Common causes are arthritis and synovitis, also called osteolysis. There is a small joint between the bones of the clavicle and the acromion, which is covered with articular cartilage and surrounded by a synovial membrane which makes fluid. Over time, wear and tear can cause the surfaces to wear down on the joint, which leads to the development of acromioclavicular joint arthritis. Many people will develop at least some arthritis with normal aging, however, in some cases, the arthritis may become painful, which may also follow an aggravating injury. This may also lead to bone spurs which may protrude inferiorly, and put pressure on the rotator cuff tendon and cause rubbing or impingement. In a second group of patients the cause of pain in the acromioclavicular joint is due to synovitis. These patients are often younger than the patients who develop degenerative arthritis. Synovitis is commonly seen in patients who are performing repetitive lifting activities, particularly in weight lifters doing heavy bench presses or overhead press activities. This can lead to an irritation of the joint, which results in significant swelling and pain.
In contrast to acromioclavicular joint arthritis, where the space between the bone is generally decreased on x-rays, the space with synovitis or osteolysis remains the same or actually increases. In severe cases the bone can start to dissolve away, resulting in cysts in the bone and ultimately destruction of the bone.
The treatment for these two problems is similar. This includes rest, ice, anti-inflammatory medications, and consideration of a corticosteroid injection into the joint. An MRI scan may be helpful in diagnosing acromioclavicular joint arthritis, and particularly in diagnosing acromioclavicular joint synovitis or osteolysis. With synovitis, increased fluid is commonly seen in the acromioclavicular joint, and there may also be fluid in the distal clavicle and some evidence of bone destruction. In many cases, symptoms can be well controlled with conservative measures. In other cases, the symptoms will persist, and continue to cause limitation in function and inability to do heavy lifting. In these patients, resection of the acromioclavicular joint can be performed arthroscopically. This may be done in conjunction with a decompression surgery of the subacromial space.