Shoulder joint replacement procedures are mostly done for osteo-arthritis of joints. In the normal joint, cartilage covers both bony surfaces of the ball and socket. When this cartilage wears away bone is exposed and bone then grinds onto bone on the other side of the joint – this is why osteo-arthritis causes pain and stiffness in the shoulder joint.
If and when this shoulder pain becomes disabling, joint replacement procedures are done. The purpose is to place material between the bone-ends to avoid this painful contact by having two smooth surfaces in contact with each other. The most common joint replacement procedure is to place a metal surface on the ball and a plastic surface on the socket.