Musculoskeletal case 45.

A 19-year-old male college basketball player experienced onset of pain after playing 4 games in 2 days. Clinical examination revealed mild swelling over the mid-left tibia. There were no signs of infection. We obtained radiographs (Fig. 1). The radiological differential diagnosis at this stage included focal cortical abscess related to infection, stress fracture and osteoid osteoma. Computed tomography (CT) scans performed for further evaluation showed a focal rounded lytic lesion with a central calcified area and associated cortical thickening (Fig. 2). We observed no cortical breaching. Based on these findings, the radiological differential diagnosis favoured osteoid osteoma. The patient received nonsteroidal anti-inflammatory drugs for 1 year. Since symptoms persisted, he underwent general anesthesia for CT-guided radiofrequency ablation of the suspected osteoid osteoma. However, there was marked change in the appearance of the lesion on the CT scan performed for localizing during the intervention, with new bone formation and a lucent cleft extending through the bone perpendicular to the axis of the tibia (Fig. 3). What is your diagnosis? Gustav Andreisek, MD*† John S. Theodoropoulos, MD‡ Walter Mak, MD* Lawrence M. White, MD*