Abstract:
An implant assembly is provided for surgical implantation into an intervertebral space, such as for stabilization of vertebrae adjacent the intervertebral space during a spinal fusion procedure. The implant assembly includes a primary segment separate from a secondary segment. These segments are elongate and of sufficiently small cross-section that they can be implanted posteriorly in a minimally invasive manner. The primary segment preferably includes a tunnel and the secondary segment preferably includes a neck with the tunnel and neck sized complementally so that the segments stabilize each other where they intersect with the neck within the tunnel. The entire implant assembly is thus provided which both widens and supports the intervertebral space and is sufficiently rigid to provide adequate support for the intervertebral space as the vertebrae are fusing together.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 10/671,640, filed on Sep. 26, 2003 and issued as U.S. Pat. No. 7,621,951 on Nov. 24, 2009; which is a continuation of International Patent Application No. PCT/US02/08845 filed on Mar. 22, 2002 claiming priority from U.S. patent application Ser. No. 09/819,461, filed on Mar. 27, 2001, now U.S. Pat. No. 6,368,351. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The following invention relates to implants which are configured to be placed within an intervertebral space between adjacent spinal vertebrae after a disk has been removed from the space and to facilitate fusion of the vertebrae together. More particularly, this invention relates to implants which can be implanted posteriorly in either a minimally invasive or open manner and spread vertebrae adjacent the intervertebral space away from each other to recreate the lumbar lordosis and support the vertebrae while they fuse together. 
       BACKGROUND OF THE INVENTION 
       [0003]    Spinal fusion procedures are known as an effective treatment for certain spinal conditions. In general, such spinal fusion procedures may involve removal of a disk within an intervertebral space between two adjacent vertebrae. After the disk has been removed an implant can be located within the intervertebral space to push the vertebrae apart. By pushing the vertebrae apart, ligaments and other body structures surrounding the vertebrae are placed in tension and tend, along with the implant, to securely hold the two vertebrae in fixed position relative to each other. It is important to restore as much as possible the height of the intervertebral space. It is also important to restore the angle or “lordosis” of the intervertebral space. Finally, fusion material is placed within the intervertebral space which induces bone growth within the intervertebral space, effectively fusing the two vertebrae together with the implant typically remaining embedded within this fused vertebra combination. 
         [0004]    Placement of the implant within the intervertebral space is accomplished in one of two general ways. First, the intervertebral space can be accessed anteriorly by performing abdominal/thoracic surgery on the patient and accessing the intervertebral space from a front side of the patient. In this anterior procedure major abdominal/thoracic surgery is typically involved. However, the intervertebral space can be generally accessed anteriorly, such that the risk of injury to the nerves is generally reduced and the surgeon has greater flexibility in positioning the implant precisely where desired. 
         [0005]    Second, the implant can be inserted posteriorly. Direct posterior access to the intervertebral space requires moving the spinal nerves within the spinal canal towards the midline and can result in nerve injury or scarring. Implantation in the intervertebral space can also be accessed from a location spaced to the left or right side of the spinal column and at an angle extending into the intervertebral space. This approach avoids the spinal canal. A minimally invasive method using small incisions can be used but is must be carefully performed to avoid sensitive spinal structures. Additionally, implants of a smaller size are typically required due to the small amount of clearance between vertebral structures. Hence, the amount of spreading of the vertebrae with a posterior implant is often less than adequate. Additionally, portions of the vertebrae typically need to be at least partially carved away to provide the access necessary to insert the implants posteriorly into the intervertebral space. 
         [0006]    Implants for the intervertebral space come in a variety of different configurations, most of which are designed for anterior implantation. One known prior art implant is described in detail in U.S. Pat. No. 5,800,550 to Sertich. The Sertich implant is configured to be implanted posteriorly and comes in two pieces. Two separate incisions are made on either side of the spine and the pieces of the overall implant are inserted generally parallel to each other, but can be angled slightly away from a parallel orientation. The Sertich implant pieces have a rectangular cross section and an elongate form. The pieces are initially implanted with a lesser dimension oriented vertically so that the pieces can easily enter the intervertebral space. The pieces are then rotated 90° so that the greater dimension is rotated to vertical, tending to spread the vertebrae vertically to enlarge the intervertebral space. 
         [0007]    The implant taught by Sertich is not entirely desirable. Because the Sertich implant involves two entirely separate pieces, they do not stabilize each other in any way and hence provide a less than ideal amount of vertebral stabilization. Additionally, the relatively parallel angle at which they are implanted typically requires removal of portions of the vertebrae and retraction of the spinal nerves to properly implant the pieces of the Sertich implant. If the two pieces of the implant are angled more towards each other, they tend to decrease further in the stability that they provide to the vertebrae. Also, the Sertich implant pieces have a size which requires a relatively large incision to insert into the intervertebral space. 
         [0008]    Accordingly, a need exists for a posteriorly placed intervertebral space implant which has a small cross-sectional profile at insertion and yet can provide a large amount of displacement between adjacent vertebrae once placed. The implant must expand sufficiently far apart to restore the height of the intervertebral space and act substantially as a single rigid structure within the intervertebral space after implantation is completed. Such an invention would additionally benefit from being capable of having a greater height in an anterior region such that lordosis can be achieved in an amount desired by the surgeon with an anterior side of the intervertebral space larger than a posterior side of the intervertebral space. 
       SUMMARY OF THE INVENTION 
       [0009]    This invention is an intervertebral space implant which is configured to be implanted posteriorly in a minimally invasive or open surgical procedure. The implant includes two separate segments including a primary segment and a secondary segment. The primary segment and the secondary segment enter the intervertebral space through separate incisions on either side of the spine and along paths which intersect within the intervertebral space. To enhance a spreading of the intervertebral space with the implant, the segments have a height between a bottom surface and a top surface which is greater than a lateral width. The segments can thus be introduced into the intervertebral space with the top and bottom surfaces spaced laterally from each other and then be rotated 90° so that the top surface is above the bottom surface and a height of the segments is maximized. 
         [0010]    Portions of the primary segment and the secondary segment adjacent where the segments intersect are removed to allow the segments to lie in a substantially common plane. Preferably, the primary segment includes a tunnel passing laterally through the primary segment near a midpoint thereof. The secondary segment is provided with a neck near a midpoint thereof which has a lesser height than other portions of the secondary segment. The tunnel is sized so that the secondary segment can pass through the tunnel in the primary segment and then be rotated with the neck of the secondary segment within the tunnel of the primary segment. 
         [0011]    After the secondary segment has been rotated the two segments are interlocking together in a crossing pattern forming the implant assembly of this invention. Hence, the implant assembly of this invention provides the advantage of having a relatively low profile for insertion posteriorly in a minimally invasive manner and yet results in an overall implant assembly which has separate segments interlocking together to form a single substantially rigid implant assembly to maximize stabilization of the vertebrae adjacent the intervertebral space. 
         [0012]    Additionally, the segments are formed in a manner which facilitates height expansion of the segments after implantation, especially at distal ends of the segments. Such additional height expansion further stabilizes vertebrae adjacent the intervertebral space and provides lordosis to the intervertebral space. 
         [0013]    Specifically, the primary segment is preferably formed with a top structure separate from a bottom structure which pivot relative to each other, such as about a hinge. A passage passes between the top structure and the bottom structure. A shim can pass along the passage and cause a distal end of the primary segment to be expanded in height when the shim enters a tapering end portion of the passage. The distal end of the primary segment is thus expanded in height to an extent desired by a surgeon to provide a desirable amount of “lordosis” for the spinal fusion procedure. 
         [0014]    Similarly, the secondary segment is preferably formed from a top jaw and a bottom jaw which can pivot relative to each other, such as about a hinge. A bore passes between the top jaw and the bottom jaw and a wedge is caused to move within the bore in a manner causing the top jaw and the bottom jaw to be spaced apart and causing a height of the secondary segment to be increased at a first distal end of the secondary segment. 
         [0015]    The insertion of the segments themselves as well as the movement of shims and wedges within the segments to enhance their height is all accomplished through a small posterior incision. A variety of different hinge arrangements, shim and wedge arrangements and other structural variations are provided for the segments of the implant assembly. 
       OBJECTS OF THE INVENTION 
       [0016]    Accordingly, a primary object of the present invention is to provide an implant for an intervertebral space which can be implanted posteriorly and still provide a substantially rigid implant assembly for spreading and stabilization of the vertebrae adjacent the intervertebral space. 
         [0017]    Another object of the present invention is to provide an implant assembly having separate segments which are as low profile as possible so that posterior implantation can be accomplished in as minimally invasive a surgical procedure as possible. 
         [0018]    Another object of the present invention is to provide an implant assembly for an intervertebral space which is initially entered into the intervertebral space in separate segments which are later interlocked together. 
         [0019]    Another object of the present invention is to provide an intervertebral space implant assembly which can be adjusted in height to maximize a size of the intervertebral space generally and to allow for selective height adjustment within different portions of the intervertebral space, to provide a surgeon with a maximum amount of flexibility in positioning vertebrae adjacent the intervertebral space as precisely as desired. 
         [0020]    Another object of the present invention is to provide an implant assembly which can be located within an intervertebral space with little risk of damage to sensitive surrounding tissues. 
         [0021]    Other further objects of the present invention will become apparent from a careful reading of the included drawing figures, the claims and detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a side elevation view of a human spine with an intervertebral space containing the implant assembly of this invention. 
           [0023]      FIGS. 2-5  are top plan views taken along line  5 - 5  of  FIG. 1  illustrating the four basic steps involved in the implantation of the implant assembly of this invention. 
           [0024]      FIG. 6  is a side elevation view of a primary segment of the implant assembly with hollow interior details shown in broken lines. 
           [0025]      FIG. 7  is a top plan view of that which is shown in  FIG. 6 . 
           [0026]      FIG. 8  is a proximal end elevation view of that which is shown in  FIG. 6 . 
           [0027]      FIG. 9  is a full sectional view of that which is shown in  FIG. 6  and with a guide wire and shim of this invention shown entering a passage within the primary segment to expand a height of the primary segment adjacent a distal end of the primary segment. 
           [0028]      FIG. 10  is a full sectional view of that which is shown in  FIG. 9  after the shim has been fully advanced into the passage of the primary segment of this invention so that the height of the distal end of the primary segment has been enhanced. 
           [0029]      FIG. 11  is a full sectional side elevation view of a secondary segment of the implant assembly of this invention along with one form of a tool utilized to enhance a height of a distal first end of the secondary segment of the implant assembly of this invention. 
           [0030]      FIG. 12  is a top plan view of that which is shown in  FIG. 11  with interior details shown with broken lines. 
           [0031]      FIG. 13  is a proximal second end view of that which is shown in  FIG. 12 . 
           [0032]      FIG. 14  is a full sectional view of that which is shown in  FIG. 11  after a wedge has been fully advanced to enhance a height of the distal first end of the secondary segment. 
           [0033]      FIG. 15  is a top plan view of a tongs identifying one form of tool utilizable to implant the primary segment or the secondary segment of this invention. 
           [0034]      FIG. 16  is a side elevation view of an alternative embodiment of that which is shown in  FIG. 6  showing an offset hinge. 
           [0035]      FIG. 17  is a proximal end view of that which is shown in  FIG. 16 . 
           [0036]      FIG. 18  is a proximal end view of a second alternative embodiment of the primary segment of this invention. 
           [0037]      FIG. 19  is a side elevation view of a third alternative embodiment of a primary segment of the implant assembly of this invention with interior details shown with broken lines. 
           [0038]      FIG. 20  is a distal end view of that which is shown in  FIG. 19 . 
           [0039]      FIG. 21  is a side elevation view of that which is shown in  FIG. 19  after full advancement of an alternative shim for use with the third alternative primary segment of the implant assembly of this invention. 
           [0040]      FIG. 22  is a full sectional side elevation view of a fourth alternative embodiment of a primary segment of the implant assembly of this invention showing a guide wire with both a shim advanced past a tunnel in the fourth alternative primary segment and a proximal shim and expanding hinge to allow height expansion of a proximal end of the fourth alternative primary segment of the implant assembly of this invention. 
           [0041]      FIG. 23  is a full sectional side elevation view of that which is shown in  FIG. 22  after insertion of the proximal shim of this embodiment into a proximal recess to enhance the proximal height of the fourth alternative primary segment of the implant assembly of this invention. 
           [0042]      FIGS. 24-27  are sectional and side elevation views of an expanding hinge of the fourth alternative primary segment of the implant assembly of this invention revealing in detail the various stages in the operation of this expanding hinge. 
           [0043]      FIGS. 28-30  are top plan views of alternatives of the implant assembly of this invention showing how various beveled surfaces and relief notches can be provided adjacent the tunnel in the primary segment and the neck in the secondary segment to facilitate rotation of the secondary segment within the tunnel of the primary segment and to facilitate orientation of the secondary segment at an angle relative to the primary segment other than purely a perpendicular angle. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    Referring to the drawings, wherein like reference numerals represent like parts throughout the various drawing figures, reference numeral  10  ( FIG. 1 ) is directed to an implant assembly for implantation into an intervertebral space S between adjacent vertebrae V after a disk D has been removed from the intervertebral space S. A primary segment  20  and a secondary segment  60  are implanted along separate pathways but interlock together within the intervertebral space S to form a single implant assembly  10 . The resulting assembly  10  securely stabilizes the vertebrae V adjacent the intervertebral space S for spinal fusion of the vertebrae V together. 
         [0045]    In essence, and with particular reference to  FIGS. 1-5 , the basic details of the implant assembly  10  are described. The implant assembly  10  includes a primary segment  20  ( FIG. 2 ) and a secondary segment  60  ( FIG. 4 ). The primary segment  20  is elongate in form extending along a primary axis A. The primary segment  20  is preferably higher than it is wide (compare  FIG. 2  with  FIG. 3 ), thus having a rectangular cross-section. The primary segment  20  can thus be inserted on its side into the intervertebral space (along arrow C of  FIG. 2 ) and then rotated within the intervertebral space (along arrow F of  FIG. 3 ) to help spread vertebrae V adjacent the intervertebral space S away from each other. The primary segment  20  additionally includes a tunnel  30  ( FIG. 2 ) passing laterally through the primary segment  20 . 
         [0046]    The secondary segment  60  ( FIG. 4 ) is elongate and has a contour generally similar to that of the primary segment  20 . However, the secondary segment  60  includes a neck  70  rather than the tunnel  30  of the primary segment  20 . The secondary segment  60  has a cross-sectional size similar to a size of the tunnel  30 . This size allows the secondary segment  60  to be inserted along secondary axis B (in the direction identified by arrow E of  FIG. 4 ) through the tunnel  30  in the primary segment  20 . The secondary segment  60  can later be rotated (along arrow G of  FIG. 5 ) in a manner similar to the rotation of the primary segment  20  so that a height of the secondary segment  60  is oriented vertically and maximizes a spacing of vertebrae V adjacent the intervertebral space S. The segments  20 ,  60  interlock together to form the implant assembly  10  with the segments  20 ,  60  stabilizing each other and allowing the implant assembly  10  to stabilize the intervertebral spaces in which the assembly  10  is implanted. 
         [0047]    More specifically, and with particular reference to  FIGS. 6-10 , details of the primary segment  20  according to a preferred embodiment of this invention are described. The primary segment  20  is an elongate substantially rigid construct formed from a top structure  22  and a bottom structure  24  which are pivotably joined together, such with a hinge  25 . The hinge  25  can take on many different forms to provide the basic function of allowing the top structure  22  and the bottom structure  24  to be pivoted relative to each other. 
         [0048]    The primary segment  20  extends from a distal end  26  to a proximal end  28 . A guide wire stop  27  can be optionally included with the bottom structure  24  at the distal end  26  and extend up beyond the top structure  22 . 
         [0049]    The tunnel  30  passes laterally through the primary segment  20  between a top surface and a bottom surface of the primary segment  20 . The tunnel  30  includes a top  32  preferably substantially parallel to a bottom  34  and sides  36  extending between the bottom  34  and the top  32 . The tunnel  30  preferably has dimensions similar to exterior dimensions of the primary segment  20  itself, but rotated 90°. The tunnel  30  is thus sized to allow secondary segments  60  with dimensions similar to the primary segment  20  to pass laterally through the tunnel  30  during formation of the implant assembly  10  of this invention within the intervertebral space S ( FIGS. 1-5 ). 
         [0050]    A passage  40  extends longitudinally within the primary segment  20  and between the top structure  22  and the bottom structure  24 . The passage  40  includes an entrance  42  at the proximal end  28  of the primary segment. The passage  40  additionally includes a roof  44  preferably substantially parallel to and spaced from a floor  46 . Preferably, the passage  40  has a constant cross-section from the entrance  42  to a location where the passage  40  intersects the tunnel  30 . The passage  40  preferably continues beyond the tunnel  30  and toward the distal end  26  of the primary segment  20 . However, portions of the passage  40  on a distal side of the tunnel  30  preferably taper to form a tapering end  48  of the passage  40 . A step  49  is preferably located in the passage  40  directly adjacent the tunnel  30 . 
         [0051]    The passage  40  is configured to receive a shim  50  therein. The shim  50  ( FIG. 9 ) preferably has a rectangular cross-section which generally fills the passage  40  ( FIG. 8 ) so that the shim does not rotate. The shim  50  preferably includes a tip  52  which is of lesser height than a tail  54 . A central pathway  56  preferably passes through the shim  50 . A guide wire  58  can be passed entirely through the passage  40  up to the stop  27  (along arrow H of  FIG. 9 ) and then the shim  50  threaded onto the guide wire  58 . The shim  50  can then be easily advanced along the guide wire  58  (arrow J of  FIG. 9 ) and directed into the passage  40 . When the shim  50  reaches the tapering end  48  of the passage  40 , with the assistance of an appropriate shim pushing tool, the shim  50  causes the top structure  22  and bottom structure  24  of the primary segment  20  to be expanded away from each other (about arrow K of  FIG. 10 ) and a height of the primary segment  20  to be enhanced at the distal end  26  of the primary segment  20 . 
         [0052]    Such distal end  26  height expansion for the primary segment  20  is desirable in many cases to provide lordosis to the intervertebral space S. Specifically, lordosis is a orientation for the intervertebral space S where an anterior edge of the intervertebral space S has a greater height than a posterior edge of the intervertebral space S. Such lordosis can be provided to a varying degree depending on the desires of the medical practitioner. With this invention the shim  50  is advanced an amount desired through the passage  40  of the primary segment  20  to provide an amount of lordosis which is desirable in the judgment of the medical practitioner. The segment  20  can be custom designed to provide the lordosis desired or can be variably expandable for adjustment during implantation. 
         [0053]    With particular reference to  FIGS. 11-14 , details of a preferred embodiment of the secondary segment  60  are described. The secondary segment  60  preferably has a general exterior contour similar to that of the primary segment  20 . Also, the secondary segment  60  is preferably divided into a top jaw  62  and a bottom jaw  64  which are pivotably connected together, such as at a hinge  65 . As with the primary segment  20 , the hinge  65  can take on a variety of different configurations. The secondary segment  60  extends from a first distal end  66  to a second proximal end  68 . 
         [0054]    The secondary segment  60  includes a neck  70  with two preferably substantially parallel surfaces  72  and side walls  74  extending between the parallel surfaces  72  of the neck  70  and top and bottom surfaces of the secondary segment  60 . The side walls  74  can be perpendicular to the parallel surfaces  72  (as depicted generally in  FIG. 4 ) or can be beveled (as shown in  FIG. 11 ). The parallel surfaces  72  are located closer to each other than a distance between top and bottom surfaces of the secondary segment  60 . The parallel surfaces  72  need not be precisely parallel, but benefit from having a lesser height than that of the top and bottom surfaces of the secondary segment  60  so that the neck  70  of the secondary segment  60  is an open region then can reside within the tunnel  30  or other open region in the primary segment  20  after rotation of the secondary segment  60  into an orientation with the top surface and the bottom surface vertically aligned along with top and bottom surfaces of the primary segment  20  ( FIG. 5 ). Preferably, the neck  70  is located near a midpoint between the distal first end  66  and the proximal second end  68  of the secondary segment  60 . 
         [0055]    The width between lateral sides of the secondary segment  60  is preferably similar to a height of the neck  70  and a height of the tunnel  30  in the primary segment  20  for a tight fit within the tunnel  30  both before and after rotation (about arrow G of  FIG. 5 ). The hinge  25  in the primary segment  20 , general slight flexibility of the segments  20 ,  60  and possible slight additional clearances can provide the relief necessary to allow the secondary segment  60  to rotate with the neck  70  within the tunnel  30 . Preferably, the secondary segment  60  tends to snap into its final position so that the segments  20 ,  60  are securely interlocked together. 
         [0056]    To provide lordosis to the intervertebral space S, the secondary segment  60  is configured to allow height expansion, particularly at the distal first end  66 . Specifically, the secondary segment  60  includes a bore  80  passing longitudinally from the proximal second end  68 , at least part of the way toward the distal first end  66 . The bore  60  includes a pin  82  therein which includes a threaded end  83  at an end thereof closest to the distal first end  66  of the secondary segment  60 . An access end  84  of the pin  82  is opposite the threaded end  83  and closest to the proximal second end  68  of the secondary segment  60 . A wrench  85  having one of a variety of different configurations ( FIG. 11 ) can be utilized to cause the pin  82  to rotate by interaction of the wrench  85  with the access end  84  of the pin  82 . Preferably, the bore  80  is slightly smaller adjacent the proximal end  68  to keep the pin  82  from sliding toward the proximal end  68  within the bore  80 . 
         [0057]    A wedge  86  is located within a tapering recess  87  in the bore  80 . The wedge  86  is preferably cylindrical and includes a threaded hole extending perpendicularly through curving sides of the wedge into which the threaded end  83  of the pin  82  is located. Hence, when the pin  82  is rotated by rotation of the tool  85  (along arrow L of  FIG. 11 ) the threaded end  83  of the pin  82  causes the wedge  86  to travel toward the distal first end  66  of the secondary segment  60  (along arrow M of  FIG. 14 ). As the wedge  86  travels toward the distal first end  66  and through the tapering recess  87 , the top jaw  62  and bottom jaw  64  are spread vertically (along arrow N of  FIG. 14 ), enhancing a height of the secondary segment  60 . 
         [0058]    While the primary segment  20  and secondary segment  60  are shown with unique systems for vertically expanding top and bottom portions of the segments  20 ,  60 , it is noted that these systems are merely one currently most preferred embodiments of a vertical height enhancement system for the segments  20 ,  60 . In fact, a variety of different systems could be utilized to enhance the vertical height of the segments  20 ,  60  after implantation. 
         [0059]    Most preferably, the segments  20 ,  60  have a height between a top and bottom surface approximately twice a width between lateral sides of the segments  20 ,  60 . A tongs  90  ( FIG. 15 ) can be utilized to properly place the segments  20 ,  60  within the intervertebral space S ( FIG. 1 ). Tongs  90  typically have fingers  92  which have tips  93  with a width similar to half of the lateral width of the segments  20 . In this way, the segments  20 ,  60  could be grasped on lateral sides with the tips  93  of the fingers  92  of the tongs  90  and the segments  20 ,  60  can be advanced through a tubular cannula with the tubular cannula having a diameter similar to a height of the segments  20 ,  60  between top and bottom surfaces of the segments  20 ,  60 . The tongs  90  might include a pivot  94  with handles  96  at ends of the tongs  90  opposite the fingers  92  for releasably grasping the segments  20 ,  60 . 
         [0060]    Alternatively, the segments  20 ,  60  could be grasped at their proximal ends  28 ,  68  through an appropriate attachment mechanism inboard of the top and bottom surfaces and lateral surfaces of the segments  20 ,  60  so that the tongs  90  or other placement tool would not add to a cross-sectional diameter needed for the cannula through which the segments  20 ,  60  would be passed. 
         [0061]    With particular reference to  FIGS. 16 and 17 , details of an alternative offset hinge  102  are described. Such an offset hinge  102  is shown on a first alternative primary segment  100 . However, the offset hinge  102  could similarly be located on a secondary segment such as a modification of the secondary segment  60  ( FIGS. 11-14 ). The offset hinge  102  advantageously allows a single pintle to pass through all leaves of the offset hinge  102  ( FIG. 17 ). The offset hinge  102  thus avoids the necessity of two partial pintles on opposite sides of a passage  40  ( FIG. 8 ) or bore  80  ( FIG. 13 ). Otherwise, the alternative primary segment  100  of  FIGS. 16 and 17  is similar to the primary segment  20  of the preferred embodiment of the implant assembly  10  of this invention. 
         [0062]      FIG. 18  shows a second alternative primary segment  110  featuring a split hinge  112 . This split hinge  112  on the second alternative primary segment  110  is generally similar to the hinge  25  of the primary segment  20  of the preferred embodiment ( FIG. 8 ). However, the overlapping leaves place the pintles of the split hinge  112  in a slightly different position. The second alternative primary segment  110  and split hinge  112  of  FIG. 8  illustrate one of the many different hinge configurations which the segments  20 ,  60  of the implant assembly  10  of this invention can have to effectively allow top and bottom portions of the segments  20 ,  60  to move relative to each other. 
         [0063]    While the material forming the segments  20 ,  60  would typically be some form of surgical grade bio-compatible stainless steel or other material, it is conceivable that the material forming the segments  20 ,  60  could be a form of hydrocarbon polymer or other plastic material, or a metallic material which has some appreciable flexibility characteristics. If the segments  20 ,  60  are made from such materials or can be machined to have sufficiently thin connection between the top and bottom portions of the segments  20 ,  60 , the hinges  25 ,  102 ,  112  of the various embodiments of this invention could be replaced with the top and bottom portions of the segments  20 ,  60  merely flexing relative to each other sufficiently to allow the height expansion at the distal ends  26 ,  66  of the segments  20 ,  60  so that an appropriate amount of lordosis can be provided to the intervertebral space S ( FIG. 1 ). 
         [0064]    With particular reference to  FIGS. 19-21  details of a third alternative primary segment are described. This third alternative primary segment  120  features an offset hinge  122  similar to the offset hinge  102  of the first alternative primary segment  100  ( FIG. 16 ). The third alternative primary segment  120  additionally includes undulating overlapping tapering surfaces  124  for portions of the top and bottom structures of the third alternative primary segment  120  adjacent the distal end. These undulating overlapping tapering surfaces  124  can be spread apart by longitudinal advancement of a first alternative shim  126  which is preferably cylindrical and as wide as the entire segment  120 . As the first alternative shim  126  is advanced (along arrow P of  FIG. 19 ) it passes through a series of steps corresponding with different stages of lordosis which can be provided to the intervertebral space S ( FIG. 1 ). 
         [0065]    Because the tapering surfaces  124  undulate, a series of locations are provided where the first alternative shim  126  can come to rest. Varying degrees of height adjustment corresponding to various different degrees of lordosis can thus be provided to the intervertebral space S ( FIG. 1 ). The first alternative shim  126  can be advanced by being pushed along through an access passage  128  with any appropriate form of pushing tool, or could be advanced with a threaded pin similar to the advancement of the wedge  86  along the pin  82  of the secondary segment  60  of the preferred embodiment. 
         [0066]    Because the tapering surfaces  124  overlap, a greater amount of height increase at the distal end of the third alternative primary segment  120  is provided (see  FIG. 20 ). This third alternative primary segment  120  height magnification system could be fitted on an alternative secondary segment having a neck rather than a tunnel in a relatively straightforward fashion due to the relatively low profile passage  128  which could pass through a neck without compromising a strength of the neck in such an alternative secondary segment. Hence, this height magnification system is merely illustrated in the context of primary segment for convenience, but could be equally well incorporated into a secondary segment. 
         [0067]    With particular reference to  FIGS. 22-27 , details of a fourth alternative primary segment are described. The fourth alternative primary segment  130  is configured to allow height adjustment both at a distal end of the fourth alternative primary segment  130  and at a proximal end of the fourth alternative primary segment  130 . Specifically, the top and bottom portions of the fourth alternative primary segment  130  are preferably joined together with an expanding hinge  132 . 
         [0068]    Function of the expanding hinge is shown in detail in  FIGS. 24-27 . The expanding hinge  132  includes two separate pintles  134  on opposite sides of a longitudinal passage extending through the fourth alternative primary segment  130 . The pintles  134  reside within slots  136 . Hence, the expanding hinge  132  allows both rotation and vertical expansion (along arrow R of  FIGS. 25 and 26 ) while still holding the top and bottom portions of the fourth alternative primary segment  130  together. 
         [0069]    A longitudinal passage passing through the fourth alternative primary segment includes a proximal recess  140  near a proximal end of the fourth alternative primary segment  130 . A proximal shim  142  can be advanced along a guide wire in a manner similar to the advancement of the shim  50  of the primary segment  20  of the preferred embodiment. The proximal shim  142  is preferably configured with a contour matching that of the proximal recess  140 . Hence, as the proximal shim  142  is advanced into the passage (along arrow Q of  FIG. 22 ), the proximal shim  142  expands the top and bottom portions of the fourth alternative primary segment  130  away from each other until the proximal shim  142  rests within the proximal recess  140 . 
         [0070]    As an alternative to providing the proximal recess  140 , the proximal shim  142  could merely have a tapering contour (shown in  FIG. 22 ) and the friction between tapering surfaces of the proximal shim  142  and upper and lower surfaces of the pathway within the fourth alternative primary segment  130  could allow the proximal shim  142  to remain in a position where it has been advanced unless specific forces are applied to the proximal shim  142 . 
         [0071]    As shown in  FIG. 22 , a shim similar to the shim  50  of the preferred embodiment would first be advanced along the guide wire into the tapering end of the passage within the fourth alternative primary segment  130 . The proximal shim  142  would then be advanced into the passageway. Hence, the fourth alternative primary segment  130  experiences height magnification both adjacent a distal end and adjacent the proximal end of the fourth alternative primary segment  130 . The proximal shim  142  could similarly be used with an expanding hinge  132  fitted into the proximal second end  68  of the secondary segment  60  to give the secondary segment  60  proximal end  68  height adjustability. 
         [0072]      FIG. 28  shows a fifth alternative primary segment  150  which uniquely includes beveled tunnel sides  152 . These beveled tunnel sides  152  allow a second alternative secondary segment  155  to pass through the tunnel in a non-perpendicular direction. Specifically, the secondary segment  155  can be angled relative to the fifth alternative primary segment  150  by an angular amount (arrow X of  FIG. 28 ) which can be less than or greater than 90°, rather than only exactly 90°. Angle X in  FIG. 8  is shown at approximately 60° but could be reduced to as little as 45° or less and still allow the secondary segment  155  to pass through the tunnel in the fifth alternative primary segment  150  without being blocked by the beveled tunnel sides  152 . The beveled tunnel sides  152  are shown angled approximately 45° away from an orientation perpendicular to a long axis of the fifth alternative primary segment  150 . However, the angles of the beveled tunnel sides  152  and the angle X that the secondary segment  155  shares relative to the fifth alternative primary segment  150  could be increased or decreased depending on the needs of the medical practitioner for the implant assembly  10 . 
         [0073]    The second alternative secondary segment  155  preferably includes a relief bevel  156  ( FIG. 28 ) which allows a side wall of the neck in the second alternative secondary segment  155  to come into contact with a side surface of the first alternative primary segment  150  after the second alternative secondary segment  155  has been rotated into its final position. The relief bevel  156  thus allows the second alternative secondary segment  155  and the fifth alternative primary segment  150  to more completely stabilize each other in a fully interlocking fashion so that the implant assembly  10  stabilizes the intervertebral space S ( FIG. 1 ) as completely as needed. 
         [0074]    A sixth alternative primary segment  160  is shown in  FIG. 29  which includes relief notches  162  in sides of the sixth alternative primary segment  160  adjacent the tunnel. The relief notches  162  are an alternative to the relief bevel  156  of the embodiment of  FIG. 28 . Specifically,  FIG. 29  illustrates how either the relief bevel  156  can be provided on the second alternative secondary segment  155  or relief notches  162  can be provided as in the sixth alternative primary segment  160  so that complete rotation of the third alternative secondary segment  164  can be achieved without the necessity of the relief bevel  156  of the second alternative secondary segment  155 . Of course a combination of the relief bevel  156  and the relief notches  162  could be resorted to so that abutting surfaces of the primary segment and the secondary segment could mesh together in a manner providing stability for the overall implant assembly  10 . 
         [0075]    A fourth alternative secondary segment  170  is shown in  FIG. 30  along with the fifth alternative primary segment  150 . This implant assembly shown in  FIG. 30  is shown with the first alternative primary segment  150  in section and clearly illustrating how the fourth alternative secondary segment  170  can fit through the tunnel in the fifth alternative primary segment  150  at an angle X ( FIG. 28 ) other than perpendicular and be rotated, about arrow T, and to the final position such as that shown in  FIG. 28 . 
         [0076]    It can be seen from  FIG. 30  that not all of the beveled tunnel sides  152  are strictly necessary for the passage of the fourth alternative secondary segment  170  through the tunnel in the fifth alternative primary segment  150 . By providing the beveled tunnel sides  152  in two directions, the fifth alternative primary segment  150  becomes reversible. However, inclusion of both beveled tunnel sides  152  would not be absolutely necessary. Rather, only one beveled tunnel side  152  could be provided on each side of the tunnel and other beveled tunnel sides  152  could be eliminated. Particularly, and as shown in  FIG. 30 , the beveled tunnel sides  152  which include reference numerals thereon could be removed and the fourth alternative secondary segment  170  could still pass through the tunnel in the fifth alternative primary segment  150  successfully. 
         [0077]    Selective relief bevels  172  similar to the relief bevels  156  ( FIG. 28 ) could be provided on some of the neck side walls, but would not need to be on all neck side walls. The selective relief bevels  172  would come to rest adjacent sides of the primary segment  150  after rotation about arrow T and provide enhanced stability between the segments  150 ,  170 . 
         [0078]    This disclosure is provided to reveal a preferred embodiment of the invention and a best mode for practicing the invention. Having thus described the invention in this way, it should be apparent that various different modifications can be made to the preferred embodiment without departing from the scope and spirit of this disclosure. For instance, while the primary segment  20  and the secondary segment  60  are described in the preferred embodiment as being expandable, a simplified variation of this invention would not require such expandability. When structures are identified as a means to perform a function, the identification is intended to include all structures which can perform the function specified.