Abstract:
A vertebral implant for interposition between two vertebral endplates comprises a first endplate assembly for engaging a first vertebral endplate, a second endplate assembly for engaging a second vertebral endplate, and a first flexible core component interposed between the first and second endplate assemblies. The first flexible core component comprises first and second end portions. The first end portion is coupled to the first endplate assembly to prevent translation of the first end portion with respect to the first endplate assembly, and the second end portion is pivotable with respect to the second endplate assembly.

Description:
BACKGROUND 
     During the past thirty years, technical advances in the design of large joint reconstructive devices has revolutionized the treatment of degenerative joint disease, moving the standard of care from arthrodesis to arthroplasty. Progress in the treatment of vertebral disc disease, however, has come at a slower pace. Currently, the standard treatment for disc disease remains discectomy followed by vertebral fusion. While this approach may alleviate a patient&#39;s present symptoms, accelerated degeneration of adjacent discs is a frequent consequence of the increased motion and forces induced by fusion. Thus, reconstructing the degenerated intervertebral disc with a functional disc prosthesis to provide motion and to reduce deterioration of the adjacent discs may be a more desirable treatment option for many patients. 
     SUMMARY 
     In one embodiment, a vertebral implant for interposition between two vertebral endplates comprises a first endplate assembly for engaging a first vertebral endplate, a second endplate assembly for engaging a second vertebral endplate, and a first flexible core component interposed between the first and second endplate assemblies. The first flexible core component comprises first and second end portions. The first end portion is coupled to the first endplate assembly to prevent translation of the first end portion with respect to the first endplate assembly, and the second end portion is pivotable with respect to the second endplate assembly. 
     In another embodiment, the vertebral implant comprises a second flexible core component. 
     In another embodiment, the vertebral implant comprises at least one tether extending between the first and second endplate assemblies to constrain the implant. 
     In another embodiment, the vertebral implant comprises a modification element for modifying the flexibility of the core component. 
     In another embodiment, a vertebral implant for interposition between two vertebral endplates comprises a first endplate assembly for engaging a first vertebral endplate, a second endplate assembly for engaging a second vertebral endplate, and a first flexible core component interposed between the first and second endplate assemblies. The first flexible core component comprises an outer surface,wherein the outer surface comprises an articulating surface covering less than the entire outer surface. The first endplate assembly comprises a coupling mechanism shaped to match a contour of the first flexible core component. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sagittal view of vertebral column having a destroyed disc. 
         FIG. 2  is a sagittal view of vertebral column with an intervertebral prosthesis replacing the destroyed disc. 
         FIG. 3  is a cross sectional side view of an exploded intervertebral disc prosthesis according to a first embodiment of the present disclosure. 
         FIG. 4  is a cross sectional side view of an assembled intervertebral disc prosthesis according to the first embodiment of the present disclosure. 
         FIG. 5  is a cross sectional top view of the intervertebral disc prosthesis according to the first embodiment of the present disclosure. 
         FIG. 6  is a side view of an endplate portion of an intervertebral disc prosthesis. 
         FIG. 7  is an anterior view of an endplate portion of an intervertebral disc prosthesis. 
         FIG. 8  is a side view of an endplate portion of an intervertebral disc prosthesis. 
         FIG. 9  is an anterior view of an endplate portion of an intervertebral disc prosthesis. 
         FIG. 10  is a side view of an endplate portion of an intervertebral disc prosthesis. 
         FIGS. 11–13  are cross sectional top views of intervertebral disc prostheses according to alternative embodiments of the present disclosure. 
         FIGS. 14–18  are cross sectional side views of intervertebral disc prostheses according to alternative embodiments of the present disclosure. 
         FIG. 19  is a cross sectional side view of an exploded intervertebral disc prosthesis according to an alternative embodiment of the present disclosure. 
         FIG. 20  is a cross sectional side view of an assembled intervertebral disc prosthesis according to the embodiment of  FIG. 19 . 
         FIG. 21  is a cross sectional top view of an intervertebral disc prosthesis according to the embodiment of  FIG. 19 . 
         FIGS. 22–30  are cross sectional side views of assembled intervertebral disc prostheses according to alternative embodiments of the present disclosure. 
         FIG. 31  is a cross sectional side view of an intervertebral disc prosthesis according to an alternative embodiment of the present invention. 
         FIG. 32  is a cross sectional top view of an intervertebral disc prosthesis according to the embodiment of  FIG. 31 . 
         FIG. 33  is a cross sectional side view of an intervertebral disc prosthesis according to an alternative embodiment of the present invention. 
         FIG. 34  is a cross sectional top view of an intervertebral disc prosthesis according to the embodiment of  FIG. 34 . 
         FIGS. 35 ,  37 ,  39 , and  41  are cross sectional top views of intervertebral disc prostheses according to alterniative embodiments of the present invention. 
         FIGS. 36 ,  38 ,  40  and  42  are cross sectional side views of the intaervertebral disc prostheses of the embodiments of  FIGS. 35 ,  37 ,  39 , and  41 , respectively. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to vertebral reconstructive devices, and more particularly, to a functional intervertebral disc prosthesis. For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments, or examples, illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring first to  FIG. 1 , the reference numeral  10  refers to a vertebral column with a damaged intervertebral disc  12  extending between two intact vertebrae  14  and  16 . In a typical surgical discectomy, the damaged disc  12  is removed creating a void between the two intact vertebrae  14  and  16 . This procedure may be performed using an anterior, anterolateral, lateral, or other approach known to one skilled in the art. Referring now to  FIG. 2 , a prosthesis  18  may be provided to fill the void between the vertebrae  14  and  16 . 
     Referring now to  FIGS. 3–5 , an intervertebral disc prosthesis  20  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  20 , according to an embodiment of the present invention, includes endplate assemblies  22 ,  24  and a core component  26 . The endplate assembly  22  may include an exterior surface  28  and an interior surface  30 . In this embodiment, the surface  30  may be relatively flat and smooth and may have a mirror surface finish. The surface  30  may further include a groove  32 . The endplate assembly  24  may have an exterior surface  34  and an interior surface  36 . The surface  36  may be relatively flat and smooth and may have a mirror surface finish. The surface  36  may further include a coupling mechanism  37  such as a groove. The articulating interior surfaces  30 ,  36  may be flat with a mirror finish as shown in this embodiment, however in alternative embodiments, the articulating surfaces may include grooves, dimples or other features to improve lubrication and reduce friction and wear. These surfaces may be treated with any of various techniques to improve wear resistance such as ion-implantation, diamond or diamond-like coating, or other methods that make the surface harder than the original surface. 
     The core component  26  may include a flexible body  38  having end surfaces  40  and  42 . As shown in  FIG. 5 , the core component  26  may have a generally circular cross-section as viewed from a plane perpendicular to a longitudinal axis  44  ( FIG. 3 ). Alternate cross-sectional shapes may be desirable, and in a single core component  26 , the cross sectional shape may vary depending upon the location of the perpendicular plane. In this embodiment, the end surfaces  40  and  42  may be relatively flat and parallel and may incorporate coupling mechanisms  46 ,  48 , respectively which may be ridges. The end surfaces  40 ,  42  may be integral with the flexible body  38  or may be mechanically or adhesively attached to the flexible body  38 . For example, as shown in  FIG. 4 , a coupling mechanism  50 , such as a ridge, formed on the flexible body  38  may engage a coupling mechanism  52 , such as a groove, formed on the end surface  42 . In alternative embodiments, the core component may have curved end surfaces or end surfaces angled with respect to one another. 
     The endplate assemblies  22 ,  24  may be formed of any suitable biocompatible material including metals such as cobalt-chromium alloys, titanium alloys, nickel titanium alloys, and/or stainless steel alloys. Ceramic materials such as aluminum oxide or alumnia, zirconium oxide or zirconia, compact of particulate diamond, and/or pyrolytic carbon may be suitable. Polymer materials may also be used, including any member of the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK), carbon-reinforced PEEK, or polyetherketoneketone (PEKK); polysulfone; polyetherimide; polyimide; ultra-high molecular weight polyethylene (UHMWPE); and/or cross-linked UHMWPE. 
     The exterior surfaces  28 ,  34  may include features or coatings (not shown) which enhance the fixation of the implanted prosthesis. For example, the surfaces may be roughened such as by chemical etching, bead-blasting, sanding, grinding, serrating, and/or diamond-cutting. All or a portion of the exterior surfaces  28 ,  34  may also be coated with a biocompatible and osteoconductive material such as hydroxyapatite (HA), tricalcium phosphate (TCP), and/or calcium carbonate to promote bone in growth and fixation. Alternatively, osteoinductive coatings, such as proteins from transforming growth factor (TGF) beta superfamily, or bone-morphogenic proteins, such as BMP2 or BMP7, may be used. Other suitable features may include spikes as shown on the endplate assembly  60  in  FIG. 6  for initial fixation; ridges or keels as shown on the endplate assembly  62  in  FIG. 7 and 8  to prevent migration in the lateral and anterior direction, for example; serrations or diamond cut surfaces as shown on the endplate assembly  64  in  FIG. 9 and 10 ; fins; posts; and/or other surface textures. 
     Referring again to  FIGS. 3–5 , flexible body  38  may be formed from one or more resilient materials which may have a lower modulus than the endplate materials. Suitable materials may include polymeric elastomers such as polyolefin rubbers; polyurethanes (including polyetherurethane, polycarbonate urethane, and polyurethane with or without surface modified endgroups); copolymers of silicone and polyurethane with or without surface modified endgroups; silicones; and hydrogels. Polyisobutylene rubber, polyisoprene rubber, neoprene rubber, nitrile rubber, and/or vulcanized rubber of 5-methyl-1,4-hexadiene may also be suitable. 
     The core component end surfaces  40 ,  42  may be modified, treated, coated or lined to enhance the wear resistant and articulating properties of the core component  26 . These wear resistant and articulation properties may be provided by cobalt-chromium alloys, titanium alloys, nickel titanium alloys, and/or stainless steel alloys. Ceramic materials such as aluminum oxide or alumnia, zirconium oxide or zirconia, compact of particulate diamond, and/or pyrolytic carbon may be suitable. Polymer materials may also be used including any member of the PAEK family such as PEEK, carbon-reinforced PAEK, or PEKK; polysulfone; polyetherimide; polyimide; UHMWPE; and/or cross-linked UHMWPE. Polyolefin rubbers, polyurethanes, copolymers of silicone and polyurethane, and hydrogels may also provide wear resistance and articulation properties. Wear resistant characteristics may also or alternatively be provided to the end surfaces  40 ,  42  by modifications such as cross-linking and metal ion implantation. 
     Although the embodiments of  FIGS. 3–5  describe circular endplate assemblies,  FIG. 11  shows a rectangular endplate assembly  66 .  FIG. 12  shows a rectangular endplate assembly  68  with curved sides.  FIG. 13  shows a kidney or heart shaped endplate assembly  70 . Other endplate geometries may be square, oval, triangular, hexagonal, or any other shape. As shown in the cross sectional top view of  FIG. 5 , the geometry of the core component may be round, oval, or any other shape which promotes constraint or articulation. 
     In the embodiment of  FIGS. 3–5 , the exterior surfaces  28  and  34  may be relatively parallel, but in other embodiments, the surfaces may be angled with respect to each other to accommodate a particular lordotic or kyphotic angle. As shown in  FIGS. 14–18 , protheses may be tapered, angled, or wedge shaped to achieve a desired lordotic or kyphotic angle. Such angles may be created by incorporating angled endplate assemblies and/or core components. The prosthesis  72  of  FIG. 14  is angled by incorporating an angled endplate. The prosthesis  74  of  FIG. 15  is angled by incorporating two angled endplates. The prosthesis  76  of  FIG. 16  is angled by incorporating flat endplates with a core component having one angled side. The prosthesis  78  of  FIG. 17  is angled by incorporating flat endplates with a core component having two angled sides. The prosthesis  80  of  FIG. 18  is angled by incorporating an angled endplate and a core component having an angled side. 
     Referring again to  FIG. 4 , the prosthesis  20  may be assembled by engaging the ridges  46 ,  48  of the core component  26  with the grooves  32 ,  37 , respectively of the endplate assemblies. The assembled prosthesis  20  may be implanted into the vertebral column  10  ( FIG. 1 ) in the void created by the removed disc  12  such that the exterior surface  28  engages an endplate of the vertebral body  14  and the exterior surface  34  engages an endplate of the vertebral body  16 . 
     In operation, the prosthesis  20  may elastically deform under compressive loads parallel to the longitudinal axis  44  and may elastically stretch in response to a force which may pull the endplate assemblies away from one another along the longitudinal axis  44 . The prosthesis  20  may also deform or flex under flexion-extension or lateral bending motion. The core component  26  may allow a variable center of rotation to permit flexion-extension and lateral bending motions. The flexible nature of the core component  26  may also reduce wear caused by cross-shearing or by articulation in flexion-extension and lateral bending motions. The core component  26  may also flex to permit anterior-posterior or lateral translational displacement of the endplate assembly  22  relative to the endplate assembly  24 . Further, as the interface between the end surfaces  40 ,  42  and the interior surfaces  30 ,  36 , respectively may be rotationally unconstrained, the core component  26  may pivot or rotate about the longitudinal axis  44 . The interface may, however, constrain translational movement at the interface. The end plate assemblies  22 ,  24  may also rotate relative to one another. In alternative embodiments, at least one of the interfaces between the end surfaces  40 ,  42 , and the interior surfaces  30 ,  36 , respectively may permit no rotational or pivotal movement. The engagement of the coupling mechanisms  46 ,  48  of the core component  26  with the coupling mechanisms  32 ,  37  may prevent ejection of the core component  26  while permitting rotation of the endplate assemblies  22 ,  24  relative to the core component. 
     Referring now to  FIGS. 19–21 , an intervertebral disc prosthesis  90  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  90 , according to an embodiment of the present invention, includes endplate assemblies  92 ,  94  and a core component  96 . The endplate assembly  92  may include an exterior surface  98  and an interior surface  100 . In this embodiment, the interior surface  100  may be relatively concave and smooth and may have a mirror surface finish. The endplate assembly  94  may have an exterior surface  104  and an interior surface  106 . The surface  106  may be relatively concave and smooth and may include a mirror surface finish. In this embodiment, the exterior surfaces  98  and  104  are relatively parallel, but in other embodiments, as described above, the surfaces may be angled with respect to each other to accommodate a particular lordotic or kyphotic angle. 
     The core component  96  may include a flexible body  108  having end surfaces  110  and  112 . As shown in  FIG. 21 , the core component  96  may have a generally circular cross-section as viewed from a plane perpendicular to a longitudinal axis  44 . Alternate cross-sectional shapes may be desirable, and in a single core component  96 , the cross sectional shape may vary depending upon the location of the perpendicular plane. In this embodiment, the end surfaces  110  and  112  may be relatively convex. The end surfaces  110 ,  112  may be integral with the flexible body  118  or may be mechanically or adhesively attached to the flexible body  118 . For example, a coupling mechanism  114 , such as a ridge, formed on the flexible body  108  may engage a coupling mechanism  116 , such as a groove, on the end surface  112 . 
     The endplate assemblies  92 ,  94  may be formed of the same or similar materials as endplate assemblies  22 ,  24  respectively, including the same or similar features or coatings. Likewise, the core component  96  may be formed from the same materials and may include the same wear resistant properties as described above for core component  26 . The prosthesis  90 ; endplate assemblies  92 ,  94 ; and the core component  96  may be shaped as described above in  FIGS. 6–18 . 
     As shown in  FIG. 20 , the prosthesis  90  may be assembled by positioning end surfaces  110 ,  112  in contact with the interior surfaces  100 ,  106  respectively. The assembled prosthesis  90  may be implanted into the vertebral column  10  ( FIG. 1 ) in the void created by the removed disc  12  such that the exterior surface  98  engages an endplate of the vertebral body  14  and the exterior surface  104  engages an endplate of the vertebral body  16 . 
     In operation, the prosthesis  90  may elastically deform under compressive loads parallel to the longitudinal axis  44 . The prosthesis  90  may also deform or flex under flexion-extension or lateral bending motion. The core component  96  may also flex to permit anterior-posterior or lateral translational displacement. The core component  96  may allow a variable center of rotation to permit flexion-extension and lateral bending motions. The flexible nature of the core component  96  may also reduce wear caused by cross-shearing or by articulation in flexion-extension and lateral bending motions. Further, as the interface between the end surfaces  110 ,  112  and the interior surfaces  100 ,  106 , respectively may be unconstrained, the core component  96  may rotate about the longitudinal axis  44 . The end plate assemblies  92 ,  94  may also rotate relative to one another. The concave interior surfaces  100 ,  106  may prevent ejection of the core component  26  while permitting rotation of the endplate assemblies  22 ,  24  relative to the core component. 
     As shown in  FIGS. 22–27 , a variety of alternative endplate assembly, core component, and coupling mechanism designs may limit lateral translation while permitting axial rotation. For example, referring now to  FIG. 22 , an intervertebral disc prosthesis  120  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  120  includes endplate assemblies  122 ,  124  and a core component  126 . The endplate assemblies  122 ,  124  may include exterior surfaces  128 ,  130 , respectively and interior surfaces  132 ,  134 , respectively. The interior surfaces  132 ,  134  may include concave and convex portions and may be smooth with a mirror surface finish. The concave and convex portions may form concentric rings on the interior surfaces. In the embodiment of  FIG. 22 , convex protrusions  136 ,  138  of the interior surfaces  132 ,  134  may function as coupling mechanisms. The endplate assemblies  122 ,  124  may be formed of the same or similar materials as endplate assemblies  22 ,  24 , respectively, including the same or similar features or coatings. 
     The core component  126  may include a flexible body  140  having end surfaces  142 ,  144 . In this embodiment, the end surfaces  142 ,  144  may comprise coupling mechanisms  146 ,  148  which may be dimples in approximately the center of the end surfaces. The core component  126  may be formed from the same materials and may include the same wear resistant properties as described above for the core component  26 . 
     The prosthesis  120  may be assembled by positioning the end surfaces  142 ,  144  in contact with the interior surfaces  132 ,  134 , respectively. Specifically, the coupling mechanisms  136 ,  138  of the endplate assemblies may engage the coupling mechanisms  146 ,  148  of the core component. The assembled prosthesis  120  may be implanted into the vertebral column  10  ( FIG. 1 ) in the void created by the removed disc  12  such that the exterior surface  128  engages an endplate of the vertebral body  14  and the exterior surface  130  engages an endplate of the vertebral body  16 . 
     Referring now to  FIG. 23 , an intervertebral disc prosthesis  150  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  150  includes endplate assemblies  152 ,  154  which may be round and a core component  156 . The endplate assemblies  152 ,  154  may include exterior surfaces  158 ,  160 , respectively and interior surfaces  162 ,  164 , respectively. Like the embodiment of  FIG. 9 , the interior surfaces  162 ,  164  may include concave and convex portions and may be smooth with a mirror surface finish. The concave and convex portions may form concentric rings on the interior surfaces. In this embodiment, convex ring protrusions  166 ,  168  of the interior surfaces  162 ,  164  may function as coupling mechanisms to engage coupling mechanisms  170 ,  172 , respectively, which may be concave rings formed on end surfaces  174 ,  176  of the core component  156 . 
     Referring now to  FIG. 24 , an intervertebral disc prosthesis  180  may be used as the prosthesis  18  of  FIG. 2 . The prosthesis  180 , according to this embodiment of the present invention, includes endplate assemblies  182 ,  184  and a core component  186 . The endplate assemblies  182 ,  184  may include exterior surfaces  188 ,  190 , respectively and interior surfaces  192 ,  194 , respectively. The interior surfaces  192 ,  194  may include concave and convex portions and may be smooth with a mirror surface finish. In the embodiment of  FIG. 24 , convex protrusions  196 ,  198  of the interior surfaces  192 ,  194  may function as coupling mechanisms. The endplate assemblies  182 ,  184  may be formed of the same or similar materials as endplate assemblies  22 ,  24 , respectively, including the same or similar features or coatings. 
     The core component  186  may include a flexible body  200  having an articulating surface  202 . In this embodiment, core component  186  may be ring shaped, having a center aperture  204  which may serve as a coupling mechanism. The core component  186  may be formed from the same materials and may include he same wear resistant properties as described above for the core component  26 . 
     The prosthesis  180  may be assembled by positioning the core component  186  between the interior surfaces  190 ,  194 , respectively. Specifically, the coupling mechanisms  196 ,  198  of the endplate assemblies may engage the coupling mechanisms  204  of the core component. The assembled prosthesis  180  may be implanted into the vertebral column  10  ( FIG. 1 ) in the void created by the removed disc  12  such that the exterior surface  188  engages an endplate of the vertebral body  14  and the exterior surface  192  engages an endplate of the vertebral body  16 . 
     In operation, the interface between the articulating surface  202  and the interior surfaces  190 ,  194  may permit both rotation about the longitudinal axis  44  and limited lateral translation. The prosthesis  180  may elastically deform under compressive loads parallel to the longitudinal axis  44  to absorb shock and provide a dampening effect. Both the articulating interface between the endplate assemblies  182 ,  184  and the core component  186  and the elasticity of the flexible body  200  may allow flexion-extension and lateral bending motions. 
     Referring now to  FIG. 25 , an intervertebral disc prosthesis  210  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  210 , according to this embodiment of the present invention, includes endplate assemblies  212 ,  214  and a core component  216 . The endplate assembly  212  may include an exterior surface  218  and an interior surface  220 . In the embodiment of  FIG. 25 , the interior surface  220  may be relatively concave and smooth and may have a mirror-surface finish. 
     The endplate assembly  214  may have an exterior surface  222  and an interior surface  224 . The interior surface  224  may be relatively concave and may include a coupling mechanism  226  which may be a protruding post. 
     The core component  216  may include a flexible layer  228  and outer articulating layers  230 ,  232  attached to the flexible layer  228 . The articulating layer  230  may be convex. The articulating layer  232  may be generally convex and may include a recess  234  which may serve as coupling mechanism. In an alternative embodiment, the recess may be formed on the articulating layer  230 . 
     The endplate assemblies  212 ,  214  may be formed of the same or similar materials as endplate assemblies  22 ,  24  respectively, including the same or similar features or coatings and therefore will not be described in further detail. The flexible layer  228  may be formed from the same flexible or elastic materials as described above for core component  26 . The articulating layers  230 ,  232  may be formed from the same or similar materials as endplate assemblies  22 ,  24 . Alternatively, the articulating layers  230 ,  232  may be formed from the same or similar materials as described above for core component  26  with modifications such as cross-linking or ion implantation to enhance wear resistance. 
     As shown in  FIG. 25 , the prosthesis  210  may be assembled by engaging the concave interior surface  220  of the endplate assembly  212  with the convex articulating layer  220  of the core component  216 . The articulating layer  232  may engage the interior surface  224  of the endplate assembly  214  with the coupling mechanism  234  engaging the coupling mechanism  226 . 
     In operation, the interface between the convex articulating layer  230  and the concave surface  220  of the endplate assembly  212 , may permit both rotation about the longitudinal axis  44  and limited lateral translation. The articulating layer  232  may be permitted to rotate about coupling mechanism  226  of the interior surface  224  while permitting little or no lateral translation between the core component  216  and the endplate assembly  214 . The prosthesis  210  may elastically deform under compressive loads parallel to the longitudinal axis  44  to absorb shock and provide a dampening effect. Both the articulating interface between the endplate assembly  212  and the core component  216  and the elasticity of the flexible layer  228  may allow flexion-extension, lateral bending, or axial rotation motion about the longitudinal axis  44 . 
     Referring now to  FIG. 26 , an intervertebral disc prosthesis  240  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  240 , according to this embodiment of the present invention, includes endplate assemblies  242 ,  244  and a core component  246 . The endplate assembly  242  may include an exterior surface  248  and an interior surface  250 . In the embodiment of  FIG. 26 , the interior surface  250  may be relatively concave and smooth and may have a mirror surface finish. 
     The endplate assembly  244  may have an exterior surface  252  and an interior surface  254 . The interior surface  254  may be relatively flat and may include a coupling mechanism  256  which may be a protrusion and a coupling mechanism  258  which may be a groove. 
     The core component  246  may include a flexible layer  260  and outer articulating layers  262 ,  264  attached to the flexible layer  260 . The articulating layer  262  may be convex. The articulating layer  264  may be relatively flat and may include a ridge  266  and an indention  268 , both of which may serve as coupling mechanisms. 
     The endplate assemblies C 2 , C 4  may be formed of the same or similar materials as endplate assemblies  22 ,  24  respectively, including the same or similar features or coatings and therefore will not be described in further detail. The flexible layer  260  may be formed from the same flexible or elastic materials as described above for core component  26 . The articulating layers  262 ,  264  may be formed from the same or similar materials as endplate assemblies  22 ,  24 . Alternatively, the articulating layers  262 ,  264  may be formed from the same or similar materials as described above for core component  26  with modifications such as cross-linking or ion implantation to enhance wear resistance. 
     As shown in  FIG. 26 , the prosthesis  240  may be assembled by engaging the concave interior surface  250  of the endplate assembly  242  with the convex articulating layer  262  of the core component  246 . The articulating layer  264  may engage the interior surface  254  of the endplate assembly  244  with the coupling mechanisms  266 ,  268  engaging the coupling mechanisms  258 ,  256  respectively. 
     In operation, the interface between the convex articulating layer  262  and the concave surface  250  of the endplate assembly  242 , may permit both rotation about the longitudinal axis  44  and limited lateral translation. The articulating layer  264  may be permitted to rotate about the axis  44  relative to the interior surface  254  or alternatively, the interface may be mechanically or adhesively fixed to prevent rotation. The prosthesis  240  may elastically deform under compressive loads parallel to the longitudinal axis  44  to absorb shock and provide a dampening effect. Both the articulating interface between the endplate assembly  242  and the core component  246  and the elasticity of the flexible layer  260  may allow flexion-extension, lateral bending, or axial rotation motion about the longitudinal axis  44 . 
     Referring now to  FIG. 27 , an intervertebral disc prosthesis  270  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  270 , according to this embodiment of the present invention, includes endplate assemblies  272 ,  274  and a core component  276 . The endplate assembly  272  may include an exterior surface  278  and an interior surface  280 . In the embodiment of  FIG. 27 , the interior surface  280  may include coupling mechanisms  282  which may concentric circular, dove tail shaped grooves. 
     The endplate assembly  274  may have an exterior surface  284  and an interior surface  286 . The interior, surface  286  may be relatively smooth and concave and may have a mirror surface finish. 
     The core component  276  may include a flexible layer  288  and outer articulating layers  290 ,  292  attached to the flexible layer  290 . The articulating layer  292  may be convex. The articulating layer  290  may include flat portions and may also include coupling mechanisms  294  which may be concentric circular, dove tail shaped projections. 
     The endplate assemblies  272 ,  274  may be formed of the same or similar materials as endplate assemblies  22 ,  24  respectively, including the same or similar features or coatings. The flexible layer  288  may be formed from the same flexible or elastic materials as described above for core component  26 . The articulating layers  290 ,  292  may be formed from the same or similar materials as endplate assemblies  22 ,  24 . Alternatively, the articulating layers  290 ,  292  may be formed from the same or similar materials as described above for core component  26  with modifications such as cross-linking or ion implantation to enhance wear resistance. 
     As shown in  FIG. 27 , the prosthesis  270  may be assembled by engaging the concave interior surface  286  of the endplate assembly  274  with the convex articulating layer  292  of the core component  276 . The articulating layer  290  may engage the interior surface  280  of the endplate assembly  272  with the coupling mechanisms  282  engaging the coupling mechanisms  294 . 
     In operation, the interface between the convex articulating layer  292  and the concave surface  286  of the endplate assembly  274 , may permit both rotation about the longitudinal axis  44  and limited lateral translation. The articulating layer  290  may permit rotation about the axis  44  relative to the interior surface  280  while the coupling mechanisms  282 ,  294  prevent or limit lateral motion. The prosthesis  270  may elastically deform under compressive loads parallel to the longitudinal axis  44  to absorb shock and provide a dampening effect. Both the articulating interface between the endplate assembly  274  and the core component  276  and the elasticity of the flexible layer  288  may allow flexion-extension, lateral bending, or axial rotation motion about the longitudinal axis  44 . In an alternative embodiment, coupling mechanisms such as those used for  282 ,  294  may be used at the interface between the endplate assembly  274  and the core component  276 . In this alternative, lateral translation may be prevented or limited while still allowing rotation about the axis  44 . The flexibility of the core component in this alternative embodiment could still enable flexion-extension and lateral bending motion. 
     Referring now to  FIG. 28 , an intervertebral disc prosthesis  300  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  300 , according to another embodiment of the present invention, includes endplate assemblies  302 ,  304  and a core component  306 . The endplate assembly  302  may include an exterior surface  308  and an interior surface  310 . In the embodiment of  FIG. 28 , the interior surface  310  may be relatively concave and smooth and may have a mirror surface finish. 
     The endplate assembly  304  may have an exterior surface  314  and an interior surface  316 . The interior surface  316  may be relatively flat and may include a coupling mechanism  318  which may be a groove. The exterior surfaces  308  and  314  may be relatively parallel or may be angled with respect to each other to accommodate a particular lordotic or kyphotic angle. 
     The core component  306  may include a flexible layer  320  and an articulating layer  322  attached to the flexible layer  320 . The articulating layer  322  may have a convex surface  324 . The flexible layer  320  may include a coupling mechanism  326  which may be a ridge. The core component  306  may have a generally circular cross-section as viewed from a plane perpendicular to a longitudinal axis  44 . Alternate cross-sectional shapes may be desirable, and in a single core component  306 , the cross sectional shape may vary depending upon the location of the perpendicular plane. 
     The endplate assemblies  302 ,  304  may be formed of the same or similar materials as endplate assemblies  22 ,  24  respectively, including the same or similar features or coatings and therefore will not be described in further detail. The flexible layer  320  may be formed from the same flexible or elastic materials as described above for core component  26 . The articulating layer  322  may be formed from the same or similar materials as endplate assemblies  22 ,  24 . Alternatively, the articulating layer  322  may be formed from the same or similar materials as described above for core component  26  with modifications such as cross-linking or ion implantation to enhance wear resistance. 
     As shown in  FIG. 28 , the prosthesis  300  may be assembled by mechanically or adhesively attaching the flexible layer  320  to the interior surface  316  of the endplate assembly  304 . The coupling mechanism  318  may engage the coupling mechanism  326 , providing mechanical attachment. Additionally or alternatively, an adhesive may be used to attached the flexible layer  320  and the interior surface  316 . The convex surface  324  of the articulating layer  322  may be positioned on the concave interior surface  310 . The assembled components  302 – 306  may be implanted into the vertebral column  10  ( FIG. 1 ) in the void created by the removed disc  12  such that the exterior surface  308  engages an endplate of the vertebral body  14  and the exterior surface  314  engages an endplate of the vertebral body  16 . 
     In operation, the convex surface  324  of the articulating layer  322  may articulate with the concave surface  310  of the endplate assembly  92 . The prosthesis  300  may elastically deform under compressive loads parallel to the longitudinal axis  44  to absorb shock and provide a dampening effect. Both the articulating interface between the endplate assembly  302  and the core component  306  and the elasticity of the flexible layer  320  may allow flexion-extension, lateral bending, or axial rotation motion about the longitudinal axis  44 . The end plate assemblies  302 ,  304  may also rotate relative to one another. 
     Referring now to  FIG. 29 , an intervertebral disc prosthesis  330  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  330 , according to another embodiment of the present invention, includes endplate assemblies  332 ,  334  and a core component  336 . The endplate assembly  332  may include an exterior surface  338  and an interior surface  340 . In the embodiment of  FIG. 16 , the interior surface  340  may be relatively flat and may include a coupling mechanism  342  which may be a groove. 
     The endplate assembly  334  may have an exterior surface  344  and an interior surface  346 . The interior surface  346  may be at least partially convex and may be smooth with a mirror finish. The exterior surfaces  338  and  344  may be relatively parallel or may be angled with respect to each other to accommodate a particular lordotic or kyphotic angle. 
     The core component  336  may include a flexible layer  350  and an articulating layer  352  attached to the flexible layer  350 . The articulating layer  352  may have a concave surface  354 . The flexible layer  350  may include a coupling mechanism  356  which may be a ridge. The core component  336  may have a generally circular cross-section as viewed from a plane perpendicular to a longitudinal axis  44 . Alternate cross-sectional shapes may be desirable, and in a single core component  336 , the cross sectional shape may vary depending upon the location of the perpendicular plane. 
     The endplate assemblies  332 ,  334  may be formed of the same or similar materials as endplate assemblies  22 ,  24  respectively, including the same or similar features or coatings and therefore will not be described in further detail. The flexible layer  350  may be formed from the same flexible or elastic materials as described above for core component  26 . The articulating layer  352  may be formed from the same or similar materials as endplate assemblies  22 ,  24 . Alternatively, the articulating layer  352  may be formed from the same or similar materials as described above for core component  26  with modifications such as cross-linking or ion implantation to enhance wear resistance. 
     As shown in  FIG. 29 , the prosthesis  330  may be assembled by mechanically or adhesively attaching the flexible layer  350  to the interior surface  340  of the endplate assembly  332 . The coupling mechanism  342  may engage the coupling mechanism  356 , providing mechanical attachment. Additionally or alternatively, an adhesive may be used to attached the flexible layer  350  and the interior surface  340 . The concave surface  354  of the articulating layer  352  may be positioned on the convex interior surface  346 . The assembled components  332 – 336  may be implanted into the vertebral column  10  ( FIG. 1 ) in the void created by the removed disc  12  such that the exterior surface  338  engages an endplate of the vertebral body  14  and the exterior surface  344  engages an endplate of the vertebral body  16 . 
     In operation, the concave surface  354  of the articulating layer  352  may articulate with the convex surface  346  of the endplate assembly  334 . The prosthesis  330  may elastically deform under compressive loads parallel to the longitudinal axis  44  to absorb shock and provide a dampening effect. Both the articulating interface between the endplate assembly  334  and the core component  336  and the elasticity of the flexible layer  350  may allow flexion-extension, lateral bending, or axial rotation motion about the longitudinal axis  44 . The end plate assemblies  332 ,  334  may also rotate relative to one another. 
     Referring now to  FIG. 30 , an intervertebral disc prosthesis  360  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  360 , according to another embodiment of the present invention, includes endplate assemblies  362 ,  364 , a core component  366 , and a core component  367 . The endplate assembly  362  may include an exterior surface  368  and an interior surface  370 . In the embodiment of  FIG. 30 , the interior surface  370  may be relatively flat and may include a coupling mechanism  372  which may be a groove. 
     The endplate assembly  364  may include an exterior surface  374  and an interior surface  376 . In the embodiment of  FIG. 30 , the interior surface  376  may be relatively flat and may include a coupling mechanism  378  which may be a groove. The exterior surfaces  368  and  374  may be relatively parallel or may be angled with respect to each other to accommodate a particular lordotic or kyphotic angle. 
     The core component  366  may include a flexible layer  380  and an articulating layer  382  attached to the flexible layer  380 . The articulating layer  382  may have a concave surface  384 . The flexible layer  380  may include a coupling mechanism  386  which may be a ridge. The core component  366  may have a generally circular cross-section as viewed from a plane perpendicular to a longitudinal axis  44 . Alternate cross-sectional shapes may be desirable, and in a single core component  366 , the cross sectional shape may vary depending upon the location of the perpendicular plane. 
     The core component  367  may include a flexible layer  388  and an articulating layer  390  attached to the flexible layer  388 . The articulating layer  390  may have a convex surface  392 . The flexible layer  388  may include a coupling mechanism  394  which may be a ridge. The core component  367  may have a generally circular cross-section as viewed from a plane perpendicular to a longitudinal axis  44 . Alternate cross-sectional shapes may be desirable, and in a single core component  367 , the cross sectional shape may vary depending upon the location of the perpendicular plane. 
     The endplate assemblies  362 ,  364  may be formed of the same or similar materials as endplate assemblies  22 ,  24  respectively, including the same or similar features or coatings and therefore will not be described in further detail. The flexible layers  380 ,  388  may be formed from the same flexible or elastic materials as described above for core component  26 . The articulating layers  382 ,  390  may be formed from the same or similar materials as endplate assemblies  22 ,  24 . Alternatively, the articulating layers  382 ,  390  may be formed from the same or similar materials as described above for core component  26  with modifications such as cross-linking or ion implantation to enhance wear resistance. 
     As shown in  FIG. 30 , the prosthesis  360  may be assembled by mechanically or adhesively attaching the flexible layer  380  to the interior surface  370  of the endplate assembly  362 . The coupling mechanism  372  may engage the coupling mechanism  386 , providing mechanical attachment. Additionally or alternatively, an adhesive may be used to attached the flexible layer  380  and the interior surface  370 . The flexible layer  388  may be mechanically and/or adhesively attached to the interior surface  376  of the endplate assembly  364 . The coupling mechanism  378  may engage the coupling mechanism  394 , providing mechanical attachment. Additionally or alternatively, an adhesive may be used to attached the flexible layer  388  and the interior surface  376 . The concave surface  384  of the articulating layer  382  may be positioned on the convex articulating surface  392 . The assembled components may be implanted into the vertebral column  10  ( FIG. 1 ) in the void created by the removed disc  12  such that the exterior surface  368  engages an endplate of the vertebral body  14  and the exterior surface  374  engages an endplate of the vertebral body  16 . 
     In operation, the concave surface  384  of the articulating layer  382  may articulate with the convex surface  392  of the articulating layer  390 . The prosthesis  360  may elastically deform under compressive loads parallel to the longitudinal axis  44  to absorb shock and provide a dampening effect. Both the articulating interface between the core component  366  and the core component  367  and the elasticity of the flexible layers  380 ,  388  may allow flexion-extension, lateral bending, or axial rotation motion about the longitudinal axis  44 . The end plate assemblies  362 ,  364  may also rotate relative to one another. 
     Referring now to  FIG. 31–32 , an intervertebral disc prosthesis  400  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  400 , according to this embodiment of the present invention, includes endplate assemblies  402 ,  404  and a core component  406 . The endplate assembly  402  may include an exterior surface  408  and an interior surface  410 . In the embodiment of  FIG. 31 , the interior surface  410  may include a relatively concave portion which may be smooth with a mirror surface finish. The endplate assembly  404  may have an exterior surface  412  and an interior surface  414 . The interior surface  414  may include a relatively concave portion. Coupling mechanisms  416  which may be bumpers may protrude from the interior surfaces  410 ,  414 . 
     The core component  406  may include a flexible layer  418  and outer articulating layers  420 ,  422  attached to the flexible layer  418 . The articulating layers  420 ,  422  may include coupling mechanisms  424  which may be grooves. One or more tethers  426  may extend between endplate assemblies  402  and  404 . 
     The endplate assemblies  402 ,  404  may be formed of the same or similar materials as endplate assemblies  22 ,  24  respectively, including the same or similar features or coatings and therefore will not be described in further detail. The flexible layer  418  may be formed from the same flexible or elastic materials as described above for core component  26 . The articulating layers  420 ,  422  may be formed from the same or similar materials as endplate assemblies  22 ,  24 . Alternatively, the articulating layers  420 ,  422  may be formed from the same or similar materials as described above for core component  26  with modifications such as cross-linking or ion implantation to enhance wear resistance. The tethers  426  may be either elastic or inelastic. They may be formed, for example, of reinforcing materials such as wire, cable, cord, bands, tape, or sheets. They may be formed of any of the materials described above for endplate assemblies  22 ,  24  or core component  26 , such as UHMWPE. In some embodiments, the tethers  426  may be braided, knitted, or woven. 
     As shown in  FIG. 31 , the prosthesis  400  may be assembled by positioning the core component  406  between the interior surfaces  410 ,  414  of the endplate assemblies  402 ,  404 . The bumpers  416  may be positioned to travel along the grooves  424  of the articulating layers  420 ,  422 . Some embodiments may have between two and four bumper/groove interfaces. The one or more tethers  26  may extend between the endplate assemblies  402 ,  404  to provide additional stability and/or to provide additional constraint to the prosthesis  400  when subjected to flexion/extension, lateral bending or axial rotation forces. As shown in  FIG. 31 , the tethers  426  may extend between the endplate assemblies  402 ,  404  without passing through the core component  406 . The assembled prosthesis  400  may be positioned within the vertebral column  10  between the vertebrae  14 ,  16 . 
     Referring again to  FIG. 31–32 , in operation, the bumpers  416  may travel along the grooves  424  of the articulating layers  420 ,  422  permitting limited rotation about the longitudinal axis  44 . The rotation may be limited by the length of the grooves  424  compared to the length of the bumpers  416 . For example, bumpers  416  that are nearly the same length as the grooves  424  will permit little or no rotation. In some embodiments, between one and twenty degrees of rotation may be permissible. Some embodiments may limit rotation to between three and ten degrees. The tethers  426  may also constrain the prosthesis  400  during flexion/extension, lateral bending and/or axial rotation movement. Within the constraints of the assembly, the prosthesis  400  may elastically deform under compressive loads parallel to the longitudinal axis  44  to absorb shock and provide a dampening effect. Both the articulating interfaces between the endplate assembly  402 ,  404  and the core component  406  and the elasticity of the flexible layer  418  may allow flexion-extension, lateral bending, or axial rotation motion about the longitudinal axis  44 . 
     Referring now to  FIG. 33–34 , an intervertebral disc prosthesis  430  may be used as the prosthesis  18  of  FIG. 2 . The intervertebral disc prosthesis  430 , according to this embodiment of the present invention, includes endplate assemblies  432 ,  434  and a core component  436 . The endplate assembly  432  may include an interior surface  438 . In the embodiment of  FIG. 33 , the interior surface  438  may include a relatively concave portion which may be smooth with a mirror surface finish. The endplate assembly  434  may have an interior surface  440 . The interior surface  440  may include a relatively concave portion. Coupling mechanisms  441  which may be bumpers may protrude from the interior surfaces  438 ,  440 . 
     The core component  436  may include a flexible layer  442  and outer articulating layers  444 ,  446  attached to the flexible layer  442 . The articulating layers  444 ,  446  may include coupling mechanisms  447  which may be grooves. One or more tethers  448  may extend between articulating layers  444 ,  446 , through the flexible layer  442  in a diagonal direction. Additionally or alternatively, one or more tethers  449  may extend between articulating layers  444 ,  446 , through the flexible layer  442 , relatively parallel to the axis  44 . 
     The endplate assemblies  432 ,  434  may be formed of the same or similar materials as endplate assemblies  22 ,  24  respectively, including the same or similar features or coatings and therefore will not be described in further detail. The flexible layer  442  may be formed from the same flexible or elastic materials as described above for core component  26 . The articulating layers  444 ,  446  may be formed from the same or similar materials as endplate assemblies  22 ,  24 . Alternatively, the articulating layers  444 ,  446  may be formed from the same or similar materials as described above for core component  26  with modifications such as cross-linking or ion implantation to enhance wear resistance. The tethers  448 ,  449  may be either elastic or inelastic. They may be formed, for example, of reinforcing materials such as wire, cable, cord, bands, tape, or sheets. They may be formed of any of the materials described above for endplate assemblies  22 ,  24  or core component  26 , such as UHMWPE. In some embodiments, the tethers  448 ,  449  may be braided, knitted, or woven. 
     As shown in  FIG. 33 , the prosthesis  430  may be assembled by positioning the core component  436  between the interior surfaces  438 ,  440  of the endplate assemblies  432 ,  434 . The bumpers  441  may be positioned to travel along the grooves  447  of the articulating layers  444 ,  446 . Some embodiments may have between two and four bumper/groove interfaces. The one or more tethers  448 ,  449  may extend between the articulating layers  444 ,  446  to provide additional stability and/or to provide additional constraint to the prosthesis  430  when subjected to flexion/extension, lateral bending or axial rotation forces. The assembled prosthesis  430  may be positioned within the vertebral column  10  between the vertebrae  14 ,  16 . 
     In operation, the bumpers  441  may travel along the grooves  447  of the articulating layers  444 ,  446  permitting limited rotation about the longitudinal axis  44 . The rotation may be limited by the length of the grooves  447  compared to the length of the bumpers  441 . For example, bumpers  441  that are nearly the same length as the grooves  447  will permit little or no rotation. In some embodiments, between one and twenty degrees of rotation may be permissible. Some embodiments may limit rotation to between three and ten degrees. The tethers  448 ,  449  may also constrain the prosthesis  430  during flexion/extension, lateral bending and/or axial rotation movement. For example, tethers  448  arranged diagonally may reinforce the flexible layer  442  against torsional shear when the groove  447  impacts the bumper  441 . The tethers  449  may reinforce the flexible layer  442  against lateral shear. The tethers  448 ,  449  may be used alone or in combination with each other. Within the constraints of the assembly, the prosthesis  430  may elastically deform under compressive loads parallel to the longitudinal axis  44  to absorb shock and provide a dampening effect. Both the articulating interfaces between the endplate assembly  432 ,  434  and the core component  436  and the elasticity of the flexible layer  442  may allow flexion-extension, lateral bending, or axial rotation motion about the longitudinal axis  44 . 
     Referring now to  FIGS. 35–36 , an intervertebral disc prosthesis  450  may be used as the prosthesis  18  of  FIG. 2 . The prosthesis  450  may include endplate assemblies  452 ,  454  and core component  456  and may include any of the structures of the prostheses described above. The core component  456  may include a flexible layer  458  which may include one or more modification elements  460 . The flexible layer  458  may be formed from the same flexible or elastic materials as described above for core component  26 . 
     As shown in  FIGS. 35–36 , prosthesis  450  includes two kidney shaped modification elements  460  as viewed from the top cross-sectional view of  FIG. 36 . This prosthesis  450  may be implanted such that one of the modification elements  460  is in an anterior position and one is in a posterior position to promote or restrict extension and/or flexion motion. Alternatively, the prosthesis  450  may be rotated and implanted such that modification elements  460  are laterally positioned, promoting or restricting lateral bending. 
     As shown in  FIGS. 37–38 , a prosthesis  470  may include a core component  472  having a single modification element  474 . In this embodiment, the single modification element  474  may be located near the center of the core component  472  causing the center area of the core component to exhibit a different degree of rigidity than the circumferential area of the core component. The single modification element  474  may be formed in any geometry including a sphere or an ellipsoid. The modification element  474  may have rounded edges to resist wear. 
     As shown in  FIGS. 39–40 , a prosthesis  490  may include a core component  492  having a single modification element  494 . In this embodiment, the modification element  494  may be a ring-shaped area within the core component  492 . 
     As shown in  FIGS. 41–42 , a prosthesis  50  may include a core component  512  having a plurality of modification elements  514  dispersed throughout the core component. 
     The modification element  460 ,  474 ,  494 ,  514  may be material and/or a void which controls, adjusts, or modifies the hardness, stiffness, flexibility, or compliance of the adjacent flexible layer. The modification element  460 ,  474 ,  494 ,  514  may be of any size, shape, or material to permit variation in the rigidity of the core component  456 ,  472 ,  492 ,  512  respectively. For example, certain areas of the core component  456 ,  472 ,  492 ,  512  may be provided with modification element  460 ,  474 ,  494 ,  514 , respectively to provide differential stiffness between the modified areas and the non-modified areas. A variety of modification element configurations may be used to alter the rigidity of the core component, just a few examples of which are described above. The modification element may be a discrete body within the flexible layer or may have a gradient quality which may allow the modification element to blend into the flexible layer, for example, in a radial direction. 
     The modification elements  460 ,  474 ,  494 , and  514  may be formed from materials different than the flexible layers  458 ,  472 ,  492 , and  512  respectively, including any of the materials described above for the endplate assemblies  22 ,  24  or the core component  26 . The materials may be stiffer or more pliable than the material of the flexible layer. The modification element  460 ,  474 ,  494 , and  514  may be a void, and in some embodiments, one or more voids may function as reservoirs for therapeutic agents such as analgesics, anti-inflammatory substances, growth factors, antibiotics, steroids, pain medications, or combinations of agents. Growth factors may comprise any member of the families of transforming growth factor beta (TGF-beta), bone morphogenic proteins (BMPs), recombinant human bone morphogenic proteins (rh BMPs), insulin-like growth factors, platelet-derived growth factors, fibroblast growth factors, or any other growth factors that help promote tissue repair of surrounding tissues. 
     Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.