MODEL RELEASED. Shoulder injury. Man resting after surgery to repair a shoulder dislocation. His shoulder is heavily bandaged and his arm is being supported by a sling. A shoulder dislocation is when the head of the humerus bone in the upper arm pops out of its socket in the scapula (shoulder blade). This ball-and-socket joint is known as the glenohumeral joint. Shoulders may become dislocated during falls, or by heavy blows to the shoulder or twists to the arm. Permanent instability of the shoulder joint can result, causing repeated dislocations. Surgery is performed to repair damaged ligaments and muscle tissue.