Transiliac–transsacral (TITS) screw fixation might be necessary in some cases involving the vertical shearing injuries with transforaminal fracture and bilateral posterior ring injuries. However, the possibility of S₁ TITS screw should be preoperatively assessed because the pelvic ring injuries with sacral dysmorphism had the insufficient osseous corridors.

AxW_S2 may predict the possibility of TITS screw fixation and be used as the new indicator to discriminate the sacral dysmorphism.

The conventional CT images of eighty-two cadaveric pelvis imported into Mimics® software to reconstruct three-dimensional (3D) models. A 7.0 mm-sized screw was processed into a 3D model using a 3D-sensor at actual size and virtually implanted as S₁ and S₂ TITS screw using Mimics® software. The cortical violation around screw path was evaluated using 3D biplanar and conventional CT images. The osseous corridor widths around TITS screws were measured in the axial plane images and defined as AxW_S1 and AxW_S2, respectively.

Despite no cortical violation in S₂ of all models, cortical violation of S₁ TITS screw was found in 20 models. Of them, 14 models (impossible models) were identified in the 3D biplanar images, and all 20 models (CT-violation models) were identified only in CT axial plane images. AxWS₁ was<7mm in the impossible models and<9.0mm in the CT-violation models. AxW_S2 negatively correlated with AxW_S1 (R -0.450, p<0.01). By receiver operating characteristic curve analysis to identify the CT-violation model using AxW_S2, the cut-off value of AxW_S2 was 13.32mm (sensitivity 0.70, specificity 0.70).

By using AxW_S2, the possibility of S₁ TITS screw fixation could be predicted and safely placed without cortical violation, if AxW_S2 was less than 13mm. Considering the negative relationship with AxW_S1, AxW_S2 should be used as a new indicator to predict safe S₁ TITS screw fixation.

III, controlled laboratory study.