Computational analysis on the feasibility of transverse iliosacral screw fixation for different sacral segments

To evaluate the feasibility of transverse iliosacral (TIS) screw placement in different segments of the sacrum and measure the parameters of the unilateral iliosacral (IS) screw in the case that cannot be inserted the TIS screw.

This study used 100 pelvic continuous computed tomography images. Mimics (Materialise Interactive Medical Image Control System) was used to reconstruct the three-dimensional pelvis model. All sacrums were divided into the normal group and dysmorphic group. Any difference in osseous fixation pathway (OFP) diameter in the first two segments between both groups was investigated. In dysmorphic sacrums, the optimal inserting angle and length of the unilateral S1 screw were measured. The number of foramen in every sacrum was recorded.

Thirty-two sacrums had sacral dysmorphism. The OFP diameter for the S2 TIS screw in the dysmorphic group was larger than that in the normal group (p = 0.02). Receiver operating characteristic curve analysis indicated the cutoff values as 20.55 mm and 15.18° for the S1 front edge height and S1S2 angle, respectively. In the dysmorphic case, the unilateral S1 IS screw should be inserted with a cephalad incline angle of 36.14 ± 5.97° and a ventrally incline angle of 37.33 ± 4.64°. S3 TIS screw placement rate was 53.1% in the dysmorphic group.

The most common cause of sacral dysmorphism is the fusion of the L5 to the true S1. In dysmorphic sacrums, the unilateral IS screw should be placed obliquely in the S1 segment, and the S2 segment usually has a sufficient OFP for the TIS screw. Using S3 TIS screw and two TIS screws in the first segment technique is not recommended because of a high risk.