Giant Solitary Synovial Osteochondroma of the Subtalar Joint.

A rapidly progressing calcified mass was found in the left sinus tarsi in a 12-year-old female after a trivial ankle sprain. The lesion mimicked an aggressive lesion clinically and radiographically. Ultrasound-guided biopsy confirmed the diagnosis of a synovial chondroma. Excision of the tumor and partial synovectomy were performed. The histologic diagnosis was a solitary synovial osteochondroma. The condition had not recurred after a follow-up period of 12 months.