[Pigmented villonodular synovitis of the shoulder joint].

A 16-year-old student presented with painful swelling over the left proximal humerus. Magnetic resonance imaging showed a malignant-looking soft tissue mass of 7.5 x 6 x 4 cm invading the shoulder capsule. The patient did not remember initially a shoulder trauma, through postoperatively he reported a distortion trauma of the affected shoulder after a mountain bike accident. Intraoperatively we found two tumors infiltrating the axillary vessel and nerve structures originating from the shoulder joint capsule. Quick examination of the tissue revealed to 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 functional deficits of the shoulder joint.