The collar bone, which is the common name for the clavicle, is a short, narrow bone extending horizontally across the top of the chest. It stretches bilaterally from the sternum, or breast bone, in the center of the chest to the acromion process of the scapula at the top of the shoulder. An S-shaped bone easily felt at the base of the neck, it forms the anterior portion of the structure known as the shoulder girdle or pectoral girdle, which also comprises the scapulae in the back. This structure is not only responsible for attaching the arms to the axial skeleton in the trunk of the body, but also for serving as a site of attachment for the muscles that move the arm at the shoulder. Additionally, the location of the collar bone, elevated above the glenohumeral or shoulder joint, allows for a large range of motion in the joint.
Found just above the topmost rib, the clavicle meets with the sternum on its interior or medial end in the middle of the chest. The articulation between the collar bone and a structure on the top of the sternum called the manubrium is known as the sternoclavicular joint, a synovial saddle joint that permits some front-to-back and up-and-down movement of the clavicle, as well as minor rotation. On the bone’s exterior or lateral end where it articulates with the shoulder blade is the acromioclavicular joint. The acromion of the scapula is the large bony protrusion, extending laterally and upward from the top of the shoulder blade, that is easily felt at the top of the shoulder above the arm. This articulation is a synovial gliding joint that allows rotation of the collar bone against the scapula when lifting the arm above the head.
Many muscles that move the arm or shoulder blades affix to the collar bone. These include, on its upper side, the trapezius and deltoid muscles. The trapezius is a large muscle of the upper back whose upper fibers shrug the shoulders, while the deltoid is the prominent shoulder muscle that raises the arm away from the body in several directions.
On the front surface of the clavicle, the pectoralis major attaches. This muscle is responsible for horizontally adducting the arms, or pulling the arms inward toward the center of the chest when they are raised to shoulder height. Muscles attaching to the rear side of the collar bone include the trapezius as well as the sternocleidomastoid, which is the muscle in the neck that flexes the head forward and turns it from side to side.
As the collar bone is prominently located, relatively unprotected by fat and muscle, and consists of mostly spongy bone, it is rather susceptible to injury. Common injuries include dislocation of the bone from either of its two joints as well as fractures. These can happen as a result of falling or sudden impact and are frequent in athletes, in babies as a consequence of pushing the shoulders through the birth canal during labor, and in pre-adolescent children, as the bone is rarely fully formed until late in the teenage years.