Abstract:
A joint prosthesis comprises a first member for engaging a first bone portion and a second member for engaging a second bone portion. The first member comprises a first surface with a first curve and the second member comprises a second surface with a second curve. The first member is translatable with respect to the second member and the second curve is positioned within the first curve to bias the first and second curves towards alignment along a first axis passing through the first and second bone portions.

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. Reconstruction of a damaged joint with a functional joint prosthesis to provide motion and to reduce deterioration of the adjacent bone and adjacent joints is a desirable treatment option for many patients. Current prosthesis designs, however, may not provide the stability needed to achieve the desired results.  
       SUMMARY  
       [0002]     In one embodiment, a joint prosthesis comprises a first member for engaging a first bone portion and a second member for engaging a second bone portion. The first member comprises a first surface with a first curve, and the second member comprises a second surface with a second curve. The first member is translatable with respect to the second member and the second curve is positioned within the first curve to bias the first and second curves towards alignment along a first axis passing through the first and second bone portions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]      FIG. 1  is a human anatomy.  
         [0004]      FIG. 2  is a block drawing of a human joint.  
         [0005]      FIG. 3  is a sagittal view of a vertebral column having a damaged disc.  
         [0006]      FIG. 4  is an exploded intervertebral assembly according to a first embodiment of the current disclosure.  
         [0007]      FIG. 5  is an assembled intervertebral assembly according to the first embodiment of the current disclosure.  
         [0008]      FIG. 6  is a sagittal view of a vertebral column implanted with the intervertebral assembly according to the first embodiment of the current disclosure.  
         [0009]      FIG. 7  is a cross sectional view of the assembled intervertebral assembly according to the first embodiment of the current disclosure.  
         [0010]      FIG. 8  is a cross sectional view of the translated intervertebral assembly according to the first embodiment of the current disclosure.  
         [0011]      FIG. 9  is a cross sectional view of an assembled intervertebral assembly according to a second embodiment of the current disclosure.  
         [0012]      FIG. 10  is a cross sectional view of an assembled intervertebral assembly according to a third embodiment of the current disclosure.  
         [0013]      FIG. 11  is a cross sectional view of an assembled intervertebral assembly according to a fourth embodiment of the current disclosure.  
         [0014]      FIG. 12  is a cross sectional view of an assembled intervertebral assembly according to a fifth embodiment of the current disclosure.  
         [0015]      FIG. 13  is a cross sectional view of an assembled intervertebral assembly according to a sixth embodiment of the current disclosure.  
         [0016]      FIG. 14  is a cross sectional view of an assembled intervertebral assembly according to a seventh embodiment of the current disclosure.  
         [0017]      FIG. 15  is an exploded intervertebral assembly according to an eighth embodiment of the current disclosure.  
         [0018]      FIG. 16  is an assembled intervertebral assembly according to the eighth embodiment of the current disclosure.  
         [0019]      FIG. 17  is a cross sectional view of the assembled intervertebral assembly of the eighth embodiment of the current disclosure in a translated position.  
         [0020]      FIG. 18  is an exploded intervertebral assembly according to a ninth embodiment of the current disclosure.  
         [0021]      FIG. 19  is an assembled intervertebral assembly according to the ninth embodiment of the current disclosure.  
         [0022]      FIG. 20  is a cross sectional view of the assembled intervertebral assembly of the ninth embodiment of the current disclosure.  
         [0023]      FIG. 21  is an exploded intervertebral assembly according to a tenth embodiment of the current disclosure.  
         [0024]      FIG. 22  is an assembled intervertebral assembly according to the tenth embodiment of the current disclosure.  
         [0025]      FIG. 23  is a cross sectional view of the assembled intervertebral assembly of the tenth embodiment of the current disclosure.  
         [0026]      FIG. 24  is an exploded intervertebral assembly according to an eleventh embodiment of the current disclosure.  
         [0027]      FIG. 25  is an assembled intervertebral assembly according to the eleventh embodiment of the current disclosure.  
         [0028]      FIG. 26  is an exploded intervertebral assembly according to a twelfth embodiment of the current disclosure.  
         [0029]      FIG. 27  is an exploded intervertebral assembly according to a twelfth embodiment of the current disclosure.  
         [0030]      FIG. 28  is an assembled intervertebral assembly according to the twelfth embodiment of the current disclosure.  
         [0031]      FIG. 29  is a cross-sectional view of the intervertebral assembly according to the twelfth embodiment of the current disclosure.  
         [0032]      FIG. 30  is a cross-sectional view of the intervertebral assembly of the twelfth embodiment of the current disclosure in an articulated position. 
     
    
     DETAILED DESCRIPTION  
       [0033]     The present disclosure relates generally to the field of orthopedic surgery, and more particularly to an apparatus and method for vertebral reconstruction using a functional intervertebral prosthesis. For the purposes of promoting an understanding of the principles of the invention, reference will now be made to 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 alteration 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.  
         [0034]     Referring first to  FIG. 1 , the numeral  10  refers to a human anatomy having one or more joint locations  12  which may be damaged by injury or disease. As shown in  FIG. 2 , in a typical arthroplasty procedure all or a portion of one of the joints  12  may be removed, creating a void between two intact bones  14 ,  16 . An implant  18  may then be inserted between the bones  14 ,  16  to at least partially fill the void.  
         [0035]     Referring now to  FIG. 3 , one example of a joint that can benefit from the present invention is a vertebral joint  12   a  with the implant  18  interposed between vertebrae  14   a ,    16   a,  corresponding to intact bones  14 ,  16 , respectively. In a typical surgical discectomy, a void is created between the two intact vertebrae  14   a  and  16   a.  This procedure may be performed using an anterior, anterolateral, lateral, or other approach known to one skilled in the art. An implant  18  according to an embodiment of the present invention may then be provided to fill the void between the two intact vertebrae  14   a  and  16   a.    
         [0036]     Other examples of joints that can benefit from the present invention include orthopedic applications in shoulder, knee, or hip arthroplasty. It is understood that other joints may require different sizes, materials, and/or shapes to fulfill specific joint requirements, as is well understood by those of ordinary skill in the art. Sizing and material selection may, for example, require consideration of the heavy load bearing requirements of hip or knee joints. Other joints, such as cervical vertebrae joints, may require materials and sizing which reflect the wide range of movement desired at the joint.  
         [0037]     The vertebral embodiments disclosed may be used in the cervical, thoracic, or lumbar spine or in other regions of the vertebral column. Although the embodiments to be described are generally premised upon the removal of a single disc, it is understood that more than one of the disclosed devices may be used in a multi-level disc replacement such as, for example, the replacement of two or more vertebral discs. The methods and apparatus of this disclosure may also be applied to the insertion of a vertebral body replacement device between two vertebrae following a corpectomy, in which at least one vertebral body has been removed. Moreover, the methods and apparatus may be used whenever motion preservation is needed or desired.  
         [0038]     Referring now to  FIG. 4 , a joint prosthesis  20 , which in this embodiment may be an intervertebral disc prosthesis, includes a center member  22  interposed between two endplate assemblies  24 ,  26 . The endplate assembly  24  may include an exterior surface  28  and an interior surface  30 . An articulation mechanism such as a protrusion  32  may extend from the interior surface  30 . In this embodiment the protrusion may be semi spherical, however protrusions may be provided in a variety of shapes, a few of which will be described in other embodiments. The surfaces  28  and  30  may be flat, angled, or curved. In this embodiment, the exterior surface  28  may be relatively flat or may be contoured to match the surface of an adjacent vertebral endplate. The interior surface  30  may taper away from or toward the protrusion  32 .  
         [0039]     The endplate assembly  26  may include a interior surface  34  and an exterior surface  36 . The surfaces  34  and  36  may be flat, angled, or curved. In this embodiment, the surface  36  may be generally flat or may be contoured to match the surface of an adjacent vertebral endplate. This surface may have other features (not shown), such as fins or keels, to secure the exterior surface  36  to the bone. The interior surface  34  may be generally concave and may serve as an articulation mechanism.  
         [0040]     The center member  22  may vary somewhat in shape, size, composition, and physical properties, depending upon the particular joint for which the implant is intended or a particular deformity which the prosthesis  20  is intended to correct. The shape of the center member  22  may complement that of the interior surfaces  30 ,  34  of the endplate assemblies  24 ,  26  to allow for a range of translational, flexural, extensional, rotational, and lateral bending motion appropriate to the particular joint being replaced. In this embodiment, the center member  22  may include a surface  38  having a cavity  40  generally conforming to the shape of the protrusion  32 . The center member  22  may also have a surface  42  which, in this embodiment, may generally conform to the shape of the interior surface  34 .  
         [0041]     The endplate assemblies  24 ,  26  and center member  22  may be formed of any suitable biocompatible material including, cobalt-chrome alloys, stainless steel, titanium alloys, alumina, zirconia, polycrystalline diamond, pyrolytic carbon, polyetheretherketone (PEEK), ultra-high molecular weight polyethylene (UHMWPE), cross-linked UHMWPE, and/or other suitable materials. The surfaces  28 ,  36  may include features or coatings which enhance the purchase of the implanted prosthesis. For example, a biocompatible and osteoconductive material such as hydroxyapatite (HA) may coat all or a portion of the surface  28 . Other suitable coatings or treatments may include a porous bead coating, a porous mesh coating, osteogenic peptide coating, growth factor coating, rh-BMP coating, and/or grit blasting. Other suitable features may include serrations, spikes, ridges, fins, and/or other surface textures.  
         [0042]     In some embodiments, the center member  22  may be formed of the relatively rigid materials listed above, and in other embodiments, the center member may permit a degree of elasticity or dampening, and accordingly, an elastomeric material may be used for the center member. Although the center member  22  may have a degree of flexibility, it may also be sufficiently stiff to effectively cooperate with the endplate assemblies to limit motion beyond an allowable range. The surface of the center member  22  may also be sufficiently durable to provide acceptable wear characteristics. In one embodiment, this combination of properties may be achieved with a center member  22  having surface regions that are harder than the material of the central body closer to its core. The portion  22  may, therefore, comprise a biocompatible composite or elastomeric material having a hardened surface.  
         [0043]     Referring now to  FIG. 5 , the components of the intervertebral disc prosthesis  20  may be assembled by engaging the protrusion  32  with the cavity  40  and by positioning the surface  42  of the center member on the surface  34  of the endplate assembly  26 . The components  26 ,  22 ,  24  may be centrally aligned along a longitudinal axis  44 .  
         [0044]     Referring now to  FIG. 6 , the intervertebral disc prosthesis  20  may used as the implant  18  and may be inserted in the void of the vertebral column  12   a  (of  FIG. 3 ) created by the discectomy. In one embodiment, the surface  36  may contact an endplate of vertebra  14   a  and the surface  28  may contact the endplate of vertebra  16   a.  In other embodiments, the prosthesis may be inverted.  
         [0045]     As shown in the cross sectional view of  FIG. 7 , the intervertebral disc prosthesis  20  may be in a neutral position when the components  26 ,  22 ,  24  are centrally aligned along the longitudinal axis  44 . The protrusion  32  may have a curve  50 , which in this embodiment may be an arc with a relatively constant radius  52  and a center point  54 . The surface  34  may have a curve  56  which in this embodiment may be an arc with a relatively constant radius  58  and a center point  60 . A distance  55  may be measured between the center points  54 ,  60 . In this example, the radius  52  is smaller than the radius  58 , and accordingly, the arc  50  is tighter than the arc  56 . In the neutral position, the center points  54  and  60  may be aligned along the longitudinal axis  44 , and the smaller curve  50  may be positioned within the curve  56 , which in this embodiment may be the area  57  defined by the sweep of the radius  58 .  
         [0046]      FIG. 8  shows the intervertebral disc prosthesis  20  in a translated position along, for example, an anterior-posterior axis  62 . Translation may, for example, occur with flexion-extension movement. As the endplate assemblies  24 ,  26  are moved out of alignment relative to axis  44 , the center member  22  may articulate between the endplate assembly interior surfaces  30 ,  34 . With the patient&#39;s body weight as a load  64  in the longitudinal direction  44  and the position of the smaller curve  50  within the larger curve  56 , the prosthesis  20  may be biased to return to the more stable, neutral position in which the curves  50 ,  56  are aligned along the longitudinal axis  44 . In this embodiment alignment may occur when the center points  54 ,  60  are aligned along the longitudinal axis  44 . In this embodiment alignment may occur when the center points  54 ,  60  are aligned along the longitudinal axis  44 . This embodiment describes curves which represent arcs of circle, but in alternative embodiments the curves may be portions of other curves, such as an arc of an ellipse. In these alternative embodiments, alignment may occur when foci, for example of an ellipse, are in alignment or when center lines bisecting the curves are in alignment.  
         [0047]     This tendency of the prosthesis  20  to self correct a spondylolisthesis or other displacement may allow freer, more natural joint movement while preventing excessive translation that could otherwise result in instability of the prosthesis  20 . Instability may result in the placement of unsustainable loads on adjacent joints or may result in the disassembly of the prosthesis  20 . The alignment bias of the prosthesis  20  may relieve excessive loads that might otherwise form in adjacent joints due to chronic over-displacement of the endplate assemblies  24 ,  26 . Although the wider arc is superior to the tighter arc in the orientation of this embodiment, in another embodiment, the orientation may be inverted with the tighter arc superior to the wider arc but with the tighter arc still falling within the curve of the wider arc.  
         [0048]     It may be appreciated that the amount of alignment bias, and accordingly the amount of stability, may be related to the distance  55  between the center points  54 ,  60 . As the distance  55  increases (for example, a sphere on a flat surface), stability, the amount of constraint within the prosthesis  20 , and the tendency to self-align may decrease. As the distance  55  decreases (for example, a sphere in a tight socket), stability, constraint within the prosthesis  20 , and the tendency to self-align may increase. Although this embodiment has been described as contemplating a displacement in the anterior-posterior direction  62 , displacements caused by translation, bending, and/or rotation in other directions or combinations of directions may be corrected using other embodiments of the invention. For example, displacement of the endplate assembly  26  relative to the endplate  24  in a lateral direction  66  may also generate constraining forces which drive the center points  54 ,  60  back into alignment. The components  22 - 26  may be selected from a kit which allows the surgeon to design a patient specific prosthesis having a patient-appropriate amount of constraint and bias.  
         [0049]     In embodiments involving multi-level disc removal, ligaments and other supportive soft tissue structures may be surgically removed or compromised. In these embodiments, replacing the discs with assemblies, such as prostheses  20 , may resupply at least some of the stability lost with the removal of the soft tissue. This restored stability may prevent excessive loading and wear in the adjacent joints and may also encourage more kinematically accurate motions.  
         [0050]     Referring now to  FIG. 9 , in this embodiment, an intervertebral disc prosthesis  70 , may include a center member  72  interposed between two endplate assemblies  74 ,  76 . The endplate assembly  74  may include a protrusion  78  having a curve  80 . In this embodiment, the curve  80  may be an arc having a centerpoint  81  and a constant radius. The endplate assembly  76  may include an interior surface  82  which may have a curve  84 . In this embodiment, the curve  84  may be an arc having a center point  86  and a constant radius.  
         [0051]     Referring now to  FIG. 10 , in this embodiment, an intervertebral disc prosthesis  90 , may include a center member  92  interposed between two endplate assemblies  94 ,  96 . The endplate assembly  94  may include a protrusion  98  having a curve  100 . In this embodiment, the curve  100  may be an arc having a center point  101  and a constant radius. The endplate assembly  96  may include an interior surface  102  which may have a curve  104 . In this embodiment, the curve  104  may be an arc having a center point  106  and a constant radius.  
         [0052]     The materials, the assembly, and the operation of prosthesis  90  may be similar to prosthesis  20  and therefore will not be described in detail. The shape of a protrusions relative to the shape of the contacted interior surfaces may correspond to the amount of constraint within the prosthesis. For example, where the arc-shaped curve  84  is wide compared to the relatively tight curve  104  in  FIG. 9 , the prosthesis  70  may be more constrained than prosthesis  90  in the embodiment of  FIG. 10  wherein the arc-shaped curve  104  more closely matches the curve  100 . Increased constraint may correspond to an increased bias for the prosthesis to return to the neutral position with the center points centrally aligned about the longitudinal axis  44 .  
         [0053]     Referring now to  FIG. 11 , in this embodiment, an intervertebral disc prosthesis  110 , may include a center member  112  interposed between two endplate assemblies  114 ,  116 . The endplate assembly  114  may include a protrusion  118  having a curve  120 . In this embodiment, the curve  120  may be a semi-ellipse or other type of curve having a focus point  121  and a variable radius. The endplate assembly  116  may include an interior surface  122  which may have a curve  124 . In this embodiment, the curve  124  may be U-shaped having a focus point  126 , a variable radius, angled flat, and/or parallel flat portions. The materials and the assembly of prosthesis  110  may be similar to prosthesis  20  and therefore will not be described in detail. In operation, the prosthesis  110  may be biased toward alignment of the foci  121 ,  126  about the longitudinal axis  44 .  
         [0054]     Referring now to  FIG. 12 , in this embodiment, an intervertebral disc prosthesis  130 , may include a center member  132  interposed between two endplate assemblies  134 ,  136 . The endplate assembly  134  may include a protrusion  138  having a curve  140 . In this embodiment, the curve  140  may be a semi-ellipse having a focus point  141  and a variable radius. The endplate assembly  136  may include an interior surface  142  which may have a curve  144 . In this embodiment, the curve  144  may be U shaped having a focus point  146 , a variable radius, angled flat, and/or parallel flat sections.  
         [0055]     The materials and the assembly of prostheses  110 ,  130  may be similar to prosthesis  20  and therefore will not be described in detail. In operation, the prosthesis  130  may be biased toward alignment of the foci  141 ,  146  about the longitudinal axis  44 . As shown in  FIGS. 11 and 12 , in some embodiments, the shape of the curves  124 ,  144  may not correspond to constant radius arcs of a circle, but rather the shape of the curve may be, for example, a U-shape, a semi-ellipse, or an elliptic curve. In  FIG. 11  where the U-shaped curve  124  is wide compared to the relatively tight curve  144  of  FIG. 12 , the prosthesis  110  may be less constrained than prosthesis  130  wherein the U-shaped curve  154  is relatively tight and more closely matches the curve  140 . It may be appreciated that the prosthesis  110  ( FIG. 11 ) may be more constrained than prosthesis  70  ( FIG. 9 ) as the walls of the U-shape may increase the bias for the prosthesis  110  to return to the neutral position.  
         [0056]     Referring now to  FIG. 13 , in this embodiment, an intervertebral disc prosthesis  150 , may include an center member  152  interposed between two endplate assemblies  154 ,  156 . The endplate assembly  154  may include a protrusion  158  having a curve  160 . In this embodiment, the curve  160  may have a combination of curved and flat surfaces and may have a center line  161  bisecting the curve  160 . The endplate assembly  156  may include an interior surface  162  which may have a curve  164 . In this embodiment, the curve  164  may have a combination of curved and flat surfaces and may have a center line  166  bisecting the curve  164 . The materials and the assembly of prosthesis  150  may be similar to prosthesis  20  and therefore will not be described in detail. In operation, the prosthesis  150  may be biased toward alignment of the center lines  161 ,  166  along the axis  44 .  
         [0057]     Referring now to  FIG. 14 , in this embodiment, an intervertebral disc prosthesis  170 , may include an center member  172  interposed between two endplate assemblies  174 ,  176 . The endplate assembly  174  may include a protrusion  178  having a curve  180 . In this embodiment, the curve  180  may have a combination of curved and flat surfaces and may have a center line  181  bisecting the curve  180 . The endplate assembly  176  may include an interior surface  182  which may have a curve  184 . In this embodiment, the curve  184  may have a combination of curved and flat surfaces and may have a center line  186  bisecting the curve  180 . The materials, the assembly, and the operation of prosthesis  170  may be similar to prosthesis  20  and therefore will not be described in detail.  
         [0058]     For prostheses  150 ,  170 , the curves  164 ,  184  are relatively pointed compared to curve  80  ( FIG. 9 ). In  FIG. 13  where the pointed curve  164  is wide compared to the relatively tight curve  184  of  FIG. 12 , the prosthesis  150  may be less constrained than prosthesis  170  wherein the U-shaped curve  184  is relatively tight and more closely matches the curve  180 .  
         [0059]     Referring now to  FIG. 15 , an intervertebral disc prosthesis  190  may include two endplate assemblies  192 ,  194  which may be identical or substantially similar to endplate assemblies  24 ,  26  ( FIG. 4 ) and therefore, will not be described in detail except to define a protrusion  196  corresponding to protrusion  32  of prosthesis  20 , and a surface  198  corresponding to surface  34 . As shown in  FIG. 16 , the prosthesis  190  may be assembled by positioning the protrusion  196  on the surface  198 . The components,  192 ,  194  may be aligned along the longitudinal axis  62 . The prosthesis  190  of this embodiment is one example of a relatively unconstrained joint (as compared to  FIG. 10 , for example). Protrusion  196  may be permitted to move unconstrained on surface  198  as the patient moves. The surface  198  may, in some embodiments as shown, have a slight lip  198   a  around the perimeter to provide a minimal amount of constraint.  FIG. 17  shows the intervertebral disc prosthesis  190  in a translated position along, for example, an anterior-posterior axis  62 . This embodiment, which may omit a bushing, center articulating portion, or other wear reduction device, may be suitable, for example, when contacting surfaces are formed of extremely durable material able to withstand point contact. This embodiment may also minimize stress on the adjacent vertebral endplates.  
         [0060]     Referring now to  FIG. 18 , a joint prosthesis  200 , which in this embodiment may be an intervertebral disc prosthesis, includes a center member  202  interposed between two endplate assemblies  204 ,  206 . The endplate assembly  204  may include an exterior surface  208  and an interior surface  210 . A protrusion  212  may extend from the interior surface  210 . In this embodiment, the protrusion  212  may be a semi-cylinder extended in the direction of axis  66 , however, as described above, protrusions may be provided in a variety of shapes suitable for a particular application or particular location in the vertebral column. The surfaces  208  and  210  may be flat, angled, or curved. In this embodiment, the exterior surface  208  may be relatively flat or may be contoured to match the surface of an adjacent vertebral endplate. The interior surface  210  may taper away from the protrusion  212 .  
         [0061]     The endplate assembly  206  may include a interior surface  214  and an exterior surface  216 . The surfaces  214  and  216  may be flat, angled, or curved. In this embodiment, the surface  216  may be generally flat or may be contoured to match the surface of an adjacent vertebral endplate. The interior surface  214  may be generally concave.  
         [0062]     The center member  202  may vary somewhat in shape, size, composition, and physical properties, depending upon the particular joint for which the implant is intended. The shape of the center member  202  may complement that of the interior surfaces  210 ,  214  of the endplate assemblies  204 ,  206 , respectively, to allow for a range of translational, flexural, extensional, rotational, and lateral bending motion appropriate to the particular joint being replaced. In this embodiment, the center member  202  may include a surface  218  having a cavity  220  generally conforming to the shape of the protrusion  212 . The center member  202  may also have a surface  222  which, in this embodiment, may generally conform to the shape of the interior surface  214 .  
         [0063]     The components  202 ,  204 ,  206  may be formed from the same materials as described above for components  22 ,  24 ,  26 , respectively. Referring now to  FIGS. 19 &amp; 20 , the components of the intervertebral disc prosthesis  200  may be assembled by engaging the protrusion  212  with the cavity  220  and positioning the surface  222  of the center member  202  on the surface  214 . The components  202 - 206  may be centrally aligned along the longitudinal axis  44 . The intervertebral disc prosthesis  200  may be inserted in the void of the vertebral column  12   a  (of  FIG. 3 ) created by discectomy. The positioning and functioning of the prosthesis  200  may be similar to that of the prosthesis  20  and therefore will not be described in detail. As described above for prosthesis  20 , the prosthesis  200  may also have a bias to return toward a neutral position centrally aligned along the axis  44 . Additionally, in this embodiment, the extension of the protrusion  212  in the lateral direction  66  may permit more stable and controlled lateral translation while decreasing the risk of dislodging the center member  202 .  
         [0064]     Referring now to  FIG. 21 , an intervertebral disc prosthesis  230  may include two endplate assemblies  232 ,  234  which may be identical or substantially similar to endplate assemblies  204 ,  206  ( FIG. 18-20 ) and therefore, will not be described in detail except to define a protrusion  236  similar to protrusion  212  of prosthesis  200 , and a surface  238  similar to surface  214 . As shown in  FIGS. 22 and 23 , the prosthesis  230  may be assembled by positioning the protrusion  236  on the surface  238 . The components  232 ,  234  may be centrally aligned along the longitudinal axis  44 . The curved surface  238  and the curve of the protrusion  236  may provide constraint in the direction  62 , but may provide relatively little constraint in direction  66 . As shown, the protrusion may be relatively linear along the axis  66 , but in other examples, the protrusion may be curved along the axis  66  to create an elliptical dome which provides constraint in both directions  62 ,  66 . Prosthesis  230 , which may omit a bushing, center articulating portion, or other wear reduction device, may be suitable, for example, when contacting surfaces are formed of extremely durable material able to withstand line contact.  
         [0065]     Referring now to  FIG. 24 , a joint prosthesis  240 , which in this embodiment may be an intervertebral disc prosthesis, includes a center member  242  interposed between two endplate assemblies  244 ,  246 . The endplate assembly  244  may include an exterior surface  248  and an interior surface  250 . A protrusion  252  may extend from the interior surface  250 . In this embodiment, the protrusion  252  may be a semi-cylinder extended along the direction of axis  66 . A restraint member  253 , which in this example may be a depression, may be formed on the protrusion  252  or the surface  250 . The restraint member  253  may extend across the protrusion  252  in the anterior-posterior direction  62  and may be flared to permit limited motion in the lateral direction  66 . The surfaces  248  and  250  may be flat, angled, or curved. In this embodiment, the exterior surface  248  may be relatively flat or may be contoured to match the surface of an adjacent vertebral endplate. The interior surface  250  may taper away from the protrusion  252 .  
         [0066]     The endplate assembly  246  may include a interior surface  254  and an exterior surface  256 . The surfaces  254  and  256  may be flat, angled, or curved. In this embodiment, the surface  256  may be generally flat or may be contoured to match the surface of an adjacent vertebral endplate. The interior surface  254  may be generally concave.  
         [0067]     The center member  242  may vary somewhat in shape, size, composition, and physical properties, depending upon the particular joint for which the implant is intended. The shape of the center member  242  may complement that of the interior surfaces  250 ,  254  of the endplate assemblies  244 ,  246 , respectively, to allow for a range of translational, flexural, extensional, rotational, and lateral bending motion appropriate to the particular joint being replaced. In this embodiment, the center member  242  may include a surface  258  having a cavity  260  generally conforming to the shape of the protrusion  252 . The cavity  260  may comprise a restraint mechanism  261  which, in this example, may be a boss. More than one restraint mechanism  261  may be used (corresponding to more than one restraint mechanism  253 ), and the one or more restraint mechanisms  261  may be located at alternative locations on center member  242 . The boss  261  may extend across the cavity  260  in the anterior-posterior direction  62  to restrict motion along the axis  66 , but in other examples a restraint mechanism may be positioned to restrict motion along the axis  62 . The center member  242  may also have a surface  262  which, in this embodiment, may generally conform to the shape of the interior surface  254 .  
         [0068]     The components  242 ,  244 ,  246  may be formed from the same materials as described above for components  22 ,  24 ,  26 , respectively. Referring now to  FIG. 25 , the components of the intervertebral disc prosthesis  240  may be assembled by engaging the protrusion  252  with the cavity  260  and further engaging the restraint mechanism  261  with the restraint member  253 . The surface  262  of the center member  242  may be positioned on the surface  254 . The components  242 - 246  may be centrally aligned along the longitudinal axis  44 .  
         [0069]     The intervertebral disc prosthesis  240  may be inserted in the void of the vertebral column  12   a  (of  FIG. 3 ) created by the removal of disc  12 . The positioning and functioning of the prosthesis  240  may be similar to that of the prosthesis  200  ( FIG. 18 ) and therefore will not be described in detail. As described above in detail for prostheses  20  and  200 , the prosthesis  240  may have a bias to return toward the neutral position aligned along the axis  44 . Additionally, in this embodiment, the extension of the protrusion  252  in the lateral direction  66  may permit more stable and controlled lateral translation while decreasing the risk of dislodging the center member  242 . The engagement of the restraint mechanism  261  and the restraint member  253  may limit lateral translation in accordance with the needs of a particular application. The lateral flare of the restraint member  253  may be varied such that embodiments having a narrow flare would permit less lateral translation than embodiments having wider flares. It is understood that a variety of other restraint mechanism  261 /restraint member  253  configurations may be employed to restrict the amount of lateral translation. For example, the restraint member  253  can protrude to engage a grooved restraint mechanism  261 .  
         [0070]     Referring now to  FIGS. 26-30 , a joint prosthesis  270 , which in this embodiment may be an intervertebral disc prosthesis, includes a center member  272  interposed between two endplate assemblies  274 ,  276 . The endplate assembly  274  may include an exterior surface  278  and an interior surface  280 . A depression  282 , may be formed on the interior surface  280 . In this embodiment, the depression  282  may be formed as a concave recess extended along the lateral direction of axis  66 . The depression  282  may also be curved along the axis  66 . The surfaces  278  and  280  may be flat, angled, or curved. In this embodiment, the exterior surface  278  may be relatively flat or may be contoured to match the surface of an adjacent vertebral endplate. The interior surface  280  may be generally flat around the depression  282 .  
         [0071]     The endplate assembly  276  may include a interior surface  284  and an exterior surface  286 . The surfaces  284  and  286  may be flat, angled, or curved. In this embodiment, the surface  286  may be generally flat or may be contoured to match the surface of an adjacent vertebral endplate. The interior surface  284  may include a concave recess  288 .  
         [0072]     The center member  272  may vary somewhat in shape, size, composition, and physical properties, depending upon the particular joint for which the implant is intended. The shape of the center member  272  may complement that of the interior surfaces  280 ,  284  of the endplate assemblies  274 ,  276 , respectively, to allow for a range of translational, flexural, extensional, rotational, and lateral bending motion appropriate to the particular joint being replaced. In this embodiment, the center member  272  may include a surface  290  generally conforming to the shape of the depression  282 . The center member  272  may also have a surface  292  which, in this embodiment, may generally conform to the shape of the concave recess  288 .  
         [0073]     As shown in  FIG. 29 , the intervertebral disc prosthesis  270  may be in a neutral position when the components  272 - 276  are centrally aligned along the longitudinal axis  44 . The surface  292  may have an arc  294  with a radius  296  and a center point  298 . The surface  290  may have an arc  300  with a radius  302  and a center point  304 . In the neutral position of  FIG. 29 , the center points  298 ,  304  are aligned along the longitudinal axis  44 . In this example, the radius  302  is smaller than the radius  296 , and accordingly, the arc  300  is tighter than the arc  294 . A distance  306  extends between the center points  298 ,  304 .  
         [0074]     The components  272 ,  274 ,  276  may be formed from the same materials as described above for components  22 ,  24 ,  26 , respectively. Referring specifically to  FIG. 28-30 , the components of the intervertebral disc prosthesis  270  may be assembled by engaging the surface  290  with the depression  282  and further engaging the surface  292  with the surface  288 . The components  272 - 276  may be centrally aligned along the longitudinal axis  44 . The intervertebral disc prosthesis  270  may be inserted in the void of the vertebral column  12   a  (of  FIG. 3 ) created by the removal of disc  12 . The surface  278  may contact an endplate of vertebra  16  and the surface  286  may contact the endplate of vertebra  14   a.    
         [0075]     Referring now to  FIG. 30 , the intervertebral disc prosthesis  270  may be articulated by, for example, flexion, extension, and/or translational movement. In response to this movement, the center member  272  may articulate between the endplate assembly interior surfaces  284 ,  280 . With the position of the tighter arc  300  within the wider arc  294 , the articulated prosthesis  270  may be constrained and biased to return to the more stable, neutral position aligned along the longitudinal axis  44  when subject to a load such as the patient&#39;s weight. This tendency of the prosthesis  270  to self align may allow more natural joint movement while preventing excessive translation that might otherwise result in the disassembly of the prosthesis  270 . Further, this alignment bias may relieve excessive loads that might otherwise form in adjacent joints due to chronic over-displacement between the center points  298 ,  304 . The depression  282  and the concave recess  288 , in addition to permitting the smooth articulation of the center member  272 , may function to limit or prohibit lateral movement along the axis  66 . The matching curvatures of surfaces  282 , 290  and  292 , 288  may distribute the loadings and enhance the wear resistance of the components  272 ,  274 ,  276 . The components  272 ,  274 ,  276  may be modular which may permit the selection of a center member  272  having a thickness which adjusts the prosthesis  270  to a desired height.  
         [0076]     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.