an intervertebral implant for implantation between an upper vertebra and a lower vertebra having a central axis . the implant may have a first member with a top surface for contacting at least a portion of the upper vertebra and a bottom surface as well as a second member with a top surface and a bottom surface for contacting at least a portion of the lower vertebra . an elastic spacer may be disposed between the first member and the second member . contraints may be employed to to limit the amount of lateral movement between the first and second members .