Artificial disc replacements (ADRs) use mobile spacers between vertebral endplates. In the preferred embodiment, spherical spacers are used between metal or ceramic plates placed over or in the endplates of the vertebrae. The spherical spacers may be made of metal, ceramics or polymers such as polyethylene. One or more of the bearings may be used in each disc replacement, and one or more disc replacements may be inserted into the disc space. The small bearings allow the preservation of the vertebral endplates. Although the preferred embodiment use mobile units preferably in the form of small spheres and two ADRs per level, alternatives are disclosed, wherein the mobile unit is non-spherical, including oblong shapes. Further alternative configurations include an elongated mobile bearing contained by interdigitating projections from superior and/or inferior endplates.