A known artificial disc is disclosed in U.S. Pat. No. 5,893,889. U.S. Pat. No. 5,893,889 discloses an artificial disc having upper and lower members for engaging adjacent vertebrae. A spherical pivot ball on a post extends upwardly from the lower member. The spherical pivot ball is received in a spherical pivot socket on the upper member. Between the upper and lower members and surrounding the post and pivot ball is an annular resilient cushion member.