Patent Publication Number: US-6669732-B2

Title: Spinal disc

Description:
This application is a continuation of application Ser. No. 09/173,282, filed Oct. 15, 1998, now abandoned, which in turn is a continuation of application Ser. No. 08/954,293, filed Oct. 17, 1997, now U.S. Pat. No. 5,824,064. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to a spinal disc prosthesis to replace a damaged or degenerated spinal disc in a spinal column of a human. 
     U.S. Pat. Nos. 5,017,437 and 5,534,030 disclose typical spinal disc prostheses to replace a damaged or degenerated spinal disc in a spinal column of a human. The discs disclosed in these patents include a pair of rigid plates adhered to opposite surfaces of a body of elastomeric material. In U.S. Pat. No. 5,534,030, the opposite surfaces of the body of elastomeric material to which the rigid plates are adhered extend at an angle to each other as they extend across the disc. The rigid plates which are adhered to the elastomeric material are not wedge-shaped, but the spinal disc comprising the elastomeric core and the rigid plates is generally wedge-shaped. 
     The disc when in use is positioned between adjacent vertebrae, and the rigid plates have bone ingrowth material for enabling bone to adhere or fuse to the rigid plates. The disc is subject to forces which act in the spine including compression forces due to loads on the spine, shear forces due to bending of the spine, and torsional forces due to twisting of the spine. These forces can be applied simultaneously to the disc. These forces may cause the rigid plates to separate from the body of elastomeric material. Such separation would be detrimental to the proper functioning of the disc. 
     Also, it is desirable to control relative displacement of the rigid plates when in use to minimize the possibility of spinal instability. An excessive amount of relative displacement would not be desirable. 
     It has been discovered that the maximum forces acting on a spinal disc, and particularly the maximum forces tending to separate the rigid plates from the body of elastomeric material, can be reduced and the relative displacement of the rigid plates can be effectively controlled by constructing the spinal disc so that the disc comprises the following: 
     1. an elastomeric core having upper and lower surfaces which are parallel to each other, 
     2. an upper rigid plate having a first surface affixed to the upper surface of the core and a second surface for adherence to a vertebra, which second surface is inclined relative to the first surface, and 
     3. a lower rigid plate having a third surface affixed to the lower surface of the core and a fourth surface for adherence to a vertebra, which fourth surface is inclined relative to the third surface, 
     4. the second and fourth surfaces being inclined relative to each other to give the disc a wedge shape. 
     When the spinal disc is in use between adjacent vertebrae, the second surface is inclined away from the first surface as the second surface extends from a posterior portion of the spinal disc toward an anterior portion of the spinal disc. Also, the fourth surface is inclined away from the third surface as the fourth surface extends from the posterior portion of the spinal disc toward the anterior portion of the spinal disc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will become more apparent to one skilled in the art upon reading the following description of a preferred embodiment with reference to the accompanying drawings, wherein: 
     FIG. 1 is an elevational view of a human spinal column having a spinal disc in accordance with the present invention between adjacent vertebrae of the spinal column; 
     FIG. 2 is a top perspective view of the spinal disc of FIG. 1; 
     FIG. 3 is a bottom plan view of the spinal disc of FIG. 1; 
     FIG. 4 is an elevational view, partly in section, of the spinal disc of FIG. 1, taken generally along line  4 — 4  of FIG. 3; 
     FIG. 5 is a sectional view of the spinal disc of FIG. 1, taken generally along line  5 — 5  of FIG.  3  and with parts removed; and 
     FIG. 6 is an enlarged view of a portion of FIG.  4 . 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     The present invention relates to an artificial spinal disc prosthesis to replace a damaged or degenerated spinal disc in a spinal column of a human. As representative of the present invention, FIG. 1 illustrates a spinal disc prosthesis, i.e. spinal disc  10 . The spinal disc  10  is illustrated in use between adjacent upper and lower vertebrae  12  and  14  of a human spinal column  16 . The vertebrae  12  and  14  have portions which face anteriorly (to the right as viewed in FIG. 1) and portions which face posteriorly (to the left as viewed in FIG.  1 ). 
     The disc  10  comprises a first or upper rigid plate  20 , a second or lower rigid plate  120 , and an elastomeric core  200  interposed between and adhered to the two plates. The upper and lower plates  20  and  120  are identical to each other, and the disc  10  is symmetrical about an anterior-posterior, horizontally extending plane A (FIG. 4) and is also symmetrical about a sagittal plane B (FIG.  3 ). The terms “upper” and “lower” are used herein with reference to the orientation of the disc  10  when it is implanted in the human body as illustrated in FIG. 1, to distinguish the two identical plates for reference purposes. 
     The upper plate  20  (FIG. 5) is rigid and is preferably made of a biocompatible metal such as a titanium-vanadium-aluminum alloy having about 90% by weight titanium, about 6% by weight aluminum and about 4% by weight vanadium. Alternatively, the upper plate  20  can be made of any suitable biocompatible material, including but not limited to a composite plastic material. The upper plate  20  is preferably milled out of a single block of metal. The upper plate  20  could, however, be made in a different manner, for example, by casting. 
     The upper plate  20  has an anterior portion  22  and a posterior portion  24 . The anterior portion  22  of the upper plate  20  is that portion of the upper plate which is disposed anteriorly in the spine  16  when the disc  10  is implanted in the spine. The posterior portion  24  of the upper plate  20  is that portion of the upper plate which is disposed posteriorly in the spine  16  when the disc  10  is implanted in the spine. The anterior portion of the upper plate can be said to be located generally on one side (to the right as viewed in FIG. 5) of an axis  28  of the disc  10 ; the posterior portion of the upper plate can be said to be located generally on the other side (to the left as viewed in FIG. 6) of the axis  28 . The axis  28  extends through the disc between the upper and lower plates  20  and  120 . The axis  28  extends generally along the length of the spinal column  16  when the disc  10  is implanted in the spinal column. 
     The configuration of the disc  10  (as viewed in plan) is designed to conform generally to the shape of a natural human spinal disc. The perimeter  30  (FIG. 3) of the disc  10  has a flat posterior portion  32 . The perimeter  30  of the disc  10  has a curved convex portion  34  which extends between opposite ends  36  and  38  of the flat portion  32  of the perimeter. The perimeter  30  of the disc  10 , including the perimeters of the core  200  and of the plates  20  and  120 , does not have any outwardly projecting lobes. The perimeter of the core  200  has the same configuration (as viewed in plan) as the perimeter of the upper and lower plates  20  and  120 . 
     The upper plate  20  has an inner major side surface  40  which is presented downward as viewed in FIG.  5 . The inner major side surface  40  includes all of the surface area of the upper plate  20  which is visible from below (in plan) as viewed in FIG.  5 . The inner major side surface  40  of the upper plate  20  includes a planar first surface  42  of the upper plate which extends perpendicular to the axis  28 . The area of the first surface  42  is at least 65% or more of the area of the inner major side surface  40  as viewed in plan, that is, with all points on the inner major side surface  40  viewed in a direction parallel to the axis  28 . Preferably, the area of the first surface  42  is 75% of the area of the inner major side surface  40 . 
     The first surface  42  is circumscribed by a first rim  44  of the upper plate  20 . The first rim  44  has a generally semi-cylindrical cross-sectional configuration as shown in FIG. 5 including an arcuate outer surface  46 . The outer surface  46  on the first rim  44 , and the first surface  42 , together define a shallow cavity or recess  48  in the inner major side surface  40  of the upper plate  20 . The first surface  42  forms the bottom of the recess  48 . The outer surface  46  on the first rim  44  forms a part of the inner major side surface  40  of the upper plate  20 . 
     The upper plate  20  has an outer major side surface  50  which is presented upward as viewed in FIG.  5 . The outer major side surface  50  includes all of the surface area of the upper plate  20  which is visible from above (in plan) as viewed in FIG.  5 . 
     The outer major side surface  50  includes a planar second surface  52  of the upper plate  20 . The second surface  52  is circumscribed by a second rim  54  of the upper plate  20 . The area of the second surface  52  is greater than the area of the first surface  42 . The area of the second surface  52  is 85% or more, and preferably 92%, of the area of the inner major side surface  10 . 
     The second rim  54  is located outward of (as viewed in plan) the first rim  44 . The second rim  54  has an inner surface  56 , which extends perpendicular to the second surface  52  and extends entirely around the upper plate  20 , and a curved outer surface  58 . The inner surface  56  of the second rim  54 , and the second surface  52 , together define a shallow cavity or recess  60  in the outer major side surface  50  of the upper plate  20 . The second surface  52  forms the bottom of the recess  60 . 
     The distance by which the second rim  54  projects from the second surface  52  is less than the distance by which the first rim  44  projects from the first surface  42 . Thus, the recess  60  in the outer major side surface  50  of the upper plate  20  is shallower than the recess  48  in the inner major side surface  40  of the upper plate. 
     The second surface  52  of the upper plate  20  is inclined relative to the first surface  42  of the upper plate. The second surface  52  is inclined at an angle in the range of from about 1.5° to about 7.5° relative to the first surface  42 . In the illustrated preferred embodiment,the second surface  52  is inclined at an angle of 5° relative to the first surface  42 . In another preferred embodiment, not illustrated, the second surface  52  is inclined at an angle of 2.5° relative to the first surface  42 . 
     The first and second surfaces  42  and  52  are oriented relative to each other so that they are closest together at the posterior portion  24  of the upper plate  20 , and farthest apart at the anterior portion  22  of the upper plate. The second surface  52  is inclined away from the first surface  42  as the second surface  52  extends from the posterior to the anterior of the disc  10 . Thus, the first and second surfaces  42  and  52  diverge as they extend from the posterior portion  24  of the upper plate  20  to the anterior portion  22  of the upper plate. This divergence of the first and second surfaces  42  and  52  gives the upper plate  20  a wedge-shaped configuration as viewed in a lateral or medial direction (FIG.  4 ). 
     A dome  62  projects from the second surface  52  of the upper plate  20 . The dome  62  has a crescent-shaped configuration including a central portion  64  and two tips  66  and  68  (see FIG.  3 ). The dome  62  is oriented on the second surface  52  so that the tips  66  and  68  of the crescent-shaped configuration point generally posteriorly and the central portion  64  of the crescent-shaped configuration is located anteriorly of the tips. The dome  62  is also located anteriorly of the axis  28 . 
     The dome  62  has a side surface  70  and a top surface  72 . The top surface  72  of the dome  62  is inclined at a small angle to the second surface  52 . The top surface  72  on the central portion  64  of the crescent-shaped configuration is farther from the second surface  52  than are the top surfaces on the tips  66  and  68  of the crescent-shaped configuration. In the illustrated embodiment, the top surface  72  of the dome  62  is inclined at an angle of 3.8° to the second surface  52  of the upper plate  20 , that is, at an angle of 8.8° to the first surface  42  of the upper plate. The top surface  72  of the dome  62  forms a part of the outer major side surface  50  of the upper plate  20 . 
     The outer surface  58  of the second rim  54  merges with an outer peripheral side surface  74  of the upper plate  20 . The outer peripheral side surface  74  of the upper plate  20  extends perpendicular to the first surface  42  of the upper plate and also extends entirely around the upper plate. Thus, the outer peripheral side surface  74  of the upper plate  20  is not perpendicular to the plane of the second surface  52 . Because of the inclination of the second surface  52  to the first surface  42 , the outer peripheral side surface  74  of the upper plate  20  has a greater axial extent in the anterior portion  22  of the upper plate (to the right as viewed in FIG. 5) than in the posterior portion  24  of the upper plate (to the left as viewed in FIG.  5 ). 
     The upper plate  20  has an outer peripheral flange  78  which extends around the periphery of the upper plate. The flange  78  has a generally planar first surface  80  which extends outward from the outer peripheral side surface  74 , in a direction parallel to the first surface  42 . The first surface  80  of the flange  78  forms a part of the outer major side surface  50  of the upper plate  20 . The flange  78  has a curved second surface  82  which extends downward (as viewed in FIG. 5) and inward from the first surface  80  of the flange. 
     A planar third surface  84  of the flange  78  extends inward from the second surface  82 , in a direction parallel to the first surface  80  of the flange and parallel to the first surface  42  of the upper plate  20 . The third surface  84  of the flange  78  lies in a plane located between the plane of the first surface  42  of the upper plate  20  and the plane of the second surface  52  of the upper plate. The third surface  84  of the flange  78  extends from a location outward of the outer peripheral side surface  74 , to a location inward of the outer peripheral side surface  74 , and merges with the outer surface  46  of the first rim  44 . The second and third surfaces  82  and  84  of the flange  78  form a part of the inner major side surface  40  of the upper plate  20 . 
     A porous coating  90  (FIGS. 4 and 6) is located in the recess  48  in the inner major side surface  40  of the upper plate  20 . The coating  90  is formed on the first surface  42  and is circumscribed by, or lies inward of, the first rim  44 . The coating  90  covers the entire extent of the first surface  42 . The coating  90  comprises a layer of small spherical particles or beads  92 . 
     The beads  92  are preferably made of commercially pure titanium, but could be made of any suitable biocompatible material. The beads  92  are sized such that none of the beads pass through a 25 mesh U.S. Series Sieve and all the beads pass through a 40 mesh U.S. Series Sieve. The beads  92  are preferably adhered to the upper plate  20  by diffusion bonding. The beads  92  can, alternatively, be applied to the upper plate  20  by any other suitable technique. 
     The coating  90  of beads  92  is firmly adhered to the upper plate  20  and is incapable of removal by normal abrasions. As described below, the coating  90  in combination with a primary adhesive interlocks with the material of the elastomeric core  200  to provide a strong bond between the upper plate  20  and the elastomeric core  16 . The coating  90  of beads  92  does not project past the first rim  44 , that is, in a downward direction as viewed in FIGS. 4 and 6. 
     A porous coating  94  (FIGS. 2,  4  and  6 ) is located in the recess  60  in the outer major side surface  50  of the upper plate  20 . The coating  94  is made from beads  96  which are the same size as, and are applied in the same manner as, the beads  92  on the first surface  42 . The coating  94  is formed on the second surface  52  of the upper plate  20  and is circumscribed by, or lies inward of, the second rim  54 . The coating  94  covers the entire extent of the second surface  52 . The coating  94  also covers the dome  62 . 
     The coating  94  on the second surface  52 , as described below, provides for ingrowth of bony tissue when the disc  10  is implanted in the spine  16 . The coating  94  of beads  96  is thicker than the depth of the recess  60 . Thus, the beads  96  of the coating  94  project axially outward past the second rim  54 . This is in contrast to the coating  90 , which does not project axially outward past the first rim  44 . 
     The lower plate  120  is identical in configuration to the upper plate. The lower plate  120  is rigid and is made from the same material as the upper plate. The lower plate  120  (FIG. 5) has an anterior portion  122  which is disposed anteriorly in the spine  16  when the disc  10  is implanted in the spine. A posterior portion  124  of the lower plate  120  is disposed posteriorly in the spine  16  when the disc  10  is implanted in the spine. 
     The configuration of the lower plate  120  as viewed in plan (FIG. 3) is the same as the configuration of the upper plate  20 . The perimeter of the lower plate  120  has a flat posterior portion and a curved convex portion which extends between opposite ends and of the flat portion of the perimeter. The lower plate  120 , like the upper plate  20 , does not have any outwardly projecting lobes. 
     The lower plate  120  has an inner major side surface  140  (FIG. 5) which is presented upward as viewed in FIG.  5 . The inner major side surface  140  includes all of the surface area of the lower plate  120  which is visible from above (in plan) as viewed in FIG.  5 . The inner major side surface  140  of the lower plate  120  includes a planar third surface  142  of the lower plate  120  which extends perpendicular to the axis  28 . The area of the first surface  142  is at least 65% or more of the area of the inner major side surface  140  as viewed in plan, that is, with all points on the inner major side surface  140  viewed in a direction parallel to the axis  28 . Preferably, the area of the third surface  142  is 75% of the area of the inner major side surface  140 . 
     The third surface  142  is circumscribed by a first rim  144  of the lower plate  20 . The first rim  144  has a generally semi-cylindrical cross-sectional configuration as shown in FIG. 5 including an arcuate outer surface  146 . The outer surface  146  on the first rim  144 , and the third surface  142 , together define a shallow cavity or recess  148  in the inner major side surface  140  of the lower plate  120 . The third surface  142  forms the bottom of the recess  148 . The outer surface  146  on the first rim  144  forms a part of the inner major side surface  140  of the lower plate  120 . 
     The lower plate  120  has an outer major side surface  150  which is presented downward as viewed in FIG.  5 . The outer major side surface  150  includes all of the surface area of the lower plate  120  which is visible from below (in plan) as viewed in FIG.  5 . 
     The outer major side surface  150  of the lower plate  120  includes a planar fourth surface  152  of the lower plate. The fourth surface  152  is circumscribed by a second rim  154  of the lower plate  120 . The area of the fourth surface  152  is greater than the area of the third surface  142 . The area of the fourth surface  152  is 85% or more, and preferably 92%, of the inner major side surface  40 . 
     The second rim  154  is located outward of (as viewed in plan) the first rim  144 . The second rim  154  has an inner surface  156 , which extends perpendicular to the second surface  152  and extends entirely around the lower plate  120 , and a curved outer surface  158 . The inner surface  156  of the second rim  154 , and the fourth surface  152 , together define a shallow cavity or recess  160  in the outer major side surface  150  of the lower plate  120 . The fourth surface  152  forms the bottom of the recess  160 . 
     The distance by which the second rim  154  projects from the fourth surface  152  is less than the distance by which the first rim  144  projects from the third surface  142 . Thus, the recess  160  in the outer major side surface  150  of the lower plate  120  is shallower than the recess  148  in the inner major side surface  140  of the lower plate. 
     The fourth surface  152  of the lower plate  120  is inclined relative to the third surface  142  of the lower plate. The fourth surface  152  is inclined at an angle in the range of from about 1.5° to about 7.5° relative to the third surface  142 . In the illustrated preferred embodiment, the fourth surface  152  is inclined at an angle of 5° relative to the third surface  142 . In another preferred embodiment, not illustrated, the fourth surface  152  is inclined at an angle of 2.5° relative to the third surface  142 . 
     The third and fourth surfaces  142  and  152  are oriented relative to each other so that they are closest together at the posterior portion  124  of the lower plate  120 , and farthest apart at the anterior portion  122  of the lower plate. The fourth surface  152  is inclined away from the third surface  142  as the fourth surface  152  extends from the posterior to the anterior of the disc  10 . Thus, the third and fourth surfaces  142  and  152  diverge as they extend from the posterior portion  124  of the lower plate  120  to the anterior portion  122  of the lower plate. This divergence of the third and fourth surfaces  142  and  152  gives the lower plate  120  the same wedge-shaped configuration as the upper plate  20 . 
     A dome  162  projects from the fourth surface  152  of the lower plate  120 . The dome  162  has a crescent-shaped configuration including a central portion  164  and two tips  166  and  168  (see FIG.  3 ). The dome  162  is oriented on the fourth surface  152  so that the tips  166  and  168  of the crescent-shaped configuration point generally posteriorly and the central portion  164  of the crescent-shaped configuration is located anteriorly of the tips. The dome  162  is also located anteriorly of the axis  28 . 
     The dome  162  has a side surface  170  and a top surface  172 . The top surface  172  of the dome  162  is inclined at a small angle to the fourth surface  152 . The top surface  172  on the central portion  164  of the crescent-shaped configuration is farther from the fourth surface  152  than are the top surfaces on the tips  166  and  168  of the crescent-shaped configuration. In the illustrated embodiment, the top surface  172  of the dome  162  is inclined at an angle of 3.8° to the fourth surface  152  of the lower plate  120 , that is, at an angle of 8.8° to the third surface  142 . The top surface  172  on the dome  162  forms a part of the outer major side surface  140  of the lower plate  120 . 
     The outer surface  158  of the second rim  154  merges with an outer peripheral side surface  174  of the lower plate  120 . The outer peripheral side surface  174  extends perpendicular to the third surface  142  of the lower plate  120  and also extends entirely around the lower plate. Thus, the outer peripheral side surface  174  of the lower plate  120  is not perpendicular to the plane of the fourth surface  152 . Because of the inclination of the fourth surface  152  to the third surface  142 , the outer peripheral side surface  174  of the lower plate  120  has a greater axial extent in the anterior portion  122  of the lower plate (to the right as viewed in FIG. 5) than in the posterior portion  124  of the lower plate (to the left as viewed in FIG.  5 ). 
     The lower plate  120  has an outer peripheral flange  178  which extends around the periphery of the lower plate. The flange  178  has a generally planar first surface  180  which extends outward from the outer peripheral side surface  174 , in a direction parallel to the third surface  142 . The first surface  180  on the flange  178  forms a part of the outer major side surface  150  of the lower plate  120 . The flange  178  has a curved second surface  182  which extends upward (as viewed in FIG. 5) and inward from the first surface  180  of the flange. 
     A planar third surface  184  of the flange  178  extends inward from the second surface  182 , in a direction parallel to the first surface  180  of the flange and parallel to the third surface  142  of the lower plate  120 . The third surface  184  of the flange  178  lies in a plane located between the plane of the third surface  142  of the lower plate  120  and the plane of the fourth surface  152  of the lower plate. The third surface  184  of the flange  178  extends from a location outward of the outer peripheral side surface  174 , to a location inward of the outer peripheral side surface  174 , and merges with the outer surface  146  of the first rim  144 . The second and third surfaces  182  and  84  of the flange  178  form a part of the inner major side surface  140  of the lower plate  120 . 
     A porous coating  190  (FIG. 4) is located in the recess  148  in the inner major side surface  140  of the lower plate  120 . The coating  190  is formed on the third surface  142  and is circumscribed by, or lies inward of, the first rim  144 . The coating  190  covers the entire extent of the third surface  142 . The coating  190  comprises a layer of small spherical particles or beads  192 . 
     The beads  192  are made from the same material as the beads  92  of the coating  90 . The beads  192  are preferably adhered to the lower plate  120  by diffusion bonding. The beads  192  can, alternatively, be applied to the lower plate  120  by any other suitable technique. 
     The coating  190  of beads  192  is firmly adhered to the lower plate  120  and is incapable of removal by normal abrasions. As described below, the coating  190  in combination with a primary adhesive interlocks with the material of the elastomeric core  200  to provide a strong bond between the lower plate  120  and the elastomeric core  16 . The coating  190  of beads  192  does not project axially outward of the first rim  144 . 
     A similar porous coating  194  (FIGS. 3 and 4) is located in the recess  60  in the outer major side surface  150  of the lower plate  120 . The coating  194  is formed on the fourth surface  152  and is circumscribed by, or lies inward of, the second rim  154 . The coating  194  covers the entire extent of the fourth surface  152 . The coating  194  also covers the dome  162 . The coating  194  is made from a plurality of beads  196  which are the same as, and are applied in the same manner as, the beads  192  on the third surface  142 . 
     The coating  194  on the fourth surface  152 , as described below, provides for ingrowth of bony tissue when the disc  10  is implanted in the spine  16 . The layer  190  of beads  196  is thicker than the depth of the recess  160 . Thus, the beads  196  of the coating  194  project axially outward past the second rim  154 . This is in contrast to the coating  190 , which does not project axially outward past the first rim  144 . 
     The elastomeric core  200  is preferably made of a polyolefin rubber or carbon black reinforced polyolefin rubber. The hardness of the elastomeric core is 56-72 shore A durometer. The ultimate tensile strength of the core is greater than 1600 psi. The core has an ultimate elongation greater than 300% using the ASTM D412-87 testing method, and a tear resistance greater than 100 psi using the ASTM D624-86 testing method. Although the elastomeric core  200  is disclosed as being made of a polyolefin rubber, it can be made of any elastomeric material that simulates the characteristics of a natural disc. 
     To construct the spinal disc  10 , the plates  20  and  120 , with the coatings  90 ,  94 ,  190  and  194  in place, are cleaned in a methyl ethyl ketone or similar reagent bath for approximately 25 minutes. The plates  20  and  120  are etched, for example with a nitric hydrofluoric acid solution, to remove any oxide coating from the plates. Thereafter, the plates  20  and  120  are rinsed in distilled water, and a primer is applied to the plates that will be bonded to the core  200 . The primer is applied within about 2 hours of the etch, and at a nominal thickness of 0.35 mils. After the primer has dried for not less than 60 minutes an adhesive is applied at a nominal thickness of 0.65 mils. The plates  20  and  120  are then placed in a mold and the elastomeric material of the core  200  is flowed into the mold and adhered to the plates. The elastomeric material of the core  200  is then cured to form the completed disc  10 . 
     The elastomeric core  200 , as thus formed, is affixed to the inner major side surface  40  of the upper plate  20 . The core  200  has a planar upper surface  202  (FIGS. 2,  4  and  6 ) which is affixed to and overlies the first surface  42  of the upper plate  20 . A portion  204  of the material of the core  200  extends into and interlocks with the first surface  42  of the upper plate  20 , as well as with the porous coating  90  on the first surface. The first surface  42  of the upper plate  20  is bonded to the upper surface  202  of the elastomeric core  200  and to the beads throughout the entire extent of the first surface. 
     Another portion  206  (FIG. 6) of the material of the core  200  extends over and is adhered to the first rim  44  on the upper plate  20 . Another portion  208  of the material of the core  200  extends over and is adhered to the planar third surface  84  of the flange  78  of the upper plate  20 . Yet another portion  210  of the material of the core  200  extends over and is adhered to the curved second surface  82  of the flange  78  of the upper plate  20 . The material portion  210  which overlies the second surface  82  of the flange  78  tapers to a zero thickness, as it approaches the first surface  80  of the flange. 
     The material of the core  200 , as thus formed, is also affixed to the inner side surface  140  of the lower plate  120 . A portion of the material of the core  200  extends into and interlocks with the third surface  142  of the lower plate  120 , as well as with the porous coating  190  on the third surface. The core  200  has a planar lower surface  212  (FIG. 4) which is affixed to the third surface  142  of the lower plate  120 . The lower surface  212  of the core  200  is parallel to the upper surface  202  of the core. The third surface  142  of the lower plate  120  is bonded to the lower surface  212  of the elastomeric core  200  throughout the entire extent of the third surface. 
     A portion  216  (FIG. 4) of the material of the core  200  extends over and is adhered to the first rim  144  on the lower plate  120 . Another portion  218  of the material of the core  200  extends over and is adhered to the planar third surface  184  of the flange  178  of the lower plate  120 . Yet another portion  220  of the material of the core  200  extends over and is adhered to the curved second surface  182  of the flange  178  of the lower plate  120 . The material portion  220  which overlies the second surface  182  of the flange  178  tapers to a zero thickness, as it approaches the first surface  180  of the flange. 
     The core  200  has an exposed outer side surface  230  (FIGS. 2,  4  and  6 ) which extends between the upper and lower plates  20  and  120 . The outer side surface  230  of the core  200  includes a first surface portion  232  (FIGS. 4 and 6) extending substantially perpendicular to the first surface  42  of the upper plate  20 . The first surface portion  232  is located outward of the flange  78  of the upper plate  20 . 
     A convex second portion  234  of the outer side surface  230  of the core  200  extends from the first surface portion  232 , in a direction toward the lower plate  120 . A concave third portion  236  of the outer side surface  230  of the core  200  extends from the second surface portion  234 , in a direction toward the lower plate  120 . 
     The outer side surface  230  of the core  200  includes a fourth surface portion  238  extending from the third surface portion  236 , in a direction substantially perpendicular to the first surface  42  of the upper plate  20  and parallel to the axis  28  of the disc  10 . The fourth surface portion  238  is disposed axially at a location between the upper plate  20  and the lower plate  120 . The fourth surface portion  238  is disposed inward of the outer periphery of the plate flanges  78  and  178 , but outward of the first rims  44  and  144  of the plates. 
     The fourth surface portion  238  merges with a concave fifth surface portion  240  which is a mirror image of the third surface portion  236 . The fifth surface portion  240  merges with a convex sixth surface portion  242  which is a mirror image of the second surface portion  234 . 
     The sixth surface portion  242  merges with a seventh surface portion  244  which is a mirror image of the first surface portion  232 . The seventh surface portion  244  is located outward of the flange  178  of the lower plate  120 . 
     The central portion of the core  200 , i.e. the portion of the core  200  located between the surface  42  and the surface  142 , is of substantially uniform thickness. Because the central portion of the core  200  is of uniform thickness and the plates  20  and  120  are wedge-shaped, the overall configuration of the disc  10  is wedge-shaped. The disc  10  is thicker in the anterior portion  22  of the disc and is thinner in the posterior portion  24  of the disc. 
     When the disc  10  is in use in the spinal column  16 , the upper plate  20  is affixed to the upper vertebra  12 . The dome  62  on the upper plate  20  is fitted into a corresponding recess or cavity (not shown) formed in the upper vertebra  12 . The engagement of the dome  62  of the upper plate  20  in the cavity in the upper vertebra  12  resists relative movement between the upper plate and the upper vertebra. 
     The porous coating  94  on the second surface  52  of the upper plate  20  promotes bone ingrowth between the upper vertebra  12  and the upper plate  20 . The second surface  52  (FIG. 6) of the upper plate  20  engages the bony material of the upper vertebra  12 . Interlocking engagement between the upper plate  20  and the bony material of the upper vertebra  12  is enhanced by the fact that the beads  96  of the coating  94  project axially outward past the second rim  54 . 
     The lower plate  120  is affixed to the lower vertebra  14 . The dome  162  on the lower plate  120  is fitted into a corresponding recess or cavity (not shown) formed in the lower vertebra  14 . The engagement of the dome  162  of the lower plate  120  in the cavity in the lower vertebra  14  resists relative movement between the lower plate and the lower vertebra. 
     The porous coating  194  on the fourth surface  152  promotes bone ingrowth between the lower vertebra  14  and the lower plate  120 . The fourth surface  152  of the lower plate  120  engages the material of the lower vertebra  14 . Interlocking engagement between the lower plate  120  and the bony material of the lower vertebra  14  is enhanced by the fact that the beads  196  of the coating  194  project axially outward past the second rim  154 . 
     The maximum stresses under load acting on the spinal disc  10  are reduced as compared to the maximum stresses acting on the spinal disc of U.S. Pat. No. 5,534,030 under an identical load. For example, finite element analysis has shown an 8.2% decrease in shear stresses in the spinal disc  10  as compared to the disc of U.S. Pat. No. 5,534,030. Thus, the disc  10  has less tendency for the plates  20  and  120  and the elastomeric core  200  to separate. The maximum principal stress in the disc  10  is reduced by about 10.25% as compared to the known prior art disc shown in U.S. Pat. No. 5,543,030. This reduction in stress also reduces the tendency of the plates to separate from the elastomeric core as compared to the disc of U.S. Pat. No. 5,543,030. 
     Further, the disc  10  has an increased resistance to anterior-posterior displacement between the upper plate and the lower plate as compared to the disc of U.S. Pat. No. 5,543,030. Specifically, the disc  10  of the present invention, has a maximum anterior to posterior displacement of the plates of 20% less than the displacement of the plates of the disc of U.S. Pat. No. 5,543,030. This reduction in anterior-posterior displacement minimizes the possibility of disc contact with the spinal cord which could cause instability of the spinal cord. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications in the invention. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.