Fig. 5.

a–c The schematic shows the type 1 sacral endplate and the measurement of pelvic incidence based on CT and plain film X-ray methods. a Type 1 sacral endplate with a concave side anteriorly. b Point A is obtained as the midpoint of the segment crossing the mid-sagittal plane of the endplate via CT. Point B is obtained as the midpoint of the projection line via plain film X-ray method. c Point A is behind point B. Thus, the pelvic incidence value measured on CT is larger than that measured via plain film X-ray