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Myositis ossificans

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.

There are a number of  conditions that are related to or share similar name to myositis ossificans 1:

The remainder of this article focuses on the former. Myositis ossificans progressiva, panniculitis ossificans and fibro-osseous pseudotumour of the digits are discussed separately.

Myositis ossificans is one of the skeletal “Don’t touch” lesions.

Epidemiology

Most cases of myositis ossificans occur as a result of trauma, and thus the main demographic is young adults 1. Another group which are particularly prone to myositis ossificans are paraplegics, usually without evidence of trauma 2.

Clinical presentation

Typically presents as a painful, tender, enlarging mass, which in 80% of cases is located in large muscles of the extremities, often following recognised local trauma, although a definite traumatic event is not always recalled 1,3. In the case of paraplegics, recognised episodes of trauma are often absent, and the disease occurs particularly around the knees and hips 2.

Pathology

Myositis ossificans is essentially extraosseous bone formation (without inflammation) which occurs in muscle. It has a zonal organisation 1:

  • peripheral well organized mature lamellar bone
  • intermediate osteoid region
  • central immature non-ossified cellular focus

Unfortunately the histologically of myositis ossificans can appear similar to osteosarcoma, and thus can actually 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 film

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 2 months. Over the following 4 or so months, they typically become smaller and denser 1,3

Cleft between it an the subjacent bone may be difficult to see on plain films.

CT

CT appearances are similar to those of plain radiography, demonstrating mineralisation proceeding from the outer margins towards the centre. The cleft between it and the subjacent bone is usually visible.

The peripheral rim of mineralisation is usually visible within 4 - 6 weeks 3.

MRI

MRI appearances change with the age of the lesion. 

Early features can be misleading because the peripheral calcification is not well seen, and oedema in the soft tissues may extend beyond the often inapparent calcific rim.

  • T1
    • ill-defined isointense to muscle mass
  • T2
    • periphery : high signal (oedema) seen up to 8 weeks 3
    • central : heterogeneous high signal, due to high proliferating cellularity and cartilaginous components 3
    • fluid-fluid levels have been reported and attributed to previous haemorrhage 1,3
  • T1 C+ (Gd) - enhancement is often present

Late appearances mimic bone 1,3:

  • T1
    • periphery - low signal (mature lamellar bone)
    • central - intermediate to high signal (bone marrow)
  • T2
    • periphery - low signal (mature lamellar bone)
    • central - intermediate to high signal (bone marrow)
  • T1 C+ (Gd) - usually none in mature lesions.
Nuclear medicine 

Non-specific increased uptake on flow and blood pool images is seen early on in the development of the lesion, and gradually decreases as the lesion matures 2. Increased uptake on delayed phase is typical.

Treatment and prognosis

Myositis ossificans is benign and there is no compelling evidence that malignant degeneration ever occurs 1. As such treatment is reserved for symptomatic lesions, and surgical resection is usually curative. 

Differential diagnosis

Imaging differential considerations include

If fluid-fluid levels are present then a different differential should be entertained (see differential of fluid fluid level containing bone lesions)

See also

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