Intraoperative estimation of varisation (abduction then internal rotation of the hip to get the ossific nucleus completely covered by the acetabulum): Hip is in neutral position (a). Hip is being abducted (b, c, d). Hip is being internally rotated (e, f). Black line is Perkin’s line. “a angle” is the amount of hip abduction which was needed to make the nucleus completely covered by the acetabulum, which equals the amount of needed varus. Note that rotation was needed (e, f) to make the ossific nucleus completely inside the acetabulum.