Patent Publication Number: US-10765488-B2

Title: Rod contouring apparatus for percutaneous pedicle screw extension

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/599,846, filed on May 19, 2017, which is a continuation of U.S. application Ser. No. 15/000,642, filed on Jan. 19, 2016, now U.S. Pat. No. 9,655,685, which is a continuation of U.S. application Ser. No. 12/316,637, filed on Dec. 15, 2008, now U.S. Pat. No. 9,247,977, which is a divisional of U.S. application Ser. No. 11/526,785, filed on Sep. 25, 2006, now U.S. Pat. No. 8,894,655, and claims the benefit of the filing date of U.S. Provisional Application No. 60/765,606, filed Feb. 6, 2006, the disclosures of which are hereby incorporated herein by reference. 
     This application relates to U.S. application Ser. No. 10/868,075, entitled “Methods and Devices For Improving Percutaneous Access In Minimally Invasive Surgeries” and filed on Jun. 15, 2004, now U.S. Pat. No. 7,955,355; U.S. application Ser. No. 11/178,035, entitled “System and Method For Orthopedic Implant Configuration” and filed on Jul. 8, 2005, now U.S. Pat. No. 8,177,817; U.S. application Ser. No. 11/202,487, entitled “System and Method For Spinal Implant Placement” and filed on Aug. 12, 2005, now U.S. Pat. No. 8,002,798; and International Application No. PCT/US2004/036640 and filed on Nov. 4, 2004, published as International Publication Number WO 2005/072081, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to methods and devices for improving percutaneous access in minimally invasive surgeries, and more particularly to methods and devices that provide a template for the extracorporeal selection and contouring of connecting devices based on landmark locations within the body, and the percutaneous transfer of connecting devices and instruments, particularly such selected or contoured devices, within one or more access channels to positions defined by particular locations within the body. 
     It is well known that traditional surgical procedures in locations deep within a patient&#39;s body require a long incision, extensive muscle stripping, prolonged retraction of muscles for visualization, and denervation and devascularization of the adjacent tissue. These procedures result in extensive tissue traumatization and consequently in prolonged recovery time, risk of infections, high hospitalization costs, pain that can be more severe than the pain due to the initial ailment, and in some cases permanent scarring. In minimally invasive surgical procedures, portals are used to access the locations deep in the patient&#39;s body. The use of portals rather than a long incision causes less trauma to the adjacent tissue, reduces the recovery time and pain and may be performed in some case under only local anesthesia. The avoidance of general anesthesia reduces post-operative recovery time and the risk of complications. 
     Minimally invasive surgical procedures are especially desirable for spine surgeries because spine pathologies are located deep within the body without clear muscle planes and there is danger of damaging the adjacent neural and vascular tissues. In treating the majority of spinal pathologies, the spinal muscles are stripped from the bony elements of the spine followed by laminectomy to expose the dura, the nerve roots, and the discs. The incision has to be wide enough and the tissues have to be retracted to maintain a channel from the skin to the floor of the spinal canal that will allow direct visualization. This is similar to an open surgery approach to the knee to expose the menisci versus minimally invasive alternatives such as an arthroscopy which uses 1 centimeter portals under illuminated magnification which results in improved visualization, reduced postoperative knee pain, recovery time, and the destruction of healthy tissue. The destruction to the spinal structures is even more extensive during fusion procedures, which require more lateral tissue dissection and exposure to access the transverse processes and pedicles for placement of pedicle screws, rod constructs for stability, and bone graft under direct vision. 
     Furthermore, in spine fusion procedures, connecting elements, such as rods, plates or wires are placed and fixed between two or more locations of the spine. Placement of these connecting elements requires open surgery, which is currently one of the major limitations of other percutaneous cannula access methodologies. Accordingly there is a need for inserting and placing these connecting elements between two or more separate spinal locations without performing open surgery. 
     A wide variety of orthopedic implants exist. Such implants are typically anchored to bones within the body. Every person has different bone structure; accordingly, implants must vary considerably in geometry to meet the needs of a broad range of patients. Connecting elements are an example of an orthopedic implant that often must be specially configured, adjusted, or selected based on the internal anatomical configuration of the patient&#39;s bone structure. Although visualization methods such as X-Rays and fluoroscopy can be utilized to help determine bone geometry, contact with the bones must often be made in order to provide a sufficiently accurate measurement of bony landmarks. 
     Trial fittings of an implant within the body are often required. In open treatment procedures, access to the operation site is typically sufficiently large to allow fitting and adjustment of implants such as connecting devices within the body. This is not feasible in minimally invasive surgical procedures because the surgeon has neither the physical access nor visibility required to test and adjust the device in situ. 
     According to new minimally invasive surgical (MIS) procedures, many orthopedic implants can be secured to bone through relatively small incisions. Unfortunately, if a larger incision must be made to permit bone measurement and implant selection or configuration, most of the beneficial effects of the MIS implantation procedure will be lost. Accordingly, there is a need in the art for bony landmark measurement and implant selection or configuration methods that can be carried out through small incisions. Such methods should be relatively simple and quick to perform, with comparatively simple instrumentation. 
     Furthermore, there is a need to provide a system, apparatus and method that solves the combined problems of using minimally invasive surgery for inserting and fastening implants such as connecting elements to bone locations such as spinal vertebrae and also allows configuration of the implants based on internal landmarks locations without performing open surgery. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features apparatus for use as connectable portals in percutaneous minimally invasive surgery performed within a patient&#39;s body. The apparatus includes a first elongated hollow tube having a proximal end and a distal end and defining a first working channel between the proximal end and the distal end when placed within the body cavity and a second working channel transverse to said first working channel comprising two slots along the length of the hollow tube. 
     In another aspect, the invention features at least a second elongated hollow tube having a proximal end and a distal end and defining a first working channel between the proximal end and the distal end when placed within the body cavity and a second working channel transverse to said first working channel comprising two slots along the length of the hollow tube. 
     In another aspect of the invention the first and second tubes are sized for delivering carrier devices, surgical instruments, medical devices, fixation devices, vertebral disc replacement devices, interbody devices, fixation tools, connecting devices, connecting tools, tissue, grafting material, or illumination devices, to a pathology location within the body cavity through either the first or second working channels. The surgical instruments may be scissors, scalpels, saws, drills, tissue dilators, biting and grabbing instruments, curettes, knot tying, or cautery. The fixation devices may be screws, hooks, loops, pins, nuts, washers, wires, sutures, or staples. The fixation tools may be screw drivers, pushers, holders, wrenches, staplers, or knot tiers. The connecting devices may be plates, rods, wires, vertebral disc replacements, interbody fusion devices, or articulating versions thereof. The connecting tools may be connecting tools carriers, pushers, screw drivers, and wrenches. The illumination devices may be light sources, fiber optic cables, infrared detectors, magnification devices, and microscopes. The tubes may further comprise a mechanism for engaging and disengaging a fixation device. The tubes may further comprise separable components that can be assembled and disassembled while at least partially within the body. 
     In an embodiment of the invention the first tube and second tube may comprise appendages at the distal end configured to releasably engage features of the fixation device and secure to the fixation device. 
     In an aspect of the method of the invention, the first tube comprises a first opening extending the entire width of the first tube and being located in a portion of the first tube within the first body cavity and wherein a cutting tool is used to incise tissue around the first body cavity through the first opening. The method may also include inserting a second elongated hollow tube within a second body cavity of the patient adjacent to the first body cavity, wherein the second tube has a proximal end and a distal end and defining a second working channel between the proximal end and the distal end when placed within the second body cavity. The method also includes incising tissue between the first body cavity and the second body cavity, thereby forming a path extending from the first body cavity to the second body cavity, then inserting a connecting device into or through the first tube and then transferring the connecting device from the first tube to the second tube through the path. The method also includes attaching a first end of the connecting device to a first bone within the first body cavity via a first fixation device and attaching a second end of the connecting device to a second bone within the second body cavity via a second fixation device. The first bone within the first body cavity may be a first vertebra, and the second bone within the second body cavity may be a second vertebra. The first and second fixation devices may be screws, hooks, loops, pins, nuts, washers, wires, sutures, or staples and in a preferred embodiment is a multiaxial pedicle screw. The connecting device may be plates, rods, wires or articulating versions thereof and in a preferred embodiment is a rod. The tissue between the first and the second body cavities may be a lumbodorsal fascia and the path is located either above or below the lumbodorsal fascia. The first and second tubes are sized for delivering carrier devices, surgical instruments, fixation devices, fixation tools, connecting devices, connecting tools, tissue, grafting material, or illumination devices, to a pathology location within the body cavity. The method may also include inserting additional elongated tubes within additional body cavities of the patient adjacent to the first and second body cavities. The method may also include making a second incision on a second location of the patient&#39;s skin, then advancing a second guide wire through the second incision, through tissue underlying the second location and into a second underlying bone, then forming the second body cavity around the second guide wire and finally removing the first and second tubes from the first and second body cavities and closing the first and the second incisions. 
     The present invention has applications in a wide range of surgical procedures, and in particular in spinal procedures such as laminotomy, laminectomy, foramenotomy, facetectomy and discectomy, fusions or disc replacements using an anterior, posterior, postero-lateral, or a lateral approach to the disc space, facet, laminas, pedicles, or transverse processes. The devices and instruments of the present invention have application to surgical techniques that permit each of these several types of surgical procedures to be performed via a single or multiple sequential working channels. The present invention also has application to surgical techniques for preparing a disc space for insertion of an implant into the disc space. 
     In another aspect, the invention performs a function similar to a surgical navigation system with simple manual instruments that create a mechanical analog of the body target sites outside the body. This invention further provides a convenient template for the shaping of an implantable device to mate with target body sites without requiring a full surgical exposure to access the target body sites. This invention also provides a suitable level of positional control of the template to allow the surgeon discretion in positioning the template and shaping the implantable device. 
     In a still further aspect the invention provides an apparatus and a method for creating an extracorporeal set of reference features that replicates the spatial positioning of a set of target sites located inside the body, outside of the body. The target sites are preferably anchor sites for an implantable fixation device, but could be preferred locations for delivering therapeutic agents or anatomic locations. 
     In one embodiment, the invention creates extracorporeal references of the preferred anchor sites within the body for a fixation member to attach to bone anchors applied to the spine. This is accomplished by attaching elongate members to each bone anchors. Typically the members are attached to a first portion of a bone anchor that articulates with respect to a second portion of the bone anchor that is anchored to the bone. In the case of the application of the invention to a spine surgery, the anchors can be a pedicle screw or a pedicle hook for example. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects and advantages of the invention will be apparent from the following description of the preferred embodiments, the drawings and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present invention will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. 
         FIG. 1  is a perspective view of two adjacent vertebrae of a spine, with guide wires implanted in the pedicles of the right side. 
         FIG. 2  is a perspective view of three guide wires in isolation, positioned as though implanted in the pedicles of the right sides of three adjacent vertebrae. 
         FIG. 3  is a perspective view of the guide wires of  FIG. 2 , with dilators advanced along the guide wires to dilate surrounding tissue. 
         FIG. 4  is a perspective view of the guide wires and dilators of  FIG. 3 , with cannulas positioned around the dilators. 
         FIG. 5  is a view as in  FIG. 4  with the dilators removed. 
         FIG. 6  is a perspective view of the guide wires and cannulas of  FIG. 5 , with pedicle screws implanted in a pedicle along a guide wire through the use of an insertion tool. 
         FIGS. 7 and 7A  are perspective views of guide wires, pedicle screws and an insertion tool as in  FIG. 6 , with retractor blades having distal ends engaged with the pedicle screws and retained in position by abutment members to form a slotted cannula. 
         FIG. 8  is a perspective view of the retractor blades, abutment members and pedicle screws of  FIG. 7 , with trough simulation members used to form assemblies for contouring a fixation member attached to the distal portion of the retractor blades. 
         FIG. 9  is a perspective view of the assemblies of  FIG. 8 , with links bridging between the trough simulation members to retain the assemblies in an axially parallel relationship. 
         FIG. 10  is a perspective view of the cannulas, pedicle screws, trough simulation members, and bridges of  FIG. 9 , with a fixation member in the form of a rod seated in troughs of the simulation members for contouring. 
         FIG. 11  shows the contoured rod being percutaneously guided through the retractor blades toward the pedicle screws. 
         FIG. 12  is a perspective view of the contoured rod, seated in the pedicle screws and being fastened by bolts using a driving tool. 
         FIG. 13  is a perspective view as in  FIG. 14  of the contoured rod fastened to the pedicle screws. 
         FIG. 14  is a perspective view as in  FIG. 15  of the contoured rod fastened to the pedicle screws after removal of the retractor blades. 
         FIG. 15  is a perspective view as in  FIG. 9  except that the trough simulation members are not used and the links engage the retractor blades to retain the assemblies in an axially parallel relationship. 
         FIG. 16  is a perspective view as in  FIG. 10  showing that the distal portion of the retractor blades may be used in place of the troughs for contouring the rod. 
         FIG. 17  is a perspective view of the assemblies as in  FIG. 8 , with extenders that replace the trough simulation members and pass through the slotted cannulas to provide a contouring feature. 
         FIG. 18  is a perspective view of the cannulas, pedicle screws, extenders, and bridges of  FIG. 17 , with a fixation member in the form of a rod seated in troughs of the simulation members for contouring. 
         FIG. 19  is a perspective view as in  FIG. 18  of the contoured rod fastened to the pedicle screws. 
         FIG. 20  is a perspective view as in  FIG. 19  of the contoured rod fastened to the pedicle screws after removal of the retractor blades. 
     
    
    
     DETAILED DESCRIPTION 
     In this application, an “anatomic point” is a location within the body. An anatomic point need not be located on any specific anatomic structure. When applied to anatomy, “proximal” refers to a position relatively closer to the center of the body, and “distal” refers to a position relatively further from the center of the body. However, when referred to a tool or similar implement, “proximal” refers to a portion relatively nearer the operator of the tool or similar implement, and “distal” refers to a portion relatively further from the operator. 
     The phrase “spatial transformation” refers to any mathematical procedure in which one or more coordinates can be transformed in a manner that permits the original coordinates to be determined based on the results of the transformation. Accordingly, a spatial transformation may involve any combination of translation and rotation of the original coordinates, as long as the transformation can be analytically reversed to permit the original coordinates to be obtained. A “translational spatial transformation” is a spatial transformation in which the original coordinates are all uniformly translated along the same vector. 
     The term “mate” refers to any type of connection in which cooperating features engage each other to restrict relative motion of the mating parts. The term “couple” is not limited to fixed attachment, but also includes sliding attachment and the like. The term “receive” does not require one item to completely capture another; rather, one item receives another if the first item engages the second item in a manner that restricts relative motion of the items. The term “substantially parallel” means that a range of adjustment is available for limited relative movement of the assemblies, as the surgeon requires, to position the assemblies and also encompasses normal mechanical tolerances and deflections that create variance from geometrically parallel assemblies. 
     Referring to  FIG. 1 , a perspective view illustrates a portion of a spine  10 .  FIG. 1  illustrates only the bony structures; accordingly, ligaments, cartilage, and other soft tissues are omitted for clarity. The spine  10  has a cephalad direction  12 , a caudal direction  14 , an anterior direction  16 , a posterior direction  18 , and a medial/lateral axis  20 , all of which are oriented as shown by the arrows bearing the same reference numerals. In this application, “left” and “right” are used with reference to a posterior view, i.e., a view from behind the spine  10 . “Medial” refers to a position or orientation toward a sagittal plane (i.e., plane of symmetry that separates left and right sides from each other) of the spine  10 , and “lateral” refers to a position or orientation relatively further from the sagittal plane. 
     As shown, the portion of the spine  10  illustrated in  FIG. 1  includes a first vertebra  24 , which may be the L5 (Fifth Lumbar) vertebra of a patient, and a second vertebra  26 , which may be the L4 (Fourth Lumbar) vertebra of the patient. The systems and methods may be applicable to any vertebra or vertebrae of the spine  10  and/or the sacrum (not shown). In this application, the term “vertebra” may be broadly interpreted to include the sacrum. 
     As shown, the first vertebra  24  has a body  28  with a generally disc-like shape and two pedicles  30  that extend posteriorly from the body  28 . A posterior arch, or lamina  32 , extends between the posterior ends of the pedicles  30  to couple the pedicles  30  together. The first vertebra  24  also has a pair of transverse processes  34  that extend laterally from the pedicles  30  generally along the medial/lateral axis  20 , and a spinous process  36  that extends from the lamina  32  along the posterior direction  18 . 
     Similarly, the second vertebra  26  has a body  48  from which two pedicles  50  extend posteriorly. A posterior arch, or lamina  52 , extends between the posterior ends of the pedicles  50  to couple the pedicles  50  together. The second vertebra  26  also has a pair of transverse processes  54 , each of which extends from the corresponding pedicle  50  generally along the medial/lateral axis  20 , and a spinous process  56  that extends from the lamina  52  along the posterior direction  18 . 
     The vertebrae  24 ,  26  and/or the intervertebral disc (not shown) between them, may be damaged or diseased in some manner that makes it desirable to secure the vertebrae  24 ,  26  together in a manner that prevents relative motion between them. Accordingly, posterior spinal fusion may be employed to secure the pedicles  30  and  50  together in a geometrical relationship that produces a fused spinal section with an appropriate bio-mechanical function. In order to allow the surgeon to provide a proper geometrical relationship between vertebrae, multi-axial pedicle screws and contoured rods connecting the screws have become the gold standard for spinal fusion hardware.  FIGS. 1 through 16  illustrate an apparatus and method of configuring and installing a posterior spinal fusion system.  FIGS. 17 through 20  illustrate an alternate embodiment for contouring the fixation member. 
     As further illustrated in  FIG. 1 , a first guide wire  70  has been inserted into the right-side pedicle  30  of the first vertebra  24 , and a second guide wire  72  has been inserted into the right-side pedicle  50  of the second vertebra  26 . The guide wires  70 ,  72  pass through the saddle points  42 ,  62 , respectively, of the pedicles  30 ,  50 . Each of the guide wires  70 ,  72  has a proximal end  74  and a distal end  76 . As shown, the proximal ends  74  are exposed, and the distal ends  76  are implanted in the pedicles  30 ,  50 . The distal ends  76  may be implanted by methods known in the surgical arts. 
     Referring to  FIG. 2 , a perspective view illustrates the first and second guide wires  70 ,  72  of  FIG. 1 , with the vertebrae  24 ,  26  not shown for clarity. The vertebrae are not shown for clarity in the subsequent  FIGS. 3-20  also. A third guide wire  78  is also shown. The third guide wire  78  is positioned adjacent to the first and second guide wires  70 ,  72  as though the third guide wire  78  were implanted in the right-hand pedicle of a vertebra (not shown) directly superior to the second vertebra  26 . Accordingly, the method of  FIGS. 1 through 20  may be used to secure together vertebrae on multiple levels, not just two adjacent vertebrae. 
     Referring to  FIG. 3 , a perspective view illustrates the guide wires  70 ,  72 ,  78 , in conjunction with a first dilator  80 , a second dilator  82 , and a third dilator  88 . Each of the dilators  80 ,  82 ,  88  has a proximal end  92  and a distal end  94 . The proximal ends  92  may be shaped for gripping by hand, or for attachment to a handle or the like. The distal ends  94  are rounded to permit relatively gentle spreading of tissues surrounding the guide wires  70 ,  72 ,  78  by the dilators  80 ,  82 ,  88 . 
     Each of the dilators  80 ,  82 ,  88  has a bore sized to receive the proximal end  74  of the corresponding guide wire  70 ,  72 , or  78 , so that the dilators  80 ,  82 ,  88  are able to slide along the guide wires  70 ,  72 ,  78  toward the distal ends  74 , thereby spreading the tissues away from the guide wires  70 ,  72 ,  78 . As an alternative to the guide wires  70 ,  72 ,  78  and the dilators  80 ,  82 ,  88 , a variety of other guiding devices and/or dilation devices may be used within the scope of the present invention. 
     Referring to  FIG. 4 , a perspective view illustrates the guide wires  70 ,  72 ,  78  and dilators  80 ,  82 ,  88 , with the addition of a first cannula  100 , a second cannula  102 , and a third cannula  108 . Each of the cannulas  143  has a proximal end  112 , a distal end  114 , with a bore passing between the proximal and distal ends  112 ,  114 . Each proximal end  112  has a port  116  in communication with the bore, and a tab  118  that may facilitate manipulation or securement of the corresponding cannula  100 ,  102 , or  108 . 
     Each distal end  114  has a taper  122  that provides a reduction in the diameter of the cannula  100 ,  102 , or  108  toward the distal end  114 . 
     The cannulas  143  are inserted around the guide wires  70 ,  72 ,  78 . The cannulas  143  may be placed by withdrawing dilators  80 ,  82 ,  88 , inserting the cannulas  143  around the proximal ends  74  of the guide wires  70 ,  72 ,  78 , inserting the distal ends  94  of the dilators  80 ,  82 ,  88  into the ports  116  of the proximal end  112  of the cannulas  143 , and then advancing the dilators  80 ,  82 ,  88  along the guide wires  70 ,  72 ,  78  to urge the cannulas  143  toward the distal ends  76  of the guide wires  70 ,  72 ,  78 , into the dilated tissue. 
     According to one alternative method, the dilators  80 ,  82 ,  88  are removed to permit placement of the cannulas  143 , and are not re-inserted. According to other alternative embodiments, cannulas (not shown) may be modular, or may have dilatable distal ends that enable placement of the cannulas around the dilators  80 ,  82 ,  88 , so that the dilators  80 ,  82 ,  88  need not be removed from the guide wires  70 ,  72 ,  78  until the cannulas are properly positioned. The present invention is not limited to use of cannulas like those of  FIG. 4 ; rather, any of a variety of cannulas may be used. 
     Referring to  FIG. 5 , a perspective view illustrates the guide wires  70 ,  72 ,  78  and cannulas  143 , after the dilators  80 ,  82 ,  88  have been removed. 
       FIG. 6  is a perspective view showing the addition of the first of three cannulated connection elements  140  installed through the cannula  100  and into the vertebra using an insertion tool  170 . 
     The connection elements may be fixation members designed to anchor a rod to the first vertebra  24 , the second vertebra  26 , and the third vertebra (not shown in  FIG. 6 ). More precisely, the connection elements may be pedicle screws  140 ,  142 , and  148  implantable in vertebral pedicles, as shown in  FIG. 7 . 
     The pedicle screws  140 ,  142 ,  148  may be designed to provide poly-axial coupling to the associated pedicles. Each of the pedicle screws  140 ,  142 ,  148  has a cage  152  shaped to receive a rod and a screw  154  that passes through an aperture (not visible) of the cage  152  in such a manner that the screw  154  is able to extend from the cage  152  along a plurality of relative orientations. Thus, after the screw  154  has been implanted in a pedicle, the orientation of the cage  152  with respect to the screw  154  can still be altered. Each of the screws  154  has a lumen passing along the axis of the screw  154  so that the screws  154  can slide along the guide wires  70 ,  72 ,  78  for accurate implantation in the pedicles. 
     As seen in  FIG. 8 , each cage  152  has two arms  156  that extend generally away from the screw  154  and define a trough  158  through which a rod (not shown in  FIG. 5 ) can pass. The closed end of the trough  158  is rounded in a manner that corresponds to the radius of the rod to be retained within the cage  152  to facilitate secure retention of the rod. The inward-facing surfaces of the arms  156  may be threaded to enable the arms  156  to receive a nut (shown in  FIG. 14 ). Tightening of the nut then presses the rod against the head  154  (shown in  FIG. 14 ) of the screw  154  to keep the rod in place within the slot  158  and to lock the orientation of the screw  154  with respect to the cage  152 . 
     The pedicle screws  140 ,  142 ,  148  represent only one of many types of connection elements that may be used in connection with the present invention. A variety of known devices may be used to secure a rod to a plurality of vertebra to provide posterior fusion. 
     Upon implantation in the pedicles, the pedicle screws  140 ,  142 ,  148  are positioned such that a first anatomic point  164 , a second anatomic point  166 , and a third anatomic point  168  are within the troughs  158  of the cages  152  of the first pedicle screw  140 , the second pedicle screw  142 , and the third pedicle screw  148 , respectively. Upon installation of the rod in the troughs, the axis of the rod is to pass through the anatomic points  164 ,  166 ,  168 . 
     Referring back to  FIG. 7 , seen extending from the connecting element  140 , is a slotted cannula  143  and an abutment member  145 . The cannula  143  is used to maintain access to the connecting element  140  after it has been implanted in the pedicle in a manner that facilitates percutaneous placement of the rod and attachment of the rod to the connecting element  140 . The abutment member  144  helps to hold the cannula  143  together and keep it secured to the connecting element  140  in a manner that will be described subsequently. Additional cannulas  143  can be attached to pedicle screws  142  and  148 . 
     Prior to the installation of the connecting element  140  shown in  FIG. 6 , the slotted cannula  143  is assembled to the connecting element  140  as visible in  FIG. 7 . Upon assembly, the cannula  143  will have a proximal end  191  and a distal end  192 . The cannula  143  may be dimensioned such that the proximal end  190  protrudes above the skin while the distal end  192  is securable to the cage  152  and is insertable through the skin along with the cage  152 . The cannula  143  includes a first retractor blade  195  and a second retractor blade  197 , which may be substantially identical to each other. Each of the blades  195 ,  197  has a proximal end corresponding to the proximal end  191  of the cannula  143 , and a distal end corresponding to the distal end  192  of the cannula  143 . 
     The retractor blades are detachably attached to the first portion of the bone anchor as shown in  FIGS. 7 and 7A . Each distal end  192  has a distal tab  202 , and each proximal end  191  has a proximal tab  204 . Each distal tab  202  has a locking ridge  206  that protrudes generally outward, and extends generally circumferentially. Each distal tab  202  is also elongated, with a thin cross section that permits bending toward and away from the axis (not shown) of the cannula. Each proximal tab  204  has bends  208  that cause proximal tab  204  to jut outward, while remaining generally parallel with the remainder of the corresponding blade  195  or  197 . 
     Each of the distal tabs  202  is insertable through the slot  174  of the adjacent arm  172  of the cage  152  when the corresponding blade  195  or  197  is tilted to position the proximal end inward relative to the distal end. Once the distal tabs  202  have passed through the slots  174 , rotation of the blades  195  or  197  back to a position generally parallel to each other, and to the axis of the cage  152 , causes the distal tabs  202  to engage the edge of the slots  174  such that the bends  208  in the tab  202  are unable to slide back through the slots  174 . Thus, the blades  195  and  197  are then in a locked configuration, and cannot be detached from the cage  152 . When they are again moved to the unlocked configuration, i.e., tilted to a position with the proximal ends  191  inward, the retractor blades can be unlocked and detached. 
     As long as the blades  195 ,  197  remain generally parallel to each other, the distal end  192  of the cannula  143  remains secured to the cage  152 . Thus, the distal tabs  202  form a docking element that removably secures the cannula  143  to the connecting element  140 . The abutment member  145  serves to keep the blades  195 ,  197  parallel to each other to keep the cannula  143  in assembled form and to simultaneously keep the cannula  143  secured to the cage  152  by keeping the blades  195 ,  197  from rotating into the unlocked configuration. When the cannula  143  is secured to the cage  152 , the cannula  143  is in its “docked configuration.” When the cannula  143  is removed from the cage  152 , the cannula  143  is in its “undocked configuration.” 
     As shown, the abutment member  145  is generally disc-shaped with a central opening and an open side that provides access to the central opening. The abutment member  145  also has a pair of arcuate slots that extend around opposing portions of the central opening and are sized to surround the first and second blades  195 ,  197  and keep the blades generally parallel to each other, and perpendicular to the abutment member  145 . Thus, the blades  195 ,  197  are unable to pivot to the unlocked configuration when the abutment member  145  is installed to create an assembly and the cannula  143  maintains a generally tubular shape. 
     After the blades  195 ,  197  have been inserted into the arcuate slots, the abutment member  145  may be positioned at any of a range of positions along the cannula  143 . Thus, upon implantation of the pedicle screw  140  in the corresponding pedicle, the abutment member  145  can be positioned abutting the outward-facing surface of the patient&#39;s skin through which the cannula  143  passes. The abutment member  144  helps to stabilize the cannula  143  with respect to the tissues it passes through. 
     Once assembled to the pedicle screw  140 , the cannula  143  has slots  220  extending along its entire longitudinal length, along opposite sides of the cannula  143 . The slots  220  extend to the cage  152 , and are therefore contiguous with the recesses defined in the arms  172  of the cage  152 . Upon installation of the cannula and pedicle screw assembly by using the cannula  100  and tool  170  as shown in  FIG. 6 , the slots  220  will extend along the entire subcutaneous length of the cannula  143  as better seen in  FIG. 8 . Therefore, the rod for connecting the pedicle screws  14 ,  142 ,  148  may be inserted percutaneously through the slots  220  along a direction transverse to the axis of the cannula  143 , and may then be moved through the slots  220  along the anterior direction  16 , directly into the trough of the cage  152 . 
     The pedicle screws  140 ,  142 ,  148 , with or without the assembled cannulas  143 , may be installed in a variety of ways. According to one method, the dilators  80 ,  82 ,  88  are first removed. Then, each of the pedicle screws  140 ,  142 ,  148  is implanted through the use of an insertion tool  170 . The insertion tool  170  has a handle  172  designed to be gripped by a hand, a distal end extending from the handle  172  and engaging the head of each of the screws  154 . Thus, torque applied to the handle can be transmitted to each of the screws  154 . 
     The stem  174  also has a lumen (not shown) sized to fit around each of the guide wires  70 ,  72 ,  78  so that the guide wires  70 ,  72 ,  78  can be used to guide implantation of the screws  154  through the use of the insertion tool  170 . Slots  178  provide access to the lumen for cleaning. 
     Each of the screws  140 ,  142 ,  148  is coupled to the insertion tool  170  by connecting the head  154  of the screws to the distal end  176  of the stem  174 . The insertion tool  170  is then moved to insert the proximal end  74  of the corresponding guide wire  70 ,  72 ,  78  through the lumen of the screw  154  and into the lumen of the stem  174 . The insertion tool  170  is used to insert the pedicle screw  140 ,  142 , or  148  through the corresponding cannula  100 ,  102 , or  108  until the screw  154  contacts the first pedicle  30 , the second pedicle  50 , or the third pedicle. Then, torque and axial pressure are applied to the tool  170  to embed the threads of the screw  154  into the bone. The same method may be used to implant all three of the pedicle screws  140 ,  142 ,  148 . After the pedicle screws  140 ,  142 ,  148  have been implanted, the guide wires  70 ,  72 ,  78  may be removed. 
     As previously discussed, the fixation member in the form of a rod for connecting the pedicle screws  140 ,  142 ,  148  must be configured in three dimensional space to match the geometrical targets  164 ,  166 ,  168 , in order to allow the pedicle screws to constrain the vertebrae in the desired positions once they are fastened to the rods. This requires that the rod be contoured. For better precision in contouring the fixation member, simulation members, such as the trough simulation members  180  with base  182 , stem  184  and troughs  188  as shown in  FIG. 8 , may be attached to the proximal end  191  of the cannula  143  to better replicate the geometry of the geometrical targets  164 ,  166 ,  168 . In conjunction with the cannula  143 , the trough simulation members  180  provide a translational spatial transformation of the troughs  158  of the pedicle screws to the troughs  188  in order to use the troughs  188  as an extracorporeal template to bend the rod. The rod will later be attached in the troughs  158  of the pedicle screws  140 , 142 , 148  attached to the vertebrae within the body of the patient, placing the central axis of the rod in the troughs  158  to match the geometrical targets  164 ,  166 ,  168  at each trough location. 
     As shown in  FIG. 8 , the particular attachment method employed for the trough simulation members  180  attaches the member to each proximal cannula end  191  with a proximal tab  204  releasably engaged with a slot in the trough simulation member. As will be later described, the trough simulation members  180  are but one example of a simulation member and other arrangements that project the positional relationship of the troughs  158  outside the body to achieve a translational spatial transformation, such as rods or cannulas that locate on the troughs  158  directly rather than through a cannula  143  are within the scope of the inventions. Regardless of the configuration of the simulation members, the members must be retained in an approximately parallel axial relationship, be of the same length, and maintain the same alignment of each set of troughs  158  and  188  when using the externally projected troughs  188  to gauge the contouring of the rod in order to provide an accurate translational spatial transformation of the troughs  158  and consequently allow the axis of the contoured rod to correctly fit within the troughs  158  in the geometrical targets  164 ,  166 ,  168 . 
     Referring to  FIG. 9 , a perspective view illustrates the cannulas  143 , pedicle screws  140 ,  142 ,  148 , and the trough simulation members  180  of  FIG. 8 , with the addition of a first link or bridge  250  and a second link or bridge  252 . The bridges  250 ,  252  are used to keep the trough simulation members  180  substantially parallel to each other to constrain the spatial transformation of the anatomic points  164 ,  166 ,  168 . The bridges  250 ,  252  are designed to constrain the trough simulation members  180  only to parallelism. Thus, the bridges  250 ,  252  do not limit relative translation or relative axial rotation of the trough simulation members  180 . 
     Each of the first and second bridges  250 ,  252  has a first slider  254  and a second slider  256 . The first slider  254  of each of the bridges  250 ,  252  has a pair of grooves  258  that face inward. The second slider  256  of each of the bridges  250 ,  252  has a pair of flanges that extend outward into the grooves  258  of the corresponding first slider  254  so that the first and second sliders  254 ,  256  are linearly slidable relative to each other to permit lengthening or shortening of the bridges  250 ,  252 . Each of the sliders  254 ,  256  also has an aperture  262  that fits around the stem  184  of the corresponding trough simulation members  180 . The apertures  262  are sized to fit around the stems  184  with relatively little clearance so that the bridges  250 ,  252  keep the trough simulation members  180  and thus the attached cannulas  143  and cages  152  parallel to each other without restricting relative axial rotation between the stems  184  and the apertures  162 . 
     The bridges  250 ,  252  embody only one of many possible configurations that may be used in connection with the invention. According to one alternative embodiment (not shown), each bridge does not have two sliders, but has two members that are rotatably coupled to each other. Each of the members has an aperture like the apertures  262  of the bridges  250 ,  252 , so that the bridges can permit relatively free relative translation and axial rotation of the trough simulation members  180 , while keeping the trough simulation members  180  parallel to each other. The bridges would simply elongate and contract through the use of rotary motion instead of linear motion. 
     Returning to the configuration of  FIG. 9 , once the bridges  250 ,  252  have been applied, the trough simulation members  180  axially are parallel. The projected points  214 ,  216 ,  218  then mimic the relative positioning of the anatomic points  164 ,  166 ,  168  within the body and each pair of real and simulation troughs corresponding to the anatomic and projected points is in the same relative orientation to achieve a translational spatial transformation. Thus, the trough simulation members  180 , in conjunction with the cannulas  143  and cages  152 , apply a translational spatial transformation to the anatomic points  164 ,  166 ,  168  to move them to a more accessible location without altering their positions relative to each other. Accordingly, a rod contoured such that its axis passes through the projected points  214 ,  216 ,  218  may be installed such that its axis passes through the anatomic points  164 ,  166 ,  168  to properly extend through the cages  152  of the pedicle screws  140 ,  142 ,  148 . An aspect of the invention is that in order for the projected points  214 ,  216 ,  218  to accurately correspond with the relative positioning of the anatomic points  164 ,  166 ,  168  within the body, the various mechanical interfaces of the intervening components between the points, such as the trough simulation members  180 , the cannulas  143  and the cages  152 , must have mechanical interfaces with suitable tolerances, such as axial concentricity and fit, to provide the necessary accuracy. 
     Referring to  FIG. 10 , a perspective view illustrates the cannulas  143 , the pedicle screws  140 ,  142 ,  148 , the trough simulation members  180 , and the bridges  250 ,  252  of  FIG. 9 , with a rod  270  seated in the trough  180  of the trough simulation members  180  for contouring. 
     Due to natural variations in spinal morphology, the troughs  158  of the pedicle screws  140 ,  142 ,  148  may not be arranged in a straight line. Thus, the simulation troughs  180  may not be arranged in a straight line. Consequently, the rod  270  may need to be bent into the proper shape, for example, through the use of tooling such as pliers, French benders, a vice, or the like, so that it will lie properly within simulation trough  180 . The process of deforming the rod  270  to the required shape may be termed “contouring.” 
     Contouring may be carried out by, first, placing the undeformed rod  270  in the troughs  180  to determine how the rod  270  should be deformed to lie properly within the troughs  180 . Then, the rod  270  is deformed, and again placed in the troughs  180  to check the fit. This process is repeated until the rod  270  is shaped to provide an optimal fit with the troughs  180 . 
     In the alternative to contouring, the rod  270  may simply be selected from a kit or the like. For example, such a kit (not shown) may include rods bent at a variety of angles. The troughs  180  could be used to select the proper rod from the kit by placing each rod, in turn, on the troughs  180  until one is identified that has the proper fit. As another alternative, the rod  270  may be custom fabricated, for example, by measuring the relative positions of the troughs  180  and using a CNC procedure to form the rod  270 . 
     After the rod  270  has been configured or selected, the rod  270  and the trough simulation members  180  may be removed from the operating site as shown in  FIG. 11 , leaving the pedicle screws  140 ,  142 ,  148  in place. The cannulas  143  may also be removed at this stage, depending on the method that will be used to implant the rod  270 . The rod  270  may be inserted subcutaneously and placed on the cages  152  by making additional incisions to connect the access passageways provided by the cannulas  143 . Alternatively, MIS (Minimally Invasive Surgical) techniques, as subsequently described, may be used to implant the rod  270  without making additional major incisions, for example, by inserting the rod  270  subcutaneously and subfascially through the slots  220  of the cannulas  143  using a rod holding tool  302 . 
     As shown in  FIGS. 12, 13 and 14 , the rod  270  has now been seated in the troughs  158  of the cages  152  such that its axis passes through the anatomic points  164 ,  166 ,  168 . The use of a persuasion tool to seat a rod in a pedicle screw trough is well known in the art. Nuts  290 ,  292 ,  298  have been rotated into engagement with the inward-facing surfaces of the arms  156  of the cages  152  of the first, second, and third pedicle screws  140 ,  142 ,  148 , respectively. The nuts  290 ,  292 ,  298  have been tightened with a tool  304  to press the rod  270  against the heads of the heads  154  of the pedicle screws  140 ,  142 ,  148 , respectively. Thus, the cages  152  are no longer freely rotatable with respect to the screws  154 , but are instead locked in their current orientations. 
     The pedicle screws  140 ,  142 ,  148  thus cooperate with the rod  270  to restrict relative motion of the vertebrae to form a posterior vertebral fusion system. If desired, a similar system may be implanted in the left-side pedicles through the method set forth previously to provide a bilateral system. Additionally, the present invention is not limited to a three-level fusion system, but may be used to fuse any number of vertebrae together. To fuse more than three vertebrae together, the steps set forth above may simply be repeated for each additional vertebra, and the rod may be placed on four or more rod interfaces for configuration or selection. 
     The foregoing is only one of many methods encompassed within the scope of the present invention. According to one alternative method, the trough simulation members  180  may be omitted entirely from the procedure. Such a method may commence with the steps outlined above in the descriptions of  FIGS. 1 through 7 , but may then include the steps illustrated in  FIGS. 15 and 16 . 
     Referring to  FIGS. 15 and 16 , a perspective view illustrates that in this embodiment the apertures  262  of the bridges  250 ,  252  are sized to fit in close sliding contact with the outer surfaces of the cannulas  143  in order to keep the cannulas parallel to each other. The rod  270  is then manually positioned at the proximal end  191  of the cannula  170  and visually evaluated to conduct the contouring or selection process described in conjunction with  FIG. 10 . While not providing the accuracy of an embodiment using simulation members, this method may be used to shorten the time necessary for the contouring step or be may be used to contour a trial rod or may be used for an initial contouring of a rod before using a simulation member. 
       FIGS. 17 through 20  depict an embodiment that uses a different type of simulation member than the trough simulation members  180  discussed above. As seen in  FIG. 17  trough simulation rods  380  project the positional relationship of the troughs  158  outside the body, by passing through cannulas  143  and locating directly on the troughs  158 . The trough simulation rod  380  has a trough interface  382 , an elongate shaft  384  and simulation troughs  388  located at the proximal end of the rod. The trough interface  382  is configured to locate on the troughs  158  of the pedicle screws  140 , 142 , 148  attached to the vertebrae within the body of the patient, in order to determine the position of the geometrical targets  164 ,  166 ,  168  for the central axis of the rod  270 . The shaft  384  projects the location of the troughs  158  to simulation troughs  388  external to the body and maintains a close concentric fit with the cannula  143  to ensure an accurate projection. Thus, similar to the previous embodiment using the trough simulation members  180 , the trough simulation rods  380  provide a spatial transformation of the troughs  158  of the pedicle screws to the simulation troughs  388  in order to use the troughs  388  as an extracorporeal template to bend the rod. As in the previous embodiment, the trough simulation rods  380  must be retained in an approximately parallel axial relationship by structures such as bridges  250 ,  252 , be of the same length, and maintain the same alignment of each set of troughs  158  and  388  when using the externally projected troughs  388  to gauge the contouring of the rod in order to provide an accurate projection of the troughs  158  and consequently allow the axis of the contoured rod to correctly fit with the troughs  158  in the geometrical targets  164 ,  166 ,  168  as previously described. The rod  270  will later be placed in the body and be attached to the pedicle screws as previously described in connection with  FIGS. 11-14  and pictured in  FIGS. 18-20 . In the latter series of figures, a three dimensionally contoured rod  270  is depicted. 
     A typical surgical procedure in accordance with the present invention will now be described. It will be understood by those of ordinary skill in the art that additional or fewer steps may be performed, the sequence of steps can be varied as appropriate and that substitute techniques and methods may be utilized. Nonetheless, during a typical surgery, a surgeon may perform the following steps:
     percutaneously installing guide wires in bones, such as adjacent vertebrae, as shown in  FIG. 1 ,   

     using blunt dilators and cannulas to open incisions and cavities as shown in  FIGS. 2-5 , 
     percutaneously installing polyaxial screws with retractor blades attached as shown in  FIGS. 6 , 
     removing the cannulas and guide wires as shown in  FIGS. 7 and 7A , 
     installing the abutment members to form slotted cannula assemblies, 
     installing the trough simulation members or rods to the cannula assembly and/or the polyaxial screw head to form contouring assemblies as shown in  FIG. 8  and alternatively, in  FIG. 17 , 
     aligning the contouring assemblies in a parallel relationship and installing the links onto the assemblies as shown in  FIG. 9  and, alternately, in  FIG. 17 , 
     contouring the fixation member to fit the trough simulation members as shown in  FIG. 10  or alternatively to fit into the trough simulation rods shown in  FIG. 18 , 
     installing the contoured fixation member percutaneously as shown in  FIG. 11 , 
     fastening the fixation member in the polyaxial screws as shown in  FIGS. 12, 13 and 19 , and 
     removing the elongate members as shown in  FIGS. 14 and 20  and thereafter completing the surgery. 
     The foregoing description discloses a number of different elements, any of which may be components of a system for configuring or selecting one or more implants for implantation in a body of a patient. Although the foregoing examples relate to the assembly and implantation of a posterior spinal fusion system, the present invention may be applied to a wide variety of implants, within and outside the orthopedic area. The present invention has particular benefits when an implant is to be configured or selected for a given patient, with reference to two or more anatomic points within the body. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.