Aneurysmal bone cyst of the sacrum

We describe the case of a 9-year-old girl presenting with a 3 month history of ingravescent pain in the sacro-iliac joint. The initial diagnosis was infective sacroiliitis and antibiotic therapy was started. As this therapy failed, computed tomography (CT) and magnetic resonance imaging (MRI) were performed, showing a roundish irregular mass with polylobate edges and apparently haemorrhagic contents. The suspected diagnosis of aneurysmal bone cyst was confirmed histologically after surgical excision of the mass. This case points to the risk of delayed diagnosis of the cause of pathological bone pain, which should be investigated with the appropriate radiological techniques, since correct diagnosis is crucial to prognosis.