constant relative distraction positions i e the elements 200 300 do not significantly move if at all in directions that are directed away from one another for example do not move in opposing axial directions from one another e g along the longitudinal axis of the spine accordingly the present invention provides a pair of articulation surfaces 204 304 that have a center of rotation above the surfaces in one mode of motion lateral bending and below the surfaces in another flexion extension consistent in these regards with a natural intervertebral joint in the cervical spine preferably the articulation surfaces 204 304 are sized and configured so that the respective ranges of angles through which flexion extension and lateral bending can be experienced are equal to or greater than the respective normal physiologic ranges for such movements in the cervical spine while the present invention is not limited to any particular dimensions a preferred embodiment has the following radii of curvature for the convex arc 312 and the convex arc 214 c 0 280 inches and b 0 340 inches such preferable radii of curvature provide the preferred embodiment with a flexion extension range of plus or minus 7 5 degrees total of 15 degrees and a lateral bending range of plus or minus 7 5 degrees total of 15 degrees while the preferred embodiment is shown with concave arc 212 having a larger constant radius of curvature a than the constant radius of curvature c of convex arc 312 for reasons.