Sacral pseudotumor complicating iliac bone harvest: radiographic, CT and MRI appearances.

We present the imaging appearances of a lytic pseudotumor in the right sacral ala presenting with referred pain to the right thigh. Subsequent imaging revealed the presence of a cystic lesion arising at the site of previous bone graft harvest; CT-guided aspiration yielded synovial fluid presumed to arise from the contiguous sacroiliac joint.