A prosthetic knee joint device of `hingeless` form having femoral and tibial components held in mutual sliding and rolling bearing engagement by natural muscle and ligaments. The femoral component includes a generally U-shaped channel member having an outer surface with a generally convex curvature in the circumferential sense relative to the channel longitudinal axial direction, and a smooth form with a single curvature in the axial sense. This single curvature need not necessarily be convex. The tibial component includes a platform member having one of its major surfaces concavely dished in generally complementary manner to at least the convex curvature of the femoral component. The inner surface and the other major surface of channel and platform members are adapted for fixation with cement to the femur and tibia.