Measurement of sacral slope (SS), pelvic tilt (PT), and pelvic incidence (PI). SS=∠ABC, PT=∠CDF, PI=∠DCE. ∠ABC is the angle between the horizontal line and the sacral end plate. ∠CDF is the angle between a vertical line and a line connecting the midpoint of the sacral end plate with the point D. ∠DCE is the angle between the perpendicular line to the sacral end plate and a line connecting point D with midpoint of the sacral end plate. Point D is defined as the midpoint of the line connecting centers of two femoral heads.