An artificial disc having a pair of opposing plates (330,400) for seating against opposing vertebral bone surfaces, separated by at least one spring mechanism. The preferred spring mechanism is at least one spirally slotted belleville washer (330b), and in some embodiments the belleville washer is also radially thinning or thickening. Various illustrated embodiments use two washers or one washer. For double washer embodiments, the wide ends of the washers seat against respective opposing plates, in some embodiments each being maintained thereagainst by a retaining wall (306) and ring (310) or a circular recess and retaining shield. For single washer embodiments, the narrow end of the washer can be modified to have a curvate socket (332b) for rotatably mounting onto a semispherical protuberance (410) extending from one of the plates.
