Paediatric ImagingMyositis ossificans
non-neoplastic, heterotopic bone formation in skeletal muscles. It is rare in children and usually occurs in adolescents and young adults, particularly those who are athletic. 60 - 75% of patients give a history of
trauma but this may not be well documented. It may occur spontaneously; other associations include peripheral and central nervous system diseases, burns and tetanus. The commonest sites are those
prone to injury, such as the elbow, thigh, buttocks, shoulder and calf. Clinically the patient presents with painful soft tissue swelling. In the early stages it can be difficult to differentiate histologically and radiologically from a
malignant lesion.
Imaging
Plain films demonstrate faint calcification 2 - 4 weeks after the onset of symptoms. The calcification is predominantly around the periphery of the lesion. A radiolucent zone of soft tissue may separate the lesion from the cortex and periosteal reaction (Fig.1). After 6 - 8 weeks a well-defined partially ossified soft tissue mass is visible. The lesion may become very large especially if in the buttock. Regression usually commences after 5 - 6 months.
CT will identify the lucent zone and the peripheral site of the calcification. These features help in the differentiation from parosteal osteosarcoma which has an ill defined periphery and calcification more centrally.
Ultrasound in myositis ossificans shows a soft tissue mass containing echodensities with acoustic shadowing. The lesion is hot on isotope bone scans even before any radiographic abnormality; activity reduces in more mature lesions.
MRI prior to the appearance of characteristic calcification on plain films shows inflammatory reaction extending beyond the central mass. MRI in the acute stage shows a mass isointense with muscle on T1-weighted sequences with peripheral enhancement following intravenous contrast administration. On T2-weighted sequences and particularly STIR sequences high signal intensity extends beyond the tumour into the surrounding soft tissues. Oedema in the adjacent bone marrow may also be seen. In the intermediate stage the lesion is isointense/hyperintense on T1-weighted images and high signal intensity on T2-weighted images. A rim of decreased intensity is seen corresponding to the peripheral calcification. More mature lesions are well defined with inhomogeneous signal intensity similar to fat and reduced signal intensity surrounding the lesion. Myositis ossificans is self-limiting and the prognosis is excellent. The differential diagnosis prior to calcification being seen is soft tissue sarcoma.
JC
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Radiograph of tibia and fibula showing periosteal new bone formation from a subperiosteal haematoma and, separately from it, an area of ossification within the muscle due to myositis ossificans
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Myositis ossificans, Fig.1 | |