Abstract:
Apparatus, systems, and methods for spine surgery employ a guide wire temporarily anchored to a contralateral side of an intervertebral disc. The guide wire establishes a reliable pathway for passage and actuation of cannulated instruments or implants through a small working channel leading to an ipsilateral side of the disc. The guide wire may be disconnected and removed from the disc after use.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the filing date of: 
         [0002]    U.S. Application No. 61/324,384 filed Apr. 15, 2010, entitled DIRECT LATERAL SPINE SYSTEM—INSTRUMENTS, IMPLANTS AND ASSOCIATED METHODS, attorney&#39;s docket no. MLI-83 PROV, which is pending. 
         [0003]    The above-identified document is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0004]    The present disclosure relates to orthopedics, and more particularly, to apparatus, systems, and methods for spinal surgery through a lateral approach. 
         [0005]    The lateral approach to the spine is appealing because it avoids the major neural and vascular structures on the anterior and posterior aspects of the spine. However, the lateral approach may encounter other sensitive structures, such as the psoas muscle, neural plexus, vascular plexus, or other peritoneal structures. The psoas muscle is of particular interest because it is highly innervated. Therefore, insult to the psoas muscle is quite painful. Surgical procedures or approaches which minimize trauma to the psoas muscle may reduce postoperative pain and improve outcomes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Various embodiments will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments and are therefore not to be considered limiting of the scope of the invention as set forth in the claims. 
           [0007]      FIG. 1  is a perspective view of a patient in a prone position on a surgical table; 
           [0008]      FIG. 2  is a perspective view of a patient in a lateral decubitus position on a surgical table; 
           [0009]      FIG. 3  is a perspective view of Caspar pins and spheres in pedicles on the same side of adjacent vertebrae in a segment of a torso; 
           [0010]      FIG. 4  is a perspective view of the pins and spheres of  FIG. 3  with a targeting apparatus oriented and positioned relative to an intervertebral disc between the vertebrae; 
           [0011]      FIG. 5A  is a perspective view of the pins and spheres of  FIG. 3  with a stylus oriented and positioned relative to the intervertebral disc; and  FIG. 5B  is the perspective view of  FIG. 5A  with the psoas muscle omitted for clarity; 
           [0012]      FIG. 6  is a perspective view of the pins, spheres, and stylus of  FIGS. 5A-5B  with a neural monitoring probe inserted inside the stylus; 
           [0013]      FIG. 7  is an enlarged detail view of the tip of the stylus and a portion of the intervertebral disc; 
           [0014]      FIG. 8  is a perspective view of the pins, spheres, and stylus of  FIGS. 5A-5B  with a retractor positioned over the stylus; 
           [0015]      FIG. 9  is a perspective view of the pins, spheres, and retractor of  FIG. 8  with the retractor docked to a sphere on a pin, and with a dilator positioned within the retractor; 
           [0016]      FIG. 10  is a lateral view of the pins, spheres, retractor, and dilator of  FIG. 9 , showing the retractor docked to both pins; 
           [0017]      FIG. 11  is a perspective view of the pins, spheres, and retractor of  FIG. 8  with the retractor docked to a rail of a surgical table, and with a dilator positioned within the retractor; 
           [0018]      FIG. 12  is a cephalad view of an anchor and inserter instrument positioned and oriented relative to the vertebrae, with the anchor in an insertion configuration; 
           [0019]      FIG. 13  is an enlarged cephalad view of the anchor of  FIG. 12  in a deployed configuration; 
           [0020]      FIG. 14  is a perspective view of the pins, spheres, retractor, and dilator of  FIG. 9  with the anchor of  FIG. 13  positioned on the contralateral side of the disc from the retractor; 
           [0021]      FIG. 15  is a perspective view of another anchor and inserter instrument positioned and oriented relative to the vertebrae, with the anchor in an insertion configuration on an ipsilateral side of the disc to the inserter; 
           [0022]      FIG. 16  is a perspective view of the anchor of  FIG. 15  positioned on the contralateral side of the disc from the inserter instrument; 
           [0023]      FIG. 17  is a perspective view of the anchor of  FIG. 15  in a deployed configuration against the contralateral side of the disc; 
           [0024]      FIG. 18A  is a perspective view of a portion of an auger over a guide wire;  FIG. 18B  is a perspective view of a portion of a paddle distractor, or spade drill, over a guide wire;  FIG. 18C  is a perspective view of a portion of a shaver over a guide wire;  FIG. 18D  is a perspective view of a portion of an expandable retrograde cutter over a guide wire; and  FIG. 18E  is a perspective view of the cutter of  FIG. 18D  in an expanded configuration; 
           [0025]      FIG. 19  is a perspective view of a trial implant and a portion of an inserter instrument shaft; 
           [0026]      FIG. 20  is a perspective view of an implant and an inserter instrument; 
           [0027]      FIG. 21A  is a perspective view of two portions of a segmented spinal fusion cage;  FIG. 21B  is a perspective view of a first portion of the cage of  FIG. 21A  on a portion of an inserter shaft, with the first cage portion positioned relative to a vertebral endplate;  FIG. 21C  is a perspective view of a second portion of the cage of  FIG. 21A  on a portion of another inserter shaft, with the second cage portion engaging the first cage portion;  FIG. 21D  is a perspective view of the first and second cage portions positioned relative to a vertebral endplate and connected to the inserter shafts; and  FIG. 21E  is a perspective view of the first and second cage portions in their final implanted configuration; 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    The present disclosure relates to systems and methods for direct lateral approach spine surgery. Those of skill in the art will recognize that the following description is merely illustrative of principles which may be applied in various ways to provide many different alternative embodiments. This description is made for the purpose of illustrating the general principles and is not meant to limit the inventive concepts in the appended claims. 
         [0029]    One principle in the present disclosure is to reduce trauma to the psoas muscle. For example, the present disclosure teaches that the discectomy portion of a spinal procedure may be performed through a very small working portal. In another example, the present disclosure teaches that all instruments may be cannulated or otherwise adapted to be guided into the disc space by a guide-wire that is anchored to the disc annulus and/or vertebral body. These guided instruments would not require visualization of the procedural site, allowing for the minimal dissection size. In yet another example, at least some of the instruments may expand at the distal (working) tip to clear a path larger than that created outside the disc space by the access corridor. Once the discectomy is complete, the corridor through the psoas may be dilated to a larger size to accommodate implant placement. 
         [0030]    A segmented fusion cage is disclosed. With a segmented implant, the entire procedure may be performed through a smaller working cannula. It may not be necessary to dilate the access corridor to a larger size after discectomy for fusion cage placement. The disclosed segmental fusion cage may be in two parts, so that half of the implant may be placed, followed by the other half. 
         [0031]    Various apparatus, systems and methods will now be described with reference to the drawings. 
         [0032]    A method of positioning a patient  2  for lateral spine surgery may place the patient  2  in a prone position, as illustrated in  FIG. 1 . A radiolucent surgical table  4  may be used in order to position the patient  2  optimally and provide an unrestricted view for imaging. The table  4  may allow the abdominal contents to hang freely for safer navigation through the retroperitoneal space, and to ease venous drainage. Arms and legs may be supported as necessary. Bony prominences may be padded. The patient  2  may be secured to the table  4  and immobilized to minimize shifting during the procedure. 
         [0033]    Another method of positioning a patient  2  for lateral spine surgery may place the patient  2  in a lateral decubitus position, as illustrated in  FIG. 2 . A bendable surgical table  4  may be used in order to position the patient  2  optimally. It may be helpful to position the patient  2  so that the iliac crest is directly over a bend, or break, in the table  4 . This may allow the pelvis to tilt away from the spine, and may provide clear access to the lumbar spine, such as the L4-L5 level. The patient  2  may be supported, padded, secured, and immobilized similar to the above description for the prone position. 
         [0034]    Regardless of the patient position, the table  4  may provide one or more rails  16  to which apparatus may be secured. Specialized adapters may be useful with certain tables, depending on rail design. 
         [0035]    After the patient  2  is prepared and draped, fluoroscopy may be used to locate the relevant spinal anatomy. The use of biplanar fluoroscopy may facilitate this step. Anatomy may be marked on the skin for later reference. 
         [0036]      FIG. 3  illustrates an optional step which may be used when the patient  2  is in the prone position. The table  4  and C-arm (not shown) may be oriented to provide distinct anterior-posterior and lateral images of the vertebral endplates  18 . A Jamshidi needle (not shown) may be used to target the contralateral pedicle  20 , and its location verified with biplanar fluoroscopy. A k-wire (not shown) may be placed through the Jamshidi outer sheath, which may then be removed. A Caspar pin  22  may be screwed into the pedicle  20  over the k-wire, and a split clamping sphere  24  attached to the Caspar pin  22 . This optional step may be repeated at each operative level. 
         [0037]    A method of locating the appropriate disc space may rely upon a targeting post  30 . This method is illustrated in  FIG. 4  for a curved lateral approach. The appropriate disc space  26  may be located prior to making the skin incision. The location of the superior edge of the transverse process-pedicle junction  28  in the anterior-posterior plane may be determined and a tiny stab hole made with a scalpel blade. A targeting post  30  with a depth stop  32  may be inserted onto the transverse process-pedicle junction  28 . The targeting post  30  may then slide over the superior edge of the transverse process-pedicle junction  28 . A nerve monitoring cable alligator clip (not shown) may be attached as well. 
         [0038]    The targeting post  30  may be positioned and oriented so that the tip  34  of the obturator  36  is seated at the midpoint of the superior endplate  18  of the inferior vertebra  38 . Lateral fluoroscopy may be used to verify the position of the obturator tip  34 . The targeting post depth stop mechanism  32  may be engaged. The obturator  36  may be oriented collinear with the superior endplate  18  of the inferior vertebra  38 . Lateral fluoroscopy may be used to verify the orientation of the obturator  36 . The micrometer  40  may be aligned with the spinous processes  42  and its position verified by anterior-posterior fluoroscopy. The targeting post  30  may be affixed to the clamp on the articulating surgical support arm. 
         [0039]    The initial dilator  48  may be inserted into the offset arm  50  of the targeting apparatus  52  using the cam clamp  54 . The initial dilator  48  shown in  FIG. 4  is curved, although a straight dilator may be used instead. The initial dilator  48  may be attached to the targeting post  30 . The telescoping offset arm  50  may be fully extended. The distal tip  56  of the initial dilator  48  may be swung onto the body. A skin marker may be used to mark the initial incision location  58 . 
         [0040]    An incision may be made through the skin into the initial fascial plane. The skin incision may be extended through the muscle fibers of the external and internal obliques and transversus abdominis. The initial incision may be large enough to permit finger dissection. Once through the abdominal muscles, blunt finger dissection may be used to access the retroperitoneal space. These anatomical structures are not shown but are well documented elsewhere, for example, in anatomy texts. Blunt finger dissection may extend down the transverse process to the psoas muscle  60 . Optionally, a slender blunt instrument, such as a Kittner probe (not shown), may be used to gently dissect through the psoas muscle  60  between the middle and anterior third. One or more peritoneal retractors (not shown) may be used, as necessary. A neural monitoring system (not shown) may be clipped to the retractors. 
         [0041]    An electrode (not shown) may be attached to the proximal end  62  of the initial dilator  48 . Some or all of the dilators disclosed herein may include insulation to decrease current shunting whenever there is an exposed distal electrode at the tip of the dilator that acts as the stimulation source. The initial dilator  48  may be inserted all the way through the psoas muscle  60  up to the disc space  26 , while observing the neural monitoring system, as shown in  FIG. 4 . The descending nerves of the lumbar plexus lie in the posterior third of the psoas muscle  60 . Intra-operative neural monitoring may facilitate safe passage past these nerves and may confirm their location with evoked electromyogram (EMG) monitoring. As the dilators advance through the psoas muscle  60 , the stimulus necessary to elicit an EMG response may vary based on the proximity of the dilator to the lumbar plexus. The closer the stimulus source is to the nerve, the less intensity may be required to elicit a response. The literature suggests that threshold values greater than 10 mA may correlate to a distance that allows for both continued nerve safety and ample working space. Values less than 10 mA may be regarded as being too close to the exiting nerve root. In this situation, it may be advisable to reposition the dilator. Cranial-caudal adjustments may be made with the micrometer  40  so that the distal tip  56  of the initial dilator  48  is in a mid-disc location of the affected disc  26 . Lateral fluroroscopy may be used to verify the position of the initial dilator tip  56 . The anterior-posterior dilator location and nerve location may be confirmed with neural monitoring prior to placing the guide pin. 
         [0042]    A guide pin or k-wire (not shown) may be placed down the first dilator  48  and may be inserted into the disc space  26  to secure the dilator  48 . The guide pin or k-wire may be inserted at least half way across the disc space  26 . The position of the guide pin or k-wire may be verified with biplanar fluoroscopy. 
         [0043]    Another method of locating the appropriate disc space may use the services of an interventional radiologist. This method is not illustrated in the present disclosure. Prior to the surgical procedure, the interventional radiologist may place catheters within each operative disc space  26 . Catheters may be placed under conscious sedation. For example, an introducer needle may be placed on or in the outer annulus, with the optional use of biplanar fluoroscopy. A guide wire may be introduced through the needle and into the disc space  26 . A catheter may be placed over the guide wire and through the introducer needle into the nucleus, after which the guide wire and needle may be removed. The balloon anchor of the catheter may be slightly inflated to anchor the catheter within the disc space. For example, the balloon may be inflated with contrast medium. The proximal end of the catheter may be capped and secured to the patient  2  with sterile tape. The patient  2  may then be transferred to the operating room for lateral access spine surgery. 
         [0044]    When the lateral access spine surgery begins, a new guide wire may be introduced through the catheter directly into the disc space  26 . The catheter may then be removed. The guide wire may be used to establish a safe operative corridor to the lateral spine. 
         [0045]    Yet another method of locating the appropriate disc space may rely upon manual finger palpation. This method is illustrated in  FIGS. 5A-7  for a straight, or direct, lateral approach. A lateral skin incision may be made through the skin into the initial fascial plane. The initial incision may be large enough to permit blunt finger dissection. Once through the abdominal muscles, blunt finger dissection may be used to access the retroperitoneal space, the transverse process, and the psoas muscle  60 . A stylus guide ring  64  may be placed over the middle phalange of the index finger  66 . The stylus tip  68  may be inserted onto the guide ring  64 , and the index finger  66  may be re-inserted to the psoas muscle  60  at the desired level. The stylus  70  may be introduced through the psoas muscle  60  and into the disc space  26 , as illustrated in  FIG. 5A , and in  FIG. 5B  with the psoas muscle  60  removed for clarity. The stylus illustrated in  FIGS. 5A-5B  is straight, although a curved stylus may be used instead. A cranial-caudal sweeping motion may be effective in separating the psoas muscle fibers during the dilation phase. The stylus  70  may slidably receive the neural monitoring probe  72  inside, as shown in  FIG. 6 . The probe  72  may transmit signals through apertures  74  at the tip  68  of the stylus  70 , as shown in  FIG. 7 . Once proper seating of the stylus  70  has been confirmed, the location may be maintained by inserting a k-wire through the central lumen of the stylus  70  and into the disc space  26 . 
         [0046]    A method of placing a retractor  76  and dilating surrounding tissues may dock the retractor  76  to the spine. This method is illustrated in  FIGS. 8-10  for a straight, or direct, lateral approach. The proper length of the dilator blades  78  may be determined from depth markings on the stylus  70 . The retractor blades  78  may be introduced slowly over the initially placed stylus  70  until the lateral aspect of the vertebral body is reached, as shown in  FIG. 8 . The retractor housing spheres  80  may be locked to the spheres  24  on the Caspar pins  22  using the articulating polyaxial clamps  44 . The location of the retractor blades  78  may be verified in both the anterior-posterior and the lateral views with fluoroscopy. The retractor  76  may be dilated to the desired diameter for discectomy using sequential tubular dilators  82  up to the desired size, as shown in  FIGS. 9-10 . The retractor blades  78  may dilate more near the retractor housing  84  and less near the disc space  26 . Alternately, the retractor blades  78  may remain parallel, or they may angle outwardly near the disc space  26 . A k-wire or shim (not shown) may be placed into the disc space  26  or vertebral body through the lumen  86  of the working dilator  82 . 
         [0047]    Another method of placing a retractor and dilating surrounding tissues may dock at least a portion of the apparatus to a rail  16  of the table  4 . This method is illustrated in  FIG. 11  for a direct lateral approach. The proper length of the dilator blades  78  may be determined from depth markings on the stylus  70 . The retractor blades  78  may be introduced slowly over the initially placed stylus  70  until the lateral aspect of the vertebral body is reached. The retractor housing spheres  80  may be locked to the surgical table rail  16  using the articulating polyaxial clamps  44 , as shown in  FIG. 11 . The location of the retractor blades  78  may be verified in both the anterior-posterior and the lateral views with fluoroscopy. The retractor  76  may be dilated to the desired diameter for discectomy using sequential tubular dilators  82  up to the desired size. The retractor blades  78  may dilate more near the retractor housing  84  and less near the disc space  26 . Alternately, the retractor blades  78  may remain parallel, or they may angle outwardly near the disc space  26 . A k-wire or shim (not shown) may be placed into the disc space  26  or vertebral body through the lumen  86  of the working dilator  82 . 
         [0048]    A method for placing an anchor  88  on a contralateral side  27  of the disc  26  may deploy an anchor  88  that resembles a grappling hook. This method is shown in  FIGS. 12-14 . A trephine or other coring tool (not shown) may be used to establish a pathway or hole through the annulus. An anchor  88  may be secured to an anchor inserter instrument  90 . The anchor  88  may also be coupled to a portion of a guide wire  98 . For example, the anchor  88  may be coupled to one end of the guide wire  98 . When the anchor  88  is secured to the anchor inserter instrument  90 , the guide wire  98  may rest in a central cannulation in the instrument  90 . The anchor  88  may resemble a grappling hook. The anchor  88  may have one or more movable hooks  92  or tabs which may move between an insertion position, in which the hooks  92  are close to a body  94  of the anchor  88 , and a deployed position, in which the hooks  92  project laterally from the body  94  of the anchor  88 . In the insertion position, the anchor  88  may be described as having a smaller projected area normal to a center longitudinal axis of the guide wire  98 . In the deployed position, the anchor  88  may be described as having a larger projected area. The anchor inserter instrument  90  may be aligned with the pathway or hole previously established by the coring tool. As shown in  FIG. 12 , the anchor  88  may be inserted into the hole and pushed across the disc space  26  by squeezing the handle  96  of the inserter instrument  90 . When the anchor  88  is through the contralateral annulus, the hooks  92  may be moved from the insertion position to the deployed position, as shown in  FIG. 13 . For example, the hooks  92  may be deployed by pulling on the guide wire  98  to rotate the hooks  92  outwardly from the body  94 . The anchor  88  may be seated against or partially within the contralateral annulus by pulling back on the inserter  90 . The anchor  88  may disassociate from the inserter instrument  90  when the pulling force exceeds a predetermined threshold value. The inserter instrument  90  may then be removed, leaving the anchor  88  and the guide wire  98  in place, as shown in  FIG. 14 . In this way, the anchor may be described as connecting the guide wire  98  to the disc  26 . The anchor may be removed when desired by sliding the inserter instrument  90  over the guide wire  98  to the anchor body  94 , reconnecting the instrument  90  to the body  94 , pushing on the instrument  90  to separate the anchor  88  slightly from the contralateral annulus, pushing on the guide wire  98  to move the hooks  92  from the deployed position to the insertion position, and pulling the anchor  88  back out of the disc space  26 . 
         [0049]    Another method for placing an anchor  100  on a contralateral side  27  of the disc  26  may deploy an anchor  100  that resembles a button. This method is illustrated in  FIGS. 15-17 . A trephine or other coring tool (not shown) may be used to establish a pathway or hole through the annulus. An anchor  100  may be secured to an anchor inserter instrument  102 . The anchor  100  may also be coupled to a portion of a guide wire  110 . For example, the anchor  100  may be coupled to one end of the guide wire  110 . When the anchor  100  is secured to the anchor inserter instrument  102 , the guide wire  110  may rest in a central cannulation in the instrument  102 . The anchor  100  may resemble a button. The anchor  100  may be elongated in one direction, so that it resembles a toggle button. The anchor  100  may move between an insertion position, in which the elongation is aligned with a center longitudinal axis of the guide wire  110 , and a deployed position, in which the elongation is transverse to the axis. The anchor  100  may be described as having a smaller projected area normal to the axis in the insertion position, and a larger projected area in the deployed position. The anchor inserter instrument  102  may be aligned with the pathway or hole previously established by the coring tool. As shown in  FIG. 16 , the anchor  100  may be inserted into the hole and pushed across the disc space  26 . When the anchor  100  is through the contralateral annulus, the anchor  100  may be deployed, or rotated, as shown in  FIG. 17 . For example, the anchor  100  may be deployed by pulling on the guide wire  110  or the inserter  102 . The anchor  100  may disassociate from the inserter instrument  102  when the pulling force exceeds a predetermined threshold value. The inserter instrument  102  may then be removed, leaving the anchor  100  and the guide wire  110  in place, as shown in  FIG. 17 . In this way, the anchor  100  may be described as connecting the guide wire  110  to the disc  26 . The anchor  100  may be removed when desired by sliding the inserter instrument  102  over the guide wire  110  to the anchor  100 , reconnecting the instrument  102  to the anchor  100 , pushing on the instrument  102  to separate the anchor  100  slightly from the contralateral annulus, pushing on the guide wire  110  to move the anchor  100  from the deployed position to the insertion position, and pulling the anchor  100  back out of the disc space  26 . 
         [0050]    Regardless of the anchor used, the guide wire  98  or  110  is thus constrained to the contralateral annulus so that cannulated instruments or additional implants may be passed over the guide wire  98 ,  110  with confidence that the instruments or implants will follow a safe trajectory. 
         [0051]    A method for performing at least a partial discectomy and/or endplate preparation may employ cannulated instruments which operate over the guide wire  98  or  110  to remove disc material and/or prepare the endplates.  FIGS. 18A-18E  illustrate working portions of several cannulated instruments which may be used in discectomy and/or endplate preparation.  FIG. 18A  shows the working portion of an auger  112  which may be rotated to simultaneously cut annulus, disc or endplate material and move the debris out of the disc space.  FIG. 18B  shows a paddle distractor  114  which may be inserted into the disc space  26  and rotated to provide localized distraction. Distractor  114  may also be an end cutter similar to a spade drill, and may be used in continuous rotation to cut a hole through an annulus or disc.  FIG. 18C  shows a shaver  116  which may be rotated in one direction or in alternating directions to cut disc or endplate material. The shaver  116  of  FIG. 18C  may be rigid, or it may be expandable in width transverse to the guide wire. For example, the shaver  116  may be inserted into the disc space  26  in a smaller diameter elongated configuration and expanded into a larger diameter but shorter configuration for material removal.  FIGS. 18D-18E  show an expandable retrograde scraper  120  which may be inserted into the disc space  26  in a closed configuration and expanded into an open configuration for material removal. The scraper  120  may be inserted into the disc space  26 , opened, and pulled out to scrape and remove material. While only the working portions of the instruments have been shown in  FIGS. 18A-18E , it can be appreciated that each instrument may include, for example, a shaft, a handle, or a drive fitting, such as a square drive. 
         [0052]    After the disc space  26  and/or endplates are prepared, the contralateral anchor  88  or  100  and guide wire  98  or  110  may be removed as described above. 
         [0053]    A method for selecting and inserting a spinal implant  122  is illustrated in  FIGS. 19-20 . One or more trial implants  124  may be used in the prepared disc space  26  to establish the desired disc height or, in other words, the desired implant size. Trial implants  124  may be selected, connected to a trial implant inserter instrument  126 , inserted into the disc space  26 , and evaluated for fit and fill. The inserter  126  in  FIG. 19  is shown in a truncated form; the complete inserter  126  may share some or all of the characteristics of inserter  128  of  FIG. 20 . Optionally, the trial implants  124  and trial inserter  126  may be cannulated. In this situation, the contralateral anchor  88  or  100  and guide wire  98  or  110  may remain secured to the contralateral annulus until an appropriate implant size has been established. The contralateral anchor  88  or  100  and guide wire  98  or  110  may be removed immediately prior to insertion of the implant  122 . The implant  122  may be a unitary fusion cage. The implant  122  may be connected to an implant inserter instrument  128 , optionally packed with bone graft material, and delivered to the prepared disc space  26 . The final position of the implant  122  may be verified using fluoroscopy. When a properly sized implant  122  is positioned as desired in the disc space  26 , the inserter instrument  128  may be disconnected from the implant and withdrawn along with the retractor  76  and any other remaining apparatus. 
         [0054]    Another method for selecting and inserting a spinal implant  130  is shown in  FIGS. 21A-21E . One or more trial implants  124  may be used in the prepared disc space  26  to establish the desired disc height or, in other words, the desired implant size. Trial implants  124  may be selected, connected to a trial implant inserter instrument  126 , inserted into the disc space  26 , and evaluated for fit and fill. Optionally, the trial implants  124  and trial inserter  126  may be cannulated. In this situation, the contralateral anchor  88  or  100  and guide wire  98  or  110  may remain secured to the contralateral annulus until an appropriate implant size has been established. The contralateral anchor  88  or  100  and guide wire  98  or  110  may be removed immediately prior to insertion of the implant  130 . The implant  130  may be a segmental fusion cage which may be inserted a piece at a time in order to minimize the insertion profile. As shown in  FIG. 21A , the implant  130  may include a first portion  138  and a second portion  140  with a complementary dovetail interface  142  for sliding connection of the first and second portions  138 ,  140 . The first portion  138  of implant  130  may be connected to an implant inserter instrument  136 , optionally packed with bone graft material, and delivered to the prepared disc space  26 , as shown in  FIG. 21B . The inserter  136  in  FIG. 21B  is shown in a truncated form; the complete inserter  136  may share some or all of the characteristics of inserter  128  of  FIG. 20 . The second portion  140  of implant  130  may be connected to another inserter instrument  136 , optionally packed with bone graft material, and slidingly connected to the first portion  138  in the prepared disc space  26 , as shown in  FIGS. 21C-21D . The final position of the implant  130  may be verified using fluoroscopy. When a properly sized implant  130  is positioned as desired in the disc space  26 , the inserter instrument  136  may be disconnected from the implant and withdrawn along with the retractor  76  and any other remaining apparatus. The complete implant  130  is shown in  FIG. 21E . 
         [0055]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. It is appreciated that various features of the above-described examples can be mixed and matched to form a variety of other alternatives. It is also appreciated that this system is not limited to spinal surgery; it may be used for surgical approaches to other body structures or in other directions. As such, the described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.