When shoulder replacement surgery is performed to treat arthritis, damaged tissue, bone and cartilage are removed and replaced with artificial materials. In some cases, the head of the humerus is replaced, while it is sometimes necessary to remove and replace both the ball and the socket of the shoulder. When one suffers from osteoarthritis in the shoulder, the cartilage that usually cushions the bones wears away. This causes the bones to rub against each other, causing pain and discomfort as well as stiffness. Another arthritic condition that can be treated with shoulder replacement surgery is rheumatoid arthritis, which causes the synovial membrane surrounding the joint to become inflamed, which can then damage the cartilage, wearing it away and causing stiffness and pain. Shoulders where the small muscles around the shoulder, the so-called rotator cuff muscles, are torn or deficient might require a reverse shoulder replacement where the ball and socket’s positions are reversed.