Inradiology, thecrescent sign is a finding on conventionalradiographs that is associated withavascular 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 curvedsubchondralradiolucent line that is often found on the proximalfemoral orhumeral 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 thenecrotic and repair processes that occur during avascular necrosis.[1][2]Osteosclerosis occurs at a margin where new bone is placed over deadtrabeculae.[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 shearfractures.[1]