Fig. 2A–B.

Lateral schematics of the pelvis in (A) standing and (B) sitting positions show variations of the sacral slope (SS) and the acetabular orientation. The SS increases in the standing position, whereas the acetabular sagittal inclination (ASI) angle decreases. The SS decreases in the sitting position and the ASI angle increases. The pelvic incidence (PI) represents the offset between the lumbosacral junction and the hips. The PI is a constant anatomic parameter. It does not vary whatever the pelvic tilt is.