Fig. 2

The measurement and calculation of ED. No matter how big the radius of the femoral head (R_FD) was, we assumed the R_FD as “3” (R_FD = “3,” without any unit) for good match of the Cleveland zone system and easy calculation. “x₀, y₀, R_ap, R_lat” were actual measured values. The ED of the cephalic fixator was calculated by the distances from the cephalic fixator tip (x, y) to the geometric central axis in the femoral neck and head on both the AP view radiograph (y = y₀/R_ap × R_FD) and the lateral view radiograph (x = x₀/R_lat × R_FD), resulting in “ED_(x,y) = (x² + y²)^1/2.” The value of “x” or “y” was defined as positive if the cephalic fixator tip was on the superior or posterior, and as negative if the cephalic fixator tip was on the inferior or anterior when compared with the axis of femoral head