Fig. 1

Radiographic parameter of spinal alignment. LL is measured from the inferior endplate of T12 to the superior endplate of S1. SS is measured as the angle between the sacral plate and the horizontal line.PT is measured by the angle between the vertical and the line through the midpoint of the sacral plate to the femoral heads axis. PI is measured as the angle between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the femoral head’s axis. “PI = PT -SS. Abbreviations: LL, lumbar lordosis; PI, pelvic incidence; SS, sacral slope; PT, pelvic tilt