An artificial spinal disc (386, 414) comprising: an upper portion (390, 418) having a top surface configured for attachment to a vertebra and a lower surface having a first articulation surface disposed thereon； a lower portion (394, 422) having a lower surface configured for attachment to a vertebra and an upper surface having a second joint surface disposed thereon, the second joint surface being in slidable engagement with the first surface joint； characterized in that: the first articulation surface comprises a plurality of projections (398, 426, 402, 430) and the second articulation surface comprises a plurality of cavities (406, 434, 410, 438), each corresponding cavity one of the plurality of protrusions； the first articulation surface comprises a rear flange (398, 426) that is centered laterally and a pair of former projections (402, 430) which are spaced laterally, and wherein the second articulation surface comprises a back cavity (406, 434) that is centered laterally and a pair of anterior cavities (410, 438) which are separated laterally； wherein each of the cavities (406, 434, 410, 438) has a generally planar portion (186) makes transitions (190) in a generally flat curved side wall (194) extending upward from the portion toward the center of the lower portion； and wherein the upper portion has a neutral position relative to the lower portion, wherein the upper portion is aligned on the lower portion, and wherein the projections are arranged in transition regions of the respective cavities when the upper portion is in the neutral position so that the sliding of a projection away from the center of the lower portion moves the projecting generally horizontally and so that the movement of the projection toward the center of the lower portion moves the projecting horizontally and up to increase the overall thickness of the artificial disc near said projection.Un disco espinal artificial (386, 414) que comprende: una porción superior (390, 418) que tiene una
