Eight months of misdiagnosis: Intra-articular osteoid osteoma in a 48-year-old woman.

Intra-articular osteoid osteoma of the femoral neck is a diagnostically challenging entity, particularly when it occurs beyond the fourth decade of life, a demographic in which this tumor is rarely considered. We report the case of a 48-year-old woman presenting with an 8-month history of persistent right hip and groin pain characterized by nocturnal exacerbation and marked responsiveness to nonsteroidal anti-inflammatory drugs (NSAIDs), who was successively misdiagnosed with lumbar disc herniation, piriformis syndrome, coxarthrosis, and femoroacetabular impingement. Magnetic resonance imaging (MRI) demonstrated nonspecific bone marrow edema of the femoral neck without a discernible nidus, whereas thin-slice computed tomography (CT) identified a well-defined 7 mm radiolucent nidus in the anterosuperior femoral neck, establishing the diagnosis of osteoid osteoma; histopathological confirmation was obtained at the time of the procedure. The patient underwent CT-guided percutaneous radiofrequency ablation (RFA) at 90 degrees Celsius for 6 minutes, achieving near-complete resolution of her characteristic osteoid osteoma pain within 24 hours (Visual Analogue Scale score declining from 10 to 1) and remaining asymptomatic at 6-week clinical follow-up, with 3-month CT demonstrating no evidence of residual or recurrent nidus. This case illustrates that osteoid osteoma should remain in the differential diagnosis of refractory nocturnal hip pain regardless of patient age and sex. Thin-slice CT should be considered when MRI findings are inconclusive and clinical suspicion persists, while CT-guided RFA provides rapid and effective early pain relief in intra-articular lesions.