REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application Ser. Nos. 60/416,749, filed Oct. 7, 2002; 60/417,346, filed Oct. 9, 2002; 60/442,971, filed Jan. 27, 2003; 60/443,815, filed Jan. 30, 2003; 60/445,489, filed Feb. 6, 2003; 60/445,958, filed Feb. 7, 2003. This application is also a continuation-in-part of U.S. patent application Ser. Nos. 10/434,931, filed May 9, 2003; 10/434,917, filed May 9, 2003; and 10/435,332, filed May 9, 2003 now U.S. Pat. No. 6,875,235 which claims priority from 60/379,462, filed May 10, 2002. The content of each application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to prosthetic implants and, in particular, to artificial intervertebral disc and joint replacement components with improved wear and kinematics. 
     BACKGROUND OF THE INVENTION 
     Improvements in prosthetic intervertebral disc and joint replacement components, and related surgical procedures, have led to dramatic increases in implant longevity. Many artificial hip and knee components now last for twenty years or more due to improved materials and greater insight into movement, load distribution and wear characteristics. 
     Many spinal conditions, including degenerative disc disease, can now be treated through artificial disc replacement (ADR), which has several advantages over spinal fusion. The most important advantage of ADR is the preservation of spinal motion. Spinal fusion eliminates motion across the fused segments of the spine. Consequently, the discs adjacent to the fused level are subjected to increased stress, which increases the changes of future surgery to treat the degeneration of the discs adjacent to the fusion. 
     One of the most important features of an artificial disc replacement (ADR) is its ability to replicate the kinematics of a natural disc. ADRs that replicate the kinematics of a normal disc are less likely to transfer additional forces above and below the replaced disc. In addition, ADRs with natural kinematics are less likely to stress the facet joints and the annulus fibrosus (AF) at the level of the disc replacement. Replicating the movements of the natural disc also decreases the risk of separation of the ADR from the vertebrae above and below the ADR. 
     The kinematics of joint replacements in general and ADRs in particular are governed by range of motion (ROM), the location of the center(s) of rotation (COR), and other such considerations. Clearly any improvements in these and other areas would be welcomed by the medical community and by patients undergoing procedures to implant prosthetic components of this kind. 
     SUMMARY OF THE INVENTION 
     This invention is broadly directed to improved artificial disc (ADR) and joint replacement components, particularly those associated with the knee and hip. Most embodiments use compressible, resilient materials to approximate joint kinematics, often with elastic or spring components to tether articulating components to avoid dislocation, excessive compression, flexion or other out-of-range movements. The preferred embodiments include a robust design, requiring only two or a few articulating components. The designs allow for normal flexion, extension, lateral bending, rotation, and translocation, depending upon the application. 
     Most ADR embodiments do not require precise alignment. The ADR can be self-centering in versions that articulate with the endplates of the vertebrae. Alternative vertebral endplate resurfacing designs are also disclosed. Typical ADR embodiments combine a compressible component and a C-shaped or other containment component, to accommodate natural axial forces, relaxation, and bending movements. Further embodiments use the ADR EPs to force or wedge apart articulating components made of metal, or other material with good wear characteristics, as loads are applied. 
     In all embodiments, the articulating components may be surrounded and/or connected by a material or materials with elastic properties. The elastic material or materials force the assembled disc or joint replacement to assume its resting size and shape once the loads are removed. Other embodiments utilize components that articulate with a spring component. Further embodiments incorporate one or more springs in conjunction with one or more dampening materials, preferably hydrogels, elastomers, polyurethanes, foam polyurethanes, or other polymers to protect against the full loads of the spine. 
     In different embodiments, membranes may be used to prevent the ingrowth of tissue, to trap debris or to contain lubricating fluid such as vegetable oil or saline, a gel other than hydrogel, or a viscoelastic fluid. The springs can be made of metals such as titanium, Nitinol, stainless steel or other metals. Polymeric elastomers or other high performance fibers can also provide spring properties. The articulating components and the ADR EPs can be made of ceramic, titanium, chrome cobalt, or other material commonly used in orthopaedic implants. A fully assembled disc or joint replacement may be inserted after compression with a tool, or cemented in the disc space with polymethylacrylate or other cement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of the lateral aspect of the spine and an artificial disc replacement (ADR) according to the invention; 
         FIG. 2  is a sagittal cross section of the ADR of  FIG. 1 ; 
         FIG. 3  is a view of the anterior aspect of the ADR of  FIG. 1 ; 
         FIG. 4  is a view of the lateral aspect of a flexed spine and the ADR of  FIG. 1 ; 
         FIG. 5  is a view of the anterior aspect of spine bent to the left and the ADR; 
         FIG. 6  is a view of the anterior surface of an artificial disc replacement (ADR) according to the invention; 
         FIG. 7  is a view of the lateral aspect of the ADR of  FIG. 6 ; 
         FIG. 8  is an exploded view of the anterior aspect of the ADR of  FIG. 6 ; 
         FIG. 9  is a lateral view of the spine and the ADR of  FIG. 6 ; 
         FIG. 10A  is a lateral view of the spine and the ADR of  FIG. 6 ; 
         FIG. 10B  is a view of the anterior aspect of the ADR of  FIG. 6 ; 
         FIG. 10C  is view of the anterior aspect of the ADR of  FIG. 6 ; 
         FIG. 11A  is a lateral view of a flexed spine and the ADR of  FIG. 6 ; 
         FIG. 11B  is a view of the lateral aspect of the ADR of  FIG. 6  in a flexed position; 
         FIG. 11C  is a view of the anterior aspect of the ADR of  FIG. 6  in a flexed position; 
         FIG. 12  is a view of the lateral aspect of the spine, an ADR, and optional resurfacing components placed over the endplates of the vertebrae; 
         FIG. 13  is a lateral view of the spine and an alternative embodiment of the invention; 
         FIG. 14A  is a view of the anterior aspect of the spine and the ADR of  FIG. 13 ; 
         FIG. 14B  is a view of the anterior aspect of the spine and the ADR of  FIG. 13 ; 
         FIG. 15  is coronal cross section of an ADR and optional link members connecting the two components of the ADR; 
         FIG. 16  is a coronal cross section of the spine and an ADR according to the invention; 
         FIG. 17  is an axial cross section of a spring component and an ADR endplate from the ADR of  FIG. 16 ; 
         FIG. 18  is a coronal cross section of another version of the ADR of  FIG. 16 ; 
         FIG. 19A  is a coronal cross section of an alternative embodiment of the invention; 
         FIG. 19B  is an exploded view of the ADR of  FIG. 19A ; 
         FIG. 20A  is a view of the top of a circular or oval spring with an anterior opening; 
         FIG. 20B  is a view of the top of an alternative spring component without an opening; 
         FIG. 20C  is a view of the top of an alternative spring component; 
         FIG. 21  is a coronal cross section of an ADR including an alternative spring component designed to resist radial forces; 
         FIG. 22A  is a coronal cross section of yet a further alternative embodiment of the invention; 
         FIG. 22B  is a coronal cross section of the ADR drawn in  FIG. 22A  during the insertion process; 
         FIG. 23  is a sagittal cross section of an embodiment with components to help prevent extrusion of the modular spring component; 
         FIG. 24  is a coronal cross section of an embodiment of the invention incorporating a composite spring; 
         FIG. 25  is a coronal cross section of an embodiment of the invention with an alternative spring component; 
         FIG. 26  is a coronal cross section of an embodiment of the invention with a different alternative spring component; 
         FIG. 27  is a coronal cross-section of yet another embodiment of the invention; 
         FIG. 28  is a coronal cross section of an alternative embodiment of the invention, wherein the spring component forms part of one of the endplates; 
         FIG. 29  is a coronal cross section of an alternative embodiment, wherein the endplates are convex; 
         FIG. 30  is a coronal cross-section of yet a further alternative embodiment of the invention incorporating a hoop-like spring; 
         FIG. 31  is a coronal cross section of another embodiment, wherein the spring component has vertical separations through the top and bottom of the spring; 
         FIG. 32  is a coronal cross-section of another embodiment of the device drawn in  FIG. 31 , wherein the vertical spring members are an integral part of one of the ADR endplates; 
         FIG. 33A  is a coronal cross-section of another embodiment of the invention, wherein the spring component utilizes multiple leaf spring-like components; 
         FIG. 33B  is a coronal cross-section of another embodiment similar to that drawn in  FIG. 33A , wherein the leaf springs articulate with one or more intermediate members; 
         FIG. 34A  is a view of the side of the spring component drawn in  FIG. 33A ; 
         FIG. 34B  is a view of the side of the spring component drawn in  FIG. 34A  during compression of the springs; 
         FIG. 35A  is a view of the top of the spring component drawn in  FIG. 34A ; 
         FIG. 35B  is a view of the top of the compressed spring component drawn in  FIG. 34B ; 
         FIG. 36  is a cross section view of the hoops and leaf springs drawn in  FIG. 34A ; 
         FIG. 37A  is a view of the side of another embodiment of an ADR according to the invention; 
         FIG. 37B  is a coronal cross section of the ADR drawn in  FIG. 37A ; 
         FIG. 37C  is a coronal cross section of the ADR drawn in  FIG. 37A  during insertion; 
         FIG. 37D  is a coronal cross section of the ADR drawn in  FIG. 37C  following insertion; 
         FIG. 38  is a coronal cross section of a Total Knee Replacement (TKR) embodiment of the invention; 
         FIG. 39A  is a view of the top of a spring component applicable to an ADR embodiment; 
         FIG. 39B  is a coronal cross section through the ADR drawn in  FIG. 39A ; 
         FIG. 39C  is a coronal cross section of another embodiment of the ADR drawn in  FIG. 39A ; 
         FIG. 40  is a top view of another ADR spring component embodiment; 
         FIG. 41A  is a top view of a further ADR spring component embodiment; 
         FIG. 41B  is a coronal cross section of the embodiment of  FIG. 41A ; 
         FIG. 42A  is a coronal cross section of another embodiment of an ADR according to the invention; 
         FIG. 42B  is a top view of the spring component of  FIG. 42A ; 
         FIG. 43A  is an axial cross section through another embodiment of a spring component, wherein the elastomer ring around the C-shaped articulating components supplies all the spring force; 
         FIG. 43B  is a coronal cross section through the embodiment of the invention of  FIG. 43A ; 
         FIG. 44  is a top view of another embodiment of the spring component, wherein the articulating C-shaped components are opposing at rest; 
         FIG. 45A  is a coronal cross section of another embodiment of the invention; 
         FIG. 45B  is a coronal cross section of another embodiment of the ADR drawn in  FIG. 45A ; 
         FIG. 46A  is a coronal cross section of yet a different embodiment of the invention; 
         FIG. 46B  is a coronal cross section of the embodiment of  FIG. 46A  with axial force applied to the ADR EPs; 
         FIG. 47A  is a coronal cross section of another embodiment of the ADR, including two wedge-shaped components; 
         FIG. 47B  is a coronal cross section of the embodiment of  FIG. 47A  with axial force applied to the ADR EPs; 
         FIG. 47C  is a view of the bottom of the wedge-shaped components and one of the components that articulates with the ADR EPs from the embodiment of  FIG. 47B ; 
         FIG. 47D  is a view of the bottom of four wedge-shaped components and an alternative embodiment of the component that articulates with the ADR EPs; 
         FIG. 48  is a coronal cross section of another embodiment of the ADR, wherein the articulating surfaces of the ADR EPs have a varying radii of curvature; 
         FIG. 49A  is the view of the top of an alternative embodiment of the C-shaped components and the elastic hoop; 
         FIG. 49B  is a view of the top of an alternative embodiment of the C-shaped components and the elastic hoop; 
         FIG. 49C  is a view of the sides of a portion of the C-shaped pieces drawn in  FIG. 49B ; 
         FIG. 49D  is the view of the top of an alternative embodiment of the C-shaped components and the elastic hoops; 
         FIG. 49E  is a cross section of a C-shaped piece and the sleeve drawn in  FIG. 49D ; 
         FIG. 50A  is a sagittal cross section of another embodiment of the C-shaped pieces and the elastic hoop; 
         FIG. 50B  is a sagittal cross section of the ADR EPs in another embodiment of the ADR; 
         FIG. 51A  is a coronal cross section of another embodiment of the device; 
         FIG. 51B  is a coronal cross section of the embodiment of the ADR drawn in  FIG. 51A  with a load being applied; 
         FIG. 52A  is a coronal cross section of another embodiment of an ADR according to the invention; 
         FIG. 52B  is a coronal cross section of the embodiment of  FIG. 52A  with the elastic hoop in its second shape; 
         FIG. 53  is a sagittal cross section of yet a further, different embodiment of an ADR according to the invention; 
         FIG. 54A  is a side view of an embodiment of the invention designed for placement in a disc space without ADR endplates; 
         FIG. 54B  is a coronal cross section of the ADR of  FIG. 54A ; 
         FIG. 54C  is a coronal cross section of another version of the ADR of  FIG. 54B ; 
         FIG. 55A  is a coronal cross section of another alternative embodiment of the invention; 
         FIG. 55B  is a coronal cross section of a different version of the ADR of  FIG. 55A ; 
         FIG. 56A  is a coronal cross section of through another embodiment of the invention; 
         FIG. 56B  is a coronal cross section of the ADR of  FIG. 56A  with an axial load applied; 
         FIG. 56C  is an alternative version of the ADR of  FIG. 56B ; 
         FIG. 57A  is a coronal cross section of yet a different alternative embodiment of the invention; 
         FIG. 57B  is a coronal cross section of the embodiment of  FIG. 57A  with an axial load applied; 
         FIG. 58A  is a coronal cross section of yet a further alternative embodiment of the invention; 
         FIG. 58B  is a view of the top of the cushioning component drawn in  FIG. 58A ; 
         FIG. 58C  is a coronal cross section of the ADR of  FIG. 58A ; 
         FIG. 58D  is a coronal cross section of the ADR of  FIG. 58C  with axial load applied; 
         FIG. 58E  is a view of the top of one of the hinged articulating components drawn in  FIG. 58B ; 
         FIG. 58F  is an enlarged coronal cross section of one of the articulating hinged components drawn in  FIG. 58A ; 
         FIG. 59  is a coronal cross section of a different further alternative embodiment of the invention; 
         FIG. 60  is a coronal cross section of another ADR embodiment with non-congruent articulating surfaces; 
         FIG. 61A  is an axial cross section through another embodiment of an ADR according to the invention; 
         FIG. 61B  is a sagittal cross section through the ADR of  FIG. 61A ; 
         FIG. 61C  is a sagittal cross section through the ADR of  FIG. 61B  showing one of many alternative cushioning components; 
         FIG. 61D  is a sagittal cross section of the ADR endplates (EPs) of  FIG. 61C ; 
         FIG. 62  is a sagittal cross section through another ADR embodiment; 
         FIG. 63A  is a view of the top of a C-shaped elastic component in a closed position; 
         FIG. 63B  is a view of the top of the C-shaped elastic component draw in an open position; 
         FIG. 64A  is a coronal cross section through another embodiment of the ADR; 
         FIG. 64B  is a coronal cross section of the ADR drawn in  FIG. 64A  with axial load applied; 
         FIG. 64C  is a view of the top of the elastic C-shaped component with shape memory, and an associated articulating component; 
         FIG. 64D  is a view of the top of the C-shaped component and the articulating component, after the C-shaped component has assumed a final shape; 
         FIG. 65A  is a sagittal cross section of an alternative sealing mechanism; 
         FIG. 65B  is an axial cross section of the embodiment of  FIG. 65A ; 
         FIG. 66  is an alternative, cushioned ADR embodiment; 
         FIG. 67A  is a sagittal cross section of an alternative embodiment of the ADR of  FIG. 66 ; 
         FIG. 67B  is a sagittal cross section of an alternative configuration of the ADR of  FIG. 67A ; 
         FIG. 68  is a sagittal cross section through another ADR embodiment; 
         FIG. 69A  is a sagittal cross section of another embodiment of the ADR; 
         FIG. 69B  is a sagittal cross section of another version of the ADR of  FIG. 69A ; 
         FIG. 70A  is a sagittal cross section through another TKR embodiment of the invention; 
         FIG. 70B  is a sagittal cross section of another version of the invention of  FIG. 70A ; 
         FIG. 70C  is a sagittal cross section of another configuration of the device of  FIG. 70B ; and 
         FIG. 70D  is an axial cross section of the embodiment of the TKR of  FIG. 70A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a view of the lateral aspect of the spine and an ADR according to the invention, including a plurality of articulating components  102 .  FIG. 2  is a sagittal cross section of the ADR, illustrating how the articulating components are connected by one or more springs or elastic bands  110 . Using this configuration, the articulating components are free to position themselves to “custom fit” the endplates of the vertebrae. 
       FIG. 3  is a view of the anterior aspect of the ADR, and  FIG. 4  is a view of the lateral aspect of the ADR in conjunction with a flexed spine. The anterior components slide against one another to decreases the height of the anterior aspect of the ADR during spinal flexion. The elastic tether returns the articulating components to their resting position as the spine is returned to its neutral position. The articulating components and the elastic tether also cooperate to dampen axial loads to the spine. 
       FIG. 5  is a view of the anterior aspect of spine bent to the left and the ADR, showing how the articulating components slide against each other to facilitate lateral bending of the spine. 
       FIG. 6  is a view of the anterior surface of an ADR according to the invention including a biconvex component  602 . A C-shaped inferior component  604  preferably includes a spring-like shape memory.  FIG. 7  is a view of the lateral aspect of the ADR of  FIG. 6 , and  FIG. 8  is an exploded view.  FIG. 9  is a lateral view of the spine and the ADR of  FIG. 6 , and  FIG. 10A  is a lateral view of the spine and the ADR of  FIG. 6 .  FIG. 10B  is a view of the anterior aspect of the ADR. The dotted line shows the position of the inferior surface of the biconvex component within the C-ring spring component.  FIG. 10C  is view of the anterior aspect of the ADR. As in  FIG. 10B , the dotted line represents the position of the inferior surface of the biconvex component. 
       FIG. 11A  is a lateral view of a flexed spine and the ADR.  FIG. 11B  is a view of the lateral aspect of the ADR in a flexed position, showing how the anterior surface of the biconvex component tilts and is forced into the C-ring spring component. Thus, the height of the anterior aspect of the ADR decreases with flexion of the spine. The C-ring spring returns the biconvex component to its neutral position as pressure is relieved from the anterior aspect of the spine. Similarly, axial loads on the spine are cushioned by the interaction of the biconvex component with the C-ring spring. 
       FIG. 11C  is a view of the anterior aspect of the ADR in a flexed position. The anterior portion biconvex component can be seen to extend closer to the inferior surface of the C-ring spring component in  FIG. 11C  when  FIG. 11C  is compared to  FIG. 10C . The opening of the C-ring spring is larger in  FIG. 11C  than the opening of the C-ring spring of the ADR in the neutral position of  FIG. 10C . 
       FIG. 12  is a view of the lateral aspect of the spine and the ADR showing optional resurfacing components  1202 ,  1204  over the endplates of the vertebrae. The endplate resurfacing components preferably include bone ingrowth on the vertebral portion of the components. The ADR can also be used with the “mobile link” described in my co-pending U.S. patent application Ser. No. 10/426,995, the entire content of which is incorporated herein by reference. 
       FIG. 13  is a lateral view of the spine and an alternative embodiment of the ADR wherein the superior component includes a bone ingrowth upper surface.  FIG. 14A  is a view of the anterior aspect of the spine and the ADR with the spine positioned to show side bending to the left.  FIG. 14B  is a view of the anterior aspect of the spine and the ADR with the spine positioned to show side bending to the right. 
       FIG. 15  is coronal cross section of an ADR according of the invention showing optional link members  1502  connecting the two ADR components  1504 ,  1506 . The portion of the link member across the C-shaped spring component allows the C-shaped to open and close. In this example, a relatively lax cable connects the two side of the C-shaped component. 
       FIG. 16  is a coronal cross section of the spine and an ADR including a horizontal, generally C-shaped spring component  102  that articulates with endplates  104  over the surfaces of vertebral bodies  110 .  FIG. 17  is an axial cross section of the spring component and an ADR endplate  104 .  FIG. 18  is a coronal cross section of another embodiment of the ADR drawn in  FIG. 16 , wherein one or both of the endplates  110  may have raised areas  112  to help the spring component open when axial loads are applied to the ADR. The anterior location of the opening of the spring component facilitates spinal flexion. 
       FIG. 19A  is a coronal cross section of an alternative embodiment of the invention including spacer components  1902  that articulate between the ADR endplates and the spring  1904 .  FIG. 19B  is an exploded view of the ADR of  FIG. 19A . The spacer components  1902  facilitate opening of the spring, while allowing translation of the ADR endplates relative to the spring with spinal movement. 
       FIG. 20A  is a view of the top of a circular or oval-shaped spring component with an anterior opening  2002 . The opening of a circular shaped spring is more likely to rotate away from an anterior location.  FIG. 20B  is a view of the top of an alternative oval-shaped spring component without an opening.  FIG. 20C  shows an alternative spring component having two or more components joined together through a joint that allows a relative degree of motion. An axle  2010  is used to connect the pieces in the example drawn. An elastic tether or spring  2020  on the anterior portion of the assembled spring component enables the use of metals with hard articulating surfaces, but poor elastic properties. 
       FIG. 21  is a coronal cross section through an embodiment of the ADR with an alternative spring component  2102  designed to resist radial forces. Furthermore, the spring component  2102  is compressible in a superior to inferior direction. The superior to inferior compressibility of the spring allow tilting and translation of the ADR endplates. A compressible, resilient material such as an elastomer or hydrogel is preferably placed with central portion  2104  of the spring in the preferred embodiment of the device. Axial load on the ADR endplates is transferred to radial force on the spring. The preferred spring component in this embodiment of the device, as illustrated in the example drawn in  FIG. 20B , does not contain an opening. If a hydrogel is used, the central component imbibes fluid after insertion and the spring with shape memory properties, increases in height after insertion between the ADR endplates. 
       FIG. 22A  is a coronal cross section of yet a further alternative embodiment of the invention featuring a differently shaped spring component  2202 . Again, the cavity within the spring component  702  is at least partially filled with an elastomer, hydrogel or other compressible, resilient material  2220 .  FIG. 22B  is a coronal cross section of the ADR drawn in  FIG. 22A  during the insertion process. As discussed in  FIG. 21 , the spring and central material expands vertically after insertion between the ADR endplates. 
       FIG. 23  is a sagittal cross section of an embodiment of the device with components to help prevent extrusion of the modular spring component, including projections  2304  from the superior endplate and attachable pieces  2302  on the inferior endplate. The projections can be spring loaded or rotated (i.e., at 90 degrees) into position. 
       FIG. 24  is a coronal cross section of an embodiment of the invention utilizing a composite spring. Area  2402  of the spring represents the hard less flexible portion of the spring, whereas the area  2404  of the spring represents the more flexible portion. The area of the spring  2410  between areas  2402 ,  2404  represents an optional area of the composite with intermediate properties. Hydrogel or other elastomer is preferably placed within the composite spring. 
       FIG. 25  is a coronal cross section of an embodiment of the device with an alternative spring component. Again, the areas  2502 ,  2504  preferably contain a hydrogel, elastomer, or other compressible, resilient material. 
       FIG. 26  is a coronal cross section of an embodiment of the device with an alternative spring component featuring closely spaced concentric rings  2602 . As in  FIGS. 21-25  the spring component is flexible in a superior to inferior direction. The spring also expands slightly in a radial direction. 
       FIG. 27  is a coronal cross-section of another embodiment of the invention including a spring component  2702 .  FIG. 28  is a coronal cross section of an alternative embodiment, wherein the spring component  2802  forms part of one of the endplates. Note that the spring components in other embodiments, such as those depicted in of  FIGS. 21-26 , can also be incorporated into one of the endplates.  FIG. 29  is a coronal cross section of an alternative embodiment, wherein the endplates  2902 ,  2904  are convex. Alternatively, the ADR endplates can be flat, concave or a more complex shape. It is also possible to use ADR endplates with different shapes in a single ADR. 
       FIG. 30  is a coronal cross section of yet a further alternative embodiment of the invention incorporating a hoop-like spring  3002 .  FIG. 31  is a coronal cross section of another embodiment, wherein the spring component has vertical separations through the top and bottom of the spring which make it easier for the spring to bend in a superior to inferior direction. Hydrogel-containing embodiments of the device may place a membrane between the hydrogel and the spring to help hold the hydrogel within the spring.  FIG. 32  is a coronal cross section of a variation of the device drawn in  FIG. 31 , wherein the vertical spring members are an integral part of one of the ADR endplates. 
       FIG. 33A  is a coronal cross section of another embodiment of the invention, wherein the spring component utilizes multiple leaf spring-like components  3302 ,  3304 . In this configuration, the leaf spring-like components are preferably connected to hoops  3310 ,  3312  at the superior and inferior ends of the springs. Additional leaf spring-like components can be coupled to the outer surface of the inside leaf spring to increase the stiffness of the assembled spring component. Alternatively, circular hoops can be attached to the outside of the leaf springs to increase the spring&#39;s resistance to radial expansion. The ends of the leaf springs articulate may articulate with one or both ADR endplates  3320 ,  3322 .  FIG. 33A  demonstrates articulation between the springs and both endplates. As discussed in  FIG. 31 , hydrogel-containing embodiments may place a membrane between the hydrogel and the spring to help hold the hydrogel within the spring. 
       FIG. 33B  is a coronal cross section of another embodiment similar to that drawn in  FIG. 33A , wherein the leaf springs articulate with intermediate member(s). The intermediate member(s)  3302  provide an improved articular surface for cooperation with the ADR endplates.  FIG. 34A  is a view of the side of the spring component drawn in  FIG. 33A . Preferably, the leaf spring components overlap to decrease the gaps that occur between the components during compression of the spring component. The leaf spring represented by the cross hatched area of the drawing sits behind the two other leaf spring components.  FIG. 34B  is a view of the side of the spring component drawn in  FIG. 34A  during compression of the springs. The drawing illustrates the cooperation between adjacent leaf spring components to eliminate gaps between the components during compression of the springs. 
       FIG. 35A  is a view of the top of the spring component drawn in  FIG. 34A .  FIG. 35B  is a view of the top of the compressed spring component drawn in  FIG. 34B .  FIG. 36  is a cross section view of the hoops and leaf springs drawn in  FIG. 34A . 
       FIG. 37A  is a view of the side of another embodiment of the invention, including a mesh or basket-like weave spring component  3702  that cooperates with two ADR endplates  3704 ,  3706 . The spring component preferably incorporates elastic and/or shape-memory materials. As with other embodiments, the spring component may also contain hoops that are capable of collapsing inside one another. 
       FIG. 37B  is a coronal cross section of the ADR drawn in  FIG. 37A . The spring component surrounds a hydrogel, elastomer, or other compressible, resilient material.  FIG. 37C  is a coronal cross section of the ADR drawn in  FIG. 37A  during its insertion. The hoops on the top and bottom of the spring component have a first shape with a central opening larger than the projections from the ADR endplates. A hydrogel would be dehydrated.  FIG. 37D  is a coronal cross section of the ADR drawn in  FIG. 37C  after its insertion. The hoops on the top and bottom of the spring component contract around the projections from the ADR endplates, thus locking the components together. The hydrogel imbibes additional fluid after insertion of the ADR into the body. 
       FIG. 38  is a coronal cross section of the Total Knee Replacement (TKR) embodiment of the invention. A hydrogel-containing spring component, similar to that drawn in  FIG. 37A , cushions the tibial component of the prosthetic knee. The articular component of the TKR pistons up and down a cylinder within the portion of the tibial component that attaches to the tibia. The articular portion of the tibia component preferably includes a reversible locking mechanism  3808  that prevents the articular component from dissociating from the component that is attached to the tibia. The articular and cushion components can be removed from the component that attaches to the tibia by rotating the piston to allow the enlargement at the end of the piston to fit through an enlarged opening in the cylinder. 
       FIG. 39A  is a top view of a different spring component used in an ADR embodiment according to the invention, wherein C-shaped metal articulating pieces  3902 ,  3904  are connected by two pieces of elastic material  3910 ,  3912 . The dotted lines within the C-shaped pieces represent sockets in the C-shaped pieces and spheres at the ends of the elastic components. The dotted area of the drawing between the C-shaped components represents a polymeric or fiber dampening material. The assembled spring component is oval in shape to keep the open areas between the C-shaped pieces from rotating into a less desired position. For example, the ADR may be designed to keep the open areas between the C-shaped components in anterior and posterior positions to facilitate spinal flexion and extension. 
       FIG. 39B  is a coronal cross section through the ADR drawn in  FIG. 39A . Forces applied to the spine are transmitted to the spring component through two ADR endplates  3902 ,  3904 . Force from the ADR EPs drive the C-ring articulating pieces apart, thus reducing the height of the ADR. The elastic tethers between the C-shaped pieces pull the C-shaped pieces together after the force is removed. The dampening material resists the C-shaped components return to their resting position, thus dampening the ADR. The dampening component is not bonded to either the ADR EPs or the C-shaped pieces.  FIG. 39C  is a coronal cross section of another embodiment of the ADR drawn in  FIG. 39A . The C-shaped articulating pieces have concave inter surfaces to allow more room for the dampening component. 
       FIG. 40  is a view of the top of another embodiment of a spring component according to the invention, wherein four articulating components  4002 ,  04 ,  06 ,  08  are connected by four springs  4110 ,  12 ,  14 ,  16 . The openings between the articulating components allow room for more springs. The openings also facilitate flexion, extension, and lateral bending. 
       FIG. 41A  is a view of the top of another embodiment of the spring component according to the invention, wherein four articulating pieces are surrounded by an elastomer  4102 . The articulating pieces fit into holes  4110 ,  12 ,  14 ,  16  in the elastomer. The articulating components, which may also have a circular shape when viewed from the top, fit into holes in the elastomer and are preferably not bonded to the elastomer.  FIG. 41B  is a coronal cross section of the embodiment of the device drawn in  FIG. 41A . 
       FIG. 42A  is a coronal cross section of another embodiment of the ADR including articulating components surrounded by an elastomer ring  4202 . Other materials with elastic or shape-memory properties, including Nitinol, may also be used for the ring.  FIG. 42B  is a view of the top of the embodiment of the spring component drawn in  FIG. 42A . The elastomer ring assists the springs in returning the C-shaped articulating pieces to their resting position after the load is removed from the spine. 
       FIG. 43A  is an axial cross section through another embodiment of the spring component wherein the elastomer ring around the C-shaped articulating components supplies substantially all the spring force. Note that the springs between the C-shaped components have been eliminated. The C-shaped articulating components are drawn with an optional joint to prevent dissociation of the components. The ends of one articulating component piston in and out of the sockets in the ends of the second articulating component. 
       FIG. 43B  is a coronal cross section through the embodiment of the ADR drawn in  FIG. 43A . Since the elastomer band  4302  does not touch the ADR EPs  4310 ,  4312 , friction is reduced. The dampening component within the articulating components is not bond to either articulating component or the ADR EPs. The elastomer band is free from compression force from the ADR EPs. The dampening component between the articulating components is exposed to minimal compression from the ADR EPs. Compression by the ADR EPs drives articulating components apart. The elastomer ring limits how far apart the articulating components can separate, thus limiting compression of the dampening component. Compression of the dampening component can be eliminated by making the component too thin to touch the ADR EPs. An elastic metal with shape memory properties, such as Nitinol, could also be used as the band. The first shape of the band could include a large center opening that enables the spring component to be inserted after insertion of the ADR EPs. The band could assume a second shape with a smaller central opening, after insertion between the ADR EPs. 
       FIG. 44  is a top view of the top of another embodiment of the spring component, wherein the articulating C-shaped components are opposing at rest. The spring components pull the articulating components together after loads are removed from the ADR. Impingement of the articulating components prevents the components from excessive recoil and repeated cycles of compression and recoil from a single quick application and removal of a load. Thus, the spring component in this embodiment does not require a dampening material. The dotted lines outline the course of the springs and the sphere shaped enlargements on the ends of the springs. 
       FIG. 45A  is a coronal cross section of another embodiment of the invention, wherein the endplates impinge at a predetermined amount of ADR compression, thereby limiting peak loads on the elastic band, bands, or cables. The concave exterior of the articulating components also protects the elastic band from friction against the ADR EPs.  FIG. 45B  is a coronal cross section of another embodiment of the ADR drawn in  FIG. 45A . The articulating components impinge against stops on one or both of the ADR EPs at a predetermined amount of ADR Compression. As mentioned in the text of  FIG. 45A , impingement of the ADR EPs limits peak loads on the elastic band, bands, or cables. 
       FIG. 46A  is a coronal cross section of another embodiment of the invention, wherein a material  4602  with elastic properties surrounds three articulating components. A center, preferably wedge-shaped articulating component  4604  fits into a wedge-shaped opening between the two components  4606 ,  4608  that articulate with the ADR EPs.  FIG. 46B  is a coronal cross section of the embodiment of the ADR drawn in  FIG. 46A  with axial force applied to the ADR EPs. The wedge shaped articulating component is forced from between the two articulating components that articulate with the ADR EPs. The elastomeric hoop forces the wedge shaped component into its resting position, when the load is removed from the ADR EPs. 
       FIG. 47A  is a coronal cross section of another embodiment of the ADR, including a plurality of wedge-shaped components.  FIG. 47B  is a coronal cross section of the embodiment of the ADR drawn in  FIG. 47A  with axial force applied to the ADR EPs.  FIG. 47C  is a view of the bottom of the wedge shaped components and one of the components that articulates with the ADR EPs from the embodiment of the ADR drawn in  FIG. 47B . 
       FIG. 47D  is a view of the bottom of four wedge-shaped components and an alternative embodiment of the component that articulates with the ADR EPs. Similar to the embodiment of the ADR drawn in  FIG. 47C , the component that articulates with the ADR EP, forces the wedge-shaped components apart as axial loads are applied to the ADR EPs. The separation between the one pair of wedge shaped components may be larger than the separation between the second pair of wedge shaped components during spinal motion in one direction. For example, spinal flexion forces the anterior pair of wedges apart. Spinal flexion also forces the anterior pair of wedges from the posterior pair of wedges. Spinal flexion does not force the posterior pair of wedges apart. 
       FIG. 48  is a coronal cross section of another embodiment of the ADR, wherein the articulating surfaces of the ADR EPs have a varying radii of curvature. In particular, the radius at the periphery of the ADR EPs is smaller than the radius in the central portion of the ADR EPs. The smaller radius in the periphery of the ADR EPs increases the rate of ADR height change to ADR diameter change ratio. The larger radius in the central portion of the ADR EPs increases the surface contact when the plates impinge at maximum loads. The central portion of the ADR EPs could also be another shape, including flat. 
       FIG. 49A  is the view of the top of an alternative configuration wherein the C-shaped components have concavities to help prevent pinching the elastic hoop.  FIG. 49B  is a view of the top of an alternative embodiment of the C-shaped components and the elastic hoop. The C-shaped components overlap to help prevent pinching the elastic hoop. The elastic cords that connect the C-shaped pieces preferably have different durometers to accommodate spinal motion in different directions. For example, the elastic cords and/or the elastic hoop could facilitate spinal flexion relative to spinal extension. 
       FIG. 49C  is a view of the sides of a portion of the C-shaped pieces drawn in  FIG. 49B .  FIG. 49D  is the view of the top of an alternative embodiment of the C-shaped components and the elastic hoops. The assembled C-shape components form an oval shape that helps prevent rotation of the C-shaped components. A flexible sleeve lies between the C-shaped pieces and the elastic hoop to prevent pinching the elastic hoop. Alternatively, the flexible sleeve could be placed over the C-shaped pieces.  FIG. 49E  is a cross section of a C-shaped piece and the sleeve drawn in  FIG. 49D . The flexible sleeve could also fully surround the C-shaped pieces or the hoop at the junction of the C-shaped pieces. 
       FIG. 50A  is a sagittal cross section of another embodiment of the C-shaped pieces and an elastic hoop. The C-shaped pieces are trapezoidal shaped to fit normal disc anatomy. The circles  5012  represent the enlarged ends of the elastic cords that connect the C-shaped pieces.  FIG. 50B  is a sagittal cross section of the ADR EPs in another embodiment of the ADR. The ADR EPs are trapezoidal to fit the normal disc anatomy. 
       FIG. 51A  is a coronal cross section of another embodiment wherein the articulating surfaces of the ADR EPs are flat at their periphery to accommodate the articulating pieces and the elastic hoop. The articulating pieces are in contact along the entire junction between the components.  FIG. 51B  is a coronal cross section of the embodiment of the ADR drawn in  FIG. 51A  with load applied to the ADR. 
       FIG. 52A  is a coronal cross section of another embodiment of the invention, wherein the elastic hoop is made of a shape-memory material such as Nitinol. The first shape of the material eases insertion of the C-shaped pieces and the elastic hoop after ADR EP insertion. The first shape of the hoop could be thinner and/or larger in diameter than the second shape of the hoop. Alternatively, the first state of the material may be more malleable than the second state of the material. The hoop could also be a composite of materials such as, Nitinol and an elastomer. 
       FIG. 52B  is a coronal cross section of the embodiment of the ADR drawn in  FIG. 52A  with the elastic hoop in its second shape.  FIG. 53  is a sagittal cross section of another embodiment of the ADR. The convexities on the ADR EPs are preferably located in the posterior half of the ADR. 
       FIG. 54A  is a side view of another embodiment designed for placement in the disc space without ADR endplates. An elastomer with bias ply fabric ( 5402 ), is attached to an elastic hoop  5406 .  FIG. 54B  is a coronal cross section of the ADR draw in  FIG. 54A . The black area of the drawing represents the elastic hoop made of a material such as Nitinol. In the preferred embodiment of the drawing the ADR is filled with a hydrogel.  FIG. 54C  is a coronal cross section of another embodiment of the ADR drawn in  FIG. 54B . The elastic hoop surrounds a portion of the superior and inferior surfaces of the hydrogel. 
       FIG. 55A  is a coronal cross section of another embodiment of an ADR including an elastic hoop  5502  that surrounds a portion of a hydrogel component  5506 . The elastic hoop articulates with the ADR endplates. The ADR endplates have raised lips to help prevent the extrusion of the cushioning component.  FIG. 55B  is a coronal cross section of another embodiment of the ADR drawn in  FIG. 55A . The ADR endplates are shaped to prevent the extrusion of the cushioning component. The shape of the ADR endplate also assists the hydrogel component in expanding the elastic hoop as axial pressure is applied to the ADR. 
       FIG. 56A  is a coronal cross section of through another ADR embodiment including three or more articulating components  5602 ,  04 ,  06  made of a material with good surface wear characteristics held together by an elastic component  5610 .  FIG. 56B  is a coronal cross section of the ADR drawn in  FIG. 56A  with axial load applied to the ADR. The superior articulating component wedges the inferior articulating components open. The elastic component forces the articulating components into their resting position as the force is removed from the ADR. In the preferred embodiment, the articulating components impinge on both of the ADR endplates at a predetermined load. Impingement of the articulating pieces, with the ADR endplates, protects the elastic component from excessive force. Furthermore, the impingement protects the facet joints from excessive force.  FIG. 56C  is an alternative embodiment including ball-and-socket joints between the articulating components that facilitate motion between the components. 
       FIG. 57A  is a coronal cross section of yet a different ADR embodiment wherein two articulating components  5702 ,  04  cooperate on an inclined plane. Axial loads on the ADR cause the articulating components to slide along the inclined plane. In an alternative embodiment, the inclined plane courses from anterior to posterior. An elastic hoop or C-shaped component forces the articulating components together. The articulating components could be tethered together by a projection from one component that slides in a slot in the second component.  FIG. 57B  is a coronal cross section of the embodiment of the ADR with an axial load applied to the ADR. 
       FIG. 58A  is a coronal cross section of an alternative ADR embodiment wherein hinged articulating components  5802 ,  04  cooperate with an elastic hoop or C-shaped component  5810 . The hinged articulating components are preferably loosely attached to the elastic hoop.  FIG. 58B  is a view of the top of the cushioning component drawn in  FIG. 58A .  FIG. 58C  is a coronal cross section of the ADR drawn in  FIG. 58A .  FIG. 58D  is a coronal cross section of the ADR drawn in  FIG. 58C  with axial load applied to the ADR.  FIG. 58E  is a view of the top of one of the hinged articulating components drawn in  FIG. 58B .  FIG. 58F  is an enlarged coronal cross section of one of the hinged articulating components drawn in  FIG. 58A . 
       FIG. 59  is a coronal cross section of an alternative embodiment of the ADR wherein an articulating component  5902  articulates between an elastic hoop or C-shaped component and the ADR endplates. Axial compression forces the articulating component to wedge open the elastic hoop or C-shaped component. Note that the elastic hoop or C-shaped component is not congruent with the inferior ADR endplate. The top of the articulating component need not be congruent with the top ADR endplate. Non-congruent surfaces between the ADR endplates and the articulating components facilitate ADR insertion. Non-congruent surfaces do not require an exact fit for proper articulation. 
       FIG. 60  is a coronal cross section of another embodiment of the ADR with non-congruent articulating surfaces. A ball-shaped projection  6002  from one ADR endplate cooperates with an elastic hoop or C-shaped component  6004  and a second ADR endplate  6006 . 
       FIG. 61A  is an axial cross section through another embodiment, wherein one or more ball-shaped projections from one ADR Endplate (EP) articulate(s) with a second ADR EP. The second ADR EP may have a socket to receive the ball-shaped component. Elastic hoops or C-shaped rings are used to cushion the ADR.  FIG. 61B  is a sagittal cross section through the ADR drawn in  FIG. 61A . Ball-shaped projections from one ADR EP cooperate with elastic hoops or C-shaped components to cushion the ADR. Articulation through the ball and socket joint in the posterior aspect of the ADR prevents transferring loads to the facet joints during axial compression of the ADR. 
       FIG. 61C  is a sagittal cross section through the ADR drawn in  FIG. 61B  with one of many alternative cushioning components. For example, a combination titanium or other metal and elastomer component may be used as a cushioning component. Shape-memory technology is preferably used to attach a removable stop to the front of the ADR. For example, the stop could be made of Nitinol. The hole in the stop could contract around a projection fro the ADR EP. The stop serves to prevent extrusion of the modular cushioning component. The stop may also impinge on the upper ADR EP during maximum loads on the ADR. Impingement of the stop on the ADR EP protects the cushion component from excessive loads.  FIG. 61D  is a sagittal cross section of the ADR EPs drawn in  FIG. 61C . Rotation through the ball-and-socket joint facilitates insertion of the ADR. 
       FIG. 62  is a sagittal cross section through another embodiment wherein a hoop or C-shaped component  6202  articulates between two ADR EPs. The hoop or C-shaped component can be made of an elastic material. One of the ADR EPs can be a cemented polyethylene component. The flat articulating surface of the ADR EP and the flat upper surface of the hoop or C-shaped articulating component facilitate insertion of the ADR. Note that the two ADR EPs do not require precise placement to permit articulation between the components. 
       FIG. 63A  is a view of the top of a C-shaped elastic component in a closed position.  FIG. 63B  is a view of the top of the C-shaped elastic component draw in an open position. The ends of the C-shaped component collide when they close to prevent recoil past a neutral, closed position. The design of the C-shaped component prevents excessive recoil and bouncing of the ADR. 
       FIG. 64A  is a coronal cross section through another ADR embodiment wherein a mobile spacer component  6402  articulates between the ADR EPs and an elastic hoop or C-shaped member  6404 . The flat surface between the articulating spacer and one of the ADR EPs allows the spacer to “self center” over the shape in the second ADR EP.  FIG. 64B  is a coronal cross section of the ADR drawn in  FIG. 64A  with axial load applied to the ADR. The drawing also illustrates how the articulating component can slide along the flat surface of the ADR EP, thus aligning the articulating component over the concavity in the second ADR EP.  FIG. 64C  is a view of the top of the elastic C-shaped component, preferably constructed with a shape-memory material, and the articulating component. The first shape of the C-shaped component facilitates insertion of the C-shaped component, and the articulating component, after insertion of the ADR EPs.  FIG. 64D  is a view of the top of the C-shaped component and the articulating component, after the C-shaped component has assumed a final shape. 
       FIG. 65A  is a sagittal cross section of an elastomerics membrane  6502  used to seal a cushioned TKR according to the invention. The membrane prevents debris from migrating out of the device. The membrane can also hold a lubricating fluid, such as vegetable oil, within the TKR. Spherical projection from the articulating component and or the second metal component, wedge open an elastic hoop component. 
       FIG. 65B  is an axial cross section of the embodiment of the device drawn in  FIG. 65A . The piston of the articulating component is represented by  6506 . The embodiment of the device drawn in  FIG. 65B  uses two C-shaped elastic hoops. Alternatively, one or more wave washers, such as, ring shaped or spiral shaped wave washers could cushion the TKR or Total Hip Replacement (THR). One or more Belleville washers could also cushion the TKR or THR. For example, Belleville washers such as spirally slotted Belleville washers with or without radially extending grooves, Belleville washers with radially spaced concentric grooves, and radially thinning Belleville washers could be used. 
       FIG. 66  is an alternative, cushioned, embodiment of an ADR wherein spherical projections from the upper ADR EP wedge open elastic hoops or elastic C-shaped components in response to axial loads on the ADR. The inferior ADR EP may have concavities to receive the spherical projections from the superior ADR EP. 
       FIG. 67A  is a sagittal cross section of an alternative configuration of the ADR of  FIG. 66 , wherein a spherical projection from one ADR EP articulates with the flat portion of a second ADR EP. The articulation between the spherical projection and the flat portion of the ADR EP allows translation of one ADR EP relative to the other ADR EP. The articulation also requires less precise placement of the ADR EPs. The anterior portion of the ADR is cushioned by one or two elastic hoops or elastic C-shaped components  6704 . The C-shaped components can be an elongated C or oval. In the preferred embodiment, the opening of the C-shaped component lies in the anterior portion of the ADR to facilitate spinal flexion. An oval shape helps prevent rotation of the opening away from an anterior location. 
       FIG. 67B  is a sagittal cross section of an alternative configuration of the ADR drawn in  FIG. 67A . The spherical projection from the superior ADR EP in this case articulates with washer  6710  with a concavity on the superior surface of the washer. The flat inferior surface of the washer articulates with the inferior ADR EP. The washer can be snapped or otherwise loosely connected to the spherical projection from the upper ADR EP. The washer may be slightly non-congruent with the spherical projection. The elastic hoop or C-shaped component lies outside of two or more C-shaped components of a harder material  6720  such as chrome cobalt. 
       FIG. 68  is a sagittal cross section through another embodiment of the ADR. The spherical projection from the superior ADR EP travels in a slot in the superior ADR. In the preferred embodiment of the ADR, the slot is located in the midline of the ADR EP as measured from left to right and in the posterior half of the ADR as measured from front to back. The spherical projection could also be fixed to the superior ADR EP at a certain point along the slot. For example a shape memory fastener could connect the spherical projection to the ADR EP. 
       FIG. 69A  is a sagittal cross section of another embodiment of the ADR wherein an elastic hoop or C-shaped component  6902  articulates with convex portions of the ADR EPs  6904 ,  06 . The flat surfaces at the center of the ADR EPs allow the ADR EPs to come closer together before impinging on one another. The articulating elastic component preferably features a spherical cross section to cooperate with spherical surfaces of the ADR EPs. The preferred shapes maintain good surface apposition during rotation of the ADR EPs and expansion of the elastic component. 
       FIG. 69B  is a sagittal cross section of another embodiment of the ADR drawn in  FIG. 69A . Spacers  6920 ,  22  articulate between the ADR EPs and the elastic hoop or C-shaped component. The spacers and the elastic hoop can be placed after ADR EP insertion. A retaining mechanism is used to hold the spacers and hoop in position. For example, using shape memory technology, a projection from the inferior ADR EP can change shape in the hole of a retaining piece, thus locking the two components together. 
       FIG. 70A  is a sagittal cross section of another TKR embodiment including a superior component that articulates with a femoral component of a TKR. Both articulating components could be made of a hard material such as zirconium oxide ceramic or aluminum oxide ceramic. A wedge-shaped central component  7008  cooperates along inclined planes of the superior and inferior components. The wedge shaped central component could be made of a wear-resistant material such as polyethylene. The inferior surface of the inferior component could be treated to promote bone growth. 
       FIG. 70B  is a sagittal cross section of another embodiment of the device drawn in  FIG. 70A . Two or more central wedge shaped components cooperate along inclined planes with the superior and inferior components. An elastic hoop or C-shaped component resists expansion of the central wedge shaped components. The wedge shaped components could be made of ceramic, chrome cobalt, or other material with good wear characteristics. 
       FIG. 70C  is a sagittal cross section of another embodiment of the device drawn in  FIG. 70B , showing how multiple elastic hoops  7080  resist the expansion of the wedge shaped central components.  FIG. 70D  is an axial cross section of the embodiment of the TKR drawn in  FIG. 70A . Two pistons are preferred in this device to prevent surface shear on the polyethylene component, and for rotation of the superior articulating component. Alternatively, a single piston that has a non-circular cross section, could also be used to prevent rotation. A single piston with a circular cross section could also be used.
Artificial disc (ADR) and joint replacement components, particularly those associated with the knee and hip, use compressible, resilient materials to approximate natural kinematics, often in conjunction with elastic or spring components to tether articulating components to avoid dislocation, excessive compression, flexion or other out-of-range movements. The preferred embodiments require only two or a few articulating components. The designs allow for normal flexion, extension, lateral bending, rotation, and translocation, depending upon the application.