A prosthetic implant device for use in replacing the ball end of a biological joint, the replacement including a spherical head for insertion into a natural socket, an insert in the head, a ball-shaped member captured in the insert and having a neck and a stem for mounting the ball-shaped member on the end of the biological member on which the ball is being replaced, and a radially-expansible ring for capturing the ball-shaped member within the insert the ring having a downwardly and inwardly sloping outer wall and seated in a recess in the insert, the recess having upper and lower end walls for defining upper and lower limits of axial movement of the ring within the recess to permit selective removal of the ball-shaped member from the insert and to prevent inadvertent disengagement of the ball-shaped member from the insert.