Crescent sign

Not to be confused with the air crescent sign.

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]

A radiograph of a left hip joint, which reveals a thin, curvilinear lucent line parallel to the cortical margin of the femoral head, in a patient with avascular necrosis.

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]