Fig. 4.

Geometric relationship between pelvic incidence, pelvic tilt and sacral slope. Pelvic tilt is defined by (1) the line through midpoint of the sacral plate and midpoint of the femoral heads axis, and (2) the vertical. Sacral slope is defined as the angle between the sacral plate and a horizontal line. Pelvic incidence is defined as the angle between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the femoral heads axis. “Pelvic incidence=pelvic tilt+sacral slope” because they are formed by lines perpendicular to each other (the horizontal and the vertical, and the lines parallel and perpendicular to the sacral plate)