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AC (Acromio-clavicular) Joint Arthritis


Three bones make up the shoulder girdle – the humerus (or arm bone), the scapula (or shoulder blade), and the clavicle (or collar bone). The upper part of the humerus attaches to a socket on the side of the acromion called the glenoid and forms the shoulder joint, termed the “glenohumeral joint.” Above the glenohumeral joint and on the top part of the shoulder, the clavicle attaches to another part of the scapula called the acromion at a smaller joint called the acromio-clavicular (AC) joint. The AC joint is the only boney attachment of the scapula to the rest of the skeleton.

AC joint arthritis is a common degenerative condition that occurs with age and activity. Similarly to arthritis in other joints, in AC arthritis there is narrowing of the space between the bone ends. This leads to abnormal rubbing between the bone ends and causes bone spurs to develop and the lining of the joint to become inflamed and painful. Occasionally, the condition may result from a shoulder (AC) separation, but more commonly it results from wear and tear on the shoulder with repetitive activity over time. Weightlifters often perform repetitive overhead lifting and place significant stress on the AC joint and are more prone to developing this condition.


Degenerative changes in the AC joint are often asymptomatic and are noted on shoulder x-rays taken to evaluate a separate shoulder problem. Some patients, however, may experience significant pain and limitation of shoulder function due to this condition. Pain and tenderness on the top of the shoulder in the area of the AC joint are the most common symptoms of AC joint arthritis. The pain is often worse when the arm is moved across the chest as this motion tends to compress the joint.


The diagnosis of AC joint arthritis is typically made from a history of the patient’s symptoms, physical examination findings of localized tenderness over the AC joint, and x-ray findings. An injection of local anesthetic may be used to confirm the location of the pain or to determine if there is a component of pain coming from other shoulder problems. MRI may be indicated to identify other potential causes or “contributors” to the shoulder pain.


Treatment of AC arthritis is initially non-operative and includes rest and avoidance of aggravating activity, icing, and anti-inflammatory medications. A cortisone injection into the AC joint is usually recommended. One or two injections may be required to give lasting benefit. If non-operative treatment fails, then surgery may be required. The surgery is performed using arthroscopic techniques or through a tiny incision over the joint. The end of the clavicle is removed and this alleviates painful rubbing of the bone ends. This procedure is called distal clavicle excision.”

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