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Myositis ossificans

Last revised by Dr Joshua Yap on 13 Jul 2022

Myositis ossificans (MO) is the most common form of 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 a similar name to, myositis ossificans 1:

  • myositis ossificans circumscripta: refers to new bone that usually appears after trauma (see below)
  • myositis ossificans progressiva: a rare, inherited disorder characterized by fibrosis and ossification of muscles, tendons and ligaments at multiple sites that is disabling and ultimately fatal
  • panniculitis ossificans: similar to myositis ossificans but occurring in subcutaneous tissues
  • fibro-osseous pseudotumor of the digits: variant of myositis ossificans occurring in the fingers and toes

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

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

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

Myositis ossificans is essentially metaplasia of the intramuscular connective tissue resulting in extraosseous bone formation (without inflammation).

There are three well-described histopathological stages 6:

  1. first month: tissue injury causes organizing granulation tissue with fibroblastic and osteoblastic differentiation and osteoid formation
  2. second month: mineralized osteoid matrix develops with immature lamellar bone
  3. third month: immature bone progresses into mature lamellar cortical and trabecular bone

It has a zonal organization 1:

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

Unfortunately, the histology of myositis ossificans can appear similar to osteosarcoma, and thus, can lead to inappropriate management.

The typical radiographic appearance of myositis ossificans is circumferential calcification with a lucent center and a radiolucent cleft (string sign) that separates the lesion from the cortex of the adjacent bone.

Initially calcification is absent but there may be soft tissue swelling. Calcification usually becomes apparent 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.

The cleft between it and the subjacent bone may be difficult to see on plain radiographs.

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

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

MRI appearances change with the age of the lesion.

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

  • T1
    • ill-defined isointense to muscle mass
  • T2
    • periphery: high signal (edema) 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 hemorrhage 1,3
  • T1 C+ (Gd): enhancement is often present

Late features 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

Non-specific increased bone scan 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.

FDG-PET can demonstrate intense uptake mimicking high-grade lesions 5.

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

Imaging differential considerations include:

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

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Cases and figures

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  •  Case 6
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  • Case 7: on MRI
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  • Case 8: right thigh
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  • Case 9: on MRI
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  • Case 10
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  • Case 11
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