Shoulder replacement removes damaged areas of bone and replaces them with parts made of metal and plastic (implants). This surgery is called shoulder arthroplasty The shoulder is a ball-and-socket joint. The round head (ball) of the upper arm bone fits into a shallow socket in the shoulder. Damage to the joint can cause pain, weakness and stiffness. Shoulder implants are available in a few different shapes and a range of sizes. Replacement options include partial and total using either anatomic or reverse implants.