Abstract:
An intervertebral implant for separating an upper vertebra and a lower vertebra. The implant includes an upper mount having a first surface sized and shaped for mounting on the upper vertebra and a second surface opposite the first surface. The implant includes a lower mount having a first surface sized and shaped for mounting on the lower vertebra and a second surface opposite the first surface. The implant includes an element positioned between the upper mount and the lower mount spacing the first surface of the upper mount from the first surface of the lower mount by a predetermined distance. The element is configured to permit the upper mount to pivot relative to the lower mount. The element allows the upper vertebra to pivot relative to the lower vertebra while maintaining spacing between the upper vertebra and the lower vertebra.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 60/681,793 filed May 17, 2005, entitled, “Ensemble of Devices and Methods for Dynamic Posterior Spinal Stabilization”, and U.S. Provisional Patent Application No. 60/700,197 filed Jul. 18, 2005, entitled, “Posteriorly Placed Intervertebral Dynamic Implant”. Both of these applications are incorporated by reference. 
    
    
     BACKGROUND 
     The present invention relates to implants and, more particularly, to intervertebral implants. 
     Vertebrae of the human spine are arranged in a column with one vertebra positioned on top of the next. An intervertebral disc is positioned between each vertebrae pair to provide a cushion between the adjacent vertebrae and transmitting forces. The discs permit movement between vertebra so the column can twist, bend, stretch and compress. Disease, injury, surgery and spinal degeneration can adversely affect the normal function of the intervertebral disc and the complex interrelationship between adjacent vertebrae of the spinal column. Sometimes pain results from diseased, injured or degenerated discs. Because the spinal column is frequently moved, resulting pain can occur frequently to afflicted humans, substantially diminishing quality of life. 
     Conventionally, surgeons treat malfunctioning discs by surgically removing the disc and immobilizing the adjoining vertebrae so they fuse together over time. Such procedures permanently prevent motion between the affected vertebrae, potentially increasing stress on other healthy spinal segments. The increased stress can accelerate disc degeneration. Over time, the increased stress can cause disc herniation, instability and arthritis in the previously healthy segments. Thus, there is a need for a system and method that treats malfunctioning discs without significantly increasing stress on other discs and adversely affecting them. 
     BRIEF SUMMARY 
     The present invention relates to an intervertebral implant for separating an upper vertebra and a lower vertebra. The implant comprises an upper mount having a first surface sized and shaped for mounting on the upper vertebra and a second surface opposite the first surface. The implant also includes a lower mount having a first surface sized and shaped for mounting on the lower vertebra and a second surface opposite the first surface. In addition, the implant has an element positioned between the upper mount and the lower mount spacing the first surface of the upper mount from the first surface of the lower mount by a predetermined distance. The element is configured to permit the upper mount to pivot relative to the lower mount and allows the upper vertebra to pivot relative to the lower vertebra while maintaining spacing between the upper vertebra and the lower vertebra. 
     In another aspect, the present invention relates to an intervertebral implant for separating an upper vertebra and a lower vertebra. The implant comprises an upper mount having a upper surface sized and shaped for mounting on the upper vertebra and a lower surface opposite the first surface. Further, the implant includes a lower mount having a lower surface sized and shaped for mounting on the lower vertebra and a upper surface opposite the first surface. The implant also comprises an element positioned between the upper mount and the lower mount spacing the upper surface of the upper mount from the lower surface of the lower mount by a predetermined distance. The element is configured to permit the upper mount to pivot relative to the lower mount, thereby allowing the upper vertebra to pivot relative to the lower vertebra while maintaining spacing between the upper vertebra and the lower vertebra. 
     In yet another aspect, the present invention relates to an intervertebral implant for separating an upper vertebra and a lower vertebra comprising an upper mount having a forward facing surface sized and shaped for mounting on the upper vertebra. The upper mount also has a rearward facing surface opposite the forward facing surface. Further, the implant includes a post extending downward from the upper mount and a lower mount having a forward facing surface sized and shaped for mounting on the upper vertebra. The lower mount also has a rearward facing surface opposite the forward facing surface and a channel sized and shaped for pivotally receiving the post so the upper mount pivots relative to the lower mount. This allows the upper vertebra to pivot relative to the lower vertebra while maintaining spacing between the upper vertebra and the lower vertebra. 
     The present invention also relates to a method of implanting an intervertebral implant comprising a plurality of components. Each of the plurality of components connects to another component of the plurality of components. The method comprises removing at least a portion of a natural disc from between an upper vertebra and a lower vertebra. A first component of the plurality of components is inserted between the upper vertebra and the lower vertebra, and a second component of the plurality of components is inserted between the upper vertebra and the lower vertebra. The first component of the intervertebral implant is connected to the second component of the intervertebral implant. 
     While the invention has been described with reference to the preferred embodiment(s) thereof, it will be appreciated by those of ordinary skill in the art that various modifications can be made to the invention itself without departing from the spirit and scope thereof. All changes and modifications that are within the spirit of the invention are desired to be protected. 
     Other aspects of the present invention will be in part apparent and in part pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a rear elevation of two lumbar vertebrae. 
         FIG. 2  is a side elevation of two lumbar vertebrae. 
         FIG. 3  is a cross section of the vertebrae taken along line  3 - 3  of  FIG. 1 . 
         FIG. 4  is a is a side elevation of an intervertebral implant of a first embodiment of the present invention. 
         FIG. 5  is a side elevation of an intervertebral implant of a second embodiment of the present invention. 
         FIG. 6  is a side elevation of an intervertebral implant of a third embodiment. 
         FIG. 7  is a side elevation of an implant of a fourth embodiment. 
         FIG. 8  is a side elevation of an implant of a fifth embodiment. 
         FIG. 9  is a side elevation of an implant of a sixth embodiment. 
         FIG. 10  is a side elevation of an implant of a seventh embodiment. 
         FIG. 11  is a rear elevation of an implant of an eighth embodiment. 
         FIG. 12  is a cross section of an implant of a ninth embodiment. 
         FIG. 13  is a top plan of an implant of a tenth embodiment. 
         FIG. 14  is a perspective of the implant of  FIG. 13 . 
         FIG. 15  is a second perspective of the implant of  FIG. 13 . 
         FIG. 16  is a top plan of an intervertebral implant of an eleventh embodiment of the present invention. 
         FIG. 17  is a perspective of the implant of  FIG. 16 . 
         FIG. 18  is a second perspective of the implant of  FIG. 16 . 
         FIG. 19  is a separated rear elevation of an implant of a twelfth embodiment. 
         FIG. 20  is an assembled rear elevation of the implant of  FIG. 19 . 
         FIG. 21  is cross section the implant of the twelfth embodiment taken along line  21 - 21  of  FIG. 20 . 
         FIG. 22  is a rear elevation of an implant of a thirteenth embodiment. 
         FIG. 23  is a cross section of the implant of the thirteenth embodiment taken along line  23 - 23  of  FIG. 22 . 
         FIG. 24  is a rear elevation of an upper portion of an implant of a fourteenth embodiment. 
         FIG. 25  is a cross section of the upper portion of the implant of the fourteenth embodiment taken along line  25 - 25  of  FIG. 24 . 
         FIG. 26  is a rear elevation of a lower portion of the implant of the fourteenth embodiment. 
         FIG. 27  is an assembled rear elevation of the implant of the fourteenth embodiment. 
         FIG. 28  is a rear elevation of an implant of a fifteenth embodiment. 
         FIGS. 29   a - 29   c  are schematics of the implant of  FIG. 28  showing relative movement between portions. 
         FIG. 30  is a rear elevation of an implant of a sixteenth embodiment. 
         FIGS. 31   a  and  31   b  are schematics of the implant of  FIG. 30  showing relative movement between portions. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Referring to the figures, and more particularly to  FIGS. 1-3 , adjacent upper (or cephalad) and lower (or caudal) vertebrae are designated in their respective entireties by reference numbers  40  and  42 . Each of the vertebra  40 ,  42  includes a body  44 . Portions of the vertebrae known as pedicles  46  extend rearward from each body  44  to lamina  48  extending across the rearward ends of the pedicles. The body, pedicles and lamina surround a spinal canal (or vertebral foramen)  50 , which receives the spinal cord (not shown). A central rearward protrusion known as a spinous process  52  extends rearward from the lamina  48 . Each vertebra  40 ,  42  also includes upper facets (or superior articular processes)  54 ,  56  (respectively) extending obliquely outward from the lamina  48  on opposite sides of the spinous processes  52 . The vertebrae  40 ,  42  also include lower facets (or inferior articular processes)  58 ,  60  (respectively) extending downward adjacent the corresponding spinous processes  52 . Portions of the upper facets  56  of the lower vertebrae  42  overlap the lower facets  58  of the upper vertebrae  40  as shown in  FIG. 3 , to form facet joints allowing the spine to bend forward and backward (i.e., to articulate), and twist. As shown in  FIG. 2 , an intervertebral disc  70  is positioned between the upper and lower vertebrae  40 ,  42 . The disc  70  separates and provides cushion between the vertebrae  40 ,  42 . A strong ligament (not shown) extends between the vertebrae  40 ,  42  along the rearward wall of the spinal canal  50  to prevent the vertebrae from separating. 
     Referring to  FIG. 4 , an intervertebral implant of a first embodiment is designated in its entirety by the reference number  100 . The implant  100  includes an upper plate or mount  102  and a lower plate or mount  104 . The upper plate  102  includes a lower face  110  and a protrusion  112  having a concave lower end  114  extending downward from the lower face. The lower plate  104  includes an upper face  116  and a protrusion  118  having a convex upper end  120  extending upward from the upper face. The concave end  114  of the upper plate protrusion  112  receives the convex end  120  of the lower plate protrusion  118 . The concave end  114  and the convex end  120  have complementary shapes (e.g., similarly sized spherical shapes) allowing the upper and lower plates  102 ,  104  to pivot with respect to each other. Although the concave end  114  and the convex end  120  may have other radii without departing from the scope of the present invention, in one embodiment the concave end has a radius of between about 3 millimeters (mm) and about 15 mm, and the convex end has a radius of between about 3 mm and about 20 mm. In one particular embodiment, the concave end  114  has a radius of about 4 mm and the convex end  120  has a radius of about 4.5 mm. Although the upper plate protrusion  112  and the lower plate protrusion  118  may have other overall lengths without departing from the scope of the present invention, in one embodiment the protrusions have equal lengths of between about 8 mm and about 20 mm. In one particular embodiment, upper plate protrusion  112  and the lower plate protrusion  118  both have a length of about 12 mm. In an alternate embodiment, it is envisioned the protrusions  112 ,  118  may have different lengths from each other and/or the convex and concave ends  120 ,  114  may be switched. 
     As further illustrated in  FIG. 4 , the implant  100  includes a forward compressible element or cushion  122  positioned between the upper and lower plates  102 ,  104  in front of (or anterior to) the protrusions  112 ,  118  and a rearward compressible element or cushion  124  positioned between the upper and lower plates behind (or posterior to) the protrusions. The compressible elements  122 ,  124  provide some resistance to movement of the plates  102 ,  104 , but compress to allow the plates to pivot. Thus, the elements  122 ,  124  stabilize the plates  102 ,  104 . Although the elements  122 ,  124  may be joined to the plates  102 ,  104  in other ways without departing from the scope of the present invention, in one embodiment the elements are adhesively bonded to the plates using a non-toxic adhesive having a suitable strength. In other embodiments, it is envisioned that other conventional fasteners and bonding materials may be used in addition to or instead of the adhesive. 
     Although the plates  102 ,  104  may be made of other materials without departing from the scope of the present invention, in one embodiment the plates are made from a cobalt alloy or a surgical steel. Although the compressible elements  122 ,  124  may be made of other materials without departing from the scope of the present invention, in one embodiment the compressible elements are made from polyurethane or other polymer. Although the compressible elements  122 ,  124  may be have other compressive stiffness modulii without departing from the scope of the present invention, in one embodiment the compressible elements have a compressive stiffness modulus approximately equal to a natural disc. In one embodiment, the compressible elements  122   124  have an undeformed thickness of between about 3 mm and about 15 mm, providing an unloaded vertical gap  124  between the concave end  114  of the upper plate protrusion  112  and the convex end  120  of the lower plate protrusion  118  of between about 0.1 mm and about 3 mm. 
     To install the implant  100 , the damaged disc  70  is removed using conventional surgical techniques. The implant  100  is inserted between the vertebrae  40 ,  42  once the disc  70  is removed. Depending upon the particular disc being replaced and other factors understood by those skilled in the art, the implant  100  may be inserted between the vertebrae from the rear or from the front. The implant  100  is anchored in place between the vertebrae  40 ,  42  using conventional techniques, including using adhesives, screws and integral fasteners. Unlike many conventional implants, which encourage the vertebrae  40 ,  42  to fuse, this implant  100  replaces the disc  60  and provides movement. 
       FIG. 5  illustrates an intervertebral implant of a second embodiment, generally designated by  130 . This implant  130  includes an upper plate  132  and a lower plate  134 . These plates  132 ,  134  are similar to those of the first embodiment but do not have protrusions. The implant  130  also includes a forward compressible element  136  and a rearward compressible element  138  behind the forward compressible element. Although the compressible elements  136 ,  138  may be made of other materials without departing from the scope of the present invention, in one embodiment the compressible elements are made from a polymer. Although the compressible elements  136 ,  138  may have other compressive stiffness modulii without departing from the scope of the present invention, in one embodiment the elements have a compressive stiffness modulus approximating that of a natural disc. In one embodiment, the compressible elements  136 ,  138  have uniform and equal undeformed thicknesses of between about 3 mm and about 15 mm. In addition, the implant  130  includes a flexible linkage  140  connecting the upper plate  132  and the lower plate  134  to limit relative motion between the plates. As will be appreciated by those skilled in the art, the configuration of the implant of the second embodiment  130  may provide different intervertebral motions than the configuration of the implant of the first embodiment  100 . Therefore, as will be appreciated by those skilled in the art, one configuration may have advantages over another configuration depending upon the specific application. Further, different components may be combined to provide a range of configurations. Because the implant  130  is identical to that of the first embodiment in all other respects, it will not be described in further detail. 
       FIG. 6  illustrates an intervertebral implant of a third embodiment, generally designated by  150 . The implant  150  includes an upper plate  152  and a lower plate  154 . The upper plate  152  includes a lower face  156  having forward and rearward protrusions  158 ,  160 , respectively, extending downward from the face. These protrusions  158 ,  160  have convex lower ends  162 . The lower plate  154  includes a lower face  164  having forward and rearward protrusions  166 ,  168 , respectively, extending upward from the face. These protrusions  166 ,  168  have concave upper ends  170  complementing and receiving the convex ends  162  of the upper plate protrusions  158 ,  160 , and allowing the upper and lower plates  152 ,  154  to pivot with respect to each other. This embodiment does not include compressible elements. Because other features of the implant  150  are identical to those of the first embodiment, they will not be described in further detail. 
     As shown in  FIG. 7 , an intervertebral implant of a fourth embodiment is designated in its entirety by the reference number  180 . The implant  180  includes an upper plate  182  and a lower plate  184 . The upper plate  182  includes a lower face  186  and a protrusion  188  having a concave lower end  190  extending downward from the lower face adjacent one end (e.g., a forward end)). The lower plate  184  includes an upper face  192  and a protrusion  194  having a convex upper end  196  extending upward from the upper face. The concave end  190  of the upper plate protrusion  188  receives the convex end  196  of the lower plate protrusion  194 , allowing the plates to pivot. Further, the implant  180  includes a compressible element  198  positioned between the upper and lower plates  182 ,  184  adjacent an end of the plates opposite the protrusions  188 ,  194 . Because other features of the implant  180  are identical to those of the first embodiment, they will not be described in further detail. 
       FIG. 8  shows an implant of a fifth embodiment, generally designated by  200 , that is identical to the implant of the first embodiment except that a compressible element  202  on one end has a greater thickness than a compressible element  204  on an opposite end. Although the compressible element  202  may be have other median thicknesses without departing from the scope of the present invention, in one embodiment the element has a median thickness of between about 3 mm and about 17 mm. Although the compressible element  204  may be have other median thicknesses without departing from the scope of the present invention, in one embodiment the element has a median thickness of between about 2 mm and about 10 mm. Further, although the elements  202 ,  204  may have other included angles without departing from the scope of the present invention, in one embodiment both elements have an included angle of between about 10 degrees and about 35 degrees. In one embodiment, the element  204  having the greater thickness is the forward element. 
       FIG. 9  shows an implant of a sixth embodiment, generally designated by  210 , that is identical to the implant of the second embodiment except that a compressible element  212  on one end has a greater thickness than a compressible element  214  on an opposite end. Although the compressible element  212  may be have other median thicknesses without departing from the scope of the present invention, in one embodiment the element has a median thickness of between about 3 mm and about 17 mm. Although the compressible element  214  may be have other median thicknesses without departing from the scope of the present invention, in one embodiment the element has a median thickness of between about 2 mm and about 12 mm. Further, although the elements  212 ,  214  may have other included angles without departing from the scope of the present invention, in one embodiment both elements have an included angle between about 10 degrees and about 35 degrees. In one embodiment, the element  214  having the greater thickness is the forward element. 
       FIG. 10  illustrates an implant of a seventh embodiment, generally designated by  220 , that is identical to the implant of the second embodiment except that the seventh embodiment has a lower plate  222  having a varying thickness. Although exterior surfaces of the plate  222  may have other included angles without departing from the scope of the present invention, in one embodiment the plate surfaces have an included angle between about 10 degrees and about 35 degrees. In one embodiment, the plate  222  has a greater thickness at its forward end. 
     As shown in  FIG. 11 , an implant of an eighth embodiment is generally designated  230 . This embodiment is similar to the implant of the second embodiment except it only has one compressible element  232  positioned between an upper plate  234  and a lower plate  236 . The element extends between a forward end and a rearward end (not shown) of the implant  230  and between opposite lateral sides  238  of the implant. In addition, the upper and lower plates  234 ,  236 , respectively, include longitudinal anchors  240  for anchoring each of the plates to its respective vertebra. In one embodiment, the upper plate  234  includes two spaced anchors  240  and the lower plate  236  includes one central anchor  240 . 
       FIG. 12  illustrates a ninth embodiment of an implant generally designated  250 . The ninth embodiment is similar to the second embodiment except it has an upper plate  252  including a rounded upper surface  254  and a lower plate  256  has a rounded lower surface  258  to increase a contact area with the corresponding vertebrae. The upper and lower plates  252 ,  256 , respectively, each include an opening  260  sized and positioned for receiving a screw fastener  262  for anchoring the plates to the respective vertebrae  40 ,  42 . In one embodiment, each of the plates  252 ,  256  include one opening  260 . In other embodiments, the plates  252 ,  256  may include more openings without departing from the scope of the present invention. As will be apparent to those skilled in the art, the implant  250  of the ninth embodiment may in installed without removing the strong ligament  264  extending between the vertebrae  40 ,  42 . 
       FIGS. 13-15  illustrate an implant of a tenth embodiment, generally designated by  270 . The implant  270  includes two upper plates  272 ,  274  overlapped at one end and arranged in a V-shape as shown. The implant  270  also includes two lower plates  276 ,  278  overlapped at one end and also arranged in a V-shape as shown. Although the upper and lower plates  272 ,  274 ,  276 ,  278  may be overlapped at other ends without departing from the scope of the present invention, in one embodiment, the plates are overlapped at their forward ends. A forward compressible element  280  is positioned between the upper and lower plates  272 ,  280 . Rearward compressible elements  282 ,  284  are positioned between respective rearward ends of the upper and lower plates as shown. The plates may be fastened together using conventional means such as interlocking parts, riveting or brazing. Although the upper plates  272 ,  274  and lower plates  276 ,  278  may be arranged to form other included angles, in one embodiment, both the upper and lower plates define included angles of between about 15 degrees and about 90 degrees. 
       FIGS. 16-18  illustrate an implant of an eleventh embodiment, generally designated by  290  including two upper plates  292 ,  294  and two lower plates  296 ,  298 . The upper plates  292 ,  294  are overlapped between their ends and arranged in an X-shape, and the lower plates  296 ,  298  are overlapped between their ends and arranged in an X-shape as shown. Forward compressible elements  300  are positioned between forward ends of the upper and lower plates  292 ,  294 ,  296 ,  298 , and rearward compressible elements  302  are positioned between rearward ends of the upper and lower plates. In one embodiment, the inner upper and lower plates  292 ,  298  have interengaging protrusions  304 ,  306  similar to the plates of the first embodiment. Because the implant  290  of the eleventh embodiment is similar to the implant  270  of the tenth embodiment in all other respects, the implant of the eleventh embodiment will not be described in further detail. 
     As will be appreciated by those skilled in the art, the tenth and eleventh embodiments may be assembled outside the patient and surgically implanted using conventional techniques. In another embodiment, at least a portion of a natural disc is removed from between an upper vertebra and a lower vertebra using conventional techniques. A first component of the implant (e.g., upper plate  272 ) is inserted between the upper vertebra and the lower vertebra. Then, a second component of the implant (e.g., upper plate  274 ) is inserted between the upper vertebra and the lower vertebra. Once both components are in place, they are connected together. As will be apparent to those skilled in the art, the first and second components may be inserted along different lines of entry, and those lines of entry may be obliquely oriented with respect to each other. In addition, other aspects and features of this method use conventional surgical techniques and will be understood by those skilled in the art from this description. 
       FIGS. 19-21  illustrate an implant of a twelfth embodiment generally designated by  310 . This implant  310  includes an upper portion (generally designated by  312 ) and a lower portion (generally designated by  314 ). The upper portion  312  includes two lobes  320 , and the lower portion  314  includes two lobes  322 . Each of the lobes  320 ,  322  has an opening  324  sized for receiving bone attachment elements such as conventional pedicle screws  326 . The openings  324  are positioned so the screws  326  may be anchored to the pedicles  46  of the vertebrae  40 ,  42 . The upper portion  312  includes a post or protrusion  330  extending downward from and centrally located between the lobes  320 . The lower portion  314  includes a central channel  332  defined by two ribs  334 . The channel  332  is sized and shaped for receiving the post  330  of the corresponding upper portion  312  so the post moves freely in the channel, allowing substantial motion between the vertebrae  40 ,  42 . The upper and lower portions  312 ,  314  may be made from any suitable material such as a cobalt alloy, surgical steel, carbon composite, ceramic or polymer and may be made in a variety of sizes to fit different size people. As illustrated in  FIG. 21 , portions of the vertebrae  40 ,  42  (e.g., portions of the spinous processes  52  and the lamina  48 ) may be removed to provide a planar surface upon which to mount the upper and lower portions,  312 ,  314  of the implant. 
       FIGS. 22 and 23  show a thirteenth embodiment of an implant, generally designated by  340 . This implant  340  is substantially identical to the twelfth embodiment except it includes a linkage or synthetic ligament  342  connecting its upper and lower portions  344 ,  346 , respectively. The linkage  342  allows the upper and lower portions  344 ,  346  to articulate, rotate, extend and contract, but prevents the portions from becoming disengaged. Although the linkage  342  may be made of other materials without departing from the scope of the present invention, in one embodiment the linkage is made of polyester. 
       FIGS. 24-27  show an implant  350  of a fourteenth embodiment of the present invention. As illustrated in  FIGS. 24 and 25 , an upper portion  352  of the fourteenth embodiment of the implant  350  is substantially identical to the upper portion  312  of the twelfth embodiment except it includes a bowtie-shaped opening  354  extending laterally across the post  356 . As shown in  FIG. 26 , a lower portion  358  of the fourteenth embodiment of the implant  350  is substantially identical to the lower portion  314  of the twelfth embodiment except it includes elastic or ligamentous elements  360 ,  362  extending across the channel  364 . Although the elastic or ligamentous elements  360 ,  362  may be made of other materials without departing from the scope of the present invention, in one embodiment the elements are made of polyester. When assembled as shown in  FIG. 27 , the post  356  of the upper portion  352  of the implant  350  is inserted in the channel  364  until the elastic elements  360 ,  362  are positioned in the opening  354  in the post to prevent the upper and lower portions from becoming disengaged and limiting excessive motion. Because the implant  350  is identical to the implant  310  of the twelfth embodiment in all other respects, it will not be described in further detail. 
       FIG. 28  shows an implant  370  of a fifteenth embodiment of the present invention. This implant  370  is substantially identical to the twelfth embodiment, having an upper portion  372  and a lower portion  374 . The lower portion  374  has a channel  376  having convex rounded sides shaped to receive a post  378  of the upper portion  372  and to allow the upper portion to pivot or tilt from side to side relative to the lower portion. Flexible or deformable components  380  are mounted on both sides of the post  378  to permit compliance and provide cushioning as the upper and lower portions move relative to each other. Although the components  380  may be made of other materials without departing from the scope of the present invention, in one embodiment the components are made of stainless steel. 
       FIG. 29   a  illustrates the upper and lower portions  372 ,  374  of the implant  370  in a neutral unloaded position. The flexible components  380  move with the upper portion  292  between a raised position shown in  FIG. 29   b  and a lowered position shown in  FIG. 29   c . The components  380  stretch and straighten as the upper and lower portions  372 ,  374  move away from the neutral position thereby increasing forces acting on the portions to move the portions back toward their neutral position. Thus, the components  380  bias the portions  372 ,  374  toward their neutral positions and limit relative movement. 
       FIG. 30  illustrates an implant  390  of a sixteenth embodiment of the present invention. This implant  390  is substantially identical to the twelfth embodiment, having an upper portion  392  and a lower portion  394 . The lower portion  394  has a channel  396  including concave rounded sides shaped to receive a post  398  of the upper portion  392  having convex sides and to allow the upper portion to pivot from side to side relative to the lower portion. Flexible or deformable components  400  are mounted on both sides of the channel  396  to permit compliance and provide cushioning as the upper and lower portions move relative to each other. Although the components  400  may be made of other materials without departing from the scope of the present invention, in one embodiment the components are made of stainless steel. 
       FIG. 31   a  shows the upper and lower portions  392 ,  394  of the implant  390  in a neutral unloaded position. The flexible or deformable components  400  move with the lower portion  394  between a raised position shown in  FIG. 31   b  and a lowered position (not shown). The components  400  deform as the upper and lower portions  392 ,  394  move away from the neutral position thereby increasing forces acting on the portions to move the portions back toward their neutral position. Thus, the components  400  bias the portions  392 ,  394  toward their neutral positions and limit relative movement. 
     When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.