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An 11 year old boy presented to the accident and emergency department complaining of left hip pain after a fall from his skateboard. Apart from a mild restriction of hip movement caused by pain, physical examination and a pelvic radiograph in the AP view were unremarkable. He was discharged home with simple analgesia.
He presented again two weeks later with worsening pain and a limp. Examination revealed exquisite tenderness in his gluteal muscles with a markedly reduced range of movement. Repeat pelvic radiographs including AP and frog lateral views were performed showing a calcified mass in the abductor mechanism of his left hip (fig 1). In view of the unresolving pain, magnetic resonance imaging was performed showing high signalling within the mass with distortion of the adjacent muscular structure associated with marked soft tissue swelling and an effusion in the hip joint (fig 2). This raised the suspicion of a tumour. Consequently, a computed tomography guided biopsy was performed showing histological features of reactive new bone formation, suggestive of myositis ossificans.1 The patient was subsequently treated with indomethacin and symptoms resolved a few weeks later.
Myositis ossificans is the formation of mature bone outside the skeleton. Although it is an entirely benign condition, it is nevertheless an important condition because in its early stages presentation may be difficult to distinguish both clinically and radiologically from a soft tissue tumour such as rhabdomyosarcoma, which is the commonest soft tissue sarcoma in this patient's age group.2 Other differential diagnoses would include a calcifying haematoma, slipped capital femoral epiphysis and septic arthritis. The other important learning point is that any child with a persisting limp should be evaluated radiologically with two views including a frog lateral because conditions such as slipped capital femoral epiphysis are easily missed on the AP view alone.