Fibrodysplasia ossificans circumscripta of the masseter muscle.

Fibrodysplasia ossificans circumscripta (FOC) is a lesion characterized by localized calcification of the investing fascia of skeletal muscle. It is often related to repeated trauma and was formerly known as traumatic myositis ossificans. Surgical procedures involving muscles are also believed to be a factor in the origin of the lesion. When FOC develops in the muscles of mastication it can lead to severe trismus. A case is reported of FOC in the left masseter muscle of a 44-year-old man who presented with marked limitation of opening. It was believed that previous fractures of the left maxilla and mandible and/or the subsequent surgical treatments were responsible for the onset of FOC. The radiographic and microscopic features of FOC are discussed in relation to recent theories on the etiology, pathogenesis, and treatment of the lesion.