Teaching Case

An 11-year-old girl was admitted with increasing pain, stiffness, and limited motion of left knee for one week, following a fall from a trampoline. Physical examination demonstrated locally increased warmness and hardness of posterior inferior thigh on palpation. Her body temperature was within normal limits.

WBC was normal (9740 x 10³/mm³) while CRP (29, 3 mg/L) and sedimentation (39 mm/hr) levels were increased. X-ray was normal. Ultrasonography (US) revealed a heterogeneous, hypoechoic mass in the posterior left thigh measuring 34x31x30 mm. Doppler US showed no vascularisation. On magnetic resonance imaging (MRI), the lesion was within biceps femoris muscle, and hyperintense on T2- (Fig. 1) and isointense on T1-weighted (Fig. 2) sequences. It enhanced vividly (Fig. 3). The biceps femoris muscle was hyperintense on T2-weighted images spanwise length (Fig. 4). Re-evaluation with US (Fig. 5) and computed tomography (CT) (Fig. 6) at first week of admission revealed typical peripheral calcification for myositis ossificans (MO). The patient underwent physical therapy and oral naprosyn sodium, and was discharged following relief of symptoms. The patient was followed up with US after 1 month. The MO lesion was stable in size with peripheral calcification.

MO is heterotopic ossification in skeletal muscle with no definite aetiology. Some cases may be associated with trauma, however, more than 80% of cases have been reported to have no causative factors [1]. Children are rarely affected [2, 3]. It is usually detected within the large muscles of the extremities [2, 3].
Patients usually present with a lump, pain, and tenderness where the lesion is located. Erythema of the overlying skin may be present. Due to inflammatory response in early stages of MO, acute phase proteins may elevate. C reactive protein is reported to better correlate with the inflammatory activity of heterotopic ossification compared to erythrocyte sedimentation rate [4].
In early stages (1-2 weeks), MO is revealed as a well-defined solid mass with MRI. It appears to be isointense on T1- and hyperintense on T2-weighted image compared to muscle. Following intravenous gadolinium-based contrast medium injection, it enhances vividly. In early stages, adjacent muscle may have increased signal on T2 compatible with oedema. Peripheral calcification detected > 2 weeks is pathognomonic of MO and CT has been reported to be superior to demonstrate compared to other imaging modalities [5]. During the maturation stage (up to 5-6 months), peripherally located calcification proceeds through central part of MO.
In the presence of a peripherally calcified mass within the skeletal muscles with/without a history of trauma, the diagnosis of MO is established. Differential diagnosis consists of pyomyositis, extraskeletal osteosarcoma, and soft tissue sarcomas. The T2 hyperintensity of the muscle (muscle oedema) adjacent to MO on MRI may raise the suspicion of pyomyositis radiologically. However, lack of systemic (e.g. fever) and laboratory (e.g. leukocytosis) findings of infection lead to the diagnosis of MO [6]. In early stages of MO, MRI may be confusing and mimic soft tissue sarcomas. When peripheral calcification appears, other soft tissue sarcomas including osteosarcoma (central calcification is expected) may be ruled out.
As MO is self-limiting and may resolve completely in time, the management is conservative, including application of ice and resting in early stages and physiotherapy with passive stretching and strengthening.
Radiological methods, particularly CT, are invaluable in making the accurate diagnosis of MO. Although rarely encountered in paediatric age group, paediatric radiologists should be familiar with the imaging findings of MO to establish the diagnosis without necessity of biopsy.