Rotator cuff tear associated with an acromioclavicular cyst in rheumatoid arthritis.

We report two cases of a full-thickness rotator cuff tear followed by acromioclavicular cyst formation in patients with longstanding erosive polyarticular rheumatoid arthritis. One of the consequences of a rotator cuff tear is articular instability with upward migration of the humeral head. The ensuing chronic friction against the undersurface of the acromioclavicular joint caused by arm movements can lead to a non-inflammatory effusion of the acromioclavicular joint with cyst formation. Clinical and ultrasonographic features and a pathogenetic hypothesis are discussed.