In the early stage of slipping of the capital femoral epiphysis, the
epiphysis slips posteriorly in relation to the neck of the femur. When this
occurs, the posterior portion of the femoral head is seen on the
anteroposterior radiograph as a crescent-shaped area of increased density
overlying the metaphysis adjacent to the epiphyseal plate. This metaphyseal
blanch sign suggests that slipping of the capital femoral epiphysis has
occurred and that a lateral radiograph should be made to confirm or exclude
the diagnosis.