Fig. 1.

Illustration of a lateral view of the pelvis and lumbar spine demonstrating the non-positional sacropelvic parameter, pelvic incidence (PI), and its positional components, sacral slope (SS) and pelvic tilt (PT). PI is defined as the angle formed between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the axis of the femoral heads. PT is defined as the angle between the vertical and the line connecting the midpoint of the sacral plate and the femoral axis. SS is defined as the angle between the superior plate of S1 and a horizontal line.