An artificial intervertebral disc for implantation between adjacent superior and inferior vertebrae of a spine, the disc having a posterior end and an opposite anterior end, the disc comprising: - a superior shell, an inferior shell, and a means for biasing (20, 120, 120a, 120b) the superior and inferior shells apart; - the superior and inferior shells being moveable with respect to each other over two or more articulating surfaces; - the inferior shell having a superior surface comprising a convex portion; - the superior shell having an inferior surface opposing the superior surface of said inferior shell, and comprising a concave portion, being opposite said convex portion; - the convex and concave portions being in articulating cooperation to form a ball and socket joint; characterized in that: - at least one of the convex or concave portions having at least one male portion (400, 400a, 400b, 400c, 400e, 400f, 400g, 402, 402a, 402b, 402c, 402e, 402f, 402g) and the other of the convex or concave portions having a corresponding number of cooperating female portions (414, 414a, 414b, 414c, 414e, 414f, 414g) adapted to receive and positively engage said at least one male portion, whereby said male and female portions are engaged in a lock and key arrangement thereby preventing separation of the superior and inferior shells; - the convex portion and the concave portion being positioned at the posterior end of the disc; - said superior shell includes a first wall (32, 132) and said inferior shell includes a second wall (26, 126), said first and second walls being positioned at the anterior end of the disc, at least a portion of said walls being overlapped wherein said first wall extends downward and said second wall extends upward and is positioned posteriorly of said first wall; and, said inferior shell includes an anteriorly extending lip for limiting anterior movement of the superior shell relative to the inferior shell. - a superior shell, an inferior shell, and a means for biasing (20, 120, 120a, 120b) the superior and inferior shells apart; - the superior and inferior shells being moveable with respect to each other over two or more articulating surfaces; - the inferior shell having a superior surface comprising a convex portion; - the superior shell having an inferior surface opposing the superior surface of said inferior shell, and comprising a concave portion, being opposite said convex portion; - the convex and concave portions being in articulating cooperation to form a ball and socket joint; characterized in that: - at least one of the convex or concave portions having at least one male portion (400, 400a, 400b, 400c, 400e, 400f, 400g, 402, 402a, 402b, 402c, 402e, 402f, 402g) and the other of the convex or concave portions having a corresponding number of cooperating female portions (414, 414a, 414b, 414c, 414e, 414f, 414g) adapted to receive and positively engage said at least one male portion, whereby said male and female portions are engaged in a lock and key arrangement thereby preventing separation of the superior and inferior shells; - the convex portion and the concave portion being positioned at the posterior end of the disc; - said superior shell includes a first wall (32, 132) and said inferior shell includes a second wall (26, 126), said first and second walls being positioned at the anterior end of the disc, at least a portion of said walls being overlapped wherein said first wall extends downward and said second wall extends upward and is positioned posteriorly of said first wall; and,