Abstract:
An intervertebral device includes a first plate having an outer face and an inner face, and a second plate juxtaposed with the first plate, the second plate having an outer face, an inner face that opposes the first plate and a concavity that opposes the first plate. The device includes an elongated member extending from the first plate toward the second plate, the elongated member having a distal end with a spherical surface that is engageable with the concavity of the second plate for providing an articulating joint between the first and second plates. The device also includes a resilient member in contact with the elongated member for counteracting compressive loads on the plates, the resilient member being surrounded by the concavity of the second plate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/708,813, filed Feb. 20, 2007, now allowed, which is a continuation of U.S. Pat. No. 7,214,244, filed Feb. 11, 2005, which claims the benefit of U.S. Provisional Application No. 60/546,027, filed Feb. 19, 2004, the disclosures of which are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to a spinal implant assembly for implantation into the intervertebral space between adjacent vertebral bones to simultaneously provide stabilization and continued flexibility and proper anatomical motion, and more specifically to such a device that has limited rotation using an uncaptured ball and socket joint with a partial ball having a large radius and substantially continuous radii of curvature. 
     The bones and connective tissue of an adult human spinal column consists of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex that consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than twenty bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are the thoracic vertebrae, and connect to the lower spine comprising the five lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions. 
     The spinal column is highly complex in that it includes these more than twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. 
     Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art that achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification. 
     Referring now to  FIGS. 1-2 , in which a side perspective view of an intervertebral body cage and an anterior perspective view of a post implantation spinal column are shown, respectively, a more complete description of these devices of the prior art is herein provided. These cages  1  generally comprise tubular metal body  2  having an external surface threading  3 . They are inserted transverse to the axis of the spine  4 , into preformed cylindrical holes at the junction of adjacent vertebral bodies (in  FIG. 14  the pair of cages  1  are inserted between the fifth lumbar vertebra (L 5 ) and the top of the sacrum (S 1 )). Two cages  1  are generally inserted side by side with the external threading  4  tapping into the lower surface of the vertebral bone above (L 5 ), and the first surface of the vertebral bone (S 1 ) below. The cages  1  include holes  5  through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior  6  of the cage  1  to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage  1 . 
     These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient&#39;s spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution. 
     It is, therefore, an object of the invention to provide an intervertebral spacer that stabilizes the spine without promoting a bone fusion across the intervertebral space. 
     It is further an object of the invention to provide an implant device that stabilizes the spine while still permitting normal motion. 
     It is further an object of the invention to provide a device for implantation into the intervertebral space that does not promote the abnormal distribution of biomechanical stresses on the patient&#39;s spine. 
     It is further an object of the invention to provide an artificial disc that provides free rotation of the baseplates relative to one another. 
     It is further an object of the invention to provide an artificial disc that supports compression loads. 
     It is further an object of the invention to provide an artificial disc that permits the baseplates to axially compress toward one another under a compressive load. 
     It is further an object of the invention to provide an artificial disc that permits the baseplates to axially compress toward one another under a compressive load and restore to their original uncompressed relative positions when the compressive load is relieved. 
     It is further an object of the invention to provide an artificial disc that prevents lateral translation of the baseplates relative to one another. 
     It is further an object of the invention to provide an artificial disc that provides a centroid of motion centrally located within the intervertebral space. 
     It is further an object of the invention to provide artificial intervertebral disc baseplates having outwardly facing surfaces that conform to the concave surface of adjacent vertebral bodies. 
     It is a further object of the present invention to provide a disc replacement device having a first element for seating against a lower endplate surface of a superior vertebral body and a second element for seating against an first end plate surface of an inferior vertebral body, said baseplates having disposed therebetween a partial spherical member having a large radius disposed in a complementary concavity such that said baseplates are articulatable against one another. 
     It is yet a further object of the present invention to provide a disc replacement device that is resistant to point loading and fatigue failure. 
     It is still a further object of the present invention to provide a disc replacement device employing ball and socket type articulation using a partial spherical member wherein said partial spherical member is not captured. 
     Other objects of the invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter. 
     SUMMARY OF THE INVENTION 
     The preceding objects are achieved by the invention, which is an artificial intervertebral disc or intervertebral spacer device comprising a pair of support members (e.g., spaced apart baseplates), each with an outwardly facing surface. Because the artificial disc is to be positioned between the facing endplates of adjacent vertebral bodies, the baseplates are arranged in a substantially parallel planar alignment (or slightly offset relative to one another in accordance with proper lordotic angulation) with the outwardly facing surfaces facing away from one another. The baseplates are to mate with the vertebral bodies so as to not rotate relative thereto, but rather to permit the spinal segments to bend (and in some embodiments, axially compress) relative to one another in manners that mimic the natural motion of the spinal segment. This natural motion is permitted by the performance of a ball and socket type joint using a partial spherical member disposed between the secured baseplates, and the securing of the baseplates to the vertebral bone is achieved through the use of a vertebral body contact element attached to the outwardly facing surface of each baseplate. 
     Preferable vertebral body contact elements include, but are not limited to, one or more of the following: a convex mesh, a convex solid dome, and one or more spikes. The convex mesh is preferably secured at its perimeter to the outwardly facing surface of the respective baseplate. This can be accomplished in any effective manner, however, laser welding and plasma coating burying are two preferred methods when the mesh is comprised of metal. While domed in its initial undeflected conformation, the mesh deflects as necessary during insertion of the artificial disc between vertebral bodies, and, once the artificial disc is seated between the vertebral bodies, the mesh deforms as necessary under anatomical loads to reshape itself to the concave surface of the vertebral endplate. Thus, the mesh is deformably reshapeable under anatomical loads such that it conformably deflects against the concave surface to securably engage the vertebral body endplate. Stated alternatively, because the mesh is convexly shaped and is secured at its perimeter to the baseplate, the mesh is biased away from the baseplate but moveable toward the plate (under a load overcoming the bias; such a load is present, for example, as an anatomical load in the intervertebral space) so that it will securably engage the vertebral body endplate when disposed in the intervertebral space. This affords the baseplate having the mesh substantially superior gripping and holding strength upon initial implantation, as compared with other artificial disc products. The convex mesh further provides an osteoconductive surface through which the bone may ultimately grow. The mesh preferably is comprised of titanium, but can also be formed from other metals and/or non-metals. Inasmuch as the mesh is domed, it does not restrict the angle at which the artificial disc can be implanted. It should be understood that while the flexible dome is described herein preferably as a wire mesh, other meshed or solid flexible elements can also be used, including flexible elements comprised of non-metals and/or other metals. Further, the flexibility, deflectability and/or deformability need not be provided by a flexible material, but can additionally or alternatively be provided mechanically or by other means. 
     It should be understood that the convex mesh attachment devices and methods described herein can be used not only with the artificial discs and artificial disc baseplates described or referred to herein, but also with other artificial discs and artificial disc baseplates, including, but not limited to, those currently known in the art. Therefore, the description of the mesh attachment devices and methods being used with the artificial discs and artificial disc baseplates described or referred to herein should not be construed as limiting the application and/or usefulness of the mesh attachment device. 
     To enhance the securing of the baseplates to the vertebral bones, each baseplate further comprises a porous area, which at least extends in a ring around the lateral rim of each outwardly facing surface. The porous area may be, for example, a sprayed deposition layer, or an adhesive applied beaded metal layer, or another suitable porous coating known in the art. The porous ring permits the long-term ingrowth of vertebral bone into the baseplate, thus permanently securing the prosthesis within the intervertebral space. The porous layer may extend beneath the domed mesh as well, but is more importantly applied to the lateral rim of the outwardly facing surface of the baseplate that seats directly against the vertebral body. 
     Some of the embodiments described herein use two baseplates each having the above described convex mesh on its outwardly facing surface, while other embodiments use two baseplates each having a convex solid dome in combination with a plurality of spikes on the lateral rim of the outwardly facing surface of the baseplates. It should be understood, however, that the various attachments devices or methods described herein (as well as any other attachment devices or methods, such as, for example, keels) can be used individually or in combination in any permutation, without departing from the scope of the present invention. 
     The ball and socket joint, employing a partial spherical member that is not captured, disposed between the baseplates permits rotation and angulation of the two baseplates relative to one another about a centroid of motion centrally located between the baseplates. A variety of embodiments are contemplated. In some embodiments, the joint is used in conjunction with a resilient member to additionally permit the two baseplates to axially compress relative to one another. Further in each of the embodiments, the assembly prevents lateral translation of the baseplates during rotation and angulation. 
     It should be understood that the described embodiments and embodiment families are merely examples that illustrate aspects and features of the present invention, and that other embodiments and embodiment families are possible without departing from the scope of the invention. 
     Each of the embodiments discussed herein share the same basic elements, some of which retain identical functionality and configuration across the embodiments, and some of which gain or lose functionality and/or configuration across the embodiments to accommodate mechanical and/or manufacturing necessities. More specifically, each of the embodiments includes two baseplates, each having an inwardly directed articulation surface, having a ball and socket joint disposed therebetween employing an uncaptured partial spherical member that is established centrally between the baseplates. The partial spherical member has a large radius and substantially continuous arc of curvature to minimize point loading and reduce the risk and incidence of fatigue failure. Each of the embodiments will be understood further in light of the additional descriptions of the embodiments herein. 
     The inwardly directed articulation surface of the first baseplate is adapted such that extending thereform is a member having at its distal end a partial spherical member. The partial spherical member is defined by a convex arc that forms the articulation surface that is complementary to a concave articulation surface of the second baseplate. 
     In a preferred embodiment the longitudinally inwardly directed articulation surface of the first baseplate comprises essentially a centrally disposed projection having a central bore for receiving and/or retaining an elongated member. The projection is sized to have a diameter less than the diameter of the inwardly directed concave articulating surface of the second baseplate. The projection preferably has a cross section that is cylindrical or frustoconical. 
     In a preferred embodiment, the elongated member comprises essentially a mushroom-shaped pin having an elongated portion and a head portion, the elongated portion thereof seated in a central bore of the first baseplate and the head portion, located distally, having a convex arc having a substantially constant radius of curvature A. The pin shaped member may be fixedly engaged in the bore or may be slidably engaged in the bore. In the embodiment in which the pin is slidably engaged in the bore, in a preferred embodiment a resilient annular member such as a resilient washer or the like is optionally deployed over the projection of the first baseplate as a shock absorber, the resilient annular member being positioned with one side facing the surface adjacent the projection of the first member and the opposite side of the annular resilient member facing the interior of the head of the pin-shaped member. 
     The elongated portion of the pin member preferably comprises a continuous cylindrical cross section; however, the cross section may vary toward the distal end thereof, such as by gradually or abruptly thickening near the juncture of the elongated member and the head portion, to provide structural strength. 
     The longitudinally inwardly directed articulation surface of the second baseplate is a substantially constant radii concave articulation surface forming a curvate socket. 
     The constant radii articulation surfaces are configured and sized to be nestable against one another and articulatable against one another, to enable adjacent vertebral bones (against which the first and second baseplates are respectively disposed in the intervertebral space) to articulate in flexion, extension, and lateral bending. More particularly, the artificial disc implant of the present invention is assembled by disposing the first and second baseplates such that the vertebral body contact surfaces are directed away from one another, and the articulation surfaces are nested against one another such that the concave arc accommodates the convex arc. 
     The curvate socket defines a spherical contour that closely accommodates the partial spherical member for free rotation and angulation. Therefore, when seated in the curvate socket, the partial spherical member can rotate and angulate freely relative to the curvate socket through a range of angles, thus permitting the opposing baseplates to rotate and angulate freely relative to one another through a corresponding range of angles equivalent to the fraction of normal human spine rotation and angulation (to mimic normal disc rotation and angulation). Because the baseplates are made angulatable relative to one another by the partial spherical member being rotatably and angulatably coupled in the curvate socket, the disc assembly provides a centroid of motion within the sphere defined by the partial spherical member. Accordingly, the centroid of motion of the disc assembly remains centrally located between the vertebral bodies, similar to the centroid of motion in a healthy natural intervertebral disc. 
     Optionally, the end of the mushroom-shaped pin element proximal to the baseplate, and the bore in which it is located, may be covered by a vertebral body contact element disposed on or as the outside surface of the baseplate. In such an embodiment it is preferable to include such a vertebral body contact element disposed on or as the opposing baseplate for purposes of symmetry. Such contact elements are preferably contoured to match the contour of the surface it contacts in the intervertebral space. 
     In other preferred embodiments of the present invention, an intervertebral device includes a first plate having an outer face and an inner face, and a second plate juxtaposed with the first plate, the second plate having an outer face, an inner face that opposes the first plate and a concavity that opposes the first plate. The device preferably includes an elongated member extending from the first plate toward the second plate, the elongated member having a distal end with a spherical surface that is engageable with the concavity of the second plate for providing an articulating joint between the first and second plates. The device also desirably includes a resilient member in contact with the elongated member for counteracting compressive loads on the plates, whereby the resilient member is surrounded by the concavity of the second plate. 
     In other preferred embodiments of the present invention, an intervertebral device includes a first plate having an outer face and an inner face, a second plate juxtaposed with the first plate, the second plate having an outer face and an inner face that opposes the first plate, and a ball and socket articulating joint provided between the first and second plates. The device also preferably includes a resilient member in contact with the ball portion of the ball and socket articulating joint for counteracting compressive loads on the plates, whereby the resilient member extends between the first and second plates and is surrounded by the socket portion of the articulating joint. 
     In still other preferred embodiments of the present invention, an intervertebral device includes a first plate having an outer face and an inner face, a second plate juxtaposed with the first plate, the second plate having an outer face and an inner face that opposes the first plate, the inner face of the second plate having a concavity, and an elongated member extending from the inner face of the first plate toward the second plate, the elongated member being slideably attached to the first plate and having a distal end with a spherical surface that forms a ball and socket-like articulating joint between the first and second plates. The device may also include a resilient member in contact with the distal end of the elongated member for counteracting compressive loads on the plates. The concavity of the second plate desirably surrounds the resilient member. The elongated member may have a mushroom-shaped head at the distal end thereof. 
     These and other preferred embodiments of the present invention will be described in more detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a side perspective view of a prior art interbody fusion device. 
         FIG. 2  shows a front view of the anterior portion of the lumbo-sacral region of a human spine, into which a pair of interbody fusion devices of  FIG. 1  have been implanted. 
         FIG. 3  is a cross sectional view of a first embodiment of the present invention, the first baseplate having an inwardly directed articulating surface having extending therefrom a mushroom-shaped pin element having a partial spherical element at the distal end thereof and a second baseplate having a circular recess within which seats the convex structure of the partial spherical element of the first baseplate. 
         FIG. 4  is a cross-sectional view of a second embodiment of the present invention in which the pin element is slidably engaged in a central bore of the first baseplate and further includes a resilient member disposed between the first and second baseplates. 
         FIG. 5  is a cross-sectional view of a preferred embodiment of the present invention. 
         FIG. 6  is a cross-sectional view of a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     While the invention will be described more fully hereinafter with reference to the accompanying drawings, in which particular embodiments and methods of implantation are shown, it is to be understood at the outset that persons skilled in the art may modify the invention herein described while achieving the functions and results of the invention. Accordingly, the descriptions that follow are to be understood as illustrative and exemplary of specific structures, aspects and features within the broad scope of the invention and not as limiting of such broad scope. Like numbers refer to similar features of like elements throughout. 
     A preferred embodiment of the present invention will now be described. 
     Referring to  FIG. 3 , the invention is shown having a first baseplate  10  and a second baseplate  30  and a pin  50 . Each baseplate  10 , 30  has an outwardly facing surface  12 , 32 . Because the artificial disc of the invention is to be positioned between the facing surfaces of adjacent vertebral bodies, the two baseplates  10 , 30  used in the artificial disc are disposed such that the outwardly facing surfaces  12 , 32  face away from one another. The two baseplates  10 , 30  are to mate with the vertebral bodies so as to not rotate relative thereto, but rather to permit the spinal segments to bend relative to one another in manners that mimic the natural motion of the spinal segment. This motion is permitted by the performance of a ball and socket joint disposed between the secured baseplates  10 , 30 . The mating of the baseplates  10 , 30  to the vertebral bodies and the construction of the ball and socket joint are described below. 
     More particularly, each baseplate  10 , 30  is a plate (preferably made of a metal or metal alloy, such as, for example, cobalt-chromium or titanium) having an overall shape that conforms to the overall shape of the respective endplate of the vertebral body with which it is to mate. Further, each baseplate  10 , 30  comprises a vertebral body contact element  80 , 82  (e.g., a convex mesh, preferably oval in shape) that is attached to the outwardly facing surface  12 , 32  of the baseplate  10 , 30  to provide a vertebral body contact surface. The mesh  80 , 82  is secured at its perimeter to the outwardly facing surface  12 , 32  of the baseplate  10 , 30 . The mesh  80 , 82  is domed in its initial undeflected conformation, but deflects as necessary during insertion of the artificial disc between vertebral bodies, and, once the artificial disc is seated between the vertebral bodies, deforms as necessary under anatomical loads to reshape itself to the concave surface of the vertebral endplate. This affords the baseplate  10 , 30  having the mesh  80 , 82  substantially superior gripping and holding strength upon initial implantation as compared with other artificial disc products. The mesh  80 , 82  further provides an osteoconductive surface through which the bone may ultimately grow. The mesh  80 , 82  is preferably comprised of titanium, but can also be formed from other metals and/or non-metals without departing from the scope of the invention. 
     Each baseplate  10 , 30  may further comprises at least a lateral ring (not shown) that is osteoconductive, which may be, for example, a sprayed deposition layer, or an adhesive applied beaded metal layer, or another suitable porous coating. This porous ring permits the long-term ingrowth of vertebral bone into the baseplate  10 , 30 , thus permanently securing the prosthesis within the intervertebral space. It shall be understood that this porous layer may extend beneath the domed mesh  80 , 82  as well, but is more importantly applied to the lateral rim of the outwardly facing surface  12 , 32  of the baseplate  10 , 30  that seats directly against the vertebral body. 
     Each of the baseplates  10 , 30  comprises features that, in conjunction with other components described below, form the ball and socket joint. The first baseplate  10  includes an inwardly facing articulating surface  18  that includes a perimeter region  20  and a projection  22  protruding from the inwardly facing surface  18 . The projection  22  preferably has a cylindrical or frustoconical cross section. The projection  22  further includes an axial bore  26  that accepts a mushroom-shaped pin  50  (or rivet, plug, dowel, or screw). 
     The second baseplate  30  comprises an inwardly facing articulation surface  34  having a peripheral surface  36  and a curvate socket  38 , the socket  38  having a substantially constant radii concave articulation surface. 
     Pin  50  further comprises an elongated portion  52  and a head  54 , the head  54  having a convex arc having a substantially constant radius of curvature. The arc of head  54  is such that the sphere it defines has a large radius, thereby minimizing point loading and the risk of fatigue failure. 
     The projection  22  of baseplate  10  is sized to have a diameter at least a portion of which is less than the diameter of the socket  38 . The projection  22  preferably has a cross section that is cylindrical or frustoconical. 
     In a first embodiment, the elongated portion  52  of mushroom-shaped pin  50  is disposed in bore  26  of the baseplate  10  and the head  54  is nested in socket  38 . Pin  50  is fixedly engaged by force fitting, welding or the like in bore  26 . Head  54  is not captured in socket  38 . Baseplates  10  and  30  are at no time connected to each other in the ball and socket joint of the present invention. 
     Optionally, the end of pin  50  proximal to the baseplate  10 , and the bore  26 , are covered by a vertebral body contact element  80  disposed over the outside surface  12  of the baseplate  10 . In such an embodiment it is preferable to include a vertebral body contact element  82  on the baseplate  30  for purposes of symmetry. Such contact elements  80  and  82  are preferably contoured to match the contour of the surface it contacts in the intervertebral space. 
     Now referring to  FIG. 4 , in a preferred embodiment, pin  50  is slidably engaged in bore  26 . In this embodiment, in a preferred embodiment a resilient annular member  60  such as a resilient washer or the like is deployed over the projection  22  (which in this embodiment is preferably cylindrical) of the first baseplate  10  as a shock absorber, the resilient annular member  60  being sized and positioned such that it functions as a force restoring element (e.g., a spring) that provides axial cushioning to the device, by deflecting under a compressive load and restoring when the load is relieved. 
     Now referring to  FIGS. 5 and 6 , in other embodiments the elongated portion  52  of pin  50  preferably has a continuous cylindrical cross section; however, the cross section may vary toward the distal end thereof, such as by gradually or abruptly thickening near the juncture of the elongated member  52  and the head  54 , to provide structural strength and/or to provide a different location for resilient member  60 . Now referring to  FIG. 5 , in a preferred embodiment resilient member  60  is a continuous collar comprising a spring having a cylindrical cross section. It is desirable, but not essential, to use a spring as the resilient member  60  because of the ability of a spring to hold its diameter when subjected to compressive force. In a most preferred embodiment resilient member  60  is retained in a retainer  62 . Retainer  62  is formed of a resilient material such as but not limited to an elastomeric material. In this embodiment elongated member  52  has a frustoconical section  56  adjacent proximal head  54  such that resilient member  60  and retainer  62  are firmly engageable in a seat formed between the frustoconical section  56  of elongated portion  52  and the end  28  of projection  22 . As forces are applied to retainer  62 , the spring comprising resilient member  60  deforms outwardly such that its diameter increases. 
     In another embodiment, now referring to  FIG. 6 , resilient member  60  is an O-ring preferably formed of an elastomeric material. Retainer  62  is a collar such as a split collar having formed thereon an exterior groove  64  to accommodate secure mounting of a resilient member  60 . In this embodiment elongated member  52  has a frustoconical section  56  adjacent proximal head  54  such that resilient member  60  and retainer  62  are firmly engageable between the frustoconical section  56  of elongated portion  52  and the end  28  of projection  22 . As forces are applied to retainer  62 , the O-ring comprising resilient member  60  deforms outwardly such that its diameter increases. 
     The substantially constant radii articulation surfaces of the head  54  and socket  38  are configured and sized to be nestable against one another and articulatable against one another, to enable adjacent vertebral bones (against which the baseplates  10  and  30  are respectively disposed in the intervertebral space) to articulate in flexion, extension, and lateral bending. More particularly, the artificial disc implant of the present invention is assembled by disposing the baseplates  10  and  30  such that the vertebral body contact surfaces  80 , 82  are directed away from one another, and the articulation surfaces (head  54  and socket  38 ) are nested against one another such that the concave arc of socket  38  accommodates the convex arc of head  54 . 
     While there has been described and illustrated specific embodiments of an artificial disc, it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad spirit and principle of the invention. The invention, therefore, shall not be limited to the specific embodiments discussed herein.