The acromioclavicular joint cyst. An unusual presentation of a rotator cuff tear.

An unusual presentation of a full-thickness tear of the rotator cuff is the acromioclavicular (AC) joint cyst. This is formed when glenohumeral joint fluid leaks through the full-thickness cuff tear and into a diseased AC joint, eventually distending the superior capsule. Often, communication between the cyst and the glenohumeral joint can be demonstrated on shoulder arthrogram. It is essential to recognize that the presenting cyst is usually indicative of an underlying full-thickness rotator cuff tear, which is often massive and which will be difficult to reconstruct. This clinical finding is reported in the following two cases to emphasize the important relation between AC joint disease and rotator cuff abnormalities. Attempted excision of this cyst without recognition of its pathogenesis usually leads to cyst recurrence and unimproved symptoms.