Subacromial bony erosion: a rare presentation of pigmented villonodular synovitis of the shoulder.

Pigmented villonodular synovitis (PVNS) rarely affects the shoulder. We describe two cases of PVNS arising from the shoulder joint, which caused rotator cuff tears and sub-acromial bony erosion, and which were treated arthroscopically. Sub-acromial erosion is frequently associated with various glenohumeral joint disorders, but it has not been reported in association with PVNS. We believe PVNS should also be considered in the differential diagnosis of patients who present with sub-acromial erosion.