In radiology , the crescent sign is a finding on conventional radiographs that is associated with avascular necrosis . ^{[1] [2] [3]} It usually occurs later in the disease, in stage III of the four-stage Ficat classification system. ^[1] It appears as a curved subchondral radiolucent line that is often found on the proximal femoral or humeral head. ^[1] Usually, this sign indicates a high likelihood of collapse of the affected bone. ^[1] The crescent sign may be best seen in an abducted (frog-legged) position. ^{[1] [4]}

The crescent sign is caused by the necrotic and repair processes that occur during avascular necrosis. ^{[1] [2]} Osteosclerosis occurs at a margin where new bone is placed over dead trabeculae . ^[1] When the trabeculae experience stress leading to microfractures and collapse, the crescent sign appears. ^[1]

The crescent sign may be seen with other bone diseases, such as shear fractures . ^[1]

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