Fig. 1

The methods of measurement for Disc Height (DH), slip ratio and Segmental Lordosis (SL). Ia. The length of posterior wall of L4 vertebral body were chosen as a reference, R mm. Ib. Draw a perpendicular line from the midpoint of superior endplate of L5. The distance between midpoint of L5 superior endplate and intersection point of this perpendicular line and L4 inferior endplate was measured as a mm. Measure the length of posterior wall of L4 vertebral body as b mm. DH was R × a/b mm. DH-A and DH-P were anterior and posterior distances of the overlapped disc space. II. draw a perpendicular line from the posteroinferior point of L4 vertebral body to the L5 superior endplate and measure the distance of intersection point and posterosuperior point of L5 vertebral body as c mm. The slip ratio defined as the ratio of c to the length of L5 superior endplate. III. angulation between the lines of L4 superior endplate and L5 superior endplate