a joint prosthesis device comprises a head configured to fit within a cup and in one embodiment includes a first member for attachment to a first bone in a joint and including a first articulation surface portion defined by a first radius of curvature and a second member for attachment to a second bone in the joint and including a second articulation surface portion defined by a second radius of curvature and a third articulation surface defined by a third radius of curvature , wherein each of the second radius of curvature and the third radius of curvature has a length that is different from the length of the first radius of curvature by less than 0 . 05 millimeters and wherein the origin of the second radius of curvature is not coincident with the origin of the third radius of curvature .