Radiographic measurement of the postoperative shift of hip joint center (HJC). The vertical axis of the pelvis (VA line) was defined by connecting the middle of the inter-sacroiliac line and the middle of the pubic symphysis. The distance (D1) between the postoperative femoral head center and the VA line was measured, the same as the distance (D2) between the contralateral intact femoral head center and the VA line. The horizontal shift (X) of the postoperative HJC was calculated as the absolute value of the difference between D1 and D2 (X = |D1 – D2|). The vertical shift (Y) was measured as the distance between the paralleled D1 line and D2 line.