Avascular Necrosis on MRI: Imaging Findings

The crescent sign is a subchondral radiolucent line visible on AP or frog-leg lateral hip radiographs, representing separation of the necrotic subchondral bone from the overlying articular cartilage. It indicates mechanical failure of the subchondral plate and imminent or actual femoral head collapse. Its presence upgrades the patient to at least Ficat stage III and dramatically changes prognosis — once the crescent sign appears, collapse typically occurs within weeks to months without intervention. Identifying the crescent sign on radiographs is therefore a critical triage point that often prompts urgent surgical consultation.

Early osteonecrosis may have normal radiographs despite MRI changes. Femoral head collapse marks progression beyond precollapse disease and changes treatment options.

The most frequent causes are corticosteroid use (cumulative doses above 2 g of prednisone equivalent or high-dose pulse therapy) and excessive alcohol consumption, together accounting for roughly 90% of non-traumatic AVN cases. Traumatic AVN results from displaced femoral neck fractures or hip dislocations that disrupt the retinacular blood supply to the femoral head. Additional risk factors include systemic lupus erythematosus, sickle cell disease, hyperlipidemia, Gaucher disease, radiation therapy, and decompression sickness (dysbaric osteonecrosis). Idiopathic AVN accounts for a minority of cases. Recognizing the underlying etiology guides treatment of the precipitating condition alongside hip-specific management.