Prosthetic femoral joint

A prosthetic hip joint replacement system has a femoral component with a tapered trunion thereon. A modular head with a part-spherical outer surface terminating in a planar surface, engages the trunion. An acetabular cup member is provided for receiving the part-spherical head of a femoral implant, the cup member has a cavity having a part-spherical inner surface for receiving the modular head. The part-spherical inner surface of the cup defines a polar axis and an equator centered about a pole of the cavity. The cavity has a part-circular opening spaced from the pole a greater distance than the equator. Thus the inner part-spherical bearing surface extends beyond the 180° which the equator extends. The configuration of the part-spherical inner bearing surface causes a movement of translation of the head during rotation so that there is a crescent shaped retention area on each opposed side thereof. The opening has a non-circular or planar portion alignable with the planar head surface during insertion of the head in the cavity and a plane parallel to the non-circular portion through the center of the portion of the opening being offset from a parallel plane containing the polar axis of the cavity.