Myositis ossificans of the temporalis muscle: case report and review of the literature.

Myositis ossificans, also known as traumatic myositis ossificans or myositis ossificans circumscripta, is the product of an unusual reactive process of mesenchymal stem cells within the muscle produced secondary to a traumatic insult or inflammatory process. In approximately 75% of cases, the process has a direct correlation with a single or repetitive, traumatic, penetrating, or crushing injury to the muscle. However, in up to 25% of cases, there is no recollection of a traumatic event. The most commonly accepted mechanism of traumatic etiology includes the embedment and lysis of bone fragments within the soft tissue causing subsequent exposure of bone morphogenic proteins to extraosseous cells. This environment stimulates osteoblasts to form true bone, dystrophic calcifications, or a calcified chondroid matrix. A concomitant mechanism that occurs involves the overproduction of bone morphogenic protein-4 produced by the muscle cells or the muscle fascia.