A joint prosthesis device comprises a head configured to fit within a cup. The device includes a first member for attachment to a first bone in a joint having 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 having a second articulation surface portion defined by a second radius of curvature and a third articulation surface defined by a third radius of curvature. Each of the second and third radii of curvature has a length that is different from the length of the first radius of curvature by less than 0.05 mm, and the origin of the second radius of curvature is not coincident with the origin of the third radius of curvature.
