Myositis ossificans (MO) is a benign process characterised by heterotopic ossification usually within large muscles. Its importance stems in large part from its ability to mimic more aggressive pathological processes. Myositis ossificans is one of the skeletal “don’t touch” lesions.
There are some conditions that are related to, or share similar name to, myositis ossificans 1:
- myositis ossificans circumscripta: refers to new bone that usually appears after trauma
- myositis ossificans progressiva: a rare, inherited disorder characterised by fibrosing and ossification of muscle, tendon and ligaments of multiple sites that are disabling and ultimately fatal
- panniculitis ossificans: similar to MO but occurring in subcutaneous tissues
- fibro-osseous pseudotumour of the digits: variant of MO occurring in the fingers and toes
Pathology
Myositis ossificans is essentially metaplasia of the intramuscular connective tissue resulting in extraosseous bone formation (without inflammation).
It has a zonal organisation 1:
- peripheral, well-organized mature lamellar bone
- intermediate osteoid region
- central immature non-ossified cellular (fibroblasts) focus
Unfortunately, the histologically of myositis ossificans can appear similar to osteosarcoma, and thus, can lead to inappropriate management.
Radiographic features
The typical radiographic appearance of myositis ossificans is circumferential calcification with a lucent centre and a radiolucent cleft (string sign) that separates the lesion from the cortex of the adjacent bone.
Plain radiograph
Calcification usually begins to become apparent on plain radiographs within 2-6 weeks, and the lesion reaches the classic well circumscribed peripherally calcified appearance by two months. Over the following 4 or so months, they typically become smaller and denser 1,3.
The cleft between it and the subjacent bone may be difficult to see on plain radiographs.
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