Total shoulder replacement surgery replaces your shoulder's damaged bone and cartilage with a metal and plastic implant. The shoulder is a ball-and-socket joint, much like the hip joint. The ball is the top of the arm bone (the humerus) and the socket is within the shoulder blade (the scapula). During shoulder replacement surgery, the ball is removed from the top of the humerus and replaced with a metal implant. This is shaped like a half-moon and attached to a stem inserted to the center of the arm bone. The socket portion of the joint is then replaced with a plastic socket that is cemented into the scapula.