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How the Shoulder Works

Posted: June 1, 2006

The shoulder is composed of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone).

Although most people think of the shoulder as a single joint, there are really two joints in the area of the shoulder. One is located where the collarbone meets the tip of the shoulder bone. This is called the acromioclavicular or AC joint. The junction of the upper arm bone with the shoulder blade is called the glenohumeral joint.

The bones of the shoulder are held in place by muscles, tendons and ligaments. Tendons are tough cords of tissue that attach the shoulder muscles to bone and assist the muscles in moving the shoulder. Ligaments attach shoulder bones to each other, providing stability.

The shoulder is the most movable joint in the body. However, it is unstable because of the range of motion allowed. It is easily subject to injury because the ball of the upper arm is larger than the shoulder socket that holds it.

Sources of shoulder pain
To remain stable, the shoulder must be anchored by its muscles, tendons and ligaments. Some shoulder problems arise from the disruption of these soft tissues as a result of injury, or from overuse or underuse of the shoulder.

Osteoarthritis can affect the joints in the shoulder, causing pain and reduced mobility. Other problems arise from a degenerative process in which tissues break down and no longer function well.

Shoulder pain may be localized or it may be felt in the areas around the shoulder or down the arm. Disease within the body (such as gallbladder, liver or heart disease, or disease of the cervical spine of the neck) also may generate pain that travels along nerves to the shoulder.

Source: National Institute of Arthritis and Musculoskeletal and Skin Diseases

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