Patent Publication Number: US-2007118119-A1

Title: Methods and device for dynamic stabilization

Description:
TECHNICAL FIELD  
      This disclosure relates generally to methods and devices for accessing an area of a patient&#39;s spinal column during a surgical procedure. More particularly, this disclosure relates to an instrument that provides an access opening to the spinal column.  
     BACKGROUND  
      A wide variety of surgical techniques have been used to access the spinal column in spinal surgery procedures. For example, some techniques included making an incision in the patient&#39;s back and distracting or separating tissue and muscle to expose a wide area of the spine in order to perform the spinal surgery procedure. Such techniques often result in excessive invasions into the patient&#39;s spine and back region causing major damage to the normal anatomy, and significant and dangerous blood loss.  
      In an attempt to minimize risks associated with spinal surgery procedures, some surgical techniques have been developed wherein only portions of the spinal column area are accessed during various stages of the surgical procedure. In these procedures, a smaller incision can be used to access the portion of the spinal column area. However, access to only a portion of the spinal column area does not provide sufficient access for all surgical procedures.  
      In general, improvement has been sought with respect to such surgical techniques, generally to better provide sufficient accessibility to a spinal column area while minimizing anatomical trauma and blood loss.  
     SUMMARY  
      In one embodiment, the present invention is a method of stabilizing a motion segment of the spine. A posterior incision is formed lateral to a spinous process of the spine. A length and width of the incision is stretched. A portal having an open inner area sized and shaped to provide access to the pedicles of adjacent vertebrae and having a long axis aligned with an axis extending between the pedicles is inserted into the incision. Finally, a dynamic spinal stabilization device is implanted onto the pedicles through the open inner area of the portal.  
      In another embodiment, the present invention is a method of stabilizing a motion segment of the spine. A posterior incision is formed lateral to a spinous process of the spine. Incrementally larger sleeve members are inserted into the incision to stretch a length and a width of the incision. A portal having an open inner area sized and shaped to provide access to the pedicles of adjacent vertebrae and having a long axis aligned with an axis extending between the pedicles is inserted into the incision. A dynamic spinal stabilization device is implanted onto the pedicles through the open inner area of the portal.  
      In yet another embodiment, the present invention is a method of stabilizing a motion segment of the spine. A posterior incision is formed lateral to a spinous process of the spine. A portal having a sleeve portion defining an open inner area is inserted into the incision. Finally, a dynamic stabilization device is implanted onto the spine through the open inner area of the portal. This is done by attaching a pair of bone-engaging members to adjacent pedicles with a gap therebetween, attaching a flexible cord to a first of the bone-engaging members, advancing a spacer over the cord into the gap between the bone-engaging members, tensioning the cord and attaching the cord to the other of the bone-engaging members.  
      A variety of examples of desirable product features or methods are set forth in part in the description that follows, and in part will be apparent from the description, or may be learned by practicing various aspects of the disclosure. The aspects of the disclosure may relate to individual features as well as combinations of features. It is to be understood that both the foregoing general description and the following detailed description are explanatory only, and are not restrictive of the claimed invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of one embodiment of a surgical instrument according to the principals of the present disclosure, shown in a nested configuration.  
       FIG. 2  is a perspective view of the surgical instrument of  FIG. 1 , shown partially exploded.  
       FIG. 3  is a perspective view of the components of the surgical instrument of  FIG. 2 , shown disassembled.  
       FIG. 4  is a top plan view of one embodiment of a blade member according to the principals of the present disclosure, and shown in  FIG. 3 .  
       FIG. 5  is a front elevational view of the blade member of  FIG. 4 .  
       FIG. 6  is a side elevational view of the blade member of  FIG. 4 .  
       FIG. 7  is a top plan view of one embodiment of an inner portal member according to the principals of the present disclosure, and shown in  FIG. 3 .  
       FIG. 8  is a front elevational view of the inner portal member of  FIG. 7 .  
       FIG. 9  is a side elevational view of the inner portal member of  FIG. 7 .  
       FIG. 10  is top plan view of one embodiment of an intermediate portal member according to the principals of the present disclosure, and shown in  FIG. 3 .  
       FIG. 11  is a front elevational view of the intermediate portal member of  FIG. 10 .  
       FIG. 12  is a side elevational view of the intermediate portal member of  FIG. 10 .  
       FIG. 13  is a rear elevational view of one embodiment of an outer portal member according to the principals of the present disclosure, and shown in  FIG. 3 .  
       FIG. 14  is a cross-sectional view of the outer portal member of  FIG. 13 , taken along line  14 - 14 .  
       FIG. 15  is a cross-sectional view of the outer portal member of  FIG. 14 , taken along line  15 - 15 .  
       FIG. 16  is a top plan view of another embodiment of an outer portal member according to the principals of the present disclosure, shown in a retracted position.  
       FIG. 17  is a top plan view of the outer portal member of  FIG. 16 , shown in a distended position.  
       FIG. 18  is a perspective view of the outer portal member of  FIG. 17 .  
       FIG. 19  is a side elevational view two vertebrae.  
       FIG. 20  is a top plan view of one of the two vertebrae of  FIG. 19 .  
       FIG. 21  is a side elevation view of the outer portal member according to another embodiment of the present invention.  
       FIG. 22  is a top plan view of the outer portal member of  FIG. 21  relative to one of the two vertebrae of  FIG. 19 .  
       FIG. 23  is a side elevation view of the outer portal member according to another embodiment of the present invention.  
       FIG. 24  is a top plan view of the outer portal member of  FIG. 23  relative to one of the two vertebrae of  FIG. 19 .  
       FIG. 25  is a side view of a dynamic spinal stabilization device for use with various embodiments of the spinal access instrument of  FIGS. 1-18  and  21 - 24 , shown disassembled.  
       FIG. 26  is a posterior view of a patient showing the outer portal member installed to access adjacent vertebrae and showing a spacer template.  
       FIG. 27  is a side partial cross-sectional view of the installed outer portal member of  FIG. 26  taken along line A-A showing the bone-engaging members installed onto the pedicles and a measuring device for measuring the space between adjacent bone-engaging members.  
       FIG. 28  is a side partial cross-sectional view of the installed outer portal member of  FIG. 27  showing the cord of  FIG. 25  threaded through a first of the bone-engaging members.  
       FIG. 29  is a side partial cross-sectional view of the installed outer portal member of  FIG. 28  showing the spacer of  FIG. 25  threaded onto the cord.  
       FIG. 30  is a side partial cross-sectional view of the installed outer portal member of  FIG. 29  showing the cord threaded through the second of the bone-engaging members and a depressor employed to position the spacer between the bone-engaging members.  
       FIG. 31  is a posterior view of a patient showing a laterally offset incision adjacent the incision of  FIG. 26 . 
    
    
     DETAILED DESCRIPTION  
      Reference will now be made in detail to various features of the present disclosure that are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
       FIGS. 1-18  illustrate surgical instrument embodiments having features that are examples of how inventive aspects in accordance with the principals of the present disclosure may be practiced. Preferred features of the embodiments are adapted for providing a sufficient access opening to a spinal column area while minimizing risks associated with spinal surgery, such as incisional invasiveness, trauma, and blood loss.  
      Referring to  FIG. 1 , one embodiment of the spinal access instrument  10  is illustrated in complete assembly. The spinal access instrument is used to dissect skin tissue and muscle and provide a sufficiently sized opening for accessing a patient&#39;s spinal column. A sufficiently sized opening is an opening that is large enough to perform the desired surgical procedure. Preferably the opening provides access to a spinal column area or region such that the surgical procedure can be performed without having to provide more than one incision or opening.  
      For example, when performing a spinal procedure involving placement of pedicle screws (schematically represented in  FIG. 20  by dashed lines), preferably the accessed spinal column area or region includes first and second pedicle sites. As shown in  FIGS. 19 and 20 , the first and second pedicle sites or locations are the two sites (A 1 , A 2  (hidden) or B 1 , B 2 ) that are vertically aligned on upper and lower vertebral bodies V 1 , V 2 . That is, the access opening is preferably sized to provide access to the spinal column area including both the first pedicle site (e.g. B 1 ) and the second pedicle site (e.g. B 2 ) of two adjacent vertebrae.  
      Referring back to  FIG. 1 , the surgical instrument  10  generally includes a nested arrangement  12 , a first guide or placement wire  14 , and a second guide or placement wire  16 . As shown in  FIGS. 2 and 3 , the nested arrangement  12  of the spinal access instrument  10  includes a plurality of components or members sized so that each member fits with the other members in a nested configuration (as shown in  FIG. 1 ). In the nested configuration, each of the members at least partially contains or is at least partially contained within the other members. The plurality of nested members includes at least one portal member ( 18 ,  24  or  26 ) and a dissector or blade member  20 . As will be discussed in greater detail, the blade member  20  is used to provide an initial incision and the portal member provides access to the spinal column area through the incision.  
      Preferably the nested arrangement  12  is configured to incrementally provide an access opening to the spinal column area. What is meant by “incrementally provide an access opening” is that the arrangement provides an initial opening, and thereafter can be used to expand the opening (i.e. increase the cross-sectional area of the opening) as needed. By incrementally expanding the opening, surgical trauma and blood loss is minimized. In contrast, some existing procedures involve making an incision much wider than the incision needed by the present disclosure. The wider incision is needed in some existing procedures so that the skin tissue and muscle can be separated or pulled apart to adequately expose the spinal column area. This excessive invasion often results in anatomical trauma to the tissue or muscle and high blood loss.  
      In the illustrated embodiment of  FIGS. 2 and 3 , the nested arrangement  12  includes the blade member  20 , and second, third, and fourth sleeve members  24 ,  26 , and  18 ; although any number of sleeve members can be used in accord with the present disclosure. The second sleeve member or inner portal member  24  is slidably positionable over the blade member  20 . The second sleeve member  24  is sized to expand the area of initial incision created by the blade member  20  to a second opening area. The second opening area is generally defined by the outer perimeter of the second sleeve member  24 . The third sleeve member or intermediate portal member  26  is slidably positionable over the second sleeve member  24 . The third sleeve member  26  is sized and configured to expand the access opening from the second opening area defined by the second sleeve member  24  to a third opening area. The third opening area is generally defined by the outer perimeter of the third sleeve member  26 . Finally, the fourth sleeve member or outer portal member  18  is slidably positionable over the third sleeve member  26 . The outer portal member  26  is sized and configured to expand the access opening from the third opening area defined by the third sleeve member  26  to a final opening area. The final opening area is generally defined by the outer perimeter of the outer portal member  18 .  
      Referring now to  FIGS. 4-6 , the blade member  20  of the surgical instrument  10  includes a first end  28  and a second end  30 . The first end  28  of the blade member  20  is typically a solid construction defining a blade edge  22 . The blade edge  22  is configured to provide an initial incision of length IL ( FIG. 4 ) in the skin tissue and muscle of a patient. A handle  32  is located at the second end  30  opposite the first end  28  of the blade member  20 . As shown in  FIGS. 4 and 6 , the handle includes recessed areas  56  and an aperture  58  for gripping. The handle  32  can include a variety of shapes and geometries configured for gripping and moving the blade member  20  during use.  
      In general, the blade member  20  has an overall width W 1 , an overall height H 1 , and an overall length L 1 , although the disclosed principles can be applied in a variety of sizes and applications. The width W 1  of the blade member  20  is shown in  FIG. 5 , and is preferably between 19 mm and 58 mm (0.75 inches and 2.25 inches); more preferably between 38 mm and 45 mm (1.5 inches and 1.75 inches). The height H 1  of the blade member  20  is shown in  FIG. 6 , and is preferably between 4 mm and 10 mm (0.175 inches and 0.375 inches); more preferably between 5 mm and 7 mm (0.200 inches and 0.250 inches). The length L 1  of the blade member  20  is generally defined between the first end  28  and the second end  30  of the blade member  20 , excluding the handle  32 . The length L 1  of the blade member  20  is preferably between 88 mm and 140 mm (3.5 inches and 5.5 inches); more preferably between 101 mm and 127 mm (4.0 inches and 5.0 inches).  
      As shown in  FIGS. 4 and 5 , the blade member  20  includes first and second apertures  34 ,  36  extending along the length L 1  of the blade member  20 . The first and second aperture  34 ,  36  are offset from edges  38 ,  40  of the blade member  20  and extend from the first end  28  to the second end  30  of the blade member  20 . Each of the first and second apertures  34 ,  36  is sized and configured for receipt of the corresponding first and second placement wires  14 ,  16  ( FIG. 2 ). In the illustrated embodiment, the first and second placement wires  14 ,  16  are approximately 2 mm (0.08 inches) in diameter; correspondingly the first and second apertures  34 ,  36  are approximately 2.3 mm (0.09 inches) in diameter.  
      Referring now to  FIGS. 7-9 , the second sleeve member or inner portal member  24  of the nested arrangement  12  is illustrated. The second sleeve member  24  is generally a tubular construction having a first end  50  and a second end  52 . The tubular construction of the second sleeve member defines an elongated aperture  42  sized and configured for receipt of the blade member  20 . In particular, the second sleeve member  24  fits over the handle and slides along the blade member to nest with or cover the blade member  20 . The first end  50  of the second sleeve member  24  is tapered. In use, the tapered first end  50  assists in gradually expanding the access opening from the initial area of the incision created by the blade member  20  to the second opening area defined by the outer perimeter P 2  ( FIG. 8 ) of the second sleeve member  24 .  
      The second sleeve member  24  is configured to slide over the blade member  20  until shoulders  44  ( FIG. 4 ) of the blade member  20  contact stop structures  46  of the second sleeve member  24 . In the illustrated embodiment, the stop structures  46  include pins  48  positioned within the elongated aperture  42 . The pins  48  are positioned adjacent to the second end  52  of the second sleeve member  24 . Each of the pins  48  is offset from sidewalls  54  of the second sleeve member  24  so that when assembled as shown in  FIGS. 1 and 2 , the first and second placement wires  14 ,  16  extend between the pins  48  and the sidewalls  54  of the second sleeve member  24 .  
      In general, the second sleeve member  24  has an overall width W 2 , an overall height H 2 , and an overall length L 2 , although the disclosed principles can be applied in a variety of sizes and applications. The width W 2  of the second sleeve member  24  is shown in  FIG. 8 , and is preferably between 24 mm and 63 mm (0.95 inches and 2.45 inches); more preferably between 43 mm and 50 mm (1.70 inches and 1.95 inches). The height H 2  of the second sleeve member  24  is shown in  FIG. 9 , and is preferably between 9 mm and 15 mm (0.375 inches and 0.575 inches); more preferably between 10 mm and 12 mm (0.400 inches and 0.450 inches). The length L 2  of the second sleeve member  24  is generally defined between the first end  50  and the second end  52  of the second sleeve member  24 . The length L 2  of the second sleeve member is preferably between 95 mm and 146 mm (3.75 inches and 5.75 inches); more preferably between 107 mm and 134 mm (4.25 inches and 5.25 inches). The outer perimeter P 2  of the second sleeve member  24  defines the second access opening area; the second access opening area is generally between 180 and 716 square mm (0.28 and 1.11 square inches).  
      Referring now to  FIGS. 10-12 , the third sleeve member or intermediate portal member  26  of the nested arrangement  12  is illustrated. The third sleeve member  26  is also generally a tubular construction having a first end  60  and a second end  62 . The tubular construction of the third sleeve member  26  defines an elongated aperture  76  sized and configured for receipt of the second sleeve member  24 . In particular, the third sleeve member  26  fits over the second sleeve member  24  to nest with or cover the second sleeve member  24 . Similar to the second sleeve member, the first end  60  of the third sleeve member is tapered to assist in gradually expanding the access opening from the second opening area to the third opening area defined by the outer perimeter P 3  of the third sleeve member  26 .  
      The third sleeve member  26  slides over the second sleeve member  24  until notches  56  ( FIG. 7 ) of the second sleeve member  24  contact stop structures  66  of the third sleeve member  26 . In the illustrated embodiment, the stop structures  66  include pins  68  positioned within the elongated aperture  76 . The pins  68  are positioned adjacent to the second end  62  of the third sleeve member  26 . Each of the pins  68  is offset from sidewalls  78  of the third sleeve member  26  so that when assembled as shown in  FIG. 2 , the first and second placement wires  14 ,  16  extend between the pins  68  and the sidewalls  78  of the third sleeve member  26 .  
      In general, the third sleeve member  24  has an overall width W 3 , an overall height H 3 , and an overall length L 3 , although the disclosed principles can be applied in a variety of sizes and applications. The width W 3  of the third sleeve member  26  is shown in  FIG. 11 , and is preferably between 27 and 66 mm (1.08 inches and 2.58 inches); more preferably between 46 mm and 53 mm (1.83 inches and 2.08 inches). The height H 3  of the third sleeve member  26  is shown in  FIG. 12 , and is preferably between 17 mm and 23 mm (0.675 inches and 0.875 inches); more preferably between 17 mm and 19 mm (0.700 inches and 0.750 inches). The length L 3  of the third sleeve member  26  is generally defined between the first end  60  and the second end  62  of the third sleeve member  26 . The length L 3  of the third sleeve member is preferably between 95 mm and 146 mm (3.75 inches and 5.75 inches); more preferably between 107 mm and 134 mm (4.25 inches and 5.25 inches). The outer perimeter P 3  of the third sleeve member  26  defines the third access opening area; the third access opening area is generally between 368 and 1148 square mm (0.57 and 1.78 square inches).  
      Referring now to  FIGS. 13-15 , the fourth sleeve member or outer portal member  18  of the nested arrangement  12  is illustrated. The outer portal member  18  generally includes a sleeve portion  70  having a first end  82  and a second end  84 . The sleeve portion  70  defines an elongated aperture  74  that extends from the first end  82  to the second end  84 .  
      A handle portion  72  of the outer portal member  18  is located at the second end  84  of the sleeve portion  70 . The handle portion  72  can include a plurality of holes  80 . The holes  80  provide locations at which other surgical tools (not shown) can be attached for use during the surgical procedure.  
      In general, the outer portal member  18  has an overall width W 4 , an overall height H 4 , and an overall length L 4 , although the disclosed principles can be applied in a variety of sizes and applications. The width W 4  of the outer portal member  18  is shown in  FIG. 15 , and is preferably between 30 mm and 68 mm (1.19 inches and 2.69 inches); more preferably between 49 mm and 56 mm (1.94 inches and 2.19 inches). The height H 4  of the outer portal member  18  is also shown in  FIG. 15 , and is preferably between 20 mm and 25 mm (0.787 inches and 0.987 inches); more preferably between 20 mm and 22 mm (0.812 inches and 0.862 inches). The length L 4  of the outer portal member  18  is generally defined between the first end  82  and the second end  84  of the outer portal member  18 . The length L 4  of the outer portal member is preferably between 97 mm and 149 mm (3.85 inches and 5.85 inches); more preferably between 110 mm and 136 mm (4.35 inches and 5.35 inches). The outer perimeter P 4  of the outer portal member  18  defines the fourth or final access opening area; the fourth or final access opening area is generally between 477 and 1348 square mm (0.74 and 2.09 square inches).  
      In use, the surgical access instrument  10  provides access to first and second pedicle sites at a spinal column area or region. To begin a procedure, the first placement wire  14  is advanced through a patient&#39;s skin tissue and muscle until the wire  14  is positioned at a selected first pedicle site (e.g. B 1  in  FIG. 19 ) of a first vertebral body V 1 . The second placement wire  16  is positioned at a corresponding upper or lower second pedicle site (e.g. B 2  in  FIG. 19 ) of an adjacent vertebral body V 2 . The first and second pedicle sites are located a general distance D apart from one another. The site of the access opening is located at the region defined generally between and adjacent to the first and second placement wires  14 ,  16 .  
      While first ends of the first and second placement wires  14 ,  16  are positioned at the first and second pedicle locations, opposite ends of the placement wires  14 ,  16  are inserted within the first and second apertures  34 ,  36  at the first end  28  of the blade member  20 . The blade member  20  slides along the first and second placement wires  14 ,  16  in a first direction (represented by arrow A in  FIG. 2 ) until the blade member  20  is adjacent to the skin tissue located between the first and second placement wires  14 ,  16 . As the blade member  20  is further advanced toward the first and second pedicle sites, the blade edge  22  provides an initial incision through the skin tissue and muscle to the spinal column area. The surgeon can use hand force or a tapping hammer, for example, to advance the blade member along the placement wires  14 ,  16  to a desired depth.  
      When the blade member  20  is position at the desired depth adjacent to the spinal column area, the first end  50  of the second sleeve member  24  is positioned over the second end  30  of the blade member  20  ( FIG. 2 ). The second sleeve member  24  slides along the blade member  20  in the first direction A until the second sleeve member  24  is adjacent to the initial incision in the skin tissue. As the second sleeve member  24  is further advanced toward the spinal column area, the tapered first end  50  of the second sleeve member  24  is introduced into the initial incision and begins to enlarge the incisional area. The incisional area is incrementally enlarged to the second opening area defined by the outer perimeter of the second sleeve member  24 .  
      The second sleeve member  24  is inserted to a desired depth adjacent to the spinal column area, however cannot be inserted a depth exceeding the depth of the blade member  20 . That is, the stop structures  46  of the second sleeve member  24  contact the shoulders  44  of the blade member  20  to limit the insertion depth of the second sleeve member.  
      When the second sleeve member  24  is position at the desired depth adjacent to the spinal column area, the first end  60  of the third sleeve member  26  is positioned over the second end  52  of the second sleeve member  24  ( FIG. 2 ). The third sleeve member  26  slides along the second sleeve member  24  in the first direction A until the third sleeve member  26  is adjacent to the access opening in the skin tissue. As the third sleeve member  26  is further advanced toward the spinal column area, the tapered first end  60  of the third sleeve member  26  is introduced into the access opening and begins to enlarge the access opening. The access opening is incrementally enlarged from the second opening area to the third opening area defined by the outer perimeter of the third sleeve member  26 .  
      The third sleeve member  26  is inserted to a desired depth adjacent to the spinal column area, however cannot be inserted a depth exceeding the depth of the second sleeve member  24 . That is, the stop structures  66  of the third sleeve member  26  engage the notches  56  of the second sleeve member  24  to limit the insertion depth of the third sleeve member  26 .  
      Similar to the preceding steps, when the third sleeve member  26  is position at the desired depth adjacent to the spinal column area, the first end  82  of the outer portal member  18  is positioned over the second end  62  of the third sleeve member  26  ( FIG. 2 ). The outer portal member  18  slides along the third sleeve member  26  in the first direction A until the outer portal member  18  is adjacent to the access opening in the skin tissue. As the outer portal member  18  is further advanced toward the spinal column area, the first end  82  of the outer portal member  18  is introduced into access opening and begins to enlarge the access opening. The access opening is incrementally enlarged from the third opening area to the final opening area defined by the outer perimeter of the outer portal member  18 .  
      When the portal member  18  has been positioned at the desired depth adjacent to the spinal column area, each of the members  18 ,  20 ,  24 , and  26  are in the nested configuration, generally shown in  FIG. 1 . The access opening to the first and second pedicle sites at the spinal column area has been incrementally expanded to minimized incisional trauma and blood loss.  
      To continue the surgical procedure, each of the blade member  20 , the second sleeve member  24 , and the third sleeve member  26 , is removed from the elongated aperture  74  of the portal member  18 . Removing all three members  20 ,  24 , and  26  can be accomplished by simply grasping the handle  32  of the blade member  20  and pulling the blade member  20  out from the aperture  74  of the outer portal member  18 .  
      In particular, each of the blade, second sleeve and third sleeve members  20 ,  24 ,  26  are interconnected when moved in a second direction B ( FIG. 1 ) relative to the outer portal member  18 . That is, the shoulders  44  of the blade member  20  contact the pins  48  of the second sleeve member  24 , and the notches  56  of the second sleeve member  24  engage the pins  68  of the third sleeve member  26  to form an interconnection that permits all three nested members  20 ,  24 ,  26  to be simultaneously removed from the aperture  74  of the outer portal member  18 . Thus, as a surgeon pulls the blade member  20  from the aperture  74 , the blade member  20  interconnects with the second sleeve member  24  and the second sleeve member interconnects with the third sleeve member  26  so that the three nested and interconnected members  20 ,  24 ,  26  can be removed at the same time.  
      When the three nested members  20 ,  24 , and  26 , are removed from the elongated aperture  74  of the outer portal member  18 , the surgeon now has access to first and second pedicle sites at the spinal column area. The access is provided through the elongated aperture  74 ; thereby the elongated aperture  74  of the outer portal member  18  is sized and configured to correspond to the distance (D) between the first and second pedicle sites. More preferably, the elongated aperture  74  provides access to each of the first and second pedicle sites and the immediate surrounding area of each pedicle site at the spinal column area. In the illustrated embodiment, the elongated aperture  74  is sized and configured to receive and guide pedicle screws into the first and second vertebral bodies at the first and second pedicle sites.  
      It is to be understood that the placement wires  14 ,  16  may or may not be removed from the elongated aperture  74  with the three nested members  20 ,  24 ,  26 . In some procedures, pedicle screws having a bore extending through the screw shaft are positioned on the placement wires. The placement wires therein act as guide wires to direct the pedicle screws to the first and second pedicle sites. In other procedures, the first and second placement wires  14 ,  16  are removed with the three nested members  20 ,  24 ,  26  and the screws are engaged by an appropriate driving tool and positioned down into the aperture to the first and second pedicle sites. In yet another alternative, the placement wires  14 ,  16  can be removed from the blade member  20  after the blade member  20  has been properly positioned adjacent to the spinal column area.  
      The pedicle screws can include a variety of pedicle screw configurations known in the art. Typically the diameter of pedicle screws range between about 5 mm and 8 mm. These specific dimensions are merely illustrative of normal configurations and can be varied as needed. Accordingly, the elongated aperture  74  of the outer portal member  18  can be varied to accommodate the variety of pedicle screw configurations.  
      Referring now to  FIGS. 16-18 , a second embodiment of an outer portal member or fourth sleeve member  118  is illustrated. In this embodiment, the outer portal member  118  generally includes a sleeve portion  170  having a first end  182  and a second end  184 . The sleeve portion  170  defines an elongated aperture  174  that extends from the first end  182  to the second end  184 . The second outer portal member embodiment  118  generally has similar overall width, height, and length dimensions as the first outer portal member  18  shown in  FIGS. 13-15 .  
      The sleeve portion  170  illustrated in the second embodiment, however, includes a first sleeve section  186  and a second sleeve section  188  that define the elongated aperture  174 . The first and second sleeve sections  186 , 188  are coupled to a flange or collar  190  at pivot locations  192 . Each of the first and second sleeve sections  186 , 188  is configured to rotate or pivot, relative to the collar  190 , from a retracted position (shown in  FIG. 16 ) to a distended position (shown in  FIGS. 17 and 18 ).  
      The second end  184  of each of the sleeve sections  186 ,  188  is angled such that an inner region  194  of each section is longer than an outer region  196 . In other words, the second end  184  of each section has an oblique edge construction  198  (partially shown in  FIG. 16 ) relative to the inner and outer regions  194 ,  196  of the first and second sleeve sections  186 ,  188 .  
      The outer portal member  118  further includes a clamp plate  210  positioned adjacent to the collar  190 . Typically, the clamp plate  210  is positioned in relation to the collar  190  so that a gap G is provided between the collar  190  and the clamp plate  210 . Alignment spacers  202  in cooperation with holes  206  formed in the clamp plate  210  properly orient the clamp plate  210  relative to the collar  190  so that an opening  212  in the clamp plate  210  is aligned with the elongated aperture  174  of the sleeve portion  170 . The alignment spacers  202  can also be configured to maintain the gap G between the collar  190  and the clamp plate  210 . For example, the alignment spacers  202  can be configured to provide a sufficient interference fit with the holes  206  formed in the clamp plate  210  such that the clamp plate  210  seats in an offset position from the collar  190  when no force is applied. In the illustrated embodiment, the spacers  202  are pegs  204  extending from a first surface  200  of the collar  190 .  
      As shown in  FIG. 16 , when the gap G is provided between the collar  190  and the clamp plate  210 , the first and second sleeve sections  186 ,  188  remain in the retracted position. In the retracted position, the outer portal member  118  can be introduced into an access opening area as previously described with respect to the first outer portal member embodiment.  
      When the outer portal member  118  is positioned adjacent to the spinal column area at the desired depth, and the three nested members  20 ,  24 ,  26  are removed from the elongated aperture  174 , the first and second sleeve sections  186 ,  188  can be outwardly distended to further expose the first and second pedicle sites. In particular, the clamp plate  210  can be forcibly positioned to contact the first surface  200  of the collar  190  ( FIGS. 17 and 18 ). As the clamp plate  210  is forced towards the collar  190 , the clamp plate  210  contacts the oblique edge construction  198  of the second end  184  of the first and second sleeve sections  186 ,  188 . The force from the clamp plate  210  pivots the first end  182  of the first and second sleeve members  186 ,  188  outward away from one another. That is, the second end  184  of the first and second sleeve members  186 ,  188  pivot about pivot locations  192 , and the first end  182  of the first and second sleeve members  186 ,  188  rotate in opposite directions from one another.  
      The clamp plate  210 , spacers  198 , and collar  190  can be configured such that a surgeon can forcibly position the outer portal member  118  in the distended position by hand, or such that a clamp (not shown) is required to press the clamp plate  210  toward the collar  190 . The pivoting design of this second outer portal member embodiment provides a greater access opening adjacent to the spinal column area without having to expand the access opening in the tissue and muscle region of the patient&#39;s back. This is advantageous in further reducing trauma in situations where access to a larger spinal column area is needed.  
      The above specification provides a complete description of SPINAL ACCESS INSTRUMENT. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.  
      Various surgical procedures may be performed on the surgical site through the outer portal member  18 . For example, spinal stabilization devices may be installed onto the spine through the outer portal member  18  to stabilize a motion segment of the spine. A dynamic stabilization is a device that acts to stabilize the spine while permitting movement and flexibility of the spine. Such device may also be known as non-fusion devices. In one embodiment, a dynamic stabilization device such as the Dyneses® Dynamic Stabilization System may be installed through the portal (available from Zimmer, Inc. of Warsaw, Ind.).  
      In one embodiment, the outer portal member  18  and in particular the length and width of the aperture  74  is sized and shaped to facilitate installing a dynamic stabilization device such as the dynamic stabilization device shown in  FIG. 26  onto the spine.  FIGS. 21-25  show examples of outer portal members  18  which are so sized and shaped and which may include additional features to facilitate installation of a dynamic stabilization device.  
       FIG. 21  shows an outer portal member  18  according to another embodiment of the present invention. The outer portal member  18  is similar to those shown in the preceding figures and like parts are given like numbering. However, the first end  82  of the outer portal member  18  is asymmetric, such that a first side  83  is longer than a second side  85 . The asymmetric first end  82  may be employed to contour around the bony anatomy of the pedicles, as shown in  FIG. 22 . The asymmetric first end  82  may also be employed to accommodate an angle of insertion of the outer portal member  18  that is angled relative to the spine, as shown in  FIG. 22 . The second side  83  of the outer portal member  18  is shown in  FIG. 21 , and is preferably between 5 and 15 mm longer than the first side  85 ; more preferably between 8 and 12 mm longer than the first side  85 .  
       FIG. 23  shows an outer portal member  18  according to another embodiment of the present invention. The outer portal member  18  is similar to those shown in the preceding figures, and like parts are given like numbering. However, the handle portion  72  of outer portal member  18  is angled relative to the sleeve portion  70 . As shown in  FIG. 23 , an outer portal member  18  having this configuration accommodates an angle of insertion that is angled relative to the spine. While the sleeve portion  70  is inserted into the patient at an angle relative to the spine that is non-perpendicular, as shown in  FIG. 24 , the handle portion  72  rests flush against the patient&#39;s skin to provide increased stability. The angle of the handle portion  72  relative to the sleeve portion  70  is shown in  FIG. 23 , and is preferably between 30° and 60°; more preferably between 40° and 50°.  
      The spinal access instrument  10  may be made of a variety of sterilizable materials, including, but not limited to, stainless steel, plastic and titanium. The outer portal member  18  is preferably made of a material that is radiolucent, such as titanium. A radiolucent material such as titanium is translucent when imaged with fluoroscopy. This construction permits the use of fluoroscopy to image or visualize the surgical site during installation of a spinal device without the outer portal member  18  blocking the view.  
       FIG. 25  illustrates the components of an exemplary dynamic stabilization system  330  which may be installed through an outer portal member  18  according to various embodiments of the present invention. Dynamic stabilization system  330  includes pedicle screws  332 , a spacer  334  and a cord  336 . The pedicle screws  332  are bone-engaging members and are attached to the vertebrae to anchor the spacer  334  and cord  336 . The pedicle screws  332  include a through-hole  338  for receiving the cord  336  and an opening  340  to the through-hole  338  for receiving a set screw  342 . The set screw  342  is threaded into the opening  340  to fix the cord  336  to the pedicle screw  332 . The spacer  334  is used to hold the segment in a more natural anatomical position and to control the spine in extension. The cord  336  controls forward flexion movement.  
       FIGS. 26-31  show various stages of a surgical procedure in which the dynamic stabilization system  330  is installed or implanted onto the spine through the outer portal member  18 . As shown in  FIG. 26 , the patient is placed into a prone position. Other positions, such as a knee-chest position, are also acceptable provided that care is taken to preserve the natural lordosis in the lumbar spine as well as to avoid any pressure on the abdominal cavity that might results in excessive bleeding. The illustrative example shows the incision at a lumbar region of the spine; however, it is contemplated that the methods and apparatuses described herein are equally adapted to use in other regions of the spine.  
      The outer portal member  18  is installed to provide access to the pedicles of adjacent vertebrae V 1  and V 2  as previously described.  
      It should be realized that in some embodiments, however, the spinal access instrument  10  is installed without the use of first and second placement wires  14 ,  16 . For example, the blade member  20  may be held in place manually and advanced toward the first and second pedicle sites. Alternately, a scalpel or other cutting device may be employed to create an incision extending between the first and second pedicle sites. The blade member  20  may then be advanced into the incision to access the first and second pedicle sites and to stretch the incision. The second, third and fourth sleeve members  24 ,  26  and  18  are slid over the blade member  20  as previously described.  
      Optionally, after making an initial incision with a scalpel or other cutting device, the surgeon may use their fingers to push aside muscle tissue above the vertebrae rather than cutting through the muscle. Once the appropriate muscle tissue has been pushed aside, the blade member  20  is advanced into the incision. The blade member  20  holds the displaced tissue away from the surgical site while the second, third and fourth sleeve members  24 ,  26  and  18  are advanced into the incision. This method may reduce trauma to the patient.  
      The spinal access instrument  10  may be initially inserted at an angle relative to the spine so that a long axis of the outer portal sleeve portion  70  is oriented at an angle to the spine as shown in  FIG. 24 . In one embodiment, the outer portal sleeve portion  70  is oriented at an angle of between 30° and 60° relative to the spine. In this manner, the pedicle screws  332  are more easily driven into the pedicles at a corresponding angle. This provides increased strength and stability of the connection between the bones of the vertebrae and the pedicle screws  332 .  
      As shown in  FIG. 26 , a lighting device  324  such as a fiber optic light source may be attached to the outer portal member  18  so as to illuminate the exposed surgical site within the elongated aperture  74 . Additional accessories may be attached to the outer portal member  18 , including, for example, a fluid removal or suction device or an endoscopic viewing device. A stabilization arm  326  may be attached to the outer portal member  18  and attached to the patient&#39;s body, the surgical table or to some other stationary device to stabilize the position of the outer portal member  18  and prevent dislodgement of the outer portal member  18  from the patient.  
      A tool may be employed to remove or push aside tissue obscuring the spine so as to expose portions of the spine within the elongated aperture  74 . For example, a scraper, rongeur or electrocautery device may be employed to expose the facets, pedicles or other appropriate portions of the vertebrae so as to permit installation of the spinal stabilization device. Such a tool is maneuvered by the physician through the outer portal member  18  to access the surgical site. Furthermore, such tools may include a lateral offset such that the tool does not obstruct the view of the surgical site through the outer portal member  18 .  
      A spacer template or guide  344  may be employed to determine the correct position of the pedicle screws  332  relative to the facet joints. For example, the template  344  may be configured to facilitate driving the pedicle screws  332  into the correction portion of the vertebrae and at a chosen angle, as previously described. With the template  344  in position, a tool such as a bone awl may be employed to pierce the cortical bone of the exposed pedicles. A probe may then be employed to establish a channel for insertion of each of the pedicle screws  332  into the pedicles. The orientation of the probe generally determines the ensuing orientation of the pedicle screws  332 . Fluoroscopy or X-ray visualization devices may be employed to determine the position of the vertebrae relative to the outer portal member  18  and to facilitate proper placement of the bone awl and probe relative to the vertebrae. Doing so increases the likelihood that the pedicle screws  332  will be subsequently installed in the correct orientation and position.  
      The length of the pedicle screws  332  depends on the patient morphology, and may be from about 35 to about 55 mm. Care should be taken so as to avoid inserting the probe deeper into the pedicles than the length of the intended pedicle screw  332 . To avoid over-insertion of the probe, the probe may be provided with depth markings corresponding to pedicle screw lengths. Alternately, a sleeve provided with depth markings at a proximal end may be fit over the probe.  
      The probe is removed and the intactness of the pedicle wall may be checked with a pedicle sound. If the pedicle wall is determined to be sufficiently intact, a first pedicle screw  332  is driven into the pedicle, as shown in  FIG. 27 . Optionally, a guide device such as a guide pin is coupled to the pedicle screw  332  (not shown). The guide device may improve the orientation possibilities and may make subsequent instrument positioning easier. However, care should be taken to avoid over-tightening the guide device onto the pedicle screw  332 , which could cause difficulty in loosening later.  
      A driving tool such as a screw driver is employed to drive the pedicle screws into the channel. A portion of the tool may be laterally offset to avoid blocking the user&#39;s view of the surgical site through the outer portal member  18  while the pedicle screws are driven into the pedicles. Optionally, a stabilizing device such as a T-handle is used in conjunction with the driving tool to facilitate insertion of the screw. Such a stabilizing device may reduce wobbling of the screw during tightening.  
      As shown in  FIG. 27 , in general, the pedicle screw  332  is advanced as deep as possible, i.e., when a head portion of the screw  332  is in contact with the bone. Care should be taken to avoid over-torquing the screws  332  or exerting an excessive bending load, as this could fracture the pedicle. After the pedicle screw  332  is driven fully into the channel, the guide pin or other guiding or driving device may be removed. Alternately, the guide device may remain coupled to the pedicle screw  332  until the remainder of the stabilization device is assembled.  
      The process described above is repeated to install the second pedicle screw  332  into place. The pedicle screws  332  should be aligned with one another so that the through-holes  338  are aligned and will allow passage of the cord  336  therethrough.  
      After insertion of the pedicle screws  332 , the distance between the pedicle screws  332  is measured to determine the appropriate length of the spacer  334 . A drag indicator  346  as shown in  FIG. 23  may be employed to measure the distance between the screws  332  and to assess the movement in the facets in distraction and compression. As shown in  FIG. 27 , the ends of the drag indicator  346  are inserted into the through-holes  338  of the pedicle screws  332 . The distance between the screws  332  is measured under a slight distraction force. The size of the spacer  334  is chosen relative to the distance between the pedicle screws  332  as well as to achieve various physiological effects. For example, the spacer  334  might be slightly oversized to distract the segments to create parallel vertebral end plates. Alternately, the spacer  334  might slightly oversized to distract the segments to create a neutral facet joint position. An appropriately-sized pre-cut spacer  334  may be chosen, or the spacer  334  may be custom cut according to the patient&#39;s measurements.  
      As shown in  FIG. 28 , the flexible cord  336  is threaded through the through-hole  338  of a first of the pedicle screws  332 . A cord threader may be used to guide the cord  336  into the through-hole  338 . Because the aperture  74  is oblong and aligned in with the an axis extending between the pedicle screws  332 , the cord  336  remains aligned to the pedicle screws  332  and organized during the procedure to reduce kinking and misalignment of the cord  336 .  
      A set screw  342  is tightened into the pedicle screw opening  340  to fix the cord  336  to the pedicle screw  332 . An anti-torque device may be inserted over the guide pin (if in place) and screw  332  to overcome any binding of the guide pin. The set screw  342  may be tightened with a laterally offset driver or other tool so as to avoid obscuring user&#39;s view of the exposed incision through the outer portal member  18 .  
      As shown in  FIG. 29 , the spacer  334  is then threaded onto the cord  336  and advanced over the cord  336  into the portal opening  14 . The spacer  334  is advanced into a position against the first screw  332 . As shown in  FIG. 30 , the cord  336  is then threaded through the through-hole  338  of the second pedicle screw  332 . The cord  336  is tensioned until the spacer  334  is positioned between the first and second pedicle screws  332 . A cord tensioner may be employed to tension the cord  336  and to pull the spacer  334  into position between the pedicle screws  332 . Other tools such as a depressor and/or forceps  348  may be employed to grip the spacer  334  and position it between the pedicle screws  332 . Again, such tools may be laterally offset to avoid blocking the user&#39;s view of the incision through the outer portal member  18 . In other embodiments, the spacer  334  may be threaded onto the cord  336  prior to the first set screw  342  being tightened onto the opening  340  of the first pedicle screw  332 .  
      Tension is maintained on the cord  336  and may be adjusted according to the desired patient result. A second set screw  342  is inserted into the opening  340  of the second pedicle screw  332  and tightened to fix the spacer  334  into position between the pedicle screws  332 .  
      In other embodiments, the cord  336  is threaded through second pedicle screw  332  before the spacer  334  is positioned between the pedicle screws. This avoids blocking the through-hole  338  of the second pedicle screw  332  while attempting to thread the cord  336 . After the cord  336  is threaded through the second pedicle screw  332 , the spacer  334  is maneuvered into position, the cord  336  is tensioned and the second set screw  342  is tightened. The cord  336  may be further tensioned and either or both of the set screws  342  tightened further.  
      Excess portions of the cord  336  may be trimmed and removed. The outer portal member  18  may then be removed and the incision closed.  
      As shown in  FIG. 31 , the procedures previously described herein may be repeated on the adjacent pedicles of the vertebrae to stabilize the adjacent segment. Thus, a second incision is made approximately 3.5 cm lateral to the spinous process on the other side of the spine. The incision is dilated as previously described, the outer portal member  18  is inserted and a second spinal stabilization device  330  is installed on the pedicles of the vertebrae.  
      The procedures previously described herein may be employed to install a multi-level spinal stabilization device. A multi-level device is one in which multiple vertebral motion segments along the longitudinal axis of the spine are stabilized. Thus, the procedures previously described herein may be employed to install spinal stabilization devices on segments above and/or below the stabilized motion segment to stabilize greater portions of the spine.  
      The present method is not limited to installation of a dynamic stabilization device as described and shown in  FIG. 21 . Other types of dynamic spinal stabilization devices that permit motion and flexion of the spine may be installed on the spine according to the methods and devices previously described.  
      Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.