Anatomy of the Shoulder

The shoulder is a ball and socket joint. The shoulder joint is comprised of the arm bone (the humerus or "ball") and shoulder blade (the scapula or "socket"). The ball top of the humerus fits into the socket of the shoulder blade (the glenoid). The rotator cuff is a group of muscles and tendons that form a cuff over the shoulder. These muscles and tendons hold the arm in its "ball and socket" joint and help the shoulder rotate.

The shoulder, or glenohumeral joint is a unique joint in the body, allowing the greatest range of motion. Because of its mobility, the shoulder is not as strong as other joints, making it susceptible to dislocations and instability, which can lead to premature wear.

The surfaces where the bones meet are covered with articular cartilage, a soft, durable tissue that provides a low friction cushion between the bones. Healthy cartilage allows smooth, frictionless movement. As the cartilage thins and wears down, bone surfaces become rough and rub against each other unprotected, causing inflammation, swelling, pain and destruction of the joint.

The joint is lined by a thin, smooth tissue called the synovium. Here, fluid is produced that acts as a lubricant to reduce wear and tear on the joint.