Fig. 2

a The anterior pelvic plane (APP) was used for the pelvic coordinates, based on the anatomic bony landmarks, such as the right anterior superior iliac spine (R-ASIS), the left anterior superior iliac spine (L-ASIS) and the midpoint of the pubic tubercles (MPT). b The midpoint of the anterior edge (MAE) and the midpoint of the posterior edge (MPE) on the sacrum were defined. c The center of rotation (COR) was defined as the centroid of the best sphere (red-covered surface) to the surface of the femoral head (the mean standard deviation (STD) of the best-fit sphere of all femoral heads < 0.4 mm). d Bony landmarks of the pelvis including anterior-superior iliac spines (ASIS), pubic tubercles (PT) and posterior-superior iliac spine (PSIS) were digitized