Figure 7.

Sagittal spinopelvic alignment includes the pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), lumbar lordosis (LL), sagittal vertical axis (SVA), and spinal inclination angle (SIA). The PI is the angle between a line perpendicular to the sacral plate at its midpoint and a line connecting this point to the bi-coxo-femoral axis. The PT is the angle between a vertical line passing through the bi-coxo-femoral axis and a line joining the bi-coxo-femoral axis with the center of the upper sacral endplate. The SS is the angle between a tangent line to the superior endplate of S1 and the horizontal plane. The LL is the angle between the superior endplate of L1 and the upper sacral endplate. The SVA is the horizontal distance between a plumb line drawn from the center of C7 and a line drawn from the center of C7 to the posterior superior corner of S1. The SIA is the angle between the true vertical and a straight line from the tip of the T1 spinous process to that of S1.