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 . 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 . 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 . various apparatus , systems and methods will now be described with reference to the drawings . a method of positioning a patient 2 for lateral spine surgery may place the patient 2 in a prone position , as illustrated in fig1 . 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 . another method of positioning a patient 2 for lateral spine surgery may place the patient 2 in a lateral decubitus position , as illustrated in fig2 . 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 . 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 . 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 . fig3 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 . a method of locating the appropriate disc space may rely upon a targeting post 30 . this method is illustrated in fig4 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 . 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 . 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 fig4 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 . 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 . 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 fig4 . 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 . 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 . 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 . 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 . yet another method of locating the appropriate disc space may rely upon manual finger palpation . this method is illustrated in fig5 a - 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 fig5 a , and in fig5 b with the psoas muscle 60 removed for clarity . the stylus illustrated in fig5 a - 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 fig6 . the probe 72 may transmit signals through apertures 74 at the tip 68 of the stylus 70 , as shown in fig7 . 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 . a method of placing a retractor 76 and dilating surrounding tissues may dock the retractor 76 to the spine . this method is illustrated in fig8 - 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 fig8 . 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 fig9 - 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 . 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 fig1 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 fig1 . 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 . 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 fig1 - 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 fig1 , 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 fig1 . 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 fig1 . 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 . 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 fig1 - 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 fig1 , 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 fig1 . 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 fig1 . 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 . 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 . 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 . fig1 a - 18e illustrate working portions of several cannulated instruments which may be used in discectomy and / or endplate preparation . fig1 a 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 . fig1 b 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 . fig1 c 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 fig1 c 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 . fig1 d - 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 fig1 a - 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 . 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 . a method for selecting and inserting a spinal implant 122 is illustrated in fig1 - 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 fig1 is shown in a truncated form ; the complete inserter 126 may share some or all of the characteristics of inserter 128 of fig2 . 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 . another method for selecting and inserting a spinal implant 130 is shown in fig2 a - 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 fig2 a , 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 fig2 b . the inserter 136 in fig2 b is shown in a truncated form ; the complete inserter 136 may share some or all of the characteristics of inserter 128 of fig2 . 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 fig2 c - 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 fig2 e . 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 .
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 .