Turret exostosis of the talus.

Turret exostosis is a benign osteocartilagenous lesion believed to arise from reactive perisoteum following relatively mild trauma. This article presents an unusual case of a turret exostosis of the talar neck in a 12-year-old child. A healthy adolescent presented with a 5-month history of a firm, painless mass about the anteromedial aspect of the right ankle 1 month after suffering a mild twisting injury to the ankle. Radiographs demonstrated a benign-appearing bony mass within the soft tissues anterior to the right ankle overlying the talar neck. An excisional biopsy of the mass was performed and pathology was consistent with turret exostosis, demonstrating a central area of mature trabecular bone maturing via enchondral ossification with a thin hypocellular peripheral rim of cartilage. The absence of a periosteal layer, abundant "blue bone," bizarre metaplastic cartilage, or marked cytologic atypia confirmed the diagnosis. The patient remains pain free with full ankle motion. Reactive periosteal lesions are well-described entities, tending to occur with the greatest frequency in the small bones of the hand. Few cases of bizarre parosteal osteochondromatous proliferation in the metatarsals and phalanges of the toes have been reported. The current case represents the first account of a turret exostosis of the hindfoot, and the youngest patient with a histologically confirmed diagnosis. It further illustrates the manner in which this case exists along a continuous spectrum of reactive periosteal lesions. The clinical, radiographic, and histologic characteristics of reactive periosteal lesions are reviewed.