Pigmented villonodular synovitis of the shoulder

A 16-year-old student presented with painful swelling over the left proxima l humerus. Magnetic resonance imaging showed a malignant-looking soft tissu e mass of 7,5 x 6 x 4 cm invading the shoulder capsule. The patient did not remember initially a shoulder trauma, though postoperatively he reported a distortion trauma of the affected shoulder after a mountainbike accident. Intraoperatively we found two tumors infiltrating the axillary vessel and n erve structures originating from the shoulder joint capsule. Quick examinat ion of the tissue revealed no signs of malignancy, further pathohistologic examination showed a local pigmented villonodular synovitis. Unusual is the clinical presentation of the lesion over the proximal humerus, leading us preoperatively to neglect the possibility of tumor origin from the shoulder capsule, especially since the patient didn't have any complaints or functi onal deficits of the shoulder joint.