Objective:To identify transverse sacroiliac screw’s safety entry point and angle for fixation of Denis Ⅲ sacral fracture by anatomical parameter measurement of S1 vertebra and virtual screws fixation of the three-dimensional reconstruction model of the sacroiliac（SI）joint. Methods:The three-dimensional models of pelvis were reconstructed and cut by Mimics software based on 50 pelvises CT data. The anatomical parameters of S1 vertebra were measured and the four virtual transverse SI screws were placed in each pelvis

then the central transverse SI screw’s entry point and angles were identified. The soft tissues on pelvic surface were reconstructed by Mimics software

then the surface projection of the screw’s entry point Q

anterior superior iliac spine’s vertex M

iliac crest point N were determined as Q

0

M

0

N

0

respectively. At the same time

the lengths of Q

0

M

0

Q

0

N

0

and M

0

N

0

were measured sectionally. Results:The lengths between center screw’s entry point Q and anterior superior iliac spine’s vertex M were（101.60±6.60）mm in male and（102.00±7.75）mm in female without statistical difference between two groups（P

>

0.05）. Point N was intersecting point between iliac crest and the line QN

which was vertical line of QM passing the entry point Q

then the lengths of QN were（58.68 ±6.93） mm in male and（47.89 ±6. 62） mm in female with statistical difference between two groups（P

<

0.05）. The lengths of the line MN from anterior superior iliac spine’s vertex M to point N

which was along the iliac crest

were（134.25±8.84）mm in male and（133.47±8.53）mm in female with no statistical difference between two groups（P

>

0.05）. The lengths between Q

0

and M

0

were（120.96±14.11）mm in male and（128.74±14.18）mm in female

and the difference between two groups is no statistical significance（P

>

0.05）

the lengths between Q

0

and N

0

were（55.29±9.96）mm in male and（46.09±5.75）mm in female with statistical difference between two groups（P

<

0.05）. The average angle of all the center screws in the forward and backward inclination was（0.93±3.24）°

and the average angle in the cephalic and caudal inclination was（0.38±2.58）°. The variation rate of S1 vertebra was 31.51%

and there is no statistical difference between male and female（P

>

0.05）. Conclusion:After the anterior superior iliac spine’s vertex M

0

and iliac crest point N

0

in body surface are identified in preoperative

the transverse screw’s entry point for fixation of Denis Ⅲ sacral fracture can be determined. In C-arm monitor

when the superior surface of S1 vertebra is defined as horizontal plane and the posterior surface as coronal plane

the transverse screw can be placed safely

which is perpendicular to the sagittal plane with no forward-backward and cephalic-caudal inclination. The preoperative pelvic CT and 3- D reconstruction is strongly recommended before the transverse screw is inserted in bilateral SI joints because of high variation of S1 vertebra.