while the invention will be described more fully hereinafter with reference to the accompanying drawings , in which particular embodiments and methods of implantation are shown , it is to be understood at the outset that persons skilled in the art may modify the invention herein described while achieving the functions and results of the invention . accordingly , the descriptions that follow are to be understood as illustrative and exemplary of specific structures , aspects and features within the broad scope of the invention and not as limiting of such broad scope . like numbers refer to similar features of like elements throughout . a preferred embodiment of the present invention will now be described . referring to fig3 , the invention is shown having a first baseplate 10 and a second baseplate 30 and a pin 50 . each baseplate 10 , 30 has an outwardly facing surface 12 , 32 . because the artificial disc of the invention is to be positioned between the facing surfaces of adjacent vertebral bodies , the two baseplates 10 , 30 used in the artificial disc are disposed such that the outwardly facing surfaces 12 , 32 face away from one another . the two baseplates 10 , 30 are to mate with the vertebral bodies so as to not rotate relative thereto , but rather to permit the spinal segments to bend relative to one another in manners that mimic the natural motion of the spinal segment . this motion is permitted by the performance of a ball and socket joint disposed between the secured baseplates 10 , 30 . the mating of the baseplates 10 , 30 to the vertebral bodies and the construction of the ball and socket joint are described below . more particularly , each baseplate 10 , 30 is a plate ( preferably made of a metal or metal alloy , such as , for example , cobalt - chromium or titanium ) having an overall shape that conforms to the overall shape of the respective endplate of the vertebral body with which it is to mate . further , each baseplate 10 , 30 comprises a vertebral body contact element 80 , 82 ( e . g ., a convex mesh , preferably oval in shape ) that is attached to the outwardly facing surface 12 , 32 of the baseplate 10 , 30 to provide a vertebral body contact surface . the mesh 80 , 82 is secured at its perimeter to the outwardly facing surface 12 , 32 of the baseplate 10 , 30 . the mesh 80 , 82 is domed in its initial undeflected conformation , but deflects as necessary during insertion of the artificial disc between vertebral bodies , and , once the artificial disc is seated between the vertebral bodies , deforms as necessary under anatomical loads to reshape itself to the concave surface of the vertebral endplate . this affords the baseplate 10 , 30 having the mesh 80 , 82 substantially superior gripping and holding strength upon initial implantation as compared with other artificial disc products . the mesh 80 , 82 further provides an osteoconductive surface through which the bone may ultimately grow . the mesh 80 , 82 is preferably comprised of titanium , but can also be formed from other metals and / or non - metals without departing from the scope of the invention . each baseplate 10 , 30 may further comprises at least a lateral ring ( not shown ) that is osteoconductive , which may be , for example , a sprayed deposition layer , or an adhesive applied beaded metal layer , or another suitable porous coating . this porous ring permits the long - term ingrowth of vertebral bone into the baseplate 10 , 30 , thus permanently securing the prosthesis within the intervertebral space . it shall be understood that this porous layer may extend beneath the domed mesh 80 , 82 as well , but is more importantly applied to the lateral rim of the outwardly facing surface 12 , 32 of the baseplate 10 , 30 that seats directly against the vertebral body . each of the baseplates 10 , 30 comprises features that , in conjunction with other components described below , form the ball and socket joint . the first baseplate 10 includes an inwardly facing articulating surface 18 that includes a perimeter region 20 and a projection 22 protruding from the inwardly facing surface 18 . the projection 22 preferably has a cylindrical or frustoconical cross section . the projection 22 further includes an axial bore 26 that accepts a mushroom - shaped pin 50 ( or rivet , plug , dowel , or screw ). the second baseplate 30 comprises an inwardly facing articulation surface 34 having a peripheral surface 36 and a curvate socket 38 , the socket 38 having a substantially constant radii concave articulation surface . pin 50 further comprises an elongated portion 52 and a head 54 , the head 54 having a convex arc having a substantially constant radius of curvature . the arc of head 54 is such that the sphere it defines has a large radius , thereby minimizing point loading and the risk of fatigue failure . the projection 22 of baseplate 10 is sized to have a diameter at least a portion of which is less than the diameter of the socket 38 . the projection 22 preferably has a cross section that is cylindrical or frustoconical . in a first embodiment , the elongated portion 52 of mushroom - shaped pin 50 is disposed in bore 26 of the baseplate 10 and the head 54 is nested in socket 38 . pin 50 is fixedly engaged by force fitting , welding or the like in bore 26 . head 54 is not captured in socket 38 . baseplates 10 and 30 are at no time connected to each other in the ball and socket joint of the present invention . optionally , the end of pin 50 proximal to the baseplate 10 , and the bore 26 , are covered by a vertebral body contact element 80 disposed over the outside surface 12 of the baseplate 10 . in such an embodiment it is preferable to include a vertebral body contact element 82 on the baseplate 30 for purposes of symmetry . such contact elements 80 and 82 are preferably contoured to match the contour of the surface it contacts in the intervertebral space . now referring to fig4 , in a preferred embodiment , pin 50 is slidably engaged in bore 26 . in this embodiment , in a preferred embodiment a resilient annular member 60 such as a resilient washer or the like is deployed over the projection 22 ( which in this embodiment is preferably cylindrical ) of the first baseplate 10 as a shock absorber , the resilient annular member 60 being sized and positioned such that it functions as a force restoring element ( e . g ., a spring ) that provides axial cushioning to the device , by deflecting under a compressive load and restoring when the load is relieved . now referring to fig5 and 6 , in other embodiments the elongated portion 52 of pin 50 preferably has a continuous cylindrical cross section ; however , the cross section may vary toward the distal end thereof , such as by gradually or abruptly thickening near the juncture of the elongated member 52 and the head 54 , to provide structural strength and / or to provide a different location for resilient member 60 . now referring to fig5 , in a preferred embodiment resilient member 60 is a continuous collar comprising a spring having a cylindrical cross section . it is desirable , but not essential , to use a spring as the resilient member 60 because of the ability of a spring to hold its diameter when subjected to compressive force . in a most preferred embodiment resilient member 60 is retained in a retainer 62 . retainer 62 is formed of a resilient material such as but not limited to an elastomeric material . in this embodiment elongated member 52 has a frustoconical section 56 adjacent proximal head 54 such that resilient member 60 and retainer 62 are firmly engageable in a seat formed between the frustoconical section 56 of elongated portion 52 and the end 28 of projection 22 . as forces are applied to retainer 62 , the spring comprising resilient member 60 deforms outwardly such that its diameter increases . in another embodiment , now referring to fig6 , resilient member 60 is an o - ring preferably formed of an elastomeric material . retainer 62 is a collar such as a split collar having formed thereon an exterior groove 64 to accommodate secure mounting of a resilient member 60 . in this embodiment elongated member 52 has a frustoconical section 56 adjacent proximal head 54 such that resilient member 60 and retainer 62 are firmly engageable between the frustoconical section 56 of elongated portion 52 and the end 28 of projection 22 . as forces are applied to retainer 62 , the o - ring comprising resilient member 60 deforms outwardly such that its diameter increases . the substantially constant radii articulation surfaces of the head 54 and socket 38 are configured and sized to be nestable against one another and articulatable against one another , to enable adjacent vertebral bones ( against which the baseplates 10 and 30 are respectively disposed in the intervertebral space ) to articulate in flexion , extension , and lateral bending . more particularly , the artificial disc implant of the present invention is assembled by disposing the baseplates 10 and 30 such that the vertebral body contact surfaces 80 , 82 are directed away from one another , and the articulation surfaces ( head 54 and socket 38 ) are nested against one another such that the concave arc of socket 38 accommodates the convex arc of head 54 . while there has been described and illustrated specific embodiments of an artificial disc , it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad spirit and principle of the invention . the invention , therefore , shall not be limited to the specific embodiments discussed herein .
an intervertebral device includes a first plate having an outer face and an inner face , and a second plate juxtaposed with the first plate , the second plate having an outer face , an inner face that opposes the first plate and a concavity that opposes the first plate . the device includes an elongated member extending from the first plate toward the second plate , the elongated member having a distal end with a spherical surface that is engageable with the concavity of the second plate for providing an articulating joint between the first and second plates . the device also includes a resilient member in contact with the elongated member for counteracting compressive loads on the plates , the resilient member being surrounded by the concavity of the second plate .