Myositis ossificans

Clinical History

25-year-old pregnant, afebrile female with no significant background arrived due to back pain with no history of trauma.
The patient was send to the radiology department where an US and non-contrast MRI were performed. IV contrast and radiographs were not requested due to the patient's pregnany.

Imaging Findings

Ultrasonography showed hypoechoic lesion in the paravertebral musculature with small hyperechoic areas within, with acoustic shadow. There was significant peripheral hyperaemia on Doppler. (Fig 1)

On MR, the mass was well-defined, heterogeneous predominantly isointense to muscle on T1; hyperintense on T2 with hypointense foci. STIR showed surrounding oedema that extended into multiple soft tissue compartments. It also showed oedema in transverse process and hypointense peripheral area. (Fig.2-4)
The differential diagnosis included tumour and infection, therefore biopsy was performed. Biopsy did not show evidence of malignancy and serology and microbiology cultivation tests were negative as well.

After ten days, on follow-up ultrasound the lesion size was the same, hyperaemia had decreased and peripheral calcification became more evident. Due to the evolution myositis ossificans was our diagnosis. US follow-up demostrated increase of the peripheral calcification, that is the (zone phenomenon explained in 'Discussion'). (Fig. 5)

Later plain film showed the lesion as calcified on the periphery and not in the center.(Fig.6)

Discussion

Myositis ossificans (MO) is a benign process characterised by heterotopic ossification usually with large muscles. Its importance stems from its ability to mimic more aggressive pathological processes, it may be clinically and histologically mistaken MO for a malignant soft tissue tumour[1] and this can lead to inappropiate management.

The aetiology and potential predisposing MO factors remain unclear. In some cases, non-causative factors can be identified, but in most cases it can occur as a result of muscular trauma, commonly in the thigh or arm.

There are several distinct stages of MO. In early stages histologically, the proliferation is composed of mesenchymal cells and fibroblasts, radiographs cannot show any anomaly, but in later stages, also called the maturation phase, bone production can be observed at the periphery of the lesion, radiographs may show the pathognomonic ossification surrounding a clear central area.This happens because the characteristic rim of calcification may be identified in 4-6 weeks after the injury.
This pattern of calcification is essential to establish the radiologic diagnosis, and allows differentiation from other mineralized lesions, such as sarcoma, that usually shows central calcification. If this peripheral calcification is not present, the diagnosis remains non-specific and an aggressive sarcoma or infection cannot be excluded.[2]

Computed tomography (CT) is more sensitive than radiography to detect calcification.

Magnetic resonance imaging (MR) appearance changes with the age of the lesion. We can differentiate between early and late stages[3]:

-Early lesions: peripheral mineralization is not visible or well-defined making the diagnosis more difficult. On T1, the lesion is isointense to muscle; on T2, signal intensity is high and secondaty to proliferating and cartilagenous components. Active lesions present enhancement after gadolinium injection and are surrounded by soft tissue oedema.

-Late lesions: both T1 and T2 show low signal intensity in the periphery corresponding to mature lamellar bone. The centre shows intermediate to high signal intensity. The difference between MO and sarcoma consists of the type of mineralization: whereas MO shows peripheral mineralization, sarcomas have a centre calcification.

Ultrasound is the most sensitive imaging modality to depict the zone phenomenon as it may suggest the diagnosis in early stage. There is a central, undifferentiated zone impossible to distinguish from sarcoma. This zone merges into oriented osteoid formation and finally into well-formed bone in the periphery of the lesion. The diagnosis has to be based not only on the history of the trauma, but also on other clinical and imaging outcomes.[4][5]

As MO is a benign process and there is no compelling evidence that malignant degeneration or tumour ever occurs, treatment is reserved for symptomatic lesions.

Differential Diagnosis List

Final Diagnosis

Myositis ossificans

URL:	https://eurorad.org/case/11379
DOI:	10.1594/EURORAD/CASE.11379
ISSN:	1563-4086