Measurement of sagittal parameters. Sagittal vertical axis (SVA) was the distance between the C7 plumb line and the posterosuperior corner of S1. Thoracic kyphosis (TK) was the Cobb's angle between upper end plate of T5 and lower end plate of T12. Lumbar lordosis (LL) was the Cobb's angle between upper end plate of L1 and S1. Pelvic incidence (PI) was the angle between the perpendicular to the sacral plate at its midpoint and the line connecting this point to the middle axis of both femoral heads. Pelvic tilt (PT) was the angle between the line connecting the midpoint of the sacral plate to the axis of the femoral heads and the line perpendicular to the floor.