The shoulder is made up of the following three bones: the upper arm bone (humerus), shoulder blade (scapula), and the collarbone (clavicle). Two joints in the shoulder allow it to move: the acromioclavicular joint, where the scapula meets the clavicle, and the glenohumeral joint, a ball and socket joint. The ball, or head, of the humerus fits into a socket called the glenoid, which is in the scapula. The arm is kept centered in the socket by the rotator cuff, a network of muscles and tendons that help to lift and rotate the arm.
The surfaces of the bones are covered with smooth, elastic cartilage that absorbs shock and allows the joint to move easily. Bursae are small, fluid-filled sacs that are located in joints and act as cushions between bones and the overlying soft tissues. They help reduce friction between the gliding muscles and the bone.