The shoulder joint is a ball-and-socket joint. The socket is fairly shallow. This puts the joint at risk for dislocation. To help deepen the socket, the shoulder joint has a rim of cartilage called a labrum. The labrum forms a cup for the end of the arm bone (humerus) to rest and move inside.

A Bankart lesion is an injury to the labrum caused by forward (anterior) shoulder dislocation. The force of the head of the humerus (upper arm bone) as it dislocates, tears the labrum and the ligaments attached to it. The shoulder joint is unstable when the labrum is torn. There isn't anything to keep the head of the humerus from slipping forward out of the socket again. Surgery to repair the damage is usually needed.

A Hill-Sachs lesion is an injury that causes damage to the head of the humerus. It also occurs with shoulder dislocation. When the shoulder dislocates, the smooth surface of the humerus hits against the bony edge of the socket (called the glenoid). The collision causes a dent in the bone of the humerus. This is also called an impaction fracture.

The Hill-Sachs lesion can usually be seen on a normal shoulder x-ray. Surgery may not be needed. Treatment is designed to prevent further damage (and later arthritis) to the surface of the humerus.