 32 , as depicted during the two-minute pause in  FIG. 13  and during the 3rd and 4th pauses B 26  in  FIG. 14B , the force declined much more rapidly during the AFR B 30  (1st and 2nd pauses B 27  of  FIG. 14B ) than during the NFR B 32 . Also, the AFR B 30  had a larger magnitude when the oscillation motion B 28  was initiated earlier in the pause, as evidenced when the 2nd pause/oscillation motion B 28  is compared to the 1st pause/oscillation motion B 28 —the oscillation motion B 28  started sooner in the 2nd pause oscillation and a larger AFR B 30  was seen. 
       FIG. 15  shows a retraction in which a different trajectory is followed than for a standard retraction, which is called an “oscillating retraction” for this discussion. Displacement, or separation of the blades B 20 , is shown by trace B 36 . Force on one blade B 20  is shown by trace B 38 . During the experiment depicted in  FIG. 15 , the incision was opened and oscillated repeatedly. Three general features observed during oscillating retractions like this are:
         1. the retraction force does not peak as high as is seen during the first opening of the standard clinical pace retraction (compare to  FIG. 13 );   2. the maximum force during oscillating retraction is frequently lower than the maximum force in a standard clinical pace (compare to  FIG. 13 ); and   3. there are fewer large, obvious breaks during oscillating retractions than during standard clinical pace retractions.       

     The latter point is shown in  FIG. 13  where breaks B 24  are marked with arrows B 24 —the breaks appear as rapid changes in the slope of the force/time trace that are almost always accompanied by loud snaps or cracks. These events are common during standard retractions, especially in the final 20 seconds of the first, one-minute opening and during the last two minutes of the second, three-minute opening. These rapid changes in slope, accompanied by loud snaps or cracks, are almost never seen/heard during oscillating retractions. This point is especially important in light of the tissue trauma that is frequently observed during normal surgical practice—broken ribs, dislocated costo-chondral joints, and torn ligaments and tendons (Vander Salm, Cutler et al. 1982; Greenwald, Baisden et al. 1983; Baisden, Greenwald et al. 1984; Woodring, Royer et al. 1985; Bolotin, Buckner et al. 2007; Lewis 2007). 
     Thus, there are several advantages conferred by oscillating retractions:
         1. accelerated force relaxation rapidly decreases the force required for opening;   2. the large peak in force seen during the first, one-minute opening of a standard retraction is not evident;   3. the maximum force experienced during retraction is frequently smaller during oscillating retraction; and   4. there are fewer, large tissue breaks during oscillating retraction.
 
All of these advantages can result in reduced tissue trauma during retraction.
 
B.1.1.2 Single-Actuator Retractors
       

       FIG. 16  shows a Finochietto retractor in the prior art (similar to retractor A 2  in  FIG. 2 ). It has a fixed retraction element B 44  attached to a rack B 45  of a rack-and-pinion drive B 46 . A moveable retraction element B 42  is attached to the rack and pinion drive B 46  that drives motion B 52  of the moveable retraction element when manual handle B 48  is rotated. Each of the retraction elements B 42 , B 44  has a single blade B 40  (similar to blade A 4  in  FIG. 2 ) that engages the tissue to be retracted. 
     One way to implement a retractor with both a retraction motion and an oscillation motion is to use a single actuator that drives both the retraction and the oscillation motions, such as the retractor shown in  FIG. 12 . 
       FIG. 17  shows another embodiment of a single actuator retractor, in which a motor B 60  replaces the hand crank B 48  of a typical Finochietto-style rack-and-pinion retractor. The motor B 60  can be any motor appropriate for generating the desired motions B 52 , such as a servo-motor or a stepper motor. The instructions to the motor generate any desired motions for retraction motions as well as oscillations for oscillation motions. Thus, the retractor can perform retraction and oscillation motions that either are separated in time or are superimposed in time. 
       FIG. 18  shows another embodiment in which the rack-and-pinion drive B 46  is replaced with a lead screw B 62  turned by a motor B 74 , either a stepper motor or a servomotor. The motor B 74  moves the moveable retraction element B 70  with respect to fixed retraction element B 72  to achieve the desired motion B 68 . Control of the motor B 74  can be either on-board with the motor B 74  or off-board connected by an electrical cable B 76 . 
     In yet another embodiment shown in  FIG. 19 , the rack-and-pinion drive B 46  is replaced by a hydraulic cylinder B 82 . A pressure pump B 94  is connected to a pressure reservoir B 88  by a pressure hose  92 , and the pressure reservoir B 88  is connected to the hydraulic cylinder B 82  by a pressure hose B 84 . Pressurization of the hydraulic fluid in the pressure pump B 94  pressurizes the pressure reservoir B 88  that feeds the hydraulic cylinder B 82  which forces a moveable retraction element B 78  and a fixed retraction element B 80  apart. A pressure gauge B 86  reports the pressure in the system, and a bleed valve B 90  permits release of pressure. Oscillation of the pressure in the hydraulic fluid generates the oscillation motion. Oscillation can be driven by one of several means, such as a piston B 85  attached to pressure reservoir B 88  that is driven in and out by motor B 87 . 
     B.1.1.3 Dual-Actuator Retractors 
     The retraction and oscillation motions can be generated by separate actuators. For example, a first actuator drives the retraction motion, and a second actuator drives the oscillation motion. This confers several advantages:
         different actuators can be matched to the different power requirements for the two different motions;   different actuators can be matched to the displacements required for the two different motions; and   different distributions of masses are permitted, e.g. removing bulky components required for the large amplitude motions of the retraction motion from the components that must be driven at higher frequency but lower amplitude for the oscillation motions.       

     In one embodiment shown in  FIG. 20 , a retraction motion is generated by a first actuator which in this example is a rack-and-pinion drive B 98  driven by hand crank B 99  along a rack B 100  on a conventional Finochietto-style retractor. The first actuator moves a moveable retraction element B 101 . The oscillation motion B 102  is generated by a motor-driven acentrically mounted cam B 104  that rides on two surfaces, a first surface B 106  attached to the rack B 100  of the retractor and a second surface B 107  attached to an oscillation motion element B 108  that is mounted to the rack B 100  by a hinge B 109 . When the acentrically mounted cam B 104  rotates, the oscillation motion element B 108  oscillates with motion B 102  with the frequency of rotation of the motor and with amplitude determined by the diameter and acentricity of the cam B 104  and the lever-arm of the oscillation motion element B 108 . 
     In another embodiment shown in  FIG. 21 , a hand-cranked rack-and-pinion drive B 110  is used for performing the retraction motion. A second actuator B 112  drives the oscillation motion. The second actuator B 112  presented here is a thin hydraulic cylinder or pressure pad B 120  mounted on each retractor blade B 114  and is driven by a hydraulic system capable of generating the necessary pressures and volumes to drive the requisite motion. In this example, a pressure hose B 124  attached to pressure pads B 120  and to an external pressure source (not shown) permits cyclic oscillation of the pressure pads B 120  via an oscillating flow B 126  of fluid. The second actuator B 112  could be any actuator that mounts to the retractor blades B 114  of retraction elements B 116 , B 118 , such as a voice coil, a linear motor, a hydraulic cylinder or other actuator capable of generating the oscillation motion. A semi-transparent view of a retractor blade  114  is provided to allow a more complete view of the assembly. 
     In another embodiment shown in  FIG. 22 , a retractor has a motorized lead screw drive B 130  that drives along the lead screw B 132  for the retraction motion. Voice coils B 134  mounted onto blades B 136  of the retraction elements drive an oscillation motion B 138 . 
     B.1.1.4 Multiple-Actuator Retractors 
       FIG. 23  shows an embodiment in which a retractor B 140  with multiple arms and actuators can apply oscillating loads. The retractor B 140  has a frame B 142  to which independent actuators B 144  and arms with attached blades B 146  are mounted. The actuators B 144  can be motors, hydraulic cylinders, or other appropriate actuators and can be actuated by one of a variety of methods, including all moving in synchrony, opposing pairs of actuators B 144  or other functional groupings of actuators B 144  moving in synchrony but not in synchrony with other functional groupings of actuators B 144 , or all actuators B 144  moving independently. The actuators B 144  perform both the retraction motion and the oscillation motion. The actuators B 144  can be wire or cable wound onto a spool that is driven by a servo-motor or by a manually driven worm drive, with the blades B 146  of the retraction elements attached to the wire or cable. Optionally, the retractor B 140  can be instrumented with sensors that measure the force on the blades B 146  of the retraction elements, or the displacements of the blades B 146  of the retraction elements, or any other parameter relevant to the motion of the blades B 146  of the retraction elements. The output from the sensors can be displayed by indicators B 148  on the frame of the retractor B 142  or on the monitor of a computer attached to the retractor B 140  via an electrical cable B 150 . 
       FIG. 24  shows another embodiment of a retractor B 160  with multiple arms and actuators. Here there is a first actuator that generates the retraction motion by separating two halves B 162 , B 164  of the retractor frame that resembles a Finochietto-style retractor. This first actuator is a rack-and-pinion B 166  driven by a hand crank, but, optionally, could be driven by a motor or other appropriate actuator. Additional actuators B 168  attached to both halves B 162 , B 164  of the retractor frame drive the oscillation motion of retractor blades B 170 . The additional actuators B 168  can be driven by any appropriate actuator, such as a motor, a voice coil, a piezoelectric driver, or a hydraulic actuator. The additional actuators B 168  can be rack-and-pinion in which the retractor blades B 170  are attached to the rack. The additional actuators B 168  can be wire or cable wound onto a spool that is driven by a servo-motor with the cable attaching to the blade B 170  of the retraction element. Alternatively, the actuators can be linear motors. 
     B.1.2 Angioplasty Balloons and Stents 
     Another common actuator for deforming anatomical tissues is the balloon used for angioplasty with or without placement of a stent. The deflated balloon is inserted via a catheter into the blood vessel to be enlarged, and the balloon is inflated such that it presses against the walls of the blood vessel, enlarging the radius of that portion of the blood vessel. Similar methods are used in valvuloplasty, where the diameter of a heart valve is enlarged. Similar methods are used in tuboplasty to enlarge portions of the urinary tract and other surgical procedures to enlarge tubular anatomical elements, such as biliary tubes. 
     In the prior art, motions of the balloons are one-directional—they are simply inflated with a sterile fluid. Sometimes several balloons of increasing diameter are used to enlarge the anatomical element in increments, but each balloon is simply opened. 
     For angioplasty and valvuloplasty, inflation of the balloon is similar to the “retraction motion” described earlier for retractors. We present inventions to impose an “oscillation motion”, as described above. To simplify the following discussion, each cycle of oscillation is divided into an “inflation phase” and a “deflation phase”. 
     In one embodiment depicted in  FIG. 25 , an angioplasty balloon B 200  is inflated by a sterile fluid that passes through a catheter B 202  from a first syringe B 204  that is used to generate the pressure to inflate the balloon B 200 . The retraction motion is inflation of the balloon B 200 , which is driven by the plunger B 206  of the first syringe B 204 . These retraction motions are shown in solid black, single-headed arrows. A second syringe B 210  is also connected to the catheter B 202 . A plunger B 212  of the second syringe B 210  oscillates in and out, cycling a pressure that drives the oscillation motion of the balloon B 200 . The oscillation motion of the balloon B 200 , and the associated oscillating drive of the plunger B 212 , are shown as asymmetric, double-headed arrows with one arrow shape depicting the inflation phase and the other arrow shape depicting the deflation phase. Thus, oscillation of the pressure is achieved with a reciprocating motion of the plunger B 212  such that the plunger B 212  stroke length determines the amplitude of the oscillation and the plunger B 212  stroke frequency determines the frequency of the oscillation motion. Motion of fluid during the deflation phase can be driven both by the combined elastic strain in the wall of the balloon B 200  and in the wall of the anatomical element and by the rearward motion of the plunger B 212 . 
     In another embodiment depicted in  FIG. 26 , motion of fluid up the catheter B 202  for inflation of the balloon B 200  is driven by a first syringe B 204  as described for  FIG. 25 . The oscillation motion is driven by a piston B 224  that impinges on a drive membrane B 222  separating fluid from the piston B 224 . This separation of fluid from the piston B 224  facilitates cleaning and sterilization of the moving parts for maintenance of sterility of the fluid. During the oscillation motion, the motion of fluid up the catheter B 202  during the inflation phase is driven by the piston B 224 . Motion of fluid in the opposite direction during the deflation phase is driven by the combined elastic strain in the wall of the balloon B 200  and in the wall of the anatomical element and by rearward motion of the piston B 224 . 
     One limitation of driving the inflation and deflation motions of the fluid up and down the lumen of the catheter is the resistance to fluid motion imposed by the long, narrow catheter lumen. This high resistance to fluid motion limits the frequencies and amplitudes attainable for the oscillation motion. 
     One means of eliminating the limitation is to restrict fluid motion during both phases of the oscillation motion to short distances through larger diameter connections. This is achieved in the embodiments depicted in  FIGS. 27 and 28 . Consider  FIG. 27 , the balloon has two compartments. A first larger diameter compartment B 207  functions to force the anatomical element B 211  open (see  FIG. 27C ), and a second smaller diameter compartment B 206  functions as a fluid reservoir. Second compartment B 206  can be placed upstream, as depicted, or downstream from the first compartment B 207 . Fluid flows easily between the two compartments through connecting channel B 208 . The retraction motion is driven, as in the prior art, by pumping fluid up the catheter B 212  to inflate the first compartment B 207 . The oscillation motion is generated by forcing fluid back and forth from the second compartment B 206  to the first compartment B 207 . The oscillation motion is thus driven by a second actuator, comprising the second compartment B 206 . 
     One means for driving the motions of the second compartment B 206  is shown in  FIG. 28A .  FIG. 28B  shows an enlarged view of second compartment B 206 . The second compartment B 222  can be helically wound B 226  with a wire or cable made of a shape memory material, such as Nitinol. Electrical actuation of the Nitinol decreases the diameter, and thus the volume, of the second compartment B 222 , forcing fluid through connecting channel B 208  into the first compartment B 207  to drive the inflation phase of the oscillation motion. The deflation phase is then driven by the combined elastic strain in the wall of first compartment B 207  and the wall of the anatomical element. The elastic strain driving the deflation phase can also be augmented by a second helical wind (not shown) of spring material around the first balloon compartment B 207 . 
     In another embodiment depicted in  FIG. 29 , the angioplasty balloon B 230  has a single compartment B 232  and is shaped as a cylinder, and the oscillation motion is generated in the walls of the compartment B 232 .  FIG. 29A  shows compartment B 232  deflated, and  FIG. 29B  shows compartment B 232  inflated. Compartment B 232  is inflated by flow B 234  through catheter B 212 , driving the retraction motion. The two phases of oscillation motion are driven by a first helical wind B 236  of shape-memory material, such as Nitinol, and a second helical wind B 238  of an elastic spring material. The first helical wind B 236  of shape-memory material causes the cylindrical compartment B 232  to decrease diameter (and elongate to maintain constant volume), thereby driving the deflation phase. The second helical wind B 238  of spring material stores elastic strain energy during the deflation phase that then drives the inflation phase when electrical actuation of the first helical wind B 236  ends. Similarly, the two helical winds B 236 , B 238  can be wound such that electrical actuation of the first helical wind of shape-memory material increases the diameter of the balloon driving the inflation phase, and elastic energy storage in the second helical wind of material decreases the diameter of the balloon driving the deflation phase. Furthermore, shape memory material and elastic spring material can be included in the first helical wind B 236  and in the second helical wind B 238  such that actuation of one helical wind B 236 , B 238  and then the other helical wind B 236 , B 238  drives both phases of oscillation. Furthermore, only shape memory material can be included in the first helical wind B 236  and in the second helical wind B 238  such that actuation of one helical wind B 236 , B 238  and then the other helical wind B 236 , B 238  drives both phases of oscillation. 
     In another embodiment depicted in  FIG. 30A , the angioplasty balloon B 250  has a single compartment B 252  and the oscillation motion is driven by a bubble generated inside the compartment B 252 . The retraction motion is driven by inflation of the compartment B 252  by fluid motion up the lumen of the catheter B 212 . The oscillation motion is driven by a small electrical heater B 254  mounted onto a wire B 256  inside catheter B 212  underlying the compartment B 252  such that a bubble B 258  of water vapor is formed, driving the inflation phase of the oscillation motion. Heat dissipation to the surrounding fluid and tissue causes the vapor bubble B 258  to collapse, driving the deflation phase of the oscillation motion. Similarly, electrolytic bubble generation of a bubble B 258  inside the compartment B 252  could drive the oscillation motion. 
     In another embodiment depicted in  FIG. 30B , high frequency of oscillation is generated by a piezoelement. Angioplasty balloon B 270  has a single compartment B 272  and the oscillation motion is driven by a piezo-vibrator B 274  mounted on a wire B 276  inside the compartment B 272 . The retraction motion is driven by inflation of the compartment B 272  by fluid motion up the lumen of the catheter B 212 . The oscillation motion is driven by actuation of piezo-vibrator B 274  which emits high-frequency pressure waves B 280  which transmit as high-frequency, low-amplitude oscillations of the wall of compartment B 272 . 
     B.2 Measurement of Tissue Properties 
     Oscillating deformation of a tissue with simultaneous measurement of selected parameters (e.g., force, displacement) can yield important information about the tissue&#39;s material properties and physiological state. 
     Leveque et al. (Leveque, Rasseneur et al. 1981) disclose oscillating loading for measurement of the Young&#39;s modulus and the internal damping factor of a viscoelastic material, including excised biological tissues, by oscillating loading. Long et al. (Long 1992; Long, Pabst et al. 1997) disclose measurement of the dynamic bending stiffness and damping coefficients of isolated intervertebral joints that are loaded by oscillating bending. 
     There are two disclosures for measuring the mechanical properties of an intact biological tissue:
         1. Brown and Holmes (Brown and Holmes 1990) disclose a method for measuring the mechanical properties of intact tissues, and they disclose only constant velocity deformation as a means for standardizing measurements for spinal instability; and   2. Huszar (Huszar 1984) discloses a modified version of the technique of Leveque et al. (Leveque, Rasseneur et al. 1981) to make a measuring device that applies a force on the uterine cervix to measure the modulus of extensibility of the tissue in situ; the purpose is to assess the status of the cervix during obstetric procedures, especially for pregnancy, and Huszar also suggests use for ear or skin.       

     Measurements on intact tissues, as opposed to excised tissues, limits direct applicability of the above techniques disclosed for measuring mechanical properties by oscillating loading. This is due to the unknown dimensions of the intact tissues, unknown mass and connectivity to surrounding tissues, etc. However, modifications we disclose permit the collection of information relevant to the mechanics and physiology of the tissue being retracted or dilated. Importantly, these modifications can provide information relevant to the processes of retraction or dilation. 
     In one embodiment, as disclosed in Section B.1.1.1 and shown in  FIG. 12 , simultaneous measurement of force and displacement during oscillating loading permit measurement of effective stiffness (the slope of force/distance of displacement) and of viscous losses (area bound by hysteresis of the force/displacement curve seen during one cycle of loading/unloading). Furthermore, accelerated force relaxation AFR can be measured as disclosed in Section B.1.1.1.2 and shown in  FIG. 31  to determine when to end an oscillation period. As shown in  FIG. 31 , a time constant τ for accelerated force relaxation AFR can be determined by fitting a decay curve to the minimum force points for each oscillation, and cyclic loading can then be terminated when a fraction of the time constant has expired. For example, when retracting a tissue, the following sequence of steps can be followed:
         (1) a retraction motion B 300  is performed;   (2) retraction is paused;   (3) an oscillation motion B 302  is performed with measurement of force and real-time calculation of the time constant τ, B 304  of the force relaxation as shown in  FIG. 31 ; such that when a time equal to the time constant τ, B 304  has elapsed;   (4) oscillation motion B 302  ceases; and   (5) retraction motion B 300  resumes.
 
This algorithm can be used to optimize the reduction of force during a retraction (maximum force decrease in the smallest amount of time). Similarly, the oscillation can be stopped when some fraction or multiple of the time constant is achieved. Conversely, the force decrease can be monitored, and the oscillation motion terminated when the force has declined by a specified amount or percentage of the starting force.
       

     In another embodiment shown in  FIG. 32 , decline in effective stiffness of a tissue can be measured arising from oscillating loading via phenomena such as work softening. Effective stiffness (force/displacement, dF/dx where F is force and x is distance) decreases in a tissue that displays work softening. Measurement of force and distance during repeated cycles of loading/unloading permit comparison of stiffness during each successive loading (or unloading). Thus, as shown in  FIG. 32 , which shows two (2) cycles of loading, the force displacement trace begins at B 310  and ends at B 312 . As an example, the effective stiffness of the material during the first cycle of loading is estimated as the slope of the line B 314 , drawn between the limits of minimum and maximum displacement for that cycle, and, again, during the second cycle as the slope of the line B 316 . The decrease in slope from line B 314  to line B 316  is then used as an estimate of the degree of work softening. Comparisons of effective stiffness can be made repeatedly, in real time, during oscillating loading. The embodiment of a retractor described in Section B.1.1.1 would serve for such measurements. Another embodiment would be an angioplasty balloon in which pressure and volume in the balloon are measured during oscillating loading, such that volume is used as an estimate of displacement and pressure is used as an estimate of force. Pressure can be measured with any appropriate pressure gauge. If loading is at a low frequency, then measurement anywhere in the fluidic system would suffice because pressure gradients that drive flow into the balloon would be small. If loading is at higher frequencies, the measurements can be made inside the balloon with several methods including miniaturized pressure sensors (membrane deflection, capacitance based, etc.) Volume can be measured by measuring the displacement of fluid in the balloon by means such as piston displacement, or with a mass flow sensor placed along the channel to the balloon. The diameter of the balloon can also be used to directly determine deformation of the anatomical part or to estimate the volume of the balloon. The diameter of the balloon can be measured acoustically or optically via reflection of radiation off the wall of the balloon. 
     Viscous losses during deformation of the tissue can be estimated by any of several methods, including: measuring the phase lag between force and displacement, measuring the area bound by the hysteresis curve during one cycle of loading/unloading, or measuring the difference in work performed by the motor during loading and unloading. 
     The resonant frequency of the materials may be measured by oscillating at different frequencies as disclosed by Leveque et al. (Leveque, Rasseneur et al. 1981), by identifying the frequency at which the force required for deformation is smallest, by identifying the frequency at which viscous loss is smallest, or by other methods known in the fields of mechanics and biomechanics. 
     Many methods of testing by oscillation require testing at multiple frequencies of oscillation. This can be accomplished by testing at multiple discrete frequencies, testing via a frequency sweep, or testing with “white noise”. 
     B.3 Tissue Deformation Via Oscillating Loading, Tissue Measurement, and Feedback 
     Information obtained by measurements such as those disclosed in Section B.2 can be used to make decisions about how best to perform a tissue deformation by either oscillating loading (e.g., an oscillation motion) or normal one-directional loading (e.g., a retraction motion). 
     In one embodiment, force and displacement are measured by a retractor. Alternating retraction motion and oscillation motion are used. A retractor similar to that in Section B.1.1.1.1 and shown if  FIG. 12  can be used. The first phase of retraction proceeds as follows:
         (1) The retraction motion starts, during which the distance is measured, and the retraction motion is stopped when a fraction of the desired opening is reached, e.g. 10% or 30%;   (2) Oscillation motion is then imposed to determine the frequency that results in the smallest time constant τ for accelerated force relaxation (AFR).   (3) Oscillation motion is then continued for a duration of 1.5τ and then stopped; and   (4) Retraction motion is resumed.
 
This cycle is repeated, possibly with different opening extents (e.g. another 10% of desired opening or another 20% of desired opening) until the desired opening is obtained.
       

     In another embodiment, the stiffness of the material is used to determine when an oscillation motion begins. Force (F) and distance (x) are measured by the retractor, and stiffness is measured in real-time as dF/dx. Alternating retraction motion and oscillation motion are used as described in the preceding paragraph. Retraction proceeds as follows:
         (1) Retraction begins with a retraction motion, and stiffness is measured throughout motion. When stiffness starts to decrease, indicating the material properties of the tissue are changing, retraction motion stops;   (2) Oscillation motion commences with an amplitude of approximately 2 mm and a frequency of approximately 5 to 10 Hz, or with an amplitude of approximately 4 to 8 mm and a frequency of approximately 0.5 to 2 Hz;   (3) Oscillation occurs for approximately 10 to 50 cycles to alter the material properties of the tissue such that stresses in the tissue are relieved and large-scale tissue components don&#39;t break; and   (4) retraction motion is resumed.
 
Other frequencies and amplitudes can be used, and frequency and amplitude can be adjusted for the tissue to be retracted, with bone, for example, being oscillated at a frequency of kHz and an amplitude of micrometers. The oscillation motion can be any combination of frequency and amplitude that achieves appropriate modification of the tissue being retracted.
       

     In another embodiment, retraction and oscillation motions are superimposed. Retraction follows a pre-determined trajectory (e.g., a trajectory in which the retractor blades move apart quickly at first but increasingly slowly as retraction proceeds such that the desired opening is achieved in a proscribed time, such as that shown in  FIG. 11B ). Force and distance are measured. Oscillation serves both to accelerate force relaxation, which now occurs concomitantly with continuous deformation, and to permit repeated stiffness measurements, as shown in  FIG. 32 ) to regulate the velocity of the retraction motion about the predefined trajectory (e.g., if stiffness is too high, then the velocity of the retraction motion slows, or if the stiffness is too low, then the retraction motion accelerates). 
     In another embodiment, pressure in the tissue is measured by a catheter sensor, such as the miniaturized sensors from Scisense, Inc. of London, ON, Canada and ADInstruments of Colorado Springs, Colo., USA. One or more pressure sensors are placed into the tissue near the retractor blades, such that the pressure sensors sense internal tissue pressure and how internal tissue pressure rises during retraction. Alternating retraction and oscillation motions are used. Retraction motion starts and proceeds until tissue pressure reaches a threshold (e.g., a level that indicates that perfusion of the tissue has stopped). Retraction motion is halted and oscillation motion is started. Oscillation motion is used for accelerated force relaxation until the pressure drops below the threshold. Retraction motion is resumed, and the process repeated until the desired opening is achieved. 
     C. Detecting Tissue Trauma During Retraction 
       FIG. 33  presents an example of a Finochietto-style retractor C 4  in the prior art. The retractor C 4  has a fixed retraction element C 6  attached to one end of a rack C 8  of a rack-and-pinion drive C 10  driven by a manual drive handle C 12 . A moveable retraction element C 14  is attached to the rack-and-pinion drive C 10  and moves along the rack C 8 . Each of the retraction elements C 6 , C 14  has a single blade C 16  that engages the tissue to be retracted. 
     A retractor C 4 , such as that shown in  FIG. 33 , can be instrumented to measure several parameters during retraction. For example, the blades C 16  of the retractor C 4  can be fitted with force sensors (such as strain gauges or a load cell), and the separation of the blades C 16  can be measured by fitting a displacement sensor onto the retraction elements C 6 , C 14  (such as a linear potentiometer or an optical encoder). The output from these sensors can be fed into a display (such as a digital numeric display or a bank of light emitting diodes LEDs) for direct readout, or the signal can be fed into an analog-to-digital converter and read by a computer for subsequent calculations and display. Multiple sensors measuring a parameter (for example, a plurality of load cells and/or accelerometers indicating forces and/or accelerations acting on a corresponding plurality of retractor blades) can provide a map in two dimensions (2D), three dimensions (3D), or four dimensions (4D, with time) of the forces and moments acting on the system consisting of the body of the patient and the retractor C 4 . 
       FIG. 34  depicts the retractor C 4  showing how it might be fitted with a set of calipers C 22  for measuring the separation of the retraction elements C 6 , C 14 . Additionally, strain gauges C 20  can be placed on each of the two blades C 16  of the retraction elements C 6 , C 14  (“retractor blades”) to measure forces on the retractor blades C 16 . 
       FIG. 35  shows a retractor C 28  that uses a bi-directional ball screw C 30  (i.e., having two followers that travel in opposite directions) that is driven by a stepper motor C 32  which in this case is a MDrive 23Plus from Intelligent Motion Systems, Inc. The bi-directional ball screw C 30  is mounted to a rail C 34  with two linear translation stages C 36  which in this case is the IKO LWHG 25 from IKO, Inc. such that each translation stage C 36  attaches to one of the bearings on bi-directional ball screw C 30 , thus when bi-directional ball screw C 30  is rotated by the stepper motor C 32 , the translation stages C 36  travel in opposite directions. A retractor arm C 38  fabricated by hand from mild steel angle iron that was cut/bent/welded into shape, is mounted to each translation stage C 36 . Each retractor arm C 38  has a retractor blade C 40  fabricated by hand with mild steel. 
     A linear potentiometer C 42  which in this case is a 5 kOhm, 100 mm linear potentiometer from Schaevitz is used to measure separation of the retractor blades C 40 . The static mount of the potentiometer C 42  is affixed to the rail C 34 , and the piston of the potentiometer C 42  is affixed to one of the translation stages C 36 . Note that any means of measuring displacement could be used here, such as optical encoders, contact and non-contact proximity sensors, digital calipers, and the like. 
     The retractor blades C 40  are instrumented with a full-bridge strain gauge assembly (not shown) which includes two (2) gauges, which in this case are model CEA-06-125UN-350 from Vishay Micro-Measurements, Inc., on each side of the blade. The signal from the strain gauges is the amplified by a signal conditioner (not shown) which in this case was model OM-2 from 1-800-LoadCells. Note that force could be measured by any of several means, such as drive current on the motor (and other means of measuring torque on the drive mechanism), fiber optic strain gauges, optical sensors of deformation, and the like. 
     All signals from the linear potentiometer C 42  and the signal conditioners/strain gauges are read by a Windows-based computer using a data acquisition card, which in this case is a model USB-6211 from National Instruments, and software, which in this case is LabVIEW from National Instruments, Inc. using a custom program prepared by Katya Prince of Prince Consulting. The stepper motor C 32  is controlled with IMS Terminal software from Intelligent Motion Systems, Inc. Note that a servo-motor could also be used. The strain gauges were calibrated by hanging known weights from the retractor blades C 40  of the retractor C 28 . The linear potentiometer C 42  was calibrated with a metric ruler. 
     A series of experiments were conducted with the prototype retractor C 28  described above using parts from pig cadavers. The parts were a “front quarter” purchased from Nahunta Pork Center (Pikeville, N.C.). A front quarter is basically a whole pig cut at the waist (forming a front half) and split down the vertebrae (forming left and right quarters); thus, each quarter had an intact rib cage (one side), spine (bisected), sternum (bisected), and shoulder. All parts had been refrigerated after slaughter, used within 24 hours of slaughter, and warmed by immersion in warm water (while wrapped in a plastic bag to prevent soaking of the tissue) to near body temperature (31° C. to 37° C.). The quarters ranged in size from 8 to 12 kg. 
     We performed thoracotomies between 3-4 rib pairs on each quarter, almost always performing an incision between ribs 5-6, 7-8, 9-10, and 11-12. Thoracotomies were performed by:
         cutting the skin with a scalpel over the range of the thoracotomy, and in a direction parallel to the ribs,   bisecting the muscles overlying the ribs with a scalpel,   cutting through the intercostal tissues with a scalpel,   pushing a finger between the ribs to make a small opening,   inserting the closed blades of the retractor into the opening,   positioning the retractor such that the blades sat approximately halfway between the spine and the sternum and the retractor&#39;s axis of opening was approximately parallel with the spine,   initiating opening according to a specified algorithm via computer control of the stepper motor.       

     Incisions were typically 110 mm to 130 mm long, with longer incisions being performed on larger quarters. 
       FIG. 36  shows data from the retractor C 38 , with force C 50  and displacement C 52  (distance measured by the linear potentiometer C 42 ) plotted with respect to time for a “standard retraction”, similar to that defined by Bolotin et al. (US Patent Application Publication Number 2006/0025656 and 2007, J. Thorac. Cardiovasc. Surg. 133:949), which proceeds as follows:
         open to 40 mm in one (1) minute (⅔ of final opening);   pause two (2) minutes for force relaxation; and   open to 60 mm in three (3) minutes (i.e., to the final opening).       

     Thus, a total opening of 60 mm is reached in 6 minutes. Each of the two moves is constant velocity (40 mm/min for the first and 6.8 mm/min for the second). These moves were controlled by a computer program executed in a computer with the IMS Terminal software. Thus, unlike Buckner and Bolotin et al. (US Patent Application Publication Number 2006/0025656 and 2007, J. Thorac. Cardiovasc. Surg. 133:949) the velocity of the retraction motions was precisely controlled. This somewhat matches the pace described to us by other thoracic surgeons, but there is no standard clinical practice. Surgeons use a procedure defined by their training, personal experience, patient condition, and sense-of-touch (i.e., non-quantitative) estimates of force applied at the handle of a hand-cranked retractor. Furthermore, surgeons have no velocity control, other than hand-eye coordination. Importantly, the self-locking Finochietto-style rack-and-pinion drive C 10  engages and advances in rather abrupt half-step turns of the handle C 10 , producing a non-linear relationship between rotation and motion of the retractor blades C 16 , making control of velocity and force difficult. 
       FIG. 37  shows the displacement and force on both arms for a Finochietto retractor instrumented like the retractor shown in  FIG. 34 , except that a linear potentiometer is used to measure displacement, instead of the caliper shown in  FIG. 34 . These measurements are from a thoracotomy performed on an anesthetized pig (female, 50 kg weight, procedures similar to those in  FIG. 36  except that opening was to 52 mm in one minute without a long pause in retraction). The force and displacement traces in  FIG. 37  are not smooth. Both traces show the step-by-step increases generated by the ½-rotations of the crank. Furthermore, even small adjustments or other motions of the crank resulted in large deflections in the force trace. For example, when the surgeon simply adjusted the position of his hand on the crank at 42 s, an approximately 30 N change in force is seen in the force trace for both retractor blades. During this retraction, a rib broke. Importantly, the point on the trace where the rib broke could not be identified in any of the traces. The point C 56  on the trace where the rib broke was identified by careful analysis of a time-correlated video recording of the procedure in which the break could be heard as a “crack”. Thus, it is not possible from these force or displacement traces to detect that a rib is about to break. Nor is it possible to determine if a rib breaks. 
     Returning to  FIG. 36 , the force of the motorized retraction rises rapidly over the first minute of retraction (opening to 40 mm). Force relaxation, as described in Buckner and Bolotin et al. (US2006/0025656 and 2007, J. Thorac. Cardiovasc. Surg. 133:949), and also illustrated in  FIG. 1 , is evident during the two-minute pause—the force required to maintain the 40 mm opening decreases with time. Force again rises when retraction is resumed at 3 minutes, rising at a time-varying rate, but the increase in force is smooth up until 60 mm retraction is achieved. No significant tissue breaks occurred during the retraction shown in  FIG. 36 . Two small breaks are evident over the first 50-60 s interval, as evidenced by small downward deflections in the force trace (marked by arrows). The absence of significant breaks is unusual. Most retractions of this type resulted in large tissue breaks, as seen in  FIGS. 38A-38C and 40A-40B  (discussed below). 
       FIG. 38A  shows data from another retraction, using the same motorized standard retraction as in  FIG. 36 . A large break is seen—a break that spanned several seconds (46-70 s on the graph, marked with an asterisk) and ended only with the start of the pause period. There are also several smaller breaks (marked with arrows), also evident as significant drops in the slope of the force plot.  FIG. 38B  shows an expanded view of the data from 20 to 70 s, showing the large break. There are two types of events that precede this large break. The first type of event is a decrease in the slope of the curve beginning at 38 s (illustrated by the two dashed lines—termed a “slope event”). The second type of event is a small break seen as a drop C 60  in force at 42 s, marked with an arrow in  FIG. 38B  (termed a “force event”). Note that there is a second force event C 62  at 44 s.  FIG. 38C  shows the interval from 41.5 to 45 seconds on an expanded scale; the two force events, the first force event C 60  at 42 s and the second force event C 62  at 44 s, are clearly visible. 
     The two types of events, slope events and force events, preceding the large break are better seen in  FIG. 39  which plots both the force (kg) and the slope of the force (kg/s) for the interval of 10 to 46 s in the retraction shown in  FIG. 38 . A slope event C 70  beginning at 38 s is more visible, and the two small breaks are now much more prominent as negative-going peaks marking the first force event C 60  and the second force event C 62 . 
       FIGS. 40A and 40B  present another example of a standard retraction— FIG. 40A  presents data from the entire retraction, and  FIG. 40B  presents a magnified view of one minute of data from 30 s to 90 s. In this retraction, there is, again, a large break C 100  at the end of the first 1-minute of retraction, beginning at about 72 s. Several small force events occur (e.g., at 57 s, 59 s, 61 s and others), but preceding these is a slope event C 102  beginning at 57 s. This drop in slope C 102  is more obvious than the slope event C 70  seen in the retraction shown in  FIGS. 38  and  39 . The slope event C 102  in  FIG. 40  is evident in the force trace, but is more easily seen in the slope trace. Another common feature is evident here—both the force trace and the slope trace become noisier; their variance increases. This provides third and fourth indicators of an imminent break—termed a “force variance event” and a “slope variance event”. 
     All four of these events, (a) a force event, (b) a slope event, (c) a force variance event, and (d) a slope variance event are frequently seen preceding a large break and can be used as indicators that a large break is about to occur. 
     Note that higher order time derivatives of the force trace (e.g., d2F/dt2, etc) also present information relevant to imminent breaks and can make a distinction from baseline simpler because the signal stays near zero.  FIGS. 41A and 41B  show the second time derivative, d 2 F/dt 2 , of the force for the retractions presented in  FIGS. 38, 39, and 40 . ( FIG. 41Aa  presents the retraction from  FIGS. 38 and 39 , and  FIG. 41B  presents the retraction from  FIG. 40 .) In  FIG. 41A , the force event C 60  at 42 s and the force event C 62  at 44 s are now clearly resolved as negative-going spikes C 200  and C 202 . In  FIG. 41B , the slope event at 57 s is now clearly resolved as a large, negative-going spike C 210 . Thus, the second time derivative of the force provides both (a) a flat baseline over much of the retraction and (b) a negative-going spike at force and slope events providing a clear signal indicating the onset of the variance. Detection of the spike can be accomplished by comparison of substantially instantaneous values of d2F/dt2 versus a time-averaged value of d2F/dt2, with the ratio of instantaneous/time-averaged values of d2F/dt2 exceeding a predefined threshold, or by comparison of instantaneous values of d2F/dt2 with variance of d2F/dt2 measured over a preceding time interval, such as the ratio of the instantaneous value of d2F/dt2 with the sum of squares of d2F/dt2 over the preceding 20 s or over the preceding 4 s. There are many such detection algorithms well-established in the art of signal processing that can be used to detect a negative-going spike in d2F/dt2. 
     Implementation of these indicators within automated control systems in medical devices would permit both (a) the presentation of indicators to the physician, permitting the physician to take corrective action before a break occurs, and (b) automated operation whereby the device contains appropriate mechanisms to implement corrective action. Software executed by microprocessors can perform appropriate signal processing (e.g., Butterworth filter, Fourier analysis, etc.) of signals from sensors to improve signal-to-noise, and this software can also perform automatic event detection with automatic response. For example, an automated system can initiate a pause in the first phase of retraction if a negative-going spike in d 2 F/dt 2  is detected, or an automated system can initiate an oscillating motion in the first phase of retraction if a negative-going spike in d2F/dt2 is detected. 
     Importantly, detection of these events requires a stable force-time trace. This requires a means of regulating the velocity of retraction to ensure that it maintains a commanded velocity free of substantial variations in velocity; for example, the retraction velocity remains constant during measurement, or the trajectory of motion during the first phase of retraction follows a substantially parabolic profile, retracting more quickly at first and increasingly slower as retraction approaches a desired opening of the surgical incision or a desired dilation of the artery. This can be accomplished with a retraction system with manual actuation permitting very smooth motion, such as a hydraulic actuator or a fine pitched lead screw. Preferably, retraction is performed by a motor-driven retractor such that velocity can be maintained at predetermined rates by internal control, such as by an open-loop system with a stepper motor that is capable of generating sufficient torque as to not be impeded by retraction forces or by a closed-loop system with a servo-motor. Closed loop control of velocity in a hydraulically-actuated system is also possible. The velocity of retraction can be constant, but this is not necessary. For example, a smoothly time-varying velocity can be used. 
       FIG. 42  depicts an example of an algorithm C 300  for detecting imminent tissue trauma. The algorithm C 300  can be used for any retraction profile (displacement over time) with any device that measures force. The algorithm C 300  searches for both a negative-going spike in the force trace and for an increased variability (“noisier”) force trace. The user inputs two thresholds, T S  for detecting the negative-going spike and T V  for detecting increased variance. The thresholds T S  and T V  allow the user to set the sensitivity of the algorithm C 300 . For example, a surgeon might choose to use a more sensitive setting for a patient expected to have fragile bones. Variability in the force signal is calculated as the root-mean-square (RMS) of the force trace, as shown in  FIG. 42 . Execution of the algorithm C 300  starts (block C 302 ) at the initiation of retraction. Retraction proceeds for N+0.1 seconds (block C 304 ), with force sampled at a rate equal to or greater than 10 Hz. The algorithm C 300  then calculates RMS of d 2 F/dt 2  over the last N seconds (block C 306 ), skipping the first 0.1 second to avoid transients from the start of retraction (e.g., motor stiction, etc.). The algorithm C 300  first looks for a negative going spike in d 2 F/dt 2  by comparing the last measurement to the RMS over the last N seconds (RMS N ) multiplied by the threshold T S  input by the user (d 2 F/dt 2 &lt;(0−TS*RMS N )) (block  308 ). If the force is more negative than this parameter, then a 20 s pause in retraction (block C 310 ) is triggered permitting force relaxation in the tissues, and the algorithm C 300  returns to the start (block C 302 ). If d 2 F/dt 2  is not more negative than this parameter, then the algorithm C 300  checks for increased variability in d 2 F/dt 2  by comparing the RMS over the past 0.5 seconds (RMS 0.5 ) to the RMS over the past N seconds (RMS N ) multiplied by the threshold T V  (block C 312 ). If RMS 0.5  is greater then a 20 second pause (block C 310 ) in retraction is triggered permitting force relaxation in the tissues, and the algorithm C 300  returns to the start (block C 302 ). If RMS 0.5  is not greater then retraction proceeds for another 0.1 second (block C 314 ) and checks again (block  306 ). Thus, force is checked for a negative-going spike in d 2 F/dt 2  and for increased variability in d 2 F/dt 2  every 0.1 seconds. The force trace can be checked more or less frequently. Other sampling frequencies can be used. The 0.1 second added to N in block C 304  can be any other time interval sufficient to avoid transients in the force trace on starting the motor or around any other event deemed spurious to detecting tissue trauma. The event triggered by the detection algorithm (a 20 second pause in this case) can be any event that is appropriate to the detected signal. For example, retraction can pause with continued measurement of force and then retraction can resume after the slope of the force trace becomes shallow, indicating that force relaxation has approached a limit. Another example is to initiate an oscillation of the retractor to accelerate force relaxation, or to pause for a first period and then to oscillate for a second period. 
     There is a fifth event for predicting imminent large breaks in tissue during retraction. Breaks are audible. Snaps and pops (“audible events”) are heard throughout a retraction. Big breaks are louder. The large break at 46-70 s in  FIG. 38B  was actually a series of repeated fractures of the tissue. This was audible as a rapid series of loud audible events. Thus, the audible events of tissues breaking can be used as an indicator of tissue trauma, and audible events, including less loud events, can be used as indicators that a larger traumatic event is about to occur. Also, the qualities of an acoustic signal (e.g., the frequency of occurrence of audible events) can be used as an indicator of impending trauma. In the preceding example, the frequency of occurrence of acoustic events becomes higher as trauma increases. 
       FIG. 43  depicts how such a trace would look—two breaks occur, one at about 55 s and another at about 255 s (marked with an asterisk). These are preceded by audible events (marked by arrows). These audible events might be distinguished from background noise by sound intensity, spectral composition, or both. In another example, the acoustic frequency (i.e., the pitch) of each acoustic event might change; for example, the pitch of earlier events might be higher than the pitch of later events as the tissue approaches a large fracture. 
     Sound measurement can be performed by microphones or other sound sensors placed in the air near the incision; on the retraction device, such as with a contact microphone; or on the patient&#39;s body, for example, with a contact microphone embedded in a gel beneath an adhesive pad, in which the gel matches the acoustic conduction of the body. If the sound measuring device is placed on the patient&#39;s body, then multiple sound measuring devices placed at distinct locations can be used to detect the position of the fracture either by relative intensity of the sound or by detecting time-of-arrival for triangulation of the location of the fracture or the propagation of a locus of damage. 
     Acceleration can serve as a sixth event indicator. When a large ligament snaps during retraction, the entire retractor suddenly shakes, as will all or a portion the body of that patient (depending on the magnitude of the tissue trauma event). With a smooth retraction, even smaller sounds can be “felt” with fingertips that lightly touch the retractor. Accelerometers are ideally suited to measure these motions. Accelerometers mounted on elements such as the body of the retractor, on the retractor blades, and/or on the body of the patient would provide an indication of the motions of any, some, or all of these elements. Acceleration is thus indicative of any number of a range of events occurring within (or to) a patient&#39;s tissues, including incipient tissue trauma. In this way, acceleration can serve prognostic goals. Acceleration can also provide feedback, to track the behavior of the device itself. 
     An accelerometer typically measures acceleration along a single axis. Accelerations acting directly along this axis produce the strongest signal, while accelerations acting exactly perpendicularly to that axis may produce little or no signal at all. In an actual patient&#39;s body, with complex tissues and force transmission paths, one may encounter the situation where one cannot expect an accelerometer associated with that body to ever register a zero output. One might mount one or more accelerometers to a surgical instrument (for example, the body of a retractor), to a portion of a surgical instrument (for example, one, two or a plurality of a retractor&#39;s blades), and/or to a patient&#39;s body. With its axis oriented at a carefully chosen angle with respect to the local axis of retraction, a given accelerometer can provide indications of not only an early warning of impending tissue trauma, but also a local direction of interest with respect to those accelerations, and a complex time series of accelerations associated with specific tissue types or tissue behaviors. As with the acoustic event detection above, one can use accelerometers to detect (a) acceleration events, (b) acceleration slope events, (c) acceleration variance events, and (d) acceleration slope variance events. 
     Attaching multiple accelerometers at multiple locations and/or angles can provide a picture, or map, in 2D, 3D, or 4D (with time) of the forces and moments acting on the system consisting of the body of the patient and the retractor. This picture can enable a surgeon (or the corrective software) to know which tissue type (or which of many tissue elements) might be involved, and/or when and where tissue trauma will occur before the onset of major damage. As one example, accelerations parallel to the surface of a given retractor blade might indicate the incipient failure of fibrous connective tissue (e.g., fascia or periosteum) oriented in that direction, while accelerations perpendicular to the surface of that retractor blade might indicate the incipient failure of the rib that that retractor blade is moving. As for corrective actions, in that example one might try to prevent snapping connective tissue by initiating oscillating loading, whereas one might instead respond to prevent rib breakage by pausing the retraction. 
     Furthermore, accelerometers might also provide independent confirmation of how well the actual behavior of a motorized instrument (such as a retractor) is conforming to the commanded behavior (whether controlled by the surgeon, the software, or some combination of the two). This could serve as an on-the-fly diagnostic to permit active self-correction and self-calibration. Accommodation and re-modulation can correct performance variances should they occur, further increasing confidence in the safe operation of the device. 
     A further aspect of self-operational feedback features (e.g., for acceleration) is that the device could adapt to the different operating styles of surgeons, for example by enabling detection of the operator&#39;s instrument handling patterns. For example, an accelerometer mounted to the retractor can be used to detect motions of the retractor arising when a surgeon inadvertently touches the retractor (e.g., when inspecting the incision) or purposefully handles the retractor (e.g., to adjust the position of the retractor). Such inadvertent touches or purposeful handling of the retractor can create transients in the signals that resemble imminent trauma. Signals from the accelerometer can be used to discriminate transients in the force and/or sound traces arising from the surgeon&#39;s actions. 
     Using any of these events for detecting an imminent tissue fracture or other damage, it is possible for a surgeon or an automated system to take corrective steps to prevent the tissue fracture. For example, upon detection of an event, retraction can be paused, permitting force relaxation, or retraction can switch from constant velocity retraction to an oscillating loading to use work softening of the tissue (or related phenomena arising from oscillating loading) to either induce an accelerated force relaxation or to create many small tissue fractures that relieve the stress in the tissue and prevent fracture of a major tissue component. 
     It is important to recognize that the techniques described here for detecting imminent tissue trauma by measuring force and sound, coupled with detection of transients, can be used without prior knowledge of a particular patient&#39;s physiology or pathology—the signals are unique to tissue trauma, but independent of a patient&#39;s unique characteristics. Thus, tissue trauma can be detected whether a patient is old or young, large or small, osteoporotic or normal. There is no requirement for determination of a threshold force to try to avoid tissue trauma, nor is there any need for databases of patients&#39; characteristics and related force-distance measurements for adjusting retraction to unique patient parameters. 
     There are any number of other tissue trauma early warning event indicators. The aforementioned examples are only intended to teach the principle of early detection, not limit the embodiments of sensing modality to force, sound, and acceleration. 
     F. Self-balancing Retractor Blades 
       FIG. 44  presents an example of a Finochietto-style retractor F 2  in the prior art. It has a fixed retraction element F 6  attached to rack of a rack-and-pinion drive F 10  that is manually driven by rotation of the drive handle F 12 . A moveable retraction element F 8  is attached to the drive of the rack-and-pinion drive F 10 . Each of the retraction elements F 6 , F 8  has a single retractor blade F 4  that engages the tissue to be retracted. 
     The forces under the retractor blades F 4  can be large. Furthermore, an edge of a retractor blade F 4  can become a point-load if the retractor blade F 4  is not well-seated or if the retractor blade F 4  contacts a curved surface, such as a rib. If a retractor blade F 4  becomes a point load, then the stress in the tissue at the point of loading can become extreme. Broken ribs are common using these types of devices. 
     Such maladjustments of a blade can be reduced if several blades are used to engage the edge of an incision.  FIG. 45  shows a retractor  300  in the prior art from the lab of Greg Buckner (Buckner and Bolotin 2006; Bolotin, Buckner et al. 2007). This retractor has six retractor blades  200  attached to a common frame  700 . Each retractor blade  200  has an intermediate member  220  that connects the retractor blade  200  to an actuator  300 . Thus, each retractor blade  200  has its own actuator  300 .  FIG. 45A  is a diagram of the retractor  300 .  FIG. 45B  is a photograph of the retractor  300  being used in a thoracotomy in a sheep, demonstrating how the retractor blades  200  engage the margins of the incision. The use of multiple retractor blades  200  along the margin of the incision distributes the retraction forces, reducing the force on any single retractor blade  200 . However, the load on any single retractor blade  200  is determined by how hard it pulls on the incision as set by the actuator  300  of that particular retractor blade  200 . Adjusting the forces to be equivalent to one another, or to have any other desired distribution of forces, requires individual adjustment of all the actuators  300  which must be made by an operator (which would be slow and irregular) or by an automated system combining force measurement, motorized actuators, and a control system (which might be expensive). 
     Discussion of the next section requires review of a piece of very old prior art, related to the harnesses of draught horses that pull wagons. A “swingletree” is a pivoted, suspended crossbar to which the two traces of a horse&#39;s harness are attached when it pulls a wagon.  FIG. 46  shows a top view of a swingletree F 94  attached to wagon F 97  by first harness component F 104 . Swingletree F 94  attaches to harness component F 104  at pivot F 98 . Two traces F 96  of the harness extend from swingletree F 94  to the collar F 92  against which horse F 90  pulls, exerting force F 106  met by reaction force F 107  and creating force F 108  on the pivot F 98  and force F 102  on the wagon F 97 . If due to uneven motion of the horse, the force on traces F 96  become unbalanced, then the moment about pivot F 98  causes swingletree F 94  to rotate until the forces on the traces F 96  become balanced. 
     Every horse F 90  attached to a wagon F 97  pulls against a swingletree F 94 . When more than one horse pulls a wagon, multiple swingletrees are tiered, as shown in  FIG. 47 . Two horses F 110  and F 112  pull a wagon F 124 . Each horse F 110  and F 112  pulls on its own (child) swingletree F 120 , and the two swingletrees F 120  are attached to a third (parent) swingletree F 122  that is also known as a “doubletree”. The entire structure connecting the swingletrees F 120  and F 122  is a tensile one, and rotation of swingletrees F 120  and F 122  balances the forces on each swingletree. Ultimately, the pivot F 130  ensures that only a tensile force is applied to wagon F 124 , and rotation of the swingletrees F 120  and F 122  isolates all unbalanced forces from wagon F 124 . 
       FIGS. 48A and 48B  show another retractor F 150  in the prior art. This is the Skyhook from Rultract (www.rultract.net, U.S. Pat. No. 4,622,955). The retractor F 150  is a hoist, suspended above a patient F 163 , with two retraction rakes F 160 , F 164 , F 166  that engage a bisected sternum F 162  at two locations, and the retraction rakes F 160 , F 164 , F 166  attach to the opposite ends of a swingletree F 156 , F 170 . A cable F 154 , F 168  attaches to the mid-point of the swingletree F 156 , F 170 , and the swingletree F 156 , F 170  is free to pivot about this attachment. As seen in  FIG. 48B , when the winch F 158  pulls the swingletree F 156 , F 170  upward, if one of the retraction rakes, such as F 164 , engages the margin of the incision F 162  first, then that retraction rake F 164  is pulled downward, which pulls the opposite retraction rake F 166  upward until both rakes F 164  and F 166  engage the margin of the incision F 162 , where the swingletree F 170  has rotated with the right retraction rake F 166  raised above the left retraction rake F 164 . Force exerted by the cable F 168 , through the swingletree F 170 , and then through the retraction rakes F 164  and F 166  pulls the bisected sternum F 162  upward to provide surgical access for the surgeon. The swingletree F 170  here ensures that the forces on the two retractor rakes F 164  and F 166  remain equal—if the force on one retraction rake, for example retraction rake F 164 , is larger, then the other retraction rake F 166  is pulled upward until the forces on the two retraction rakes F 164  and F 166  are balanced. More specifically, the swingletree F 160 , F 170  rotates whenever the moment about the pivoting attachment to the cable F 154 , F 168  become unbalanced. This occurs automatically. One drawback of the retractor F 150  is that it requires a large derrick-like arm F 152  that is bolted to an operating table F 153  that suspends the winch F 158  over the patient F 163 , or some similar superstructure over the operating table F 153 . Such structures can obstruct the surgical field, making access difficult from some angles, and present the risk of dropping the requisite fasteners into the patient&#39;s open chest cavity. 
     A means for automatically adjusting the force exerted by each retractor element without the large, table-mounted hardware of the retractor F 150  and with fewer actuators than the device of  FIGS. 45A &amp; 45B  is desirable. 
       FIGS. 49A and 49B  illustrate one embodiment that balances the forces on multiple retractor blades without table-mounted hardware and with fewer actuators. This is a retractor F 172  that uses a mechanical system for balancing forces on the opposing arms of a Finochietto-style retractor in the prior art (see  FIG. 44 ). Four retractor blades F 184  engage the margins F 202  of the incision to be retracted and create a surgical aperture F 200 . Retraction is manually driven by rotation of the drive handle F 174  acting on rack-and-pinion drive F 175  which moves along rack F 176 . There is a pair of blades F 184  (also labeled F 196  and F 198  in view F 190  in  FIGS. 49B . 1  and  49 B. 2 , respectively) on each retraction element F 178  and F 180  of the retractor F 172 . A first balancing assembly F 186  is comprised of two retractor blades F 184  in each pair which are attached to a balance bar F 188 , and the first balancing assembly F 186  on the fixed retraction element F 178  opposes a second balancing assembly F 185  on the moveable retraction element F 180 . Each balance bar F 188  is attached to its respective retraction element F 178  or F 180  by a pivoting mount F 187  or F 189  so that the balance bar F 188  is able to rotate in the plane of the page in  FIG. 49A , but rotation of the balancing bar F 188  in the two planes perpendicular to the plane of  FIG. 49A  is not permitted. Prohibition of rotation in those other two planes permits the use of rigid mounts to retractor blades, retraction hooks, or retractor rakes. In  FIG. 49B . 1 , a balance bar F 193  will rotate F 195  about a pivot point F 194 , and within the plane of the page, to balance forces F 204  and F 206  on the two retractor blades F 196  and F 198  attached to the balance bar F 193 , and will stop rotating F 195  when the two forces F 204  and F 206  are balanced. Additionally, should the two forces F 204  and F 206  again become unbalanced as retraction F 212  proceeds, the balance bar F 193  will, again, automatically rotate F 195  the retractor blades F 196  and F 198  to balance the forces F 204  and F 206  on the blades F 196  and F 198 . In  FIG. 49B . 2  depicts a subsequent state of view F 190  showing a balanced state for a pair of forces F 208  and F 210  during retraction F 214 , such that F 208  and F 210  have equalized due to the accommodation via rotation F 218  of balance bar F 216  about pivot point F 217 . 
     Referring now to  FIGS. 50A through 50C ,  FIG. 50A  shows how a balance bar F 226 , F 236  and F 246  can be adjusted such that the balance bar F 226 , F 236  or F 246  maintains an approximately constant ratio of forces F 232 , F 242 , F 252  versus F 234 , F 244 , F  254  between two retractor blades (not shown) located at the ends of the balance bar F 226 , F 236  or F 246 . As shown in  FIG. 50A  balance bar F 226  rotates, not due to an imbalance of the forces F 232  and F 234  on the retractor blades, but due to an imbalance of moment M F 227  about the pivot point F 228 . Thus, if the length L 1  of a first side of balance bar F 226  is longer than the length L 2  of a second side of balance bar F 226 , then the force F 1  F 232  on that first side will be smaller than the force F 2  F 234  on the second side when the moment M F 227  is zero. Any ratio of forces can thus be accommodated. Additionally, the geometry of the balance bar F 226  determines a “righting moment”, a moment that returns the position of the balance bar F 226  to neutral when displaced from neutral, and thereby makes the balance bar F 226  “self righting.” As shown in  FIG. 50B , the righting moment is determined by the angle θ (θ=θ 1 +θ 2 ) formed by lines L 1  and L 2  and by the length of lines L 1  and L 2 . For example, the moment generated by F 2 L 2  sin θ 2  is maximal when L 2  is a long as possible and θ 2  equals 90°, and the moment generated by F 1 L 1  sin θ is minimal when θ 1  equals 0° regardless of the length of L 1 ; therefore, the balance bar F 236  will be maximally self-aligning when θ equals 90° (see  FIG. 50C ). However, if a 90° rotation is not anticipated when the balance bar is used, then θ=180°−2θ e  (θ e =the maximum angle of rotation in use) provides the largest righting moment. 
     As shown in  FIG. 51 , more than two retractor blades F 270 , shown as F 270  B 1 , F 270  B 2 , F 270  B 3  and F 270  B 4 , located on a retraction element F 263 , to be retracted in the direction F 278 , and another four retractor blades, shown as F 272  B 1 ′, F 272  B 2 ′, F 272  B 3 ′, and F 272  B 4 ′, located on the retraction element F 265  and to be retracted in the opposite direction F 280  can be placed onto each retraction element F 263 ,  265  of a retractor F 260 . This is accomplished by tiering balance bars F 262 , F 266  and F 267  onto which retraction blades F 270  B 1  to B 4  are mounted and also tiering balance bars F 264 , F 268  and F 269  onto which retractor blades F 272  B 1 ′ to B 4 ′ are mounted. Multiple tiers of balance bars are possible. 
       FIG. 52  shows how retractor blade numbers that are not multiples of 2 can be arranged so that the forces and moments still balance one another. As shown in  FIG. 52  the force (not shown) generated by a retraction F 300  on a blade F 298  B 3  equals the combined forces (not shown) on two more blades F 288  B 1  and F 294  B 2 . Similarly to that shown in  FIG. 51 , multiple tiers of balance bars are possible, including those creating uneven numbers of retractor blades. Again, all balance bar elements will stop rotating when the moments about their respective pivot points equalize. 
     Blades can be mounted to balance bars such that they are fixed or pivoting. As shown in  FIGS. 53 and 54 , balance bars can in some instances also be mounted by a tensile element such as a cable, chain, or wire, permitting rotation out of the plane of the page in FIGS.  53  and  54 , similar to a swingletree.  FIG. 53  shows in more detail a retractor F 302  with retraction elements F 306  and F 308 , a rack-and-pinion drive F 305  with a drive handle F 304 , and four retractor blades F 316  associated with two opposing balance bars F 310 . The balance bars F 310  are connected to the retraction elements F 306  and F 308  by tensile elements, cables F 312  and F 314 . Cables F 312  and F 314  permit easy, generous reorientation of the retractor blades F 316  to forces and accommodation of moments by the balance bars F 310  while still transmitting the forces arising out of the motion F 318  of the movable retraction element F 308 .  FIG. 54  shows how multiple balance bars F 338 , F 330 , F 340  can be tiered (similar to  FIG. 51 ) by the use of chains F 332 , F 334 , and F 336  attached by pivoting joint F 324  on balance bar F 338  and pivoting joints F 328  on balance bars F 330  and F 340 . 
       FIGS. 55A through 55C  show a top view, a side view, and a front view, respectively, of another embodiment in which an entire retractor element F 348  is able to rotate around the axis of retraction F 351 ; additionally, the retractor blades F 362  are shaped like hooks that engage a rib F 364  to avoid damage to a neurovascular bundle (not shown), as described more fully in Section H. In  FIG. 55A , the base element F 350  of the retractor element F 348  is attached by a rotational joint F 352  that allows the entire retractor element F 349  to rotate out of the plane of the page in the top view ( FIG. 55A ) and within the plane of the page in the front view ( FIG. 55C ). Thus, rotational joint F 352  permits the base element F 350  of the retraction element F 349  to rotate within a plane perpendicular to the axis of retraction. Base element F 350  attaches to a first balance bar F 354  by rotatable joint F 358 , and first balance bar F 354  attaches to second balance bars F 356  by rotatable joints F 358 . Two (2) hook-shaped retractor blades F 364  descend from each second balance bar F 356 . Rotational joints  352 , or their equivalents, can be placed at every rotatable joint F 358  providing tremendous freedom of movement for the balance bars F 354  and F 356  and the hook-shaped retractor blades F 362 . 
       FIGS. 56A through 56C  show a top view, a side view, and a front view, respectively, of an embodiment similar to that shown in  FIGS. 55A through 55C , but an articulation F 400  has been added to balance bar F 354  allowing it to bend to conform the balance bar F 354  to a patient&#39;s rib F 364  that curves in the plane perpendicular to the plane of the page as seen in the top view ( FIG. 56A ). Again, note that a cable, chain, or wire, as depicted in  FIG. F54 , could also permit rotation of the type shown at rotational joint F 352 . 
       FIG. 57  shows a Finochietto-style retractor F 430 , similar to retractor F 172  shown in  FIGS. 49A through 49B , with an opposing pair of swingletrees F 437  and F 439 . Retraction elements F 436  and F 438  have retractor arms F 433  with articulations F 434  that allow the retractor arms F 433  to conform to the curve of a patient&#39;s body. These articulations F 434  could be passive, starting out with the retractor arms F 433  straight and then conforming to the body when encountering the body, or the articulations F 434  could be preset by the surgeon and rigidly fixed in a patient-body conformal shape beforehand, or they could be self-controlled via sensor feedback. The articulations F 434  might be formed as hinges, with two discrete sections interdigitating as shown in the  FIG. 574 , or the articulations F 434  might be formed as elastomeric regions that bend smoothly from one section of a retraction element to another. Another embodiment might possess retraction elements F 436 , F 438  which are continuously, smoothly flexible (along their length) in one plane, while rigid in the others. 
       FIG. 58  shows another embodiment of a retraction element F 442  that permits more complex force distribution. Balance bars F 443  and F 445  form a second (child) tier F 449  to a first (parent) tier F 447 , connecting at rotatable joint F 446 . Each balance bar F 443 , F 448  has two retraction blades F 448  attached by rotatable mounts F 451 . Balance bars F 443  and F 445  are overlapped at F 450 , presenting opportunities for generating a broader range of moment arms to distribute the pattern of forces along the margin of the incision. A broad range of overlap, bar length, and pivot position is possible; preferred embodiments arrange bar lengths, amount of overlap, and pivot positions so that all moments equalize, but this need not be the case. Surgical situations may arise such that a clinician wishes to apply forces irregularly, for example if one is forced to simultaneously retract both exposed bone and soft muscle or adipose tissue in the same incision, or for example if a surgeon wishes to create a surgical aperture with purposefully nonparallel incision margins. Note also that besides varying the foregoing items in a surgical instrument design, the number of hierarchical levels is not restricted. It may be advantageous to provide many ‘child’ levels of balance bars below the ‘parent’ level, forming a balance bar cascade of arbitrary fineness, for example to ensure that dozens of tiny retraction hooks engage a patient&#39;s tissues, providing for nearly continuous support across the tissue face. Combining all four design variables permits the design of retractors of arbitrary complexity that apply appropriate arrangements of forces in useful directions to a variety of tissues and anatomical structures without incurring tissue trauma. 
       FIGS. 59A through 59E  show another embodiment of a retraction element that achieves automatic balancing of loads. Rather than using a swingletree, this retractor uses a cable F 466  to transmit loads between retractor blades, posts, or hooks F 468  that are mounted onto retraction arm units F 462  by a rotational mount F 460  formed by pin F 470  which attaches retraction hook F 468  to retraction arm unit F 462 .  FIG. 59A  shows a side view showing one retraction hook F 468  attached to a retraction arm unit F 462  by a rotatable mount F 460 . The retraction hook F 468  engages a rib F 456 , directly against that bone, such as in a thoracotomy. The cable F 466  attaches to the retraction hook F 468  by passing through a hole F 480  in the shaft F 469  of the retraction hook F 468 .  FIG. 59B  shows a front view, with three retraction hooks F 468  attaching to the retraction arm unit F 462 . The retraction arm unit F 462  has two articulations F 474 , permitting the retractor arm units F 462  to independently align to the curvature of the rib. Optionally, the retractor arms can be solid, without articulations F 474 . Referring to  FIG. 59B , the cable F 466  attaches at one end to a retraction element F 462  and then courses through the holes F 480  in the retraction hook shafts F 469  and over pins F 464  in the retraction arm units F 462 ; finally, cable F 466  attaches at its other end to a capstan F 478  used to adjust the tension of the cable F 466 , and thereby adjust the magnitude of the swinging of the retraction hooks F 468 .  FIG. 59C  shows a top view, illustrating how the cable F 466  travels from an attachment F 482  at one end, then zig-zags left to right, back and forth as it passes from holes F 480  in the shafts F 469  of the retraction hooks F 468  to pins F 464  inside recessed holes in the retraction arm units and finally to a capstan F 478 . Thus, as illustrated in  FIG. 59D , when a first retraction hook F 468  is pushed (by the tissues at the margin of the incision) toward the left, it tensions the cable F 466 , which then pulls a neighboring retraction hook (F 468 ′) to the right. This repositioning of the retraction hooks F 468  and F 468 ′ will continue until the force on both retraction hooks equalizes. Again, changes in the position of the through hole F 480  in the shaft of the retraction hook F 468  and F 468 ′ will control the ratio of forces between those retraction hooks.  FIG. 59E  shows how the articulations F 474  between retraction arm units F 462  permit the retraction arm units F 462  to conform to the curvature of the patient&#39;s body. 
       FIGS. 60A and 60B  show a physical model of a retractor F 540  of the type described in  FIG. 59A through 59E .  FIG. 60A  shows a top view of the retractor F 540 , and  FIG. 60B  shows an oblique side view of the retractor F 540 , showing the retraction hooks F 558  (similar to F 468 ) and cables F 556  (similar to F 466 ). Paired retraction elements F 544  are attached to and ride along a dual-thrust lead screw F 546 . Rotation of the dual-thrust lead screw F 546  with respect to the retraction elements F 544  causes the retraction elements F 544  to move F 548  apart for retraction or back together for closure. The retraction elements F 544  have articulations F 552 , like articulations F 464  in  FIGS. 59A-59E . Retraction hooks F 558  are attached to the retraction elements F 544  in the same manner as described in  FIG. 59 . The retraction hooks F 558  are rotatable about their long axes, such that prior to insertion into an incision to create a surgical aperture, a surgeon can first align the hook-shaped tips of the retraction hooks F 558  all pointing parallel to the direction of the incision (and so parallel to the margins of the incision, making that part of the retractor F 540  that actually descends into the patient as thin as possible) for easy insertion into the incision and then, secondarily, the surgeon can rotate the retraction hooks F 558  such that the hook shapes swing out and under the ribs adjacent to the retraction elements F 544  (on the left and right side, respectively) to engage the ribs for retraction. Margins F 555  of the two retraction elements F 544  can be shaped such that the retraction hooks F 558  on one retraction element F 544  interdigitate with the retraction hooks F 558  of the opposing retraction element F 544 , decreasing the separation of the axes of the retraction hooks F 558  to zero when they are inserted into the incision.  FIG. F60B  shows the retraction hooks F 558  aligned in this instance parallel to the direction of the incision; the retraction elements F 544  here have been somewhat differentially rotated about the dual-thrust lead screw F 546  to make it clear how the shape of the margin F 555  of the retraction elements F 544  can be sinuous, permitting the interdigitation of the left and right retraction elements F 544 . 
       FIG. 61  shows another embodiment of a retractor F 560  that achieves automatic balancing of loads. Multiple retractor blades F 566  are mounted onto hydraulic cylinders F 573  having pistons F 572  that move in response to pressure F 567  in the hydraulic cylinder F 573 , and the hydraulic cylinders F 573  are fluidically F 569  connected by hydraulic interconnects F 574  and arrayed in opposing gangs  570  of hydraulic cylinders F 573 . The gangs F 570  of hydraulic cylinders F 573  are positioned on a fixed retraction element F 562  and a moveable retraction element F 564  of a Finochietto-style retractor driven by a handle F 568 . When, for example during retraction, the tissue resistance force on an arbitrary first retractor blade F 566  draws out the first retractor blade F 566  such that the first hydraulic piston F 572  to which that the first retractor blade F 566  is attached is also pulled a portion of the length of hydraulic piston F 572  out of the first hydraulic cylinder F 573 , then the pressure F 567  inside the first hydraulic cylinder F 573  decreases. This decrease in pressure F 567 , communicated to the other hydraulic cylinders F 573  via the hydraulic interconnects F 574 , causes internal fluid F 569  to flow into this first hydraulic cylinder F 573  from the other hydraulic cylinders F 573 . Flow of the internal fluid F 569  out of the other hydraulic cylinders F 573  decreases their internal pressures F 567  consequently pulling the other hydraulic pistons F 572  inward, so causing the other retractor blades F 566  attached to the other hydraulic pistons F 572  to move F 576  in a direction opposite that of the first retractor blade F 566 . As with the embodiments above, the ratios of forces between all the retractor blades F 566  can be designed to be any ratio desired, for example by the use of hydraulic cylinders F 573  with different radii. In another embodiment, the hydraulic cylinders F 573  can be a single hollow fluid-filled housing with four pistons (or other number of pistons) emitting from the housing, with the housing acting as a fluidic plenum keeping all four pistons in hydraulic communication. The hydraulic fluid in these systems can be oil, sterile water, sterile saline, or a gas, such as air. Air further provides compressibility which acts like a “spring” in such a system, enabling compliance appropriate when loading tissues, for example. 
       FIG. 62  shows another embodiment of a retractor F 580  that achieves automatic balancing of loads with hydraulics. Similar to the cabled device depicted in  FIGS. 59A through 59E , retraction hooks F 584  and F 586  are attached to retraction elements F 582  by rotatable mounts F 588 ; however, now the cables are replaced by a series of hydraulic cylinders F 590  that compress or elongate (i.e., change total length) as the retraction hooks F 584  and F 586  rotate about the rotatable mount F 588 . The hydraulic cylinders F 590  are fluidically connected at fluidic connection F 599 , so as one hydraulic cylinder F 590  is elongated, for example, it pulls hydraulic fluid F 591  from the other hydraulic cylinders F 590 , causing them to shorten. Thus, as shown in  FIG. 62 , as a first retraction hook F 584  is pushed to the left (movement F 594 ), causing this first retraction hook F 584  to rotate clockwise about the rotatable mount F 588 , the hydraulic cylinder F 590  of retraction hook F 584  elongates, making the other hydraulic cylinders F 590  (such as that associated with the second retraction hook F 586 ) shorten, thereby rotating second retraction hook F 586  counter-clockwise about the rotatable joint F 588 , making second retraction hook F 586  move to the right (movement F 592 ). Alternatively, the hydraulic elements F 590  and F 599  can be arranged to be compressed under load instead of pulled, driving fluid F 591  out of the hydraulic cylinder F 590  of the first (engaging) retraction hook F 584  and into the hydraulic cylinder F 590  of the second (reacting) retraction hook F 586 . 
       FIGS. 63A through 63E  show another embodiment or a retraction element F 608  with retraction posts F 602  that compensate for one another&#39;s motion via retrograde action. Fenestrated bars F 604  link retraction posts F 602 , and motion of one retraction post F 602  causes the other retraction posts F 602  to adjust via a mechanical linkage through fenestrated bars F 604 .  FIG. 63A  shows a model with the fenestrated bars F 604  mounted on a fulcrum F 606  and the retraction posts F 602  passing through the fenestrated bars F 604  via holes F 605 , with the counteracting offsets of the retraction posts F 602  being evident.  FIG. 63B  shows a top view and  FIG. 63C  shows a side view of a retraction element F 608 . Each fenestrated bar F 604  in this example possesses two holes F 605  through which pass two retraction posts F 602 . Each fenestrated bar F 604  then further possesses one more hole F 631  admitting a fulcrum pin F 630 , forming the fulcrum F 606  upon which and about which the fenestrated bar F 604  is free to rotate. The fenestrated bar F 604  resides in this example close to the base of the retraction arm F 612 , to which each retraction post F 602  is connected via a hinge F 632  which allows each retraction post F 602  to swing back and forth along the axis of retraction F 639 .  FIG. 63D  shows the action for one retraction element F 608 . Consider the middle retraction post F 602  and its two fenestrated bars F 602 . As a the middle retraction post F 602  gets pushed backwards by the impinging tissue, the middle retraction post F 602  moves backwards, and this motion is transmitted as a moment by both fenestrated bars F 604  around the fulcrum F 606  to the top and bottom retraction posts F 602 , pushing that the top and bottom retraction post F 602  forward to meet the oncoming tissue. As with some of the other embodiments disclosed above, the motion of the retraction posts F 602  ceases when the moments equalize.  FIG. 63E  shows the counter motion of that shown in  FIG. 63D . This embodiment possesses two fenestrated bars F 604  that together link the motions (and so the countermotions) of three retraction posts F 602 . Note that one may design the fenestrated bar system with an arbitrary number n of fenestrated bars linking n+1 retraction posts. Note also that one may combine fenestrated bars of arbitrary lengths and proportions so creating useful variations of motion of the retraction posts without departing from the intent of the present invention. 
       FIGS. 64 through 66B  show still another embodiment of a retraction element of the current invention, this time providing swingletrees with the ability to automatically, dynamically and continuously adjust the position of their pivots to accommodate changing loads. In all  FIGS. 64 through 66B  the direction of retraction would be “up” towards the top of the page, and the patient&#39;s tissues would thus react by pulling “down” towards the bottom of the page. The retractor blades shown in  FIGS. 64 through 66B  thus engage an incision along the bottom of the page. 
       FIG. 64  A shows retraction element F 700  having a rectractor arm F 702  that is used to pull up in the direction of retraction F 701  F 722 . A two-tiered assembly of swingletrees, comprised of first swingletree F 704  (“parent swingletree”) and second swingletrees F 706  (“child swingletree”) hold four (4) retractor blades F 708 . First singletree F 704  attaches to retractor arm F 702  via pivot F 710 , here shown as a sheave. Second swingletrees F 706  attach to first swingletree F 704  also via pivot F 710 , here shown as a sheave. Retractor arms F 708  attach to second swingletrees F 706  via a pivot point F 712 , here shown as a rotating mount formed by a pin and a bushing. Swingletrees F 704  and F 706  still pivot within the plane of the page about a pivot F 710  that acts as a fulcrum, shown here as a freely rotating sheave. 
       FIGS. 65A through 65C  show side views of two different embodiments of retraction element F 700 .  FIGS. 65A and 65B  show side views of the retraction assembly F 700  shown in  FIG. 64 .  FIG. 65C  shows another embodiment of retractor assembly F 700  that captures first swingletree F 704  and second swingletree F 706  such that the assembly is held together. The sheave F 720  at pivot F 710  can be a bearing-mounted roller. As shown in  FIG. 65B , the sheave F 720  includes a provision (such as a groove F 722  or channel around its rim) for cupping, nestling, or riding along and otherwise retaining its association with that edge F 724  of each swingletree F 704 , F 706  that is closest to the incision. The first and second swingletrees F 704  and F 706 , respectively, includes a provision so that it mates with the sheave F 720 . As shown in  FIG. 25B , the lower edge F 724  of swingletrees F 704 , F 706  can be convexly radiused and otherwise shaped to accept the concavely shaped groove F 722  of the sheave F 720 . Given this arrangement, the lower edge F 724  of swingletrees F 704 , F 706  ride in the groove F 722  of the sheave F 720 , such that the loading by the patient&#39;s tissues retraction actually seats the swingletrees F 704 , F 706  more securely in the sheave F 720 . 
       FIG. 25C  shows another embodiment of the retractor assembly F 700 , here labeled as retractor assembly F 730 . To avoid swingletrees F 704 ,F 706  disengaging from sheaves F 722 , first swingletree F 704  is mated with another swingletee F 732 , creating a stacked assembly with two sheaves F 722  connected to each other by a pin F 732  through retractor arm F 702 . First swingletrees F 704 , F 732  are thus captured by retractor arm F 702 , and second swingletree F 706  is thus captured by doubled first swingletrees F 704 , F 732 . Another means of capturing each swingletree F 704 , F 706  is to have sheave F 720  ride in a restrictive slot formed within the child swingletree bar, instead of riding along the lower edge of the swingletree. 
     A “child” swingletree (e.g., second swingletree F 706 ) can serve as the “parent” of other swingletrees (in this case, F 676  and F 677 ) lower down in the hierarchy, creating as many levels as desired. Properly sized and assembled, such a network of swingletrees automatically assures so that no excess or imbalance of forces can remain. In this way, any excess force applied against the tissue of the patient is reduced. 
     One problem with retractor blades and the like is their tendency to apply not only forces directly against the tissue of the patient, but to shear along (or roughly parallel to) the raw surface of the incision. As a retraction proceeds, the relative motion or loading of the retractor blades may induce sliding along the edge of the margin of the incision (or an attempt by one or more retraction elements to do so), shearing the tissue in that plane (or tearing it outright). 
       FIGS. 66A and 66B  show another embodiment that uses distributed curvature of the freely riding swingletrees to limit this shearing motion of the retractor blades.  FIG. 66A  shows a retraction assembly F 740  having swingletrees with tightly curved arms that make retraction assembly F 740  more prone to shearing of the retractor blades, and  FIG. 66B  shows a retraction assembly F 760  having swingletrees with more gently curved arms that that make retraction assembly F 760  less prone to shearing of the retractor blades. The local curvature of the swingletree surface (riding in the parent sheave) influences the magnitude of the shear applied by rectractor blades F 712  along the surface of the incision (i.e., the behavior of the swingletree hierarchy is a function of the curvature of the swingletrees comprising it). Consider  FIG. 66A , swingletrees F 742  and F 744  are shaped with a substantially high curvature near the center of the swingletree, and lower curvature near their tips; thus, sheaves F 720  experience strong centering forces F 748  and remain more tightly centered under load, behaving much (but not all) of the time as if the pivots F 710  formed by sheaves F 720  were simply drilled through the bodies of the swingletrees. Under this circumstance shear is more likely to develop along the surface of the incision. Consider now  FIG. 66B  with swingletrees F 762 , F 764  shaped with a much gentler distribution of curvature along the swingletree bar, then the centering forces F 768  are smaller. Shear is instead relieved as the pivots F 710  of the swingletrees F 762 , F 764  can more easily shift laterally to suit owing to the smaller centering forces F 768 . Ideally, shear applied to the margin of the incision is minimal and the pivots F 710  supporting a given swingletree F 762 , F 764  remain substantially near the center of its respective swingletree F 762 , F 764 , thereby allowing the swingletree to rotate about the axis of the pivot F 710 , and within the plane of the page, to accommodate irregular loading as before. The gently curved swingletrees F 762 , F 764  thus permit simultaneous accommodation of rotation and sliding, thereby eliminating both excessive forces and shear. 
     Note that the intersection between the parent sheave and the child swingletree can be formed of two smooth surfaces, or it could be formed like a rack-and-pinion, where the parent sheave is a toothed like a pinion gear and the mating lower surface of the child swingletree bar is a toothed rack. Given this, one could further arrange for the active sensing and actuator control of the sheave rotation such that the position of the child is influenced by the active rotation (or clutching) of the sheave. This example admits active modulation of the play of forces and moments through a swingletree cascade. In some instances it may prove advantageous to apply shear on purpose, or to imbalance the forces applied to the patient&#39;s tissues, according to the needs of the surgeon. 
     G. Reducing Inappropriate Forces 
     G.1 Forces Exerted by Retractors on Tissues 
     When a surgeon performs a thoracotomy, she must deform a patient&#39;s body wall to move the apposed ribs aside far enough to permit her hands to access the thoracic cavity (see  FIG. 67 ). Current medical practice dictates that a surgeon (1) makes an incision between and parallel to two apposed, adjacent ribs; (2) simultaneously inserts the opposing blades of a rib spreader, or “retractor”, into the incision; and (3) turns the crank to force open the opposing blades, and the ribs, creating a hole. The hole or surgical “aperture” is typically about 10 centimeters across, and can range from 5 cm to 20 cm. Modern retractors are essentially rigid metal devices sporting hand-cranked jack elements. Today&#39;s spreaders, such as Finochietto-style retractors (see  FIGS. 2 and 68A ) are typically rack-and-pinion devices that, while constructed of polished stainless steel, operate on simple mechanical principles similar to those of 2,000-year-old bronze medical instruments found in ancient Greece (a vaginal speculum, see  FIG. 68B ), that is, a hand-cranked jack driving projecting blades. The retractor shown in  FIGS. 2 and 68A  are widely used; this retractor uses a lockable rack-and-pinion crank, as first disclosed by Finochietto in 1936 and published in 1941 (Bonfils-Roberts 1972). The principle remains the same as the ancient ones: equip a frame, otherwise rigid in all directions and along all axes, with the ability to expand along a single axis to simply overpower the tissues to force access to the inside of the patient&#39;s body. Thus, referring now to  FIG. 69 , the force required for opening is considered to be simply opposing forces F 1  G 8  and F 2  G 10 , applied at two points P 1  G 12  and P 2  G 14  lying on a single line of action. The only accommodations for more complex forces provided by the prior art are curved retractor blades, providing for example, non-point loading such as on a Cooley retractor G 16  ( FIG. 70A ), and swiveling retractor blades such as on older retractors that are no longer used (e.g. the retractors of Sauerbruch G 18  ( FIG. 70B ), De Quervain G 20  ( FIG. 70C ), and Meyer G 22  ( FIG. 70D ). Archeological museums and current medical supply catalogs visibly demonstrate that this one-dimensional thinking has underlain retraction device design for millennia. 
     Retractors work—they do force open bodies, but their design does not take into account the complex loading regime imposed on (and, in reaction, by) the patient&#39;s body. The result is that today&#39;s patient&#39;s tissues are bearing substantial loads that are not directly related to, or required for, opening; therefore, these retractors are causing unnecessary tissue trauma. 
     The inventors have measured forces during thoracotomy and observed for the first time that the actual forces of retraction are not the simple, one-dimensional case depicted in  FIG. 69 . It can now be appreciated that a complex set of forces and torques interact on the retractor, and thus on the patient&#39;s tissues. There are two lines of evidence for this claim. First, the force on a retractor (see  FIG. 71 , discussed below) is usually sufficient to lift the body of the retractor off the patient&#39;s body, such as in  FIG. 67 . Second, our measurements reveal that the forces acting on opposing retractor arms are not the same.  FIG. 72  shows our new data that were collected with the retractor shown in  FIG. 12 , which is fitted with a computer-controlled stepper motor B 8  to provide smooth motion and with a linear potentiometer B 16  to measure the distance of separation of the retractor blades B 20  and with strain gauges on the retractor blades B 20  to measure the forces on each of the two retractor blades B 20 . This retractor was used to perform thoracotomies on the carcass of a pig. In the retraction shown in  FIG. 72 , retraction occurred over 6 minutes, starting at 10 seconds, with a two-minute pause in retraction from 70 seconds to 190 seconds. The difference in the forces G 64 , G 66  measured on the two retractor blades is maximal at the end of retraction (at time=480 seconds, 19.5 kg versus 16.0 kg, a difference of about 20%). These force measurements demonstrate that retractors in the real world do not behave like perfect force diagrams out of a physics book as shown in  FIG. 69 , with two endpoints of zero extent (and equal force) connected by a one-dimensional line. 
     Applying pure tension or compression with today&#39;s retractors seems impossible. In light of the observations and measurements presented in  FIGS. 71 and 72 , it is difficult to imagine that one could ever see equal forces acting on the two blades of a conventional retractor placed inside a real patient. 
     Why is this so? Refer to  FIG. 71 . First, retractors such as retractor G 24  possess significant mass that is distributed unevenly, and they have blades G 33  and G 31  with non-zero dimensions and corners. Second, the patient&#39;s body is a sculpturally and structurally complex composite of heterogeneous biomaterials. Every structure inside a patient (e.g., ribs G 26  and G 28 ) is anisotropic and almost nothing behaves linearly. When the blades G 31  and G 33  of the retractor G 24  engage the two sides of an incision, the body forcefully opposes motion of the blades G 31  and G 33 . The patient&#39;s tissues (e.g., ribs G 26  and G 28 ) grew and developed alongside each other and the forces they generate tend to restore their apposed relationships. While the retractor G 24  drives its retractor blades G 31 , G 33  apart in a straight line, such as displacement G 32  and G 30 , the there are numerous forces G 36 , G 38 , G 40 , G 42 , G 52  G 56  and G 58  and torques G 27 , G 34 , G 44 , G 46 , G 48 , G 50 , G 54  and G 60  acting on the retractor blades G 31  and G 32  that arise from the deformations of the heterogeneous, three-dimensionally complex tissues surrounding the incision. Consequently, these forces are similarly three-dimensional and complex. 
     In the act of forcing open a living body with a conventional retractor G 24 , first one corner of one of the inserted retractor blades G 31  will strike some part of a rib G 26  and settle onto that rib G 26  and the intervening muscle tissue in an irregular fashion. Once that happens, and since the retractor G 24  is a rigid object, the retractor G 24  will react to the first contact, shifting position, until the other retractor blade G 33  encounters and settles somewhere onto its own opposing rib G 28  and muscle. The retractor G 24  then reacts and shifts again, with the blades G 31 , G 33  sliding along and shearing muscle against bone, back and forth in concert, as the surgeon applies torque to the retractor handle G 35  (and so the entire retractor) as the patient&#39;s body forcefully opposes motion of the retractor blades G 31 , G 33 . All the while, the patient&#39;s body deforms unevenly under the loads imposed by the retractor G 24 . The structures of the patient&#39;s body are deforming, which affects re-seating of the retractor blades G 31 , G 33 , which affects the deformation of the body, and so forth. All elements are shifting at once, but not evenly (i.e., not rectilinearly). The retractor G 24  is essentially a rigid object; at any time, there is little or no provision for the complex mechanical behaviors that are the hallmark of living tissue. Because of this, and crucially, the retractor blades G 31  and G 33  apply uneven forces to the body throughout spreading, and the forces are uneven when the surgeon achieves the required opening. 
     The apparent intention of the designers of conventional retractors was to apply large forces along a single line of action (the “retraction axis”). However, they do not accomplish this because they do not consider the response of the patient&#39;s body. The forces on the retractor are those imposed by the reaction of the patient&#39;s body to the displacement of its tissues, and the patient&#39;s body does not respond along a single line of action—it generates complex, three-dimensional forces in response to deformation. Furthermore, these forces change as deformation proceeds while the retractor remains in contact with the patient&#39;s body tissues. The retractor, in return, opposes these forces by moving (e.g. lifting off of the patient&#39;s body) or by accumulating stresses in the retractor. Consequently, the patient&#39;s tissues bear substantial stresses beyond those required for opening, leading to tissue trauma (e.g., broken ribs) that, otherwise, should be avoidable. 
     Clearly, minimizing undue stresses during a thoracotomy or other surgical procedure would be beneficial to the patient, reducing tissue trauma to the barest minimum required to generate an adequate surgical aperture. This can be accomplished by (a) generating force along a line of action to retract the tissues to achieve a desired surgical opening (the “retraction axis”), (b) accommodating motions (e.g. translations and rotations) of the retraction axis such that there is minimal opposing force from the retractor to these motions, and (c) accommodating motions (e.g. translations and rotations) of the retractor blades, and of the underlying tissues, that are not parallel to the retraction axis such that these non-parallel motions occur with minimal opposing force from the retractor. To this end, the retractor should also be as lightweight as is practicable. Thus, the retraction axis is free to move in space and there is minimal force opposing motions not on the retraction axis. 
     This can be accomplished by a lightweight retractor that is free to move, or its parts are free to move, as the patient&#39;s body exerts forces that are not along the retraction axis. Such a retractor, thus, automatically aligns itself (e.g., its blades) such that the retraction axis is always oriented along a direction that achieves the desired surgical opening while reducing the magnitudes of all forces. 
     Disclosed herein are apparatus and methods for automatically minimizing the imposed deformation forces applied to the patient to the minimum required for surgical access. With the various embodiments of the present invention one can readily apply forces sufficient to deform the patient&#39;s tissues in the manner appropriate for medical procedures while minimizing forces arising from or leading to undesired deformations of the patient&#39;s tissues. 
     G.2 Swing Blade Retractor—Dual-Thrust Lead Screws 
       FIG. 73  shows one embodiment of a retractor G 68  that possesses a new degree of freedom of motion, allowing the retractor blades G 76  to automatically realign to reduce forces that are not parallel to the retraction axis. Retractor G 68  is functionally divided into three units: a dual-thrust lead screw G 80 , a first retraction unit G 70 , and a second retraction unit G 72 . Collectively, retractor G 68  is referred to as a “Swing Blade Retractor”. The dual-thrust lead screw G 80  is a lead screw having at least left-hand threads on one end and right-hand threads on the other end (such as those offered by the Universal Thread Grinding Company, Fairfield, Conn.). 
     Each retraction unit G 70 , G 72  is comprised of a retraction body G 78 , G 79  having hollow ‘female’ threads that engage the outside surface of the dual-thrust lead screw G 80 , a retractor arm G 74 , G 75 , and a retractor blade G 76 , G 77 . The retraction bodies G 78 , G 79  have either a left-hand thread or a right-hand thread with which to follow the travel of the threads on the outside of the dual-thrust lead screw G 80 . When the dual-thrust lead screw G 80  is rotated, the dual-thrust lead screw&#39;s threads (which are engaged with the threads in the retraction bodies G 78 , G 79 ) force the two retraction units G 70 , G 72  to move away from each other to displace the (now formerly) apposed tissues. Rotation of the dual-thrust lead screw G 80  about its long axis can be accomplished by any of several means, including a hand crank mounted to one end of the dual-thrust lead screw G 80 , a motor mounted to one end, a hand crank attached to a gear inside one retraction body unit G 78 , or a motor attached to a gear inside one body unit (or both). The gears might be helical gears, crown gears, friction drives, or other means permitting a retractor body to simultaneously drive dual-thrust lead screw G 80  rotation and follow the motions of the threads on the outside of the lead screw. 
     Note that a dual-thrust lead screw G 80  could be made to have an arbitrary number of regions of both left- and right-handed threads, with arbitrary pitches (and so advance ratios), such that a plurality of retraction units G 70  could be made to move all at once on a single lead screw G 80 , at different speeds and directions relative to one another. For example, one might wish to engage more than two ribs at once, say, four or six, and move them all in concert to distribute deformations and loading, and to prevent crushing of soft tissues between sequential sets of ribs. 
     While away from the body of the patient and not locked together, each of the Swing Blade Retractor&#39;s G 68  retraction units G 70  and G 72  is able to swing freely about the long axis of the dual-thrust lead screw G 80 . Note that rotation of both retraction units G 70 , G 72  about the long axis of the dual-thrust lead screw G 80  is constrained when the Swing Blade Retractor G 68  is placed against the patient&#39;s body or if the retractor blades G 76  and G 77  are engaged with the tissues. The result of this constraint is that when the dual-thrust lead screw G 80  rotates, while both retractor blades G 76  and G 77  are against or inside the patient&#39;s body, both retraction units G 70  and G 72  move apart, opening the incision to create the surgical aperture. Furthermore, if a crank or motor is placed inside one retraction body G 78  or G 79  of only one retraction unit G 70  or G 72 , respectively, then both retraction units G 70  and G 72  still move apart under rotation of the dual-thrust lead screw G 80 . The retraction units G 70  and G 72  will come back together when the dual-thrust lead screw&#39;s G 80  direction of rotation about its own long axis is reversed. Thus, a motor or crank in one retraction body G 78  of one retraction unit G 70  can be used to rotate the dual-thrust lead screw G 80  and, thereby, drive both retraction units G 70  and G 72  apart. 
       FIG. 74  depicts how a Swing Blade Retractor G 68  has an additional degree of freedom, relative to a conventional retractor, such as those shown in  FIGS. 67, 68A, 68B, and 70A through 70C . For the Swing Blade Retractor G 68 , retraction units G 78 , G 79  are mounted to the dual-thrust lead screw G 80  only by the threads in each retraction body G 78 , G 79 , so the retraction units G 78 , G 79  are free to rotate about the long axis of the dual-thrust lead screw G 80 . The retractor blades G 76 , G 77  are, thus, able to rise and fall in a direction G 98  approximately perpendicular to the axis of retraction G 100  and perpendicular to the surface of the body of the patient (i.e., in and out of the incision). In contrast, the arms of a conventional retractor are always constrained to move towards or away with respect to one another within that single axis of retraction; any tendency of the retractor blades to move in any other direction is strongly resisted by the substantial structure of the retractor. Importantly, any tendency of the body wall in contact with the retractor blades to move in some direction other than the axis of retraction is also resisted by a conventional retractor, and, subsequently, substantial stresses can form in the body&#39;s tissues that are unrelated to the force required to obtain the surgical opening. In other words, the minimum amount of force and/or trauma to open the body wall might require a curved path, or a slightly shifting path, as opposed to a unidirectional, rectilinear path. 
     An additional advantage of a Swing Blade Retractor is that it can assist insertion of the retractor blades into the incision during preparation for retraction. When inserting the retractor blades of the prior art into an incision through a patient&#39;s body wall, the surgeon is forced, by the rigidity of the retractor frame, to jam both retractor blades in at once. This is a problem because this cannot be done until the surgeon first uses her fingers to pry open the incision to be wide enough to be able to fit in both blades, which may themselves have wide edges. However, for the Swing Blade Retractor G 68 , because the two retraction units G 70 , G 72  swing freely and independently, each retractor blade can be inserted one at a time as desired, allowing a surgeon to begin with a smaller opening. 
     Another advantage of the Swing Blade Retractor G 68  is that the hollow threads of the retraction bodies G 78 , G 79  can be formed of more than one piece. For example, the hollow threads can be made of two halves, each a semi-circle in section, that are brought together inside the retraction body G 78 , G 79  to enclose, embrace and engage the threads of the dual-thrust lead screw G 80 . This enables another improvement over the rack-and-pinion retractors, which must be laboriously cranked back all the way shut to be removed, in that one or both of the Swing Blade Retractor&#39;s G 68  retractor bodies G 78 , G 79  can be instantly removed from the dual-thrust lead screw G 80  by disengaging the two-piece hollow threads. For example the two-piece hollow threads can separate such that the dual-thrust lead screw G 80  can pass through a gap made by the separation. The means of thread disengagement might be a button, lever, motor or flap that when closed retains and stabilizes the threaded halves around the dual-thrust lead screw G 80 . This enables the surgeon to rapidly lift one or both retraction bodies G 78 , G 79  away to clear the surgical field in an emergency, facilitating removing the entire retractor G 68 . Similarly, the hollow threads, rather than being composed of two halves that fully or almost fully wrap the dual-thrust lead screw G 80 , can engage only one side of the dual-thrust lead screw G 80 , wrapping only ⅕th, for example, of the circumference of the dual-thrust lead screw G 80 . This facilitates disengagement of the threads from the dual-thrust lead screw G 80 —the threads need only be lifted away from the dual-thrust lead screw G 80  to permit free motion of the retraction unit G 70 , G 72  along the length of the dual-thrust lead screw G 80 . 
     The advancement of the retraction bodies G 78 , G 79  usually proceeds from the rotation of the dual-thrust lead screw G 80  about the dual-thrust lead screw&#39;s G 80  long axis. The rotation of the dual-thrust lead screw G 80  can be the result of a source of torque such as a hand crank, a motor, or the like. The source of torque can be external to the retraction body G 78 , G 79 . In one embodiment, the source of torque is located inside one retraction body G 78  or G 79 . In this case, the retraction body G 78  or G 79  thus possesses its normal capability to be driven along the dual-thrust lead screw G 80  while simultaneously being the agent that drives the rotation of the dual-thrust lead screw G 80  about its own long axis. For example, one may modify the dual-thrust lead screw G 80  by further providing rotation means co-located with the threads along the shaft so that the retraction body G 78  or G 79  may engage both the threads for advancement and the rotation means for rotation. One example of rotation means would be splines cut along the length of the shaft. The threads of the dual-thrust lead screw G 80  and splines (not shown) can co-exist on the same driveshaft, are not mutually exclusive, and can be engaged by separate mechanisms housed within the retraction body G 78  or G 79 . The hollow threads disclosed above can provide the engagement for advancement upon rotation of the dual-thrust lead screw G 80 , while a toothed ring drive (not shown) surrounding the lead screw but engaging only the splines provides the rotation. The hollow threads “see” only the threads of the dual-thrust lead screw G 80  while the toothed ring “sees” only the splines, i.e., the surface gaps forming the splines do not present occlusions to the threaded follower and the surface gaps forming the threads do not present occlusions to the toothed ring drive. This form of the dual-thrust lead screw G 80 , called a splined dual-thrust lead screw, can be made by first cutting, machining, or rolling helical threads into a plain metal rod or cylinder, and then cutting splines in the same cylinder. Other means are possible, but the intent is to provide in one device (and even in one component of the device) simultaneous dual-thrust lead screw G 80  thread following and lead screw rotation. 
     Another benefit of the Swing Blade Retractor G 68  design is that it is self-aligning. For stability&#39;s sake, the Swing Blade Retractor G 68  exploits the tendency of the edges of the patient&#39;s body wall to re-appose once separated. When a surgeon retracts the body wall, the apposed or touching edges of the incision now move apart. The body&#39;s mechanical reaction is to re-appose the edges of the incision, i.e., the distance between the edges of the incision “tries” to return to zero. Crucially, this re-apposition occurs in three dimensions. No matter the initial orientation of the retractor blades G 76 , G 77 , they cannot swing apart once engaged with the patient&#39;s body wall; thus, the natural forces at work in the patient&#39;s body automatically align the retractor blades G 76 , G 77  (and indeed, the entire axis of retraction G 100 ) to exactly that angle in three dimensions that minimizes the distance between the retractor blades G 76 , G 77 , and so, the force required for retraction. 
     G.3 Swing Blade Retractor—Roller Drives 
       FIG. 75  shows another means G 104  for driving retraction units in a retractor. Rather than using a dual-thrust lead screw, a roller drive G 106  is used. A roller drive combines thrust and rotation, like a dual-thrust lead screw, but can be more efficient, and it offers the ability to variably adjust the pitch of drive. Roller drive G 106  has three or more rollers G 110  engaging a shaft G 114  with at least one of the rollers, a driver roller G 112 , coupled to a torque source, such as a motor or a hand crank, and with the other rollers G 110  acting as idler rollers which passively roll along the shaft G 114 . The rollers G 110  and G 112  can have collars that help guide the rollers G 110  and G 112  along the shaft G 114 , ensuring that the rollers G 110  and G 112  remain engaged with the shaft G 114 . The shaft G 114  can be substantially rectangular in cross section, as in  FIG. 75 , or shaft G 114  can have any other cross-sectional shape matched to the rollers G 110  and G 112 . The rollers G 110  and G 112  are forced against the shaft G 114  such that friction between the driver roller G 112  and the shaft G 114  causes the driver roller G 112  to impel the shaft G 114  when the driver roller G 112  rotates under the action of its torque source. Note that motion is relative, so the roller drive G 112  can move along a stationary shaft G 114 , or a shaft G 114  can be pushed by a stationary roller drive G 112 . The rollers G 110  and G 112  can be fitted with appropriate bearings to permit substantial force pushing the rollers G 110  and G 112  against the shaft G 114  to generate substantial friction between the shaft G 114  and the driver roller G 112 . The rollers G 110  and G 112  can be forced against the shaft G 114  either by precise manufacture of the mounts holding the rollers G 110  and G 112 , or the rollers G 110  and G 112  can be pressed into position by, for example, a cam that variably moves the rollers G 110  and G 112  away from the shaft G 114 , releasing the shaft, or presses rollers G 110  and G 112  against the shaft G 114  to hold or drive the shaft G 114 . 
       FIG. 76  shows how a roller drive G 106  can be used in a retractor G 116 . Retraction unit G 117  has a roller drive comprised of a first idler roller G 120 , a second idler roller G 121 , and a drive roller G 124 . Idler rollers G 120 , G 121  and drive roller G 124  engage shaft G 122 , and torque on drive roller G 124  drives retraction against the retraction force G 126  from the tissues. This configuration of idler rollers G 120 , G 121  and drive roller G 124  provides several advantages. First, retraction force G 126  results in a torque G 118  on retraction unit G 117  that then applies a force G 128  on the drive roller G 124  and the first idler roller G 120  that increases drive friction for drive roller G 124 , thereby improving engagement between the driver roller G 124  and the shaft G 122 . Second, the shaft G 124  is smooth, decreasing chances for snagging items in the surgical field. Fourth, the rollers G 120 , G 121 , and G 124  and shaft G 122  are easier to manufacture precisely, decreasing cost. 
       FIG. 77  shows another embodiment of a roller drive G 130 . The idler rollers G 138  and drive roller G 139  do not have collars as in  FIGS. 75 and 76 ; rather, the idler rollers G 138  and are circular cylinders. In  FIG. 77 , the shaft G 136  is circular in cross-section. The rollers G 138  have an axis of rotation G 140  defining the orientation of rotation G 142  of the rollers G 138 . On the left-hand side of  FIG. 77 , the rollers G 138  and shaft G 136  are configured such that the roller axes of rotation G 140  are all perpendicular to the long axis of the shaft G 136 ; thus, when the driver roller is actuated, the shaft moves out of the page plane (see rotation-indicating arrows). On the right-hand side of  FIG. 77 , the roller axes of rotation G 140  are aligned oblique to the long axis of the shaft G 136 ; thus, when the driver roller G 139  is actuated, the shaft G 136  moves helically out of the page plane. In other words, the motion of the shaft G 136  imparted by the rollers G 138  and G 139  has two components, one that translates the shaft G 136  out of the page plane and one that rotates the shaft G 136  around the long axis of the shaft G 136 . 
       FIG. 78  shows that the relative degree of each motion of the shaft (translation and rotation) is determined by the angle between the roller axis of rotation G 140  and the long axis of the shaft G 136 . The angle between the roller axis of rotation G 140  and the long axis of the shaft G 136  is shown to vary from left to right in  FIG. 78 . On the left-hand of  FIG. 78 , the roller axis of rotation G 140  is perpendicular to the long axis of the shaft G 136  resulting in translation of the shaft directly out of the page towards the viewer (without rotation). On the right-hand of  FIG. 78  roller axes of rotation G 140  are perpendicular to the long axis of the shaft G 136 , resulting in rotation of the shaft G 136  within the plane of the page (without translation) when the rollers&#39; axes of rotation G 140  are parallel to the long axis of the shaft G 136 , and the shaft G 136  cannot be moved through the rollers G 139 , G 140  regardless of whether rollers G 139 , G 140  are turning, i.e., the shaft G 136  is locked, thus this embodiment of the retractor G 130  is self-retaining. At all other angles between the roller axis of rotation G 140  and the long axis of the shaft G 136 , rotation of the rollers G 139 , G 140  results in a combination of rotation and translation of shaft G 136 . 
     Note in  FIG. 78  that varying the angle between the rollers&#39; axes of rotation G 140  and the long axis of the shaft G 136  effectively varies how the driver roller&#39;s G 139  power is spent—when the roller axis of rotation G 140  is perpendicular to the long axis of the shaft G 136 , all of the power of driver roller G 139  is spent translating the shaft G 136 , and when the roller axis of rotation G 140  is parallel to the long axis of the shaft G 36 , all of the power of driver roller G 139  is spent rotating the shaft G 136  in place. In motions in which one motion (translation or rotation) of the shaft G 136  is strongly opposed and the other motion is not, then varying the angle between the roller axes of rotation G 140  and the long axis of the shaft G 136  effectively gears the roller driver G 130 , allowing the roller driver&#39;s axis of rotation G 140  to be adjusted such that the power of the roller driver G 139  is sufficient to generate the desired force and motion. For example, the roller axes of rotation G 140  can initially be parallel to the long axis of the shaft G 136  when the torque source of the driver roller G 139  starts rotating the driver roller G 139 . This causes the shaft G 136  to rotate without translation. While the driver roller G 139  continues rolling, the angle of the roller axes of rotation G 140  can be continuously changed such that the shaft G 136  slowly starts translating. The angle of the roller axes of rotation G 140  can, thus, be adjusted to place more of the power of the driver roller G 139  into translating the shaft G 136 . Note that controlling the angle between the rollers&#39; axes of rotation G 140  and the long axis of the shaft G 136  can also be used to control the velocity of translation of the shaft G 136 . 
       FIG. 79  shows one embodiment that uses roller drives in a retractor G 160 . Retractor G 160  has two opposed retraction units a first retraction unit G 161  and a second retraction unit G 162 , each comprised of a retractor arm G 168  and G 170 , respectively, and a retractor body G 183  and G 182 , respectively, mounted on shaft G 184 . Consider first retraction unit G 161 : retractor body G 183  houses a roller drive G 171  comprised of two idler rollers G 172  and a drive roller G 175 , each oriented with its roller axis G 176  of rotation oblique to the long axis of the shaft G 184 . The second retraction body G 182  contains a set of three idler rollers G 172 , each oriented with its roller axis of rotation G 176  oblique to the long axis of the shaft G 184 . Both retraction units G 161  and G 162  are, thus, driven by the one drive roller G 175  in the first retraction body G 183 . The angle between the rollers&#39; axes of rotation G 176  in the first retraction body G 183  and the long axis of the shaft G 184  determines the motion G 186  of the shaft G 184  relative to the retractor body G 183 . The motion G 186  of the shaft G 184  can be broken into its two components of rotation G 189  and translation G 187  relative to first retractor body G 183  and rotation G 190  and translation G 188  relative to second retractor body G 182 . The second retraction body G 182  does not drive the shaft G 184 ; rather, the rotation of the shaft G 184  drives the translation G 188  of the second retraction body G 182  and, thus, the second retraction unit G 162 . Engagement of the retractor arms G 168  and G 170  with the patient&#39;s tissues prevents rotation of the second retraction unit G 162  about the long axis of the shaft G 184 , so the angle between the rollers&#39; axes of rotation G 176  in the second retraction body G 182  determines the translation rate G 188 . Thus, the driver roller G 175  in the first retraction body G 183  drives apart both retraction units G 161  and G 162  with a relative velocity of G 187  plus G 188 , thereby providing retraction G 164 , G 165 . 
     Note that the angle between the rollers&#39; axes of rotation G 176  and the shaft G 184  in the first retraction body G 183  need not match the angle in the second retraction body G 182 . The angle can be such that the first retraction body G 183  generates only rotation of the shaft G 184 , and the angle in the second retraction body G 182  can be such that the second retraction unit G 162  moves away from the first retraction unit, or any other range of combinations. Infinitely fine and smooth control of the rate of retraction by the rate of rotation of the driver roller (e.g. by a motor that actuates it) is thereby achieved by varying the angle between the rollers&#39; axes of rotation G 176  and the shaft G 184  in the first retraction body G 183 , and also by varying the angle between the rollers&#39; axes of rotation G 176  and the shaft G 184  in the second retraction body G 182 . A mechanism that variably changes the angle between the rollers&#39; axes of rotation G 176  and the shaft G 184  in either or both retraction body G 182 , G 183  can thus be used to control both the rate of retraction and the magnitude of the thrust (retraction force). 
     G.4 Dovetails 
     Another embodiment of a retractor G 190  is shown in  FIG. 80 . Retractor G 190  uses an alternative means for providing additional degrees of freedom of motion to the retractor arms G 194 . The two arms G 194  of retractor G 190  are mounted to the frame G 192  of the retractor G 190  via dovetail slides G 196  and G 198 , the axes of which are perpendicular to the axis of the motion of the retractor blades (i.e., the axis of retraction). Each retractor arm G 194  is thus free to slide out and back, i.e., perpendicular to the axis (or direction) of retraction. Much of the forgoing concerning the features and benefits of the Swing Blade Retractors G 68  and G 160  and applies here, except that the accommodating motions G 200  of the retractor blades G 199  enabled by the dovetails G 196 , G 198  can be perpendicular to that of the Swing Blade Retractors G 68  and G 160 . Additionally, the motions G 200  are directly translational as opposed to rotational, as was the case for the Swing Blade Retractors G 68  and G 160 , and the two may be combined as desired to increase a retractor&#39;s ability to accommodate the patient&#39;s reconfiguring tissues. 
       FIG. 81  shows another retractor G 202  that is fitted with two dovetail slides. First dovetail G 204  permits motion G 208  of retractor arm G 214  and retractor blade G 216 , matching the motion G 200  of dovetails G 196 , G 198  in  FIG. 80 . Second dovetail G 206  permits motion G 210  of retractor arm G 214  in a direction at right angles to the first dovetail G 204 , with both motions G 208  and G 210  being perpendicular to the axis of retraction. This means that this retractor can accommodate both a rise and fall of the body wall and a relative sliding of the edges of the incision parallel to the incision and within the plane of the skin of the patient, while still delivering retraction forces to the patient&#39;s body wall. This design still achieves stability and force minimization (now in 2 axes) by exploiting the tendency of the patient&#39;s body wall to re-appose. 
     G.5 Parallelograms 
     In yet another embodiment of a retractor G 218  shown in  FIGS. 82A and 82B , another mechanism is disclosed for providing an additional degree of freedom to the retractor arms G 224 , G 226 . Retractor G 218  is comprised of two parallel dual-thrust lead screws G 230  and G 232  held by captured swiveling nuts G 228  in a first retraction body G 220  and a second retraction body G 222 . Each retraction body G 220 , G 222  bears a retractor arm G 224 , G 226 , and a retractor blade G 250 , G 252 . Captured, swiveling nuts G 228  are similar to those found in a Jorgenson clamp used for woodwork, such as those offered by Woodworker&#39;s Supply of Albuquerque, N. Mex. These captured, swiveling nuts G 228  allow movement G 254  of the retractor blades G 250 , G 252  in a direction approximately perpendicular to the axis of retraction G 256  (see  FIG. 82 b   ). 
     G.6 Tension Straps 
     Another embodiment is shown in  FIG. 83 , which shows a different retractor configuration we call a “tension strap retractor” G 258  that automatically aligns to the forces on the retractor blades G 264 . Tension straps include mechanisms integral to the strap that are capable of generating the force for retraction. Here, the retractor G 258  takes the form of two or more thin straps, cranial strap G 262  and caudal strap G 266 , that wrap around, and maybe behind, a portion of the body of the patient G 260 . Cranial strap G 262  and caudal strap G 266  are connected to retractor blades G 264  which are inserted into the incision G 269  to pull on the cranial rib G 263  and caudal rib G 265 . The cranial strap G 262  can be held in position by wrapping around a portion of the patient&#39;s G 260  body, such as around the neck and/or shoulder. The caudal strap G 266  can be held in position by wrapping around a portion of the patient&#39;s G 260  body, such as around one leg. Alternatively, the straps G 262  and G 266  might anchor on the dermis of the patient G 260  or on a bedframe. In this embodiment, retractor G 258  self-aligns with the natural resistance of the body wall. Tension strap retractor G 258  operates in tension, as opposed to a traditional compression- and bending-resisting frame. One benefit of a tension strap retractor G 258  is that the volume of material required to withstand the retraction forces in tension is a small fraction of the volume of material required to withstand similar forces in, say, bending. Given this, a tension strap retractor can be very lightweight, further reducing unnecessary loading of the patient&#39;s tissues. 
       FIGS. 84 and 85  show another embodiment of a tension strap retractor G 270  adapted for sternotomy.  FIG. 84  shows a front view of tension strap retractor G 270 , and  FIG. 85  shows a cross-sectional view through a patient&#39;s body G 272 . Tension strap retractor G 270  simply wraps around behind the back of the patient G 272  and automatically orients to open up an incision G 275  that bisects the sternum into two halves G 281  and G 282 . Retractor blades G 278  and G 280  reach into and/or around the margin of the incision G 275 , pulling back on sternum halves G 281 , G 282 . Tension strap retractor G 270  pulls along the surface of the body of the patient G 272 . In this arrangement, the straps G 274 , G 276  and G 306  of the tension strap retractor G 290  load the body wall such that straps G 274 , G 276  and G 306  remain aligned with the body wall and, thus, with the retraction forces for opening incision G 275 . 
     For tension strap retractor G 270 , the straps can be any thin and strong fabric, such as nylon webbing, that can resist tension. The straps can develop tension via a pull strap with sliding buckle, a ratchet pull, a winch, or by direct shortening of the fibers of the strap (for example by using shape memory alloy for the fibers). To this end, the strap might be fibrous netting surrounding pressure bladders G 296 , for example elastomer balloons residing within two-layer (or hollow) nylon webbing. In this case, the netting can be formed of fibers that run helically around the strap G 276 , G 278 , and G 306  as a whole. In this example, the trajectory of the helical fibers forms an angle with respect to the path of the main strap; the angle can be very low (10 to 30 degrees) to facilitate developing significant force when the bladders G 296  are inflated. Retraction forces can be generated by inflation alone if desired. Inflating the pressure bladders G 296  would swell them, developing tension in the straps G 276 , G 278 , and G 306 , and so loading the retractor blades G 278  and G 280 . The swollen bladders G 296  can also provide a moment enhancer G 303  (i.e., a stand-off) to reduce the magnitude of the tension that must be developed to create the forces sufficient to operate the tension strap retractor G 290 . Alternately, the stand-off G 303  function might be achieved more directly by placing pads, pillows, blocks, or other compression-resisting members between the straps G 276 , Gs 78 , and G 306  and the body of the patient G 272 . Saddles and pads can be added to the straps G 274 , G 276  and G 306  to distribute loading of the straps over the patient&#39;s body G 272 , or to concentrate the loads in particular areas, for example those areas that can withstand more concentrated pressure. 
     Another advantage of the tension strap retractor G 270  is that it offers greater access to the surgical field because it has few components near the surgical field and these components lay close to the body of the patient G 272  with tension strap retractor G 270  having an extremely low profile, perhaps projecting no taller than 2 or 3 millimeters above the skin of the patient. 
     Tension strap retractors G 290  can also be used for non-thoracic surgery. A common use of retractors, such as a Weitland retractor G 312  (see  FIG. 86 ) is to pull open an incision through the skin G 314  to provide access to the anatomy beneath the skin for plastic surgery, orthopedic surgery, neurosurgery, and others. Retraction of the skin frequently requires only small forces, but conventional retractors in the prior art, such as a Weitland retractor G 312 , are typically scissor-like devices made of steel and are heavy, thereby interfering with surgical access and exerting unnecessary loads, especially during the second phase of retraction. 
       FIG. 87  shows a tension strap retractor G 320  that is small and lightweight to, for example, open the skin on an arm for vascular surgery. The tension for retraction of the retractor blades G 326  and G 332  can be generated by pull tabs G 324  and G 334  that pull the strap G 322  through a self-cinching buckle G 328  and G 330 . Alternatively, tension could be generated by pulling the strap through a loop and then securing back onto the strap with Velcro. 
     H. Hard Tissue Engagers 
     Retractors, by their very nature, are typically made of rigid stainless steel to withstand the stresses of forcing open incision, including incisions through rigid structures like rib cages. Rib cages are themselves made largely of rigid bone and built to withstand the stresses of human locomotion or lifting large loads. The ribs, as it happens, are intermingled with several much softer tissues, including muscles which provide actuation for breathing and modifying posture, connective tissues which transmit forces from one rib to another and to the spinal column, vessels and arteries which supply nutrients and remove waste products, nerves providing signaling to and from the spinal chord; and all these are covered with skin and adipose tissues. During a thoracotomy in which a surgeon inserts the retractor blades and then cranks to spread the patient&#39;s ribs apart, the muscles apposed to those ribs and the nerves running along the surface of those ribs are often damaged when compressed between the rigid rib and the metal blades of the retractor. The soft tissues, supposedly protected by the ribs, are instead caught in the middle when the retractor blades push against the bones during retraction. 
     In more detail, our ribs lie in a closely packed row deep under the skin, spaced about as far apart as they are wide, forming serial bony bars embedded in the muscle and other soft tissues that the ribs in turn support. As shown in  FIG. 88A , running under the skin H 42 , cranial rib H 46  and caudal rib H 47  are roughly oval in cross section with the long axis of the oval aligned more-or-less parallel to the surface of the skin H 42 . (The following description uses the terms “caudal” H 45  and “cranial” H 43 , which refer to relative position in the body, with cranial being closer to the head and caudal being closer to the feet.) Intercostal tissues H 44 , which are mostly muscle and connective tissues, span the space between the cranial margin H 54  of the caudal rib H 47  and the caudal margin H 50  of the cranial rib H 46 . A delicate bundle of nerves and arteries (the neurovascular bundle H 48 , which includes the intercostal nerve, lays just inside the caudal margin of each rib H 46 , H 47 . 
     Surgeons, aware that the neurovascular bundle H 48  can be easily damaged, prepare to insert the retractor by slicing the intervening intercostal tissues H 44  closer to the cranial margin H 54  of the caudal rib H 47 . This lessens the probability of accidentally cutting the neurovascular bundle H 48  during the incision, and it provides a pad of muscle on the caudal margin H 50  of the rib H 46  that is cranial to the incision H 52 , in order to protect the neurovascular bundle H 48 . 
     As shown in  FIG. 88B , the retractor is inserted into the incision H 52 , with retractor blades H 30  positioned to push against the two ribs H 46  and H 48 . Retractor blades H 30  are attached to retractor arms H 32  by fasteners H 38  such that retraction pries apart the ribs H 46 , H 47  when retraction arms H 32  separate during the first phase of retraction. 
     Damage to the neurovascular bundle H 48 , nevertheless, occurs. As depicted in  FIGS. 89 and 90 .  FIG. 89  shows a cross-sectional view, and  FIG. 90  shows a top view. Regions of high pressure H 60  are created in the intercostal tissues H 44  that are compressed between the ribs H 46 , H 47  and the hard retractor blades H 30 . The pressures are large, owing to the large forces used to separate the ribs. Subsequently, tissues are mechanically crushed. The neurovascular bundle H 48  can be pinched, especially at pinch points H 82  created by the edges (i.e., corners) of the blades H 30  where they intersect the ribs H 46  and H 47 . The tissue pressures underlying the retractor blades H 30  can be sufficiently high to block both blood flow through the vessels of neurovascular bundle H 48  and perfusion of all this tissue underlying the retractor blades H 30 . Lack of perfusion causes anoxia in theses tissues, which damages all tissues, especially nerves. Additionally, movement of ribs H 46 , H 47  during retraction can be sufficiently large that a rib H 46 , H 47  can impinge on the adjacent rib further from the incision, as shown in  FIG. 91 . Again, the resulting regions of high tissue pressure H 60  between ribs can be sufficiently large that intercostal tissues H 44 , including the intercostal nerves in neurovascular bundles H 48 , can be damaged one or even several ribs removed from the incision (Rogers, Henderson et al. 2002). 
     The regions of high pressure H 60  and the pinch points H 82  are established during the first phase of retraction and are then sustained during the second phase of retraction for the duration of the surgical procedure, which can often be hours. 
     Damage to intercostal tissues caused by the regions of high pressure H 60  and by pinch points H 82  is thought to underlie much of the pain caused by thoracotomies, especially damage to the intercostal nerves of the neurovascular bundles H 48 . Thoracotomies are considered one of the most painful of all surgical procedures. Pain is always intense for days after surgery and, unfortunately, can last for months to years, and sometimes is permanent. The long-lasting pain after a thoracotomy has lead to the identification of a “post-thoracotomy pain syndrome”. 
     This great pain following thoracotomies, and the associated morbidity and mortality, are the main drivers for alternatives to these open-chest procedures, including minimally invasive surgery (MIS). While many MIS procedures have been developed, such as mini-thoracotomies, endoscopic surgeries, and the like, their adoption rates have been low. 
     An improved retractor blade that decreases tissue damage during retraction, especially to the intercostal nerve, would be of great benefit. It would reduce post-operative pain while retaining full surgical access. 
     To these ends, we disclose apparatus and methods for attaining favorable alignments and positive engagements with a patient&#39;s hard tissues, for example bones (e.g. ribs) or teeth. With the various embodiments of the present invention one can rapidly and assuredly apply forces sufficient to displace or deform the patient&#39;s tissues for medical procedures while entirely avoiding compressing, crushing, or compromising adjacent soft tissues, thus preventing post-surgical pain. 
     In one embodiment shown in  FIG. 92A  and  FIG. 92B , holes H 118  are drilled from above into the ribs H 46  and H 47  and rigid posts H 120  are inserted into these holes to serve as anchors for the retractor. Holes H 118  can be drilled at an angle H 119  such that, after the posts H 130  are inserted, the posts possess an angle with respect to the axis of loading to ensure the posts don&#39;t slip out of the holes during retraction. The posts H 120  can be made such that they snugly fit into the holes H 118  to ensure good purchase in the bone of ribs H 46  and H 47 . Optionally, the posts H 120  can possess threads and be screwed into position in the ribs H 46  and H 47  to ensure good purchase in the bone of ribs H 46  and H 47 . A jig can be used when drilling the holes to ensure appropriate angle, depth, and position of the holes H 118 . 
     As shown in  FIG. 92C , the posts H 120 , after placement in the holes H 118  drilled in the ribs H 46  and H 47 , are then used as secure anchors for retractor arms H 100  that push against the posts H 120  to move the ribs H 46  and H 47  without pushing on soft tissue. The posts H 120  can be used for closing the incision as well. 
     Another embodiment is shown in  FIG. 93 . The posts H 160  are attached by mechanical fasteners H 166  to the retractor arms H 170 . Also, the holes H 155  are drilled all the way through the ribs H 46 , H 47 . Note that different depths of the holes H 155 , including holes that pass through the ribs H 46 , H 47 , can be used with any configuration of posts H 160 . Note, also, that when the holes H 155  are drilled all the way through the ribs H 46 , H 47 , holes H 155  can be used during closing, whereby sutures pass through the holes H 155 , running from caudal rib H 47  to caudal rib H 46  to re-appose the ribs and to secure them into position. (It is prior art that such holes are drilled specifically for re-apposing and securing the ribs with sutures, but the holes are not used for retraction.) 
       FIGS. 94A and 94B  show another embodiment of a device to engage ribs but for which holes need not be drilled. Rather, elastic circumferentially surrounding clips H 192  can be attached to the ribs H 46 , H 47  such that each rib H 46 , H 47  is firmly clasped without exerting pressure on soft tissues H 44  surrounding ribs H 46 , H 47 . As shown in  FIG. 94A , clip H 192  possesses points, or spikes, including a top spike H 196  that engages the outer surface of the rib H 46  and a bottom spike H 194  that engages the inner surface of the rib H 46 . The spikes H 194  and H 196  automatically seat onto or into the surface of the bone (i.e., rib H 46 ), crucially, away from the neurovascular bundle H 48 . The clip H 192  is attached to a descenders H 186  that are attached to retractor arm H 182  by mechanical fasteners H 184 . Descender H 186  descends from the retractor arm H 182 . Clip H 192  is attached to descender H 186  by a flexibly bendable, tensily stiff element, such as cable or chain H 188 , which runs tangentially around and attaches to the clip H 192  at point H 190 . Clip H 192  is hooked into, and so loads, the ribs H 180 . The clip H 192  possesses a roughly even radius that is a function of the tension in the chain H 188 , that is, the radial distance separating the clip H 192  from the surface of the rib H 46  changes as the tension in the chain H 188  changes. Use of descender H 186  ensures an advantageous angle for pulling on the chain H 188  around the circumference of the clip H 192 . When the spikes H 194  and H 196  are loaded for retraction of the rib H 46 , by tension on the chain H 188 , the resulting torque on the clip H 192  acts as if to rotate the entire clip H 192 . But, since the clip&#39;s H 192  rotation is largely restrained by the spikes H 194  and H 196  inserted onto or into the surface of the rib H 46 , the torque instead causes the clip H 192  to “rise up on tiptoes,” i.e., the elastic circumferential clip H 192  increases its radius away from the surface of rib H 180 , doing so by angling the spikes H 194  and H 196  and firmly driving the spikes H 194  and H 196  into the bone H 46 , thereby more securely engaging with the rib H 46 . Bottom spike H 194  effectively serves as a pivot point for the clip H 192  which, when combined with rotation H 191  of the clip H 192 , forces the top spike H 196  into the bone H 46 . Thus, slippage of the clip H 192  is prevented by the spikes H 194  and H 196 , and sufficient force can be exerted on the rib H 46  to achieve retraction without loading the soft tissues H 44 , including the neurovascular bundle H 48 . 
     As shown in  FIG. 94B , clips are placed on the both ribs H 46  and H 47  on both sides of the incision H 52 , and retraction moves the ribs apart. 
       FIGS. 95A and 95B  illustrate different configurations of spikes on clips to achieve more secure engagement with the rib.  FIG. 95A  shows a clip H 192  like those in  FIGS. 94A and 94B . Clip H 192  has single spikes H 194 , H 196 . Alternatively,  FIG. 95B  shows that a clip H 256  can have multiple spikes on each end, double spikes H 258  and H 260  in this example, which distribute loading of the spikes H 188  on a rib and also prevent sideways rolling of the clip H 256  if chain H 188  should pull a bit sideways. 
     Clips H 192  and H 256  are positioned after the intercostal incision is made, and then the chains H 188  are attached to the descender H 186 . Similarly, the chains H 188 , or other tensile element, can be attached to the descender H 186  with sufficient length of chain H 188  to provide slack during placement of the clip H 192  or H 256 . After placement of the clip H 192  or H 256 , the slack is then removed before retraction commences. Removal of the slack can be by any of several one-way slip attachments, such as a ratcheting cable tie or “zip tie” as offered for sale by Nelco, Inc. of Pembroke, Mass. 
       FIG. 95  illustrates the placement of one or more clips to distribute loading along the ribs during retraction. Multiple single spike clips H 192  are placed on each rib H 46  and H 47 , possibly with different numbers on each rib H 46  and H 47 . Multiple clips H 192  are attached to retractor arms H 182  and are, thus, spaced along the rib margin facing the incision H 52 . When retractor arms H 182  move apart during retraction, the multiple clips distribute loading of the rib to decrease the chance of rib fracture. 
       FIGS. 97A through 97D  show another embodiment of a device that engages the ribs directly, minimizing trauma to intercostal tissues.  FIG. 97A  shows a side view and  FIG. 97B  shows a top view. A device can cut through soft tissues H 44  to abut hard tissues. The cut through the soft tissues is a small trauma relative to the compressive loading of a larger retractor blade. Retractor arms H 300  can have descender posts attached, a first descender post H 304  abutting cranial rib H 46  and a second descender post H 306  abutting caudal rib H 47 . Descender posts H 304  and H 306  can have their positions adjusted (arrows H 314  and H 316 ) closer or farther from the centerline of the incision H 52  (i.e., left-right in  FIGS. 97A and 97B ) to permit automatic balancing of loads as disclosed in Section F. Descender posts H 304  and H 306  can have sharpened edges H 310  and H 312 , respectively, that face the ribs H 46  and H 47 , respectively, such that the descender posts H 304  and H 306  can penetrate laterally through the margin of soft intercostal tissues H 44  to abut the ribs H 46  and H 47  directly. Thus, rather than crush the intercostal tissue, descender posts H 304  and H 306  slice into the intercostal tissues H 44 . Because descender post H 302  has a vertically straight margin oriented towards the cranial rib H 46 , descender post H 302  can abut the cranial rib H 46 , but does not impinge on the neurovascular bundle H 48  that lies just underneath the caudal edge of the cranial rib H 46  (see  FIG. 97A ). The sharp edges H 310 , H 312  of descender posts H 304 , H 306  can be serrated, or possess other structures to engage the ribs, to prevent the descender post H 304 , H 306  from slipping off the rib H 46 , H 47 . Alternatively,  FIG. 97C  shows a different descender post H 334  descending from retractor arm H 330 . Descender arm H 334  has a retraction hook H 336  to facilitate positioning against the rib H 46  and to prevent slipping. Preferably, such structures include appropriate stand-offs H 338  and curvature such as to create a hollow H 340  to avoid impinging on the neurovascular bundle H 48 . 
     Descender posts can be mounted to the retractor arms such that their positions can be adjusted left-right (indicated by arrows H 314  and H 316  in  FIGS. 97A and 97B ), variably pushing through the soft tissues H 44  at the margins H 320  of the incision H 52  to accommodate curvature of the ribs H 46 , H 47  and thereby ensure distribution of loads on the ribs H 46 , H 47 . Any of the embodiments disclosed in Section F are appropriate. Additionally, as shown in  FIG. 97D , a retractor arm H 330  can have descender posts H 350  and H 352  attached by lockable, telescoping, rotatable mounts H 354 . Thus, after the descender posts H 350  and H 352  have been placed into an incision, each descender post H 350  or H 352  can be swung up against the rib H 46 , telescoped by a motion shown by arrow H 356  to bring the hook H 336  into contact with the bottom of the rib H 46 , and then locked into position. 
     Refer now to  FIGS. 98A and 98B  which show another embodiment using retraction hooks that have an additional degree of freedom of motion facilitating placement onto the rib.  FIG. 98A  shows a side view, and  FIG. 98B  presents a top view showing the steps of placing the retraction hook onto the rib. Consider  FIG. 98A , a descender post has a shaft H 366  that attaches to retractor arm H 364  via swivel joint H 374 . Shaft H 366  has a Retraction hook H 336 , including a stand-off H 338 , that extends under the rib H 46  and forms a hollow H 340  to avoid contact with or compression of neurovascular bundle H 48 . Swivel joint H 374  permits rotation H 376  of shaft H 366  around an axis of rotation H 375  parallel to the axis of the shaft of the descender post H 366  (as shown in  FIG. 983A ). As shown in  FIG. 98B , Step 1 (block  390 ), the retraction hook H 336  on shaft H 366  can be aligned such that, before insertion into the incision H 52 , the retraction hook H 336  is parallel to both the rib H 46  and the incision H 52 . In Step 2 (block  392 ) of  FIG. 98B , after insertion into the incision H 52 , the retraction hook H 336  can be rotated to position the retraction hook H 336  under the rib H 46 . Additionally, the length of the shaft H 366 , or the distance between the retraction hook H 336  and the retractor arm H 364 , can be adjusted (arrow H 378 ), as shown in  FIG. 98A , to further permit positioning of the retraction hook H 336  relative to the rib H 46 . 
       FIG. 99  shows a three-dimensional working model of a retractor H 412  equipped with first and second retractor arms H 414  and H 416 , respectively, each having three (3) descender post shafts H 366  with rotatable, height-adjustable, retraction hooks H 336 . 
       FIG. 100  discloses yet another embodiment having a plurality of descender posts to distribute loads on a rib. Descender posts H 428  can be very thin, each not capable of displacing the rib H 46 , but with several arrayed in parallel on retractor arm H 426  such that, combined, descender posts H 428  can displace the rib H 46 . Descender posts H 428  can be, for example, stiff wires arrayed into a comb that can be placed against the rib H 46 . The descender posts H 428  can be sufficiently sharp on their ends that they are placed by piercing the soft tissues H 44  next to the rib H 46 . The positions of the descender posts H 428  can be independently adjustable, or they can be flexible such that they automatically seat against the rib. As also shown in  FIG. 100 , the shape of the descender posts H 428  can be such that they press against the rib H 46  without impinging on the space below it, thus again avoiding loading the neurovascular bundle residing in the soft tissues H 44  there. Any number of hard tissue engagers can be arrayed to avoid crushing, damaging, or traumatizing any soft tissues associated with a hard tissue. 
       FIGS. 101A through 101E  show another embodiment of a device for engaging a hard tissue, like a rib, without damaging neighboring soft tissues. Retractor arm  432  has a descender post H 434  with a turn or bend H 435  going to a projection H 436  that engages the rib H 46 . The projection H 436  is thin and pointed such that it easily penetrates the soft tissues H 44  adjacent to the rib H 46 . The projection H 436  can include a tip H 442  with a sharp point to engage the edge of the rib H 46  such that projection H 436  of the descender post H 434  firmly engages rib H 46  but does not touch the neurovascular bundle H 48 . As shown in  FIGS. 101B through 101D , the sharp point of tip H 442  can be configured such that it pierces only the surface of the rib H 46  to prevent slipping, but can not pierce further. For example, as depicted in  FIG. 101C , the tip H 442  of the projection H 436  can terminate with a spike H 470  that penetrates the rib H 46 , but the tip of the projection itself H 442  it too dull to further penetrate the rib H 46 . As shown in  FIG. 101D , the tip H 442  can have multiple spikes H 472  to ensure engagement of the rib H 46 . As shown in  FIG. 101E , projection H 436  is depicted engaging the rib H 46  with a spike H 470 , which ensures engagement of projection H 436  with rib H 46  even as rib H 46  moves (shown by arrow H 488 ). 
       FIG. 102  shows a section view of a thoracotomy performed by another embodiment in which the rib is firmly grasped on both sides to permit deterministic control of its motion, i.e. the position and orientation of the rib is always controlled. Retractor arms H 500  each have a spacer bar H 506  to which are attached paired descender posts H 502  and H 504 . The paired descender posts H 502  and H 504  engage both sides of the ribs (for example, H 46 ). The directions of retraction (i.e., rib spreading) are shown by arrows. Both descender posts H 502  and H 504  are straight and engage the ribs H 46  laterally. The sides of the descender posts H 502  and H 504  can have serrations H 503  where they engage the ribs to reduce slipping of the descender posts H 502  and H 504  on the ribs H 46 . The distance H 491  separating the paired descender posts H 504  and H 502  can be adjustable to permit positioning the paired descender posts H 502  and H 504  firmly against the margins of rib H 46  thereby accommodating variations in cranial-caudal width of rib H 46 . This embodiment allows a retractor to firmly grasp the rib H 46 , permitting deterministic control of rib position throughout retraction. 
     Referring now to  FIG. 103 , a retractor H 510  can also engage ribs H 519  and H 525  further from the incision H 526 , in addition to the ribs H 521  and H 523  adjacent to the incision. As shown in  FIG. 103 , a retractor H 510  can possess a plurality of retractor arms (H 528 , H 530 , H 532  and H 534 ), each equipped with descender posts (H 529 , H 531 , H 533  and H 535 , respectively). The retractor arms H 528 , H 530 , H 532  and H 534  can have coordinated motion, with respect to one another, that includes different speeds such that the retractor arms H 528 , H 530 , H 532  and H 534  each effect a different displacement over the same time. For example, retractor arms 1 (H 528 ) and 2 (H 530 ) form a pair on one side (the cranial side, H 537 ) of the incision H 526 , and both retractor arms 1 (H 528 ) and retractor arms 2 (H 530 ) can move in the same direction of retraction (H 536 ), but the displacement (H 511 ) of retractor arm 1 H 528  can be less than the displacement (H 513 ) for retractor arm 2 H 530  (e.g., retractor arm 1 can move more slowly than retractor arm 2). (Retractor arms 3 H 532  and 4 H 534  can, similarly, form a pair on the opposite, or caudal H 539 , side of the incision H 526 .) This varying displacement of the retractor arms reduces the pressure in intercostal space A H 520  and intercostal space B H 522  (and their respective neurovascular bundles, grouped H 524 ) arising from ribs H 518 , H 519  and H 521  (cranially) and H 523 , H 25  and H 527  (caudally) impinging on one another during retraction. 
       FIGS. 104A, 104B and 104C  show another embodiment in which an articulating clip is used to grasp a rib without compressing neighboring soft tissues.  FIGS. 104A through 104C  show an action sequence of an articulated clip H 553  meeting, closing, and locking down onto a rib H 46 . In the sequence, a descender post H 544  is fitted with a moveable, deeply curved, articulated clip H 553  having a top half jaw H 554  and a bottom half jaw H 555  attached by a pivot H 546  to the descender post H 544 . Each half H 554  and H 555  possesses a sharp point H 562  and H 560 , respectively, for engaging the rib H 46 , and a tab (one tab, H 556 , projects up and is associated with the bottom clip half H 555 , and another tab, H 558 , projects down and is associated with the top clip half, H 554 . The clip halves H 554  and H 555  can be spring-loaded such that they stay open (with jaws spread wide) before loading (i.e., before contacting the rib H 46 ). The clip half tabs H 556  and H 558  are cams shaped so as to generate a torque when the descender post H 544  with all associated components is pushed against the rib H 46 . When the rib H 46  impinges on the clip half tabs H 556  and H 558 , each tab is pushed apart and away from the other, causing each half of the clip H 554  and H 555  to rotate in the opposite direction (i.e., the top half clip H 554  comes down and the bottom half clip H 555  comes up), thus using the force applied by the rib H 46  to bring the sharp points H 560  and H 562  into position against the rib H 46  to forcefully secure it. The shapes of tabs H 556  and H 558  cooperate with that of the clip jaw halves H 554  and H 555  such that a space is created protecting the neurovascular bundle H 48  from any contact while retraction proceeds. The sharp points H 560  and H 562  are configured such that they penetrate only the surface of the rib H 46 , far behind the neurovascular bundle H 48  as the descender post H 544  is pushed more firmly against the rib H 46 . Thus, further force (or travel) of the descender post H 544  against the rib H 46  is born by the sharp points H 560  and H 562  loaded into hard, resistant bone, and not just the tabs H 556  and H 558 , thereby limiting any pressure exerted by the tabs against the soft tissues adjacent to the rib H 46  and, thus, protecting the neurovascular bundle H 48 . 
     J. Compensating for Retractor Deformation 
     Many biological tissues are very rigid. For example, rib cages are made of rigid bone connected by numerous ligaments, muscles, and tendons—strong enough to withstand the stresses of human locomotion or lifting large loads. Thus, the forces required to deform these tissues during procedures such as sternotomy or thoracotomy are significant. We have measured forces of up to 500 N during thoracotomies and 250 N during sternotomies on pigs weighing 50 to 60 kg. Forces on vertebral distractors are also large. 
     Retractors, by their very nature, are typically made of rigid stainless steel to withstand the stresses of forcing open rib cages. However, retractors deform under load. Deformation of a typical Finochietto-style retractor (e.g. a Finochietto, see  FIG. 2 , Burford, Ankeney, or other thoracic retractor) is primarily of two types—the arms bend and twist. Deformation of the retractor is more complex, e.g. including bending of the rack of the rack and pinion. 
       FIG. 105A  shows the bending of the retractor arms J 6 , J 10  of a Finochietto-style retractor J 2  in the prior art when loaded during retraction. Retractor J 2  has two retractor arms J 6  and J 10 . Retractor arm J 6  is fixed to the rack J 8  of a rack-and-pinion drive J 4  that is driven by a manual handle J 5 . Each retractor arm J 6  and J 10  has an attached retractor blade J 12  and J 14 , respectively, that engages the patient&#39;s tissues during retraction. Bending J 20  of the retractor arms J 6  and J 10  causes the distance J 36  between the retractor blades J 12 , J 14  to decrease, whereas the distance J 38  between the retractor arms J 6 , J 10  is not greatly effected. 
       FIG. 105B  shows twisting of the retractor arms J 6 , J 10  of a Finochietto-style retractor J 2  in the prior art when loaded during retraction. Retractor blades J 12  and J 14  push against ribs J 30  and J 32 , respectively. The force on the retractor blades J 12 , J 14  twists the retractor arms J 6 , J 10 , causing the distance J 36  to decrease. 
     We have measured deformations, decreases of J 36 , of up to 2 cm when the retractor blades J 12 , J 14  are first separated 10 cm and then loaded with 500 N; in other words, the retractor blades J 12 , J 14  are forced together 20% of the separation J 36  when loaded with forces seen in thoracotomies. 
     These deformations of the retractor J 2  are elastic; the retractor J 2  deforms like a spring under load. If the load decreases, the deformation of the retractor J 2  decreases, and the retractor blades J 12 , J 14  move further apart. 
     The tissues against which the retractor applies this load are viscoelastic. Unlike an elastic material, a viscoelastic material will continue to deform when loaded, even if the load is constant—it will exhibit creep. 
     This combination of an elastically deformed retractor J 2  pushing against a viscoelastic material creates a problem. Consider a sternotomy, a surgeon retracts (first phase of retraction) to a desired opening of the incision and then stops (second phase of retraction), striving not to retract wider than necessary to reduce damage to the tissues of the chest. Many retractors are self-locking or have a lock mechanism designed to hold the incision at that desired opening during the second phase of retraction. However, the retractor J 2  has deformed during the first phase of retraction, and now the elastic deformation of the retractor J 2  continues to push against the viscoelastic materials of the chest during the second phase of retraction, causing the thoracic opening to further widen as the viscoelastic materials of the chest wall creep under the elastic force of the retractor. This results in an unnecessarily wide thoracic opening, increasing damage to the tissues of the chest wall. For example, cardiothoracic surgeons report that they will hear a “pop” or “snap” as a rib breaks, sometimes minutes after cessation of the first phase of retraction. 
     This problem is encountered whenever any surgical instrument or medical device is used to deform a biological tissue because biological tissues are viscoelastic. Examples include, but are not limited to, retraction of skin for access to subdermal tissues, distraction of vertebrae for surgeries on intervertebral discs or to manipulate vertebrae for fusion or for other fixation, separation of joints for surgery on cartilage or for joint replacement, forcing open an annulus or tube with an inflatable device, such as angioplasty, or any other procedure requiring deformation of a biological tissue. 
     We disclose apparatus and methods for deforming a patient&#39;s tissues to the degree defined by the physician, reducing any further deformation of the tissue after the desired deformation is attained, and, thereby, reducing the chances of unnecessary tissue trauma. 
     One solution to this problem is to make the retractor exceedingly rigid through the use of materials having large Young&#39;s modulus (e.g., titanium which is expensive) or by the use of members having large cross-sectional area (e.g., wide, thick members which adds weight to the retractor). This retractor will still deform elastically, but the elastic deformation of the retractor will be small, so it will impose only a short distance of creep on the tissues of the chest. 
     An alternative solution is a thoracic retractor that deforms elastically but the surgical opening is controlled by a servo-mechanism that maintains the opening at the point set by the surgeon. Thus, as the tissue begins to creep, causing the retractor blades to move apart elastically, the servo-mechanism causes the retractor to close slightly, thereby decreasing the elastic deformation of the retractor and applying only the force required to maintain the surgical opening at the point set by the surgeon. 
       FIG. 106  shows one embodiment in which the servo-mechanism controls the distance J 36  between the retractor blades J 12 , J 14  of a retractor J 40 . The servo-mechanism is comprised of a motor J 44  that opens the retractor J 40 , a distance measuring device J 41  that measures the surgical opening (e.g. a linear potentiometer), and a servo-controller J 46  that receives the signal from the distance measuring device J 41  and then adjusts the position of motor J 44  to maintain the desired surgical opening, even as the tissue creeps. Any actuator that effects the deformation of the tissue (i.e. that powers the first phase of the retraction) can be used; thus, for the embodiment shown in  FIG. 106 , the actuator that opens the retractor J 40  is also the actuator controlled by the servo-controller J 44 . 
     An alternative to direct measurement of the surgical opening for servo-control, as shown in  FIG. 106 , is to use the force-deformation relationship of the retractor (e.g. a spring constant or a force-deformation curve). During use of the retractor, the force on the retractor is measured with a force measuring device, and then the deformation of the retractor is determined from the measured force using the force-deformation relationship. This has the advantage that no measuring device is present in or near the surgical opening. 
       FIG. 107  presents an algorithm J 50  illustrating one way to implement a servo-mechanism that uses force measurement with a force-deformation relationship. A surgeon retracts to a desired opening (block J 51 ) and then activates the servo-mechanism (block J 52 ). The position of the motor is recorded by the servo-mechanism (block J 52 ), the force on the retractor is measured (block J 52 ), and the motor begins to hold position (block J 54 ). Force is then measured continuously (block J 56 ). If force remains constant, then the motor continues to hold position (decision at block J 57  then to step J 58 ). If force decreases (due to creep of the biological tissue) (block J 57  then to step J 59 ), then a translator (e.g. an electrical circuit, possibly including a microprocessor) uses the force-deformation relationship to convert the change in force on the retractor into a change in deformation of the retractor (block J 60 ). The translator then instructs the motor to move to correct for the change in deformation, thereby keeping the surgical opening at the separation set by the surgeon (block J 62 ) returning the feedback loop to block J 54 . 
     A similar algorithm can be used to correct for deformation throughout retraction. Thus, rather than correct for changes in deformation on entering the second phase of retraction, the algorithm can correct for deformation throughout both the first and second phases of retraction. With such an algorithm, the separation achieved by the retractor blades (i.e., distance J 36 ) will then match the separation seen by the surgeon when looking at the retractor arms (i.e., distance J 38 ). This algorithm also can be part of an automated retraction system to ensure that separations J 36  intended by the automatic retraction system are, in fact, the separations achieved. 
       FIG. 108  shows a retractor J 70  that can control the distance J 36  between the retractor blades J 73 , J 77  by using servo-loop J 78  that implements an algorithm such as algorithm J 50 . The retractor has a first retractor arm J 72  and a second retractor arm J 76  that is driven relative to the first retractor arm J 72  by a motorized rack-and-pinion drive J 78  comprised of a rack J 74  attached to the first retractor arm J 72  and a drive housing J 80  attached to the second retractor arm J 76 . Retractor arms J 72  and J 76  have retractor blades J 73  and J 77 , respectively. The drive housing J 80  houses a servo-motor J 84  and a servo-controller J 86  that controls the servo-motor J 84 . The surgeon opens the retractor J 70  by providing instructions to the servo-controller J 86 , for example with a rotating knob J 88  that replaces the crank of a manual rack-and-pinion. A force measuring device J 90 , for example a strain gauge, is placed on the second retractor arm J 76  to measure force on the retractor J 70 . A translator J 82  in the drive housing J 80  receives signals from the force measuring device J 90  and implements the algorithm J 50 . Placement of the force measuring device and the translator can be at any of several locations, such as on the fixed retractor arm, but placement of all three components—servo-motor, translator, and force measuring device, on the moveable retractor arm simplifies the design. 
       FIG. 109  shows another retractor J 100  that uses two actuators, a first actuator J 101  to drive the first phase of retraction and a second actuator J 108  to adjust for changes in deformation arising from creep in the tissues during the second phase of retraction. During the first phase of retraction the first actuator J 101 , a hand-cranked rack-and-pinion in this example, is used to by the surgeon. A second actuator J 108  attached to one arm of the retractor J 100 , for example the first arm of the retractor J 6 , is controlled by a servo-mechanism J 110  and is used to correct for changes in the deformation of the retractor J 100  due to creep of the tissue during the second phase of retraction. A force measuring device J 102 , for example a strain gauge, measures force on the first retractor arm J 6 . A translator J 104  detects changes in force, translates these to changes in the deformation of the retractor J 100 , and signals the servo-controller J 106  to instruct the second actuator J 108  to move and thereby remove the change in the surgical opening resulting from the change in the deformation of the retractor J 100 . The second actuator J 108  can be a servo-motor, a voice coil, a hydraulic cylinder from which fluid is released to move the retractor blade J 12 , or any other actuator that can move the retractor blade J 12  such that the retractor blade J 12  moves closer to the opposing retractor blade J 14  when a decrease in force on the retractor is detected by the translator. Such a device as shown in  FIG. 109  can either be integrated into a retractor or be a component that attaches to an existing retractor. 
     K. A Thoracic Retractor Combining Elements of the Earlier Sections 
       FIGS. 110 through 114  present a thoracic retractor K 2  used for thoracotomies. This thoracic retractor K 2  combines components disclosed in earlier sections. Thoracic retractor K 2  comprises two opposing retractor arms K 4  and K 6  attached to retraction driver K 60 . 
       FIG. 111  shows retraction driver K 60 . Retraction driver K 60  comprises a motor-driven rack-and-pinion, with the pinion driven by a servo-motor K 40  controlled by servo-controller K 42  and powered by battery K 44 . Processor K 46  receives input from strain gauge sensors K 14  and K 18  that measure the forces on the retractor arms K 6  and K 4 , respectively. Strain gauge sensors K 14  and K 18  can be single gauges or multiple gauges located in multiple locations and arrayed in, for example, a full bridge configuration; additionally, strain gauge sensors K 14  and K 18  can be mounted where the strains in the underlying material are expected or designed to be large to increase the sensitivity of force measurement. Processor K 46  is in communication with servo-controller K 42  for automatic control of the servo-motor K 40 . Retractor arm K 4  attaches to the rack K 8  connector K 5  via and then rotatable mount K 16 . Retractor arm K 6  attaches to driver housing K 10  via connector K 7 . The attachment of retractor arms K 4  and K 6  to connectors K 5  and K 7 , respectively, is secured with fasteners K 12 . Examples of fasteners include screws, clips, or any other appropriate mechanical fastener. 
       FIG. 112  shows retractor arm assembly K 50  that attaches via retractor arm K 4  to connector K 5  to rotatable mount K 16 . A similar retractor arm assembly is attached via retractor arm K 6  to driver housing K 10 . Retractor arm assembly K 50  comprises retractor arm K 4  which attaches to balance arm K 20  via rotating mount K 24 ; two daughter balance arms K 26  which attach to balance arm K 20  via rotating mounts K 30 ; and two descender posts K 52  each with hook K 54  attaching to each daughter balance arm K 26  via rotatable mount K 34 . Thus there are four descender posts K 52  each with hook K 54 . Each descender post K 52  is attached to the daughter balance arm K 26  by rotatable mount K 36  such that hook K 54  rotates as shown in K 56 . Rotatable mount K 36  can include a heavy sleeve K 34  that reinforces the joint. 
       FIG. 113  shows an enlarged view of rotatable mount K 16  which provides retractor arm assembly K 50  an additional degree of rotational freedom, as disclosed in Section G. Rotatable mount K 16  comprises rod K 62  that is rigidly coupled to rack K 8 . Sleeve K 64  is attached to rod K 62  and secured by an E-clip (not shown). Rotatable mount K 16 , therefore, provides for rotation K 66  of retractor arm assembly K 50 . 
       FIG. 114  shows the shapes and sequence of attachment of retractor arm K 4  to balance arm K 20  and daughter balance arms K 26  for retractor arm assembly K 50 . Rotatable mounts K 24  and K 30  can be made by connector pins; alternatively, rotatable bushings or bearings can be used. Rotatable mounts K 24  and K 30  can be made loose to provide some freedom of alignment out of the plane of the page of  FIG. 114 . 
     Connectors K 5  and K 7  can be replaced by appropriate snap-together fittings permitting the retractor arms K 4  and K 6  to be easily attached and removed. Furthermore, connectors K 5  and K 7  can include electrical connectors for transmission of power or electrical signals to electrical components on or connected to the retractor arms K 4  and K 6  or to different retractor arm assemblies K 50 . Such electrical components can include sensors, processors, motors or other actuators, data input interfaces, data or status indicators, or other advantageous electrical components. 
     Retractor assembly K 50  is designed to distribute the forces along a rib during a thoracotomy. Other retractor assemblies designed for other procedures, such as a sternotomy, can be attached to rack K 8  and to retraction driver K 60 , optionally with rotating mount K 16   a  replaced by a rigid connection or other moveable mount. 
     Retractor arm assembly K 50  can be a disposable component. Retractor arm assembly can include the battery K 44 , and if K 50  is a disposable component, this would permit the attachment of a fresh battery for every use. 
     Instructions to the servo-controller K 42  and processor K 46  can be via a user interface that is integral to retraction driver K 60 . An example of a user interface is shown in  FIG. 115 . The interface can include a panel in which membrane buttons activate functions such as start retraction K 72 , pause retraction K 74 , fast forward or accelerate retraction K 76 , emergency open K 78 , rewind or reverse retraction K 80 , fully close the retractor K 82 , set duration for retraction K 84 , set distance of retraction K 86 , and a display K 80  for showing information to the user, for example retraction progress with a progress bar, force on the retractor, or other information. 
     The thoracic retractor K 2  shown in  FIG. 110  has been constructed to demonstrate selected embodiments described above and, specifically, to demonstrate functioning of the self-balancing retractor arms as described in Section F (e.g., see  FIG. 55 ), the rotating retraction arm and retraction assembly as described in Section G (e.g. see  FIG. 74 ), hook-shaped tissue engagers as described in Section H (e.g.  FIG. 98 ), and automated retraction with detection of trauma as described in Section C (e.g. algorithm C 300  in  FIG. 42 ). Motor K 40  is a model EC22 50 W from Maxon Precision Motor Inc. The prototype does not have battery K 44  or servo-controller K 42  or processor K 46  housed in driver housing K 10 . Rather, these functions are provided by an off-board power supply (16V, 4.5 A), servo-controller (EPOS 24/5 motor controller from Maxon Precision Motor, Inc), and computer connected by a cable. Strain gauges from Vishay Micro-Measurements, Inc. are placed at locations K 14  and K 18 , arranged as full bridges, to measure forces on arms K 6  and K 4 , respectively. Power to and signals from these strain gauges is provided by signal conditioners (Model OM-2-115 from 1-800-LoadCell), which then send signals to a data acquisition card (Model USB-6211 from National Instruments, Inc.) attached to a laptop computer. Custom software for motor control and data acquisition is written in LabVIEW from National Instruments, Inc. 
       FIGS. 116A and 116B  show retractions from two thoracotomies. These are fully automated retractions. The surgeon placed the retractor into the incision, rotated the hooks K 54  on the descender posts K 52  under the ribs, and then a remote operator initiated computer-controlled retraction—the computer controlled the rest of the retraction. The retraction trajectory was programmed to be parabolic (distance as a function of time, retraction speed initially higher, continuously decreasing throughout retraction, and approaching zero speed as full retraction is approached). The algorithm C 300  described in  FIG. 42  was used to automatically pause the retractor, thereby acting as a detector and automatic response to imminent tissue trauma. If a pause was triggered, then after the pause, the computer calculated a new parabolic trajectory starting at the current position and reaching the desired end point; for example, consider the retraction in  FIG. 116B :
         the desired endpoint was 62 mm;   retraction was to occur over 45 seconds;   a pause occurred at 43 mm after 20 seconds had elapsed, with the pause being 20 seconds long; thus   the remaining retraction distance (62−43=19 mm) was to be covered in (45−20−20)=5 seconds.       

     Alternate algorithms for desired endpoints, desired retraction duration, pause durations, and means of recalculating the trajectory after the pause can be used. Alternate algorithms could be:
         Desired endpoint=50 mm; desired retraction duration=50 s; pause duration=15 s; if pause occurs in last 30 s of retraction, then set the pause duration to be equal to half the remaining time; or   Desired endpoint=100 mm; desired retraction duration=2 minutes (120 seconds); pause duration=one-third of time remaining at the initiation of a pause.
 
The complexity of the algorithm is limited only by such things as the processing power of the processor K 46 , the numbers and types of sensors used, etc.
       

       FIG. 116A  shows the displacement K 100  and forces on the right arm K 102  and left arm K 104  for a retraction to 50 mm over about 35 seconds. This retraction should be compared to a similar retraction performed with an instrumented Finochietto retractor (shown in  FIG. 37 ) for retraction to 52 mm over about 50 seconds. Both retractions in  FIGS. 37 and 116A  were performed on the same animal. The retraction in  FIG. 37  was at rib pair 4/5 on the left side, and the retraction in  FIG. 116A  was at rib pair 4/5 on the right side. Returning to  FIG. 116A , retraction starts K 106  at 2 seconds and follows a substantially parabolic trajectory, with retraction ending at K 108 . We have found that such substantially parabolic trajectories have less evidence of tissue trauma than other trajectories, such as linear trajectories or, as in  FIG. 37 , stepped trajectories. It is important to note several things:
         (1) No pause was triggered.   (2) The maximum force generated during retraction was about 300 N, about 25% less than the 400 N observed with the instrumented Finochietto during retraction to 52 mm over 50 seconds ( FIG. 37 ). This lower force of retraction is especially noteworthy because the more rapid retraction in  FIG. 116A  should have required more force than the slower retraction in  FIG. 37 .   (3) The forces on the two retractor arms are nearly equal, unlike the unequal forces seen on the retractor arms in  FIG. 37 .   (4) The force traces K 102  and K 104  in  FIG. 116A  are exceedingly smooth, unlike the extremely jagged traces seen in  FIG. 37 . Note that all the data presented in  FIGS. 116 and 117  are raw data—the data are not smoothed, the traces are not fitted curves.       

       FIG. 116B  shows another retraction with retractor K 2 . This retraction was at rib pair 5/6, right hand side, of the same animal as in  FIGS. 37 and 116A . We performed multiple retractions on this rib pair, going to increasingly wider endpoints, in an attempt to get a pause to be initiated by the algorithm C 300 .  FIG. 116B  shows the third retraction which was to an endpoint of 62 mm over 45 seconds. Retraction started at K 110  and ended at K 111 . A pause K 112  was triggered by a negative-going spike (too small to see in this figure) at the point marked by the arrow K 113  approximately 20 seconds after starting retraction. Retraction before the pause produced a very smooth displacement trace K 114  and force traces (right and left arms collectively labeled K 118 ). Only a small amount of force relaxation is evident in the pause. The retraction after the pause was very rapid, due to the short time allowed by the algorithm (about 5 seconds), but again produced a very smooth displacement trace K 116  and force traces, right and left arms collectively numbered as K 120 . 
     It is noteworthy that the forces on the retractor relaxed only slightly during the pause in  FIG. 116B  and also at the end of the retractions in both  FIGS. 116A and 116B , relative to the relaxation seen after each ½-rotation of the crank in  FIG. 37 . This indicates that slow, steady pulling permits force relaxation to occur simultaneously with retraction and, therefore, also indicates that there is an optimum retraction speed that maximizes force relaxation and thereby reduces forces during retraction. A substantially parabolic trajectory, as described above, provides such an optimal retraction. 
       FIGS. 117A, 117B, and 117C  present the same data from the retractions shown in  FIGS. 116A and 116B , but show only force for the left retractor arm; these figures also show the second derivative of the force, d 2 F/dt 2 , referred to here as the Fracture Predicting Signal, FPS, where fracture can be of any tissue (e.g. rib, ligament, tendon, muscle) giving rise to tissue trauma. 
       FIG. 117A  shows the retraction from  FIG. 116A . Traces for both the FPS K 130  and force K 104  on the left retractor arm are presented. The FPS trace K 130  is constant, with low noise, at zero throughout the retraction. There are no negative-going spikes and no increase in variance of the signal that would trigger a pause, so there was no pause in this retraction. 
       FIG. 117B  shows the retraction from  FIG. 116B . The force trace K 132  is presented from the left retractor arm only. Here, there is a prominent event K 134  at about 24 seconds that triggers the pause K 112 . 
       FIG. 117C  shows the event K 134  with an expanded scale. A small drop in the force K 132  occurs in event K 134 , a decrease of only about 3N (˜1% of the maximum retraction force during this retraction). This creates a negative going spike K 136  in the FPS K 130  that triggers the pause. Retraction stopped 0.2 seconds after the drop in force K 132  (displacement trace not shown). 
     Returning to  FIG. 117B , there is another event K 140  in the FPS during retraction after the pause. This event was not sufficient to trigger a second pause, and retraction proceeded to completion K 111 . 
     L. Tissue Engagers: “Safe Tissue Retraction Elements for Thoracotomy” 
     Introduction to the Problem 
     Tissue Engagers (Safe Tissue Retraction Elements) For Thoracotomy 
     Introduction to the Problem 
     Every 15 seconds, a thoracic surgeon enters a patient&#39;s chest by spreading the space between two adjacent ribs. After choosing a location and slicing through the patient&#39;s intercostal musculature, the surgeon first (1) inserts the retractor&#39;s blades into the incision, then (2) ensures the retractor blades are parallel to and apposed to the incision&#39;s margins so that the blades will open along an axis (the retraction axis) lying perpendicular to the line of the incision, and (3) forcefully cranks open the retractor to widen the intercostal space. 
     Unfortunately, the design of thoracic retractors hasn&#39;t changed much in 75 years. Tissue trauma is common, including broken ribs, crushed nerves and vessels, and torn muscles, ligaments and cartilage. Knowing this, many surgeons preemptively cut nerves or remove rib sections in an attempt to confine the damage. Clearly there is a need for improvements to thoracic retractor design. 
     Issues with Current Thoracic Retractor Blade Designs 
     Today&#39;s thoracic retractors employ stiff retractor blades stamped out of stainless steel plate, usually rectangular and possessing fenestrations through the plate. They are minimally finished; in fact, the blades&#39; corners are intentionally left sharp to bite into the exposed muscle tissue to prevent slippage under load. Further, the retractor blades possess wide, right-angled shelves at their tips (also with sharp corners), the better to catch hold of the incision&#39;s margins, to prevent the retractor blades&#39; rising up and out of the incision as the ribs are spread open. With the retractor blades cranked together, closed for insertion, these shelves are much wider than the initial gap created by the incision through the intercostals. To insert the blades the surgeon must therefore force the wide edges of the shelves past the patient&#39;s freshly cut muscle, jamming that muscle against the patient&#39;s ribs. Once inserted, these retractor blades continue to damage tissues, for example, when the retractor blades are settled in place, sharp edges are adjacent to the fragile, respiring lungs. The aforementioned tissue trauma of then spreading the ribs leads to severe pain, prolonging the patient&#39;s recovery and trading quality of life for quantity. The resulting healthcare costs are unnecessarily high. Is this avoidable? 
     Solution 
     Physcient here discloses novel devices and methods in the field of Tissue Retraction Elements (TRE) enabling and improving the process of:
         (1) Inserting a retractor&#39;s Tissue Retraction Elements into a thoracoscopic incision,   (2) Orienting and Settling those Tissue Retraction Elements into place against the margins of the thoracoscopic incision to be retracted, and   (3) Applying Force against those margins to expose the chest cavity.       

     We propose a device that is easy to insert without damaging the patient&#39;s tissues, that gently and securely self-orients and engages the tissues forming the margins of the patient&#39;s incision, and that safely applies force throughout the retraction process. Further, the improved Tissue Retraction Elements are also easier for the surgeon to use, are easier to remove, are self-adjusting and improve overall patient recovery. Surgeon, Patient and Hospital all benefit. 
     Inserting the Tissue Retraction Elements without Trauma 
     Physcient discloses here multiple devices and methods for inserting retraction elements of a thoracic retractor. The portion of the thoracic retractor that is to be inserted into the patient&#39;s chest cavity (hereinafter referred to as the Tissue Retraction Elements, or TRE) can possess certain attributes to prevent tissue trauma from insertion: 
     First, the TRE can be thin in profile (“profile” here defined as the plane view of the incision in section, see  FIG. 118  and  FIG. 119 ), so as not to spread or stretch the ribs apart as the device is inserted parallel to the incision. The TRE might also be thin enough in profile to avoid any contact with the freshly exposed margin of the patient&#39;s incision ( FIG. 118 ). 
     Second, the TRE can possess an overall, grossly rounded shape, free of any corners, rectilinear portions, or other substantial protrusions that could catch hold of, impinge on, slice, cut, dig into, pierce, hook into, grab or otherwise injure tissues on the way in, i.e., during the insertion step ( FIG. 119 ). 
     Third, the surface or surfaces of the TRE can be free of fine projections, surface imperfections, edges, mold lines, or textures; the TRE can be polished smooth, for example as shiny and smooth as glass, to more easily slide easily into place without friction or impediment ( FIG. 116C ). 
     Fourth, the TRE can be constructed all or in part of a very low friction material, so as to minimize shearing of the exposed margins of the incision should any contact occur during insertion ( FIG. 116D ). 
     Fifth, the TRE can be lubricated, to reduce friction between the TRE and the patient&#39;s tissues even further ( FIG. 116E-G ). Any number of lubricants could be used, including FDA-cleared lubricants. The lubricants could be applied to the surface of the TRE during the procedure or before (for example during factory-packaging the TRE in a sterile bag) ( FIG. 116E ), or the device could be designed so that lubricants emerge from the TRE ( FIG. 116F ). Further, the TRE could be coated or made with a substance that, when contacting the wet surfaces of the freshly exposed margins of the patient&#39;s incision, creates there a lubricant or lubricating effect ( FIG. 116G ). For example, the TRE can incorporate a hydrogel that takes up water and becomes slippery. 
     Sixth, the TRE can move, flex, bend, or otherwise behave in a compliant manner ( FIGS. 116H-I ). Since the patient&#39;s tissues are diverse and include some highly flexible, compliant tissues, the TRE can itself be constructed all or in part of an elastomer, rubber or other soft, compliant material, so as to give way upon contact with the patient&#39;s tissues ( FIG. 116H ). The Young&#39;s modulus of the compliant material can be substantially similar to one or more of the patient&#39;s tissues. For example, the modulus of the TRE could be close to the modulus of the freshly exposed muscle, to match the muscle&#39;s shearing behavior, preventing steep shear gradients there. The modulus of the TRE (or a portion thereof) might also be made lower than, or greater than, the exposed tissues so as to give way, or push and guide, as is deemed appropriate. Any number of FDA-cleared polymers might be used, including plasticizers for varying their modulus. Alternatively, the TRE might include a substantially rigid component(s) working in concert with a compliant component(s) ( FIG. 116I ), for example associated with joints. Further, the compliant component might not be in direct contact with the patient&#39;s tissue. 
     Seventh, a TRE could be capable of changing shape. While a thin or flattened profile permits easy entry into the patient&#39;s incision, a TRE designed solely for easy entry might not be optimal for engaging nor holding the patient&#39;s tissues under load. The TRE can be designed to be capable of changing shape from a first, thin profile favoring easy insertion to another, subsequent, second profile favoring securely retracting the patient&#39;s tissues. The shape change can be achieved any number of ways. We&#39;ll describe at least two here. 
     Articulated Joints—Finger 
     One embodiment ( FIGS. 121-126C ) of a shape-changing Tissue Retraction Element is an Articulated Safety Finger (ASF). The ASF mimics the structure of the surgeon&#39;s own fingers ( FIG. 121 ), which were likely the first “retractors” employed. The ASF can be constructed with substantially rigid, jointed segments (i.e., “bones”) held together by a compliant sheath (i.e., a “skin”) and actuated by a cable (i.e., “tendon”) ( FIG. 122 ). The cable can be very stiff in tension, or it may be designed to be somewhat compliant to permit some accommodation of the load of the patient&#39;s tissues on the Articulated Safety Finger. 
     The ASF starts out straight, for insertion ( FIG. 123 ). Once inserted, the tendon can pull on the segments ( FIG. 124A ) to flex them until the tip of the Articulated Safety Finger rests against the inside of the patient&#39;s rib ( FIG. 124B ).  FIG. 125  shows an embodiment of the Articulated Safety Finger ready for retraction. 
     The ASF&#39;s cable (i.e., tendon) could be designed to automatically pull and flex the ASF as a part of the insertion or retraction process, or the action of the cable could be under the manual control of the surgeon ( FIG. 126A-C , showing a surgeon pushing the Finger Flexing Lever). See also  FIG. 127  for a more detailed drawing of the parts of the ASF&#39;s Finger Flexing Handle (aka Finger Flexing Lever). See  FIG. 128  for a depiction of one way the Finger Flexing Handle (Lever) can drive the cable motion. 
     The flexing action and proportions of the ASF can be designed to automatically create a gap that avoids applying any force to the neurovascular bundle (refer again to  FIG. 122 ). 
     Further, Settling the Tissue Retraction Elements into Place without Trauma 
     Swinging Safety Fingers 
     Another embodiment ( FIGS. 129-130B ) of a shape-changing Tissue Retraction Element is a Swinging Safety Finger (SSF) (oblique view,  FIG. 129 ). Designed without any sharp surfaces or projections, the SSF starts out folded flat in profile for easy insertion ( FIG. 130A ). One advantage of the SSF is that the surface area of the tissue retraction elements that are projecting down into the incision can be designed to make the SSF TRE respond to initial loading (e.g., near the beginning of the rib retraction process) by automatically reorienting the SSF TRE to present a new profile shape designed to safely engage and capture the tissues as a part of the retraction process ( FIG. 130B ). This reorientation can occur within the space of the incision and can involve SSF TRE rotation about one or more axes. This rotation can be passive, driven by rib retraction, or it can be actively controlled. 
     One way for the SSF TRE to accomplish automatic, passive reorientation is (1) to arrange an axis (about which a SSF TRE might swing, called a swing axis) oriented and projecting substantially up and out of the chest cavity, and (2) arranging a substantial majority of the area of the tissue retraction element to be located both within the depth of the incision (and so impinging upon the surface of the margin of the incision) and located some distance to one side of a line drawn from the swing axis perpendicular to the retraction axis ( FIG. 131 ). The portion of the SSF TRE that contains the actual joint about which the SSF TRE swings can be located up out of the incision (for example, above the skin). Looking down into the incision, the deep, off-center portion of the SSF TRE thus forms a moment arm with its fulcrum at the swing axis. 
     Refer to the deployment sequence in  FIGS. 132A-C . Before beginning rib retraction, the SSF TRE is oriented flat and parallel to the line of the incision for easy insertion ( FIG. 132A ). As the rib retraction process starts ( FIG. 132B ), the high, joint section passes freely and easily across the patient&#39;s skin, perpendicularly away from the incision and parallel to the retraction axis. At the same time, however, the margin of the incision resists passage of the deeper, flat area of the SSF TRE. Given the moment created by the off-center portion of the SSF TRE, the resistance of the tissue causes the entire TRE to swing backwards about the swing joint as retraction proceeds. The SSF TRE will swing through whatever angle that that design permits. If the motion about the SSF TRE joint is unrestricted, the force of the oncoming chest wall tissues means that the whole SSF TRE will naturally swing through an angle of around 90 degrees, until the deep portion of the SSF TRE trails directly behind the SSF TRE joint ( FIG. 132C ), pointing back at the incision. Note that the amount of SSF TRE rotation can be limited to any arbitrary angle by providing limit stops in the swing joint, by providing progressive resistance of a compliant element associated with the SSF TRE, by providing a SSF shape that reaches torque equilibrium with the patient&#39;s tissues at a chosen angle, or any other rotation or torque-limiting means. 
     The shape of the deep portion of the SSF can be so designed so that as it reorients as it swings about the swing joint, it presents the impinging chest wall tissues with a profile that changes over time ( FIGS. 133A-F ). During initial retraction, such a changing profile can automatically guide the relative positions of the SSF TRE and the patient&#39;s rib, gradually developing a safe hold on the rib while protecting the neurovascular bundle throughout. 
     Ribs rotate about their attachments to the spine and sternum, presenting a challenge to avoiding contact with the neurovascular bundle. Addressing this, another important element of our design can include a gap that avoids applying pressure to the neurovascular bundle regardless of the relative rotations of the rib and SSF TRE ( FIG. 134 ). Such a gap can be formed by the space between a curved, descending SSF TRE shaft and a substantially flattened, or oblate spheroid fingertip mounted near the end of the shaft, making it impossible for the neurovascular bundle to impinge on the SSF TRE. The shape of such a space preserves a gap for the neurovascular bundle regardless of the orientation of the patient&#39;s rib as it contacts the SSF TRE. Even more, this shape preserves a protective gap for the neurovascular bundle over a wide range of rib sizes (i.e., patient sizes). 
     Still further, if a SSF TRE shaft is curved in a first plane, such a fingertip might or might not be oriented substantially in that same plane ( FIG. 135 ). When retraction begins, a deviation of the oblate spheroid fingertip from the plane of the curve of the SSF TRE shaft can be advantageous. For example the rounded, blunt limb of the oblate spheroid fingertip can be oriented to impinge very early (i.e., at very low SSF TRE swing angles) upon the underside of the near margin of the adjacent rib, keeping the oblate spheroid fingertip low and the rib high right from the start, thus immediately presenting the protective gap to the neurovascular bundle even at very low degrees of SSF TRE rotation. 
     So, the SSF TRE incorporates a first, flat profile shape for easy insertion, a further, second SSF TRE profile shape providing mechanically automatic reorienting to the patient&#39;s tissues, and a final, achieved SSF TRE profile shape that gently and safely applies force to the patient&#39;s tissues without impinging upon the neurovascular bundle under load. 
     Yet another feature of our device is that the SSF TRE can be designed so that the tissue retraction elements are gathered up by, guided by, or contained within a compliant, elastic, flexible sheath, for example an elastomeric cover (above). One advantage of including a rubber sheath would be that the re-orientable tissue retraction elements are managed easily in a flat, thin form during handling in an operating room, as the surgeon inserts the tissue retraction elements. Another advantage is that a compliant material can keep the surfaces of the SSF TRE smooth. 
     Still another advantage of a compliant sheath is that, if resilient, the SSF TRE automatically re-flattens itself upon removing it from the patient&#39;s incision, making the process of completing the surgery faster and easier. 
     Helical Shape Rotates on Oblique Axis to Grab Tissues 
     The Tissue Retraction Element, all or in part, can be substantially helical, creating a Helical Tissue Retraction Element (HTRE) ( FIG. 136A ). The advantage of the HTRE shape is that a helix (or a portion thereof) can present to the incision a nearly straight, thin first profile shape in a first rotational position, and a curving, grasping, or engaging profile shape when in a second rotational position. The helically shaped TRE might be designed to rotate from the first rotational position to the second rotational position under the influence of the forces experienced in the retraction process. This can be facilitated by off-center loading as above, so that automatic reorienting is achieved as retraction begins. 
     The axis of rotation can be arbitrarily oblique, as desired. Further, helical shapes are naturally gradually curved, so that the transition from a first profile orientation to a second profile orientation can be extremely smooth, gradual and without sudden changes in aspect or loading to the patient&#39;s tissues. 
     Still further, a smooth, helical TRE form can be designed so that all or a part of the process of inserting the tissue retraction elements into the incision automatically reorients the HTRE ( FIGS. 136B and 136C ). To facilitate this, the radius of curvature of such a helix forming the HTRE need not be constant over its depth. For example, if the lower, more vertical portion of an HTRE (with a small radius and a high pitch angle) presents a thin profile shape to the incision, so easing insertion ( FIG. 136B ), the overall helical form can be such that the upper portion of that HTRE (with a larger radius and lower pitch angle) smoothly impinges on the margin of the incision to drive HTRE rotation from a first rotational position to a second rotational position ( FIG. 136C ). 
     Tissue Retraction Elements on an Improved Thoracic Retractor 
       FIG. 137  shows some improved Tissue Retraction Elements as mounted on a automated thoracic retractor L 50 . The retractor L 50  is motorized and automated. Tissue engagers L 60  are for thoracotomy. A hand-held controller L 70  communicates with retractor body L 80  through cable L 90 . 
       FIG. 138  shows an embodiment of a complete retractor L 00  for reducing the trauma to ribs during retraction for thoracotomy. Retractor L 100  comprises a linear drive element L 05  aligned with the direction of retraction L 10 . Retractor L 00  has a first arm L 15  and a second arm L 20  oriented substantially perpendicular to the direction of retraction L 10  with at least one of the arms L 15  and L 20  being moveable along linear drive element L 05 . For the purposes of this discussion two axes are important: first, the direction of retraction L 10  and a vertical axis L 11  that is approximately normal to a plane that is parallel with the skin of a patient. Each arm L 15  and L 20  have a self-balancing tissue engager associated with each arm. The two arms and associated self-balancing tissue engagers presented here are symmetrical, with the two self-balancing tissue engagers being mirror images, but asymmetrical assemblies can be made. Due to the symmetry here, description of only one arm and balancing assembly will suffice. 
     Consider first arm L 15  in  FIG. 139 . Self-balancing tissue engager L 25  comprises a first rotatable joint L 105  that joins arm L 15  to first balance bar L 110  such that the middle L 115  of first balance bar L 110  attaches to the end L 120  of arm L 15 . First rotatable joint L 105  thus permits rotation about an axis L 125  that is oriented approximately perpendicular to the direction of retraction L 10  and approximately parallel to normal axis L 11 . First balance bar L 110  has a first end L 130  and a second end L 135 . Two second rotary joints L 140  are located on first balance bar L 110 , with a second rotary joint L 140  being placed at each of the two ends, first end L 130  and second end L 135 , of balance bar L 110 . Both second rotary joints L 140  permit rotation about an axis L 145  that is oriented approximately perpendicular to the direction of retraction L 10  and approximately parallel to normal axis L 11 . A second balance bar L 150  attaches at its middle L 155  to each of the rotary joints L 140  on first end L 130  and on second end L 135  of first balance bar L 110 . Two third rotary joints L 160  are located on second balance bar L 150 , with a third rotary joint L 160  being placed at each of the two ends, first end L 165  and second end L 170 , of second balance bar L 150 . Both third rotary joints L 160  permit rotation about an axis L 175  that also is oriented approximately perpendicular to the direction of retraction L 10  and approximately parallel to normal axis L 11 . A descender post L 180  attaches to third rotatory joints L 160  as shown in  FIG. 140  to permit rotation L 185  of descender post L 180  in the incision of the patient. 
     Note that there are, thus, four descender posts L 180  in each self-balancing tissue engager L 25 . The combination of rotary joints L 105 , L 140 , and L 160  with the first and second balance arms L 110  and L 150 , respectively, creates a doubletree as described in Section F—Self-balancing Retractor Blades. Thus, the first balance bar L 110  is a doubletree balance bar and the second balance bars L 150  are swingletrees. 
       FIG. 140  shows a descender post L 180  having a unique shape that enables sure engagement of a rib L 205  without touching the neurovascular bundle L 210 . The cranial direction is the direction of retraction L 10 . Rib L 205  is the cranial-most rib at the incision. Descender post L 180  pushes against caudal margin L 215  of rib L 205  to retract rib L 205 . Descender post L 180  comprises an elongate member L 220  having a first end L 225  and a second end L 230 , a first rib-forcing surface L 235  adjacent to second end L 230 , a hook element L 240  disposed adjacent the second end L 230  of the elongate member L 220 , the hook element L 240  comprising a first hook end L 245  and a second hook end L 250 , and a second rib-forcing surface L 255  between first hook end L 245  second hook end L 250 , thereby defining a gap region L 260  between the first rib-forcing surface L 235  and the second rib-forcing surface L 255 . The gap region L 260  is concave and possesses a length L 265  along the direction of retraction L 10  configured to place the second rib-forcing surface L 255  substantially away from the neurovascular bundle L 210 . 
     The placement of second rib-forcing surface L 255  should be such that second rib-forcing surface L 255  contacts the bottom of rib L 205  somewhere in the mid-region along the chord L 270  of the rib L 205 , from approximately 20% to 80% from the caudal margin L 215  of rib L 205 . This placement is important to ensure that neurovascular bundle L 210  is positioned in the gap region L 260  such that no part of descender post L 180  contacts or in any ways exerts a force on neurovascular bundle L 210 . A descender post L 180  on the opposite side of the incision retracting a caudal rib (not shown) does not have this consideration because the neurovascular bundle L 210  is not on the cranial margin L 280  of a rib. Nevertheless, descender posts L 180  work well for the caudal rib, too. 
       FIG. 141  shows a descender post having a different shape, being more hook-shaped, like the descender post shown in  FIG. 98A .  FIG. 141  is a photograph from a thoracotomy in a 50 kg pig. It is clear in this picture there is a large gap region and that the neurovascular bundle is not being touched. 
     The axis of rotation L 175  of third rotary joint L 160  is shown in  FIG. 140  and is positioned at the first end L 225  of elongate member L 220 . Rotation about axis L 175  causes first rib-forcing surface L 235  to swing through an arc having substantial radius L 275 .  FIG. 139  shows a descender post L 180  in two positions, position L 300  with the descender post aligned approximately perpendicular to the direction of retraction L 10  and position L 310  with the descender post L 180  aligned approximately parallel to the direction of retraction L 10 . Position L 310  is the deployed position, the position the descender post L 180  assumes during retraction. Position L 300  is the undeployed position. If all descender posts are in position L 300 , then the hook element L 240  aligns approximately parallel with the incision, easing insertion between the ribs. This is the situation depicted in  FIG. 130A . As retraction commences, the force at first rib forcing surface L 235  causes descender post L 180  to rotate into position L 310 . Thus descender posts L 180  in self-balancing tissue engager L 25  can self-deploy. The surgeon can insert both self-balancing tissue engagers into the incision with hook elements L 240  of all descender posts L 180  aligned approximately perpendicular to the direction of retraction (and thus parallel with the margins of the two ribs adjacent the incision). When retraction commences, the force applied to the first rib-forcing surfaces L 235  on all descender posts L 180  on both sides of the incision causes all descender posts L 180  to automatically rotate into the deployed position L 310  with second rib-forcing surface L 255  coming into proper position under the rib, as depicted in  FIG. 130B . 
     An elastic element can be added to self-balancing tissue engager L 25  both to hold all components (balance arms and descender posts) in their undeployed position L 300 . This makes retractor L 00  easier to handle. When the retractor is loaded, the elastic element only lightly opposes the forces at first rib-forcing surfaces, allowing the descender posts L 180  to rotate into position L 310  and the balance arms to balance the forces on the descender posts L 180 . Furthermore, when retraction is released, the elastic element will exert a light force to return the descender posts L 180  to their undeployed position L 300  facilitating removal from the incision. The elastic element can include simple rubber bands or other elastomeric components deployed at joints. Alternately, an elastomeric layer could be placed over the entire self-balancing tissue engager L 25 , such as would occur on coating the self-balancing tissue engager during a dip or molding process. 
     Elastomeric components can also be placed at the rib-forcing surfaces to pad the rib at those surfaces. These pads can be soft, but the pad at the first rib-forcing surface L 235  should not be so thick as to deform into the gap region L 260  and apply pressure to the neurovascular bundle L 210 . 
     N. Preventing Trauma During Thorascopic Procedures 
     Problems with Current Port Designs 
     The inventions described in this section are directed to methods and devices for gaining surgical access for thoracoscopic surgery or minimally invasive surgery. More particularly, they are directed to new devices acting as retractors and ports that introduce into the chest and hold surgical instruments during thoracoscopic surgery. Further, these inventions include a device that provides a temporary stable platform for holding surgical instruments that are introduced between ribs into the chest without damaging the surrounding tissues, including the intercostal nerve and vasculature. 
     Thoracoscopic surgery commonly uses “ports”, which are used to provide an annular opening into a patient&#39;s chest. Ports provide both a route into the chest for thoracoscopic instruments and a device that holds instruments, such as an endoscope, in place during the surgery. Alternatively, many minimally invasive surgeries use smaller thoracic retractors to retract smaller incisions for surgery. 
     A port, in its simplest form, is a rigid tube placed between the ribs through which surgical instruments are introduced into the chest, as depicted in  FIGS. 142A through 142C . The tissues of the thorax include the skin H 42 , subcutaneous tissues N 2 , and a layer N 3  containing the ribs H 46  and H 47  and intercostal muscle H 44 . The patient&#39;s head is in the cranial direction H 43  and the feet are in the caudal direction H 45 . An incision is made between two ribs, such that one rib is designated the cranial rib H 46  having a caudal margin H 50  adjacent the incision and the opposing rib is the caudal rib H 47  having a cranial margin H 54  adjacent the incision. A neurovascular bundle H 48  lies just inside and below the caudal-most edge of each rib H 46  and H 47 . The neurovascular bundle H 48  contains the intercostal nerve and intercostal vasculature. Importantly, damage of the intercostal nerve has been implicated in post-thoracotomy pain, especially long-term post-thoracotomy pain (e.g. Wildgaard et al. 2009. Eur. J. Cardiothorac. Surg. 55(1):60-68; Virginie et al. 2011, Pain. 152(1):74-81.) 
     As shown in  FIG. 142A , a port N 210  is inserted between ribs H 46  and H 47 , either after a small incision is made by a scalpel or by pushing the port N 210 , which can have a sharp tip, through the tissues H 42 , N 2 , and N 3 . The port N 210  has an internal lumen N 220  formed by a shaft N 230  that permits the introduction of surgical instruments into the pleural cavity of the chest. The shaft N 230  of the port N 210  impinges on the margins H 50  and H 54  of ribs H 46  and H 47 , including the caudal edge H 50  of cranial rib H 46  thereby also impinging on the neurovascular bundle H 48  of cranial rib H 46 . The shaft N 230  directly impinges on the delicate periosteum wrapping the ribs, and the shaft N 230  can contact the neurovascular bundle H 48 . Subsequently, the intercostal muscles between ribs H 46  and H 47  and shaft N 230  are crushed, ribs H 46  and H 47  can be bruised, and the neurovascular bundle H 48  of rib H 46  can be damaged. Additionally, as the intercostal muscles H 44  are separated by the port N 210 , a region of high pressure H 60  forms around the port N 210  which also can damage the intercostal nerve or prevent the flow of blood through the intercostal vasculature in the neurovascular bundle H 48  of rib H 46 . 
     As shown in  FIG. 142B , ports frequently have an enlarged head N 250  that can be pushed against the skin H 42  to better secure port N 210  in position. Additionally, as shown in  FIG. 142C , the shaft N 230  of a port N 211  can be helically threaded like a screw with the thread N 260  used to attach additional components to shaft N 230  but also to assist with engaging ribs H 46  and H 47  to firmly pull head N 250  against the skin H 42  to firmly secure port N 210  and thus any instruments the port holds. One problem with this design is that the edge of the thread almost certainly crushes the neurovascular bundle H 48  of rib H 46  as shown in the enlarged portion of  FIG. 142C . Examples of such threaded ports include the Trocar Sleeves from Germed USA (part number 70-00510, Garden City Park, N.Y., USA) and the Thoracoports from Covidien (order code 179301, Mansfield, Mass., USA). 
     Patients suffer significant pain after thoracoscopy. As with thoracotomy, pain after thoracoscopy occurs in the immediate post-operative period and also can be long-term (Steegers et al. 2008. J. Pain. 9(10):955-961; Wildgaard et al. 2011. Acta Anaesthesiol. Scand. 55(1):60-68). One possible cause of this pain, especially long-term pain, is thought to be damage to the intercostal nerve as described earlier, and this damage can be caused by the design of current ports. 
     Improved devices and methods of introducing surgical instruments into the chest and also of temporarily holding the surgical instruments, especially avoiding damage to the neurovascular bundle, are thus desired. 
     Improved Port Designs 
       FIG. 143  shows a rib engager N 310  used to engage the cranial rib H 46  such that the neurovascular bundle H 48  is not crushed. Rib engager N 310  has descender post H 334  having a first end N 321  and a second end N 322  and a retraction hook H 336 . There are thus two rib forcing surfaces formed between rib engager N 310  and rib H 46 : a first rib forcing surface N 360  at the point of contact between the descender post H 334  and the caudal H 50  of cranial rib H 46 , located adjacent the second shaft end N 322 , and a second rib forcing surface N 370  between the tip of the retraction hook H 336  and the bottom (pleural face) of rib H 46 . First rib forcing surface N 360  is the point of force application to rib H 46  for retraction force N 300 , and second rib forcing surface N 370  provides a point of application of a much smaller force that prevents the rib engager N 310  from sliding upward. (See  FIG. 147  for a more complete description of these two forces.) Retraction hook H 336  has a recurved shape such that it forms a protective gap or hollow region H 340 , around the neurovascular bundle H 48  that prevents crushing of the neurovascular bundle H 48  both from direct contact between the descender post H 334  or from formation of a region of high tissue pressure H 60  such as is depicted in  FIGS. 142A and 142B . Rib engager N 310  can include tab N 350  near first end N 321  of descender post H 334  to facilitate handling of rib engager N 310 . 
       FIGS. 144A through 144E  show a retractor N 600  that permits independent actuation of either side of retractor N 600 , thereby allowing an operator to vary the separation between opposing rib engagers N 310 . Furthermore, retractor N 600  can be made very small to provide the small openings desired for thoracoscopic surgery.  FIGS. 144A  through C present top views without ribs, and  FIGS. 144D through 144E  show side views with ribs. A first pair of rib engagers N 310  are rotatably mounted to a first retraction element N 610 , and second pair of rib engagers N 310 ′ are attached to a second opposing retraction element N 620 . (Prime notation on numbers, e.g. N 310 ′, indicates components of rib engagers on right hand side of diagrams connecting to second opposing retraction element N 620 ). Retraction elements N 610  and N 620  are driven apart by screw drives N 630  and N 640  which are actuated by screws N 650  and N 660 , respectively. Screws N 650  and N 660  can be turned by hand via knurled knobs N 655  and N 665 , respectively, or alternatively could be driven by motors. Screw N 660  couples to retraction element N 620  by a rotatable screw mount N 680  which permits rotation of screw N 660  with respect to retraction element N 620  but without advancement of retraction element N 620  along screw N 660 . Screw N 660  couples to retraction element N 610  by rotatable captured threads N 670  which converts rotation of screw N 660  into advancement of retraction element N 610  along screw N 660 . Thus turning of screw N 660  changes the separation of retraction elements N 610  and N 620  at screw drive N 640 . Similarly, screw N 650  couples to retraction element N 610  by a rotatable screw mount N 680  which permits rotation of screw N 650  with respect to retraction element N 610  but without advancement of retraction element N 610  along screw N 650 . However, screw N 650  couples to retraction element N 620  by fixed threads N 690  which convert rotation of screw N 650  into advancement of retraction element N 620  along screw N 650 . Thus turning of screw N 650  changes the separation of retraction elements N 610  and N 620  at screw drive N 630 . Note that fixed threads N 690  where screw N 650  attaches to second retraction element N 620  prevent the retractor N 600  from collapsing closed as a parallelogram. 
     The change in the angle formed by screw N 660  and retraction elements N 610  and N 620  seen in  FIGS. 144A and 144B  illustrates how rotatable screw mount N 680  and rotatable captured threads N 670  rotate with respect to retraction elements N 610  and N 620 . Alternatively, screws N 650  and N 660  could be screws having threads of opposite handedness on each end that are coupled to retraction elements N 610  and N 620  only by rotatable captured threads N 670  (i.e. without rotating screw mounts N 680 ) in a manner similar to a Jorgensen hand screw clamp used commonly in woodworking. Thus, rotation of a single screw N 650  or N 660  would advance both retraction elements N 610  and N 620  relative to the screw. 
       FIG. 144A through 144C  show how actuation of the two screw drives permits independent control of the spacing of opposing rib engagers N 310 /N 310 ′. In  FIG. 144A , the first and second retraction elements N 610  and N 620 , respectively, are close together. As shown in  FIG. 144B , activation of screw drive N 640  by rotation of screw N 660  increases the spacing between the opposing rib engagers N 310  and N 310 ′ on the end of retractor N 600  closest to screw drive N 640 . Similarly, as shown in  FIG. 144C , activation of screw drive N 630  by rotation of screw N 650  increases the spacing between the opposing rib engagers N 310  and N 310 ′ on the end of retractor N 600  closest to screw drive N 650 . 
       FIG. 144D  shows a side view of retractor N 600  placed between opposing ribs H 46  and H 47  with the rib engagers N 310  and N 310 ′ rotated such that the retraction hooks H 336  are positioned under the ribs. Rotation of the rib engagers N 310  and N 310 ′ about a vertical axis is enabled by rotational joints N 687  and N 687 ′ which allow rotation of the rib engagers N 310  and N 310 ′ as indicated by the arrows encircling descender posts H 334  and H 334 ′. Activation of either screw drive to increase the separation of the two retraction elements N 610  and N 620  can push the ribs apart. Tabs N 350  and N 350 ′ facilitate manual rotation of rib engagers N 310  and N 310 ′. 
       FIG. 144E  shows retractor N 600  with retraction elements N 610  and N 620  spaced more fully apart such that the ribs H 46  and H 47  have been pushed apart. The retraction forces N 300  required to push rib H 46  creates a moment about rib engager N 310  such that it rotates from an initial orientation shown by dashed line N 685  to a slanted orientation shown by dashed line N 686 . Rib engager N 300 ′ rotates in similar fashion, but opposite sense, in response to force N 300 ′. 
       FIGS. 145A and 145B  show how rotation of rib engagers N 310  and N 310 ′ facilitates insertion of retractor N 600  into an intercostal incision cut along line N 51 .  FIG. 145A  presents a top view of retractor N 600  with the caudal margin H 50  of cranial rib H 46  and the cranial margin H 54  of caudal rib H 47  (i.e. the opposing edges of the two ribs). In  FIG. 145A , rib engagers N 310  and N 310 ′ are oriented such that the tabs N 350  and N 350 ′ and retraction hooks H 336  and H 336 ′ are aligned approximately parallel to the rib margins H 50  and H 54 . This facilitates insertion of retractor N 600  into the intercostal incision along line N 51 .  FIG. 145B  shows the orientation of rib engagers N 310  and N 310 ′ after they have been turned such that the retraction hooks H 336  and H 336 ′ are now positioned under the margins H 50  and H 54  of the ribs, respectively. This is the position that would be used during retraction of ribs. 
       FIGS. 146A through 146C  show another means for orienting rib engagers N 605  that automatically orient to the rib as retraction proceeds.  FIGS. 146A and 146B  present a similar series of top-view illustrations as in  FIGS. 145A and 145B , but here a torsion spring pivot N 690  initially holds the hook element L 240  of rib engagers N 605  such that the plane in which hook element L 240  lies is approximately parallel to the rib margins H 50  and H 54 .  FIG. 146B  shows the rib engagers N 605  now deployed such that hook elements L 240  are now deployed under rib margins H 50  and H 54 .  FIG. 146C  shows a side view of retractor N 600 . The descender posts L 180  of rib engagers N 605  are curved such that application of retraction force at the first rib forcing surface L 235  generates a torque around axis N 691  of torsion spring pivot N 690  thereby turning rib engager N 605  automatically, without the need of tabs N 350 , such that hook elements automatically deploy under the ribs. The opposing rib engager N 605 ′ turns in similar fashion. 
       FIG. 147  diagrams the forces on rib engagers N 310  and N 310 ′ when the ribs H 46  and H 47  have been spread by retractor N 600 . As mentioned when discussing  FIGS. 143 and 144E , retraction of rib H 46  is driven by a force N 700  that is countered by a reaction force N 700 ′ on the rib engager N 310 ′. The reaction force N 700  applied at first rib forcing surface N 360  creates a moment that torques rib engager N 310  into an angled position, as described in  FIG. 144E , such that it is now oriented along tilted axis N 686 . Retraction force N 700  is thus approximately perpendicular to axis N 686  and can now be considered to be composed of two component forces, horizontal component force N 710  and vertical component force N 720 . Similarly, retraction force N 700 ′ is approximately perpendicular to axis N 686 ′ and can now be considered to be composed of two component forces, horizontal component force N 710 ′ and vertical component force N 720 ′. (Note that the relative magnitudes of forces N 710 /N 710 ′ and N 720 /N 720 ′ are for illustration only.) Vertical component force N 720  causes rib engager N 310  to slide upward approximately in the direction of arrow N 688  along axis N 686 . Retraction hook H 336  halts this sliding when tip N 370  of retraction hook H 336  contacts rib H 46 , such that vertical component force N 720  is countered by a downward force (not shown). Similar forces cause the opposing rib engager N 311  to slide upward along rib H 47 —retraction force N 700 ′ has two components, horizontal component force N 710 ′ and vertical component force N 720 ′. Thus, when ribs are spread apart by retractor N 600 , there is a vertical force (N 720 +N 720 ′) acting on retractor N 600 , and this force secures retractor N 600  against the pleural surfaces of ribs H 46  and H 47 , creating a stable platform from which thoracoscopic instruments can be mounted, as described later. 
     The stronger the forces N 720  and N 720 ′, the more stable the platform. Forces N 720  and N 720 ′ increase with larger retraction forces N 700  and N 700 ′ and as the tilt angle N 740  between N 700  and N 710  (and of the tilt angle N 740 ′ on the opposing descender post H 334 ′) increases from 0 degrees to a maximum at 45 degrees. The degree of tilt of axes N 688  and N 688 ′, respectively, are determined first by the amount of play in the joints of retractor N 600 , such as in rotational joints N 687  and N 687 ′. 
       FIGS. 148A and 148B  present an alternative means by which a retractor N 700  can be secured into position without spreading the ribs. This is important because one goal of thoracoscopic surgery is to minimize rib spreading.  FIG. 148A  presents a side view, and  FIG. 148B  presents a top view. Plates N 750  and N 750 ′ are mounted onto rib engagers N 310  and N 310 ′, respectively, by sliding joint N 755 /N 755 ′. Considering only plate N 750 , plate N 750  must be bound to descender post H 334  by sliding joint N 755  such that plates N 750  are able to move up and down along descender post H 334  as indicated by arrows N 760 . The means by which plate N 750  is bound to descender post H 334  can include friction mounts, manually actuated cams, lock screws, or other devices well known in the art, such that plate N 750  can press against skin H 42 . When plate N 750  is pushed down manually by an operator it presses against skin H 42 , and retraction hook H 336  is pulled upward to form second rib forcing surface N 370  on the bottom of rib H 46 . The tissues between plates N 750  and the ribs (e.g. the skin H 42 , subcutaneous tissue N 2 , and layer N 3 ) thus become compressed, securing retractor N 700  into place. Other means of mounting plate N 750  to retractor N 700  to forcibly press plate N 750  against skin H 42  can include screw drives connecting plate N 750  to retraction element N 610  and other mechanisms known in the art. 
       FIGS. 149A and 149B  show how a retractor N 600  can be used to hold a thoracoscopic instrument in the incision such that is securely held and does not touch the neurovascular bundle N 40 .  FIG. 149A  shows a top view, and  FIG. 149B  shows a side view. Mounting plate N 910  spans the gap N 911  between retraction elements N 610  and N 620 . Mounting plate N 910  has a mounting slot N 920  that is secured to retraction element N 610  with first set screw N 930 . When first set screw N 930  is released, mounting plate N 910  is capable of sliding left-right (perpendicular to the ribs&#39; axes) and rotating around set screw N 930 , permitting ball joint mount N 940  to be positioned anywhere between retraction elements N 610  and N 620 . Mounting plate N 910  holds ball joint mount N 940  over the incision between the ribs. Referring now to  FIG. 149B , ball joint mount N 940  is comprised of an outer sphere N 950  containing an inner sphere N 960  that can be secured relative to outer sphere N 950  by second set screw N 970 . Inner sphere N 960  has an annulus N 980  for mounting thoracoscopic instruments. Ball joint mount N 940  permits control of the angular position of a thoracoscopic instrument. 
       FIGS. 150A and 150B  show another embodiment of a retractor N 700  that is similar to retractor N 600 , but now ball joint N 940  is mounted in the plane of the ribs H 46  and H 47  rather than above this plane. This placement of ball joint N 940  ensures maximum angular travel of instruments held by retractor N 700  inside annulus N 980  without impinging on neurovascular bundle H 48 . Ball joint N 940  is suspended in position from screws N 650  and N 660  by suspension hooks N 990 . 
       FIG. 151  shows another embodiment of a retractor N 800  that is similar to retractor N 800  but now screws N 650  and N 660  are replaced by bars N 850  and N 860 , which can be either smooth bars or striated bars or threaded screws or other bars with textured surfaces for engaging a clasp. Clasps N 810  can be actuated to alternately grasp or release bars N 850  and N 860 . As shown here, clasps N 810  normally grasp the bars and a finger press releases the grasp, permitting retraction elements N 610  and N 620  to be separated by a surgeon who pulls the retraction elements N 610  and N 620  apart. Ball joint N 940  can then be placed into position by suspending it by suspension hooks N 990  from bars N 850  and N 860 . 
     The embodiments disclosed thus far are primarily devices that resemble small retractors with many different parts. These permit a broad range of adjustments of position, orientation, and forces on the ribs, but smaller and simpler devices are possible, devices with only a few moving parts that fit almost entirely inside the intercostal incision, in most cases leaving no parts projecting out of the skin incision. From here forward, we disclose such smaller, simpler devices. Despite being smaller and simpler, these devices still manage to protect the surface of the rib and the intercostal neurovascular bundle from damage by preventing impingement of surgical instruments inserted between the cranial and caudal ribs, impingement that can damage fragile tissues and lead to post-surgical pain and complications. 
       FIG. 152  establishes the relevant anatomy, illustrating an intercostal incision, in cross-section, passing through the tissues of the thorax, between adjacent ribs, and into the pleural cavity.  FIG. 152  is presented solely to identify the appropriate anatomy and incision and does not present any inventions. The tissues of the thoracic wall N 10  include the skin H 42 , subcutaneous tissues N 2 , and a layer N 3  containing ribs H 46  and H 47  and intercostal muscle H 44 . The space interior to the thoracic wall N 10  is the pleural cavity N 20 . The patient&#39;s head is in the cranial direction H 43  and the feet are in the caudal direction H 45 . A thoracic wall incision N 5  is made through the thoracic wall N 10  passing between ribs H 46  and H 47 , such that one rib is designated the cranial rib H 46  having a caudal margin H 50  adjacent the thoracic wall incision N 5 , and the opposing rib is the caudal rib H 47  having a cranial margin H 54  adjacent the thoracic wall incision N 5 . Each rib H 46  and H 47  has a longitudinal axis N 30  (shown here coming out of the page), a pleural surface (dashed line) N 40  facing pleural space N 20 , and a skin surface N 50  facing skin H 42 . A neurovascular bundle H 48  lies just inside and below (pleurally) the caudal-most edge of each rib H 46  and H 47 . The neurovascular bundle H 48  contains the intercostal nerve and intercostal vasculature. For purposes of this discussion, the thoracic incision N 5  through all the tissues of the thoracic wall N 10  can be sub-divided into the skin incision N 6  through the skin H 42  and the intercostal incision N 7  through the intercostal muscle H 44 . 
       FIGS. 153A through 153E  shows a rib-protecting clip N 900  that resides solely in the intercostal incision N 7  with no part projecting through the skin incision N 4  and thus no part above the skin H 42 .  FIGS. 153A through 153E  show rib-protecting clip N 900  apposed to the caudal margin H 50  of a cranial rib H 46 , but rib-protecting clip N 900  can attach to the cranial margin H 54  of the caudal rib H 47  in similar fashion. 
       FIG. 153A  shows an oblique view of the rib-protecting clip N 900 .  FIG. 153B  shows the rib-protecting clip N 900  with a rib, here cranial rib H 46 .  FIG. 153C  shows a cross-sectional view, including select anatomy around the incision. Rib-protecting clip N 900  comprises a rib-engaging channel N 901  having a first end N 902  and a second end N 903  and engages cranial rib H 46 , generally along longitudinal axis N 30  of the cranial rib H 46 , wrapping around the caudal margin H 50  of cranial rib H 46 . Rib-engaging channel N 901  thus serves as a barrier to prevent impingement of any object that is inserted into intercostal incision N 7  onto cranial rib H 46 . When an object, such as a surgical instrument, is inserted into intercostal incision N 7 , rib-protecting clip N 900  prevents direct impingement by that object onto cranial rib H 46 , thereby protecting cranial rib H 46 . Nevertheless, that object can exert a force N 905  that pushes rib-engaging channel N 901  into cranial rib H 46 , thereby crushing intercostal muscle H 44  and other associated tissues into rib H 46  and creating a high pressure region H 60 , all of which can potentially damage these tissues and the neurovascular bundle H 48 .  FIGS. 153D and 153E  show modifications that reduce this broad crushing of intercostal muscle H 44  and the neurovascular bundle H 48 .  FIGS. 153D and 153E  show identical views as  FIGS. 153B and 153C , respectively, but now rib-engaging channel N 901  sports raised members N 910  that are disposed along a portion of rib-engaging channel N 901  between first end N 902  and second end N 903  and that are elevated from rib-engaging channel N 901  toward cranial rib H 46 . Raised members N 910  here are depicted as raised ridges on rib-engaging channel N 901  that place loading from force N 902  onto discrete rib-contacting surfaces, seen in  FIG. 153E  as rib-contacting surface N 920 . Other than rib-contacting surfaces N 920 , no other portion of rib-protecting clip N 900  contacts cranial rib H 46 . Raised members N 910  thus relieve pressure on (and damage to) intercostal muscle H 44  and neurovascular bundle H 48  caused by the rest of rib-engaging clip N 900 . 
       FIGS. 154A through 154C  shows another rib-protecting clip N 1000  with components to attach reversibly to a rib.  FIGS. 154A through 154C  show rib-protecting clip N 1000  apposed to the caudal margin H 50  of a cranial rib H 46 , but rib-protecting clip N 1000  can attach to the cranial margin H 54  of the caudal rib H 47  in similar fashion. 
     Rib-protecting clip N 1000  is comprised of a rib-engaging channel N 1005  formed by a first rib-clip N 1010 , a second rib-clip N 1020 , and a barrier member N 1030 .  FIG. 154A  shows an oblique view of rib-protecting clip N 1000  apposed to cranial rib H 46  generally along the longitudinal axis N 30  of cranial rib H 46 .  FIG. 154B  shows four panels, each presenting a cross-section through rib-protecting clip N 1000 . Panel “Rib-clip Only” shows a rib-clip N 1010  or N 1020  in isolation in side view. Panel “Rib-clip and Rib” shows a rib-clip N 1010  or N 1020  and cranial rib H 46  in cross section. Panel “Barrier Member and Rib” shows a cross-section through cranial rib H 46  and a portion of barrier member N 1030  such that barrier member N 1030  is seen in isolation with cranial rib H 46 . Panel “Barrier Member, Rib-clip, Rib, and Incision” shows a cross section through rib-protecting clip N 1000  through one end of barrier member N 1030  such that all components are seen in cross-section.  FIG. 154C  shows a sequence of illustrations depicting how rib-protecting clip N 1000  is applied to a rib. 
     Barrier member N 1030  is preferably an elongated plate curved as shown here, but can be any elongated component such as a cylindrical bar. Barrier member N 1030  has a first end N 1032 , a second end N 1034 , a rib-facing surface N 1036 , and an incision-facing surface N 1038 . The length N 1031  of barrier member N 1030  is thus the distance separating first end N 1032  and second end N 1034 . First rib-clip N 1010  is disposed on rib-facing surface N 1036  at first end N 1032 , and second rib-clip N 1020  is disposed on rib-facing surface N 1036  at second end N 1034 . Barrier member N 1030  can have one of several different lengths N 1031  to accommodate different incision sizes. For example, barrier member N 1030  can have a length N 1031  ranging from 2 to 15 mm for the camera port for thoracoscopic surgery (fitting into a very small incision), or it can have a length N 1031  of 100 mm for a utility incision for thoracoscopic surgery (permitting access for larger instruments or for multiple instruments simultaneously). 
     First rib-clip N 1010  and second rib-clip N 1020  have similar components. Each rib-clip N 1010  or N 1020  is comprised of bent elongated member N 1050  that wraps cranial rib H 46 . Bent elongated member N 1050  has an apex N 1051 , an upper end N 1052 , and a lower end N 1053 . Upper segment N 1055  is that portion of bent elongated member N 1050  that spans from apex N 1051  to upper end N 1052 , and lower segment N 1056  is that portion of bent elongated member N 1050  that spans from apex N 1051  to lower end N 1053 . Either or both ends N 1052  and N 1053  can be sharp or slightly blunted. Apex N 1051  contacts the caudal margin H 54  of cranial rib H 46  at apex contact area N 1057 . Upper end N 1052  contacts the skin surface N 50  of cranial rib H 46  at upper contact area N 1058 , and lower end N 1053  contacts the pleural surface N 40  of cranial rib H 46  at lower contact area N 1059 . Apex contact area N 1057 , upper contact area N 1058 , and lower contact area N 1059  are substantially away from the neurovascular bundle H 48  of cranial rib H 46 . Upper segment N 1055  and lower segment N 1056  are curved such they create upper gap N 1060  and lower gap N 1061 , respectively, and do not touch either cranial rib H 46  or neurovascular bundle H 48 . 
     First and second rib clips N 1010  and N 1020  attach reversibly to cranial rib H 46  and appose barrier member N 1030  along the caudal margin H 50  of cranial rib H 46 , but without barrier member N 1030  touching cranial rib H 46 . Effectively, first rib clip N 1010  and second rib clip N 1020  act as a raised member (as described for  FIGS. 153D and 153E ). There is thus a barrier member gap N 1062  between cranial rib H 46  and the rib-facing surface N 1036  of barrier member N 1030 . Barrier member N 1030  thus does not crush tissue attached to cranial rib H 46 , but barrier member N 1030  still functions as a shield to protect cranial rib H 46  and any tissue lying in barrier member gap N 1050 . Upper gap N 1060 , lower gap N 1061 , and barrier gap N 1062  thus prevent rib-protecting clip N 1000  from contacting cranial rib anywhere but at the apex contact area N 1057 , upper contact area N 1058 , and lower contact area N 1059  for first rib clip N 1010  and for second rib clip N 1020 . 
       FIG. 154C  shows a sequence of illustrations depicting how rib-protecting clip N 1000  is applied to a rib. Rib-protecting clip N 1000  can be made of metal or polymer or any material with sufficient rigidity and toughness to (a) elastically deform and (b) hold instruments away from cranial rib H 46 . In Position 1, rib-protecting clip N 1000  is separate from cranial rib H 46 . In Position 2, rib-protecting clip N 1000  has been deformed to separate upper end N 1052  from lower end N 1053 , and rib-protecting clip N 1000  is inserted onto cranial rib H 46  until it contacts the caudal margin H 50  of the cranial rib H 46 . The actions of deforming rib-protecting clip N 1000  and placing it onto cranial rib H 46  can be performed by hand or with the assistance of a separate clip applier for this purpose. In Position 3, rib-protecting clip N 1000  has been allowed to elastically return to its undeformed shape, and now upper end N 1052  and lower end N 1053  are in contact with cranial rib H 46 , holding barrier member N 1030  firmly in place, aligned generally along the longitudinal axis N 30  of the cranial rib H 46 . Barrier member N 1030  is now firmly held in position against cranial rib H 46 , creating upper gaps  1060 , lower gaps,  1061 , and barrier member gap N 1062  and shielding cranial rib H 46  and neurovascular bundle H 48  from impingement by surgical instrument N 60 . 
     A rib-protecting clip, like rib-protecting clip N 1000 , can be configured in several ways. Barrier member N 1030  can instead be supported by a multiple rib-clips with some disposed between first end N 1032  and second end N 1034  of barrier member N 1030 , which would help support a barrier member N 1030  with longer length N 1031 . Similarly, the rib-clips need not have both an upper segment N 1055 /upper end N 1052  and a lower segment N 1056 /lower end N 1053 . For example, the rib-protecting clip N 1001  in  FIG. 154D  is comprised of a barrier member N 1030 , a first rib-clip N 1081  on first end N 1032  of barrier member N 1030 , a second rib-clip N 1082  on second end N 1034  of barrier member N 1030 , and a third rib-clip N 1083  halfway between first end N 1032  and second end N 1034 . First rib-clip N 1081  and second rib-clip N 1082  have only an upper segment N 1055 /upper end N 1052  while third rib-clip N 1083  has only a lower segment N 1056 /lower end N 1053 . These three ends of the three clips would comprise a “3-point rib clip” connected by the barrier member N 1030 , and rib-engaging channel N 1005  is formed by first rib-clip N 1110 , second rib-clip N 1120 , third rib-clip N 1130  and barrier member N 1030 . 
       FIGS. 155A through 155H  illustrate a different device for protecting the cranial rib H 46  and the caudal rib H 47 , a temporary rib-protecting port N 1100  that defines the entire aperture of a surgical access opening. Rib-protecting port N 1100  possesses two stable configurations, a collapsed configuration that facilitates insertion into an intercostal incision and a deployed configuration that opens the intercostal incision into a surgical access opening.  FIG. 155A  presents an oblique view,  FIG. 155B  presents an end view,  FIG. 155C  presents a top view.  FIG. 155D  presents a side view.  FIG. 155E  shows a side piece N 1102  in isolation with a cranial rib H 46 .  FIG. 155F  shows an end piece N 1114  in isolation with cranial rib H 46 .  FIG. 155G  shows an end view with cranial rib H 46 , caudal rib H 47  and a surgical instrument N 1099 .  FIG. 155H  shows a top view with cranial rib H 46  and caudal rib H 47 , presenting a 3-step sequence showing how rib-protecting port N 1100  transitions from a collapsed configuration to an expanded configuration. Rib-protecting port N 1100  comprises a chassis of six main parts: a first side piece N 1101 , a second side piece N 1102 , a first end piece N 1111 , a second end piece N 1112 , a third end piece N 1113 , and a fourth end piece N 1114 . 
     Several paragraphs describing components and their arrangements are needed first. Note that pleural space N 20  is labeled in some figures to assist with orientation. 
     Referring now primarily to  FIGS. 155D and 155E . Both side pieces N 1101  and N 1102  have similar structure.  FIG. 155E  shows second side piece N 1102  in isolation as an example. It has a first hinged end N 1181 , a second hinged end N 1182 , a bottom edge N 1162  and a top edge N 1163 . Two wing members, first wing member N 1131  and second wing member N 1132 , are disposed on top edge N 1163 . Second side piece N 1102  thus comprises a barrier member, having a rib-facing surface N 1165  and an incision-facing surface N 1166  (not visible in this view) on the side opposite rib-facing surface N 1165 . Second side piece N 1102  also has two raised members, first raised member N 1171  and second raised member N 1172 , having first rib-contacting surface N 1181  and second rib-contacting surface N 1182  formed on contact with the caudal margin H 50  of cranial rib H 46 . Similarly, first side piece N 1101  (not shown) has a first hinged end N 1181 , a second hinged end N 1182 , a bottom edge N 1162  and a top edge N 1163 , two wing members (first wing member N 1131  and second wing member N 1132  being disposed on top edge N 1163 ), first raised member N 1171  and second raised member N 1172  (having first rib-contacting surface N 1181  and second rib-contacting surface N 1182 ), rib-facing surface N 1165 , incision-facing surface N 1166 , and all other structures of second side piece N 1102 . First side piece N 1101  is against caudal rib H 47  when rib-protecting port N 1100  is deployed. Notably, as can be seen in  FIG. 155C , the incision-facing surfaces N 1166  of first side piece N 1101  and of second side piece N 1102  delimit the sides of thoracic surgical access space N 1198 . 
     Referring now primarily to  FIG. 155F . All four end pieces N 1111 , N 1112 , N 1113 , and N 1114  have similar structure. Second end piece N 1112  is presented as an example in  FIG. 155F . Second end piece N 1112  has a first hinged end N 1181 , a second hinged end N 1182 , a top portion N 1183 , and a bottom portion N 1184 . Second end piece N 1112  has a rib-engaging hook N 1120  disposed on the bottom portion N 1184  nearest the first hinged end N 1181 . Rib-engaging hook N 1120  comprises an elongated member N 1121  having a first hook end N 1122  disposed on bottom portion N 1184  near first hinged end N 1181  and a second hook end N 1123  disposed under the pleural surface N 40  of cranial rib H 46  and contacting the pleural surface at rib-contacting surface N 1124 . Elongated member N 1121  of rib-engaging hook N 1120  thus projects outward from first hinged end N 1181  in a direction approximately parallel with the bottom portion N 1184  and away from second end piece N 1112 . 
     Referring now primarily to  FIG. 155A , rib-protecting port N 1100  has six hinged joints configured as follows:
         1. a first hinged corner joint N 1141  formed by second side piece N 1102  and first end piece N 1111 ,   2. a second hinged corner joint N 1142  formed by first side piece N 1101  and second end piece N 1112 ,   3. a third hinged corner joint N 1143  formed by first side piece N 1101  and third end piece N 1113 ,   4. a fourth hinged corner joint N 1144  formed by second side piece N 1102  and fourth end piece N 1114 ,   5. a first end joint N 1151  formed by first end piece N 1111  and second end piece N 1112 , and   6. a second end joint N 1152  formed by third end piece N 1113  and fourth end piece N 1114 .       

     The first and second end pieces N 1111  and N 1112 , connected by first end joint N 1151  form a first end N 1105 . The third and fourth end pieces N 1113  and N 1114 , connected by second end joint N 1152  form a second end N 1106 . Note that hinged joints are presented for rib-protecting device N 1100 , but other joint types can be used, such as living joints. 
     Referring now to  FIG. 155C , the side pieces (N 1101 , N 1102 ), end pieces (N 1111 , N 1112 , N 1113 , N 1114 ), and joints (N 1141 , N 1142 , N 1143 , N 1144 , N 1151 , N 1152 ) thus form a single chassis N 1190  and define surgical access opening N 1198 . Additionally, the side pieces (N 1101 , N 1102 ), end pieces (N 1111 , N 1112 , N 1113 , N 1114 ), and joints (N 1141 , N 1142 , N 1143 , N 1144 , N 1151 , N 1152 ) are preferably made of metal or rigid plastic such that chassis N 1190  effectively armors surgical access opening N 1198 , especially including cranial rib H 46  and caudal rib H 47  and the neurovascular bundle H 48  of cranial rib H 46 . 
     Referring now to  FIGS. 155B, 155F, and 155G , chassis N 1190  has two rib-engaging channels, first rib-engaging channel N 1191  and second rib-engaging channel N 1192  (noted by dashed lines). First rib-engaging channel N 1191  is formed collectively by rib-engaging hook N 1120  of second end piece N 1112 , rib-facing surface N 1165  of first side piece N 1101 , and second wing member N 1132  of first side piece N 1101  (see  FIG. 155B ). Similarly, second rib-engaging channel N 1192  is formed collectively by rib-engaging hook N 1120  of first end piece N 1111 , rib-facing surface N 1165  of second side piece N 1102 , and first wing member N 1131  of second side piece N 1102  (see  FIG. 155B ). The side pieces N 1101  and N 1102 , as stated above, thus act like barrier members of a rib-engaging channel. Additionally, wing members N 1131  and N 1132  are disposed on the top portion (skin-facing) of chassis N 1190 , and the rib-engaging hooks N 1120  are disposed on the bottom portion (pleural-facing) of chassis N 1190 . Rib-engaging channels N 1191  and N 1182  form gaps N 1174  around ribs H 46  and H 47 . Again, as stated above, with rigid composition, chassis N 1190  thereby armors the perimeter of the surgical access opening N 1198 , eliminating contact with the neurovascular bundle H 48 , compression/crushing of intercostal muscle, and other sources of tissue trauma caused by impingement by a surgical instrument N 1199  or other object inserted into the surgical access opening N 1198 . 
       FIG. 155H  illustrates that rib-protecting port N 1100  is aligned generally along the longitudinal axis of both ribs H 46  and H 47 . Thus many features, of side pieces N 1101  and N 1102 , which act as barrier members, are generally aligned along the longitudinal axis of their respective rib; for example bottom edge N 1162 . Furthermore, bottom edge N 1162  (and thus side pieces N 1101  and N 1102 ) can be of different lengths to accommodate different incision sizes. For example, bottom edge N 1162  can range from 2 mm to 15 mm long for the camera port for thoracoscopic surgery, or it can be 100 mm long for a utility incision for thoracoscopic surgery. 
     Importantly, when deployed between the ribs, rib-protecting port N 1100  is firmly affixed to each rib, and moves with the rib if the forces on the chassis are sufficiently large to displace a rib—something that occurs regularly in thoracoscopic surgery, especially when the endoscopic camera impinges on the rib as a surgeon attempts to look around inside the chest. This means that rib-protecting port is well-positioned on the ribs despite the forces applied and when large forces, which carry the greatest risk of trauma, are applied. 
       FIG. 155H  shows how rib-protecting port N 1100  is deployed between ribs H 46  and H 47 . In Step 1, the hinged joints permit retractor N 1100  to collapse into a narrow configuration. This is inserted into an intercostal incision (N 7 , shown by the dashed line) between ribs H 46  and H 47 . Note that wing members N 1131  and N 1132  come to bear on the tops of ribs H 46  and H 47 , preventing rib-protecting port N 1100  from passing completely through the intercostal incision. An important feature of rib-protecting port N 1100  is that the minimum dimension across any perimeter (i.e. the shortest dimension from side-to-side in any direction) should be greater than the distance separating ribs H 46  and H 47 . This prevents rib-protecting port N 1100  from inadvertently being pushed between the ribs and into the pleural space. In Step 2, retractor N 1100  is expanded by rotation around the hinged joints (N 1141 , N 1142 , N 1143 , N 1144 , N 1151 , N 1152 ) such that first end joint N 1151  moves outward in the direction of single-head block arrow N 1195 , and second end joint N 1152  moves outward, in the direction of the single-head block arrow N 1196 . This motion results in:
         separation of first side piece N 1101  from second side piece N 1102  (double-headed block arrow N 1194 ) which opens the surgical access opening N 1198  from intercostal incision N 7 ,   side pieces N 1101  and N 1102  appose against ribs H 46  and H 47 , respectively, and   all four fingers N 1120  rotate under ribs H 46  and H 47  to contact their pleural surfaces.       

     It is this motion that forms rib-engaging channel N 1192 , as described above. In Step 3, end joints N 1151  and N 1152  evert slightly and stop at this position due to stops in the joint (not shown). Opposing forces N 1197  and N 1198  are created by ribs H 47  and H 46 , respectively due to expansion of the intercostal incision N 7 . These opposing forces N 1197  and N 1198  thus lock rib-protecting port N 1100  into the deployed configuration shown in Step 3 because they force end joints N 1151  and N 1152  against their stops. 
       FIGS. 156A through 156D  illustrate a different temporary rib-protecting port N 1200  that defines the entire aperture of a surgical access opening Like rib-protecting port N 1100 , this rib-protecting port N 1200  possesses two stable configurations, a first configuration that facilitates insertion into an intercostal incision and a second configuration that opens the intercostal incision into a surgical access opening.  FIG. 156A  presents an oblique and end views showing the shape and fit of the pieces.  FIG. 156B  presents components of some pieces.  FIG. 156C  presents the sequence for inserting rib-protecting port N 1200  into an intercostal incision.  FIG. 156D  presents how rib-protecting port N 1200  protects the margins of the ribs from an instrument in the surgical access opening. 
     Referring first to  FIG. 156A , rib-protecting port N 1200  comprises a chassis of two main parts: a cranial port half N 1201  and a caudal port half N 1202 . Cranial port half N 1201  and a caudal port half N 1202  are monoliths in this example, and each can be formed as a single injection molded part, for example. Cranial port half N 1201  has three major components: right cranial endplate N 1211 , left cranial endplate N 1212 , and cranial barrier member N 1213 . Caudal port half N 1202  similarly has three major components: right caudal endplate N 1221 , left caudal endplate N 1222 , and caudal barrier member N 1223 . Cranial port half N 1201  and a caudal port half N 1202  are joined by two coaxial rotatable joints, right end joint N 1231  and left end joint N 1235 , so cranial port half N 1201  and caudal port half N 1202  can rotate about the common axis N 1230  of coaxial rotatable joints N 1205 . Right end joint N 1231  is formed by right cranial joint hole N 1232  and right caudal joint hole N 1233  held by a hinge pin N 1234 . Left end joint N 1235  is formed by left cranial joint hole N 1232  and left caudal joint hole N 1233  held by a hinge pin N 1233 . Cranial barrier member N 1213  has a top edge N 1214  and a bottom edge N 1215 . 
     Referring now to  FIG. 156B , right cranial endplate N 1211  and right caudal endplate N 1221  are shown as they would be oriented when positioned against the ribs and in greater detail, with the barrier members N 1213  and N 1223  shown in cross-section. The view at the top shows right cranial endplate N 1211  and right caudal endplate N 1221  disjoined without the ribs, the view at the bottom shows right cranial endplate N 1211  and right caudal endplate N 1221  joined and with the ribs. Each endplate has a rib-engaging hook N 1240 , a wing member N 1250 , a bottom portion N 1260 , and a top portion N 1261 . Rib-engaging hook N 1240  comprises an elongated member N 1241  having a first hook end N 1242  disposed on bottom portion N 1260  and a second hook end N 1243 . When rib-protecting port N 1200  is positioned against the ribs, second hook end N 1243  is disposed under the pleural surface N 40  of cranial rib H 46  (or of caudal rib H 47 ) and contacts the pleural surface at rib-contacting surface N 1244 . Cranial rib H 46  also contacts the right cranial endplate N 1211  at rib-contacting surface N 1245  on the upper portion N 1261  (and similarly on left cranial endplate N 1212 ), thereby forming gap N 1280 . Cranial barrier member N 1213  is set back from the curved rib-facing surface of right cranial endplate N 1211  (and also from the curved rib-facing surface of left cranial endplate N 1212 ). The curved rib-facing surfaces of the cranial endplates N 1211  and N 1212  (including rib-contacting surfaces N 1244  and N 1245 ) and the rib-facing surface of cranial barrier member N 1213  thus collectively form a cranial rib-engaging channel N 1270 , and the curved rib-facing surfaces of the cranial endplates N 1211  and N 1212  act as raised members. Similarly, caudal barrier member N 1223  is set back from the curved rib-facing surfaces of right caudal endplate N 1221  (and also from the rib-facing surface of left caudal endplate N 1222 ), thus curved rib-facing surfaces of the caudal endplates N 1221  and N 1222  (including rib-contacting surfaces N 1244  and N 1245 ) and the rib-facing surface of caudal barrier member N 1213  collectively form a second caudal rib-engaging channel N 1271 , and the curved rib-facing surfaces of the caudal endplates N 1221  and N 1222  act as raised members. 
     Referring now to  FIG. 156C , a series of 5 snapshots is presented showing how rib-protecting port N 1200  is inserted through an intercostal incision N 7  and attached to ribs H 46  and H 47 . Initially, rib-protecting port N 1200  is in a folded configuration (Step 1) and is applied to the top of ribs H 46  and H 47  over the intercostal incision N 7 . Note that wing members N 1150  come to bear on the tops of ribs H 46  and H 47 , preventing rib-protecting port N 1200  from passing completely through the intercostal incision. As before, an important feature of rib-protecting port N 1200  is that the minimum dimension across any perimeter (i.e. the shortest dimension from side-to-side in any direction) should be greater than the distance separating ribs H 46  and H 47 . This prevents rib-protecting port N 1200  from inadvertently being pushed between the ribs and into the pleural space. In Step 2, a downward force N 1299  is applied at the uppermost part of rib-protecting port N 1200  (here shown as acting through right end joint N 1231 ). Downward force N 1299  pushes wing members N 1250  against ribs H 46  and H 47 , applying a moment about right end joint N 1231  that makes right cranial endplate N 1211  rotate clockwise and left cranial endplate N 1221  rotate counter-clockwise. In Steps 3 and 4, downward force N 1299  continues to be applied, causing the endplates N 1211  and N 1221  to rotate further, spreading wing members N 1250  further apart and causing rib-engaging hooks N 1240  to extend under the ribs H 46  and H 47 . Finally, in Step 5, rib-protecting port N 1200  is fully deployed, with rib-engaging hooks N 1240  having contacted ribs H 46  and H 47  at rib contact surfaces N 1244  and endplates N 1211  and N 1221  having contacted ribs H 46  and H 47  at rib contact surfaces N 1245 . Furthermore, gaps N 1280  have been formed adjacent to the ribs H 46  and H 47 . If end joints N 1231  and N 1235  evert slightly, and then reach rotational stops (not shown), with the center of end joints N 1231  and N 1235  more in the pleural direction than rib contact surfaces N 1245 , then the deployed orientation shown in Step 5 will be stable because any opposing forces from the ribs at rib contact surfaces N 1245  will force end joints N 1231  and N 1235  against their stops. Alternatively, end joints N 1231  and N 1235  can be ratchets to hold position. 
     One will note that cranial first rib-engaging channel N 1270  is different than caudal second rib-engaging channel N 1270  due to the differing extents (distance from top edge to bottom edge) of cranial barrier member N 1213  and caudal barrier member N 1223 . (Caudal barrier member N 1223  is narrower, with its bottom edge N 1225  sitting above caudal rib H 47 .) This reduced extent of caudal barrier member N 1223  is required to prevent interference between caudal barrier member N 1223  and cranial barrier member N 1221  when rib-protecting port N 1200  rotates from the folded configuration of Step 1 to the deployed configuration of Step 5 in  FIG. 156C . Note that any number of complimentary shapes can be used for cranial barrier member N 1213  and caudal barrier member N 1223 . 
     The cranial margin of caudal rib H 47  is, thus, exposed with this design of rib-protecting port N 1200 , as shown in  FIG. 156D . Rib-protecting port N 1200 , nevertheless still protects the neurovascular bundle H 48  of cranial rib H 46 . Three surgical instruments N 1291 , N 1292 , and N 1293  are shown in surgical access opening N 1295 . Surgical instrument N 1291  is totally blocked from touching the cranial rib H 46  and the caudal rib H 47 . Surgical instrument N 1293 , however, while still being blocked from touching cranial rib H 46 , it can contact caudal rib H 47 . A surgeon would have to exhibit greater caution with an instrument oriented like surgical instrument N 1293 ; however, surgeons usually only need access in one direction, and an appropriate rib-protecting port N 1200  can be selected (e.g. one with full coverage for the caudal rib H 47 , instead). 
     Importantly, when deployed and locked in place, rib-protecting port N 1200 , like rib-protecting port N 1100 , is firmly affixed to each rib, and moves with each rib if the forces on the chassis are sufficiently large to displace a rib—something that occurs regularly in thoracoscopic surgery, especially when the endoscopic camera impinges on the rib as a surgeon attempts to look around inside the chest. This means that rib-protecting port is well-positioned on the ribs despite the forces applied and maintain protection when large forces, which carry the greatest risk of trauma, are applied. 
       FIGS. 156A through 156D  illustrate that rib-protecting port N 1200  is aligned generally along the longitudinal axis of both ribs H 46  and H 47 . Furthermore, bottom edge N 1162  (and thus barrier members N 1213  and N 1223 ) can be of different lengths to accommodate different incision sizes. For example, the lengths of bottom edges N 1215  and N 1225  can be different for different versions of rib-protecting port N 1200 . For example, it can range from 2 mm to 15 mm long for the camera port for thoracoscopic surgery (fitting into a very small incision), or it can be 100 mm long for a utility incision for thoracoscopic surgery (permitting access for larger instruments or for multiple instruments simultaneously). 
     Rib-protecting ports, like N 1100  and N 1200 , require force (e.g. force N 1299  in  FIG. 156C ) to reconfigure from folded to deployed. Insertion and application of this force can be performed by hand, or an instrument can be used. If an instrument, then a special instrument designed for the purpose can be used, or standard surgical forceps and hemostats can suffice also. 
       FIGS. 157A and 157B  show another rib-protecting port N 1300  that is similar to rib-protecting port N 1200  but that scissors to reconfigure. Rib-protecting port N 1300  comprises a chassis of two main parts: a first port half N 1301  and a second port half N 1302 . First port half N 1301  and second port half N 1302  are monoliths in this example. Because of the scissoring action, first and second rib-engaging channels N 1350  and N 1351  are formed in a different manner, using the rib-engaging foot of one monolith in coordination with the wing member and barrier member of the other monolith. Thus first rib-engaging channel N 1350  is formed from the wing member N 1311  and barrier member N 1331  of first port half N 1301  but the rib-engaging foot N 1322  of the second port half N 1302 . Similarly, second rib-engaging channel N 1351  is formed from the wing member N 1312  and barrier member N 1332  of second port half N 1302  but the rib-engaging foot N 1312  of the first port half N 1301 . 
       FIGS. 158A and 158B  show how a slant on the component touching the margin of a rib can create a force that self-seats a rib-protecting port. (This is similar to the situation presented in  FIGS. 147A and 147B .) Rib-protecting ports like N 1100 , N 1200 , and N 1300  can be self-seating if a force is generated during deployment that pushes on the margins H 50  and H 54  of the ribs H 46  and H 47 .  FIG. 158A  shows opposing rib-engaging channels, first rib-engaging channel N 1401  and second rib-engaging channel N 1402  engaged with the cranial rib H 46  and the caudal rib H 47 , respectively. Rib-engaging channels N 1401  and N 1402  are parts of a chassis N 1403  that transmits force from one rib-engaging channel to the other. Thus, a force applied to one rib is the about same as the force applied to the other rib. When a force is applied to a rib, a reaction force is also applied to the rib-engaging channel. In  FIG. 158A , a force applied at first rib-contacting surface N 1410  produces an upward component N 1420  on each rib-engaging channel N 1401  and N 1402  that causes rib-protecting port N 1400  to rise upward (away from the pleural cavity) until, as shown in  FIG. 158B , rib-engaging channels contact the ribs H 46  and H 47  at second rib-contacting surfaces N 1411  and produce a net downward force N 1421  of equal magnitude to upward component N 1420 . If rib-protecting port N 1400  is initially mis-aligned such that, for example, first rib-engaging channel N 1401  makes contact at rib-contacting surface N 1411  first, then the unbalance upward force N 1420  on second rib-engaging channel N 1402  will cause rib-protecting port N 1400  to rotate, bringing second rib-engaging channel N 1402  into contact with caudal rib H 47  at rib-contacting surface N 1411 . Again, this means that rib-protecting port N 1400  can be self-seating. The force applied by a rib-protecting port can be just sufficient to develop seating and anchoring behaviors without noticeably spreading the ribs. Further, the force can be quite small if the shape and size of the components of a rib-protecting port are designed to fit the shape and size of the ribs. This self-seating behavior will occur whenever the pleural extent of a first rib-contacting surface (the side of first rib-contacting surface closer to the pleural cavity N 20 ) of a rib-engaging channel or of a rib-engaging hook is further from the rib than the skin extent of first rib-contacting surface (the side of first rib-contacting surface furthest from the pleural cavity). 
       FIG. 159  illustrates the aperture available to a surgical instrument inserted through the surgical access opening of a rib-protecting port N 1500 . Presented here are the components forming a first rib-engaging channel N 1511  apposed to cranial rib H 46  and a second rib-engaging channel N 1512  apposed to caudal rib H 47 . Barrier members N 1520  of first rib engaging channel N 1511  and of second rib engaging channel N 1512 , have incision-facing surfaces N 1521  that define the aperture of surgical access opening N 1530 . The aperture of the surgical access opening N 1530  is defined here as the half-angle N 1531  of the limits of rotation for a surgical instrument N 1599 , shown here at its clockwise most position and counter-clockwise most position. The aperture N 1531  increases as (a) the diameter N 1540  of the instrument N 1599  decreases, (b) as the height N 1541  of barrier member N 1520  decreases, and (c) as the width N 1542  of surgical access opening N 1530 . One advantageous relationship has the height N 1541  less than or equal to the intercostal spacing, which is the distance separating the caudal rib H 47  from the cranial rib H 46 . Alternately, given the variability of sizes of human ribs, the barrier member should have a height N 1451  greater than 1 mm and less than 20 mm or greater than 0.5 mm and less than 10 mm. Surgeons want the largest possible aperture N 1531 , allowing the greatest range of motion, thus it is advantageous to have a rib-retracting port N 1500  with the largest width N 1542  of the surgical aperture N 1530  that a surgeon is willing to stretch the intercostal incision and the smallest height N 1541  of barrier member N 1520  that still manages to protect ribs H 46  and H 47 . 
       FIGS. 160A and 160B  illustrate how flexible straps can be attached to a rib-protecting port, thereby adding new functionality. Rib-protecting port N 1200  is used as an example, any rib-protecting port can be equipped with straps.  FIG. 160A  shows an oblique view of a rib-protecting port N 1200  to which flexible straps N 1600  have been affixed. Each flexible strap N 1600  has a first strap end N 1601  and a second strap end N 1602 . First strap end N 1601  of one flexible strap N 1600  is affixed to one barrier member, e.g. cranial barrier member N 1213 . First strap end N 1601  of another flexible strap N 1600  is affixed to the other barrier member, e.g. caudal barrier member N 1223 . Second strap end N 1602  of each flexible strap N 1600  has an adhesive patch N 1620 . After rib-protecting port N 1200  is placed into intercostal incision N 7  and affixed to ribs H 46  and H 47 , the second strap ends N 1602  of the flexible straps N 1600  will stick up through the skin H 42 . Straps N 1600  can then be pulled upward, out of the skin H 42 , placing tension onto straps N 1600  and then pulled away from the thoracic wall incision N 5  to hold thoracic wall incision N 5  open as shown in  FIG. 160B . Adhesive patches N 1620  can then be used to maintain tension in flexible straps N 1600  by attaching second strap end N 1602  of flexible strap N 1600  to the patient&#39;s skin H 42  or to a surgical drape. In such an implementation, flexible straps N 1600  accomplish several important tasks:
         they hold thoracic incision N 5  open, facilitating the introduction of surgical instruments into the incision, which is important for the long, slender instruments used in thoracoscopic surgeries which can be awkward to insert into a small incision;   they protect the freshly cut tissues on the sides of the thoracic incision N 5  from abrasion by the surgical instruments;   they act as a barrier to infectious agents, preventing infectious agents from reaching the freshly cut tissues on the sides of the thoracic incision N 5 ;   if they are vapor impermeable, then they prevent desiccation of freshly cut tissues on the sides of the thoracic incision N 5 ;   they further stabilize rib-protecting port N 1200  (for example by providing an alternative and/or enhanced upward force contributing to the seating of the hooks) and also facilitate removal of rib-protecting port N 1200  from thoracic incision N 5 .
 
 FIG. 161  illustrates how a rib-protecting port N 1200  can act as the platform for an instrument mount N 1700  that holds a surgical instrument N 1599 . As described above in  FIGS. 158A, 158B, 160A and 160B , rib-protecting port N 1200  can be firmly engaged with ribs H 46  and H 47 . Rib-protecting port N 1200  can, thus, act as a platform to stabilize a surgical instrument, such as an endoscopic camera. Stabilization is improved with the addition of instrument mount N 1700  that is firmly fixed to rib-protecting port N 1200 , shown as attached to cranial barrier member N 1213 . Instrument mount N 1700  can be one of any type known in the art, like a ball joint, a friction joint, an articulating joint, etc. Engagement of surgical instrument N 1599  by instrument mount N 1700  can be via a simple pass-through, as shown here, or a clip or other clamping/holding mechanism known in the art.
       

     The embodiments set forth herein are examples and are not intended to encompass the entirety of the invention. Many modifications and embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teaching presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are used herein, they are used in a generic and descriptive sense only and not for the purposes of limitation. 
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