Osteoid Osteoma of the Femoral Neck: Use of the Half-Moon Sign in MRI Diagnosis

Abstract

OBJECTIVE. Osteoid osteoma is a benign bone tumor that is commonly located in the proximal femur. Although CT is helpful in the diagnosis of osteoid osteoma, patients in whom the clinical presentation of the tumor is atypical are first referred to undergo MRI, which yields a high percentage of false-negative or inconclusive results. We sought to explore the association between a half-moon sign of bone marrow edema and the presence of osteoid osteoma of the femoral neck. SUBJECTS AND METHODS. The MRI examinations of 11 consecutive patients with osteoid osteoma (group 1) were prospectively included in the study. In addition, 950 consecutive hip MRI examinations of 485 patients were retrospectively analyzed for the presence of bone marrow edema of the femoral neck, with 19 patients identified as having bone marrow edema resulting from causes other than osteoid osteoma (group 2). Patients in both study groups were evaluated for the presence of the half-moon sign. RESULTS. The half-moon sign of bone marrow edema was significantly associated with the presence of osteoid osteoma (p < 0.0001), having been seen on the MR images of all 11 patients in group 1 and only one of 19 patients in group 2. MRI examination was therefore 94.7% specific and 100% sensitive for the detection of osteoid osteoma, with positive and negative predictive values of 91.7% and 100%, respectively. CONCLUSION. The half-moon sign is an MRI finding that is highly specific and sensitive for an accurate diagnosis of osteoid osteoma of the femoral neck.