Extra-articular localized nodular synovitis (giant cell tumor of tendon sheath origin) attached to the subtalar joint.

A large soft tissue mass of the left ankle was documented by ultrasonography in a 24-year-old man. Histologic examination of biopsy samples was consistent with pigmented villonodular synovitis. Magnetic resonance, performed to establish the extension of the mass, revealed an extra-articular lesion involving the posteromedial extra-articular soft tissues, which, by means of a thin peduncle, extended into the subtalar joint space. The mass was excised; gross and microscopic examinations were characteristic of localized nodular synovitis. Bone and cartilage abnormalities were not revealed by either preoperative imaging or surgery. At 15-month follow-up, the patient was free of recurrences. This case is an uncommon example of localized nodular synovitis of the subtalar joint, characterized by the absence of bone and articular cartilage abnormalities. In addition, this case reveals that the pattern of growth (i.e., the shape of the lesion) is an important factor in determining secondary lesions of bone and cartilage.