Patent Publication Number: US-6217509-B1

Title: Devices and methods for percutaneous surgery

Description:
RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 08/736,626, filed on Oct. 24, 1996, now issued as U.S. Pat. No. 5,902,231; which is a continuation-in-part of application Ser. No. 08/620,933, filed on Mar. 22, 1996, now issued as U.S. Pat. No. 5,792,044; each entitled “Devices and Methods for Percutaneous Surgery.” 
    
    
     The present invention relates to devices, instruments and methods for performing percutaneous surgeries, particularly at locations deep within the body. One specific application of the invention concern devices, instruments and techniques for percutaneous, minimally invasive spinal surgery. In another aspect of the invention, the percutaneous surgery is performed under direct vision at any location in the body. 
     Traditional surgical procedures for pathologies located deep within the body can cause significant trauma to the intervening tissues. These open procedures often require a long incision, extensive muscle stripping, prolonged retraction of tissues, denervation and devascularization of tissue. Most of these surgeries require a recovery room time of several hours and several weeks of post-operative recovery time due to the use of general anesthesia and the destruction of tissue during the surgical procedure. In some cases, these invasive procedures lead to permanent scarring and pain that can be more severe than the pain leading to the surgical intervention. 
     Minimally invasive alternatives such as arthroscopic techniques reduce pain, post-operative recovery time and the destruction of healthy tissue. Orthopedic surgical patients have particularly benefited from minimally invasive surgical techniques. The site of pathology is accessed through portals rather than through a significant incision thus preserving the integrity of the intervening tissues. These minimally invasive techniques also often require only local anesthesia. The avoidance of general anesthesia reduces post-operative recovery time and the risk of complications. 
     Minimally invasive surgical techniques are particularly desirable for spinal and neurosurgical applications because of the need for access to locations deep within the body and the danger of damage to vital intervening tissues. For example, a common open procedure for disc herniation, laminectomy followed by discectomy requires stripping or dissection of the major muscles of the back to expose the spine. In a posterior approach, tissue including spinal nerves and blood vessels around the dural sac, ligaments and muscle must be retracted to clear a channel from the skin to the disc. These procedures normally take at least one-two hours to perform under general anesthesia and require post-operative recovery periods of at least several weeks. In addition to the long recovery time, the destruction of tissue is a major disadvantage of open spinal procedures. This aspect of open procedures is even more invasive when the discectomy is accompanied by fusion of the adjacent vertebrae. Many patients are reluctant to seek surgery as a solution to pain caused by herniated discs and other spinal conditions because of the severe pain sometimes associated with the muscle dissection. 
     In order to reduce the post-operative recovery time and pain associated with spinal and other procedures, micro-surgical techniques have been developed. For example, in micro-surgical discectomies, the disc is accessed by cutting a channel from the surface of the patient&#39;s back to the disc through a small incision. An operating microscope or loupe is used to visualize the surgical field. Small diameter micro-surgical instruments are passed through the small incision and between two laminae and into the disc. The intervening tissues are disrupted less because the incision is smaller. Although these micro-surgical procedures are less invasive, they still involve some of the same complications associated with open procedures, such as injury to the nerve root and dural sac, perineural scar formation, reherniation at the surgical site and instability due to excess bone removal. 
     Other attempts have been made for minimally invasive procedures to correct symptomatic spinal conditions. One example is chemonucleolysis which involved the injection of an enzyme into the disc to partially dissolve the nucleus to alleviate disc herniation. Unfortunately, the enzyme, chymopapain, has been plagued by concerns about both its effectiveness and complications such as severe spasms, post-operative pain and sensitivity reactions including anaphylactic shock. 
     The development of percutaneous spinal procedures has yielded a major improvement in reducing recovery time and post-operative pain because they require minimal, if any, muscle dissection and they can be performed under local anesthesia. For example, U.S. Pat. No. 4,545,374 to Jacobson discloses a percutaneous lumbar discectomy using a lateral approach, preferably under fluoroscopic X-ray. This procedure is limited because it does not provide direct visualization of the discectomy site. 
     Other procedures have been developed which include arthroscopic visualization of the spine and intervening structures. U.S. Pat. Nos. 4,573,448 and 5,395,317 to Kambin disclose percutaneous decompression of herniated discs with a posterolateral approach. Fragments of the herniated disc are evacuated through a cannula positioned against the annulus. The &#39;317 Kambin patent discloses a biportal procedure which involves percutaneously placing both a working cannula and a visualization cannula for an endoscope. This procedure allows simultaneous visualization and suction, irrigation and resection in disc procedures. 
     Unfortunately, disadvantages remain with these procedures and the accompanying tools because they are limited to a specific application or approach. For example, Jacobson, Kambin and other references require a lateral or a posterolateral approach for percutaneous discectomy. These approaches seek to avoid damage to soft tissue structures and the need for bone removal because it was thought to be impractical to cut and remove bone through a channel. However, these approaches do not address other spinal conditions which may require a mid-line approach, removal of bone or implants. 
     U.S. Pat. No. 5,439,464 to Shapiro discloses a method and instruments for performing arthroscopic spinal surgeries such as laminectomies and fusions with a mid-line or medial posterior approach using three cannulae. Each of the cannulae requires a separate incision. While Shapiro discloses an improvement over prior procedures which were limited to a posterolateral or lateral approach for disc work, Shapiro&#39;s procedure still suffers from many of the disadvantages of known prior percutaneous spinal surgery techniques and tools. One disadvantage of the Shapiro procedure is its requirement of a fluid working space. Another significant detriment is that the procedure requires multiple portals into the patient. 
     Fluid is required in these prior procedures to maintain the working space for proper function of optics fixed within a prior art cannula and inserted percutaneously. Irrigation, or the introduction of fluid into the working space, can often be logistically disadvantageous and even dangerous to the patient for several reasons. The introduction of fluid into the working space makes hemostasis more difficult and may damage surrounding tissue. Excess fluid may dangerously dilute the sodium concentration of the patient&#39;s blood supply which can cause seizures or worse. The fluid environment can also make drilling difficult due to cavitation. The requirement for a fluid environment generally increases expenses associated with the surgery and adds to the complexity of the surgery, due in part to the relatively high volume of fluid required. 
     A need has remained for devices and methods that provide for percutaneous minimally invasive surgery for all applications and approaches. A need has also remained for percutaneous methods and devices which do not require a fluid-filled working space, but that can be adapted to a fluid environment if necessary. 
     A significant need is present in this field for techniques and instruments that permit surgical procedures in the working space under direct vision. Procedures that reduce the number of entries into the patient are also highly desirable. The fields of spinal and neuro surgery have particularly sought devices and techniques that minimize the invasion into the patient and that are streamlined and concise in their application. 
     SUMMARY OF THE INVENTION 
     Briefly describing one aspect of the invention, there is provided devices and method for performing percutaneous procedures under direct visualization, even at locations deep within a patient. In one embodiment,, a device for use in percutaneous surgery includes an elongated cannula having a first inner dimension and an outer dimension sized for percutaneous introduction into a patient. The cannula further includes a distal working end and an opposite proximal end and defines a working channel between the ends having a second dimension which is equal to the first inner dimension. The working channel is sized to receive a tool therethrough. The device also includes a viewing element mounted inside the cannula adjacent the working channel. The viewing element has a first end connectable to a viewing apparatus and an opposite second end disposed adjacent the distal working end of the cannula. In some embodiments, the viewing element can be a fiber optic cable, a GRIN rod, a rod-lens device or a remote optics (“chip on a stick”) device. 
     In another aspect, a fixture is provided for mounting the viewing element to the cannula. The fixture includes a housing attachable to the proximal end of the cannula. The housing defines a working channel opening therethrough in communication with the working channel. The working channel opening is sized to substantially correspond to the second dimension of the working channel. The housing also defines an optics bore adjacent the working channel opening. The optics bore is sized to receive the elongated viewing element therethrough. 
     In some embodiments, the fixture supports the viewing device for movement within the optics bore along the longitudinal axis of the bore to extend or retract the lens relative to the distal working end of the cannula. In other embodiments, the fixture supports the viewing device for rotation within the optics bore about the longitudinal axis of the bore. In some embodiments, the housing is rotatable relative to the cannula so that the longitudinal axis of the optics bore is rotatable about the longitudinal axis of the working channel. 
     In one aspect of the invention, the working channel can be created by components other than a tubular cannula. For example, an expanding tissue dilator or tissue retractor is also contemplated. With this modification, the fixture would engage the dilator or retractor in its expanded condition. 
     In a further aspect of the invention, the optical viewing device is connected to a tissue retractor, such as a speculum. An apparatus of this type can be particularly useful in various applications such as transnasal transphenoidal surgery and pituitary procedures. 
     In a further aspect of the invention, the working channel maintained by the cannula or similar components can have a length calibrated so that the surgeon can maintain a tactile feel for instruments manipulated through the working channel. In spinal applications, certain beneficial aspects of the invention are attained by providing a cannula having a length slightly greater than the distance from the lamina of a vertebra to the surface of the patient&#39;s skin for posterior procedures. The viewing device is dimensioned relative to the cannula so that the viewing end of the device can project beyond the distal working end of the cannula or working channel to allow the surgeon to selectively survey the surgical site. 
     In accordance with one embodiment, the fixture includes at least one irrigation/aspiration port. Preferably, the port(s) can communicate with at least one itrigation/aspiration channel in the optical viewing device. In this manner, irrigation and/or aspiration can also be applied at the surgical site. When aspiration alone is applied, the port is connected to a vacuum or suction source. The aspiration will draw ambient air through the working channel, across the distal working space, and into the irrigation/aspiration channel of the viewing device. One benefit is that this ambient air aspiration eliminates smoke generated by various working tools and clears the optics lens of fog and debris. 
     In one embodiment, the fixture is mounted over and supported by the proximal end of the cannula. 
     In another embodiment, the fixture can be supported adjacent the proximal end of the cannula by a clamp engaging the outer surface of the cannula. In one specific embodiment, the clamp is a barrel clamp mechanism that is selectively operated by a lever arm and barrel cam. With this embodiment, the fixture itself can be translated along the length of the cannula to extend or retract the lens of the viewing device relative to the end of the working channel. 
     Novel tools are also provided which are insertable into the working channel of the cannula. A tissue retractor in one embodiment includes a body and an integral working tip configured to atraumatically displace tissue as the retractor is manipulated through tissue. The body has a convex surface configured to conform to the inner cylindrical surface of the cannula and an opposite concave surface which does not obstruct the working channel or visualization of the working space. Cannulated tissue dilators are also provided which are insertable over a guidewire or another dilator as well as insertable into the working channel. In some embodiments, the tissue dilators include a tapered working end to displace tissue and a gripping portion having a number of circumferential grooves to enhance gripping and manipulation of the dilator. 
     According to the methods of this invention, spinal and other surgeries can be performed percutaneously with direct visualization without the requirement for a fluid-maintained working space. In another aspect of the inventive surgical techniques, all steps of a surgical procedure are conducted under direct vision through a single working channel cannula. An optical scope or viewing device is moved within the working channel and throughout the working space from a variety of angles and orientations to provide a clear view of the operative steps. 
     The techniques of the present invention also encompass passing multiple tools and instruments through the single working channel cannula and manipulating the instruments and tools within the working space. In one specific embodiment, a tissue retractor is provided that extends through the working channel without significantly reducing the dimensions of the channel. 
     It is an object of the invention to provide devices and methods for percutaneous spinal surgery for all applications and approaches. One advantage of this invention is that percutaneous procedures can be accomplished in a dry environment because a fluid working space is not required for the proper function of the optics. One benefit of this invention is that it provides instruments and methods which reduce the cost, risk, pain and recovery time associated with surgery. These and other objects, advantages and features are accomplished according to the devices and methods of the present invention. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a side elevational view of a device according to this invention. 
     FIG. 2 is a top elevational view of a fixture for supporting a viewing device within a cannula according to this invention. 
     FIG. 3 is a side cross-sectional view of the fixture shown in FIG.  2 . 
     FIG. 4 is a side elevational view of a retractor according to one embodiment of this invention. 
     FIG. 4A is an end cross-sectional view of the retractor of FIG. 4 taken along lines A—A. 
     FIG. 5 is a top elevational view of the retractor shown in FIG.  4 . 
     FIG. 6 is an end elevational view of the retractor shown in FIGS. 4 and 5. 
     FIG. 7 is a side elevational view of a retractor according to another embodiment of this invention.. 
     FIG. 7A is an end cross-sectional view of the retractor of FIG. 7 taken along lines A—A. 
     FIG. 7B is an end cross-sectional view of the retractor of FIG. 7 taken along lines B—B. 
     FIG. 8 is a top elevational view of the retractor shown in FIG.  7 . 
     FIG. 9 is a side elevational view of a dilator according to this invention. 
     FIGS.  10 ( a )-( i ) depicts the steps of a method according to this invention. 
     FIG. 11 is a side cross-sectional view of a device according to one embodiment of this invention. 
     FIG. 12 is a side cross-sectional view of an aspiration cap as shown in FIG.  11 . 
     FIG. 13 is a top perspective view of a device according to another embodiment of the present invention. 
     FIG. 14 is a side perspective view of a fixture for supporting a viewing device forming part of the device shown in FIG.  13 . 
     FIG. 15 is a side elevational view of the device depicted in FIG. 13 with the device shown connected to optical equipment depicted in phantom lines. 
     FIG. 16 is a side elevational view of a scope body forming part of the fixture depicted in FIGS. 13 and 14. 
     FIG. 17 is a bottom elevational view of the scope body shown in FIG.  16 . 
     FIG. 18 is a top elevation view of a lever arm forming part of a barrel clamp mechanism used with the fixture depicted in FIG.  14 . 
     FIG. 19 is an end cross-sectional view of the lever arm shown in FIG. 18 taken along line  19 — 19  as viewed in the direction of the arrows. 
     FIG. 20 is a top elevational view of a barrel cam forming part of a barrel clamp mechanism incorporated into the fixture depicted in FIG.  14 . 
     FIG. 21 is a side elevational view of the barrel cam shown in FIG.  20 . 
     FIG. 22 is a bottom assembly view showing the assembly of the lever arm of FIGS. 18-19, the barrel cam of FIGS. 20-21 with the scope body shown in FIG.  14 . 
     FIG. 23 is a side elevational view of a scope body as depicted in FIG. 14 connected to an aspiration circuit. 
     FIG. 24 is a cross-sectional view of a human patient at a lumbar vertebral level with a device according to one embodiment of the invention situated within the patient to define a working channel above the laminae of the vertebra. 
     FIG. 25 is a side elevational view of a tissue retractor incorporating an optical viewing device. 
     FIG. 26 is a top elevational view of the tissue retractor incorporating an optical viewing device as shown in FIG.  25 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated devices and described methods, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     The present invention provides instruments and methods for performing percutaneous surgery, including spinal applications such as laminotomy, laminectomy, foramenotomy, facetectomy or discectomy, with a single working channel endoscope. The present inventors have discovered that many percutaneous surgeries may be performed without a fluid workspace through the use of optics which move independently of the cannula. The present invention contemplates techniques and instruments that can be implemented with or without a fluid environment. 
     This invention also brings the advantages of percutaneous procedures to applications that previously required open surgery. One advantage is based upon the further discovery that bone work can be performed percutaneously through a large working channel. Another advantage is realized in the use of a single portal within the patient to perform a wide range of simultaneous procedures. 
     According to one embodiment of the present invention, as depicted in FIG. 1, a device  10  is provided for use in percutaneous surgery which includes an elongated cannula  20  having a first inner diameter D I  and an outer diameter D O  sized for percutaneous introduction into a patient. The cannula  20  also includes a distal working end  21  and an opposite proximal end  22 . The cannula defines a working channel  25  between the ends  21 ,  22  having a second diameter d 2  equal to the first inner diameter D I  sized for receiving a tool therethrough. The cannula has a length along its longitudinal axis L that is sized to pass through the patient from the skin to an operative site or working space. In some cases, the working space may be adjacent a vertebra or disc, or in the spinal canal. 
     An elongated viewing element  50  is mountable inside cannula  20  adjacent the working channel  25 . The viewing element  50  has a first end  51  connectable to a viewing apparatus, such as an eyepiece or camera, and an opposite second end  52  disposed or positionable adjacent the distal working end  21  of the cannula  20 . The particular elongated viewing element  50  is not critical to the invention. Any suitable viewing element is contemplated that creates an optical or image transmission channel. In one embodiment, the elongated viewing element  50  includes a fiber optic scope  54  and a lens  55  at the second end  52 . Preferably, the fiber optic scope includes illumination fibers and image transmission fibers (not shown). Alternatively, the viewing element may be a rigid endoscope or an endoscope having a steerable or bendable tip. 
     One advantage of this invention is that it provides optics which are movable relative to the cannula  20 . Because the optics are movable, it is not necessary to provide a fluid-maintained work space. The optics can be removed, cleaned and replaced while the cannula is percutaneously positioned within the patient over the working space. Any configuration which allows the optics to be movably supported adjacent the working channel  25  is contemplated. In one embodiment, shown in FIGS. 1-3, a fixture  30  is provided for mounting the elongated viewing element  50  to the cannula  20 . Preferably, the fixture  30  includes a housing  31  attachable to the proximal end  22  of the cannula  20 . The working channel opening  35  is sized to substantially correspond to the second diameter d 2  of the working channel  25  to receive tools. The fixture  30  includes a housing  31  which defines a working channel opening  35  arranged to communicate with the working channel  25  when the fixture  30  is mounted to the cannula  20 . The working channel opening  35  is sized to receive tools therethrough for passage through the working channel  25 . In the embodiments shown in FIGS. 1-3, the fixture  30  is configured to mount the viewing element  50  within the working channel  25 . 
     The housing  31  also defines an optics bore  60  adjacent the working channel opening  35 . The optics bore  60  has a longitudinal axis f that is preferably substantially parallel to the axis L of the cannula and working channel. The optics bore  60  is preferably sized to removably receive the elongated viewing element  50  therethrough. The fixture  30  preferably supports the viewing element  50  for movement within the optics bore  60  along the longitudinal axis l of the bore  60  to extend or retract the lens  55  relative to the distal working end  21  of the cannula  20 . The retractable/extendable feature of the optics of this invention provides an advantage over prior endoscopes because it eliminates the requirement for a fluid workspace. While the device  10  and its viewing element  50  can be easily used in a fluid environment, the fluid is not essential for the system to operate, contrary to prior systems. Furthermore, many of the prior endoscopes were not suited to access certain areas because of their large diameters. For example, prior endoscopes could not access the spinal canal. However, with this invention, access to the spinal canal is not limited by the diameter of the channel or cannula. The cannula  20  can be left behind in the soft tissue or supported by the lamina while the second end  52  of the elongated viewing element  50  can be advanced into the spinal canal along with any spinal instruments which have been inserted into the working channel  25 . 
     Preferably the fixture  30  also supports the viewing element  50  for rotation within the optics bore  60  about the longitudinal axis l of the bore  60 . The lens  55  of the viewing element  50  defines an optical axis A O . As in many endoscopes, the optical axis A O can be offset at an angle relative to the longitudinal axis l of the optics bore  60 . This feature allows the optical axis A O  of the lens to be swept through a conical field of view F for greater visibility of the working space. The fixture  30  can further be configured so that the viewing element  50  is rotatable relative to the cannula  20 . In this embodiment, the housing  31  is rotatable relative to the cannula  20  so that the second longitudinal axis l of the optics bore  60  rotates about the longitudinal axis L of the working channel  25 . The rotatable features of this invention allows visualization of the entire working space. This feature also aids in simplifying the surgical procedure because the optics  50  and accompanying fittings can be moved out of the way of the surgeon&#39;s hands and tools passing through the working channel. 
     In one embodiment depicted in FIG. 3, the housing  31  defines a receive r bore  40  having an inner diameter d I  slightly larger than the outer diameter D O  of the cannula  20 . In this configuration, the proximal end  22  of the cannula  20  can be received within the receiver bore  40  so that the housing  31  can rotate about the proximal end  22  of the cannula  20 . As shown in FIG. 3, the housing  31  also includes an upper bore  41  which is contiguous with the working channel opening  35  and the receiver bore  40 . In one embodiment, the optics bore  60  is disposed within the upper bore  41  of the housing  31 . 
     In a preferred embodiment depicted in FIG. 2, the optics bore  60  is defined by a C-shaped clip  61  disposed within the upper bore  41 . Preferably, the C-shaped clip  61  is formed of a resilient material and the optics bore  60  defined by the clip  61  has an inner diameter D i  that is slightly less than the outer diameter of the elongated viewing element  50 . When the viewing element  50  is pushed into the optics bore  60  it resiliently deflects the C-shaped clip  61 . The resilience of the clip  61  provides a gripping force on the element  50  to hold it in the desired position, while still allowing the element  50  to be repositioned. 
     Alternatively, the optics bore  60  can have an inner diameter larger than the outer diameter of the viewing element. In this instance, the viewing element  50  can be supported outside the device  20 , either manually or by a separate support fixture. 
     Preferably the device  10  provides engagement means for securely yet rotatably engaging the fixture  30  to the cannula  20 . Most preferably, the fixture  30  is configured to engage a standard cannula  20 . Engagement means can be disposed between the housing  31  and the cannula  20  when the fixture  30  is mounted to the proximal end  22  of the cannula  20  for providing gripping engagement between the housing  31  and the cannula  20 . In one embodiment depicted in FIG. 3 the engagement means includes a number of grooves  32  within the receiver bore  40  and a resilient sealing member, such as an O-ring (see FIG. 11) disposed in each groove  32 . The sealing members, or O-rings, disposed between the housing  31  and the outer diameter D O  of the cannula  20  rotatably secure the fixture  30  to the cannula  20 . The O-rings provide sufficient resistance to movement to hold the fixture  30  in a selectable position on the cannula. In another embodiment, the housing  31  defines a receiver bore  40  which has an inner diameter d I  which is only slightly larger than the outer diameter D O  of the cannula  20  so that the housing  31  can rotate freely about the cannula  20 . 
     The working channel  25  and the working channel opening  35  are both sized to receive a tool or instrument therethrough. Preferably, the working channel opening  35  of the housing  31  has a diameter Dw which is substantially equal to the inner diameter d 2  of the working channel  25  so that the effective diameter of the working channel is not reduced by the fixture  30 . This configuration provides a maximum amount of space for the insertion of tools into the working channel  25 . The present invention is advantageous because standard micro-surgical spinal tools can be inserted into the working channel and manipulated to perform a surgical procedure. The present invention is particularly advantageous because the working channel  25  will simultaneously accept a plurality of movable instruments. No other known prior art device has a working channel that accepts more than one movable instrument at a time through a single port. Therefore, according to this invention, an entire percutaneous surgical procedure can be performed through the working channel  25  of the device  10  under direct visualization using the viewing element  50  disposed within the optics bore  60 . 
     In accordance with the present embodiment, the components of the device  10  are cylindrical in configuration. In other words, the cannula  20 , working channel  25  and fixture  30  have corresponding cylindrical configurations which yield the various diameters D i , D o , D w  and d 2 . In accordance with other embodiments contemplated as part of the invention, these diameters may be non-circular inner and outer dimensions, such as oval or square shaped. For example, a cannula  20  modified to a square cross-section would still provide a large working channel, such as working channel  25 . Likewise, a corresponding fixture  30  have a square cross-section would also provide a large working channel opening D w . In the case of non-circular configurations, the fixture  30  in accordance with the present embodiment would be unable to rotate around the circumference of the cannula  20 , as permitted by the circular configurations. On the other hand, even the non-circular configurations will permit axial movement of the optical viewing element and rotation of the viewing element about its own axis, as set forth more fully herein. 
     In accordance with a further variation of the present invention, the cannula  20  can be replaced by a similar device that is capable of maintaining a large working channel  25 . For example, the cannula  20  can be replaced by an expanding cannula or dilator apparatus. In one specific embodiment, the apparatus can be a spiral wound tube that is unwound or expanded to provide the working channel dimension. Alternatively, multiple tissue dilators, such as speculae, can be expanded to create a working space. In these configurations, the fixture  30  may still be used to support the optical viewing element  50  once the expandable dilator or tissue retractor reaches its full working channel dimension. 
     Although standard micro-surgical instruments may be used with the present invention, this invention also contemplates certain novel tools which capitalize on and enhance the advantages of this invention. 
     According to one preferred embodiment of the invention, a tissue retractor  70  is provided as depicted in FIGS. 4-6. The retractor  70  is removably and rotatably insertable through the working channel  25  and the working channel opening  35  of the device  10 . The tissue retractor  70  includes a working tip  75  configured to atraumatically displace tissue as the retractor  70  is manipulated through the tissue and a body  76  having a proximal first end  77  and a distal second end  78 . The second end  78  can be integral with the working tip  75  which preferably has a blunt curved end  82 . In addition, the working tip  75  is also preferably bent or curved away from the body  76 , as shown in FIG.  7 . The body  76  is sized to be rotatably received within the cannula  20  and has a length B from the first end  77  to the second end  78  sufficient so that the first end  77  and the working tip  75  can both extend outside the cannula  20  when the body  76  is within the cannula  20 . 
     This invention contemplates any suitable retractor for use through the working channel  25 . However, retractors such as the retractor  70  depicted in FIGS. 4-6 are preferred in which the body  76  includes a curved plate  84  that is configured to conform to the inner cylindrical surface  26  of the cannula without substantially blocking the working channel  25 . The curved plate  84  has a convex surface  80  and an opposite concave surface  81 . In one embodiment, the curved plate  84  includes a first plate portion  85  defining a first convex surface  80  and an opposite first concave surface  81 . A second plate portion  86  is integral with the first plate portion  85  and is disposed between the first plate portion  85  and the working tip  75 . The second plate portion  86  defines a second convex surface (not shown) and an opposite second concave surface  81 ′. Both the first plate portion  85  and the second plate portion  86  include opposite edges  90  extending substantially parallel to the length B of the body  76 . 
     Preferably, the curved plate  84  subtends an arc A 1  between the opposite edges  90  of at least 200 degrees, and most preferably 270 degrees. In a specific embodiment, the second plate portion  86  and specifically the second concave surface  81 ′ subtends an angle that decreases along the length of the retractor. Thus, in an embodiment, the second concave surface  81 ′ subtends an angle of about 200 degrees adjacent the first plate portion  85 , decreasing to an angle of less than about 10 degrees at end  78 . 
     An alternate embodiment of a tissue retractor according to this invention is depicted in FIGS. 8-11. This retractor  100  has a body. 106  which includes a first plate portion  115  defining a first convex surface  110  and an opposite first concave surface  111  and includes first opposite edges  120  extending substantially parallel to the length B of the body  106 . The first plate portion  115  subtends a first arc A 2  between the first opposite edges  120 . The retractor body  106  also includes a second plate portion  116  which is integral with the first plate portion  115  and is disposed between the first plate portion  115  and a working tip  105 . The second plate portion  116  defines a second convex surface  110 ′ and an opposite second concave surface  111 ′ and includes second opposite edges  120 ′ extending substantially parallel to the length B. The second plate portion  116  subtends a second arc A 3  between the second opposite edges  120 ′ that is different from the first arc A 2  in this embodiment. Preferably, the first arc A 2  subtends an angle of less than 180 degrees and the second arc A 3  subtends an angle of more than 180 degrees. Most preferably, the first arc A 2  subtends an angle of about 90 degrees and the second arc A 3  subtends an angle of about 270 degrees. 
     The retractors of this invention may be provided with means for engaging the retractors  70 ,  100  within the working channel  25  of the cannula  20 . For example, the convex surfaces  80 ,  110  can be configured to have a diameter that is greater than the diameter D I  of the inner cylindrical surface  26  of the cannula  20 . In that case, the body  76 ,  106  may be formed of a resilient material that is deformable to be insertable into the cannula  20  so that the convex surface  80 ,  110  is in contact with the inner cylindrical surface  26  of the cannula  20 . When the body  76 ,  106  is deformed, it exerts an outward force against the surface  26  to frictionally hold the retractor in its selected position. 
     The preferred components provided by this invention are configured so that multiple tools and instruments can be accepted and manipulated within the working channel  25  of the cannula  20 . The components are also configured so that more than one surgeon may manipulate instruments through the working channel  25  of the cannula  20  at one time. For example, one surgeon may be manipulating the retractor while another surgeon is drilling into a bone. The curvature of the body  76 ,  106  of the retractors  70 ,  100  provides more working space and increases visibility. Another feature is that the long axis of the component can be placed in the working channel  25  while a bend in the handle portion keeps hands away from the channel  25  so that more than one surgeon can work in the channel  25  and more tools can be placed in the channel  25 . The retractors shown in FIGS. 4-8 each comprise an arm  71 ,  101  attached to the proximal first end  77 ,  107  of the body  76 ,  106 . Preferably, as shown in FIGS. 4-8, the arm  71 ,  101  is at an angle ax which is less than 180 degrees from the longitudinal axis of the length L of the body  76 . Most preferably, the angle a is about 90 degrees so that the arm  71 ,  101  is substantially perpendicular to the length L of the body  76 ,  106 . Preferably, the arm  71 ,  101  has a gripping surface  72 ,  102  to facilitate manipulation of the retractor  70 ,  100 . 
     The present invention also provides tissue dilators usable with the device  10 . Any dilator which is insertable into the working channel  25  of the cannula  20  is contemplated; however, a preferred dilator provided by this invention is depicted in FIG. 12. A dilator  130  preferably includes a hollow sleeve  135  defining a channel  131 . The channel  131  allows the dilator  130  to be placed over a guidewire (not shown) or other dilators. The hollow sleeve  135  has a working end  136  defining a first opening  132  in communication with the channel  131  and an opposite end  137  defining a second opening  133 . The working end  136  is tapered to a tapered tip  138  to atraumatically displace tissue. Preferably, a gripping portion  140  is provided on the outer surface  141  of the sleeve  135  adjacent the opposite end  137 . In one embodiment, the gripping portion  140  is defined by a plurality of circumferential grooves  142  defined in the outer surface  141 . The grooves  142  are configured for manual gripping of the dilator  130  to manipulate the dilator  130  through tissue. Preferably, the grooves  142  are partially cylindrical. In the embodiment shown in FIG. 9, the gripping portion  140  includes a number of circumferential flats  143  each of the circumferential grooves  142 . The grooves  142  have a first width W 1  along the length of the sleeve  135  and the flats  143  have a second width W 2  146 along the length. Preferably, the first and second widths W 1  and W 2  are substantially equal. 
     The present invention has application to a wide range of surgical procedures, and particularly spinal procedures such as laminotomy, laminectomy, foramenotomy, facetectomy and discectomy. Prior surgical techniques for each of these procedures has evolved from a grossly invasive open surgeries to the minimally invasive techniques represented by the patents of Kambin and Shapiro. However, in each of these minimally invasive techniques, multiple entries into the patient is required. Moreover, most of the prior minimally invasive techniques are readily adapted only for a posterolateral approach to the spine. The devices and instruments of the present invention have application in an inventive surgical technique that permits each of these several types of surgical procedures to be performed via a single working channel. This invention can also be used from any approach and in other regions besides the spine. For instance, the invention contemplates apparatus appropriately sized for use in transnasal, transphenoidal and pituitary surgeries. 
     The steps of a spinal surgical procedure in accordance with one aspect of the present invention are depicted in FIG.  10 . As can be readily seen from each of the depicted steps (a)-(i), the present embodiment of the invention permits a substantially mid-line or medial posterior approach to the spine. Of course, it is understood that many of the following surgical steps can be performed from other approaches to the spine, such as posterolateral and anterior. In a first step of the technique, a guidewire  150  can be advanced through the skin and tissue into the laminae M of a vertebral body V. Preferably, a small incision is made in the skin to facilitate penetration of the guidewire through the skin. In addition, most preferably the guidewire, which may be a K-wire, is inserted under radiographic or image guided control to verify its proper positioning within the laminae L of the vertebra V. It is, of course, understood that the guidewire  150  can be positioned at virtually any location in the spine and in any portion of a vertebra V. The positioning of the guidewire is dependent upon the surgical procedure to be conducted through the working channel cannula of the present invention. Preferably, the guidewire  150  is solidly anchored into the vertebral bone, being tapped by a mallet if necessary. 
     In subsequent steps of the preferred method, a series of tissue dilators are advanced over the guidewire  150 , as depicted in steps (b)-(d) in FIG.  10 . Alternatively, the dilators can be advanced through the incision without the aid of a guidewire, followed by blunt dissection of the underlying tissues. In the specific illustrated embodiment, a series of successively larger dilators  151 ,  152  and  153  are concentrically disposed over each other and over the guidewire  150  and advanced into the body to sequentially dilate the perispinous soft tissues. Most preferably, the tissue dilators are of the type shown in FIG. 9 of the present application. In a specific embodiment, the dilators have successively larger diameters, ranging from 5 mm, to 9 mm to 12.5 mm for the largest dilator. Other dilator sizes are contemplated depending upon the anatomical approach and upon the desired size of the working channel. 
     In the next step of the illustrated technique, the working channel cannula  20  is advanced over the largest dilator  153 , as shown in step (e), and the dilators and guidewire  150  are removed, as shown in step (f). Preferably, the working channel cannula  20  has an inner diameter D I  of 12.7 mm so that it can be easily advanced over the 12.5 mm outer diameter of the large dilator  153 . Larger working channel cannulas are contemplated depending upon the anatomical region and surgical procedure. 
     With the cannula  20  in position, a working channel is formed between the skin of the patient to a working space adjacent the spine. It is understood that the length of the cannula  20  is determined by the particular surgical operation being performed and the anatomy surrounding the working space. For instance, in the lumbar spine the distance between the laminae M of a vertebra V to the skin of the patient requires a longer cannula  20  than a similar procedure performed in the cervical spine where the vertebral body is closer to the skin. In one specific embodiment in which the cannula  20  is used in a lumbar discectomy procedure, the cannula has a length of 87 mm, although generally only about half of the length of the cannula will be situated within the patient during the procedure. 
     In accordance with the present surgical technique, the working channel cannula  20  is at least initially only supported by the soft tissue and skin of the patient. Thus, in one aspect of the preferred embodiment, the cannula  20  can include a mounting bracket  27  affixed to the outer surface of the cannula (FIG.  10 ( f ), FIG.  11 ). This mounting bracket  27  can be fastened to a flexible support arm  160 , which can be of known design. Preferably, the flexible support arm  160  is engaged to the bracket  27  by way of a bolt and wing nut  161 , as shown in FIG.  10 ( i ) and in more detail in FIG. 11, although other fasteners are also contemplated. This flexible arm  160  can be mounted on the surgical table and can be readily adjusted into a fixed position to provide firm support for the cannula  20 . The flexible arm  160  is preferred so that it can be contoured as required to stay clear of the surgical site and to allow the surgeons adequate room to manipulate the variety of tools that would be used throughout the procedure. 
     Returning to FIG. 10, once the cannula  20  is seated within the patient, the fixture  30  can be engaged over the proximal end of the cannula  20 . The fixture  30 , as shown in FIGS. 2 and 3 and as described above, provides an optics bore  60  for supporting an elongated viewing element, such as element  50  shown in step h. In accordance with the invention, the viewing element  50  is advanced into the fixture  30  and supported by the optics bore  60  (FIG.  2 ). In one specific embodiment, the element  50  is most preferably a fiber optic scope, although a rod lens scope, “chip on a stick” or other viewing scopes may be utilized. In the final step (i) of the procedure shown in FIG. 10, the flexible arm  160  is mounted to the bracket  27  to support the cannula  20  which in turn supports the optical viewing element  50 . This final position of step (i) in FIG. 10 is shown in more detail in FIG.  11 . The viewing element  50  can be of a variety of types, including a rigid endoscope or a flexible and steerable scope. 
     With the viewing element or scope  50  supported by the fixture  30  the surgeon can directly visualize the area beneath the working channel  25  of the cannula  20 . The surgeon can freely manipulate the viewing element  50  within the working channel  25  or beyond the distal end of the cannula into the working space. In the case of a steerable tip scope, the second end  52  of the viewing element  50 , which carries the lens  55 , can be manipulated to different positions, such as shown in FIG.  11 . With virtually any type of viewing element, the manipulation and positioning of the scope is not limited by the working channel  25 , in contrast to prior systems. 
     Preferably, the positioning capability provided by the fixture  30  is utilized to allow extension of the lens  55  into the working space or retraction back within the cannula  20 , as depicted by the arrows T in FIG.  1 . Also the fixture preferably accommodates rotation of the element  50  about its own axis (arrows R in FIG. 1) to vary the viewing angle provided by the angled lens  55 , or rotation of the entire viewing element  50  about the cannula  20  and around the circumference of the working channel  25 , as shown by the arrows N in FIG.  1 . In this manner, the surgeon is provided with a complete and unrestricted view of the entire working space beneath the working channel  25 . In instances when the fixture  30  is rotated about the cannula  20 , the viewing orientation of the optics (i.e., left-right and up-down) is not altered so the surgeon&#39;s view of the procedure and surrounding anatomy is not disturbed. 
     Another advantage provided by the single working channel cannula  20  of the present invention, is that the cannula can be readily positioned over an appropriate target tissue or bone, to thereby move the working space as necessary for the surgical procedure. In other words, since the working channel cannula  20  is freely situated within the patient&#39;s skin and tissue, it can be manipulated so that the working space beneath the cannula  20  is more appropriately centered over the target region of the spine. Repositioning of the cannula  20  can be performed under fluoroscopic guidance. Alternatively, the cannula may be fitted with position sensing devices, such as LEDs, to be guided stereotactically. As the cannula is being repositioned, the surgeon can also directly visualize the spine through the viewing element  50 . 
     Once the position of the cannula  20  is established and a working space is oriented over the proper target tissue, a variety of tools and instruments can be extended through the working channel  25  to accomplish the particular surgical procedure to be performed. For instance, in the case of a laminotomy, laminectomy, foramenotomy or facetectomy, a variety of rongeurs, curettes, and trephines can be extended through the working channel opening  35  (see FIG. 2) and through the working channel  25  of the cannula  20  (see FIG. 11) into the working space. It is understood that these various tools and instruments are designed to fit through the working channel. For instance, in one specific embodiment, the working channel  25  through the cannula  20  can have a maximum diameter d 2  of 12.7 mm. However, with the viewing element  50  extending into the working channel  25 , the effective diameter is about 8 mm in the specific illustrated embodiment, although adequate space is provided within the working channel  25  around the viewing element  50  to allow a wide range of movement of the tool or instrument within the working channel. The present invention is not limited to particular sizes for the working channel and effective diameter, since the dimensions of the components will depend upon the anatomy of the surgical site and the type of procedure being performed. 
     Preferably, each of the tools and instruments used with the working channel cannula  20  are designed to minimize obstruction of the surgeon&#39;s visualization of and access to the working space at the distal end of the working channel cannula. Likewise, the instruments and tools are designed so that their actuating ends which are manipulated by the surgeon are displaced from the working channel cannula  20 . One such example is the tissue retractor shown in FIGS. 4-8. with these retractors, the handles that are manually gripped by the surgeon are offset at about a 90 degree angle relative to the longitudinal axis of the tool itself. 
     In accordance with once aspect of the present invention, the surgical procedures conducted through the working channel cannula  20  and within the working space at the distal end of the cannula are performed “dry”—that is, without the use of irrigation fluid. In prior surgical techniques, the working space at the surgical site is fluid filled to maintain the working space and to assist in the use of the visualization optics. However, in these prior systems the visualization optics were fixed within the endoscope. In contrast, the device  10  of the present invention allows a wide range of movement for the viewing element  50  so that the lens  55  can be retracted completely within the working channel  25  of the cannula  20  to protect it from contact with the perispinous tissue or blood that may be generated at the surgical site. 
     Moreover, since the viewing element  50  is removable and replaceable, the element  50  can be completely removed from the fixture  30  so that the lens  55  can be cleaned, after which the viewing element  50  can be reinserted into the fixture and advanced back to the working space. Under these circumstances, then, the need for irrigation is less critical. This feature can be of particular value when cutting operations are being performed by a power drill. It has been found in prior surgical procedures that the use of a power drill in a fluid environment can cause turbulence or cavitation of the fluid. This turbulence can completely shroud the surgeon&#39;s view of the surgical site at least while the drill is being operated. With the present invention, the dry environment allows continuous viewing of the operation of the power drill so that the surgeon can quickly and efficiently perform the necessary cutting procedures. 
     While the present invention permits the surgeon to conduct surgical procedures in the working space under a dry environment, irrigation may be provided separately through the working channel  25 . Alternatively, the viewing device  50  itself may include a tube  54  supported by the fitting  53  through which modest amounts of fluid can be provided to keep the visualization space clear. In addition, during a discectomy, aspiration of the excised tissue is preferred, and irrigation will frequently assist in rapid removal of this tissue. Thus, separate irrigation and aspiration elements can also be inserted through the working channel  25  as required by the procedure. 
     As necessary, aspiration can be conducted directly through the working channel  25  of the cannula  20 . In one specific embodiment, an aspiration cap  165  is provided as shown in FIGS. 11 and 12. The cap  165  includes a body  166  which defines a mating bore  167  having an inner diameter d b  larger than the outer diameter D h  of the housing  31  of fitting  30 . A tool opening  168  is provided in communication with the mating bore  167 . When the aspiration cap  165  is mounted over the housing  31 , as shown in FIG. 11, the tool opening  168  communicates directly with the upper bore  41  and provides the same entry capabilities as the working channel opening  35  of the housing  31 . The aspiration cap  165  is also provided with a tube receiver bore  169  which intersects the mating bore  167 . The receiver bore  169  is configured to receive an aspiration tube through which a vacuum or suction is applied. In certain instances, the tool opening  168  may be covered while suction is applied through the tool receiver bore  169  and mating bore  167 , and ultimately through the working channel  25 . Covering the opening  168  can optimize the aspiration effect through the working channel. 
     Returning again to the surgical technique of one embodiment of the present invention, once the working channel cannula  20  and the optics  50  are in position, as depicted in FIG. 10 step (i) and FIG. 11, the paraspinous tissue can be reflected using instruments as described above, and a laminectomy performed using various rongeurs, curettes and drills. As necessary, the cannula  20  can be angled to allow a greater region of bone removal, which may be necessary for access to other portions of the spinal anatomy. In some instances, access to the spinal canal and the posterior medial aspects of the disc annulus may require cutting a portion of the vertebral bone that is greater than the inner diameter of the working channel  25 . Thus, some manipulation of the cannula  20  may be necessary to permit removal of a greater portion of bone. In other operations, multi-level laminectomies or foramenotomies may be necessary. In this instance, these multi-level procedures can be conducted by sequentially inserting the working channel cannula  20  through several small cutaneous incisions along the spinal mid-line. Alternatively, several working channel cannulas  20  can be placed at each of the small cutaneous incisions to perform the multi-level bone removal procedures. 
     Again, in accordance with the preferred illustrated surgical technique, an opening is cut into the laminae M of the vertebra V providing direct visual access to the spinal canal itself. As necessary, tissue surrounding the spinal nerve root can be removed utilizing micro surgical knives and curettes. Once the spinal nerve root is exposed, a retractor, such as the retractors shown in FIGS. 4-8, can be used to gently move and hold the nerve root outside the working space. In one important aspect of the two retractors  70 ,  100 , the portion of the retractor passing through the working channel  25  generally conforms to the inner surface of the cannula  20  so that the working channel  25  is not disrupted by the retractor tool. Specifically, the effective diameter within the working channel  25  is reduced only by the thickness of the curved plates  84 ,  114  of the retractors  70 ,  100 . In one specific embodiment, this thickness is about 0.3 mm, so it can be seen that the tissue retractors do not significantly reduce the space available in the working channel  25  for insertion of other tools and instruments. 
     With the tissue retractor in place within the working channel  25 , bone within the spinal canal, such as may occur in a burst fracture, can be removed with a curette or a high speed drill. Alternatively, the fractured bone may be impacted back into the vertebral body with a bone impactor. At this point, if the spinal procedure to be performed is the removal of epidural spinal tumors, the tumors can be resected utilizing various micro-surgical instruments. In other procedures, the dura may be opened and the intradural pathology may be approached with micro-surgical instruments passing through the working channel cannula  20 . In accordance with the specific illustrated technique, with the nerve root retracted posterior medial disc herniations can be readily excised directly at the site of the herniation. 
     In another embodiment of the invention, a working channel cannula, such as cannula  20 , is provided with a fixture  170  for supporting optics and irrigation/aspiration components. In accordance with this embodiment, the fixture  170  includes a scope body  171  which is shown most clearly in FIGS. 13,  14 ,  16  and  17 . The scope body  171  includes a clamping ring  172  configured to encircle the outer surface  23  of the cannula  20 . In particular, the clamping ring  172  includes an inner clamping surface  175  (see FIG.  14 ). That clamping surface  175  has substantially the same configuration and dimension as the outer surface  23  of the cannula  20 . The clamping ring  172  includes clamp arms  173   a, b  at the free ends of the ring. The clamp arms  173   a, b  define a slot  174  (see FIG. 17) therebetween. 
     The clamping ring  172  is integral with a support column  176  forming part of the scope body  171 . A column slot  177  is formed in the support column  176 , with the column slot  177  being contiguous with the slot  174  between the clamp arms  173   a , b. As described in more detail herein, the slots  174  and  177  permit the clamp arms  173   a , b to be compressed toward each other to thereby compress the clamping surface  175  of the ring  172  about the outer surface  23  of the cannula  20 . In this manner, the fixture  170  can be affixed at a specific position on the cannula  20 . It is understood that when the clamping ring  172  is loosened, the fixture  170  is free to rotate about the circumference of the cannula  20  in the direction of the arrow N. In addition, the fixture  170  can translate along the longitudinal length of the cannula  20  in the direction of the arrow T. Of course, the direction of the travel distance of the fixture  170  along the length of the cannula  20  is limited by the proximal end  22  and the bracket  27  used to engage a supporting flexible arm  160  as described above. 
     Returning to FIGS. 13-17, additional details of the fixture  170  can be discerned. In particular, the fixture  170  includes an optics mounting body  178  that is supported by and preferably integral with the support column  176 . The optics mounting body  178  defines a stop edge  179  at the interface between the support column  176  and the mounting body  178 . This stop edge defines the height of the support column from the clamping ring  172  to the stop edge  179 . The stop edge  179  of the optics mounting body  178  can be used to limit the downward travel of the fixture  171  in the direction of the arrow T, which can be particularly important in embodiments of the cannula  20  that do not include the bracket  27 ,. 
     In accordance with the present embodiment, the optics mounting body  178  defines an optics bore  180  which is configured to receive and support an optics cannula  190 . The optics bore  180  can communicate with an illumination port  181  which can receive an illumination source, such as a fiber optic light cable. The optics bore  180  also communicates with an optics coupling bore  182  projecting from a front face of the fixture  170 . In accordance with one specific embodiment, the fixture  170  also includes a coupling body  183  that is preferably press-fit within the optics coupling bore  182 . As shown in FIG. 15, the coupling body  183  can be engaged by a coupler  184  to support a camera  185  thereon. 
     In a further aspect of the optics mounting body  178 , an aspiration port  186  and an irrigation port  187  can be provided that communicates with the optics bore  180 . Preferably, the optics cannula  190  includes channels along its length to correspond to the various ports in the optics mounting body  178 . In one specific embodiment, the port  181  is not used, with the port  186  being used to receive an illumination element. As shown more particularly in FIG. 23, the port  187  can be connected to an aspiration circuit. In particular, the port  187  can be engaged to an aspiration tube  225  which carries a flow control valve  226  and Luer® fitting  227  at its free end. The Luer® fitting  227  can engage a source of irrigation fluid or aspiration vacuum pressure depending upon the particular use envisioned for the port  187  and a corresponding channel within the optics cannula  190 . 
     In accordance with a method of the present invention, the port  187  is used as an aspiration port with the Luer® fitting  227  connected to a vacuum source. It is understood that the port  187  is in fluid communication with a corresponding channel in the optics cannula  190  so that suction applied through the tube  225  and port  187  is drawn through the distal or working end  192  of the optics cannula  190 . The working end  192  is at the surgical site so that the suction draws air through the working channel  25  of the cannula  20 , to the surgical site and through the aspiration/irrigation channel in the optics cannula  190 . It has been found that providing aspiration suction in this manner eliminates smoke that may be developed during operation of certain instruments, such as a Bovie. Moreover, the suction applied through the port  187  can draw air across the lens  191  (see FIG. 14,  15 ) of the optics cannula  190 , to prevent fogging of the lens. If a separate aspiration tube is extended through the working channel, defogging of the lens  191  is best achieved with the opening of the aspiration tube adjacent the lens. In this manner, the provision of aspiration vacuum through the working channel and working space virtually eliminates the need to retract the optics cannula  190  to clean the lens  191 . This is in contrast to prior devices in which either the lens had to be removed from the surgical site for cleaning or devices in which substantial flow of fluid is required to keep the lens clean and clear. 
     Looking now to FIGS. 18-22, details of a barrel clamp mechanism  195  are shown. The barrel clamp mechanism  195  compresses the arms  173   a, b  of the clamping ring  172  together to clamp the fixture  170  to the cannula  20 . The barrel clamp mechanism  195  includes a barrel cam  196  disposed immediately adjacent one of the clamp arms  173   b , and a lever arm  197  that operates to compress the barrel cam  196  against the clamp arm  173 . A shoulder screw  198  fixes each of these components together. Specifically, the shoulder screw  198  includes a threaded shank  199  that is configured to engage a mating threaded bore  202  in one of the clamp arms  173   a.  The shoulder screw  198  includes a bearing shank  200  that is smooth or non-threaded. The bearing shank  200  is received within a bearing bore  203  in the clamp arm  173   b , a colinear bearing bore  204  in the barrel cam  196 , and a bearing bore  205  in the lever arm  197 . The shoulder screw  198  further includes an enlarged head  201  which is preferably received within a head recess  206  in the lever arm  197  (see FIG.  19 ). Preferably, the enlarged head  201  of the shoulder screw includes a driving tool recess to mate with a driving tool to thread the threaded shank  199  of the screw into the mating threaded bore  202  of the clamp arm  173   a . It is understood that the barrel cam  196  and lever arm  197  are free to rotate about the bearing shank  200  of the shoulder screw  198 . 
     Referring specifically to FIGS. 18-19, the lever arm  197  includes an arm  210  that is integral with a body  211 . The bearing bore  205  and head recess  206  are defined in the body  211 . The body  211  defines a pair of projections  212  on opposite sides of the bearing bore  205 . As depicted in FIG. 19, each of the projections  212  includes a rounded tip  213  to provide a smooth sliding surface. 
     Referring specifically to FIGS. 20-21, the barrel cam  196  includes a flat face  215  that faces the clamp arm  173   b . Preferably, the flat face provides for smooth rotation of the barrel cam  196  relative to the stationary arm  173   b . The opposite face of the barrel cam  196  is a cam face  216  that includes a pair of diametrically opposite cam portions  217 . In accordance with the preferred embodiment, the cam portions  217  define a ramp  218  that is inclined upward to a detent recess  219 . Each detent recess  219  terminates in a stop  220  that is higher relative to the base detent recess  219  than the ramp  218 . 
     In the assembled configuration, the barrel clamp mechanism  195  operates to compress the arms  173   a, b  of the clamping ring  172  together when the lever arm  197  is rotated about the shoulder screw  198 . Specifically, as the lever arm  197  is rotated, the projections  212  slide on their rounded tip  213  along the ramps  218  until the rounded tips  213  fall within the opposite detents  219 . As the projections  212  move up the ramps  218 , the projections  212  push the barrel cam  196  toward the clamp arms  173   a, b . More specifically, since the opposite clamp arm  173   a  is held relatively fixed by the threaded shank  199  of the shoulder screw  198 , the movement of the barrel cam  196  presses the clamp arm  173   b  against the relatively stationary clamp arm  173   a . As this occurs, the clamping ring  172  is tightened around the outer surface  23  of the cannula  20 . When the projections  212  are seated within the recesses  219  of the barrel cam  196 , the fixture is locked onto the cannula  20 . It is understood that the recesses  219  are shallow enough to permit ready manual disengagement of the projections  212  from the recesses  219  as the lever arm  197  is rotated in the opposite direction. 
     In one specific embodiment, the detent recesses  219  are 180° opposite each other. The ramps  218  are curved and subtend an angle of about 90°. Thus, the lever arm  197  rotates through 90° to move the projections  212  from one end of the cam ramps  218  to the recesses  219 . In the preferred embodiment, the lever arm ninety degree movement (arrow J in FIG. 15) moves the arm from a first position in which the arm  197  is substantially parallel to the cannula, to a second position in which the arm is substantially perpendicular to the cannula. Most preferably, in the second position the arm is oriented immediately adjacent the cannula, rather than projecting away. In the first and second positions, the lever arm  197  maintains a low profile so as not to interfere with the surgeon&#39;s manipulation of tools and instruments through the working channel. In a specific embodiment, the first position of the lever arm corresponds to the loose or unlocked position of the barrel clamp mechanism  195 , while the second position corresponds to the locked configuration. 
     In order for the barrel clamp mechanism  195  to function properly, it is preferred that the barrel cam  196  remain stationary relative to the moveable lever arm  197 , with the exception that the barrel cam  196  is free to translate along the length of the shoulder screw  198 . Consequently, the clamp arm  173   b  includes a recess  222  that has a configuration substantially similar to the outer periphery of the barrel cam  196 . In this manner, the barrel cam can be slightly indented within the clamp arm  173   b  so that the cam is unable to rotate about the shoulder screw  198  as the lever arm  197  is pivoted. 
     In accordance with one specific embodiment of the invention, the components of the fixture  170  are formed of a flexible and resilient material. For example, the scope body  171  can be formed of a plastic, such as polycarbonate. The scope body  171  lends itself particularly well to typical plastic molding techniques. Likewise, the barrel cam  196  and lever arm  197  can be molded from a plastic material. In one specific embodiment, these components are formed of Delrin®, since Delrin® provides a smooth surface for the relative movement between the projections  212  on the lever arm  197  and the cam face  216  of the barrel cam  196 . 
     It is understood that the travel of the barrel clamp mechanism  195  can be calibrated sufficient to tightly compress the clamping rings  172  about the cannula  20 . It is also understood that this compression must not be so great as to compromise the integrity or strength of the cannula  20 . In one specific embodiment, the slot  174  is larger than the maximum travel of the barrel clamp mechanism  195  so that the projections  212  of the lever arm  197  can rest solidly within the detent recesses  219  of the barrel cam  196 . In accordance with one specific embodiment, the slot  174  has a dimension of 2.0 mm while the throw of the barrel clamp mechanism  195  achieved by the barrel cam  196  is 1.0 mm. 
     In accordance with the present embodiment of the invention, the fixture  170  supports an optics cannula  190  in a fixed orientation relative to the scope body  171 . In other words in this specific embodiment, the optics cannula  190  is not permitted to rotate about its axis as could the scope  50  of the embodiment shown in FIG.  1 . The lens  191  is therefore mounted at an angle B relative to the distal end of the optics cannula  190 . In one specific embodiment, the lens  191  is situated at an angle B of 30°. In addition, in the specific embodiment, the lens has an optical axis that is angled toward the center of the working space  25  or the cannula  20 . While the lens  191  has a fixed orientation relative to the scope body  171 , the lens can still be rotated around the working space by rotation of the fixture  170  about the outer surface  23  of the cannula  20 . In addition, the lens  191  and the optical system provide a depth of field of view that allows the surgeon to view anatomy outside the working channel  25 . 
     Even in the present specific embodiments, the fixture  170  allows rotation of the optics cannula  190  around the working space and translation of the optics cannula  190  and  191  along the longitudinal axis of the working channel  25 . Of course, it is understood that the surgeon can achieve these motions by releasing the barrel clamp mechanism  195  and then re-engaging the clamp by rotating the lever arm  197  to its locked position. Preferably, the optics cannula  190  is sized so that the lens  191  can project beyond the distal end  21  of the cannula  20 . Similarly, in the preferred embodiment, the fixture  170  allows the retraction of the lens  191  and optics cannula  190  within the working channel  25  and cannula  20 . 
     In one specific embodiment, the fixture  170  permits up to 15 mm travel along the direction of the arrow T with 7.5 mm of the travel being within the working space  25  and 7.5 mm of the travel being beyond the distal end  21  of the cannula  20 . In accordance with the specific embodiment, this 15 mm travel distance is related to the height of the support column  176  from the top of the clamping ring  172  to the stop edge  179  of the optics mounting body  178 . The amount of extension of the lens  191  of the optics cannula  190  beyond the distal end  21  of the cannula  20  is also based upon the overall length of the optics cannula  190  relative to the overall length of the working channel cannula  20 . In one specific embodiment, the optics cannula  190  has a length of 100 mm measured from the lens  191  to the stop edge  179  of the optics mounting bore  178 . Of course, it is understood that the optics cannula is longer than this 100 mm distance because a portion of the cannula is supported within the optics bore  180  of the optics mounting body  178 . Again in the specific embodiment, the cannula  20  has an overall length of 92 mm from its distal end  21  to its proximal end  22  (see FIG.  15 ). 
     In a further aspect of the invention, the overall length of the cannula, and consequently the optics cannula  190 , is determined, in part, by the spinal anatomy. In particular, for applications of the present invention in the field of spinal surgery, it has been found that placement of the proximal end  22  of the working channel  25  too distant from the surgical site at the distal end  21 , causes the surgeon to lose tactile feel while manipulating certain instruments. In other words, when the surgeon passes instruments through the working channel and manipulates them at the surgical site, a certain amount of “feel” is required so that the surgeon can accurately perform the respective operations with the instrument. If the distance between the surgical site and the manual end of the instrument is too great, the surgeon will not be able to stably and comfortably operate the instrument. 
     In accordance one beneficial aspect of the present invention, it has been found that the working channel cannula  20  must have a length that is limited relative to the distance L (FIG. 24) between the vertebral laminae and the surface of the skin. In the lumbar region of the spine, this distance is approximately 65-75 mm. Consequently, in one embodiment of the invention, the working channel cannula  20  has first portion of its length somewhat less than the anatomic distance. In one specific embodiment, this length of the first portion is about 66 mm from the distal end  21  to the mounting bracket  27 . In some surgical applications, the mounting bracket  27  may actually rest against the skin of the patient so that the distal end  21  of the working channel cannula can be closer to the surgical site. 
     Further in accordance with the present invention, the remaining second portion of the length of the cannula  20  above the mounting bracket  27  is minimized. In accordance with the invention, this distance must be sufficient to permit extension and retraction of the lens  191  relative to the distal end  21  of the cannula  20 . As described above, the travel of the optical lens  191  is preferably 15 mm, so that the remaining length of the cannula  20  is about 26 mm to accommodate this travel and to provide adequate surface for engagement by the clamping ring  172 . Thus, in the preferred embodiment, the working channel cannula  20  has an overall length of 92 mm. In accordance with one aspect of the invention, it has been found that the relative length between the first portion of the cannula disposed within the patient to the second portion of the cannula length situated outside the patient have a ratio of 2:1 to 3:1. In other words, the length of the first portion is between two to three times longer than the length of the second portion. 
     It has also been found that it is desirable to minimize the height of the fixture  170  beyond the end of the working channel cannula  20 . In accordance with the present invention, the optics mounting body  178  has a height of about 21 mm between the stop edge  179  and the top face of the body  178 . This distance is not so great that the surgeon is restrained from manipulating instruments directly above the fixture  170 . Of course, it is preferable that the surgeon manipulate the instruments directly above the proximal end  22  of the working channel  20  immediately adjacent to the fixture  170 . 
     In the present preferred embodiment, the working channel cannula has an inner diameter of about 15 mm and an outer diameter of about 16 mm. Alternatively, the cannula can be provided in a smaller size for other regions of the spine. In a further specific embodiment, the cannula inner diameter is 12.7 mm with a 14 mm outer diameter. In another aspect of the invention, the overall length and diameter of the working channel cannula  20  is calibrated again relative to the distance L of the spinal anatomy. With the larger diameter working channel, the surgeon can orient certain instruments at an angle relative to the longitudinal axis of the cannula  20 . In specific embodiments, this angle is approximately 5-6°. It has been found that this angle, together with the large working channel  25 , gives the surgeon greater flexibility and mobility within the surgical site to perform various operations. To that end, the length and diameter of the working channel cannula  20  is appropriately sized to maintain this flexibility, without getting too large. A working channel cannula  20  that has too large a diameter is less adaptable to the spinal anatomy. 
     In accordance with preferred methods using the device  10  of the present invention, the working space is generally limited to the region directly adjacent the laminae of a vertebra. A cannula having a diameter that is too large will interfere with the spinous process when the working space is created, and will require resection of greater amounts of tissue than is preferred for an optimal percutaneous procedure. Therefore, in accordance with one aspect of the invention, the working channel cannula has a relationship between its length and its diameter to permit tool angles through the cannula of between 5-8°. In accordance with one specific aspect of the present invention, the cannula can have a length to diameter ratio of between about 5.5:1 to 7:1. Further in accordance with the present invention, the working channel cannula has a length that is no more than 20-30 mm greater than the distance L (FIG. 24) between the laminae and the skin of the patient. 
     One important feature of the present invention is achieved by the large diameter of the working channel  25  in the cannula  20 . This large diameter allows the surgeon or surgeons conducting the surgical procedure to introduce a plurality of instruments or tools into the working space. For example, as described above, a tissue retractor and discectomy instruments can be simultaneously extended through the working channel. In that illustrated embodiment, the discectomy instruments could include a trephine for boring a hole through the disc annulus and a powered tissue cutter for excising the herniated disc nucleus. Likewise, the present invention contemplates the simultaneous introduction of other types of instruments or tools as may be dictated by the particular surgical procedure to be performed. For example, an appropriately sized curette and a rongeur may be simultaneously extended through the working channel into the working space. Since all operations being conducted in the working space are under direct visualization through the viewing element  50 , the surgeon can readily manipulate each of the instruments to perform tissue removal and bone cutting operations, without having to remove one tool and insert the other. In addition, since the surgical procedures can be conducted without the necessity of irrigation fluid, the surgeon has a clear view through the working space of the target tissue. Furthermore, aspects of the invention which permit a wide range of motion to the viewing element  50  allow the surgeon to clearly visualize the target tissue and clearly observe the surgical procedures being conducted in the working space. 
     The surgeon can capitalize on the same advantages in conducting a wide range of procedures at a wide range of locations in the human body. For example, facetectomies could be conducted through the working channel by simply orienting the working channel cannula  20  over the particular facet joints. The insertion of vertebral fixation elements can also be accomplished through the device  10 . In this type of procedure, an incision can be made in the skin posterior to the location of the vertebra at which the fixation element is to be implanted. Implementing the steps shown in FIG. 10, the cannula  20  can be positioned through the incision and tissue directly above the particular location on the vertebra to be instrumented. With the optics extending through the working channel, an insertion tool holding the vertebral fixation element can be projected through the cannula  20  and manipulated at the vertebra. In one specific embodiment, the fixation element can be a bone screw. The working channel  25  has a diameter that is large enough to accept most bone screws and their associated insertion tools. In many instances, the location of the bone screw within the vertebra is critical, so identification of the position of the cannula  20  over the bony site is necessary. As mentioned above, this position can be verified fluoroscopically or using stereotactic technology. 
     In many prior procedures, cannulated bone screws are driven into the vertebra along K-wires. The present invention eliminates the need for the K-wire and for a cannulated screw. The working channel itself can effectively operate as a positioning guide, once the cannula  20  is properly oriented with respect to the vertebra. Moreover, the device  10  allows insertion of the bone screw into the vertebra to be conducted under direct vision. The surgeon can then readily verify that the screw is passing into the vertebra properly. This can be particularly important for bone screws being threaded into the pedicle of a vertebra. The working channel cannula  20  can be used to directly insert a self-tapping bone screw into the pedicle, or can accept a variety of tools to prepare a threaded bore within the pedicle to receive a bone screw. 
     The device  10  can also be used to prepare a site for fusion of two adjacent vertebrae, and for implantation of a fusion device or material. For example, in one surgical technique, an incision can be made in the skin posterior to a particular disc space to be fused. The incision can be made anteriorly, posteriorly or posterior laterally. If the incision is made anteriorly for anterior insertion of the working channel, it is anticipated that care will be taken to retract tissues, muscle and organs that may follow the path of the incision to the disc space. However, the device  10  of the present invention allows this tissue retraction to occur under direct vision so that the surgeon can easily and accurately guide the cannula  20  to the disc space without fear of injury to the surrounding tissue. As the tissue beneath the skin is successively excised or retracted, the working channel cannula  20  can be progressively advanced toward the anticipated working space adjacent the vertebral disc. Again under direct vision, the disc space can be prepared for implantation of fusion materials or a fusion device. Typically, this preparation includes preparing an opening in the disc annulus, and excising all or part of the disc nucleus through this opening. 
     In subsequent steps, a bore is cut through the disc annulus and into the endplates of the adjacent vertebrae. A fusion device, such as a bone dowel, a push-in implant or a threaded implant can then be advanced through the working channel of device  10  and into the prepared bore at the subject disc space. In some instances, the preparatory steps involve preparing the vertebral endplates by reducing the endplates to bleeding bone. In this instance, some aspiration and irrigation may be beneficial. All of these procedures can be conducted by tools and instruments extending through the working channel cannula  20  and under direct vision from the viewing element  50 . 
     In some instances, graft material is simply placed within the prepared bore. This graft material can also be passed through the working channel cannula  20  into the disc space location. In other procedures, graft material or bone chips are positioned across posterior aspects of the spine. Again, this procedure can be conducted through the working channel cannula particularly given the capability of the cannula to be moved to different angles from a single incision site in the skin. 
     The present invention provides instruments and techniques for conducting a variety of surgical procedures. In the illustrated embodiments, these procedures are conducted on the spine. However, the same devices and techniques can be used at other places in the body. For example, an appropriately sized working channel device  10  can be used to remove lesions in the brain. The present invention has particular value for percutaneous procedures where minimal invasion into the patient is desirable and where accurate manipulation of tools and instruments at the surgical site is required. While the preferred embodiments illustrated above concern spinal procedures, the present invention and techniques can be used throughout the body, such as in the cranial cavity, the pituitary regions, the gastro-intestinal tract, etc. The. ability to reposition the viewing optics as required to visualize the surgical site allows for much greater accuracy and control of the surgical procedure. The present invention allows the use of but a single entry into the patient which greatly reduces the risk associated with open surgery or multiple invasions through the patient&#39;s skin. 
     In accordance with yet another aspect of the present invention, a tissue retractor apparatus  230  is provided that combines a tissue retractor  231  with an optical viewing device  232 . Referring to FIGS. 25-26, the retractor apparatus  230  includes a retractor plate  234  that is affixed to a grip  235  for manual support of manipulation of the retractor. The grip  235  is at the proximal end  236  of the plate. The distal end  237  of the retractor plate preferably has a blunt tip  238  to avoid trauma to tissue upon insertion and manipulation of the tissue retractor. Preferably, the blunt tip  238  is angled slightly away from the plate  234 . The retractor plate  234  defines an outer retraction surface  239  that can be configured according to the type of surgery being performed. In a preferred embodiment, the plate  234  is semi-cylindrical in configuration to permit atraumatic retraction of tissue adjacent a surgical site. In addition, the retractor plate  234  defines a channel  240  that helps define a working channel. As thus far described, the retractor  231  is substantially similar to the retractor  70  depicted in FIGS. 4-6 and as described above. 
     In accordance with this embodiment of the invention, an optical viewing device  232  is supported within the retractor  231  by way of a number of C-clips  245 . Preferably, the C-clips  245  are formed of a resilient material, such as plastic or thin flexible metal, and are affixed to the channel  240  of the retractor plate  234 . In accordance with one specific embodiment, two such C-clips  245  are provided to stably mount the optical viewing device  232  relative to the retractor  231 . Preferably, the clips  245  are sized to support an optical viewing device  232  that is configured substantially identical to the viewing device  50  described above. In the preferred embodiment, the viewing device  232  has a distal tip  52  with an angled lens  54 . In accordance with this embodiment, the C-clips  245  provide a resilient friction fit to the optical viewing device  232  while still permitting relative sliding and rotation of the viewing device  232  relative to the retractor  231 . 
     In accordance with the present invention, the tissue retractor apparatus  230  can be used in a variety of applications, including non-spinal applications. For example, this tissue retractor can have application in transnasal and transphenoidal surgeries, and in pituitary procedures. In surgeries of this type, it is not necessarily desirable to provide a closed cannula, such as the working channel cannula  20 . Moreover, the smaller working space does not lend itself to the use of a closed cannula which would tend to restrict the space available for manipulation of surgical instruments. Consequently, a tissue retractor or speculum of the type shown in FIGS. 25-26 may be very adequate for surgeries of this type. In this instance, then the working channel is defined in part by the patient&#39;s body itself, and in part by the tissue retractor. The optical viewing device  232  is supported relative to the retractor to permit the same degrees of motion as are available with the device  10  described above. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.