Fig. 1

Hip internal rotation view. For the right hip, neck-shaft angle (NSA) was defined as the angle between a line passing through the center of the femoral shaft and another line connecting the femoral head center and the midpoint of the femoral neck. The femoral head center was the center of the largest best-fitting circle inside the femoral head. For the left hip, migration percentage was calculated by dividing the width of the femoral head lateral to Perkin’s line (A) by the total width of the femoral head (B). Head-shaft angle (HSA) was defined as the angle between a line passing through the center of the femoral shaft and another line perpendicular to the proximal femoral physis passing through the center of the proximal femoral epiphysis