Abstract:
An expandable interbody fusion device for implantation into the intradiscal space between two opposing vertebral bodies of a spine comprises a superior endplate member having an upper surface for engaging a superior vertebral body in a spine, and an inferior endplate member having a lower surface for engaging an inferior vertebral body in the spine. The superior endplate member and the inferior endplate member are releasably coupled and define a cavity therebetween. At least one expansion member is configured to be introduced into the cavity to move the superior endplate and the inferior endplate members relatively apart upon introduction and to thereby decouple the superior endplate member and the inferior endplate member. An inserter may be releasably coupled to the device to facilitate insertion of the device as well as to provide a track for insertion of the expansion members.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to co-pending provisional application No. 60/604,422, filed on Aug. 25, 2004, and entitled “Expandable Interbody Fusion Device”. The disclosure of this provisional application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to devices and methods for stabilization of spinal motion segments and most particularly for stabilization of the intervertebral disc space.  
         [0003]     The number of spinal surgeries to correct the causes of low back pain has steadily increased over the last several years. Most often, low back pain originates from damage or defects in the spinal disc between adjacent vertebrae. The disc can be herniated or can be suffering from a variety of degenerative conditions, so that in either case the anatomical function of the spinal disc is disrupted. The most prevalent surgical treatment for these types of conditions has been to fuse the two vertebrae surrounding the affected disc. In most cases, the entire disc will be removed, except for the annulus, by way of a discectomy procedure. Since the damaged disc material has been removed, something must be positioned within the intra-discal space, otherwise the space may collapse resulting in damage to the nerves extending along the spinal column.  
         [0004]     In order to prevent this disc space collapse, the intra-discal space has been filled with bone or a bone substitute in order to fuse the two adjacent vertebrae together. In early techniques, bone material was simply disposed between the adjacent vertebrae, typically at the posterior aspect of the vertebrae, and the spinal column was stabilized by way of a plate or a rod spanning the affected vertebrae. With this technique once fusion has occurred the hardware used to maintain the stability of the segment became superfluous. Moreover, the surgical procedures necessary to implant a rod or plate to stabilize the level during fusion were frequently lengthy and involved.  
         [0005]     It was therefore determined that a more optimum solution to the stabilization of an excised disc space is to fuse the vertebrae between their respective end plates, most optimally without the need for anterior or posterior plating. There have been an extensive number of attempts to develop an acceptable intra-discal implant that could be used to replace a damaged disc and yet maintain the stability of the disc interspace between the adjacent vertebrae, at least until complete arthrodesis is achieved. These “interbody fusion devices” have taken many forms, but many have had difficulty in achieving fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of these devices are not structurally strong enough to support the heavy loads and bending moments applied at the most frequently fused vertebral levels, namely those in the lower lumbar spine.  
         [0006]     The interbody fusion devices (IBFDs) that have overcome these difficulties are typically bulky, at least with respect to the intervertebral space. In particular, these devices have been configured to completely fill the space and to restore the normal spinal anatomy at the instrumented level. One drawback of this approach is that the implant device is not exactly sized to the anatomy of the particular patient, thus typically requiring pre-distraction of opposed vertebrae in order to increase the disc space for device implantation. While a collection of differently sized IBFDs can be provided, it is unwieldy and impractical to provide an IBFD sized for every intervertebral disc space height.  
         [0007]     Another drawback of these prior devices is that that he surgical insertion site must be at least as big as the IBFD. Minimally invasive and working channel surgical techniques have been recently developed that have significantly reduced the surgical invasion, but even more improvement is needed. The present invention provides an IBFD that achieves all of the benefits of prior IBFD designs, while also addressing the above-noted drawbacks.  
       SUMMARY OF THE INVENTION  
       [0008]     In order to address these drawbacks, the present invention contemplates an expandable interbody fusion device for implantation into the intradiscal space between two opposing vertebral bodies of a spine which comprises a superior endplate member having an upper surface for engaging a superior vertebral body in a spine and an inferior endplate member having a lower surface for engaging an inferior vertebral body in the spine, the superior endplate member and the inferior endplate member being releasably coupled and defining a cavity therebetween. The device is further provided with at least one expansion member configured to be introduced into the cavity to move the superior endplate and the inferior endplate member relatively apart upon introduction and to thereby decouple the superior endplate member and the inferior endplate member. In one embodiment, the superior endplate member has a pair of opposing spaced apart sidewalls depending downwardly from the upper surface, while the inferior endplate member has a pair of opposing spaced apart sidewalls projecting upwardly from the lower surface. The depending sidewalls of the two endplate members are configured to overlap for an extent as the superior endplate member and the inferior endplate member are moved apart.  
         [0009]     In one feature, at least one of the sidewalls on one of the superior endplate member or the inferior endplate member comprises a projecting prong while an overlapping sidewall of the other of the superior endplate member or the inferior endplate member defines a complementary notch for receipt of the prong. The prong and the notch thus form a releasable coupling between the superior endplate member and the inferior endplate member.  
         [0010]     In a further embodiment, the superior endplate member has at least one end wall depending downwardly from the upper surface and the inferior endplate member has at least one end wall projecting upwardly from the lower surface. The depending end walls are configured to overlap for an extent as the superior endplate member and the inferior endplate member are moved apart.  
         [0011]     In certain embodiments of the invention, the upper surface of the superior endplate member and the lower surface of the inferior endplate member each comprise gripping surfaces for engagement with the respective superior and inferior vertebral bodies. These gripping surfaces may be defined by ribs having a generally saw-toothed configuration. Furthermore, at least one of the upper surface of the superior endplate member or the lower surface of the inferior endplate member may be angled to provide a particular angle between the opposing vertebral bodies. At least one of the upper surface of the superior endplate member or the lower surface of the inferior endplate member may be curved to provide anatomical support of the vertebral bodies.  
         [0012]     In accordance with certain features of the invention, the at least one expansion member is a generally flat wafer configured for sliding insertion into the cavity under sufficient pressure to move the superior endplate member and the inferior endplate member apart. The wafer may comprise a surface for cooperative engagement with at least the superior endplate member. Preferably, the device comprises a plurality of wafers slidably received in contact to form a stack of wafers within the cavity and to separate the superior plate from the inferior plate when the height of the stack exceeds the size of the cavity in the device.  
         [0013]     In one specific embodiment, each of the wafers has an upper generally flat surface and a lower generally flat surface. In another specific embodiment, a lower flat surface of a wafer in the stack and an upper flat surface of a contacting wafer comprise complementary interdigitating configurations to provide at least lateral and rotational stability to the stack of wafers. These complementary configurations may be defined by a ridge on at least one of the wafer surfaces and a trough for receiving the ridge on a surface of a contacting wafer.  
         [0014]     In a further embodiment of the invention, an expandable interbody fusion device is provided for implantation into the intradiscal space between two opposing vertebral bodies of a spine, in which the device comprises a superior endplate member having an upper surface for engaging a superior vertebral body in a spine and an inferior endplate member having a lower surface for engaging an inferior vertebral body in the spine, the superior endplate member and the inferior endplate member defining a cavity therebetween. The device further comprises at least one expansion member configured to be introduced into the cavity and upon introduction to move the superior endplate member and the inferior endplate member relatively apart. The superior endplate member and the inferior endplate member define cooperative surfaces that overlap for an extent as the superior endplate member and the inferior endplate member move apart to thereby provide stability to the device upon expansion.  
         [0015]     In accordance with other features of the invention, an apparatus is provided for use in restoring the anatomical height of a damaged or diseased disc space between two opposing vertebral bodies in a spine. The apparatus comprises an expandable interbody fusion device according to embodiments described above, together with an inserter releasably connected to the fusion device. The inserter may comprise a track along which the expansion element is conveyed for introduction into the cavity of the fusion device. In one aspect, a separable interface is provided between the track and the fusion device. That interface may be a connector plate supported by the inferior endplate member. The connector plate includes a support surface on one side for supporting the expansion member and at least one severable member on the other side for temporarily holding the track to the fusion device. The connector plate may interface with a movable release plate supported by the track and having a cutting surface operable upon movement to sever the at least one severable member on the connector plate, to thereby allow removal of the track from the fusion device.  
         [0016]     An expandable interbody fusion device for implantation into the intradiscal space between two opposing vertebral bodies of a spine comprises a curved superior endplate member and a curved inferior endplate member, wherein at least one of the superior endplate member or the inferior endplate member has a thickness at one side different from the thickness at the opposite side, thereby defining a lordotic angle between opposing vertebral bodies. A plurality of wafers may be stacked between the superior endplate member and the inferior endplate member, the wafers each being of relatively constant thickness from one side to the other. In certain embodiments, both the superior endplate member and the inferior endplate member have a thickness on one side different from the thickness on the other side.  
         [0017]     In one feature, the device defines cooperative interlocking surfaces between the wafers. Cooperative interlocking surfaces may also be defined between the wafers and the superior and inferior endplate members. The cooperative interlocking surfaces may constitute dovetail configurations.  
         [0018]     Additional embodiments of the invention reside in an expandable interbody fusion device for implantation into the intradiscal space between two opposing vertebral bodies of a spine that comprises a superior endplate member, an inferior endplate member, and a plurality of wafers stacked between the superior endplate member and the inferior endplate member, wherein each of the wafers has a different thickness from side to side to thereby provide upon disposition between the superior endplate member and the inferior endplate member a lordotic angle between the vertebral bodies. Again, the device may define cooperative interlocking surfaces between the wafers, as well as between the wafers and the endplate members.  
         [0019]     In a further embodiment, a device is provided for distracting a body tissue space between opposing tissue surfaces which comprises an upper plate having an outer surface configured to contact one of the opposing surfaces and a lower plate having an outer surface configured to contact the other of the opposing surfaces, the lower plate having opposite side walls configured to removably support the upper plate thereon. The upper and lower plates define a cavity when the upper plate is supported on the lower plate. The lower plate defines a wafer support surface for supporting at least one wafer within the cavity, and a channel communicating with the cavity and configured to receive a wafer conveyed therethrough for placement on the surface of the lower plate. In one feature of this embodiment, the upper plate defines a contact surface for contacting a wafer within the cavity to displace the upper plate from the lower plate.  
         [0020]     It is one object of the invention to provide an expandable device that may be manipulated percutaneously to distract the space between two tissue surfaces, such as the intervertebral disc space. Another object resides in features of the invention that provides for controlled expansion of superior and inferior plates configured to engage the tissue surfaces.  
         [0021]     One benefit of the various embodiments disclosed herein is that all of the components are configured for easy introduction to the surgical site through a working channel cannula and without the need for traditional open surgical procedures. Another benefit is that the overall height of the expandable device, and thus the amount of distraction applied to the tissue surfaces, may be easily controlled during the distraction procedure. Other objects and benefits of the invention will become apparent from the following written description and accompanying figures.  
     
    
     DESCRIPTION OF THE FIGURES  
       [0022]     FIG. 1  is a bottom perspective view of an interbody fusion device (IBFD) according to one embodiment of the invention.  
         [0023]      FIG. 2  is a side view of the IBFD shown in  FIG. 1 .  
         [0024]      FIG. 3  is a top view of the IBFD of  FIGS. 1-2  mounted on an insertion apparatus in accordance with one aspect of the invention.  
         [0025]      FIG. 4  is a side view of the IBFD and insertion apparatus shown in  FIG. 3 .  
         [0026]      FIGS. 5   a - 5   f  include perspective, side, end, top and bottom views of a superior endplate portion of the IBFD shown in  FIGS. 1-2 , and including a cross-sectional and enlarged view of portions thereof.  
         [0027]      FIGS. 6   a - 6   e  include perspective, side, end, top and bottom views of an inferior endplate portion of the IBFD shown in  FIGS. 1-2 , including an enlarged view of a portion thereof.  
         [0028]      FIGS. 7   a - 7   e  include side, top and cross-sectional views of the inferior endplate portion of the IBFD shown in  FIGS. 6   a - 6   e.    
         [0029]      FIGS. 8   a - 8   f  include side, top, bottom and perspective views of a track connector used in connection with the insertion apparatus shown in  FIGS. 3-4 , including cross-sectional views of portions thereof.  
         [0030]      FIG. 8   g  is a bottom perspective view of an alternative embodiment of a track connector used in connection with the insertion apparatus shown in  FIGS. 3-4 .  
         [0031]      FIG. 9  is a side perspective partial cut-away view of the IBFD and insertion apparatus shown in  FIGS. 3-4  with the track connector shown in  FIG. 8   b  in accordance with one embodiment of the invention.  
         [0032]      FIG. 10  is a side view of the IBFD and insertion apparatus shown in  FIGS. 3-4 .  
         [0033]      FIGS. 11   a,    11   b  are top perspective and bottom views of a wafer for introduction into the IBFD of  FIGS. 1-2  using the insertion apparatus as shown in  FIGS. 3-4  and  9 .  
         [0034]      FIG. 12  is a side cut-away view of the structure shown in  FIG. 9 .  
         [0035]      FIG. 13  is a top view of the distal end of the wafer-track portion of the insertion apparatus shown in the prior figures.  
         [0036]      FIGS. 14   a - c  are top, top perspective and top-perspective cut-away views of components of the insertion apparatus engaged with the inferior endplate portion of the IBFD illustrated in  FIGS. 6-7  and including the distal end of the wafer track shown in  FIG. 13 .  
         [0037]      FIG. 15   a  is a top perspective view of a release plate, driver and the distal end of the wafer track of  FIG. 13 .  
         [0038]      FIG. 15   b  is a top view of components of the insertion apparatus engaged with the inferior endplate, including the release plate of  FIG. 15   a.  The track connector is removed to show the position of the release plate and the distal end of the wafer track in the inserter cavity.  
         [0039]      FIG. 16   a  is a bottom perspective view of the distal end of the wafer track of  FIG. 13  with the track connector of  FIGS. 8   a,    8   b  mounted thereon.  
         [0040]      FIGS. 16   b - d  are top, top perspective and top perspective cut-away views of components of the insertion apparatus engaged with the inferior endplate portion and including the track connector of  FIG. 8   b  prior to wafer insertion.  
         [0041]      FIG. 17  is a top view of the insertion apparatus with a wafer situated within the inferior endplate portion of the IBFD. The superior endplate is removed to show the position of the wafer in the wafer cavity.  
         [0042]      FIG. 18  is a perspective cut-away view of the insertion apparatus, the inferior endplate portion of the IBFD, including the track connector, and wafer shown in  FIG. 17 .  
         [0043]      FIG. 19  is a side pictorial view of the insertion apparatus being used to insert an IBFD in accordance with the present invention into an intervertebral space.  
         [0044]      FIGS. 20   a - 20   c  include side, top and end views of a disc space distractor for use with the insertion apparatus shown in the above identified figures.  
         [0045]      FIGS. 21   a - 21   b  are side and end cross-sectional views of an IBFD in accordance with one embodiment of the present invention with a stack of wafers introduced therein to one pre-determined height.  
         [0046]      FIGS. 21   c - 21   d  are side and end cross-sectional views of the IBFD shown in  FIGS. 21   a - 21   b  stacked to a different height in which all of the wafers are contained within the endplates.  
         [0047]      FIGS. 22   a - d  include side and end views of the IBFD shown in  FIGS. 21   a - 21   b.    
         [0048]      FIGS. 23   a - 23   d  include top and bottom perspective views, a side view and a cross-sectional view of a superior endplate for a sagittally curved embodiment of an IBFD of the present invention.  
         [0049]      FIGS. 24   a - 24   d  include side, top perspective, top and end views of an inferior endplate for a sagittally curved embodiment of an IBFD of the present invention.  
         [0050]      FIGS. 25   a - c  are perspective, top and cross-sectional views of a transversely curved wafer for use with an IBFD of the present invention.  
         [0051]      FIG. 26  is a side representation of an IBFD implanted in an intervertebral space with wafers as shown in  FIGS. 25   a - c.    
         [0052]      FIGS. 27   a - c  are perspective, top and cross-sectional views of a transversely curved and angled wafer for use with an IBFD of the present invention.  
         [0053]      FIG. 27   d  is a side representation of an IBFD implanted in an intervertebral space with wafers as shown in  FIGS. 27   a - c.    
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0054]     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains.  
         [0055]     In accordance with one embodiment of the invention, an interbody fusion device (IBFD)  10  includes a superior endplate  12  and an inferior endplate  14  that define a wafer cavity  19 , as shown in  FIGS. 1-2 . The superior and inferior surfaces of the endplates define engagement ribs  16   U  and  16   L  that are configured to engage or grip the vertebral endplates of opposed vertebrae in a spine. Preferably, the ribs  16   U  and  16   L  are configured to prevent expulsion of the IBFD under normal spinal loads. For instance, the ribs may have a saw tooth shape that is inclined toward the opening through which the IBFD is inserted into the interbody space. Angling the ribs toward the opening also angles them away from the direction of insertion so that the IBFD can be easily inserted into a collapsed space.  
         [0056]     The IBFD  10  also defines an inserter cavity  18  that engages a portion of an inserter apparatus  50 , as shown in  FIGS. 3-4 . The inserter apparatus  50  defines a wafer track  52  along which a plurality of expansion members, or wafers, are conveyed to fill the wafer cavity  19 .  
         [0057]     In accordance with one aspect of the invention, the IBFD  10  has a height across the superior and inferior endplates  12 ,  14  that is less than the normal anatomic height of a typical intervertebral disc space. The invention contemplates that a series of expansion members, such as wafers, are introduced into the wafer cavity  19  to at least fill all or part of the cavity, and to distract the opposing vertebrae, separating the superior and inferior endplates. Insertion of the wafers separates the endplates to expand the height of the IBFD within the intervertebral or interbody space and to ultimately restore the normal anatomic height of the instrumented disc space.  
         [0058]     Details of the superior and inferior endplates can be seen in  FIGS. 5-7 . Referring to  FIGS. 5   a - 5   f,  and in particular to  FIG. 5   d,  the superior endplate  12  includes an upper wall  22  on which the engagement ribs  16   U  are defined. The interior face of the upper wall is thickened in a reinforcement region  23 . This region helps maintain the integrity of the superior endplate  12  and provides a strong surface against which a lifting force can be applied by successive insertion of the wafer. Region  23  is also configured to contain and to cooperate with the wafers, as described below, to provide lateral and torsional stability to the wafer stack.  
         [0059]     The upper wall terminates in an anatomically anterior end wall  24  and an anatomically posterior end wall  25  that integrate with the inferior endplate  14  as described below. In addition, the reinforcement region  23  defines outwardly and laterally projecting prongs  27  that engage cooperating notches  36  defined in the interior of the inferior endplate  14 . Details of the inferior endplate are shown in  FIGS. 6-7 . The endplate  14  includes a bottom wall  30  on which the engagement ribs  16   L  are defined. The bottom wall  30  terminates in an end wall  32  and a ledge  33 . As shown in  FIGS. 2 and 9 , the anterior end wall  24  of the superior endplate  12  overlaps the end wall  32  and end ledge  33  when the endplates are initially assembled. The two end walls  24  and  30  overlap over the majority of the height of the end wall  32  so that as the superior and inferior endplates are pushed apart the two endplates remain in contact and continue to define the wafer cavity  19 , providing stability to the IBFD as it expands.  
         [0060]     The inferior endplate  14  also includes side walls  35  that define the wafer cavity and ultimately help retain the wafers within the cavity as they are sequentially inserted. The inner face of the side walls define notches  36  that are aligned for engagement by the prongs  27  in the superior endplate  12 . Thus, when the IBFD is initially assembled prior to insertion into the interbody space, the prongs and notches  27 ,  36  hold the two endplates together. The interface between the prongs and notches is adequate to hold the IBFD together as it is inserted into the space, but is sufficiently weak to be dislodged under pressure from the inserted wafers.  
         [0061]     The interior of the inferior endplate  14  includes opposite surfaces  38  that structurally reinforce the IBFD under large compressive loads. Slightly offset from the walls  38  are support rails  40  ( FIG. 6   b ) that support the track connector  46  shown in  FIGS. 8   a - 8   f.  The top surface  49  of the track connector  46  is configured to be superior to surface  38  such that any compressive load from the wafer stack is transmitted through the bottom surface of the track connector to the support rails  40 . The end walls  38  of the endplate  14  also form end notches  43  ( FIG. 7   c ) that are complementary to the end edges of the track connector  46  in one embodiment of the invention. The end walls  38  and rails  40  of the endplate  14  define a connector channel  42 , as shown in  FIG. 7   a,  which is configured to receive the distal end of the wafer track of inserter apparatus  50 , as described below.  
         [0062]     The superior and inferior endplates  12 ,  14  can be formed of a biocompatible material with sufficient strength to support the adjacent vertebrae without fatigue and fracture. Preferably, the two endplates are molded from a biocompatible polymeric material, such as, for example, PEEK or a biocompatible composite material, such as, for example carbon-fiber-reinforced PEEK. The material may also be selected to permit tissue ingrowth to integrate with the vertebral endplates. The endplates can further be formed from a moldable or formable biologic material, such as bone.  
         [0063]     In accordance with one aspect of this invention, the IBFD  10  is configured to be introduced into the interbody space by an introducer or inserter apparatus  50 . The inserter can be constructed and operated like the insertion apparatus disclosed in U.S. Pat. No. 6,595,998, entitled “Tissue Distraction Device”, which issued on Jul. 22, 2003, to the assignee of the present invention. The disclosure of this patent, and particularly its discussion of the wafer inserter, is incorporated herein by reference. Alternatively, the inserter can be constructed and operated like the insertion apparatus disclosed in co-pending application Ser. No. 10/813,819, entitled “Tissue Distraction Device”, filed on May 31, 2004, and assigned to the assignee of the present invention. The disclosure of this co-pending application is incorporated herein by reference.  
         [0064]     For purposes of illustration, certain details of the inserter  50  will be explained herein. As shown in  FIG. 3 , the apparatus includes a wafer track  52  along which wafers are conveyed to fill the wafer cavity  19  within the IBFD and ultimately to expand the height of the IBFD. Once the last wafer has been introduced into the IBFD it is necessary to remove the inserter  50 . The preferred embodiment of the invention contemplates a track connector  46  that helps to integrate the wafer track  52  with the interior cavity of the IBFD and to provide a support surface for the wafer stack within the IBFD.  
         [0065]     Details of the track connector  46  are shown in  FIGS. 8   a - 8   f  and  FIG. 9 . In particular, the connector  46  includes connector posts  47  that project downward with the IBFD, as best seen in  FIG. 9 . These posts engage corresponding openings  71  in an insertion plate  70  (see  FIG. 12 ) to provide an interface between the inserter apparatus  50  and the IBFD. In one embodiment, the track connector  46  defines interface edges  48  at its opposite ends that are configured to conform to wall  38  in the inferior endplate  14  (see  FIG. 6   b ). The track connector may also include end edges  46   a  flanking the interface edges that contact wall edges  38   a  of the endplate  14  to limit the movement of the track connector into the endplate. The track support includes a ramp  49   a  that helps direct incoming wafers upward from the wafer track  52  to the wafer support surface  49  within the IBFD.  
         [0066]     In an alternative embodiment shown in  FIG. 8   g,  a track connector  46 ′ includes a modified proximal end  48 ′ and distal end  48 ″, but still retains the connector posts  47 , wafer support surface  49  and ramp  49   a.  The modified distal end  48 ″ catches against a lip  39  formed in the inferior endplate, as shown in  FIGS. 9, 12  to prevent removal of the track connector  46 ′ once it is positioned with the assembled IBFD. The distal end of the track connector  46 ′ further defines end edges  46 ′ a  that contact the wall edges  38   a,  as depicted in  FIG. 16   b,  in the same manner as the end edges  46   a  described above.  
         [0067]     As shown in  FIGS. 9, 10  and  12 , the wafer inserter apparatus  50  provides an avenue for passage of wafers  55  from a wafer cartridge  54  into the IBFD. The inserter apparatus includes a cartridge gun that extracts wafers  55  consecutively from a stack within the cartridge  54  and conveys them along the track  52  to the IBFD. As shown in  FIGS. 11   a - b,  the wafers  55  are configured for transition along the track  52  and for interlocking engagement within the IBFD. In particular, the wafers include a leading bevel  56  and an opposite trailing bevel  57  to facilitate movement of each successive wafer underneath the immediately prior inserted wafer. The bevels  56 ,  57  help the incoming wafer dislodge and slide underneath the wafer stack already resident within the IBFD. In certain embodiments, a wafer driver  65  may be provided within the wafer track  52  to advance each wafer into the wafer cavity. The driver  65  can also help hold the lowermost wafer of the stack in position as the inserter apparatus  50  is removed.  
         [0068]     The wafers  55  also include interdigitating upper and lower surfaces  58 ,  59 , respectively. The surfaces can assume a variety of configurations intended to prevent relative longitudinal movement between wafers in the stack as well as for lateral and rotational stability. The wafers  55  and their respective surfaces can be constructed as disclosed in U.S. Pat. No. 6,595,998 cited above. The disclosure of this patent, and most particularly its discussion of the construction of the wafers, is incorporated herein by reference. In the preferred embodiment, the upper surface  58  defines a ridge  60  and spaced rib  61  extending along the longitudinal axis of the wafer. Similarly, the lower surface defines a linear trough  62  that receives the ridge  60 , and a notch  63  that receives the rib  61 .  
         [0069]     The insertion configuration for the IBFD and wafer inserter apparatus is generally depicted in  FIG. 12 . The wafer track  52  of the inserter apparatus engaged the IBFD with the track end  53  contacting the proximal faces of both the inferior endplate  14  and the superior endplate  12 . A wafer  55  is shown resting on the wafer support surface  49  of the track connector  46 ′. The track connector  46  rests on the support rail  40  (see  FIG. 6 ) with its posts  47  projecting downward toward the post openings  44  in the inferior endplate  14 . As shown in the figures, the posts do not necessary extend into the openings  44 . Instead, the post openings  44  facilitate the assembly of insertion apparatus to the track connector prior to use.  
         [0070]     Beneath the track connector  46  reside an insertion plate  70  and a release plate  75  immediately adjacent the connector  46 . Both plates provide openings to receive the connector posts  47  therethrough, including openings  71  in the insertion plate and openings  76   a - c  in the release plate. The insertion plate  70  may define a release track  72  (as shown in  FIG. 14   c ) within which the release plate  75  slides. The release track may be provided to increase the stiffness of the insertion plate, or may be eliminated to permit a reduction in width of the components.  
         [0071]     The assembly of the components of the inserter apparatus  50  within the IBFD  10  is depicted sequentially in  FIGS. 13-18 . The insertion plate  70  is shown in  FIG. 13 . Preferably, the plate  70  is integral with the wafer track  52 . As shown in  FIG. 12 , the insertion plate  70  essentially supports the IBFD with the plate  70  extending into the wafer cavity and the track end  53  abutting the IBFD. This plate  70  will be removed with the inserter apparatus  50 , leaving the IBFD within the interbody space. The post openings  71  are sized to receive the connector posts  47  therethrough. As can be seen in  FIGS. 14   a - c,  the insertion plate  70  sits below the support rail  40  in the inferior endplate  14  with its post openings  71  aligned with the post openings  44  in the endplate  14 .  
         [0072]     The release plate  75 , as shown in  FIGS. 15   a - b,  is slidably disposed within the release track  72  in the insertion plate  70 . In an alternate embodiment, the release plate  75  is slidably disposed on top of the insertion plate  70  without any release track  72 . The release plate  75  includes openings  76   a - c  corresponding to each of the connector posts  47 . The distal edge  77   a - c  of each opening is sharpened so that they will sever the posts  47  from the connector plate  46  when the release plate is pulled proximally, or out of the IBFD. The opening  76   a  is generally sized slightly larger than the post  47 , while the other two openings  76   b - c  are increasingly elongated. This configuration allows the distal-most post to be cleanly severed before the middle post is severed, and the middle post to be severed before the proximal post. This approach reduces the force needed to sever the posts. Once the posts are severed, they are retained within the post openings  71  via an interference fit, since they are no longer needed to hold the track connector within the IBFD. When the posts are severed, the inserter apparatus  50  can be removed from the implanted IBFD without risk of retracting the IBFD.  
         [0073]     The next series of figures,  FIGS. 16   a - d,  show the placement of the track connector on top of the insertion plate  70  and release plate  75 . As can be seen in  FIG. 16   d,  the wafer support surface  49  is generally contiguous with wall  38  of the inferior endplate  14 . In an alternate embodiment the wafer support surface  49  is superior to wall  38  of the inferior endplate  14 . This alternate embodiment ensures that the compressive load from the wafer stack is transmitted through the wafer support surface  49  and not through wall  38 . A first wafer  55  is added in  FIGS. 17-18 .  
         [0074]     The inserter apparatus  50  and the IBFD  10  are shown in position for implanting the IBFD within an interbody space. It is contemplated that the interbody or intradiscal space will be prepared in a known manner. In particular, the disc nucleus is removed by known means, preferably leaving the disc annulus A relatively intact. A portal is formed in the annulus that is sized to the dimensions of the IBFD  10  in its un-expanded configuration (as shown in  FIGS. 1-2 ).  
         [0075]     In the preferred arrangement, the IBFD is sized to be received in the unexpanded state through the portal into the disc space without any pre-distraction. In certain situations where the disc space height is smaller than the height of the unexpanded IBFD, pre-distraction may be used to slightly elevate the disc space so as to allow receipt of the unexpanded IBFD through the portal. Such pre-distraction, which can occur using conventional techniques, is not intended to achieve the final disc space height. One approach is to use the distractor  80  shown in  FIGS. 20   a - 20   c.  This distractor includes a distal end  82  having a height H greater than its width W. The height H of the distal end  82  is substantially constant over the insertion length L. The distractor is inserted into the disc space at a location adjacent to but laterally spaced from the location where the IBFD is to be inserted with its larger dimension parallel to the vertebral endplates. As such, no distraction occurs during insertion of the distractor  80 . The handle  84  is used to rotate the distractor  80  until the larger dimension contacts and pushes apart the vertebral endplates. The distractor  80  can be held in position as the IBFD is maneuvered into the interbody space using the inserter apparatus  50 . After removal of the distractor, a second IBFD may be inserted adjacent to the first implanted IBFD.  
         [0076]     As shown in  FIGS. 21   a - d  and  FIGS. 22   a - d,  the IBFD can be expanded to a specific height, with its height being determined by the number of wafers  55  inserted into the IBFD. In the preferred embodiment, the superior and inferior endplates  12 ,  14  and the wafers have a pre-determined height or thickness. As explained above, the endplates include overlapping portions to help stabilize the stack, in particular the end walls  24  and  32 . After implanting the IBFD a biomaterial, such as bone chips or other osteogenetic materials, such as bone morphogenic proteins or adipose-derived adult stromal cells, may be introduced adjacent to or in contact with the IBFD so as to promote fusion between the opposing vertebrae.  
         [0077]     As indicated in the figures, in certain embodiments of the invention, the stack height will change when the inserter apparatus is dislodged from the IBFD and removed. In particular, the wafer stack will shift slightly downward when the insertion plate and release plates are removed, allowing the track connector  46  to drop down.  
         [0078]     The IBFD  90  shown in  FIGS. 21   a - d  and  FIGS. 22   a - d  includes superior and inferior endplates  92 ,  94  that are angled. These endplates are configured to restore or maintain a particular angle of the vertebral motion segment. For instance, if the IBFD  90  is used in the lumbar spine, the endplates are defined at a lordotic angle. The endplates  80 ,  82  in  FIGS. 23   a - d  and  FIGS. 24   a - d  are also configured to have arcuate upper and lower surfaces for introduction into and anatomical support of the lumbar spine.  
         [0079]     Alternative concepts for the endplates and the wafers are shown in  FIGS. 25   a - 27   d.  In  FIGS. 25   a - c,  a curved wafer  100  is provided. The wafer includes interlocking dovetail features  101  and  104  and locking notches  102  to help hold the wafer stack together. As shown in  FIG. 26 , the endplates  105 ,  106  can be angled to restore the lordotic angle of the motion segment with the wafer stack therebetween.  
         [0080]     As an alternative, the wafers can provide the lordotic angle, such as the wafer  110  shown in  FIGS. 27   a - c.  The wafer  110  includes one end  111  that is thicker than the opposite end  112 . The wafers can be contained within endplates  115 ,  116  that are planar—i.e., that do not incorporate the lordotic angle.  
         [0081]     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.  
         [0082]     This invention contemplates an interbody fusion device configured for implantation within an interbody space that incorporates a cavity for receipt of bio-compatible wafers. The wafers can be used to fill the cavity and provide additional strength for the IBFD components, and to increase the height of the IBFD. In this way, a smaller IBFD can be initially introduced into the interbody space, preferably minimally invasively, and then a series of wafers can be introduced to incrementally increase the height of the IBFD in situ, to thereby increase the disc space substantially to its natural height.