The shoulder is made up of three bones: the scapula (shoulder blade), clavicle (collarbone) and humerus (upper arm bone).

The glenohumeral joint is the one most people think of as the shoulder joint. It is formed where a ball (head) at the top of the humerus fits into a shallow cup-like socket (glenoid) in the scapula, allowing a wide range of movement. The surfaces of the bones where the ball and socket meet are covered with smooth, elastic cartilage that absorbs shock and allows the joint to move easily.

Because the socket of the shoulder is shallow, the joint relies on surrounding soft tissues to support it and hold the components in place. Many of these soft tissues surround the joint to form a protective capsule, which is lined with a thin membrane which produces a fluid (synovial fluid) to cushion and lubricate the joint.