Your shoulder is a complex joint that is capable of more motion than any other joint in your body. It is made up of three bones: your upper arm bone (humerus), your shoulder blade (scapula), and your collarbone (clavicle).

Ball and socket: The head of your upper arm bone fits into a rounded socket in your shoulder blade. This socket is called the glenoid. A slippery tissue called articular cartilage covers the surface of the ball and the socket. It creates a smooth, frictionless surface that helps the bones glide easily across each other. The glenoid is ringed by strong fibrous cartilage called the labrum. The labrum forms a gasket around the socket, adds stability, and cushions the joint.

Shoulder capsule: The joint is surrounded by bands of tissue called ligaments. They form a capsule that holds the joint together. The undersurface of the capsule is lined by a thin membrane called the synovium. It produces synovial fluid that lubricates the shoulder joint.

Rotator cuff: Four tendons surround the shoulder capsule and help keep your arm bone centered in your shoulder socket. This thick tendon material is called the rotator cuff. The cuff covers the head of the humerus and attaches it to your shoulder blade.

Bursa: There is a lubricating sac called a bursa between the rotator cuff and the bone on top of your shoulder (acromion). The bursa helps the rotator cuff tendons glide smoothly when you move your arm.