Abstract:
A vertebral implant for interposition between two vertebral endplates comprises a first endplate component having a first exterior surface for engaging a first vertebral endplate and having a first interior surface. The implant further comprises a second endplate component having a second exterior surface for engaging a second vertebral endplate and having a second interior surface. The implant also comprises a plurality of movable bearing members attached to the second interior surface and movably engaged with the first interior surface.

Description:
BACKGROUND  
       [0001]     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  
       [0002]     In one embodiment of the present disclosure, a vertebral implant for interposition between two vertebral endplates comprises a first endplate component having a first exterior surface for engaging a first vertebral endplate and having a first interior surface. The implant further comprises a second endplate component having a second exterior surface for engaging a second vertebral endplate and having a second interior surface. The implant also comprises a plurality of movable bearing members attached to the second interior surface and movably engaged with the first interior surface.  
         [0003]     In another embodiment of the present invention, a vertebral implant for interposition between two vertebral endplates comprises a first endplate component having a first exterior surface for engaging a first vertebral endplate and having a first interior surface. The implant further comprises a second endplate component having a second exterior surface for engaging a second vertebral endplate and having a second interior surface. The implant also comprises a central body disposed between the first and second interior surfaces and a plurality of movable bearing members disposed between the central body and the first and second interior surfaces.  
         [0004]     In another embodiment of the present disclosure, a system for preserving motion in a vertebral joint comprises a first means for attaching to a first vertebral endplate, a second means for attaching to a second vertebral endplate, and a plurality of third means disposed between the first and second means for providing rolling contact between the first means and the second means.  
         [0005]     In another embodiment of the present disclosure, a vertebral implant for interposition between two vertebral endplates comprises a first endplate component having a first exterior surface for engaging a first vertebral endplate and having a first interior surface. The implant also comprises a second endplate component having a second exterior surface for engaging a second vertebral endplate and having a second interior surface. The second interior surface has a convex or concave portion. The implant also comprises a plurality of movable bearing members interposed between the second interior surface and the first interior surface.  
         [0006]     In another embodiment of the present disclosure, a method for repairing an intervertebral joint comprises accessing an intervertebral disc space, removing at least a portion of a natural intervertebral disc from the intervertebral disc space, and inserting an intervertebral implant into the intervertebral disc space. The intervertebral implant comprises a first endplate component having a first exterior surface for engaging a first vertebral endplate and having a first interior surface and a second endplate component having a second exterior surface for engaging a second vertebral endplate and having a second interior surface. The second interior surface has a convex or concave portion. The implant further includes a plurality of movable bearing members interposed between the second interior surface and the first interior surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a side view of vertebral column with an intervertebral implant.  
         [0008]      FIG. 2  is an exploded side sectional view of an intervertebral implant according to one embodiment of this disclosure.  
         [0009]      FIG. 3  is an exploded side sectional view of an intervertebral implant according to another embodiment of this disclosure.  
         [0010]      FIG. 4  is an exploded side sectional view of an intervertebral implant according to another embodiment of this disclosure.  
         [0011]      FIG. 5  is an exploded side sectional view of an intervertebral implant according to another embodiment of this disclosure.  
         [0012]      FIG. 6  is a side sectional view of an intervertebral implant according to another embodiment of this disclosure.  
         [0013]      FIG. 7  is a side sectional view of an intervertebral implant according to another embodiment of this disclosure.  
         [0014]      FIG. 8  is a top sectional view of an intervertebral implant according to another embodiment of this disclosure.  
         [0015]      FIG. 9  is a top sectional view of an intervertebral implant according to another embodiment of this disclosure.  
         [0016]      FIG. 10  is a top sectional view of an intervertebral implant according to another embodiment of this disclosure. 
     
    
     DETAILED DESCRIPTION  
       [0017]     The present invention relates generally to vertebral reconstructive devices, and more particularly, to a functional intervertebral disc prosthesis or implant. 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.  
         [0018]     Referring first to  FIG. 1 , the numeral  10  refers to a vertebral joint which includes an intervertebral disc  12  extending between vertebrae  14 ,  16 . The disc  12  may be partially or entirely removed and an intervertebral implant  18  may be inserted between the vertebrae  14 ,  16  to preserve motion within the joint  10 . Although the illustration of  FIG. 1  generally depicts the vertebral joint  10  as a lumbar vertebral joint, it is understood that the devices, systems, and methods of this disclosure may also be applied to all regions of the vertebral column, including the cervical and thoracic regions. Additionally, although the illustration of  FIG. 1  generally depicts an anterior approach for insertion of the implant  18 , other approaches including posterior, posterolateral, lateral, and anterolateral are also contemplated. Furthermore, the devices, systems, and methods of this disclosure may be used in non-spinal orthopedic applications.  
         [0019]     Referring now to  FIG. 2 , an intervertebral implant  20  may be used as the prosthesis  18  of  FIG. 1  according to one embodiment of the present disclosure. The intervertebral disc prosthesis  20  includes endplate components  22 ,  24 . The endplate component  22  includes an exterior surface  26  and an interior surface  28 . A race  30  may be formed in the surface  28 . The race  30  may have a circular or oval path as viewed from a top sectional view. In this embodiment, the race  30  may be relatively smooth and may have a mirror surface finish. The endplate component  24  may have an exterior surface  32  and an interior surface  34 . The surface  34  may further include a plurality of retaining components  36 . The surfaces  28 ,  34 , the race  30 , and/or the retaining components  36  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 implant  20  may further include a plurality of bearing members  38  interposed between the interior surfaces  28 ,  34  to provide an articulating interface between the endplate components  22 ,  24 . The bearing members  38  in this embodiment may be spherical ball bearings. Acceptable alternatives to spherical ball bearings may include a variety of roller bearings.  
         [0020]     The endplate components  22 ,  24  and the bearing members  38  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. Certain 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.  
         [0021]     The exterior surfaces  26 ,  32  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 portions of the exterior surfaces  26 ,  32  may receive a coating of a metallic substance which may be applied by sintering or by a spray coating such as a plasma spray. All or a portion of the exterior surfaces  26 ,  32  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 for initial fixation; ridges or keels to prevent migration in the lateral and anterior direction, for example; serrations or diamond cut surfaces; fins; posts; and/or other surface textures.  
         [0022]     The prosthesis  20  may be assembled by inserting the bearing members  38  into the retaining components  36 . Within the retaining components, the bearing members may be permitted to freely rotate. A portion of the bearing members  38  may extend from the retaining component, and the endplate component  22  may be positioned such that the race  30  engages the bearing members  38  in rolling contact. The assembled prosthesis  20  may be implanted into the vertebral joint  10  in the void created by the removed disc  12  such that the exterior surface  26  engages an endplate of the vertebral body  14  and the exterior surface  32  engages an endplate of the vertebral body  16 . The prosthesis  20  may be implanted using any of the approaches described above.  
         [0023]     In operation, the bearing members  38  may permit rotational motion of the endplate component  22  relative to the endplate component  24 , thus promoting axial rotation at the vertebral joint  10  about a longitudinal axis  39 . The bearing members  38  are permitted to roll within the race  30  as the endplates  22 ,  24  rotate relative to each other. This rolling motion of the bearing members  38  results in single point, rolling contact between the bearing members and the race  30 . This minimum contact may reduce friction and the associated generation of wear debris, particularly as compared to surface contact and sliding motion.  
         [0024]     In an alternative embodiment, the retaining component may be a retaining race, rather than individual retaining pockets. In this embodiment, the bearing members would still be retained by the retaining race, but would be permitted to roll along the circuit of the retaining race. In another alternative embodiment, a viscous fluid may be applied to the bearing members to provide lubrication. It is understood that the prosthesis may be implanted completely assembled, partially assembled or unassembled. All or portions of the assembly may be completed by the surgeon within the disc space.  
         [0025]     Referring now to  FIG. 3 , an intervertebral implant  40  may be used as the prosthesis  18  of  FIG. 1  according to another embodiment of the present disclosure. The intervertebral disc prosthesis  40  includes endplate components  42 ,  44 . The endplate component  42  includes an exterior surface  46  and an interior surface  48 . In this embodiment, the surface  48  may be relatively smooth and may have a mirror surface finish. The endplate component  44  may have an exterior surface  52  and an interior surface  54 . The surface  54  may be relatively smooth and may have a mirror surface finish. The surfaces  48 ,  54  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. A central body  50  extends between the interior surfaces  48 ,  54 . The central body  50  may comprise a plurality of retaining components  58 . A plurality of bearing members  60  may be located in the retaining components  58 . The bearing members  60  in this embodiment may be spherical ball bearings. Acceptable alternatives to spherical ball bearings may include a variety of roller bearings.  
         [0026]     The central body  50  may have an inner portion  62  and outer surfaces  64 ,  65 . The central body  50  may have a generally circular cross-section as viewed from a plane perpendicular to the longitudinal axis  39 . The inner portion  62  may be flexible and formed from one or more resilient materials which may have a lower modulus than the outer surfaces. 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. In an alternative embodiment, the inner portion  62  may be rigid and formed of any of the materials described below for the outer surfaces or the endplate components.  
         [0027]     The outer surfaces  64 ,  65  may also be formed of the resilient and flexible materials described above, but in the alternative, they may be modified, treated, coated or lined to enhance the wear resistant and articulating properties of the core component  50 . 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 outer surfaces  64 ,  65  by modifications such as cross-linking and metal ion implantation.  
         [0028]     The endplate components  42 ,  44  and the bearing members  60  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.  
         [0029]     The exterior surfaces  46 ,  52  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 portions of the exterior surfaces  46 ,  52  may receive a coating of a metallic substance which may be applied by sintering or by a spray coating such as a plasma spray. All or a portion of the exterior surfaces  46 ,  52  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 for initial fixation; ridges or keels to prevent migration in the lateral and anterior direction, for example; serrations or diamond cut surfaces; fins; posts; and/or other surface textures.  
         [0030]     The prosthesis  40  may be assembled by inserting the bearing members  60  into the retaining components  58 . Within the retaining components  58 , the bearing members  60  may be permitted to freely rotate. A portion of the bearing members  60  may extend from the retaining component. The central body  50  may then be inserted between the interior surfaces  48 ,  54  such that the bearing members  60  engage the surfaces  48 ,  54  in rolling contact. The assembled prosthesis  40  may be implanted into the vertebral joint  10  in the void created by the removed disc  12  such that the exterior surface  46  engages an endplate of the vertebral body  14  and the exterior surface  52  engages an endplate of the vertebral body  16 . The prosthesis  40  may be implanted using any of the approaches described above.  
         [0031]     In operation, the central body  50  may allow a variable center of rotation to permit flexion-extension and lateral bending motions. The bearing members  60  may provide a rolling interface between the central body  50  and the interior surfaces  48 ,  54  which permits rotational motion, translational motion, and bending motion of the endplate component  42  relative to the endplate component  44 . This, in turn, may permit axial rotation, lateral bending, and flexion-extension motion at the vertebral joint  10 . The rolling motion of the bearing members  60  results in single point contact between the bearing members and interior surfaces  48 ,  54 . This minimum contact may reduce friction and the associated generation of wear debris, particularly as compared to sliding, shear-generating motion. The flexible nature of the core component  50  may also reduce wear caused by cross-shearing or by articulation in flexion-extension and lateral bending motions.  
         [0032]     Referring now to  FIG. 4 , an intervertebral implant  70  may be used as the prosthesis  18  of  FIG. 1  according to another embodiment of the present disclosure. The intervertebral disc prosthesis  70  includes endplate components  72 ,  74 . The endplate component  72  includes an exterior surface  76  and an interior surface  78 . The endplate component  44  may have an exterior surface  82  and an interior surface  84 . A central body  80  extends between the interior surfaces  78 ,  84 . The interior surfaces  78 ,  84  may comprise a plurality of retaining components  88 . A plurality of bearing members  90  may be located in the retaining components  88 . The bearing members  90  in this embodiment may be spherical ball bearings. Acceptable alternatives to spherical ball bearings may include a variety of roller bearings.  
         [0033]     The central body  80  may have an inner portion  92  and outer surfaces  94 ,  95 . The central body  80  may have a generally circular cross-section as viewed from a plane perpendicular to a longitudinal axis  39 . The inner portion  92  and the outer surfaces  94 ,  95  may be formed of any of the materials described above for inner portion  62  and outer surfaces  64 ,  65 , respectively. The surfaces  94 ,  95  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 endplate components  72 ,  74  and the bearing members  90  may be formed of any suitable material including those materials described above for endplate components  42 ,  44 . The exterior surfaces  76 ,  82  may include features or coatings (not shown) which enhance the fixation of the implanted prosthesis such as those described above for surfaces  46 ,  52 .  
         [0034]     The prosthesis  70  may be assembled by inserting the bearing members  90  into the retaining components  88 . Within the retaining components  88 , the bearing members  90  may be permitted to freely rotate. A portion of the bearing members  90  may extend from the retaining component. The central body  80  may then be inserted between the interior surfaces  78 ,  84  such that the bearing members  80  engage the surfaces  94 ,  95  in rolling contact. The assembled prosthesis  70  may be implanted into the vertebral joint  10  in the void created by the removed disc  12  such that the exterior surface  76  engages an endplate of the vertebral body  14  and the exterior surface  82  engages an endplate of the vertebral body  16 . The prosthesis  70  may be implanted using any of the approaches described above.  
         [0035]     In operation, the central body  80  may allow a variable center of rotation to permit flexion-extension and lateral bending motions. The bearing members  90  may provide a rolling interface between the central body  80  and the interior surfaces  78 ,  84  which permits rotational motion, translational motion, and bending motion of the endplate component  72  relative to the endplate component  74 . This, in turn, may permit axial rotation, lateral bending, and flexion-extension motion at the vertebral joint  10 . The rolling motion of the bearing members  90  results in single point contact between each of the bearing members and central body  80 . This minimum contact may reduce friction and the associated generation of wear debris, particularly as compared to sliding motion. A flexible core component  80  may also reduce wear caused by cross-shearing or by articulation in flexion-extension and lateral bending motions.  
         [0036]     Referring now to  FIG. 5 , an intervertebral implant  100  may be used as the prosthesis  18  of  FIG. 1  according to one embodiment of the present disclosure. The intervertebral disc prosthesis  100  includes endplate components  102 ,  104 . The endplate component  102  includes an exterior surface  106  and a concave interior surface  108 . A recessed portion  110  may be formed in the surface  108 . In this embodiment, the recessed portion  110  may be relatively smooth and may have a mirror surface finish. The recessed portion  110  may have a generally circular or oval shape. The endplate component  104  may have an exterior surface  112  and a convex interior surface  114 . The surface  114  may be relatively smooth and may have a mirror surface finish. The surface  114  may further include a plurality of retaining components  116 . The surfaces  108 ,  104 , the recessed portion  110 , and/or the retaining components  116  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 implant  100  may further include a plurality of bearing members  118  interposed between the interior surfaces  108 ,  114  to provide an articulating interface between the components  102 ,  104 . The bearing members  108  in this embodiment may be spherical ball bearings. Acceptable alternatives to spherical ball bearings may include a variety of roller bearings.  
         [0037]     The endplate components  102 ,  104  and the bearing members  118  may be formed of any suitable biocompatible material including those described above for endplate components  22 ,  24 . The exterior surfaces  106 ,  112  may include features or coatings (not shown) which enhance the fixation of the implanted prosthesis such as those described above for surfaces  26 ,  32 .  
         [0038]     The prosthesis  20  may be assembled by inserting the bearing members  118  into the retaining components  116 . Within the retaining component, the bearing members may be permitted to freely rotate. A portion of the bearing members  118  may extend from the retaining components  116 , and the endplate component  102  may be positioned such that the recessed portion  110  engages the bearing members  118  in rolling contact. The assembled prosthesis  100  may be implanted into the vertebral joint  10  in the void created by the removed disc  12  such that the exterior surface  106  engages an endplate of the vertebral body  14  and the exterior surface  112  engages an endplate of the vertebral body  16 . The prosthesis  100  may be implanted using any of the approaches described above.  
         [0039]     In operation, the bearing members  118  may move within the boundaries of the recessed portion  110 . The bearing members  118  may provide a rolling interface between the interior surfaces  108 ,  114  which permits rotational motion, translational motion, and bending motion of the endplate component  102  relative to the endplate component  104 . This, in turn, may permit axial rotation, lateral bending, and flexion-extension motion at the vertebral joint  10 . The rolling motion of the bearing members  118  results in single point contact between the bearing members and interior surfaces  108 ,  114 . This minimum contact may reduce friction and the associated generation of wear debris, particularly as compared to sliding motion.  
         [0040]     Referring now to  FIG. 6 , an intervertebral implant  120  may be used as the prosthesis  18  of  FIG. 1  according to another embodiment of the present disclosure. The intervertebral disc prosthesis  120  includes endplate components  122 ,  124 ; a central body  126 ; and a plurality of bearing members  128 . This prosthesis  120  may be substantially similar to the structure, assembly, and operation of the prosthesis  70  described above, except for the differences to be described. The prosthesis  120  further includes a circumferential sheath  130  that extends between the endplate components  122 ,  124  and encapsulates the central body  126  between the endplate components. The sheath  130  may be filled with a lubricating fluid  132  which may lubricate the bearing members  128  and promote motion of the central body  126 . The sheath  130  may serve to retain both the fluid  132  and any generated wear debris. Further the sheath  130  may serve as a barrier preventing unwanted tissue or fluid from contacting the central body  126  and the bearing members  128 .  
         [0041]     Referring now to  FIG. 7 , an intervertebral implant  140  may be used as the prosthesis  18  of  FIG. 1  according to another embodiment of the present disclosure. Except for the differences to be described, the implant  140  may be substantially similar to an implant disclosed in U.S. patent application Ser. No. 09/924,298, entitled “Implantable Joint Prosthesis” which is incorporated herein by reference. Specifically, the implant  140  includes opposing shells or endplate components  142 ,  144  and a central body  146 . In this embodiment, a plurality of bearing members  148  may be rotatably attached to the central body  146  to reduce the shear forces and the friction at the interface between the central body  146  and the endplate components  142 ,  144  during translation, lateral bending, flexion-extension, or rotational motion. In an alternative embodiment, the bearing members  148  may be rotatably attached to the endplate components  142 ,  144  rather than the central body  146 .  
         [0042]     Referring now to  FIG. 8 , an intervertebral implant  150  may be used as the prosthesis  18  of  FIG. 1  according to another embodiment of the present disclosure. The implant  150  includes a lower endplate component  152  fitted with a plurality of bearing members  154  which in this embodiment may be roller bearings. An upper endplate component (not shown) may also be provided, with the bearing members  154  serving as an interface between the upper and lower bearings. The endplate component  152  may be substantially similar to any of the endplate components  24 ,  74 , or  104 , except for the differences to be described. The bearing members  154  may be arranged to promote certain motions and limit other types of motion. For example, to restrict translation of the lower endplate component  152  with respect to the upper component in an anterior-posterior direction  156 , the roller bearings  154  may be arranged such that rolling occurs in a lateral direction  158 , thus permitting lateral translation and/or lateral bending. The roller bearings  154 , in this configuration, may limit anterior-posterior translation or flexion-extension motion.  
         [0043]     Referring now to  FIG. 9 , an intervertebral implant  160  may be used as the prosthesis  18  of  FIG. 1  according to another embodiment of the present disclosure. The implant  160  includes a lower endplate component  162  fitted with a plurality of bearing members  164  which in this embodiment may be roller bearings. An upper endplate component (not shown) may also be provided, with the bearing members  164  serving as an interface between the upper and lower bearings. The endplate component  162  may be substantially similar to any of the endplate components  24 ,  74 , or  104 , except for the differences to be described. The bearing members  164  may be arranged to promote certain motions and limit other types of motion. For example, to restrict translation of the lower endplate component  162  with respect to the upper component in the lateral direction  158 , the roller bearings  164  may be arranged such that rolling occurs in the anterior-posterior direction  156 , thus permitting anterior-posterior translation and/or flexion-extension motion. The roller bearings  164 , in this configuration, may limiting lateral translation or lateral bending.  
         [0044]     Referring now to  FIG. 10 , an intervertebral implant  170  may be used as the prosthesis  18  of  FIG. 1  according to another embodiment of the present disclosure. The implant  170  includes a lower endplate component  172  fitted with a plurality of bearing members  174  which in this embodiment may be roller bearings. An upper endplate component (not shown) may also be provided, with the bearing members  174  serving as an interface between the upper and lower bearings. The endplate component  172  may be substantially similar to any of the endplate components  24 ,  74 , or  104 , except for the differences to be described. The bearing members  174  may be arranged to promote certain motions and limit other types of motion. For example, the roller bearings  174  may be arranged such that rolling occurs about the longitudinal axis  39 , thus limiting anterior-posterior translation and flexion-extension motion. The roller bearings  164 , in this configuration, may limit, although not necessarily entirely restrict, lateral translation, lateral bending, anterior-posterior translation, or flexion-extension motion.  
         [0045]     In alternative embodiments, other arrangements of roller bearings, spherical ball bearings, races, or rolling surfaces may be arranged to promote or limit certain types of motion. In still other alternative embodiments, the motion limiting bearings arrangement may be rotatably attached to an upper endplate component or a central body.  
         [0046]     Although only a few exemplary embodiments 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 disclosure. Accordingly, all such modifications and alternative are intended to be included within the scope of the invention as defined in the following claims. Those skilled in the art should also realize that such modifications and equivalent constructions or methods do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. It is understood that all spatial references, such as “horizontal,” “vertical,” “top,” “upper,” “lower,” “bottom,” “left,” “right,” “rostral,” “caudal,” “upper,” and “lower,” are for illustrative purposes only and can be varied within the scope of the disclosure. In the claims, means-plus-function clauses are intended to cover the elements described herein as performing the recited function and not only structural equivalents, but also equivalent elements.