Abstract: A ball-and-socket joint prosthesis, e.g. a hip joint prosthesis, which comprises an outer shell element, a ball component, and a replaceable bearing insert element positioned between the shell and the ball component. The bearing insert has a generally spherical inside surface which encompasses more than half of the ball component to thereby retain it. The insert has an outside surface shaped to mate with the inside surface of the shell, and the shell element and the insert element have a locking mechanism consisting of a protruding portion which extends a major distance around a side surface of one element which locks into a mating depression in the other element.