An artificial spinal disc (130, 350) comprising: an upper portion (134, 354) having an upper surface configured for attachment to a vertebra and a lower surface having a first articulation surface disposed thereon; a lower portion (142, 366) having a lower surface configured for attachment to a vertebra and upper surface having a second articulation surface disposed thereon, the second articulation surface being in slidable engagement with the first articulation surface; characterized in that: the first articulation surface comprises a plurality of projections (138a, 138b, 138c, 358, 362) and the second articulation surface comprises a plurality of recesses (146a, 146b, 146c, 370, 374), each recess corresponding to one of the plurality of projections; the first articulation surface comprises an anterior projection (138a, 358) which is centered laterally and a pair of posterior projections (138b, 138c, 362) which are spaced apart laterally and wherein the second articulation surface comprises an anterior recess (146a, 370) which is centered laterally and a pair of posterior recesses (146b, 146c, 374) which are spaced apart laterally; wherein each of the recesses (146a, 146b, 146c, 370, 374) has a generally flat portion which transitions into a curved sidewall that extends upwardly from the generally flat portion toward the center of the lower portion; and wherein the upper portion has a neutral position relative to the lower portion where the upper portion is aligned over the lower portion, and wherein the projections (138a, 138b, 138c, 358, 362) are disposed at the transition regions of the corresponding recesses (146a, 146b, 146c, 370, 374) when the upper portion is at the neutral position such that sliding a projection away from the center of the lower portion moves the projection generally horizontally and such that moving the projection towards the center of the lower portion moves the projection horizontally and upwardly to increase overall thickness of the artificial disc near said projection. an upper portion (134, 354) having an upper surface configured for attachment to a vertebra and a lower surface having a first articulation surface disposed thereon; a lower portion (142, 366) having a lower surface configured for attachment to a vertebra and upper surface having a second articulation surface disposed thereon, the second articulation surface being in slidable engagement with the first articulation surface; characterized in that: the first articulation surface comprises a plurality of projections (138a, 138b, 138c, 358, 362) and the second articulation surface comprises a plurality of recesses (146a, 146b, 146c, 370, 374), each recess corresponding to one of the plurality of projections; the first articulation surface comprises an anterior projection (138a, 358) which is centered laterally and a pair of posterior projections (138b, 138c, 362) which are spaced apart laterally and wherein the second articulation surface comprises an anterior recess (146a, 370) which is centered laterally and a pair of posterior recesses (146b, 146c, 374) which are spaced apart laterally; wherein each of the recesses (146a, 146b, 146c, 370, 374) has a generally flat portion which transitions into a curved sidewall that extends upwardly from the generally flat portion toward the center of the lower portion; and wherein the upper portion has a neutral position relative to the lower portion where the upper portion is aligned over the lower portion, and wherein the projections (138a, 138b, 138c, 358, 362) are disposed at the transition regions of the corresponding recesses (146a, 146b, 146c, 370, 374) when the upper portion is at the neutral position such that sliding a projection away from the center of the lower portion moves the projection generally horizontally and such that moving the projection towards the center of the lower portion moves the projection horizontally and upwardly to increase overall thickness of the artificial disc near said projection. the first articulation surface comprises a plurality of projections (138a, 138b, 138c, 358, 362) and the second articulation surface comprises a plurality of recesses (146a, 146b, 146c, 370, 374), each recess corresponding to one of the plurality of projections; the first articulation surface comprises an anterior projection (138a, 358) which is centered laterally and a pair of posterior projections (138b, 138c, 362) which are spaced apart laterally and wherein the second articulation surface comprises an anterior recess (146a, 370) which is centered laterally and a pair of posterior recesses (146b, 146c, 374) which are spaced apart laterally; wherein each of the recesses (146a, 146b, 146c, 370, 374) has a generally flat portion which transitions into a curved sidewall that extends upwardly from the generally flat portion toward the center of the lower portion; and wherein the upper portion has a neutral position relative to the lower portion where the upper portion is aligned over the lower portion, and wherein the projections (138a, 138b, 138c, 358, 362) are disposed at the transition regions of the corresponding recesses (146a, 146b, 146c, 370, 374) when the upper portion is at the neutral position such that sliding a projection away from the center of the lower portion moves the projection generally horizontally and such that moving the projection towards the center of the lower portion moves the projection horizontally and upwardly to increase overall thickness of the artificial disc near said projection.