Myositis ossificans in a young child mimicking monoarthritis of the hip.

This report describes apparent acute monoarthritis of the left hip in a 6-year-old boy that was caused by myositis ossificans (MO). His initial presentation, with pain on movement of the left hip and an elevated sedimentation rate, led to consideration of an infection or inflammatory process affecting the left hip. Initial evaluation, including MRI and muscle biopsy, did not successfully lead to a diagnosis. MO became evident as an explanation for this child's problem on the 13th hospital day, 4 weeks after symptoms began, when typical radiologic manifestations became apparent on x-ray. This child's presentation, together with a review of recent literature, emphasizes that this condition a) is not as rare as previously described, b) occurs without a history of trauma, c) can be associated with acute phase reactants, and d) should be included in the differential diagnosis of apparent monoarthritis of the hip in children.