| Discussion: |
Myositis ossificans occurs in 2 forms: Myositis ossificans circumscripta / traumatica (secondary to soft tissue injury or without known injury) and Myositis ossificans progressiva (an autosomal dominant genetic disorder with complete penetrance and variable expression).
Nonhereditary myositis ossificans
* Rare in children < 10 yoa
* Pain, tenderness, focal swelling, fever (rare)
* History of trauma may be difficult to elicit.
* 80% ossifications occur in the thigh or arm
* ESR and WBC count are rarely elevated.
* Progressiva: Short metacarpals and metatarsals, vertebral anomalies, increased incidence of enchondromas
* Early examination may be unremarkable.
* CT scan shows fascial plane edema before ossification occurs.
* Floccular calcified mass at 2-6 weeks from onset.
* AT 6-8 weeks, calcification becomes sharply circumscribed.
* MRI early shows low T1 signal and high T2 intensity (edema). Late imaging shows peripheral rim of low intensity on all sequences. Irregular areas of increase T2 signal then central signal intensity similar to fat on all sequences with areas of low signal due to ossification.
* Biopsy may be performed to exclude osteosarcoma.
* Surgery only for nonhereditary myositis ossificans and only after maturation of the lesion (6-24 mo). Do not attempt surgical care for patients with myositis ossificans progressiva. Surgery is indicated when lesions mechanically interfere with joint movement or impinge on nerves.