The shoulder is made up of three bones—the clavicle, scapula, and humerus—held together by muscles, tendons, ligaments, and a joint capsule.

• The collarbone (clavicle) attaches the shoulder to the breastbone (sternum). It connects with the large, flat, triangular shoulder blade (scapula) at its upper, outer corner (acromion).
• The acromion extends from the scapula to form the roof of the shoulder. It lies above the shoulder socket, called the glenoid fossa. The top (head) of the upper arm bone (humerus) is cradled in this socket, forming the shoulder joint (glenohumeral joint).
• The rotator cuff is a group of four tendons and their related muscles—the supraspinatus, infraspinatus, subscapularis, and teres minor—that stabilize the shoulder joint and allow raising and rotating of the arm.