A femoral prosthetic component of a replacement hip joint comprises a stem for fixing in a medullary cavity. A separate proximal element is provided with a neck for receiving a modular ball head or having an integral ball head. A dove tail-shaped system is provided for securing the proximal component to the stem which includes a sliding tongue and groove joint. The tongue being provided on the stem by an undercut rail which extends in a medial-lateral direction and the groove being provided in the separate proximal element. The tongue and groove being tapered along their lengths.