Fibrodysplasia ossificans progressiva: a distinctive bone-forming lesion of the soft tissue.

A 3-month-old white female presented with a bone-forming lesion of the soft tissue of the left cheek. Skeletal survey revealed numerous associated radiographic abnormalities, predominantly involving the phalanges. Mineral metabolism and endocrine function were within normal limits. Multiple operations were performed over a 17-year period for palliation of sequelae arising from inability to open the mouth due to ankylosis of the temporomandibular joint by the progressively ossifying lesion. Histologically, the pathologic material had features resembling those of periosteal grafts, with all stages of membranous bone formation and a tendency for more mature lesions later in the course. .The clinico-pathologic features are those of fibrodysplasia ossificans progressiva (FOP). The differential diagnosis of this rare condition from other bone-forming lesions of the soft tissue such as myositis ossificans, extra-skeletal osteosarcoma and osseous metaplasia is discussed.