Fig. 1

The pelvic incidence (PI) is the angle between the line perpendicular to the midpoint of the superior sacral endplate and a line through the midpoint of the superior sacral endplate to the center of the femoral heads. Pelvic tilt (PT) is the angle between a line drawn from the center of the S1 endplate to the center of the femoral head and a second vertical reference line intersecting the center of the femoral head. Sacral slope (SS) is the angle between a line drawn parallel to the S1 endplate and a second horizontal reference line