The subject invention provides a modular six-degrees-of-freedom spatial mechanism for spinal disc prosthesis, with three rotational and three translational degrees-of-freedom. The prosthesis anchors to the superior and inferior vertebrae of an FSU and maintains mechanical linkage between those vertebrae for all normal motions and positions of the FSU. The prosthetic disc uses a novel, multi-curvate “ball-and-socket” joint with joint limits on all three rotational axes integrated into the design. A planar joint can further provide two, orthogonal linear degrees of freedom. Resilient spring elements, self-adjusting, in position and orientation, in conjunction with a double-layered, fiber and ring reinforced boot and toroidal belt, and a unique hydraulic damping system using dual segmented-walls can accommodate dynamic and static forces and sudden shocks on the FSU.