Osteoid Osteoma of acetabulum, a rare presentation.

The case of a relatively frequent benign osteoblastic tumour, osteoid osteoma at an atypical site is presented. It was in a 20 year old man who attended the outpatient department of Aga Khan University Hospital, Karachi with pain in left groin since last 6 months which often occurred at night and was relieved by taking NSAIDs. X-ray failed to reveal the cause. CT scan showed central radiolucent nidus with surrounding sclerosis suggestive of osteoid osteoma in the left acetabulum. Acetabulum is a rare site for osteoid osteoma, the usual sites are diaphysis of long bones. Hence, the diagnosis is often difficult and delayed in such cases. CT scan, Bone scan and MRI, helped in diagnosing the condition. Resection of the lesion by open technique under fluoroscopic control was performed and histopathology confirmed the diagnosis.