This joint complex includes three bones, the humerus (ball portion of arm bone), scapula (shoulder blade), and the clavicle (collarbone). There are two main joints in the shoulder the glenohumeral joint (ball and socket) between the humerus and scapula, and the acromioclavicular joint (AC joint) between the collarbone and scapula. The shoulder complex allows for significant motion and is loosely stabilized by capsule and ligaments. The main stability of the ball and socket joint of the shoulder is provided by the four rotator cuff muscles and the biceps tendon attachment. A labrum (cartilage structure) provides a cushion and acts to increase stability of the glenohumeral joint.