A spinal implant prosthesis includes a pair of end plates for affixation to adjacent vertebral bone structures and respectively defining inter-engaged convex and concave articulatory surfaces of elliptical profile. These elliptical articulatory surfaces are elongated in an anterior-posterior direction, and are comparatively shorter in a medial-lateral direction. With this configuration, in response to angular displacement and/or axial rotation, the elliptical surfaces displace in a manner increasing the distance between the adjacent vertebral bone structures, thereby tensioning the prosthesis and producing counteracting forces which urge the components back toward a substantially centered or neutral position. In addition, in the preferred form, the anterior-posterior length of the concave articulatory surface in incrementally greater than the anterior-posterior length of the convex articulatory surface to accommodate a limited range of anterior-posterior translation.
