Bursting through barriers: A tale of silent swelling, ruptured tendons, and unveiling the geyser within the shoulder

Acromioclavicular (AC) joint cysts are rare clinical entities, with fewer than 50 cases documented in the literature. They are most commonly associated with chronic, full-thickness rotator cuff tears and degenerative changes of the AC joint. These cysts form due to increased intra-articular synovial fluid escaping through a defect in the AC joint capsule, functioning as a one-way valve. This mechanism allows fluid to track superiorly, resulting in the formation of a subcutaneous cyst. A hallmark imaging feature, known as the Geyser sign, demonstrates fluid ascending from the glenohumeral joint, through a disrupted AC joint capsule, and into a subcutaneous cyst resembling a geyser eruption. This finding is highly suggestive of a benign process and strongly indicative of chronic underlying rotator cuff pathology. Although typically painless, AC joint cysts may cause discomfort due to their superficial location. Importantly, they can mimic malignant lesions, particularly in elderly patients. As such, magnetic resonance imaging (MRI) plays a vital role in accurate diagnosis and in excluding neoplastic processes. Recognition of the Geyser sign is critical to avoid misdiagnosis and to guide appropriate management strategies. In this report, we present a rare case of an adult female with a shoulder mass, occurring in the absence of any prior trauma or known rotator cuff tear. This case highlights the pivotal role of imaging in diagnosing this uncommon condition.