Buttock pain: a missed diagnosis.

A 13-year-old school boy presented with right-sided buttock pain, features of sepsis and Staphylococcus aureus positive blood cultures. On examination, he was febrile and in severe pain, with limited hip rotation and positive sacroiliac stress tests. Initial imaging with pelvic x-ray, hip ultrasound and MRI were normal. Despite this, a diagnosis of septic arthritis of the hip was presumed, and the patient underwent a washout of the right hip. When the imaging was reviewed in more detail, it was noted that a section of the sacroiliac joint was abnormal. Subsequent pelvic MRI confirmed that this was, in fact, septic sacroiliitis. The patient made a good recovery following washout of the right sacroiliac joint and 6 weeks of antibiotics.