Synovial osteochondromatosis presenting as a mass in the buttock

A 42-year-old woman, who had received a radiologic diagnosis of synovial osteochondromatosis many years before, was examined for a painful right hip and a large immobile mass in the right buttock. Findings of radiologic and scintigraphic imaging were consistent with synovial osteochondromatosis. The patient underwent arthrotomy and excision of the large mass and several smaller masses to control pain and to identify possible malignant change.