Fig. 1

The measurements of sagittal parameters on lateral X-ray and S2AI pathway parameters on CT imaging for AS patients a GK, PT and SS were measured on a standard standing lateral radiograph. b Three-dimensional reconstruction of the pelvis. The starting point of the S2AI pathway was 1 cm inferior and 1 cm lateral to the S1 dorsal foramen. c CT imaging in the sagittal plane. Sag angle (angle α) is determined by the caudal trajectory angulation in the sagittal plane. d CT imaging in the transverse plane. Tsv angle (angle β) was defined as lateral trajectory angulation; e Max-length (line 1) determined by the maximal distance of the trajectory from S2 ala to the anterior inferior iliac spine; Sacral distance (line 2) was defined as intrasacral trajectory length; Iliac width (line 3) was defined as the narrowest iliac width measured between the inner cortices; S2 midline (line 4) was measured from the starting point lateral to the middle line of S2; f Skin distance (line 5) was measured from the starting point lateral to the skin; Iliac wing (line 6) was defined as the distance of the starting point lateral from the nearest iliac wing; PSIS distance (line 7) was measured from PSIS to nearest skin