What’s Acromioclavicular Arthritis?

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Acromioclavicular arthritis is a degenerative disease characterized by painful inflammation of the acromioclavicular (AC) joint, the joint where the clavicle, or clavicle, meets the acromion process on the top of the scapula. A type of synovial or movable joint known as a sliding joint, the AC joint contains fibrous cartilage meant to cushion the bones against each other. Over time, repetitive motion, and injury, this cartilage can wear away, causing joint pain and stiffness when moving the shoulder. Acromioclavicular arthritis pain can be managed with anti-inflammatory medications and therapy, but cases of moderate to severe arthritis may require surgery.

The human shoulder is made up of two separate joints: the glenohumeral joint and the acromioclavicular joint. The glenohumeral joint is the ball and socket joint formed by the humerus or upper arm bone and a socket on the scapula or scapula called the glenoid fossa. Just above the glenohumeral joint is the AC joint, which is made up of the lateral or outer end of the clavicle and the acromion process of the scapula. The latter is a projection of the scapula that projects up and out from the scapular spine on the posterior side of the bone and curves forward over the top of the glenohumeral joint. On the inner surface of the acromion process, the end of the clavicle articulates with the scapula, the two bones separated only—and sometimes not at all—by a thin layer of cartilage much like the menisci of the knee joint.

Together, these two joints work to facilitate arm movement during movements such as raising and lowering the arms on either side of the body and pressing overhead. Since the acromioclavicular joint is a sliding joint, it allows the clavicle to slide and rotate against the scapula so that the scapula and therefore the arm can move freely. Over time, however, the repetitive arm-lifting motions can wear down the cartilage between the bones. Like osteoarthritis affecting the lower back and knees, age-related joint strain is the main factor leading to acromioclavicular arthritis.

Acromioclavicular arthritis is also more likely to occur in people who have sustained blunt force trauma to the joint, such as a fall with an outstretched arm. Whether the tissue damage to the joint is caused by gradual wear and tear or an acute injury, the symptoms are the same. The pain may be felt during particular movements, such as pulling the arm horizontally inward so that it crosses the chest, or during movement after long periods of inactivity. The anterior aspect of the shoulder may feel stiff or inflamed, and range of motion may feel limited. To treat acromioclavicular arthritis, doctors may recommend anti-inflammatory treatments such as ice, ibuprofen, and stretching for mild cases, and surgery in severe cases to remove the end of the clavicle bone and thereby eliminate bone-on-bone contact.

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