Anatomy of the Shoulder
The shoulder is a complex structure made up of three bones; the humerus (arm bone), the scapula (shoulder blade), and the clavicle (collar bone). At the top of the humerus is a large ball called the humeral head, which glides and rotates against a small, slightly curved part of the scapula called the glenoid (shoulder socket); the relatively small socket allows significant joint movement and flexibility. A firm, rubbery ring of tissue called the labrum is attached to the edge of the glenoid, and the humeral head and glenoid are covered in a smooth, gliding surface called articular cartilage (joint surface). The bones of the shoulder form the glenohumeral joint (the true shoulder joint) and the acromioclavicular joint (the small joint between the acromial process of the scapula and the clavicle), and are joined together by ligaments (glenohumeral, coracoclavicular and acromioclavicular ligaments) and joint capsule. The four rotator cuff muscles arise from the scapula and insert onto the humeral head, surrounding and supporting the glenohumeral joint and controlling fine shoulder movements. The strong chest wall muscles (deltoid, pectoralis major, lattisimus dorsi) and arm muscles (biceps, triceps) provide additional support and strength to the shoulder joint.