An acromioclavicular (AC) joint cyst is an unusual presentation of full-thickness rotator cuff tears in patients with degenerative changes of the AC joint. It is important to understand the relationship between AC joint cysts and rotator cuff tears because there is a high recurrence rate if the cyst is treated without addressing the rotator cuff tear. Furthermore, recurrence or draining sinus caused by failure to treat the cyst may lead to infection. To our knowledge, there have been no reports of infected AC joint cyst as a serious complication. We present 2 cases involving an infected AC joint cyst with a massive rotator cuff tear caused by simple cyst excision. When patients have an AC joint cyst, physicians should consider the following points: (1) There is the possibility of an underlying rotator cuff tear, (2) Surgical treatment is indicated for a symptomatic cyst, (3) Aspiration and simple cyst excision are not recommended, because of the potential for postoperative complications such as recurrence, a draining sinus, and infection, and (4) Lateral clavicle resection should be performed if there is an irreparable rotator cuff tear.

Chul-Hyun Cho, MD, PhD