What radiographic finding is most characteristic of avascular necrosis of the femoral head?

• A. Crescent sign on X-ray (MRI earliest)
• B. Osteophyte formation
• C. Subchondral fracture
• D. Joint effusion

Answer: (A)

Avascular necrosis of the femoral head shows a subchondral process where the necrotic bone weakens and cracks under the weight-bearing cartilage. The most characteristic radiographic finding is a crescent-shaped radiolucent line just beneath the articular surface, known as the crescent sign. This crescent represents a subchondral fracture of the necrotic bone and signals impending collapse of the femoral head. While MRI can detect AVN earlier by showing changes in the bone marrow, the crescent sign on X-ray is the classic, characteristic radiographic indicator when it appears. Osteophyte formation is more typical of osteoarthritis, and a simple joint effusion is nonspecific.