PATENT ABSTRACT
A vertebral body contact element comprises a domed wire mesh convexly extendable from an orthopedic device for securing the orthopedic device to a vertebral body endplate. In preferred embodiments, the wire mesh is made of titanium and is laser-welded or buried in a plasma coating at its perimeter to the orthpedic device, and a porous coating is provided underneath the wire mesh. The wire mesh is deformably reshapeable under anatomical loads so that it conformably deflects against a concave surface of the vertebral body endplate to securably engage the endplate.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is a continuing application of U.S. patent application Ser. No. 10/256,160 (filed Sep. 26, 2002) entitled “Artificial Intervertebral Disc Having Limited Rotation Using a Captured Ball and Socket Joint With a Solid Ball and Compression Locking Post”, which is a continuing application of U.S. patent application Ser. No. 10/175,417 (filed Jun. 19, 2002) entitled “Artificial Intervertebral Disc Utilizing a Ball Joint Coupling”, which is a continuing application of U.S. patent application Ser. No. 10/151,280 (filed May 20, 2002) entitled “Tension Bearing Artificial Disc Providing a Centroid of Motion Centrally Located Within an Intervertebral Space”, which is a continuing application of both U.S. patent application Ser. No. 09/970,479 (filed Oct. 4, 2001) entitled “Intervertebral Spacer Device Utilizing a Spirally Slotted Belleville Washer Having Radially Extending Grooves” as well as U.S. patent application Ser. No. 10/140,153 (filed May 7, 2002) entitled “Artificial Intervertebral Disc Having a Flexible Wire Mesh Vertebral Body Contact Element”, the former being a continuing application of U.S. patent application Ser. No. 09/968,046 (filed Oct. 1, 2001) entitled “Intervertebral Spacer Device Utilizing a Belleville Washer Having Radially Extending Grooves” and the latter being a continuing application of both U.S. patent application Ser. No. 09/970,479 (detailed above) as well as U.S. patent application Ser. No. 10/128,619 (filed Apr. 23, 2002) entitled “Intervertebral Spacer Having a Flexible Wire Mesh Vertebral Body Contact Element”, which is a continuing application of both U.S. patent application Ser. No. 09/906,119 (filed Jul. 16, 2001) and entitled “Trial Intervertebral Distraction Spacers” as well as U.S. patent application Ser. No. 09/982,148 (filed Oct. 18, 2001) and entitled “Intervertebral Spacer Device Having Arch Shaped Spring Elements”. All of the above mentioned applications are hereby incorporated by reference herein in their respective entireties. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to spinal implant assemblies for implantation into the intervertebral space between adjacent vertebral bones, and more specifically to a flexible element for use as a vertebral body contact surface for an orthopedic device.  
         BACKGROUND OF THE INVENTION  
         [0003]    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.  
           [0004]    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 dose 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.  
           [0005]    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.  
           [0006]    Referring now to FIGS.  13 - 14 , 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 (L5) and the top of the sacrum (S1)). Two cages  1  are generally inserted side by side with the external threading  4  tapping into the lower surface of the vertebral bone above (L5), and the upper surface of the vertebral bone (S1) 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 .  
           [0007]    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.  
           [0008]    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.  
           [0009]    It is further an object of the invention to provide an implant device that stabilizes the spine while still permitting normal motion.  
           [0010]    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.  
           [0011]    It is further an object of the invention to provide an artificial disc that provides free rotation of the baseplates relative to one another.  
           [0012]    It is further an object of the invention to provide an artificial disc that provides limited rotation of the baseplates relative to one another.  
           [0013]    It is further an object of the invention to provide an artificial disc that supports compression loads.  
           [0014]    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.  
           [0015]    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.  
           [0016]    It is further an object of the invention to provide an artificial disc that supports tension loads.  
           [0017]    It is further an object of the invention to provide an artificial disc that prevents lateral translation of the baseplates relative to one another.  
           [0018]    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.  
           [0019]    It is further an object of the invention to provide an artificial disc baseplate attachment device (for attaching the baseplates of the artificial disc to the vertebral bones between which the disc is implanted) with superior gripping and holding strength upon initial implantation and thereafter.  
           [0020]    It is further an object of the invention to provide an artificial disc baseplate attachment device that deflects during insertion of the artificial disc between vertebral bodies.  
           [0021]    It is further an object of the invention to provide an artificial disc baseplate attachment device that conforms to the concave surface of a vertebral body.  
           [0022]    It is further an object of the invention to provide an artificial disc baseplate attachment device that does not restrict the angle at which the artificial disc can be implanted.  
           [0023]    It is further an object of the invention to provide an implant attachment device (for attaching the implant to bone) with superior gripping and holding strength upon initial implantation and thereafter.  
           [0024]    It is further an object of the invention to provide an implant attachment device that is deflectable.  
           [0025]    It is further an object of the invention to provide an implant attachment device that conforms to a concave bone surface.  
           [0026]    Other objects of the invention not explicitly stated will be set forth and will be more dearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.  
         SUMMARY OF THE INVENTION  
         [0027]    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 joint (and in some embodiments, a spring 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.  
           [0028]    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.  
           [0029]    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.  
           [0030]    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.  
           [0031]    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.  
           [0032]    Some of the embodiments described herein uses 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.  
           [0033]    The ball and socket joint 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 wide variety of embodiments are contemplated, some in which the ball and socket joint permits free relative rotation of the baseplates, and others in which the ball and socket joint limits relative rotation of the baseplates to a certain range. Further in some embodiments, the ball and socket joint is used in conjunction with a spring member to additionally permit the two baseplates to axially compress relative to one another. Further in each of the embodiments, the assembly will not separate under tension loading, and prevents lateral translation of the baseplates during rotation and angulation.  
           [0034]    More particularly, four embodiment families are described herein as examples of the present invention, with a preferred embodiment for the first embodiment family, a preferred embodiment for the second embodiment family, five preferred embodiments for the third embodiment family, and five embodiments for the fourth embodiment family, each being described in detail. However, 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.  
           [0035]    Each of the embodiments in the four embodiment families 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 joined to one another by a ball and socket joint that is established centrally between the baseplates. Each ball and socket joint is established by a socket being formed at the peak (or in the peak) of a convex structure extending from the second baseplate, and by a ball being secured to the first baseplate and being captured in the socket so that when the joint is placed under a tension or compression force, the ball remains rotatably and angulatably secure in the socket. However, the convex structure is configured differently in each of the embodiment families, and the manner in which the ball is captured in the socket is different in each of the embodiment families. Each of these two variations (the configuration of the convex structure and the manner of capturing the ball in the socket) among the embodiments families is summarized immediately below, and will be understood further in light of the additional descriptions of the embodiments herein. It should be noted that although each of the embodiment families uses a preferred shape for the convex structure (e.g., in the first and second embodiment families, the preferred shape is frusto-conical, and in the third and fourth embodiment families, the preferred shape is a shape having a curved taper), the convex structure in each of the embodiment families is not limited to a particular shape. For example, shapes including, but not limited to, frusto-conical, hemispherical or semispherical shapes, shapes having sloped tapers or curved tapers, or shapes having non-uniform, irregular or dimensionally varying tapers or contours, would also be suitable in any of the embodiment families.  
           [0036]    With regard to the first embodiment family, the convex structure is configured as a flexible element and functions as a spring element that provides axial cushioning to the device. The convex structure has the socket of the ball and socket joint at its peak. In order to permit the flexible convex structure to flex under compressive loads applied to the device, it is separated from the second baseplate. In the preferred embodiment, the flexible convex structure is a belleville washer that has a frusto-conical shape. Other flexible convex structures are also contemplated as being suitable, such as, for example, convex structures that flex because of the resilience of the material from which they are made, because of the shape into which they are formed, and/or or because of the mechanical interaction between sub-elements of an assembly forming the convex structure. Although the convex structure is a separate element from the second baseplate in this embodiment family (because it must be allowed to flex), it is preferably maintained near the second baseplate so that the device does not separate in tension. Therefore, an extension of the second baseplate is provided (in the form of a shield element) to cover enough of the convex structure to so maintain it. Stated alternatively, the shield is a separate element from the second baseplate to ease manufacturing (during assembly, the flexible convex structure is first placed against the second baseplate, and then the shield is placed over the convex structure and secured to the second baseplate so that the convex structure is maintained between the second baseplate and the shield), but once the device is assembled, the second baseplate and the shield are effectively one element. That is, the second baseplate and shield can be considered to be a single integral housing within which the separate flexible convex structure flexes, because but for the sake of achieving desirable manufacturing efficiencies, the second baseplate and shield would be one piece.  
           [0037]    Also with regard to the first embodiment family, the manner of capturing the ball in the socket is effected by the ball being selectively radially compressible. That is, the ball is radially compressible to fit into the socket and thereafter receives a deflection preventing element to prevent subsequent radial compression, so that the ball remains captured in the socket. A more detailed description of the preferred manner in which this is accomplished is described below. Because the socket is formed at the peak of the flexible convex structure discussed immediately above, the capturing of the ball in the socket in this manner allows the ball to remain securely held for rotation and angulation even though the socket moves upward and downward with the flexing of the convex structure. The second baseplate preferably includes an access hole that facilitates the capture of the ball in the socket; in this embodiment family, it facilitates the capture by accommodating placement of the deflection preventing element, so that the same can be applied to the ball after the ball is fitted into the socket. Accordingly, the ball is maintained in the socket.  
           [0038]    With regard to the second embodiment family, the convex structure is configured as a non-flexible element that is integral with the second baseplate, and has the socket of the ball and socket joint at its peak. More clearly stated, the devices of this second embodiment family do not feature a flexible convex structure, and therefore (and also because of the manner in which the ball is captured in this second embodiment family, discussed immediately below) there is no need for the convex structure to be a separate element from the second baseplate. (By contrast, in the first embodiment family, as discussed above, because the convex structure is flexible, it is a separate element than the second baseplate so that it is able to flex.) In the preferred embodiment, the convex structure has a frusto-conical shape. The manner of capturing the ball in the socket in this second embodiment family is identical to that of the first embodiment family.  
           [0039]    With regard to the third embodiment family, the convex structure is configured as a non-flexible element that is integral with the second baseplate, and has the socket of the ball and socket joint in its peak, similar to the configuration of the convex structure in the second embodiment family. In the preferred embodiment, the convex structure is shaped to have a curved taper. The manner of capturing the ball in the socket of this third embodiment family is effected through the use of a solid ball. In order to permit the seating of the ball into the socket, the second baseplate has an access hole that facilitates the capture of the ball in the socket; in this embodiment family, the access hole facilitates the capture in that it has a diameter that accommodates the diameter of the ball, and leads to the interior of the peak, which interior is formed as a concavity having an opening diameter that accommodates the diameter of the ball. (Preferably, the concavity has a curvature closely accommodating the contour of the ball, and the concavity is either hemispherical or less-than-hemispherical so that the ball can easily be placed into it.) Further, in order to maintain the ball in the socket, an extension of the second baseplate (in the form of a cap element) is provided for sealing the access hole in the second baseplate (or reducing the opening diameter of the access hole to a size that does not accommodate the diameter of the ball). The cap has an interior face that preferably has a concavity (that has a curvature that closely accommodates the contour of the ball) to complete the socket. The peak of the convex structure also has a bore that accommodates a post to which the ball and the first baseplate are attached (one to each end of the post), but does not accommodate the ball for passage through the bore. Accordingly, the ball is maintained in the socket.  
           [0040]    With regard to the fourth embodiment family, the convex structure is configured as a non-flexible element that is a separate element from, but attached to, the second baseplate, and has the socket of the ball and socket joint in its peak. In the preferred embodiment, the convex structure is shaped to have a curved taper, similar to the configuration of the convex structure in the third embodiment family. The convex structure in this fourth embodiment family is separate from the second baseplate during assembly of the device, for reasons related to the manner in which the ball is captured in the socket, but is attached to the second baseplate by the time assembly is complete. The manner of capturing the ball in the socket of this fourth embodiment family is effected through the use of a solid ball. The ball is first seated against the central portion of the second baseplate (which central portion preferably has a concavity that has a curvature that closely accommodates the contour of the ball), and then the convex structure is placed over the ball to seat the ball in the socket formed in the interior of the peak of the convex structure (the interior is preferably formed as a concavity that is either hemispherical or less-than-hemispherical so that the ball can easily fit into it). After the convex structure is placed over the ball, the convex structure is attached to the second baseplate to secure the ball in the socket. As in the third embodiment family, the peak of the convex structure also has a bore that accommodates a post to which the ball and the first baseplate are attached (one to each end of the post), but does not accommodate the ball for passage through the bore. Accordingly, the ball is maintained in the socket.  
           [0041]    It should be understood that each of the features of each of the embodiments described herein, including, but not limited to, formations and functions of convex structures, manners of capturing the ball in the socket, types of spring elements, and manners of limiting rotation of the baseplates relative to one another, can be included in other embodiments, individually or with one or more others of the features, in other permutations of the features, including permutations that are not specifically described herein, without departing from the scope of the present invention.  
           [0042]    Each of the embodiment families will now be summarized in greater detail.  
           [0043]    In the first embodiment family, the ball and socket joint includes a radially compressible ball (which, in some embodiments, is shaped as a semisphere), mounted to protrude from an inwardly facing surface of a first baseplate, and a curvate socket formed at a peak of a flexible convex structure that is flexibly maintained near a second baseplate, within which curvate socket the ball is capturable for free rotation and angulation therein. Because the convex structure is flexible, 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. The flexible convex structure is preferably a belleville washer that has a frusto-conical shape. In general, a belleville washer is one of the strongest configurations for a spring, and is highly suitable for use as a restoring force providing element in an artificial intervertebral disc which must endure considerable cyclical loading in an active human adult.  
           [0044]    Belleville washers are washers that are generally bowed in the radial direction (e.g., have a hemispherical or semispherical shape) or sloped in the radial direction (e.g., have a frusto-conical shape). Bowed belleville washers have a radial convexity (i.e., the height of the washer is not linearly related to the radial distance, but may, for example, be parabolic in shape). In a sloped belleville washer, the height of the washer is linearly related to the radial distance. Of course, other shape variations of belleville washers are suitable (such as, but not limited to, belleville washers having non-uniform tapers or irregular overall shapes). The restoring force of a belleville washer is proportional to the elastic properties of the material. In addition, the magnitude of the compressive load support and the restoring force provided by the belleville washer may be modified by providing slots and/or grooves in the washer. The belleville washer utilized as the force restoring member in the illustrated embodiment is spirally slotted, with the slots initiating on the periphery of the washer and extending along arcs that are generally radially inwardly directed a distance toward the center of the bowed disc, and has radially extending grooves that decrease in width and depth from the outside edge of the washer toward the center of the washer. As a compressive load is applied to a belleville washer, the forces are directed into a hoop stress that tends to radially expand the washer. This hoop stress is counterbalanced by the material strength of the washer, and the strain of the material causes a deflection in the height of the washer. Stated equivalently, a belleville washer responds to a compressive load by deflecting compressively, but provides a restoring force that is proportional to the elastic modulus of the material in a hoop stressed condition. With slots and/or grooves formed in the washer, it expands and restores itself far more elastically than a solid washer.  
           [0045]    In order to permit the flexible convex structure to flex under compressive loads applied to the device, it is a separate element from the second baseplate in the preferred embodiment. To provide room for the flexible convex structure to expand in unrestricted fashion when it is compressed, while generally maintaining the flexible convex structure within a central area near the second baseplate, the wide end of the flexible convex structure is housed in the second baseplate through the use of an extension of the second baseplate structure (in the form of a shield element that is secured to the second baseplate). More particularly, a circular recess is provided on an inwardly facing surface of the second baseplate, and the wide end of the flexible convex structure is seated into the recess. The extension of the second baseplate (e.g., a shield) is placed over the flexible convex structure to cover enough of the convex structure to prevent it from escaping the recess, and then is attached to the second baseplate. As stated above, the shield is a separate element from the second baseplate to ease manufacturing, but once the device is assembled, the second baseplate and the shield are effectively one element. That is, the second baseplate and shield can be considered to be a single integral housing within which the separate flexible convex structure flexes, because but for the sake of achieving desirable manufacturing efficiencies, the second baseplate and shield would be one piece.  
           [0046]    More particularly with regard to the ball, the ball includes a series of slots that render it radially compressible and expandable in correspondence with a radial pressure. The ball further includes an axial bore that accepts a deflection preventing element (e.g., a rivet). Prior to the insertion of the rivet, the ball can deflect radially inward because the slots will narrow under a radial pressure. The insertion of the rivet eliminates the capacity for this deflection. Therefore, the ball, before receiving the rivet, can be compressed to pass into, and thereafter seat in, the curvate socket of the second baseplate. (The curvate socket has an opening diameter that accommodates passage therethrough of the ball in a radially compressed state (but not in an uncompressed state), and a larger inner diameter that accommodates the ball in the uncompressed state.) Once the ball has been seated in the curvate socket, the rivet can be inserted into the axial bore to ensure that the ball remains held in the curvate socket. The second baseplate preferably includes an access hole that accommodates placement of the deflection preventing element, so that the same can be applied to the ball after the ball is fitted into the socket  
           [0047]    The curvate socket defines a spherical contour that closely accommodates the ball for free rotation and angulation in its uncompressed state. Therefore, when seated in the curvate socket, the ball 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). The flexible convex structure serving as a force restoring device further provides spring-like performance with respect to axial compressive loads, as well as long cycle life to mimic the axial biomechanical performance of the normal human intervertebral disc. Because the ball is held within the curvate socket by a rivet in the axial bore preventing radial compression of the protuberance, the artificial disc can withstand tension loading of the baseplates—the assembly does not come apart under normally experienced tension loads. Thus, in combination with the securing of the baseplates to the adjacent vertebral bones via the mesh domes, the disc assembly has an integrity similar to the tension-bearing integrity of a healthy natural intervertebral disc. Also because the ball is laterally captured in the curvate socket, lateral translation of the baseplates relative to one another is prevented during rotation and angulation, similar to the performance of healthy natural intervertebral disc. Because the baseplates are made angulatable relative to one another by the ball being rotatably and angulatably coupled in the curvate socket, the disc assembly provides a centroid of motion within the sphere defined by the ball. 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.  
           [0048]    In the second embodiment family, the ball and socket joint includes a radially compressible ball (or in some embodiments, a semisphere) mounted to protrude from an inwardly facing surface of a first baseplate, and a curvate socket formed at a peak of a non-flexible convex structure that is integral with a second baseplate, within which curvate socket the ball is capturable for free rotation and angulation therein. Because the convex structure is not flexible, it does not serve as a force restoring element (e.g., a spring). In the preferred embodiment, the convex structure has a frusto-conical shape. The formation of the curvate socket, the configuration of the ball for use therewith, and the manner in which the ball is captured in the socket, are preferably identical to that of the first embodiment family. Accordingly, the embodiments of the second embodiment family enjoy the characteristics and performance features of the embodiments of the first embodiment family, except for the axial cushioning.  
           [0049]    In the third embodiment family, the ball and socket joint includes a solid ball (which, in some embodiments, is shaped as a semisphere) mounted to protrude from an inwardly facing surface of a first baseplate, and a curvate socket formed in a peak of a non-flexible convex structure that is integral with a second baseplate, within which curvate socket the ball is capturable for free rotation and angulation therein. In the preferred embodiment, the convex structure is shaped to have a curved taper. With regard to the mounting of the ball, the mounting includes a central post. A tail end of the post is (as a final step in the preferred assembly process) secured within a bore through the first baseplate, from the inwardly facing surface of the first baseplate to its outwardly facing surface. The ball is mounted at a head end of the post. The curvate socket defines a spherical contour, and is formed by opposing curvate pockets, one formed on a central portion of an outwardly facing surface of the convex structure and one formed on an inwardly facing surface of an extension of the second baseplate (the extension being in the form of a cap element) that secures to the outwardly facing surface of the second baseplate. When the cap is secured to the outwardly facing surface of the second baseplate, the opposing curvate pockets together form the curvate socket within which the ball freely rotates and angulates. Each curvate pocket is semispherically (preferably hemispherically) contoured to closely accommodate the spherical contour defined by the ball, so that the ball can freely rotate in the socket about the longitudinal axis of the post, and can freely angulate in the socket about a centroid of motion located at the center of the sphere defined by the ball.  
           [0050]    In order to enable the seating of the ball into the curvate socket, the access hole in the second baseplate leading to the outwardly facing surface of the convex structure has a diameter that accommodates the diameter of the ball, and the curvate pocket on the outwardly facing surface of the convex structure has an opening diameter that accommodates the ball for seating in the pocket. Thus, the ball can be placed through the access hole and into the curvate pocket. Thereafter, the cap is applied to seal the access hole in the second baseplate (or reduce the diameter of the access hole to a size that does not accommodate the diameter of the ball). With regard to the attachment of the post to the first baseplate, the peak of the convex structure has a central bore that accommodates the diameter of the post, but not the diameter of the ball. Therefore, as the ball is being placed into the curvate pocket on the outwardly facing surface of the convex structure, the post fits through the bore, but the ball does not. After the cap is secured, the tail end of the post that is protruding from the bore is secured to the inwardly facing surface of the first baseplate by the tail end of the post preferably compression locking into a central bore in the first baseplate.  
           [0051]    In some embodiments of the third embodiment family, the cap element includes a spring member, preferably disposed on the curvate pocket or between the curvate pocket and the remaining structure of the cap element. The spring member can be attached to the curvate pocket and/or the remaining structure of the cap element, or the spring member can be a separate element that is captured or maintained at least in part between the curvate pocket and the remaining structure of the cap element (in which embodiment the cap element may include multiple pieces). While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. Accordingly, in such embodiments, part or all of a compressive load applied to the baseplates will be borne by the spring member, which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
           [0052]    Accordingly, the baseplates are rotatable relative to one another because the ball rotates freely within the socket, and angulatable relative to one another because the ball angulates freely within the socket. (In the embodiments further having the spring member, the baseplates are also axially compressible relative to one another.) Because the ball is held within the socket by the securing of the tail end of the post to the first baseplate and the securing of the cap to the second baseplate, the artificial disc can withstand tension loading of the baseplates—the assembly does not come apart under normally experienced tension loads. Thus, in combination with the securing of the baseplates to the adjacent vertebral bones, the disc assembly has an integrity similar to the tension-bearing integrity of a healthy natural intervertebral disc. Also because the ball is laterally captured in the socket, lateral translation of the baseplates relative to one another is prevented during rotation and angulation, similar to the performance of healthy natural intervertebral disc. Because the baseplates are made angulatable relative to one another by the ball being rotatably and angulatably coupled in the socket, the disc assembly provides a centroid of motion within the ball. 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.  
           [0053]    Some embodiments in the third embodiment family limit the rotation (but preferably not the angulation) of the ball in the socket. Each embodiment accomplishes this in a different manner, but each embodiment utilizes interference between a protrusion and a recess to limit the rotation. In some embodiments, the protrusion is preferably hemispherical, and the recess preferably has a semicylindrical contour within which the protrusion fits. In other embodiments, the protrusion is preferably hemispherical, and the recess preferably has a curvate contour that is not semicylindrical. (It should be understood that the described formations of the recess and the protrusion are merely preferred, and that alternate formations, curvate or otherwise, for each are contemplated by the present invention; a particular shape or location of recess or a particular shape or location of protrusion is not required; any shape can be used so long as the recess and protrusion interact as desired.) The boundaries of the recess define the limits of rotation of the ball within the socket, by allowing movement of the protrusion relative to the recess as the ball rotates through a certain range in the socket, but providing interference with the protrusion to prevent rotation of the ball beyond that range in the socket. At the same time, the boundaries of the recess preferably do not limit the angulation of the ball within the socket, at least until the perimeter regions of the inwardly facing surfaces meet.  
           [0054]    More particularly with respect to the manner in which these embodiments limit rotation, in some embodiments the ball has a protrusion that interferes with a recess adjacent the socket, the recess being formed by a curvate recess adjacent the curvate pocket on the central portion of the outwardly facing surface of the convex structure and a curvate recess adjacent the curvate pocket on the cap. In other embodiments, the housing (e.g., the second baseplate/convex structure and/or the cap) has a protrusion (e.g., a hemispherical protrusion or a hemispherical head of a pin secured in a pin hole in the housing) that interferes with a recess on the ball. In still other embodiments, each of the housing (e.g., the second baseplate/convex structure and/or the cap) and the ball has a recess, and a ball bearing fits within the recesses, so that the ball bearing functions as a protrusion that interferes with one or both of the recesses.  
           [0055]    Therefore, when assembled, these embodiments of the third embodiment family enable angulation and limited rotation of the baseplates relative to one another about a centroid of motion that remains centrally located between the baseplates (at the center of the sphere defined by the ball), similar to the centroid of motion in a healthy natural intervertebral disc that is limited in its rotation by surrounding body structures. A benefit of limiting the relative rotation of the baseplates is that relative rotation beyond a certain range in a healthy natural disc is neither needed nor desired, because, for example, excess strain can be placed on the facet joints or ligaments thereby. As described with the rotationally free embodiments of the second embodiment family, the construction also prevents translation and separation of the baseplates relative to one another during rotation and angulation.  
           [0056]    In the fourth embodiment family, the ball and socket joint includes a solid ball (which, in some embodiments, is shaped as a semisphere) mounted to protrude from an inwardly facing surface of a first baseplate, and a curvate socket formed in a peak of a non-flexible convex structure that is attached to an inwardly facing surface of a second baseplate, within which curvate socket the ball is capturable for free rotation and angulation therein. In the preferred embodiment, the convex structure is shaped to have a curved taper. With regard to the mounting of the ball, the mounting includes a central post that extends from the inwardly facing surface of the first baseplate. The ball is (as a final step in the preferred assembly process) mounted at a head end of the post, by the head end preferably compression locking into a central bore in the ball. The curvate socket defines a spherical contour, and is formed by opposing curvate pockets, one formed on an inwardly facing surface of the second baseplate, and one formed as a curvate tapered lip of a central bore that passes through a central portion of the convex structure from the convex structure&#39;s outwardly facing surface (having the curvate tapered lip) to its inwardly facing surface. When the convex structure is secured to the inwardly facing surface of the second baseplate, the opposing curvate pockets together form the curvate socket within which the ball freely rotates and angulates. Each curvate pocket is semispherically (preferably hemispherically) contoured to closely accommodate the spherical contour defined by the ball, so that the ball can freely rotate in each pocket about the longitudinal axis of the post, and can freely angulate in each pocket about a centroid of motion located at the center of the sphere defined by the ball.  
           [0057]    In order to enable the seating of the ball into the curvate socket, the curvate pocket on the inwardly facing surface of the second baseplate has an opening diameter that accommodates the ball for seating in the pocket. Thus, the ball can be placed into the curvate pocket before the convex structure is attached to the second baseplate. Thereafter, the convex structure is attached to the inwardly facing surface of the second baseplate with the convex structure&#39;s curvate pocket (the curvate tapered lip of the convex structure&#39;s central bore) fitting against the ball to complete the ball and socket joint. With regard to completing the assembly, the central bore of the convex structure has a diameter that accommodates the diameter of the post, but not the diameter of the ball. Therefore, after the ball is secured in the curvate socket, the post fits through the bore so that the head end of the post can be compression locked to the ball, but the ball is prevented from escaping the socket through the central bore of the convex structure.  
           [0058]    In some embodiments of the fourth embodiment family, the second baseplate includes a spring member, preferably disposed on the curvate pocket or between the curvate pocket and the remaining structure of the second baseplate. The spring member can be attached to the curvate pocket and/or the remaining structure of the second baseplate, or the spring member can be a separate element that is captured or maintained at least in part between the curvate pocket and the remaining structure of the second baseplate (in which embodiment the second baseplate may include multiple pieces). While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. Accordingly, in such embodiments, part or all of a compressive load applied to the baseplates will be borne by the spring member, which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
           [0059]    Accordingly, the baseplates are rotatable relative to one another because the ball rotates freely within the socket, and angulatable relative to one another because the ball angulates freely within the socket. (In the embodiments further having the spring member, the baseplates are also axially compressible relative to one another.) Because the ball is held within the socket by the securing of the central post of the first baseplate to the ball and the securing of the convex structure to the second baseplate, the artificial disc can withstand tension loading of the baseplates—the assembly does not come apart under normally experienced tension loads. Thus, in combination with the securing of the baseplates to the adjacent vertebral bones, the disc assembly has an integrity similar to the tension-bearing integrity of a healthy natural intervertebral disc. Also because the ball is laterally captured in the socket, lateral translation of the baseplates relative to one another is prevented during rotation and angulation, similar to the performance of healthy natural intervertebral disc. Because the baseplates are made angulatable relative to one another by the ball being rotatably and angulatably coupled in the socket, the disc assembly provides a centroid of motion within the sphere defined by the ball. 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.  
           [0060]    Some embodiments in the fourth embodiment family limit the rotation (but preferably not the angulation) of the ball in the socket formed by the curvate taper of the convex structure and the hemispherical contour of the curvate pocket of the second baseplate. Each embodiment accomplishes this in a different manner, but each embodiment utilizes interference between a protrusion and a recess to limit the rotation, similar to the manner in which such interference is utilized in the third embodiment family. In some embodiments, the protrusion is preferably hemispherical, and the recess preferably has a semicylindrical contour within which the protrusion fits. In other embodiments, the protrusion is preferably hemispherical, and the recess preferably has a curvate contour that is not semicylindrical. (It should be understood that the described formations of the recess and the protrusion are merely preferred, and that alternate formations, curvate or otherwise, for each are contemplated by the present invention; a particular shape or location of recess or a particular shape or location of protrusion is not required; any shape can be used so long as the recess and protrusion interact as desired.) The boundaries of the recess define the limits of rotation of the ball within the socket, by allowing movement of the protrusion relative to the recess as the ball rotates through a certain range in the socket, but providing interference with the protrusion to prevent rotation of the ball beyond that range in the socket. At the same time, the boundaries of the recess preferably do not limit the angulation of the ball within the socket, at least until the perimeter regions of the inwardly facing surface of the convex structure and the inwardly facing surface of the first baseplate meet.  
           [0061]    More particularly with respect to the manner in which these embodiments limit rotation, in some embodiments the ball has a protrusion that interferes with a recess adjacent the socket, the recess being formed by a curvate recess adjacent the curvate pocket on the second baseplate and a curvate recess adjacent the curvate taper on the convex structure. In other embodiments, the housing (e.g., the second baseplate and/or the convex structure) has a protrusion (e.g., a hemispherical protrusion or a hemispherical head of a pin secured in a pin hole in the housing) that interferes with a recess on the ball. In still other embodiments, each of the housing (e.g., the second baseplate and/or the convex structure) and the ball has a recess, and a ball bearing fits within the recesses, so that the ball bearing functions as a protrusion that interferes with one or both of the recesses.  
           [0062]    Therefore, when assembled, these embodiments of the fourth embodiment family enable angulation and limited rotation of the baseplates relative to one another about a centroid of motion that remains centrally located between the baseplates (at the center of the sphere defined by the ball), similar to the centroid of motion in a healthy natural intervertebral disc that is limited in its rotation by surrounding body structures. A benefit of limiting the relative rotation of the baseplates is that relative rotation beyond a certain range in a healthy natural disc is neither needed nor desired, because, for example, excess strain can be placed on the facet joints or ligaments thereby. As described with the rotationally free embodiments of the third embodiment family, the construction also prevents translation and separation of the baseplates relative to one another during rotation and angulation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0063]    [0063]FIGS. 1 a - c  show top (FIG. 1 a ), side cutaway (FIG. 1 b ) and bottom (FIG. 1 c ) views of a first baseplate of a first embodiment family of the present invention, the first baseplate having an inwardly directed radially compressible ball.  
         [0064]    [0064]FIGS. 1 d - f  show top (FIG. 1 d ), side cutaway (FIG. 1 e ) and bottom (FIG. 1 f ) views of a second baseplate of the first embodiment family, the second baseplate having a circular recess within which seats a flexible convex structure.  
         [0065]    [0065]FIGS. 1 g - h  show side cutaway (FIG. 1 g ) and top perspective (FIG. 1 h ) views of a flexible convex structure of the first embodiment family, the flexible convex structure having spiral slots and radially extending grooves.  
         [0066]    [0066]FIGS. 1 i - j  show exploded (FIG. 1 i ) and assembled (FIG. 1 j ) views of a preferred embodiment of the first embodiment family.  
         [0067]    [0067]FIGS. 2 a - c  show top (FIG. 2 a ), side cutaway (FIG. 2 b ) and bottom (FIG. 2 c ) views of a first baseplate of a second embodiment family of the present invention, the first baseplate having an inwardly directed radially compressible ball.  
         [0068]    [0068]FIGS. 2 d - f  show top (FIG. 2 d ), side cutaway (FIG. 2 e ) and bottom (FIG. 2 views of a second baseplate of the second embodiment family, the second baseplate having a curvate socket within which the ball is capturable for free rotation and angulation therein.  
         [0069]    [0069]FIGS. 2 g - h  show exploded (FIG. 2 g ) and assembled (FIG. 2 h ) views of a preferred embodiment of the second embodiment family.  
         [0070]    [0070]FIGS. 3 a - e  show top (FIG. 3 a ), side (FIG. 3 b ), side cutaway (FIG. 3 c ), perspective cutaway (FIG. 3 d ) and perspective (FIG. 3 e ) views of a first baseplate of a third embodiment family of the present invention.  
         [0071]    [0071]FIGS. 3 f - j  show top (FIG. 3 f ), side (FIG. 3 g ), side cutaway (FIG. 3 h ), perspective cutaway (FIG. 3 i ) and perspective (FIG. 3 j ) views of a first type of a second baseplate of the third embodiment family, the first type of second baseplate having a convex structure of the third embodiment family integrated therewith.  
         [0072]    [0072]FIGS. 3 k - o  show top (FIG. 3 k ), side (FIG. 31), side cutaway (FIG. 3 m ), perspective cutaway (FIG. 3 n ) and perspective (FIG. 3 o ) views of a first type of a ball of the third embodiment family.  
         [0073]    [0073]FIGS. 3 p - t  show top (FIG. 3 p ), side (FIG. 3 q ), side cutaway (FIG. 3 r ), perspective cutaway (FIG. 3 s ) and perspective (FIG. 3 t ) views of a first type of a cap of the third embodiment family.  
         [0074]    [0074]FIGS. 3 u - y  show top (FIG. 3 u ), side (FIG. 3 v ), side cutaway (FIG. 3 w ), perspective cutaway (FIG. 3 x ) and perspective (FIG. 3 y ) views of an assembled first preferred embodiment of the third embodiment family. FIG. 3 z  shows a side cutaway of an alternate assembled first preferred embodiment of the third embodiment family, having a bifurcated cap housing a spring member.  
         [0075]    [0075]FIGS. 4 a - e  show top (FIG. 4 a ), side (FIG. 4 b ), side cutaway (FIG. 4 c ), perspective cutaway (FIG. 4 d ) and perspective (FIG. 4 e ) views of a second type of the second baseplate of the third embodiment family, the second type of the second baseplate having the convex structure integrated therewith and also having a curvate recess.  
         [0076]    [0076]FIGS. 4 f - j  show top (FIG. 4 f ), side (FIG. 4 g ), side cutaway (FIG. 4 h ), perspective cutaway (FIG. 4 i ) and perspective (FIG. 4 j ) views of a second type of the ball of the third embodiment family, the second type of the ball having a protrusion.  
         [0077]    [0077]FIGS. 4 k - o  show top (FIG. 4 k ), side (FIG. 4 l ), side cutaway (FIG. 4 m ), perspective cutaway (FIG. 4 n ) and perspective (FIG. 4 o ) views of a second type of a cap of the third embodiment family, the second type of cap having a curvate recess.  
         [0078]    [0078]FIGS. 4 p - t  show top (FIG. 4 p ), side (FIG. 4 q ), side cutaway (FIG. 4 r ), perspective cutaway (FIG. 4 s ) and perspective (FIG. 4 t ) views of an assembled second preferred embodiment of the third embodiment family. FIG. 4 u  shows a side cutaway of an alternate assembled second preferred embodiment of the third embodiment family, having a bifurcated cap housing a spring member.  
         [0079]    [0079]FIGS. 5 a - e  show top (FIG. 5 a ), side (FIG. 5 b ), side cutaway (FIG. 5 c ), perspective cutaway (FIG. 5 d ) and perspective (FIG. 5 e ) views of a third type of the second baseplate of the third embodiment family, the third type of the second baseplate having the convex structure integrated therewith and also having a protrusion.  
         [0080]    [0080]FIGS. 5 f - j  show top (FIG. 5 f ), side (FIG. 5 g ), side cutaway (FIG. 5 h ), perspective cutaway (FIG. 5 i ) and perspective (FIG. 5 j ) views of a third type of the ball of the third embodiment family, the third type of the ball having a curvate recess.  
         [0081]    [0081]FIGS. 5 k - o  show top (FIG. 5 k ), side (FIG. 51), side cutaway (FIG. 5 m ), perspective cutaway (FIG. 5 n ) and perspective (FIG. 5 o ) views of an assembled third preferred embodiment of the third embodiment family. FIG. 5 p  shows a side cutaway of an alternate assembled third preferred embodiment of the third embodiment family, having a bifurcated cap housing a spring member.  
         [0082]    [0082]FIGS. 6 a - e  show top (FIG. 6 a ), side (FIG. 6 b ), side cutaway (FIG. 6 c ), perspective cutaway (FIG. 6 d ) and perspective (FIG. 6 e ) views of a fourth type of the second baseplate of the third embodiment family, the fourth type of the second baseplate having the convex structure integrated therewith and also having a pin through hole for housing a pin.  
         [0083]    [0083]FIGS. 6 f - j  show top (FIG. 6 f ), side (FIG. 6 g ), side cutaway (FIG. 6 h ), perspective cutaway (FIG. 6 i ) and perspective (FIG. 6 j ) views of an assembled fourth preferred embodiment of the third embodiment family. FIG. 6 k  shows a side cutaway of an alternate assembled fourth preferred embodiment of the third embodiment family, having a bifurcated cap housing a spring member.  
         [0084]    [0084]FIGS. 7 a - e  show top (FIG. 7 a ), side (FIG. 7 b ), side cutaway (FIG. 7 c ), perspective cutaway (FIG. 7 d ) and perspective (FIG. 7 e ) views of a fifth type of the second baseplate of the third embodiment family, the fifth type of the second baseplate having the convex structure integrated therewith and also having a recess.  
         [0085]    [0085]FIGS. 7 f - j  show top (FIG. 7 f ), side (FIG. 7 g ), side cutaway (FIG. 7 h ), perspective cutaway (FIG. 7 i ) and perspective (FIG. 7 j ) views of an assembled fifth preferred embodiment of the third embodiment family. FIG. 7 k  shows a side cutaway of an alternate assembled fifth preferred embodiment of the third embodiment family, having a bifurcated cap housing a spring member.  
         [0086]    [0086]FIGS. 8 a - e  show top (FIG. 8 a ), side (FIG. 8 b ), side cutaway (FIG. 8 c ), perspective cutaway (FIG. 8 d ) and perspective (FIG. 8 e ) views of a first baseplate of a fourth embodiment family of the present invention.  
         [0087]    [0087]FIGS. 8 f - j  show top (FIG. 8 f ), side (FIG. 8 g ), side cutaway (FIG. 8 h ), perspective cutaway (FIG. 8 i ) and perspective (FIG. 8 j ) views of a first type of second baseplate of the fourth embodiment family, the first type of the second baseplate having a central curvate pocket of the fourth embodiment family.  
         [0088]    [0088]FIGS. 8 k - o  show top (FIG. 8 k ), side (FIG. 81), side cutaway (FIG. 8 m ), perspective cutaway (FIG. 8 n ) and perspective (FIG. 8 o ) views of a first type of a ball of the fourth embodiment family.  
         [0089]    [0089]FIGS. 8 p - t  show top (FIG. 8 p ), side (FIG. 8 q ), side cutaway (FIG. 8 r ), perspective cutaway (FIG. 8 s ) and perspective (FIG. 8 t ) views of a first type of a convex structure of the fourth embodiment family.  
         [0090]    [0090]FIGS. 8 u - y  show top (FIG. 8 u ), side (FIG. 8 v ), side cutaway (FIG. 8 w ), perspective cutaway (FIG. 8 x ) and perspective (FIG. 8 y ) views of an assembled first preferred embodiment of the fourth embodiment family. FIG. 8 z  shows a side cutaway of an alternate assembled first preferred embodiment of the fourth embodiment family, having a bifurcated second baseplate housing a spring member.  
         [0091]    [0091]FIGS. 9 a - e  show top (FIG. 9 a ), side (FIG. 9 b ), side cutaway (FIG. 9 c ), perspective cutaway (FIG. 9 d ) and perspective (FIG. 9 e ) views of a second type of second baseplate of the fourth embodiment family, the second type of the second baseplate having the central curvate pocket and also having a curvate recess.  
         [0092]    [0092]FIGS. 9 f - j  show top (FIG. 9 f ), side (FIG. 9 g ), side cutaway (FIG. 9 h ), perspective cutaway (FIG. 9 i ) and perspective (FIG. 9 j ) views of a second type of the ball of the fourth embodiment family, the second type of the ball having a protrusion.  
         [0093]    [0093]FIGS. 9 k - o  show top (FIG. 9 k ), side (FIG. 91), side cutaway (FIG. 9 m ), perspective cutaway (FIG. 9 n ) and perspective (FIG. 9 o ) views of a second type of the convex structure of the fourth embodiment family, the second type of the convex structure having a curvate recess.  
         [0094]    [0094]FIGS. 9 p - t  show top (FIG. 9 p ), side (FIG. 9 q ), side cutaway (FIG. 9 r ), perspective cutaway (FIG. 9 s ) and perspective (FIG. 9 t ) views of an assembled second preferred embodiment of the fourth embodiment family. FIG. 9 u  shows a side cutaway of an alternate assembled second preferred embodiment of the fourth embodiment family, having a bifurcated second baseplate housing a spring member.  
         [0095]    [0095]FIGS. 10 a - e  show top (FIG. 10 a ), side (FIG. 10 b ), side cutaway (FIG. 10 c ), perspective cutaway (FIG. 10 d ) and perspective (FIG. 10 e ) views of a third type of second baseplate of the fourth embodiment family, the third type of the second baseplate having the central curvate pocket and also having a recess on a circumferential wall around the curvate pocket.  
         [0096]    [0096]FIGS. 10 f - j  show top (FIG. 10 f ), side (FIG. 10 g ), side cutaway (FIG. 10 h ), perspective cutaway (FIG. 10 i ) and perspective (FIG. 10 j ) views of a third type of the ball of the fourth embodiment family, the third type of the ball having a curvate recess.  
         [0097]    [0097]FIGS. 10 k - o  show top (FIG. 10 k ), side (FIG. 101), side cutaway (FIG. 10 m ), perspective cutaway (FIG. 10 n ) and perspective (FIG. 10 o ) views of a third type of the convex structure of the fourth embodiment family, the third type of the convex structure having a protrusion.  
         [0098]    [0098]FIGS. 10 p - t  show top (FIG. 10 p ), side (FIG. 10 q ), side cutaway (FIG. 10 r ), perspective cutaway (FIG. 10 s ) and perspective (FIG. 10 t ) views of an assembled third preferred embodiment of the fourth embodiment family. FIG. 10 u  shows a side cutaway of an alternate assembled third preferred embodiment of the fourth embodiment family, having a bifurcated second baseplate housing a spring member.  
         [0099]    [0099]FIGS. 11 a - e  show top (FIG. 11 a ), side (FIG. 11 b ), side cutaway (FIG. 11 c ), perspective cutaway (FIG. 11 d ) and perspective (FIG. 11 e ) views of a fourth type of the convex structure of the fourth embodiment family, the fourth type of the convex structure having a pin through hole for housing a pin.  
         [0100]    [0100]FIGS. 11 f - j  show top (FIG. 1 f ), side (FIG. 11 g ), side cutaway (FIG. 11 h ), perspective cutaway (FIG. 11 i ) and perspective (FIG. 11 j ) views of an assembled fourth preferred embodiment of the fourth embodiment family. FIG. 11 k  shows a side cutaway of an alternate assembled fourth preferred embodiment of the fourth embodiment family, having a bifurcated second baseplate housing a spring member.  
         [0101]    [0101]FIGS. 12 a - e  show top (FIG. 12 a ), side (FIG. 12 b ), side cutaway (FIG. 12 c ), perspective cutaway (FIG. 12 d ) and perspective (FIG. 12 e ) views of a fifth type of the convex structure of the fourth embodiment family, the fifth type of the convex structure having a recess adjacent a curvate taper.  
         [0102]    [0102]FIGS. 12 f - j  show top (FIG. 12 f ), side (FIG. 12 g ), side cutaway (FIG. 12 h ), perspective cutaway (FIG. 12 i ) and perspective (FIG. 12 j ) views of fourth type of ball of the fourth embodiment family, the fourth type of ball having a curvate recess.  
         [0103]    [0103]FIGS. 12 k - o  show top (FIG. 12 k ), side (FIG. 12 l ), side cutaway (FIG. 12 m ), perspective cutaway (FIG. 12 n ) and perspective (FIG. 12 o ) views of an assembled fifth preferred embodiment of the fourth embodiment family. FIG. 12 p  shows a side cutaway of an alternate assembled fifth preferred embodiment of the fourth embodiment family, having a bifurcated second baseplate housing a spring member.  
         [0104]    [0104]FIG. 13 shows a side perspective view of a prior art interbody fusion device.  
         [0105]    [0105]FIG. 14 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. 13 have been implanted. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0106]    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.  
         [0107]    A preferred embodiment of a first embodiment family of the present invention will now be described.  
         [0108]    Referring to FIGS. 1 a - c , a first baseplate  10  of a first embodiment family of the present invention is shown in top (FIG. 1 a ), side cutaway (FIG. 1 b ) and bottom (FIG. 1 c ) views. Also referring to FIGS. 1 d - f , a second baseplate  30  of the first embodiment family is shown in top (FIG. 1 d ), side cutaway (FIG. 1 e ) and bottom (FIG. 1 f ) views.  
         [0109]    More specifically, 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 (as best seen in exploded view in FIG. 1 g  and in assembly view in FIG. 1 h ). 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.  
         [0110]    More particularly, each baseplate  10 , 30  is a flat plate (preferably made of a metal 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 (e.g., a convex mesh  14 , 34 , 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  14 , 34  is secured at its perimeter to the outwardly facing surface  12 , 32  of the baseplate  10 , 30 . The mesh  14 , 34  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  14 , 34  substantially superior gripping and holding strength upon initial implantation as compared with other artificial disc products. The mesh  14 , 34  further provides an osteoconductive surface through which the bone may ultimately grow. The mesh  14 , 34  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.  
         [0111]    Each baseplate  10 , 30  further comprises at least a lateral ring  16 , 36  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  16 , 36  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  16 , 36  may extend beneath the domed mesh  14 , 34  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.  
         [0112]    As summarized above, each of the embodiments in the four embodiment families 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 has the two baseplates joined to one another by a ball and socket joint that is established centrally between the baseplates. Each ball and socket joint is established by a socket being formed at the peak (or, in some embodiments, in the peak) of a convex structure extending from the second baseplate, and by a ball being secured to the first baseplate and being captured in the socket so that when the joint is placed under a tension or compression force, the ball remains rotatably and angulatably secure in the socket. However, the convex structure is configured differently in each of the embodiment families, and the manner in which the ball is captured in the socket is different in each of the embodiment families. Each of these two variations (the configuration of the convex structure and the manner of capturing the ball in the socket) among the embodiments families will be understood further in light of the detailed descriptions hereinbelow. It should be noted that although each of the embodiment families uses a preferred shape for the convex structure (e.g., in the first and second embodiment families, the preferred shape is frusto-conical, and in the third and fourth embodiment families, the preferred shape is a shape having a curved taper), the convex structure in each of the embodiment families is not limited to a particular shape. For example, shapes including, but not limited to, frusto-conical, hemispherical or semispherical shapes, shapes having sloped tapers or curved tapers, or shapes having non-uniform, irregular, or dimensionally varying tapers or contours, would also be suitable in any of the embodiment families.  
         [0113]    In this regard, in this first embodiment family, the convex structure is configured as a flexible element and functions as a spring element that provides axial cushioning to the device. The convex structure has the socket of the ball and socket joint at its peak. In order to permit the flexible convex structure to flex under compressive loads applied to the device, it is a separate element from the second baseplate. In the preferred embodiment, the flexible convex structure is a belleville washer that has a frusto-conical shape. Other flexible convex structures are also contemplated as being suitable, such as, for example, convex structures that flex because of the resilience of the material from which they are made, because of the shape into which they are formed, and/or or because of the mechanical interaction between sub-elements of an assembly forming the convex structure. Although the convex structure is a separate element from the second baseplate in this embodiment family (so that it is able to flex), it is preferably maintained near the second baseplate so that the device does not separate in tension. Therefore, an extension of the second baseplate is provided (in the form of a shield element) to cover enough of the convex structure to so maintain it. Stated alternatively, the shield is a separate element from the second baseplate to ease manufacturing (during assembly, the flexible convex structure is first placed against the second baseplate, and then the shield is placed over the convex structure and secured to the second baseplate so that the convex structure is maintained between the second baseplate and the shield), but once the device is assembled, the second baseplate and the shield are effectively one element. That is, the second baseplate and shield can be considered to be a single integral housing within which the separate flexible convex structure flexes, because but for the sake of achieving desirable manufacturing efficiencies, the second baseplate and shield would be one piece.  
         [0114]    Also in this regard, in the first embodiment family, the manner of capturing the ball in the socket is effected by the ball being selectively radially compressible. That is, the ball is radially compressed to fit into the socket and thereafter receives a deflection preventing element to prevent subsequent radial compression, so that the ball remains captured in the socket. A more detailed description of the preferred manner in which this is accomplished is described below. Because the socket is formed at the peak of the flexible convex structure discussed immediately above, the capturing of the ball in the socket in this manner allows the ball to remain securely held for rotation and angulation even though the socket moves upward and downward with the flexing of the convex structure. The second baseplate preferably includes an access hole that accommodates placement of the deflection preventing element, so that the same can be applied to the ball after the ball is fitted into the socket. Accordingly, the ball is maintained in the socket.  
         [0115]    More specifically, in this preferred embodiment of the first embodiment family, with regard to joining the two baseplates  10 , 30  with a ball and socket joint, each of the baseplates  10 , 30  comprises features that, in conjunction with other components described below, form the ball and socket joint. More specifically, the first baseplate  10  includes an inwardly facing surface  18  that includes a perimeter region  20  and a ball  22  mounted to protrude from the inwardly facing surface  18 . The ball  22  preferably has a semispherical shape defining a spherical contour. The ball  22  includes a series of slots  24  that render the ball  22  radially compressible and expandable in correspondence with a radial pressure (or a radial component of a pressure applied thereto and released therefrom). The ball  22  further includes an axial bore  26  that accepts a deflection preventing element (e.g., rivet, plug, dowel, or screw; a rivet  28  is used herein as an example) (shown in FIGS. 1 i - j ). (Alternatively, the axial bore can be threaded to accept a screw.) Prior to the insertion of the rivet  28 , the ball  22  can deflect radially inward because the slots  24  will narrow under a radial pressure. The insertion of the rivet  28  eliminates the capacity for this deflection. Therefore, the ball  22 , before receiving the rivet  28 , can be compressed to pass into, and thereafter seat in, a central curvate socket of a convex structure (described below). Once the ball  22  has been seated in the curvate socket, the rivet  28  can be inserted into the axial bore  26  to ensure that the ball  22  remains held in the curvate socket As described below, an access hole is preferably provided in the second baseplate  30  so that the interior of the device may be readily accessed for inserting the rivet  28  into the axial bore  26 , or for other purposes.  
         [0116]    The second baseplate  30  includes an inwardly facing surface  38  that includes a perimeter region  40  and a central circular recess  42  within which the wide end of the convex structure resides, and a pair of holes  44  through which rivets  46  (shown in FIGS. 1 g - h ) may be provided for securing a shield element  48  that is placed over the convex structure, which shield  48  thus serves as an extension of the second baseplate  30  (the shield  48  is more fully set forth below with and shown on FIGS. 1 i - j ).  
         [0117]    Referring now to FIGS. 1 g - h , the convex structure  31  that resides in the circular recess  42  is shown in side cutaway (FIG. 1 g ) and top perspective (FIG. 1 h ) views. In this embodiment, the convex structure  31  is frusto-conical and is flexible. Because the convex structure  31  is flexible, 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. The flexible convex structure  31  is preferably, as shown, a belleville washer that has a frusto-conical shape. The belleville washer  31  preferably, as shown, has spiral slots and radially extending grooves. The restoring force of the belleville washer  31  is proportional to the elastic properties of the material or materials from which it is made. It should be understood that belleville washers having the configuration shown can be used with the present invention, but that belleville washers having other conformations, that is, without or without slots and/or grooves, and/or with other groove and slots configurations, including the same or different numbers of grooves and/or slots, can also be used with and are encompassed by the present invention.  
         [0118]    The belleville washer  31  comprises a series of spiral slots  33  formed therein. The slots  33  extend from the outer edge of the belleville washer  31 , inward along arcs generally directed toward the center of the element. The slots  33  do not extend fully to the center of the element. Preferably, the slots  33  extend anywhere from a quarter to three quarters of the overall radius of the washer  31 , depending upon the requirements of the patient, and the anatomical requirements of the device.  
         [0119]    The belleville washer  31  further comprises a series of grooves  35  formed therein. The grooves  35  extend radially from the outer edge of the belleville washer  31  toward the center of the element. Preferably, the width and depth of each groove  35  decreases along the length of the groove  35  from the outer edge of the washer  31  toward the center of the washer  31 , such that the center of the washer  31  is flat, while the outer edge of the washer  31  has grooves of a maximum groove depth. It should be understood that in other embodiments, one or both of the depth and the width of each groove can be (1) increasing along the length of the groove from the outer edge of the washer toward the center of the washer, (2) uniform along the length of the groove from the outer edge of the washer toward the center of the washer, or (3) varied along the length of each groove from the outer edge of the washer toward the center of the washer, either randomly or according to a pattern. Moreover, in other embodiments, it can be the case that each groove is not formed similarly to one or more other grooves, but rather one or more grooves are formed in any of the above-mentioned fashions, while one or more other grooves are formed in another of the above-mentioned fashions or other fashions. It should be dear that any groove pattern can be implemented without departing from the scope of the present invention, including, but not limited to, at least one radially spaced concentric groove, including, but not limited to, at least one such groove having at least one dimension that varies along the length of the groove. Belleville washers having circumferential extents that radially vary in at least one dimension, are also contemplated by the present invention.  
         [0120]    As a compressive load is applied to the belleville washer  31 , the forces are directed into a hoop stress which tends to radially expand the washer  31 . This hoop stress is counterbalanced by the material strength of the washer  31 , and the force necessary to widen the spiral slots  33  and the radial grooves  35  along with the strain of the material causes a deflection in the height of the washer  31 . Stated equivalently, the belleville washer  31  responds to a compressive load by deflecting compressively; the spiral slots and/or radial grooves cause the washer to further respond to the load by spreading as the slots and/or the grooves in the washer expand under the load. The spring, therefore, provides a restoring force which is proportional to the elastic modulus of the material in a hoop stressed condition.  
         [0121]    With regard to the above discussion regarding the curvate socket that receives the ball  22  of the first baseplate  10 , the curvate socket is formed at the peak of the convex structure  31 . The curvate socket  37  is provided inasmuch as the central opening of the belleville washer  31  is enlarged. This central opening includes a curvate volume  37  for receiving therein the ball  22  of the first baseplate  10 . More particularly, the curvate volume  37  has a substantially constant radius of curvature that is also substantially equivalent to the radius of the ball  22 . In this embodiment, the spiral slots  33  of the washer  31  do not extend all the way to the central opening, and approach the opening only as far as the material strength of the washer  31  can handle without plastically deforming under the expected anatomical loading. Further in this embodiment, the depth of each groove  35  of the washer  31  decreases along the length of the groove  35  from the outer edge of the washer  31  toward the center of the washer  31 , such that the center of the washer  31  is flat, while the outer edge of the washer  31  has grooves of a maximum groove depth. Therefore, the central opening can be formed from flat edges. It should be understood that this is not required, but rather is preferred for this embodiment.  
         [0122]    The curvate socket  37  has an opening diameter that accommodates passage therethrough of the ball  22  in a radially compressed state (but not in an uncompressed state), and a larger inner diameter that accommodates the ball  22  in the uncompressed state. Therefore, the ball  22  can be radially compressed to pass into the curvate socket  37  under force, and then will radially expand to the uncompressed state once in the curvate socket  37 . Once the rivet  28  is then secured into the axial bore  26 , the rivet  28  prevents the ball  22  from radially compressing, and therefore the ball  22  cannot back out through the opening. An access hole  39  in the second baseplate  30  below the curvate socket  37  has a diameter that accommodates the diameter of the rivet  28  and thereby provides easy access to insert the rivet  28  in the axial bore  26  after the ball  22  has been seated in the curvate socket  37 . To prevent the ball  22  from escaping the curvate socket  37  through the second baseplate  30 , the diameter of the access hole  39  is smaller than the inner diameter of the curvate socket  37 .  
         [0123]    The curvate socket  37  defines a spherical contour that closely accommodates the ball  22  for free rotation and angulation in its uncompressed state. Therefore, when seated in the curvate socket  37 , the ball  22  can rotate and angulate freely relative to the curvate socket  37  through a range of angles, thus permitting the opposing baseplates  10 , 30  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). Further preferably, the perimeter regions  20 , 40  have corresponding contours, so that the meeting of the perimeter regions  20 , 40  as a result of the angulation of the baseplates  10 , 30  reduces any surface wearing.  
         [0124]    Referring to FIGS. 1 i - j , exploded (FIG. 1 i ) and assembled (FIG. 1 i ) views of the preferred embodiment of the first embodiment family are shown. Included in these views are the shield  48  and the corresponding rivets  46 . More particularly, assembly of the disc is preferably as follows. The first and second baseplates  10 , 30  are disposed so that their outwardly facing surfaces  12 , 32  face away from one another and their inwardly facing surfaces  18 , 38  are directed toward one another. The convex structure  31  is then positioned with its wide end in the circular recess  42  of the second baseplate, so that the curvate socket  37  of the convex structure  31  is aligned with the ball  22  of the first baseplate  10 . Then, the shield  48  is secured over the belleville washer  31  (the shield  48  is preferably frusto-conical to follow the shape of the belleville washer  31 , although other shield shapes are suitable and contemplated by the present invention) by passing the central hole  41  of the shield  48  over the curvate socket  37  and applying the rivets  46  through rivet holes  43  in the shield  48  and into the rivet holes  44  in the second baseplate  30 . Then, the ball  22  is pressed into the curvate socket  37  under a force sufficient to narrow the slots  24  and thereby radially compress the ball  22  until the ball  22  fits through and passes through the opening of the curvate socket  37 . Once the ball  22  is inside the curvate socket  37 , the ball  22  will radially expand as the slots  24  widen until it has returned to its uncompressed state and the spherical contour defined by the ball  22  is closely accommodated by the spherical contour defined by the curvate socket  37  and the ball  22  can rotate and angulate freely relative to the curvate socket  37 . Thereafter, the rivet  28  is passed through the access hole  39  and pressed into the axial bore  26  of the ball  22  to prevent any subsequent radially compression of the ball  22  and therefore any escape from the curvate socket  37  thereby. Because the diameter of the circular recess  42  is greater than the diameter of the wide end of the belleville washer  31 , compressive loading of the device (and therefore the belleville washer) can result in an unrestrained radial deflection of the belleville washer  31 . The spiral slots  33  and radial grooves  35  of the belleville washer  31  enhance this deflection. When the load is removed, the belleville washer  31  springs back to its original shape.  
         [0125]    Accordingly, when the device of the preferred embodiment of the first embodiment family is assembled, the baseplates  10 , 30  are rotatable relative to one another because the ball  22  rotates freely within the curvate socket  37 , and angulatable relative to one another because the ball  22  angulates freely within the socket  37 . Because the convex structure  31  is flexible (and is housed in the second baseplate  30  in a manner that permits it to flex), the baseplates  10 , 30  are also axially compressible relative to one another. Because the ball  22  is held within the curvate socket  37  by a rivet  28  in the axial bore  26  preventing radial compression of the ball  22 , the artificial disc can withstand tension loading of the baseplates  10 , 30 . More particularly, when a tension load is applied to the baseplates  10 , 30 , the ball  22  in the curvate socket  37  seeks to radially compress to fit through the opening of the curvate socket  37 . However, the rivet  28  in the axial bore  26  of the ball  22  prevents the radial compression, thereby preventing the ball  22  from exiting the curvate socket  37 . Therefore, the assembly does not come apart under normally experienced tension loads. This ensures that no individual parts of the assembly will pop out or slip out from between the vertebral bodies when, e.g., the patient stretches or hangs while exercising or performing other activities. Thus, in combination with the securing of the baseplates  10 , 30  to the adjacent vertebral bones via the mesh domes  14 , 34 , the disc assembly has an integrity similar to the tension-bearing integrity of a healthy natural intervertebral disc. Also, because the ball  22  is laterally captured in the curvate socket  37 , lateral translation of the baseplates  10 , 30  relative to one another is prevented during rotation and angulation, similar to the performance of healthy natural intervertebral disc. Because the baseplates  10 , 30  are made angulatable relative to one another by the ball  22  being rotatably and angulatably coupled in the curvate socket  37 , the disc assembly provides a centroid of motion within the ball  22 . 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.  
         [0126]    A preferred embodiment of a second embodiment family of the present invention will now be described.  
         [0127]    Referring to FIGS. 2 a - c , a first baseplate  50  of a second embodiment family of the present invention is shown in top (FIG. 2 a ), side cutaway (FIG. 2 b ) and bottom (FIG. 2 c ) views. Also referring to FIGS. 2 d - f , a second baseplate  70  of the second embodiment family is shown in top (FIG. 2 d ), side cutaway (FIG. 2 e ) and bottom (FIG. 2 f ) views.  
         [0128]    With regard to the configuration of the convex structure in this second embodiment family, and the manner in which the ball is captured in the socket in this second embodiment family, the convex structure is configured as a non-flexible element that is integral with the second baseplate, and has the socket of the ball and socket joint at its peak. More dearly stated, the devices of this second embodiment family do not feature a flexible convex structure, and therefore (and also because of the manner in which the ball is captured in this second embodiment family, discussed immediately below) there is no need for the convex structure to be a separate element from the second baseplate. (By contrast, in the first embodiment family, as discussed above, because the convex structure is flexible, it is separated from the second baseplate so that it is able to flex.) In the preferred embodiment, the convex structure has a frusto-conical shape. The manner of capturing the ball in the socket in this second embodiment family is identical to that of the first embodiment family.  
         [0129]    More specifically, the first and second baseplates  50 , 70  are similar to the first and second baseplates  10 , 30  of the first embodiment family described above with regard to each outwardly facing surface  52 , 72  having a vertebral body contact element  54 , 74  and an adjacent osteoconductive ring  56 , 76 , and each inwardly facing surface  58 , 78  having a perimeter region  60 , 80 , all of which elements in the second embodiment family are, for example, identical to the corresponding elements in the first embodiment family as described above.  
         [0130]    Further, as with the first embodiment family, the two baseplates  50 , 70  are joined with a ball and socket joint, and therefore each of the baseplates  50 , 70  comprises features that, in conjunction with other components described below, form the ball and socket joint. More specifically, the first baseplate  50  is formed similarly to the first baseplate  10  of the first embodiment family, having a ball  62  mounted to protrude from the inwardly facing surface  58 . The ball  62  preferably has a semispherical shape defining a spherical contour. The ball  62  is structurally and functionally identical to the ball  22  of the first embodiment family, and as such is selectively radially compressible in the same manner as the ball  22  of the first embodiment family. As with the ball  22  of the first embodiment family, the ball  62  is capturable in a curvate socket  77  formed at the peak of a convex structure  71  protruding from the second baseplate  70 . The curvate socket  77  is functionally and structurally identical to the curvate socket  37  of the first embodiment family. However, in this second embodiment family, the convex structure  77  of the device, rather than being a flexible separate element from the second baseplate as in the first embodiment family, is integral with the second baseplate  70 . The convex structure  77  is frusto-conical, but is not flexible, and therefore does not function as a force restoring element as does the flexible convex structure  37  in the first embodiment family. Access to the convex structure  77  for providing easy access to insert the rivet  68  in the axial bore  66  of the ball  62  after the ball  62  has been seated in the curvate socket  77  is provided by an access hole  79  in the second baseplate  70  below and leading to the curvate socket  77 . The access hole  79  is otherwise structurally identical to the access hole  39  in the second baseplate  30  of the first embodiment family.  
         [0131]    Referring to FIGS. 2 g - h , an assembled preferred embodiment of the second embodiment family is shown in exploded (FIG. 2 g ) and assembled (FIG. 2 h ) views. More particularly, assembly of the disc is preferably as follows. The first and second baseplates  50 , 70  are disposed so that their outwardly facing surfaces  52 , 72  face away from one another and their inwardly facing surfaces  58 , 78  are directed toward one another, and so that the ball  62  of the first baseplate  50  is aligned with the curvate socket  77  of the convex structure  71  of the second baseplate  70 . Then, the ball  62  is pressed into the curvate socket  77  under a force sufficient to narrow the slots  64  and thereby radially compress the ball  62  until the ball  62  fits through and passes through the opening of the curvate socket  77 . Once the ball  62  is inside the curvate socket  77 , the ball  62  will radially expand as the slots  64  widen until it has returned to its uncompressed state and the spherical contour defined by the ball  62  is closely accommodated by the spherical contour defined by the curvate socket  77  and the ball  62  can rotate and angulate freely relative to the curvate socket  77 . Thereafter, the rivet  68  is passed through the access hole  79  and pressed into the axial bore  66  of the ball  62  to prevent any subsequent radially compression of the ball  62  and therefore any escape from the curvate socket  77  thereby.  
         [0132]    Accordingly, when the device of the preferred embodiment of the second embodiment family is assembled, the baseplates  50 , 70  are rotatable relative to one another because the ball  62  rotates freely within the curvate socket  77 , and angulatable relative to one another because the ball  62  angulates freely within the socket  77 . Because the ball  62  is held within the curvate socket  77  by a rivet  68  in the axial bore  66  preventing radial compression of the ball  62 , the artificial disc can withstand tension loading of the baseplates  50 , 70 . More particularly, when a tension load is applied to the baseplates  50 , 70 , the ball  62  in the curvate socket  77  seeks to radially compress to fit through the opening of the curvate socket  77 . However, the rivet  68  in the axial bore  66  of the ball  62  prevents the radial compression, thereby preventing the ball  62  from exiting the curvate socket  77 . Therefore, the assembly does not come apart under normally experienced tension loads. This ensures that no individual parts of the assembly will pop out or slip out from between the vertebral bodies when, e.g., the patient stretches or hangs while exercising or performing other activities. Thus, in combination with the securing of the baseplates  50 , 70  to the adjacent vertebral bones via the mesh domes  54 , 74 , the disc assembly has an integrity similar to the tension-bearing integrity of a healthy natural intervertebral disc. Also because the ball  62  is laterally captured in the curvate socket  77 , lateral translation of the baseplates  50 , 70  relative to one another is prevented during rotation and angulation, similar to the performance of healthy natural intervertebral disc. Because the baseplates  50 , 70  are made angulatable relative to one another by the ball  62  being rotatably and angulatably coupled in the curvate socket  77 , the disc assembly provides a centroid of motion within the ball  62 . 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.  
         [0133]    Embodiments of the third embodiment family of the present invention will now be described.  
         [0134]    With regard to the configuration of the convex structure in the third embodiment family, the convex structure is configured as a non-flexible element, that is integral with the second baseplate, and has the socket of the ball and socket joint at its peak, similar to the configuration of the convex structure in the second embodiment family. In the preferred embodiment, the convex structure is shaped to have a curved taper.  
         [0135]    With regard to the manner in which the ball is captured in the socket in the third embodiment family, the capturing is effected through the use of a solid ball. In order to permit the seating of the ball into the socket formed at the peak of the convex structure, the access hole in the second baseplate has a diameter that accommodates the diameter of the ball, and leads to the interior of the peak, which interior is formed as a concavity having an opening diameter that accommodates the diameter of the ball. (Preferably, the concavity has a curvature closely accommodating the contour of the ball, and the concavity is either hemispherical or less-than-hemispherical so that the ball can easily be placed into it.) Further, in order to maintain the ball in the socket, an extension of the second baseplate (in the form of a cap element) is provided for sealing the access hole in the second baseplate (or reducing the opening diameter of the hole to a size that does not accommodate the diameter of the ball). The cap has an interior face that preferably has a concavity (that has a curvature that closely accommodates the contour of the ball) to complete the socket. The peak of the convex structure has a bore that accommodates a post to which the ball and the first baseplate are attached (one to each end of the post), but does not accommodate the ball for passage through the bore. Accordingly, the ball is maintained in the socket.  
         [0136]    A first preferred embodiment of a third embodiment family of the present invention will now be described.  
         [0137]    Referring to FIGS. 3 a - e , a first baseplate  100  of the third embodiment family of the present invention is shown in top (FIG. 3 a ), side (FIG. 3 b ), side cutaway (FIG. 3 c ), perspective cutaway (FIG. 3 d ) and perspective (FIG. 3 e ) views. Also referring to FIGS. 3 f - j , a first type  200  of a second baseplate of the third embodiment family is shown in top (FIG. 3 f ), side (FIG. 3 g ), side cutaway (FIG. 3 h ), perspective cutaway (FIG. 3 i ) and perspective (FIG. 3 j ) views.  
         [0138]    More specifically, the first and second baseplates  100 , 200  are similar to the first and second baseplates  50 , 70  of the second embodiment family described above with regard to each having an outwardly facing surface  102 , 202 , and each inwardly facing surface  108 , 208  having a perimeter region  110 , 210 , all of which elements in the third embodiment family are, for example, identical to the corresponding elements in the first embodiment family as described above. However, each of the first and second baseplates  100 , 200  in this second embodiment family instead of having a convex mesh as a vertebral body contact element, have a convex solid dome  103 , 203  and a plurality of spikes  105 , 205  as vertebral body contact element. Preferably, the dome  103 , 203  is covered with an osteoconductive layer of a type known in the art. It should be noted that the convex solid dome  203  of the second baseplate  200  is provided in this embodiment (and the other embodiments in this family) by the cap element (described below) that serves as an extension of the second baseplate  200  to capture the ball (described below), as best shown in FIGS. 3 u - y . It should also be noted that the convex mesh used in other embodiments of the present invention is suitable for use with these other vertebral body contact elements, and can be attached over the convex dome  103 , 203  by laser welding, or more preferably, by plasma burying (where the perimeter region of the convex mesh is buried under a plasma coating, which coating secures to the outwardly facing surface of the baseplate to which it is applied, and thus secures the convex mesh to the outwardly facing surface).  
         [0139]    Further, as with the first embodiment family, the two baseplates  100 , 200  are joined with a ball and socket joint, and therefore each of the baseplates  100 , 200  comprises features that, in conjunction with other components described below, form the ball and socket joint. The ball and socket joint includes a solid ball (described below) mounted to protrude from the inwardly facing surface  108  of the first baseplate  100 , and a curvate socket formed at a peak of a non-flexible convex structure (described below) that is integral with the second baseplate  200 , within which curvate socket the ball is capturable for free rotation and angulation therein. As shown in FIGS. 3 a - e , the mounting for the ball includes a central hole  112  on the inwardly facing surface  108  of the first baseplate  100 , which hole  112  accepts a tail end of a post (described below) that has the ball at a head end of the post. Preferably, the tail end compression locks into the hole  112 . As shown in FIGS. 3 f - j , the convex structure  201  is integral with the second baseplate  200  and includes a curvate pocket  212  formed by a central portion of the inwardly facing surface  209  of the convex structure  201  convexing inwardly and by a central portion of an outwardly facing surface  213  of the convex structure  201  concaving inwardly. The pocket  212  has a semispherical contour on the central portion of the outwardly facing surface  213  and an apex at the center of the semispherical contour. Further, the convex structure  201  has a bore  214  through the apex of the pocket  212 , to accommodate the post. Further, the second baseplate  200  has on its outwardly facing surface  202  an access hole  209  surrounded by a circular recess  216  leading to the pocket  212 , which recess  216  accepts the cap (described below) that serves as an extension of the second baseplate  200 .  
         [0140]    Referring now to FIGS. 3 k - o , a first type  300  of the ball of the third embodiment family is shown in top (FIG. 3 k ), side (FIG. 31), side cutaway (FIG. 3 m ), perspective cutaway (FIG. 3 n ) and perspective (FIG. 3 o ) views. The ball  300  is mounted at a head end  306  of a post  302  that also has a tail end  304 . The ball  300  defines a spherical contour that is interrupted by the shaft of the post  302 .  
         [0141]    Referring now to FIGS. 3 p - t , a first type  400  of the cap of the third embodiment family is shown in top (FIG. 3 p ), side (FIG. 3 q ), side cutaway (FIG. 3 r ), perspective cutaway (FIG. 3 s ) and perspective (FIG. 3 t ) views. The cap  400  includes an outwardly facing surface  402  that complements the outwardly facing surface  202  of the second baseplate  200  when the cap  400  is secured in the circular recess  216  of the second baseplate  200  (preferably, as shown, the outwardly facing surface  402  of the cap  400  provides the second baseplate  200  with the convex dome  203 , as best shown in FIGS. 3 u - y ). The cap  400  further includes an inwardly facing surface  404 , and a curvate pocket  406  formed by a central portion of the inwardly facing surface  404  of the cap  400  concaving outwardly. The pocket  406  has a semispherical contour that closely accommodates the spherical contour defined by the ball  300 . The semispherical contour of the pocket  406  of the cap  400  opposes the semispherical contour of the pocket  212  of the convex structure  201  such that when the cap  400  is secured in the circular recess  216  of the second baseplate  200 , the semispherical contours together define a socket  207  defining a spherical contour that closely accommodates the spherical contour defined by the ball  300  for free rotation and angulation of the ball  300  in the pockets  406 , 212 . Each of the semispherical contour of the pocket  406  and the semispherical contour of the pocket  212  are preferably no greater than hemispherical, to make easier the assembly of the device.  
         [0142]    Referring now to FIGS. 3 u - y , an assembled first preferred embodiment of the third embodiment family is shown in top (FIG. 3 u ), side (FIG. 3 v ), side cutaway (FIG. 3 w ), perspective cutaway (FIG. 3 x ) and perspective (FIG. 3 y ) views. More particularly, assembly of the disc is preferably as follows. The tail end  304  of the post  302  is passed through the access hole  209  in the second baseplate  200  and through the bore  214  at the apex of the curvate pocket  212  of the convex structure  201 , and the tail end  304  is thereafter secured to the central hole  112  in the first baseplate  100 . (The access hole  209  has a diameter that accommodates the diameter of the ball  300  at the head  306  of the post  302 , and the curvate pocket  212  on the outwardly facing surface  213  of the convex structure  201  has an opening diameter that accommodates the ball  300  for seating in the pocket  212  when the tail end  304  is fully passed through the bore  214 . Thus, the ball  300  can be placed through the access hole  209  and into the curvate pocket during this step.) The bore  214  at the apex of the curvate pocket  212  has a diameter greater than the diameter of the post  302  but smaller than the diameter of the ball  300  at the head  306  of the post  302 . Therefore, as the ball  300  is being placed into the curvate pocket  212 , the post  302  fits through the bore  214 , but the ball  300  does not, and the convex structure  201  (and the second baseplate  200 ) cannot be freed from the ball  300  once the tail end  304  of the post  302  is secured to the first baseplate  100 . Although any suitable method is contemplated by the present invention, the attachment of the tail end  304  of the post  302  is preferably accomplished by compression locking (if accomplished alternatively or additionally by laser welding, the laser weld can, e.g., be applied from the outwardly facing surface  102  of the first baseplate  100  if the hole  112  passes completely through the first baseplate  100 ). The tail end  304  can also alternatively or additionally be threaded into the central hole  112  for increased stability of the attachment.  
         [0143]    The semispherical contour of the pocket  212  closely accommodates the spherical contour defined by the ball  300 , so that the ball  300  can freely rotate in the pocket  212  about the longitudinal axis of the post  302 , and can freely angulate in the pocket  212  about a centroid of motion located at the center of the ball  300 . Further, the bore  214  is tapered to a larger diameter toward the first baseplate  100 , to permit the post  302  to angulate (about the centroid of motion at the center of the ball  300 ) with respect to the bore  214  as the ball  300  angulates in the pocket  212 . Preferably, the conformation of the taper accommodates angulation of the post  302  at least until the perimeter regions  110 , 210  of the inwardly facing surfaces  108 , 208 / 211  meet.  
         [0144]    Finally, the cap  400  is secured in the circular recess  216  of the second baseplate  200 , so that the curvate pocket  406  of the cap  400  and the opposing curvate pocket  212  of the convex structure  201  together form the socket  207  defining the spherical contour within which the ball  300  at the head  306  of the post  302  freely rotates and angulates as described above. The application of the cap  400  also seals the access hole  209  in the second baseplate (or, if the cap  400  has a bore, it preferably reduces the diameter of the access hole  209  to a size that does not accommodate the diameter of the ball  300 ). Although any suitable method is contemplated by the present invention, the cap  400  preferably is secured in the circular recess  216  by compression locking (a laser weld can alternatively or additionally be used, or other suitable attachment means). As stated above, the cap  400  preferably has an outwardly facing surface  402  that complements the outwardly facing surface  202  of the second baseplate  200  for surface uniformity once the cap  400  is secured. The cap  400  may also additionally or alternatively be threaded into the circular recess  216  for increased stability of the attachment.  
         [0145]    Referring now to FIG. 3 z , an assembled alternate first preferred embodiment of the third embodiment family is shown in side cutaway view. This alternate first preferred embodiment incorporates a multi-part cap (with first part  4000   a  and second part  4000   b ) housing a spring member  4100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  4100 . Elements of this alternate first preferred embodiment that are also elements found in the first preferred embodiment are like numbered, and the assembly of this alternate first preferred embodiment is identical to that of the first preferred embodiment, with some differences due to the incorporation of the spring member  4100 . (For example, the cap features are numbered in the 4000&#39;s rather than the 400&#39;s.) More particularly, assembly of the disc is preferably as follows. The tail end  304  of the post  302  is passed through the access hole  209  in the second baseplate  200  and through the bore  214  at the apex of the curvate pocket  212  of the convex structure  201 , and the tail end  304  is thereafter secured to the central hole  112  in the first baseplate  100 . (The access hole  209  has a diameter that accommodates the diameter of the ball  300  at the head  306  of the post  302 , and the curvate pocket  212  on the outwardly facing surface  213  of the convex structure  201  has an opening diameter that accommodates the ball  300  for seating in the pocket  212  when the tail end  304  is fully passed through the bore  214 . Thus, the ball  300  can be placed through the access hole  209  and into the curvate pocket during this step.) The bore  214  at the apex of the curvate pocket  212  has a diameter greater than the diameter of the post  302  but smaller than the diameter of the ball  300  at the head  306  of the post  302 . Therefore, as the ball  300  is being placed into the curvate pocket  212 , the post  302  fits through the bore  214 , but the ball  300  does not, and the convex structure  201  (and the second baseplate  200 ) cannot be freed from the ball  300  once the tail end  304  of the post  302  is secured to the first baseplate  100 . Although any suitable method is contemplated by the present invention, the attachment of the tail end  304  of the post  302  is preferably accomplished by compression locking (if accomplished alternatively or additionally by laser welding, the laser weld can, e.g., be applied from the outwardly facing surface  102  of the first baseplate  100  if the hole  112  passes completely through the first baseplate  100 ). The tail end  304  can also alternatively or additionally be threaded into the central hole  112  for increased stability of the attachment.  
         [0146]    The semispherical contour of the pocket  212  closely accommodates the spherical contour defined by the ball  300 , so that the ball  300  can freely rotate in the pocket  212  about the longitudinal axis of the post  302 , and can freely angulate in the pocket  212  about a centroid of motion located at the center of the ball  300 . Further, the bore  214  is tapered to a larger diameter toward the first baseplate  100 , to permit the post  302  to angulate (about the centroid of motion at the center of the ball  300 ) with respect to the bore  214  as the ball  300  angulates in the pocket  212 . Preferably, the conformation of the taper accommodates angulation of the post  302  at least until the perimeter regions  110 , 210  of the inwardly facing surfaces  108 , 208 / 211  meet.  
         [0147]    The second part  4000   b  of the multi-part cap is secured in the circular recess  216  of the second baseplate  200 , so that the curvate pocket  4060  of the inwardly facing surface  4040   b  of the cap second part  4000   b  and the opposing curvate pocket  212  of the convex structure  201  together form the socket  207  defining the spherical contour within which the ball  300  at the head  306  of the post  302  freely rotates and angulates as described above. The application of the cap second part  4000   b  (and the cap first part  4000   a ) also seals the access hole  209  in the second baseplate (or, if the cap second and first parts  4000   b ,  4000   a  have bores, it preferably reduces the diameter of the access hole  209  to a size that does not accommodate the diameter of the ball  300 ). The cap second part  4000   b  is preferably not compressed into, but rather fits loosely within the boundaries of, the circular recess  216 , so that when the first baseplate  100  is compressed toward the second baseplate  200 , the cap second part  4000   b  may travel toward the cap first part  4000   a  as the spring member  4100  compresses (due to the cap first part  4000   a  being secured in the circular recess  216  to the second baseplate  200 ). The spring member  4100  is then disposed on the outwardly facing surface  4020   b  of the cap second part  4000   b . While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  4100  is formed of an elastomeric material, for example. The illustrated spring member  4100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  4020   b  of the cap second part  4000   b  as shown.  
         [0148]    Finally, the cap first part  4000   a  is secured in the circular recess  216  of the second baseplate  200  to incarcerate the cap second part  4000   b , and the spring member  4100  between the outwardly facing surface  4020   b  of the cap second part  4000   b  and the inwardly facing surface  4040   a  of the cap first part  4000   a . Although any suitable method is contemplated by the present invention, the cap first part  4000   a  preferably is secured in the circular recess  216  by compression locking (a laser weld can alternatively or additionally be used, or other suitable attachment means). The cap second part  4000   b  should be dimensioned such that, and the spring member  4100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  4020   b  of the cap second part  4000   b  and the inwardly facing surface  4040   a  of the cap first part  4000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  4100  will compress under an anticipated load. The cap first part  4000   a  preferably has an outwardly facing surface  4020   a  that complements the outwardly facing surface  202  of the second baseplate  200  for surface uniformity once the cap first part  4000   a  is secured. The cap first part  4000   a  may also additionally or alternatively be threaded into the circular recess  216  for increased stability of the attachment. Accordingly, in this alternate first preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  4100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0149]    Accordingly, when a device of the first preferred embodiment of the third embodiment family is assembled, the baseplates are rotatable relative to one another because the ball  300  rotates freely within the socket  207 , and angulatable relative to one another because the ball  300  angulates freely within the socket  207 . Because the ball  300  is held within the socket  207  by the securing of the tail end  304  of the post  302  to the first baseplate  100  and the securing of the cap  400  (or cap first part  4000   a ) to the second baseplate  200 , the artificial disc can withstand tension loading of the baseplates  100 , 200 . More particularly, when a tension load is applied to the baseplates  100 , 200  the ball  300  seeks to pass through the bore  214  at the apex of the curvate pocket  212 . However, the smaller diameter of the bore  214  relative to the diameter of the ball  300  prevents the ball  300  from exiting the socket  207 . Therefore, the assembly does not come apart under normally experienced tension loads. This ensures that no individual parts of the assembly will pop out or slip out from between the vertebral bodies when, e.g., the patient stretches or hangs while exercising or performing other activities. Thus, in combination with the securing of the baseplates  100 , 200  to the adjacent vertebral bones via the domes  103 , 203  and spikes  105 , 205 , the disc assembly has an integrity similar to the tension-bearing integrity of a healthy natural intervertebral disc. Also because the ball  300  is laterally captured in the socket  207 , lateral translation of the baseplates  100 , 200  relative to one another is prevented during rotation and angulation, similar to the performance of healthy natural intervertebral disc. Because the baseplates  100 , 200  are made angulatable relative to one another by the ball  300  being rotatably and angulatably coupled in the socket  207 , the disc assembly provides a centroid of motion within the ball  300 . 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.  
         [0150]    The remaining embodiments in the third embodiment family of the present invention limit the rotation (but preferably not the angulation) of the ball in the socket defined by the pocket of the convex structure and the pocket of the cap. Each embodiment accomplishes this in a different manner, but each embodiment utilizes interference between a protrusion and a recess to limit the rotation. In some embodiments, the protrusion is preferably hemispherical, and the recess preferably has a semicylindrical contour within which the protrusion fits. In other embodiments, the protrusion is preferably hemispherical, and the recess preferably has a curvate contour that is not semicylindrical. (It should be understood that the described formations of the recess and the protrusion are merely preferred, and that alternate formations, curvate or otherwise, for each are contemplated by the present invention; a particular shape or location of recess or a particular shape or location of protrusion is not required; any shape can be used so long as the recess and protrusion interact as desired. For example, the recess in the second preferred embodiment of the third embodiment family has a curvate contour that is not semicylindrical so that it optimally interacts with the protrusion in that embodiment.) The boundaries of the recess define the limits of rotation of the ball within the socket, by allowing movement of the protrusion relative to the recess as the ball rotates through a certain range in the socket, but providing interference with the protrusion to prevent rotation of the ball beyond that range in the socket. Preferably, for example, the recess has a depth equivalent to the radius of the protrusion, but a radius of curvature greater than that of the protrusion. At the same time, the boundaries of the recess preferably do not limit the angulation of the ball within the socket, at least until the perimeter regions of the inwardly facing surfaces meet. Preferably for example, the recess has a length greater than the range of movement of the protrusion relative to the recess as the ball angulates in the socket.  
         [0151]    Therefore, when assembled, the discs of the remaining preferred embodiments of the third embodiment family enable angulation and limited rotation of the baseplates relative to one another about a centroid of motion that remains centrally located between the baseplates (at the center of the sphere defined by the ball), similar to the centroid of motion in a healthy natural intervertebral disc that is limited in its rotation by surrounding body structures. A benefit of limiting the relative rotation of the baseplates is that relative rotation beyond a certain range in a healthy natural disc is neither needed nor desired, because, for example, excess strain can be placed on the facet joints or ligaments thereby. As described with the first preferred embodiment of the third embodiment family, the construction also prevents translation and separation of the baseplates relative to one another during rotation and angulation.  
         [0152]    As noted above, each of the remaining preferred embodiments in this third embodiment family forms the protrusion and corresponding recess in a different manner, utilizing components that are either identical or similar to the components of the first preferred embodiment, and some embodiments utilize additional components. Each of the remaining preferred embodiments will now be described in greater detail.  
         [0153]    In the second preferred embodiment of the third embodiment family of the present invention, a hemispherical protrusion is formed on the ball itself, and interacts in the above-described manner with a curvate recess formed adjacent the socket defined by the pocket of the convex structure and the pocket of the cap. More particularly, this second preferred embodiment uses the same first baseplate  100  as the first preferred embodiment of the third embodiment family described above. Referring to FIGS. 4 a - e , a second type  500  of second baseplate of the third embodiment family is shown in top (FIG. 4 a ), side (FIG. 4 b ), side cutaway (FIG. 4 c ), perspective cutaway (FIG. 4 d ) and perspective (FIG. 4 e ) views. This second type  500  of second baseplate is identical to the first type  200  of second baseplate described above (and thus similar features are reference numbered similar to those of the first type  200  of second baseplate, but in the 500s rather than the 200s), except that this second type  500  of second baseplate has a curvate recess  518  adjacent the curvate pocket  512  in the convex structure  501 .  
         [0154]    Referring now to FIGS. 4 f - j , a second type  600  of ball of the third embodiment family is shown in top (FIG. 4 f ), side (FIG. 4 g ), side cutaway (FIG. 4 h ), perspective cutaway (FIG. 4 i ) and perspective (FIG. 4 j ) views. The ball  600  is identical to the first type  300  of ball described above (and thus similar features are reference numbered similar to those of the first type  300  of ball, but in the 600s rather than the 300s), except that the spherical contour defined by this second type  600  of ball is also interrupted by a hemispherical protrusion  608 .  
         [0155]    Referring now to FIGS. 4 k - o , a second type  700  of cap of the third embodiment family is shown in top (FIG. 4 k ), side (FIG. 41), side cutaway (FIG. 4 m ), perspective cutaway (FIG. 4 n ) and perspective (FIG. 4 o ) views. This second type  700  of cap is identical to the first type  400  of cap described above (and thus similar features are reference numbered similar to those of the first type  400  of cap, but in the 700s rather than the 400s), except that this second type  700  of cap has a curvate recess  708  adjacent the curvate pocket  706 .  
         [0156]    Referring now to FIGS. 4 p - t , an assembled second preferred embodiment of the third embodiment family is shown in top (FIG. 4 p ), side (FIG. 4 q ), side cutaway (FIG. 4 r ), perspective cutaway (FIG. 4 s ) and perspective (FIG. 4 t ) views. It can be seen that the curvate recesses  518 , 708  together form the recess described above in the discussion of the manner in which these remaining embodiments limit rotation of the ball in the socket, and that the protrusion  608  serves as the protrusion described above in the same discussion. Thus, the protrusion  608  and recesses  518 , 708  interact in the above described manner to limit the rotation of the ball  600  in the socket  507  defined by the curvate pockets  512 , 706 . Assembly of the disc is identical to that of the first preferred embodiment of the third embodiment family, except that the protrusion  608  is longitudinally aligned with the recess  518 , and the recess  708  is similarly aligned, so that when the cap  700  is secured to the second baseplate  500 , the protrusion  608  is fitted within the recesses  518 , 708  for interaction as described above as the ball  600  rotates and angulates in the socket  507 .  
         [0157]    Referring now to FIG. 4 u , an assembled alternate second preferred embodiment of the third embodiment family is shown in side cutaway view. This alternate second preferred embodiment incorporates a multi-part cap (with first part  7000   a  and second part  7000   b ) housing a spring member  7100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  7100 . Elements of this alternate second preferred embodiment that are also elements found in the second preferred embodiment are like numbered. (The cap features are numbered in the 7000&#39;s rather than the 700&#39;s.) The curvate recesses  518 , 7080  together form the recess described above, and the protrusion  608  serves as the protrusion described above, and thus the protrusion  608  and the recesses  518 , 7080  interact in the above described manner to limit the rotation of the ball  600  in the socket  507  defined by the curvate pockets  512 , 7060 .  
         [0158]    Assembly of this alternate second preferred embodiment is identical to that of the alternate first preferred embodiment of the third embodiment family, except that the protrusion  608  is longitudinally aligned with the recess  518 , and the recess  7080  is similarly aligned, so that when the cap second part  7000   b  is disposed in the circular recess  516  of the second baseplate  500 , the protrusion  608  is fitted within the recesses  518 , 7080  for interaction as described above as the ball  600  rotates and angulates in the socket  507 . The cap second part  7000   b  is preferably not compressed into, but rather fits loosely within, the circular recess  516 , so that when the first baseplate  100  is compressed toward the second baseplate  500 , the cap second part  7000   b  may travel toward the cap first part  7000   a  as the spring member  7100  compresses (due to the cap first part  7000   a  being secured in the circular recess  516  to the second baseplate  500 ). The spring member  7100  is then disposed on the outwardly facing surface  7020   b  of the cap second part  7000   b . While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  7100  is formed of an elastomeric material, for example. The illustrated spring member  7100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  7020   b  of the cap second part  7000   b  as shown.  
         [0159]    Finally, the cap first part  7000   a  is secured in the circular recess  516  of the second baseplate  500  to incarcerate the cap second part  7000   b , and the spring member  7100  between the outwardly facing surface  7020   b  of the cap second part  7000   b  and the inwardly facing surface  7040   a  of the cap first part  7000   a . Although any suitable method is contemplated by the present invention, the cap first part  7000   a  preferably is secured in the circular recess  516  by compression locking (a laser weld can alternatively or additionally be used, or other suitable attachment means). The cap second part  7000   b  should be dimensioned such that, and the spring member  7100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  7020   b  of the cap second part  7000   b  and the inwardly facing surface  7040   a  of the cap first part  7000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  7100  will compress under an anticipated load. The cap first part  7000   a  preferably has an outwardly facing surface  7020   a  that complements the outwardly facing surface  502  of the second baseplate  500  for surface uniformity once the cap first part  7000   a  is secured. The cap first part  7000   a  may also additionally or alternatively be threaded into the circular recess  516  for increased stability of the attachment. Accordingly, in this alternate first preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  7100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0160]    In the third preferred embodiment of the third embodiment family of the present invention, a hemispherical protrusion is formed to protrude into the socket defined by the pocket of the convex structure and the pocket of the cap, and interacts in the above-described manner with a semicylindrical recess formed on the ball. More particularly, this third preferred embodiment uses the same first baseplate  100  and the same cap  400  as the first preferred embodiment of the third embodiment family. Referring to FIGS. 5 a - e , a third type  800  of second baseplate of the third embodiment family is shown in top (FIG. 5 a ), side (FIG. 5 b ), side cutaway (FIG. 5 c ), perspective cutaway (FIG. 5 d ) and perspective (FIG. 5 e ) views. This third type  800  of second baseplate is identical to the first type  200  of second baseplate described above (and thus similar features are reference numbered similar to those of the first type  200  of second baseplate, but in the 800s rather than the 200s), except that this third type  800  of second baseplate has a protrusion  818  jutting out from the wall of the pocket  812  in the convex structure  801 .  
         [0161]    Referring now to FIGS. 5 f - j , a third type  900  of ball of the third embodiment family is shown in top (FIG. 5 f ), side (FIG. 5 g ), side cutaway (FIG. 5 h ), perspective cutaway (FIG. 5 i ) and perspective (FIG. 5 j ) views. The ball  900  is identical to the first type  300  of ball described above (and thus similar features are reference numbered similar to those of the first type  300  of ball, but in the 900s rather than the 300s), except that the spherical contour of this third type  900  of ball is also interrupted by a curvate recess  908 .  
         [0162]    Referring now to FIGS. 5 k - o , an assembled third preferred embodiment of the third embodiment family is shown in top (FIG. 5 k ), side (FIG. 51), side cutaway (FIG. 5 m ), perspective cutaway (FIG. 5 n ) and perspective (FIG. 5 o ) views. It can be seen that the curvate recess  908  forms the recess described above in the discussion of the manner in which these remaining embodiments limit rotation of the ball in the socket, and that the protrusion  818  serves as the protrusion described above in the same discussion. Thus, the protrusion  818  and recess  908  interact in the above described manner to limit the rotation of the ball  900  in the socket  807  defined by the curvate pockets  812 , 406 . Assembly of the disc is identical to that of the first preferred embodiment of the third embodiment family, except that the protrusion  818  is longitudinally aligned with the recess  908  during assembly so that the protrusion  818  is fitted within the recess  908  for interaction as described above as the ball  900  rotates and angulates in the socket  807 .  
         [0163]    Referring now to FIG. 5 p , an assembled alternate third preferred embodiment of the third embodiment family is shown in side cutaway view. This alternate third preferred embodiment incorporates a multi-part cap (with first part  4000   a  and second part  4000   b ) housing a spring member  4100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  4100 . Elements of this alternate third preferred embodiment that are also elements found in the third preferred embodiment are like numbered. (The cap features are numbered in the 4000&#39;s rather than the 400&#39;s.) The curvate recess  908  forms the recess described above, and the protrusion  818  serves as the protrusion described above, and thus the protrusion  818  and the recess  908  interact in the above described manner to limit the rotation of the ball  900  in the socket  807  defined by the curvate pockets  812 , 4060 .  
         [0164]    Assembly of this alternate third preferred embodiment is identical to that of the alternate first preferred embodiment of the third embodiment family, except that the protrusion  818  is longitudinally aligned with the recess  908  during assembly so that the protrusion  818  is fitted within the recess  908  for interaction as described above as the ball  900  rotates and angulates in the socket  807 . The cap second part  4000   b  is preferably not compressed into, but rather fits loosely within, the circular recess  816 , so that when the first baseplate  100  is compressed toward the second baseplate  800 , the cap second part  4000   b  may travel toward the cap first part  4000   a  as the spring member  4100  compresses (due to the cap first part  4000   a  being secured in the circular recess  816  to the second baseplate  800 ). The spring member  4100  is then disposed on the outwardly facing surface  4020   b  of the cap second part  4000   b . While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  4100  is formed of an elastomeric material, for example. The illustrated spring member  4100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  4020   b  of the cap second part  4000   b  as shown.  
         [0165]    Finally, the cap first part  4000   a  is secured in the circular recess  816  of the second baseplate  800  to incarcerate the cap second part  4000   b , and the spring member  4100  between the outwardly facing surface  4020   b  of the cap second part  4000   b  and the inwardly facing surface  4040   a  of the cap first part  4000   a . Although any suitable method is contemplated by the present invention, the cap first part  4000   a  preferably is secured in the circular recess  816  by compression locking (a laser weld can alternatively or additionally be used, or other suitable attachment means). The cap second part  4000   b  should be dimensioned such that, and the spring member  4100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  4020   b  of the cap second part  4000   b  and the inwardly facing surface  4040   a  of the cap first part  4000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  4100  will compress under an anticipated load. The cap first part  4000   a  preferably has an outwardly facing surface  4020   a  that complements the outwardly facing surface  802  of the second baseplate  800  for surface uniformity once the cap first part  4000   a  is secured. The cap first part  4000   a  may also additionally or alternatively be threaded into the circular recess  816  for increased stability of the attachment. Accordingly, in this alternate first preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  4100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0166]    In the fourth preferred embodiment of the third embodiment family of the present invention, a pin is secured in a pin hole so that the hemispherical head of the pin protrudes into the socket defined by the pocket of the convex structure and the pocket of the cap, and interacts in the above-described manner with a semicylindrical recess formed on the ball. More particularly, this fourth preferred embodiment uses the same first baseplate  100  and cap  400  of the first preferred embodiment, and the same ball  900  of the third preferred embodiment, but utilizes a fourth type of second baseplate of the third embodiment family. Referring to FIGS. 6 a - e , the fourth type  1000  of second baseplate is shown in top (FIG. 6 a ), side (FIG. 6 b ), side cutaway (FIG. 6 c ), perspective cutaway (FIG. 6 d ) and perspective (FIG. 6 e ) views. This fourth type  1000  of second baseplate is identical to the first type  200  of second baseplate described above (and thus similar features are reference numbered similar to those of the first type  200  of second baseplate, but in the 1000s rather than the 200s), except that this fourth type  1000  of second baseplate has a lateral through hole (e.g., a pin hole  1020 ) and a protrusion (e.g., a pin  1018 ) secured in the pin hole  1020  (as shown in FIGS. 6 f - j ) with the hemispherical head of the pin  1018  jutting out from the wall of the pocket  1012  toward the center of the pocket  1012  in the convex structure  1001 .  
         [0167]    Referring now to FIGS. 6 f - j , an assembled fourth preferred embodiment of the third embodiment family is shown in top (FIG. 6 f ), side (FIG. 6 g ), side cutaway (FIG. 6 h ), perspective cutaway (FIG. 6 i ) and perspective (FIG. 6 j ) views. It can be seen that the curvate recess  908  of the ball  900  forms the recess described above in the discussion of the manner in which these remaining embodiments limit rotation of the ball in the socket, and that the head of the pin  1018  serves as the protrusion described above in the same discussion. Thus, the head of the pin  1018  and the recess  908  interact in the above described manner to limit the rotation of the ball  900  in the socket  1007  defined by the curvate pockets  1012 , 406 . Assembly of the disc is identical to that of the first preferred embodiment of the third embodiment family, except that the head of the pin  1018  is longitudinally aligned with the recess  908  during assembly so that the head of the pin  1018  is fitted within the recess  908  for interaction as described above as the ball  900  rotates and angulates in the socket  1007 .  
         [0168]    Referring now to FIG. 6 k , an assembled alternate fourth preferred embodiment of the third embodiment family is shown in side cutaway view. This alternate fourth preferred embodiment incorporates a multi-part cap (with first part  4000   a  and second part  4000   b ) housing a spring member  4100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  4100 . Elements of this alternate fourth preferred embodiment that are also elements found in the fourth preferred embodiment are like numbered. (The cap features are numbered in the 4000&#39;s rather than the 400&#39;s.) The curvate recess  908  of the ball  900  forms the recess described above, and the head of the pin  1018  serves as the protrusion described above, and thus the head of the pin  1018  and the recess  908  interact in the above described manner to limit the rotation of the ball  900  in the socket  1007  defined by the curvate pockets  1012 , 4060 .  
         [0169]    Assembly of this alternate fourth preferred embodiment is identical to that of the alternate first preferred embodiment of the third embodiment family, except that the head of the pin  1018  is longitudinally aligned with the recess  908  during assembly so that the head of the pin  1018  is fitted within the recess  908  for interaction as described above as the ball  900  rotates and angulates in the socket  1007 . The cap second part  4000   b  is preferably not compressed into, but rather fits loosely within, the circular recess  1016 , so that when the first baseplate  100  is compressed toward the second baseplate  1000 , the cap second part  4000   b  may travel toward the cap first part  4000   a  as the spring member  4100  compresses (due to the cap first part  4000   a  being secured in the circular recess  1016  to the second baseplate  1000 ). The spring member  4100  is then disposed on the outwardly facing surface  4020   b  of the cap second part  4000   b . While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  4100  is formed of an elastomeric material, for example. The illustrated spring member  4100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  4020   b  of the cap second part  4000   b  as shown.  
         [0170]    Finally, the cap first part  4000   a  is secured in the circular recess  1016  of the second baseplate  1000  to incarcerate the cap second part  4000   b , and the spring member  4100  between the outwardly facing surface  4020   b  of the cap second part  4000   b  and the inwardly facing surface  4040   a  of the cap first part  4000   a . Although any suitable method is contemplated by the present invention, the cap first part  4000   a  preferably is secured in the circular recess  1016  by compression locking (a laser weld can alternatively or additionally be used, or other suitable attachment means). The cap second part  4000   b  should be dimensioned such that, and the spring member  4100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  4020   b  of the cap second part  4000   b  and the inwardly facing surface  4040   a  of the cap first part  4000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  4100  will compress under an anticipated load. The cap first part  4000   a  preferably has an outwardly facing surface  4020   a  that complements the outwardly facing surface  1002  of the second baseplate  1000  for surface uniformity once the cap first part  4000   a  is secured. The cap first part  4000   a  may also additionally or alternatively be threaded into the circular recess  1016  for increased stability of the attachment. Accordingly, in this alternate first preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  4100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0171]    In the fifth preferred embodiment of the third embodiment family of the present invention, a ball bearing protrudes into the socket defined by the pocket of the convex structure and the pocket of the cap, and interacts in the above-described manner with a semicylindrical recess formed on the ball. More particularly, this fifth preferred embodiment uses the same first baseplate  100  and cap  400  of the first preferred embodiment, and the same ball  900  of the third preferred embodiment, but utilizes a fifth type of second baseplate of the third embodiment family. Referring to FIGS. 7 a - e , the fifth type  1200  of second baseplate is shown in top (FIG. 7 a ), side (FIG. 7 b ), side cutaway (FIG. 7 c ), perspective cutaway (FIG. 7 d ) and perspective (FIG. 7 e ) views. This fifth type  1200  of second baseplate is identical to the first type  200  of second baseplate described above (and thus similar features are reference numbered similar to those of the first type  200  of second baseplate, but in the 1200s rather than the 200s), except that this fifth type  1200  of second baseplate has a recess  1218  adjacent the curvate pocket  1212  in the convex structure  1201 , the recess  1218  preferably being semicylindrical as shown.  
         [0172]    Referring now to FIGS. 7 f - j , an assembled fifth preferred embodiment of the third embodiment family is shown in top (FIG. 7 f ), side (FIG. 7 g ), side cutaway (FIG. 7 h ), perspective cutaway (FIG. 7 i ) and perspective (FIG. 7 j ) views. A ball bearing  1300  of the third embodiment family is captured for free rotation and angulation with one part closely accommodated in the semicylindrical recess  1218  and one part protruding into the curvate pocket  1212  to interact with the curvate recess  908  of the ball  900 . It can be seen that the curvate recess  908  of the ball  900  forms the recess described above in the discussion of the manner in which these remaining embodiments limit rotation of the ball in the socket, and that the ball bearing  1300  serves as the protrusion described above in the same discussion. Thus, the ball bearing  1300  and the recess  908  interact in the above described manner to limit the rotation of the ball  900  in the socket  1207  defined by the curvate pockets  1212 , 406 . Assembly of the disc is identical to that of the first preferred embodiment of the third embodiment family, except that the semicylindrical recess  1218  is longitudinally aligned with the curvate recess  908  during assembly so that the ball bearing  1300  can be and is then placed into the recesses  1218 , 908  for interaction as described above as the ball  900  rotates and angulates in the socket  1207 .  
         [0173]    Referring now to FIG. 7 k , an assembled alternate fifth preferred embodiment of the third embodiment family is shown in side cutaway view. This alternate fifth preferred embodiment incorporates a multi-part cap (with first part  4000   a  and second part  4000   b ) housing a spring member  4100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  4100 . Elements of this alternate fourth preferred embodiment that are also elements found in the fourth preferred embodiment are like numbered. (The cap features are numbered in the 4000&#39;s rather than the 400&#39;s.) The curvate recess  908  of the ball  900  forms the recess described above, and the ball bearing  1300  serves as the protrusion described above, and thus the ball bearing  1300  and the recess  908  interact in the above described manner to limit the rotation of the ball  900  in the socket  1207  defined by the curvate pockets  1212 , 4060 .  
         [0174]    Assembly of this alternate fifth preferred embodiment is identical to that of the alternate first preferred embodiment of the third embodiment family, except that the semicylindrical recess  1218  is longitudinally aligned with the curvate recess  908  during assembly so that the ball bearing  1300  can be and is then placed into the recesses  1218 , 908  for interaction as described above as the ball  900  rotates and angulates in the socket  1207 . The cap second part  4000   b  is preferably not compressed into, but rather fits loosely within, the circular recess  1216 , so that when the first baseplate  100  is compressed toward the second baseplate  1200 , the cap second part  4000   b  may travel toward the cap first part  4000   a  as the spring member  4100  compresses (due to the cap first part  4000   a  being secured in the circular recess  1216  to the second baseplate  1200 ). The spring member  4100  is then disposed on the outwardly facing surface  4020   b  of the cap second part  4000   b . While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  4100  is formed of an elastomeric material, for example. The illustrated spring member  4100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  4020   b  of the cap second part  4000   b  as shown.  
         [0175]    Finally, the cap first part  4000   a  is secured in the circular recess  1216  of the second baseplate  1200  to incarcerate the cap second part  4000   b , and the spring member  4100  between the outwardly facing surface  4020   b  of the cap second part  4000   b  and the inwardly facing surface  4040   a  of the cap first part  4000   a . Although any suitable method is contemplated by the present invention, the cap first part  4000   a  preferably is secured in the circular recess  1216  by compression locking (a laser weld can alternatively or additionally be used, or other suitable attachment means). The cap second part  4000   b  should be dimensioned such that, and the spring member  4100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  4020   b  of the cap second part  4000   b  and the inwardly facing surface  4040   a  of the cap first part  4000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  4100  will compress under an anticipated load. The cap first part  4000   a  preferably has an outwardly facing surface  4020   a  that complements the outwardly facing surface  1202  of the second baseplate  1200  for surface uniformity once the cap first part  4000   a  is secured. The cap first part  4000   a  may also additionally or alternatively be threaded into the circular recess  1216  for increased stability of the attachment. Accordingly, in this alternate first preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  4100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0176]    Embodiments of the fourth embodiment family of the present invention will now be described.  
         [0177]    With regard to the configuration of the convex structure in the fourth embodiment family, the convex structure is configured as a non-flexible element that has the socket of the ball and socket joint at its peak. In the preferred embodiment, the convex structure is shaped to have a curved taper, similar to the configuration of the convex structure in the third embodiment family. The convex structure in the fourth embodiment family is separated from the second baseplate during assembly of the device, for reasons related to the manner in which the ball is captured in the socket, but is attached to the second baseplate by the time assembly is complete.  
         [0178]    With regard to the manner in which the ball is captured in the socket in the fourth embodiment family, the capturing is effected through the use of a solid ball. In order to permit the seating of the ball into the socket formed at the peak of the convex structure, the convex structure is a separate element from the second baseplate. The ball is first seated against the central portion of the second baseplate (which central portion preferably has a concavity that has a curvature that closely accommodates the contour of the ball), and then the convex structure is placed over the ball to seat the ball in the socket formed in the interior of the peak of the convex structure (the interior is preferably formed as a concavity that is either hemispherical or less-than-hemispherical so that the ball can easily fit into it). After the convex structure is placed over the ball, the convex structure is attached to the second baseplate to secure the ball in the socket As in the third embodiment family, the peak of the convex structure has a bore that accommodates a post to which the ball and the first baseplate are attached (one to each end of the post), but does not accommodate the ball for passage through the bore. Accordingly, the ball is maintained in the socket.  
         [0179]    A first preferred embodiment of a fourth embodiment family of the present invention will now be described.  
         [0180]    Referring to FIGS. 8 a - e , a first baseplate  1400  of a fourth embodiment family of the present invention is shown in top (FIG. 8 a ), side (FIG. 8 b ), side cutaway (FIG. 8 c ), perspective cutaway (FIG. 8 d ) and perspective (FIG. 8 e ) views. Also referring to FIGS. 8 f - j , a first type  1500  of a second baseplate of the fourth embodiment family is shown in top (FIG. 8 f ), side (FIG. 8 g ), side cutaway (FIG. 8 h ), perspective cutaway (FIG. 8 i ) and perspective (FIG. 8 j ) views.  
         [0181]    More specifically, the first and second baseplates  1400 , 1500  are similar to the first and second baseplates of the third embodiment family described above with regard to their outwardly facing surfaces  1402 , 1502  having a convex dome  1403 , 1503  and a plurality of spikes  1405 , 1505  as vertebral body contact elements, and the inwardly facing surface  1408  of the first baseplate having a perimeter region  1410 , all of which elements in the fourth embodiment family are, for example, identical to the corresponding elements in the third embodiment family as described above. Preferably, the dome  1403 , 1503  is covered with an osteoconductive layer of a type known in the art. It should be noted that the convex mesh used in other embodiments of the present invention is suitable for use with these other vertebral body contact elements, and can be attached over the convex dome  1403 , 1503  by laser welding, or more preferably, by plasma burying (where the perimeter region of the convex mesh is buried under a plasma coating, which coating secures to the outwardly facing surface of the baseplate to which it is applied, and thus secures the convex mesh to the outwardly facing surface).  
         [0182]    For example, and referring now to FIGS. 8 aa - 8   dd , an alternate first baseplate  9400  of the fourth embodiment family is shown in top (FIG. 8 aa ) and side cutaway (FIG. 8 bb ) views, respectively, and an alternate second baseplate  9500  of the fourth embodiment family is shown in top (FIG. 8 cc ) and side cutaway (FIG. 8 dd ) views, respectively. The alternate first and second baseplates  9400 , 9500  are similar to the first and second baseplates of the fourth embodiment family described above, having identical features numbered in the 9400&#39;s and 9500&#39;s rather than the 1400&#39;s and 1500&#39;s, respectively. However, the alternate baseplates are different in that each has a convex mesh  9450 , 9550  attached to the outwardly facing surface  9402 , 9502  by burying the perimeter of the mesh  9450 , 9550  in a plasma coating (or other suitable material, preferably having an osteoconductive surface)  9452 , 9552  that is secured to both the outwardly facing surface  9402 , 9502  and the mesh  9450 , 9550 . The plasma coating  9452 , 9552  serves not only to secure the mesh  9450 , 9550 , but also to facilitate securing of the baseplates to the adjacent vertebral endplates. It should be understood that these alternate baseplates can be used in place of the other baseplates discussed herein, to construct artificial discs contemplated by the present invention. It should further be understood that the described manner of attaching the wire mesh can be applied to other orthopedic devices, such as but not limited to intervertebral spacers that prevent motion, or for other intervertebral spacers that preserve motion.  
         [0183]    Further, as with the first embodiment family, the two baseplates  1400 , 1500  are joined with a ball and socket joint, and therefore each of the baseplates  1400 , 1500  comprises features that, in conjunction with other components described below, form the ball and socket joint. The ball and socket joint includes a solid ball (described below) mounted to protrude from the inwardly facing surface  1408  of the first baseplate  1400 , and a curvate socket formed at a peak of a non-flexible convex structure (described below) that is attached to the inwardly facing surface  1508  of the second baseplate  1500 , within which curvate socket the ball is capturable for free rotation and angulation therein. As shown in FIGS. 8 a - d , the mounting for the ball includes a central inwardly directed post  1412  that extends from the inwardly facing surface  1408  of the first baseplate  1400 , which post&#39;s head end compression locks into a central bore in the ball (described below). As shown in FIGS. 8 e - h , the second baseplate  1500  includes an inwardly facing surface  1508  and a curvate pocket  1512  formed by a central portion of the inwardly facing surface  1508  concaving outwardly with a semispherical contour (preferably a hemispherical contour). Preferably, as shown, the curvate pocket  1512  is surrounded by a circumferential wall  1514  and a circumferential recess  1516  that cooperate with the convex structure to attach the convex structure to the second baseplate  1500 .  
         [0184]    Referring now to FIGS. 8 k - o , a first type  1600  of a ball of the fourth embodiment family is shown in top (FIG. 8 k ), side (FIG. 81), side cutaway (FIG. 8 m ), perspective cutaway (FIG. 8 n ) and perspective (FIG. 8 o ) views. The ball  1600  is semispherical (preferably greater than hemispherical as shown) and therefore defines a spherical contour, and has a central bore  1610  within which the first baseplate&#39;s post&#39;s head end is securable. The ball  1600  seats in the curvate pocket  1512  of the second baseplate  1500  with the spherical contour defined by the ball  1600  closely accommodated by the hemispherical contour of the curvate pocket  1512  for free rotation and free angulation of the ball  1600  in the curvate pocket  1512 .  
         [0185]    Referring now to FIGS. 8 p - t , a first type  1700  of a convex structure of the fourth embodiment family is shown in top (FIG. 8 p ), side (FIG. 8 q ), side cutaway (FIG. 8 r ), perspective cutaway (FIG. 8 s ) and perspective (FIG. 8 t ) views. The convex structure  1700  is shaped to have a curved taper on its inwardly facing surface  1706  (as opposed to the frusto-conical shape of the convex structure in the first and second embodiment families) and includes a central bore  1702  extending from an outwardly facing surface  1704  of the convex structure  1700  to an inwardly facing surface  1706  of the convex structure  1700 , the bore  1702  being surrounded by a curvate taper  1708  on the outwardly facing surface  1704 , and the curvate taper  1708  being surrounded by a circumferential recess  1710  and a circumferential wall  1712 . The convex structure  1700  is securable to the second baseplate  1500  with the circumferential recess  1710  of the convex structure  1700  mating with the circumferential wall  1514  of the second baseplate  1600  and the circumferential wall  1712  of the convex structure  1700  mating with the circumferential recess  1516  of the second baseplate  1500 , so that when the convex structure  1700  is so secured, the curvate taper  1708  of the convex structure  1700  serves as a curvate pocket opposite the curvate pocket  1512  of the second baseplate  1500 . That is, the curvate pocket  1708  complements the hemispherical contour of the curvate pocket  1512  of the second baseplate  1500  to form a semispherical (and preferably greater than hemispherical as shown) socket  1707  defining a spherical contour that closely accommodates the spherical contour defined by the ball  1600  so that the ball  1600  is captured in the socket  1707  for free rotation and free angulation of the ball  1600  therein. (When the formed socket  1707  is greater than hemispherical, and the shape of the ball  1600  is greater than hemispherical, the ball  1600  cannot escape the formed socket  1707 .) Further, the inwardly facing surface  1706  of the convex structure  1700  has a perimeter region  1714  that faces the perimeter region  1410  of the first baseplate  1400  when the convex structure  1700  is secured to the second baseplate  1500 .  
         [0186]    Referring now to FIGS. 8 u - y , an assembled first preferred embodiment of the fourth embodiment family is shown in top (FIG. 8 u ), side (FIG. 8 v ), side cutaway (FIG. 8 w ), perspective cutaway (FIG. 8 x ) and perspective (FIG. 8 y ) views. More particularly, assembly of the disc is preferably as follows. The ball  1600  is seated within the curvate pocket  1512  of the second baseplate  1500  (the curvate pocket  1512  has an opening diameter that accommodates the ball  1600 ) so that the spherical contour defined by the ball  1600  is closely accommodated by the hemispherical contour of the curvate pocket  1512 . Thereafter, the convex structure  1700  is secured to the second baseplate  1500  as described above with the convex structure&#39;s curvate pocket  1708  (the curvate tapered lip  1708  of the convex structure&#39;s central bore  1702 ) fitting against the ball  1600  so that the ball  1600  is captured in the socket  1707  (formed by the curvate taper  1708  and the curvate pocket  1512 ) for free rotation and free angulation of the ball  1600  therein. Thereafter, the first baseplate&#39;s post&#39;s head end is secured into the bore  1602  of the ball  1600 . The central bore  1702  of the convex structure  1700  has a diameter that accommodates the diameter of the post  1412 , but not the diameter of the ball  1600 . Therefore, after the ball  1600  is secured in the socket  1707 , the post  1412  fits through the bore  1702  so that the head end of the post  1412  can be compression locked to the ball  1600 , but the ball  1600  is prevented from escaping the socket  1707  through the central bore  1702  of the convex structure  1700 .  
         [0187]    Accordingly, the ball  1600  is captured in the socket  1707  (so that the device will not separate in tension), can freely rotate in the socket  1707  about the longitudinal axis of the post  1412 , and can freely angulate in the socket  1707  about a centroid of motion located at the center of the sphere defined by the ball  1600 . Further, the opening of the bore  1702  of the cap  1700  on the inwardly facing surface  1706  of the convex structure  1700  is large enough to permit the post  1412  to angulate (about the centroid of motion at the center of the sphere defined by the ball  1600 ) with respect to the bore  1702  as the ball  1600  angulates in the socket  1707 . Preferably, the conformation of the bore  1702  accommodates angulation of the post  1412  at least until the perimeter regions  1410 , 1714  of the inwardly facing surfaces  1408 , 1508 / 1706  meet. Further preferably, the perimeter regions  1410 , 1714  have corresponding contours, so that the meeting of the perimeter regions reduces any surface wearing.  
         [0188]    Referring now to FIG. 8 z , an assembled alternate first preferred embodiment of the fourth embodiment family is shown in side cutaway view. This alternate first preferred embodiment incorporates a multi-part second baseplate (with first part  15000   a  and second part  15000   b ) housing a spring member  15100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  15100 . Elements of this alternate first preferred embodiment that are also elements found in the first preferred embodiment of the fourth embodiment family are like numbered, and the assembly of this alternate first preferred embodiment is identical to that of the first preferred embodiment, with some differences due to the incorporation of the spring member  15100 . (For example, the second baseplate features are numbered in the 15000&#39;s rather than the 1500&#39;s.) More particularly, assembly of the disc is preferably as follows. The ball  1600  is seated within the curvate pocket  15120  of the inwardly facing surface  15090   b  to the second baseplate second part  15000   b  (the curvate pocket  15120  has an opening diameter that accommodates the ball  1600 ) so that the spherical contour defined by the ball  1600  is closely accommodated by the hemispherical contour of the curvate pocket  15120 . The spring member  15100  is then disposed on the outwardly facing surface  15020   b  of the second baseplate second part  15000   b . While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  15100  is formed of an elastomeric material, for example. The illustrated spring member  15100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  15020   b  of the second baseplate second part  15000   b  as shown.  
         [0189]    The ball  1600 , second baseplate second part  15000   b , and spring member  15100  are then disposed on the inwardly facing surface  15090   a  of the second baseplate first part  15000   a , such that the spring member  15100  is incarcerated between the inwardly facing surface  15090   a  of the second baseplate first part  15000   a  and the outwardly facing surface  15020   b  of the second baseplate second part  15000   b . The second baseplate second part  15000   b  should be dimensioned such that, and the spring member  15100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  15020   b  of the second baseplate second part  15000   b  and the inwardly facing surface  15090   a  of the second baseplate first part  15000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  15100  will compress under an anticipated load. Thereafter, the convex structure  1700  is secured to the second baseplate first part  15000   a , with the convex structure&#39;s curvate pocket  1708  (the curvate tapered lip  1708  of the convex structure&#39;s central bore  1702 ) fitting against the ball  1600  so that the ball  1600  is captured in the socket  1707  (formed by the curvate taper  1708  and the curvate pocket  15120 ) for free rotation and free angulation of the ball  1600  therein. Although any suitable method is contemplated by the present invention, the convex structure  1700  preferably is secured by compression locking (a laser weld can alternatively or additionally be used, or other suitable attachment means). The second baseplate first part  15000   a  may also additionally or alternatively be threaded to the convex structure  1700  for increased stability of the attachment. It should be understood that the second baseplate second part  15000   b  preferably fits loosely within the convex structure  1700  and the second baseplate first part  15000   a , so that when the first baseplate  1400  is compressed toward the second baseplate first part  15000   a , the second baseplate second part  15000   b  may travel toward the second baseplate first part  15000   a  as the spring member  15100  compresses. Thereafter, the first baseplate&#39;s post&#39;s head end is secured into the bore  1602  of the ball  1600 . The central bore  1702  of the convex structure  1700  has a diameter that accommodates the diameter of the post  1412 , but not the diameter of the ball  1600 . Therefore, after the ball  1600  is secured in the socket  1707 , the post  1412  fits through the bore  1702  so that the head end of the post  1412  can be compression locked to the ball  1600 , but the ball  1600  is prevented from escaping the socket  1707  through the central bore  1702  of the convex structure  1700 .  
         [0190]    Accordingly, the ball  1600  is captured in the socket  1707  (so that the device will not separate in tension), can freely rotate in the socket  1707  about the longitudinal axis of the post  1412 , and can freely angulate in the socket  1707  about a centroid of motion located at the center of the sphere defined by the ball  1600 . Further, the opening of the bore  1702  of the convex structure  1700  on the inwardly facing surface  1706  of the convex structure  1700  is large enough to permit the post  1412  to angulate (about the centroid of motion at the center of the sphere defined by the ball  1600 ) with respect to the bore  1702  as the ball  1600  angulates in the socket  1707 . Preferably, the conformation of the bore  1702  accommodates angulation of the post  1412  at least until the perimeter regions  1410 , 1714  of the inwardly facing surfaces  1408 , 15080 / 1706  meet. Further preferably, the perimeter regions  1410 , 1714  have corresponding contours, so that the meeting of the perimeter regions reduces any surface wearing. Further accordingly, in this alternate first preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  15100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0191]    Accordingly, when a device of the first preferred embodiment of the fourth embodiment family is assembled, the baseplates  1400 , 1500  (or  1400 , 15000   a ) are rotatable relative to one another because the ball  1600  rotates freely within the socket  1707 , and angulatable relative to one another because the ball  1600  angulates freely within the socket  1707 . Because the ball  1600  is held within the socket  1707  by the securing of the tail end of the central post  1412  of the first baseplate  1400  to the ball  1600  and the securing of the convex structure  1700  to the second baseplate  1500  (or second baseplate first part  15000   a ), the artificial disc can withstand tension loading of the baseplates  1400 , 1500  (or  1400 , 15000   a ). More particularly, when a tension load is applied to the baseplates  1400 , 1500  (or  1400 , 15000   a ) the ball  1600  seeks to pass through the bore  1702  in the convex structure  1700 . However, the curvate taper  1708  of the bore  1702  prevents the ball  1600  from exiting the socket  1707 . Therefore, the assembly does not come apart under normally experienced tension loads. This ensures that no individual parts of the assembly will pop out or slip out from between the vertebral bodies when, e.g., the patient stretches or hangs while exercising or performing other activities. Thus, in combination with the securing of the baseplates  1400 , 1500  (or  1400 , 15000   a ) to the adjacent vertebral bones via the domes  1403 , 1503  (or  1403 , 15030 ) and spikes  1405 , 1505  (or  1405 , 15050 ), the disc assembly has an integrity similar to the tension-bearing integrity of a healthy natural intervertebral disc. Also, because the ball  1600  is laterally captured in the socket  1707 , lateral translation of the baseplates  1400 , 1500  (or  1400 , 15000   a ) relative to one another is prevented during rotation and angulation, similar to the performance of healthy natural intervertebral disc. Because the baseplates  1400 , 1500  (or  1400 , 15000   a ) are made angulatable relative to one another by the ball  1600  being rotatably and angulatably coupled in the socket  1707 , the disc assembly provides a centroid of motion within the sphere defined by the ball  1600 . 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.  
         [0192]    The remaining embodiments in the fourth embodiment family of the present invention limit the rotation (but preferably not the angulation) of the ball in the socket formed by the curvate taper of the convex structure and the hemispherical contour of the curvate pocket of the second baseplate. Each embodiment accomplishes this in a different manner, but each embodiment utilizes interference between a protrusion and a recess to limit the rotation, similar to the manner in which such interference is utilized in the third embodiment family. In some embodiments, the protrusion is preferably hemispherical, and the recess preferably has a semicylindrical contour within which the protrusion fits. In other embodiments, the protrusion is preferably hemispherical, and the recess preferably has a curvate contour that is not semicylindrical. (It should be understood that the described formations of the recess and the protrusion are merely preferred, and that alternate formations, curvate or otherwise, for each are contemplated by the present invention; a particular shape or location of recess or a particular shape or location of protrusion is not required; any shape can be used so long as the recess and protrusion interact as desired. For example, the recess in the second preferred embodiment of the fourth embodiment family has a curvate contour that is not semicylindrical, and the recess in the fifth preferred embodiment of the fourth embodiment family has a different curvate contour that is not semicylindrical, each being formed so that it optimally interacts with the protrusion in its respective embodiment.) The boundaries of the recess define the limits of rotation of the ball within the socket, by allowing movement of the protrusion relative to the recess as the ball rotates through a certain range in the socket, but providing interference with the protrusion to prevent rotation of the ball beyond that range in the socket. Preferably, for example, the recess has a depth equivalent to the radius of the hemispherical protrusion, but a radius of curvature greater than that of the protrusion. At the same time, the boundaries of the recess preferably do not limit the angulation of the ball within the socket, at least until the perimeter regions of the inwardly facing surface of the convex structure and the inwardly facing surface of the first baseplate meet. Preferably, for example, the recess has a length greater than the range of movement of the protrusion relative to the recess as the ball angulates in the socket.  
         [0193]    Therefore, when assembled, the discs of the remaining preferred embodiments of the fourth embodiment family enable angulation and limited rotation of the baseplates relative to one another about a centroid of motion that remains centrally located between the baseplates (at the center of the sphere defined by the ball), similar to the centroid of motion in a healthy natural intervertebral disc that is limited in its rotation by surrounding body structures. A benefit of limiting the relative rotation of the baseplates is that relative rotation beyond a certain range in a healthy natural disc is neither needed nor desired, because, for example, excess strain can be placed on the facet joints or ligaments thereby. As described with the first preferred embodiment of the fourth embodiment family, the construction also prevents translation and separation of the baseplates relative to one another during rotation and angulation.  
         [0194]    As noted above, each of the remaining preferred embodiments in this fourth embodiment family forms the protrusion and corresponding recess in a different manner, utilizing components that are either identical or similar to the components of the first preferred embodiment, and some embodiments utilize additional components. Each of the remaining preferred embodiments will now be described in greater detail.  
         [0195]    In the second preferred embodiment of the fourth embodiment family of the present invention, a hemispherical protrusion is formed on the ball, and interacts in the above-described manner with a recess formed adjacent the socket formed by the curvate taper of the convex structure and the hemispherical contour of the curvate pocket of the second baseplate. More particularly, this second preferred embodiment uses the same first baseplate  1400  as the first preferred embodiment of the fourth embodiment family described above. Referring to FIGS. 9 a - e , a second type  1800  of second baseplate of the fourth embodiment family is shown in to top (FIG. 9 a ), side (FIG. 9 b ), side cutaway (FIG. 9 c ), perspective cutaway (FIG. 9 d ) and perspective (FIG. 9 e ) views. This second type  1800  of second baseplate is identical to the first type  1500  of second baseplate described above (and thus similar features are reference numbered similar to those of the first type  1500  of second baseplate, but in the 1800s rather than the 1500s), except that this second type  1800  of second baseplate has a curvate recess  1818  adjacent the curvate pocket  1812 , and preferably in the circumferential wall  1814 .  
         [0196]    Referring now to FIGS. 9 f - j , a second type  1900  of ball of the fourth embodiment family is shown in top (FIG. 9 f ), side (FIG. 9 g ), side cutaway (FIG. 9 h ), perspective cutaway (FIG. 9 i ) and perspective (FIG. 9 j ) views. The ball  1900  is identical to the first type  1600  of ball described above (and thus similar features are reference numbered similar to those of the first type  1600  of ball, but in the 1900s rather than the 1600s), except that the semispherical contour of this second type  1900  of ball is also interrupted by a hemispherical protrusion  1904 .  
         [0197]    Referring now to FIGS. 9 k - o , a second type  2000  of convex structure of the fourth embodiment family is shown in top (FIG. 9 k ), side (FIG. 91), side cutaway (FIG. 9 m ), perspective cutaway (FIG. 9 n ) and perspective (FIG. 9 o ) views. This second type  2000  of convex structure is identical to the first type  1700  of convex structure described above (and thus similar features are reference numbered similar to those of the first type  1700  of convex structure, but in the 2000s rather than the 1700s), except that this second type  2000  of convex structure has a curvate recess  2016  adjacent the curvate taper  2008 .  
         [0198]    Referring now to FIGS. 9 p - t , an assembled second preferred embodiment of the fourth embodiment family is shown in top (FIG. 9 p ), side (FIG. 9 q ), side cutaway (FIG. 9 r ), perspective cutaway (FIG. 9 s ) and perspective (FIG. 9 t ) views. It can be seen that the curvate recesses  1818 , 2016  together form the recess described above in the discussion of the manner in which these remaining embodiments limit rotation of the ball in the socket formed by the curvate taper of the convex structure and the hemispherical contour of the curvate pocket of the second baseplate, and that the protrusion  1904  serves as the protrusion described above in the same discussion. Thus, the protrusion  1904  and recesses  1818 , 2016  interact in the above described manner to limit the rotation of the ball  1900  in the socket  2007 . Assembly of the disc is identical to that of the first preferred embodiment of the fourth embodiment family, except that the protrusion  1904  is longitudinally aligned with the recess  1818 , and the recess  2016  is similarly aligned, so that when the convex structure  2000  is secured to the second baseplate  1800 , the protrusion  1904  is fitted within the recesses  1818 , 2016  for interaction as described above as the ball  1900  rotates and angulates in the socket  2007 .  
         [0199]    Referring now to FIG. 9 u , an assembled alternate second preferred embodiment of the fourth embodiment family is shown in side cutaway view. This alternate second preferred embodiment incorporates a multi-part second baseplate (with first part  18000   a  and second part  18000   b ) housing a spring member  18100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  18100 . Elements of this alternate second preferred embodiment that are also elements found in the second preferred embodiment of the fourth embodiment family are like numbered. (The second baseplate features are numbered in the 18000&#39;s rather than the 1800&#39;s.) The curvate recesses  18180 , 2016  together form the recess described above, and the protrusion  1904  serves as the protrusion described above, and thus the protrusion  1904  and recesses  18180 , 2016  interact in the above described manner to limit the rotation of the ball  1900  in the socket  2007 .  
         [0200]    Assembly of this alternate second preferred embodiment is identical to that of the first preferred embodiment of the fourth embodiment family, except that the protrusion  1904  is longitudinally aligned with the recess  18180 , and the recess  2016  is similarly aligned, so that when the convex structure  2000  is secured to the second baseplate first part  18000   a , the protrusion  1904  is fitted within the recesses  18180 , 2016  for interaction as described above as the ball  1900  rotates and angulates in the socket  2007 . It should be understood that the second baseplate second part  18000   b  preferably fits loosely within the convex structure  2000  and the second baseplate first part  18000   a , so that when the first baseplate  1400  is compressed toward the second baseplate first part  18000   a , the second baseplate second part  18000   b  may travel toward the second baseplate first part  18000   a  as the spring member  18100  compresses. While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  18100  is formed of an elastomeric material, for example. The illustrated spring member  18100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  18020   b  of the second baseplate second part  18000   b  as shown. The second baseplate second part  18000   b  should be dimensioned such that, and the spring member  18100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  18020   b  of the second baseplate second part  18000   b  and the inwardly facing surface  18090   a  of the second baseplate first part  18000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  18100  will compress under an anticipated load. Accordingly, in this alternate second preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  18100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0201]    In the third preferred embodiment of the fourth embodiment family of the present invention, a hemispherical protrusion is formed to protrude into the socket formed by the curvate taper of the convex structure and the hemispherical contour of the curvate pocket of the second baseplate, and interacts in the above-described manner with a semicylindrical recess formed on the ball. More particularly, this third preferred embodiment uses the same first baseplate  1400  as the first preferred embodiment of the fourth embodiment family described above. Referring to FIGS. 10 a - e , a third type  2100  of second baseplate of the fourth embodiment family is shown in top (FIG. 10 a ), side (FIG. 10 b ), side cutaway (FIG. 10 c ), perspective cutaway (FIG. 10 d ) and perspective (FIG. 10 e ) views. This third type  2100  of second baseplate is identical to the first type  1500  of second baseplate described above (and thus similar features are reference numbered similar to those of the first type  1500  of second baseplate, but in the 2100s rather than the 1500s), except that this third type  2100  of second baseplate has a recess  2118  adjacent the curvate pocket  2112 , and preferably in the circumferential wall  2114  as shown.  
         [0202]    Referring now to FIGS. 10 f - j , a third type  2200  of ball of the fourth embodiment family is shown in top (FIG. 10 f ), side (FIG. 10 g ), side cutaway (FIG. 10 h ), perspective cutaway (FIG. 10 i ) and perspective (FIG. 10 j ) views. The ball  2200  is identical to the first type  1600  of ball described above (and thus similar features are reference numbered similar to those of the first type  1600  of ball, but in the 2200s rather than the 1600s), except that the semispherical contour of this third type  2200  of ball is also interrupted by a curvate recess  2204 .  
         [0203]    Referring now to FIGS. 10 k - o , a third type  2300  of convex structure of the fourth embodiment family is shown in top (FIG. 10 k ), side (FIG. 10 l ), side cutaway (FIG. 10 m ), perspective cutaway (FIG. 10 n ) and perspective (FIG. 10 o ) views. This third type  2300  of convex structure is identical to the first type  1700  of convex structure described above (and thus similar features are reference numbered similar to those of the first type  1700  of convex structure, but in the 2300s rather than the 1700s), except that this third type  2300  of convex structure has a protrusion  2316  adjacent the curvate taper  2008 .  
         [0204]    Referring now to FIGS. 10 p - t , an assembled third preferred embodiment of the fourth embodiment family is shown in top (FIG. 10 p ), side (FIG. 10 q ), side cutaway (FIG. 10 r ), perspective cutaway (FIG. 10 s ) and perspective (FIG. 10 t ) views. It can be seen that the curvate recess  2204  of the ball  2200  forms the recess described above in the discussion of the manner in which these remaining embodiments limit rotation of the ball in the socket formed by the curvate taper of the convex structure and the hemispherical contour of the curvate pocket of the second baseplate, and that the protrusion  2316  fits into the recess  2118  to serve as the protrusion described above in the same discussion. Thus, the protrusion  2316  and the recess  2204  interact in the above described manner to limit the rotation of the ball  2200  in the socket  2307 . Assembly of the disc is identical to that of the first preferred embodiment of the fourth embodiment family, except that the protrusion  2316  is longitudinally aligned with the recess  2204  and the recess  2118  during assembly so that the protrusion  2316  fits into the recess  2118  to extend into the recess  2204  for interaction as described above as the ball  2200  rotates and angulates in the socket  2307 .  
         [0205]    Referring now to FIG. 10 u , an assembled alternate third preferred embodiment of the fourth embodiment family is shown in side cutaway view. This alternate third preferred embodiment incorporates a multi-part second baseplate (with first part  21000   a  and second part  21000   b ) housing a spring member  21100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  21100 . Elements of this alternate third preferred embodiment that are also elements found in the third preferred embodiment of the fourth embodiment family are like numbered. (The second baseplate features are numbered in the 21000&#39;s rather than the 2100&#39;s.) The curvate recess  2204  of the ball  2200  forms the recess described above, and the protrusion  2316  fits into the recess  21180  to serve as the protrusion described above, and thus, the protrusion  2316  and the recess  2204  interact in the above described manner to limit the rotation of the ball  2200  in the socket  2307 .  
         [0206]    Assembly of this alternate third preferred embodiment is identical to that of the first preferred embodiment of the fourth embodiment family, except that the protrusion  2316  is longitudinally aligned with the recess  2204  and the recess  21180  during assembly so that the protrusion  2316  fits into the recess  21180  to extend into the recess  2204  for interaction as described above as the ball  2200  rotates and angulates in the socket  2307 . It should be understood that the second baseplate second part  21000   b  preferably fits loosely within the convex structure  2300  and the second baseplate first part  21000   a , so that when the first baseplate  1400  is compressed toward the second baseplate first part  21000   a , the second baseplate second part  21000   b  may travel toward the second baseplate first part  21000   a  as the spring member  21100  compresses. While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  21100  is formed of an elastomeric material, for example. The illustrated spring member  21100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  21020   b  of the second baseplate second part  21000   b  as shown. The second baseplate second part  21000   b  should be dimensioned such that, and the spring member  21100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  21020   b  of the second baseplate second part  21000   b  and the inwardly facing surface  21090   a  of the second baseplate first part  21000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  21100  will compress under an anticipated load. Accordingly, in this alternate third preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  21100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0207]    In the fourth preferred embodiment of the fourth embodiment family of the present invention, a pin is secured in a pin hole so that the hemispherical head of the pin protrudes into the socket formed by the curvate taper of the convex structure and the hemispherical contour of the curvate pocket of the second baseplate, and interacts in the above-described manner with a semicylindrical recess formed on the ball. More particularly, this fourth preferred embodiment uses the same first baseplate  1400  of the first preferred embodiment, and the same ball  2200  and second baseplate  2100  of the fourth preferred embodiment. Referring to FIGS. 11 a - e , a fourth type  2400  of convex structure of the fourth embodiment family is shown in top (FIG. 11 a ), side (FIG. 11 b ), side cutaway (FIG. 11 c ), perspective cutaway (FIG. 11 d ) and perspective (FIG. 11 e ) views. This fourth type  2400  of convex structure is identical to the first type  1700  of convex structure described above (and thus similar features are reference numbered similar to those of the first type  1700  of convex structure, but in the 2400s rather than the 1700s), except that this fourth type  2400  of convex structure has a lateral through hole (e.g., a pin hole  2416 ) and a protrusion (e.g., a pin  2418 ) secured in the pin hole  2416  (as shown in FIGS. 11 f - j ) and jutting into the socket  2407 .  
         [0208]    Referring now to FIGS. 11 f - j , an assembled fourth preferred embodiment of the fourth embodiment family is shown in top (FIG. 11 f ), side (FIG. 11 g ), side cutaway (FIG. 11 h ), perspective cutaway (FIG. 11 i ) and perspective (FIG. 11 j ) views. It can be seen that the curvate recess  2204  of the ball  2200  forms the recess described above in the discussion of the manner in which these remaining embodiments limit rotation of the ball in the socket formed by the curvate taper of the convex structure and the hemispherical contour of the curvate pocket of the second baseplate, and that the head of the pin  2418  serves as the protrusion described above in the same discussion. Thus, the head of the pin  2418  and the recess  2204  interact in the above described manner to limit the rotation of the ball  2200  in the socket  2407 . Assembly of the disc is identical to that of the first preferred embodiment of the fourth embodiment family, except that the head of the pin  2418  is longitudinally aligned with the recess  2204  and the recess  2118  during assembly so that the head of the pin  2418  fits into the recess  2118  to extend into the recess  2204  for interaction as described above as the ball  2200  rotates and angulates in the socket  2407 .  
         [0209]    Referring now to FIG. 11 k , an assembled alternate fourth preferred embodiment of the fourth embodiment family is shown in side cutaway view. This alternate fourth preferred embodiment incorporates a multi-part second baseplate (with first part  21000   a  and second part  21000   b ) housing a spring member  21100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  21100 . Elements of this alternate fourth preferred embodiment that are also elements found in the fourth preferred embodiment of the fourth embodiment family are like numbered. (The second baseplate features are numbered in the 21000&#39;s rather than the 2100&#39;s.) The curvate recess  2204  of the ball  2200  forms the recess described above, and the head of the pin  2418  serves as the protrusion described above, and thus, the head of the pin  2418  and the recess  2204  interact in the above described manner to limit the rotation of the ball  2200  in the socket  2407 .  
         [0210]    Assembly of this alternate fourth preferred embodiment is identical to that of the first preferred embodiment of the fourth embodiment family, except that the head of the pin  2418  is longitudinally aligned with the recess  2204  and the recess  21180  during assembly so that the head of the pin  2418  fits into the recess  21180  to extend into the recess  2204  for interaction as described above as the ball  2200  rotates and angulates in the socket  2407 . It should be understood that the second baseplate second part  21000   b  preferably fits loosely within the convex structure  2400  and the second baseplate first part  21000   a , so that when the first baseplate  1400  is compressed toward the second baseplate first part  21000   a , the second baseplate second part  21000   b  may travel toward the second baseplate first part  21000   a  as the spring member  21100  compresses. While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  21100  is formed of an elastomeric material, for example. The illustrated spring member  21100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  21020   b  of the second baseplate second part  21000   b  as shown. The second baseplate second part  21000   b  should be dimensioned such that, and the spring member  21100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  21020   b  of the second baseplate second part  21000   b  and the inwardly facing surface  21090   a  of the second baseplate first part  21000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  21100  will compress under an anticipated load. Accordingly, in this alternate first preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  21100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0211]    In the fifth preferred embodiment of the fourth embodiment family of the present invention, a ball bearing protrudes into the socket formed by the curvate taper of the convex structure and the hemispherical contour of the curvate pocket of the second baseplate, and interacts in the above-described manner with a recess formed on the ball. More particularly, this fifth preferred embodiment uses the same first baseplate  1400  of the first preferred embodiment, and the same second baseplate  2100  of the third preferred embodiment. Referring to FIGS. 12 a - e , a fifth type  2500  of convex structure of the fourth embodiment family is shown in top (FIG. 12 a ), side (FIG. 12 b ), side cutaway (FIG. 12 c ), perspective cutaway (FIG. 12 d ) and perspective (FIG. 12 e ) views. This fifth type  2500  of convex structure is identical to the first type  1700  of convex structure described above (and thus similar features are reference numbered similar to those of the first type  1700  of convex structure, but in the 2500s rather than the 1700s), except that this fifth type  2500  of convex structure has a has a recess  2516  adjacent the curvate taper  2508 .  
         [0212]    Referring to FIGS. 12 f - j , a fourth type of ball  2700  of the fourth embodiment family is shown in top (FIG. 12 f ), side (FIG. 12 g ), side cutaway (FIG. 12 h ), perspective cutaway (FIG. 12 i ) and perspective (FIG. 12 j ) views. The ball  2700  is identical to the first type  1600  of ball described above (and thus similar features are reference numbered similar to those of the first type  1600  of ball, but in the 2700s rather than the 1600s), except that the semispherical contour of this third type  2700  of ball is also interrupted by a curvate recess  2704 .  
         [0213]    Referring now to FIGS. 12 k - o , an assembled fifth preferred embodiment of the fourth embodiment family is shown in top (FIG. 12 k ), side (FIG. 121), side cutaway (FIG. 12 m ), perspective cutaway (FIG. 12 n ) and perspective (FIG. 12 o ) views. A ball bearing  2600  of the fourth embodiment family is captured for free rotation and angulation, with one part of the ball bearing  2600  closely accommodated in the recesses  2118 , 2516 , and another part of the ball bearing  2600  protruding into the socket to interact with the curvate recess  2704  of the ball  2700 . It can be seen that the curvate recess  2704  of the ball  2700  forms the recess described above in the discussion of the manner in which these remaining embodiments limit rotation of the ball in the socket, and that the ball bearing  2600  serves as the protrusion described above in the same discussion. Thus, the ball bearing  2600  and the recess  2704  interact in the above described manner to limit the rotation of the ball  2700  in the socket  2507 . Assembly of the disc is identical to that of the first preferred embodiment of the fourth embodiment family, except that the recess  2704  is aligned with the curvate recess  2118  during assembly so that the ball bearing  2600  can be and is then placed into the recesses  2118 , 2704  (and then captured in the recess  2118  by the recess  2516  of the convex structure  2500 ) for interaction as described above as the ball  2700  rotates and angulates in the socket  2507 .  
         [0214]    Referring now to FIG. 12 p , an assembled alternate fifth preferred embodiment of the fourth embodiment family is shown in side cutaway view. This alternate fifth preferred embodiment incorporates a multi-part second baseplate (with first part  21000   a  and second part  21000   b ) housing a spring member  21100  that provides axial compressibility, such that a compressive load applied to the baseplates is borne by the spring member  21100 . Elements of this alternate fifth preferred embodiment that are also elements found in the fifth preferred embodiment of the fourth embodiment family are like numbered. (The second baseplate features are numbered in the 21000&#39;s rather than the 2100&#39;s.) The curvate recess  2704  of the ball  2700  forms the recess described above, and the ball bearing  2600  serves as the protrusion described above, and thus, the ball bearing  2600  and the recess  2704  interact in the above described manner to limit the rotation of the ball  2700  in the socket  2507 .  
         [0215]    Assembly of this alternate fifth preferred embodiment is identical to that of the first preferred embodiment of the fourth embodiment family, except that the recess  2704  is aligned with the curvate recess  21180  during assembly so that the ball bearing  2600  can be and is then placed into the recesses  21180 , 2704  (and then captured in the recess  21180  by the recess  2516  of the convex structure  2500 ) for interaction as described above as the ball  2700  rotates and angulates in the socket  2507 . It should be understood that the second baseplate second part  21000   b  preferably fits loosely within the convex structure  2500  and the second baseplate first part  21000   a , so that when the first baseplate  1400  is compressed toward the second baseplate first part  21000   a , the second baseplate second part  21000   b  may travel toward the second baseplate first part  21000   a  as the spring member  21100  compresses. While not limited to any particular structure, assembly, or material, a spring member providing shock absorption preferably includes an elastomeric material, such as, for example, polyurethane or silicon, and a spring member providing shock dampening preferably includes a plastic material, such as, for example, polyethylene. It should be understood that metal springs may alternatively or additionally be used. The illustrated spring member  21100  is formed of an elastomeric material, for example. The illustrated spring member  21100  is ring-shaped, for example, such that it fits just inside the circumferential edge of the outwardly facing surface  21020   b  of the second baseplate second part  21000   b  as shown. The second baseplate second part  21000   b  should be dimensioned such that, and the spring member  21100  should have an uncompressed height such that, a gap is present between the outwardly facing surface  21020   b  of the second baseplate second part  21000   b  and the inwardly facing surface  21090   a  of the second baseplate first part  21000   a  when the disc is assembled. The gap preferably has a height equivalent to the anticipated distance that the spring member  21100  will compress under an anticipated load. Accordingly, in this alternate first preferred embodiment, part or all of a compressive load applied to the baseplates will be borne by the spring member  21100 , which will dampen the load and/or absorb the load and preferably help return the baseplates to their original uncompressed relative positions.  
         [0216]    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.