Abstract: A prosthetic joint composed of a rigid joint head and a rigid socket. The joint head has a flat base face for engaging a flat surface prepared on the bone; the socket has a pair of shanks straddling the joint head. A hemispherical knob on the outer extremity of each of the shanks engages a groove in a respective one of a pair of flattened lateral faces of the joint head. One end of the groove extends to and forms an indentation into the edge of the base surface to allow insertion of the knob into the groove prior to the mounting of the joint head on the bone.