In shoulder replacement surgery, the painful surfaces of the damaged shoulder are resurfaced with artificial shoulder parts. The part that replaces the ball consists of a stem with a rounded metal head. The part that replaces the socket consists of a smooth plastic concave shell that matches the round head of the ball. When both sides of the joint are resurfaced, we call it a total shoulder replacement. However, your doctor may determine that only the humeral side of the joint (ball) should be resurfaced. We call this a partial shoulder replacement.