Osteoid osteomas of the femoral neck: report of four cases evaluated with isotopic bone scanning, CT, and MR imaging.

Four patients, ranging in age from 6 to 32 years, with surgically proved parosteal lesions of the femoral neck are presented. In one case, magnetic resonance (MR) imaging was the initial study ordered prior to plain radiography, and in three cases MR imaging was performed after plain radiography. Findings in the MR images were abnormal in all four cases; however, the nidus was not identified prospectively. Incorrect diagnoses based on findings at MR imaging of secondary marrow edema and/or synovitis included Ewing tumor, osteonecrosis, stress fracture, Lyme arthritis, and juvenile inflammatory arthritis. In all four cases, the correct diagnosis of intracapsular osteoid osteoma was made following (a) review of plain radiographs leading to a high index of suspicion and (b) performance of thin-section computed tomographic (CT) studies (in one case a screening CT study with 1-cm-thick sections failed to depict the lesion). In three cases, isotopic bone scans provided useful guidance for planning the CT examination.