To better understand shoulder problems and how they occur, it helps to begin with an explanation of the shoulder’s structure and how it functions.

The shoulder joint is composed of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone). (See diagram.) Two joints facilitate shoulder movement. The acromioclavicular (ah-KRO-me-o-klah-VIK-u-lahr; AC) joint is located between the acromion (ah-KRO-me-on; part of the scapula that forms the highest point of the shoulder) and the clavicle. The glenohumeral joint, commonly called the shoulder joint, is a ball-and-socket-type joint that helps move the shoulder forward and backward and allows the arm to rotate in a circular fashion or hinge out and up away from the body. (The “ball,” or humerus, is the top, rounded portion of the upper arm bone; the “socket,” or glenoid, is a dish-shaped part of the outer edge of the scapula into which the ball fits.) The capsule is a soft tissue envelope that encircles the glenohumeral joint. It is lined by a thin, smooth synovial membrane.

Shoulder

In contrast to the hip joint, which more closely approximates a true ball and socket joint, the shoulder joint can be compared to a golf ball and tee, in which the ball can easily slip off the flat tee. Because the bones provide little inherent stability to the shoulder joint, it is highly dependent on surrounding soft tissues such as capsule ligaments and the muscles surrounding the rotator cuff to hold the ball in place. Whereas the hip joint is inherently quite stable because of the encircling bony anatomy, it also is relatively immobile. The shoulder, on the other hand, is relatively unstable but highly mobile, allowing an individual to place the hand in numerous positions. It is in fact, one of the most mobile joints in the human body.

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. For example, the front of the joint capsule is anchored by three glenohumeral ligaments. The rotator cuff is a structure composed of tendons that work along with associated muscles to hold the ball at the top of the humerus in the glenoid socket and provide mobility and strength to the shoulder joint. Two filmy sac-like structures called bursae permit smooth gliding between bones, muscles, and tendons. They cushion and protect the rotator cuff from the bony arch of the acromion.