Objective To investigate the transiliac-transsacral screw insertion pathway (TSIP) so as to provide an anatomical basis for clinical surgical practice.Methods CT scanning data of normal pelvis in 90 Chinese adults were selected by random number table.There were 45 males and 45 females,aged from 21 to 82 years (mean,53.88 years).After computed tomography scan,pelvic data of the 90 subjects were entered into Mimics software 16.0 for three-dimensional reconstruction.The outer frames of sacrum and ilium at sagittal plane were depicted.Maximum inscribed circle method was applied to determine the screw insertion pathways which traversed 1st (S1) and 2nd (S2) sacral segments,respectively.The diameters and lengths of screw pathways,the distances from screw insertion points to both anterior superior iliac spine and posterior superior iliac spine,as well as the angles between pathways and anatomic surface were measured.Results Most subjects had the condition of a trans-S1 TSIP (male 78％,female 76％),and all the tested pelvis had the condition of trans-S2 TSIP.There was no statistically significant difference between the left and right side parameters in male group,and so was in female group (P ＞0.05).TSIP radius:there was no statistically significant difference among male S1 [(5.52 ± 1.91)mm],male S2 [(5.35 ± 1.05)mm],and female S1 [(5.49± 1.34)mm] (P＞0.05),but each of them was greater than female S2 [(4.79 ± 1.40) mm] (P ＜ 0.05).TSIP length:male S1 [(158.25 ±9.84) mm] was larger than male S2 [(138.94 ± 9.75) mm],and female S1 [(154.91 ± 9.40) mm] was larger than female S2 [(141.01 ±8.60)mm].The screw insertion point was located at the outer ilium side.The distances from S1 to anterior superior iliac spine and to posterior superior iliac spine were (96.49 ±6.91)mm and (68.22 ±6.35)mm in males but (100.48 ±8.15)mm and (61.57 ±6.84) mm in females.The distances from S2 to anterior superior iliac spine and to posterior superior iliac spine were (114.43 ±8.77)mm and (49.62 ±8.54)mm in males but (114.75 ± 10.19)mm and (44.52 ±8.36)mm in females.Compared with those with a condition of TSIP in S1,a pelvis without a condition of an S1 TSIP had larger S2 TSIP radius.The S2 TSIP radius in one with such condition in male was (5.10 ±0.84)mm and that in one without such condition in male was (6.22 ± 1.27) mm.The corresponding female data was (4.37 ± 0.92)mm and (6.11 ± 1.84)mm (P ＜ 0.05).Both S1 and S2 TSIP were almost vertical to sagittal plane and parallel to coronal plane.Conclusions Anatomically,a S1 or S2 transiliactranssacral screw is available in most Chinese adults for sacroiliac joint fixation on both sides.Mimics software can be helpful to ensure the pathway of screw fixation,which provides reference for transiliactranssacral screw fixation technique.