Intra-articular Osteoid Osteoma of Hip – A Lesson Learnt From “Half-moon Sign” in Early Diagnosis

Abstract

We emphasize the importance of “half-moon sign” which was seen on magnetic resonance imaging (MRI) done at an early stage of osteoid osteoma in a young patient with hip pain. The hip radiograph and computed tomogram (CT) obtained during his initial presentation were unremarkable. MRI showed a half-moon shaped marrow edema within the femoral neck which was misinterpreted as osteomyelitis. After a prolonged morbidity, the patient again underwent a CT study which picked the lesion as it was evident by that stage. The recent literature has shown the half-moon sign to be a highly specific sign for osteoid osteoma, a knowledge of which, if were known to us at the time of patient’s initial presentation would have significantly reduced his period of pain and disability. Confirming the patient is a not an athlete and does not have a recent history of physical overactivity is crucial, as the sign can also be seen due to stress reaction.