Abstract:
A multipart intervertebral implant is provided which includes an implant portion and an implant extender portion. The implant portion and the implant extender portion can be fastened together using any known fastening means including pins, interlocking structure (e.g., dovetail, tongue and groove, etc.), adhesives, etc. The size of the implant extender portion can be selected during a surgical procedure to provide an implant suitable for a particular intervertebral receiving bed. An intervertebral implant is also provided which may be formed from a multiplicity of implant sections which are fastened together to provide an implant having a desired length. Implants having surface configurations which more closely correspond to the configuration of vertebral endplates are also provided.

Description:
This application claims priority as a divisional application from and incorporates by reference U.S. Utility application Ser. No. 09/754,038, filed Jan. 2, 2001, now abandoned, which claims priority from U.S. Provisional Application Ser. No. 60/173,973, filed Dec. 30, 1999. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates generally to biocompatible implants and, more particularly, to intervertebral implants suitable for implantation into the lumbar, thoracic and/or cervical regions of the spine during a spinal fusion procedure. 
     2. Background of Related Art 
     Intervertebral implants for fusing together adjacent vertebrae of a spinal column are well known in the art. Such implants are formed in a variety of different shapes and sizes and are configured for insertion into receiving beds formed in the lumbar, thoracic and cervical regions of the spine. The implants may be formed from a variety of different biologically compatible materials including ceramics, polymers, human or animal bone, composites, etc. The implants may also be shaped to maintain the natural lordoses of the spine or to prevent the implant from backing out of an intervertebral receiving bed in which it will be implanted. 
     Examples of known implants are disclosed in U.S. Pat. No. 4,877,020 to Vich and U.S. Pat. No. 4,878,915 to Brantigan. Vich and Brantigan each disclose cylindrical implants having an outer helical thread formed thereabout. The Vich implant is formed from autogenic bone taken from the iliac crest of a patient. The Brantigan implant is formed of an inert metal such as stainless steel, cobalt-chromium-molybdenum alloys and titanium. 
     One problem associated with known implants is the difficulty in adapting an implant to meet the size requirements of a particular intervertebral receiving bed. For example, because anatomically all patients are different, the specific size of implant required for a surgical procedure will not be known to any certainty until a surgeon has prepared the intervertebral space for implantation. Thus, a surgeon must keep a variety of different size implants available for use or have means to alter the dimensions of the implant at his disposal. 
     Another problem associated with known implants constricted from bone is that the anatomical limitations of donor bone limit the size of the implant which can be formed from bone. As a result, bone having satisfactory strength characteristics may not be available for use as an implant because of size limitations. 
     Finally, yet another problem associated with known implants is their inability to accurately maintain the natural lordoses of the spine. Because of the irregular shape of the vertebral endplates, wedge-shaped implants and cylindrical dowels are incapable of supporting adjacent vertebrae in their natural orientation without substantially altering the shape of the vertebral endplate(s). 
     Accordingly, a continuing need exists for an intervertebral implant whose size may be easily altered by a surgeon during a surgical procedure to meet the size requirements of a particular implant receiving bed and for an implant capable of maintaining the natural lordoses of the spine without substantially altering the shape of the vertebral endplates. 
     SUMMARY 
     In accordance with the present disclosure, intervertebral implants are provided which more precisely correspond in shape to the shape of the vertebral endplates. In one preferred embodiment, an implant is formed from a ring of material having a top surface and a bottom surface. The top and/or bottom surfaces of the implant include a series of annular stepped surfaces which together define a convex configuration which closely corresponds to the concave shape of the vertebral endplates. In an alternate embodiment, the annular stepped surfaces on the top and/or bottom surfaces of the implant can be replaced by a single helical pathway. 
     In another preferred embodiment, a multipart intervertebral implant is provided which includes an implant portion and an implant extender portion. The implant portion and the implant extender portion can be fastened together using any known fastening means including pins, interlocking structure (e.g., dovetail, tongue and groove, etc.), adhesives, etc. The size of the implant extender portion can be selected during a surgical procedure to provide an implant suitable for a particular intervertebral receiving bed. 
     In yet another preferred embodiment, an intervertebral implant may be formed from a multiplicity of implant sections which are fastened together to provide an implant having a desired length. These implant sections and the implants described above can be formed of any biocompatible material including bone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the presently disclosed intervertebral implants are described herein with reference to the drawings, wherein: 
         FIG. 1  is a perspective view of one preferred embodiment of the presently disclosed intervertebral implant; 
         FIG. 2  is a top view of the intervertebral implant shown in  FIG. 1 ; 
         FIG. 3  is a side view from a first side of the intervertebral implant shown in  FIG. 1 ; 
         FIG. 4  is a top perspective view of the intervertebral implant shown in  FIG. 1 ; 
         FIG. 5  is a side view from the other side of the intervertebral implant shown in  FIG. 1 ; 
         FIG. 6  is a perspective view of another preferred embodiment of the presently disclosed intervertebral implant; 
         FIG. 6A  is a perspective view of another preferred embodiment of the presently disclosed intervertebral implant; 
         FIG. 7  is a top view of the intervertebral implant shown in  FIG. 6 ; 
         FIG. 8  is a side view of the intervertebral implant shown in  FIG. 6 ; 
         FIG. 9  is a top perspective view of the intervertebral implant shown in  FIG. 6 ; 
         FIG. 10  is another side view of the intervertebral implant shown in  FIG. 6 ; 
         FIG. 11  is an elevational view of the intervertebral implant shown in  FIG. 6  positioned between adjacent vertebrae; 
         FIG. 12  is a perspective view of another embodiment of the presently disclosed intervertebral implant; 
         FIG. 13  is a side view of the intervertebral implant shown in  FIG. 12 ; 
         FIG. 14  is a top view of the intervertebral implant shown in  FIG. 12 ; 
         FIG. 15  is a rear end view of the intervertebral implant shown in  FIG. 12 ; 
         FIG. 16  is a perspective view of yet another embodiment of the presently disclosed intervertebral implant; 
         FIG. 17  is a side view of the intervertebral implant shown in  FIG. 16 ; 
         FIG. 18  is a top view of the intervertebral implant shown in  FIG. 16 ; 
         FIG. 19  is a rear end view of the intervertebral implant shown in  FIG. 16 ; 
         FIG. 20  is a perspective view of yet another embodiment of the presently disclosed intervertebral implant. 
         FIG. 21  is a side view of the intervertebral implant shown in  FIG. 20 ; 
         FIG. 22  is a top view of the intervertebral implant shown in  FIG. 20 ; 
         FIG. 23  is a rear end view of the intervertebral implant shown in  FIG. 20 ; 
         FIG. 24  is a top view of yet another embodiment of the presently disclosed intervertebral implant; 
         FIG. 25  is yet another embodiment of the presently disclosed intervertebral implant; 
         FIG. 26  is a side view of the intervertebral implant shown in  FIG. 25  positioned between adjacent vertebrae; 
         FIG. 27  is a side view of the presently disclosed intervertebral implant and implant extender positioned between adjacent vertebrae with the implant extender positioned adjacent the leading end of the intervertebral implant; 
         FIG. 28  is a side view of the intervertebral implant and implant extender shown in  FIG. 27  positioned between adjacent vertebrae with the implant extender positioned adjacent the trailing end of the intervertebral implant; 
         FIG. 29  is a side view of an alternate embodiment of the presently disclosed intervertebral implant and implant extender in an interlocked configuration; 
         FIG. 30  is a side view of the intervertebral implant and implant extender shown in  FIG. 29  in a partially assembled configuration; and 
         FIG. 31  is a perspective view of yet another embodiment of the presently disclosed intervertebral implant. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the presently disclosed intervertebral implant and implant extender will now be described in detail with reference to the drawings in which like reference numerals designate identical or corresponding elements in each of the several views. 
       FIGS. 1-5  illustrate a preferred embodiment of the presently disclosed intervertebral implant shown generally as  10 . Briefly, implant  10  includes an upper surface  12 , a lower surface  14  and a sidewall  15  positioned between the upper and lower surfaces. Upper and lower surfaces  12  and  14  each include a series of circular steps  16  which move upwardly from the outer periphery  18  of implant  10  to the center  20  of implant  10 . The steps  16  include a first plane  13   a  and a second plane  13   b . The first plane  13   a  is defined by the crest portion of the curvilinear projection or step  16 . As shown, the plane  13   a  may be generally flat. In some embodiments, the first plane  13   a  and second plane  13   b  may make a tapered configuration. In some embodiments, steps  16  may not be centered about the center of implant  10  and adjacent steps may not be of the same height. A throughbore  22  extends between upper and lower surfaces  12  and  14  of implant  10 . Throughbore  22  is dimensioned to receive growth factors including autograft, allograft, DBM, etc. . . . , to stimulate bone growth. In some embodiments, at least a portion of the sidewall  15  is free of slits and extends continuously generally parallel relative to a longitudinal axis extending between the upper surface  12  and the lower surface  14  of the implant  10 . 
       FIGS. 6-10  also illustrate an implant  100  having stepped upper and lower surfaces  112  and  114 . In contrast to the implant shown in  FIGS. 1-5 , implant  100  includes a greater number of steps  116  which define a more gradual taper than steps  16  of implant  10 . As shown, the implant may include an axis A extending between an upper and lower surface of the implant, wherein the curvilinear stepped surfaces  116  include a surface that is generally perpendicular to the axis A. It is to be appreciated that such axis A may be included in any of the embodiments disclosed herein, including, for example, the embodiment of  FIGS. 1-5 . It is envisioned that any number of steps may be provided on the upper and/or lower surfaces of the implant to provide any desired surface curvature. For example, each stop may have a height of from about 0.15 mm to about 3 mm. Other step dimensions are also envisioned. The implant  100  comprises a body  121  and includes a sidewall  119 . As shown, the body  121  is substantially solid and the sidewall  119  is substantially continuous. 
       FIG. 6A  illustrates another alternate embodiment of the stepped implant shown in  FIGS. 1-5  shown generally as  100 ′. Implant  100 ′ includes a helical pathway  116 ′ which extends from the outer periphery  118 ′ of implant  100 ′ towards the center of implant  100 ′. As discussed above, the helical pathway need not be centered about the central axis of implant  100 ′ but rather it may be oriented to provide the desired curvature on the upper and/or lower surface of implant  100 ′. Moreover, the height of the step defined by the helical pathway  116 ′ may vary along the length of pathway  116 ′. 
     Implants  10 ,  100  and  100 ′ may be formed from a variety of different biologically compatible materials including ceramics, polymers, human or animal bone, carbon fiber tantalum composites, etc. using a variety of known processes including molding, casting, machining, etc. Preferably, implant  10  is formed from cadaveric human or animal bone by making a transverse cut through the diaphysis or metaphysis of a long bone, e.g., tibia, fibula, femur, ulna, radius, etc., to form a ring and thereafter machining the upper and/or lower surfaces of the implant, e.g., milling the stepped or helical configuration into the upper and lower surfaces of the ring. Alternately, only one of the upper and lower surfaces of the ring may be provided with a stepped configuration. 
     The bone used to form implants  10 ,  100  and  100 ′ may be partially or fully demineralized bone. Preferably the bone is surface demineralized. By surface demineralizing the bone, the osteoconductivity and the conformability of the outer surfaces of the implant are improved while the strength of the inner portion of the implant is retained. 
     Referring to  FIG. 11 , implants  10 ,  100  and  100 ′ may be positioned within a receiving bed formed between adjacent vertebrae  32  and  34 . Because the upper and lower surfaces of the implants conform to the natural concavity of the vertebral endplates, only minimal preparation of the vertebrae is required by a surgeon prior to insertion of an implant to maintain the natural lordoses of the spine. 
       FIGS. 12-15  illustrate an alternate embodiment of the intervertebral implant shown generally as  200 . Intervertebral implant  200  is preferably formed from a cortical ring allograft cut from the diaphysis or metaphysis of a long bone but may be formed from any biocompatible material having the requisite strength requirements. Implant  200  includes a tapered, ring-shaped body  212  having flat top and bottom surfaces  214  and  216 , respectively. Anterior end  218  of implant  200  has a height which is greater than the height of posterior end  220 . The taper of the implant should be such as to conform to the vertebral end plates of adjacent vertebrae. Mating structure  222  for engaging corresponding structure of an insertion tool is formed in a sidewall  224  of implant  200  in the anterior end  218  of implant  200 . A plurality of concentric rings  226  are formed in top and bottom surfaces  214  and  216 . Rings  226  are preferably V-shaped, although other configurations are also envisioned, i.e., U-shaped, rectangular, etc. A throughbore  228  extends between top and bottom surfaces  214  and  216  of implant  200 . If implant  200  is formed from bone, throughbore  228  may be defined by the intramedullary canal of the bone from which implant  200  is cut. Implant  200  is configured for anterior insertion into the intervertebral space. Growth factors including autograft, allograft, and demineralized bone particles may be positioned in throughbore  228  and/or rings  226  to stimulate bone growth. 
       FIGS. 16-19  illustrate an alternate embodiment of implant  200  shown generally as  300 . Implant  300  is similar to implant  200  but includes convex top and bottom surfaces  314  and  316  which are configured to engage the vertebral end plates of adjacent vertebrae. Top and bottom surfaces  314  and  316  also include concentric rings  326  similar to those described above with respect to implant  200 . 
       FIGS. 20-23  illustrate another alternate embodiment of implant  200  shown generally as  400 . Implant  400  is similar to implant  200  except that anterior end  418  is approximately equal to the height of posterior end  420 . 
       FIG. 24  illustrates a partially threaded stepped implant shown generally as  500 . Implant  500  is preferably formed from cortical bone by making a transverse cut through the diaphysis or metaphysis of a long bone to obtain a cylindrical bone plug and thereafter machining and threading the bone plug. Alternately, implant  500  may be formed from any biocompatible material having the requisite strength requirements using any known process including machining, molding, etc. Implant  500  includes a cylindrical body  510  having a first end portion  512  having a first outer diameter and a second end portion  513  having a second outer diameter larger than the first diameter. A variety of different diameter implants are envisioned. A throughbore  514  extends through first end portion  512  of cylindrical body  510 . First end portion  512  has screw threads  516  at one end thereof, but does not include threads in the area about throughbore  514 . Second end portion  513  includes screw threads  518 . When implant  500  is formed by cutting a bone plug from a long bone, the absence of screw threads in the area of throughbore  514  facilitates the use of long bones having a thinner wall section, i.e., the bone wall between the intramedullary canal of a long bone and the outer surface of the bone plug cut therefrom can be thinner. 
     When implant  500  is inserted into intervertebral space between adjacent vertebrae, second end portion  513  will sit in the vertebral wall and provide the majority of the retaining force. First end portion  512  will also screw into adjacent endplates but thread engagement may be minimal, especially if the intervertebral space is very concave. A stepped reamer and tap can be used to prepare the intervertebral space. 
       FIGS. 25 and 26  illustrate an alternate embodiment of implant  500  shown generally as  600 . Implant  600  is similar to implant  500  except that second end  613  of implant  600  is tapered from one end to the other. Implant  600  may be used to vary the spine geometry. Referring to  FIG. 26 , during insertion of implant  600  into the intervertebral space, implant  600  will directly force adjacent vertebral surfaces  630  and  632  apart. The particular taper of second end portion  613  of implant  600  can be chosen to provide the desired spacing of the adjacent vertebrae. 
     Intervertebral implants in the form of threaded, cylindrical dowels formed of bone, specifically, human or animal cadaveric bone, are well known in the surgical arts. Typically, such implants are formed by making a transverse cut through the diaphysis or metaphysis of a long bone, i.e., the femur, tibia, fibula, ulna or radius, using a cylindrical drill bit. One problem associated with forming and using bone dowel implants is that anatomical limitations make it difficult to recover bone dowels having the desired length needed for intervertebral fusion procedures. This is especially true when performing procedures in the cervical region of the spine wherein small diameter dowels are required. 
     In order to compensate for anatomical limitations, a dowel extender portion may be provided. Referring to  FIGS. 27 and 28 , a cylindrical dowel extender  710  may be implanted within the intervertebral space prior to implantation of the main dowel portion D ( FIG. 27 ), or alternately, after implantation of main dowel portion D ( FIG. 28 ). Preferably, dowel extender portion  710  includes helical threads  712  to engage vertebral end plates  714  and  716  and retain the dowel extender portion in place. However, non-threaded dowel extender portions are also envisioned. Each dowel extender portion  710  preferably includes engagement structure, such as slot  718 , for engaging an insertion tool (not shown). 
     Generally, dowel extenders having a length of from about 4 to 8 mm are needed to supplement the main dowel, although other length dowel extenders may also be needed depending upon the particular surgical procedure being performed. A common thread pattern associated with intervertebral dowels is 10 threads per inch. Thus, a dowel extender having a length of 4 mm will only have about 1.57 threads and a dowel extender having a length of 6 mm will have only about 2.35 threads. Because of the limited number of threads and the short thread engagement length, it may be difficult to stabilize a dowel extender in the intervertebral space and problems may result. For example, if the dowel extender is not firmly seated in the intervertebral space between adjoining vertebrae when contacted by the main dowel, it may tip over. 
     In order to provide greater stability, an alternate embodiment of the presently disclosed dowel extender is described herein. Referring to  FIGS. 29 and 30 , dowel extender  800  includes a cylindrical body  810  having a first end  812  and a second end  814 . Second end  814  includes a projection  816  configured and dimensioned to be received within a correspondingly shaped slot formed on one end of main dowel D. Although illustrated as having a dove-tail configuration, projection  816  may assume other configurations capable of interlocking with a correspondingly shaped slot. Referring to  FIG. 29 , locking pins  820  may be provided to further secure dowel extender portion  800  to main dowel portion D. Alternately, a locking pin or pins may be used to entirely replace projection  816  and secure dowel extender  800  to main dowel D. 
     In an alternate embodiment, a bone dowel is constructed from multiple dowel segments which are secured together using interlocking structure. The interlocking structure may be formed integrally with each dowel segment, e.g., each dowel segment may have a slotted front end and a correspondingly shaped projection formed at a rear end. Each dowel segment has a predetermined length and is joined to one or more other dowel segments to form a dowel having a desired length. For example, dowel segments may be formed having lengths of 2, 4 and 6 mm. In order to form a dowel having a length of 20 mm, three 6 mm dowel segments and a 2 mm dowel segment can be joined together. The dowel segments are preferably formed from bone, although other biocompatible materials listed above are also envisioned. The dowel segments may be cylindrical, rectangular, wedge-shaped, etc. For example,  FIG. 31  illustrates a wedge shaped intervertebral implant  900  formed of multiple implant segments  910  which are fastened together in the manner described above. 
     It will be understood that various modifications may be made to the embodiments disclosed herein. For example, the configuration of the sidewall of any of the implants described above may be modified to better suit a particular procedure, i.e., the sidewalls can be formed to be rectangular, circular, triangular, semi-circular, etc. Moreover, the implants described above, although disclosed in the context of spinal implantation, may be suitable for other implantation procedures not specifically listed here but obvious to those of ordinary skill in the art. Therefore, the above description should not be construed as limiting, but merely as exemplifications of preferred embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.