Avascular Necrosis: AI-Powered MRI Analysis

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.

The Ficat and Arlet classification describes four stages of avascular necrosis on radiographs. Stage I shows normal radiographs with clinical symptoms. Stage II demonstrates sclerosis and cystic changes without contour alteration. Stage IIb includes the crescent sign (subchondral collapse). Stage III shows femoral head flattening. Stage IV adds joint space narrowing with acetabular involvement. Because MRI detects AVN in Ficat stage I — when radiographs are still normal — it is the diagnostic gold standard for early disease. The characteristic double-line sign on T2-weighted MRI (an inner hyperintense rim of granulation tissue surrounded by a hypointense sclerotic border) is pathognomonic for AVN. MRI also quantifies the size of the necrotic lesion, a key predictor of collapse risk.

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.