Pelvic incidence (PI) is defined as an angle subtended by line oα, which is drawn from the center of the femoral head to the midpoint of the sacral endplate and a line perpendicular to the central of the sacral endplate (α). The sacral endplate is defined by the line segment bc constructed between the posterior superior corner of the sacrum and the anterior tip of the S1 endplate at the sacral promontory.