Can the possibility of transverse iliosacral screw fixation for first sacral segment be predicted preoperatively? Results of a computational cadaveric study.

Abstract

The purpose of this study was to predict the possibility of transverse iliosacral (TIS) screw fixation into the first sacral segment (S₁) and introduce practical anatomical variables using conventional computed tomography (CT) scans. A total of 82 cadaveric sacra (42 males and 40 females) were used for continuous 1.0-mm slice CT scans, which were imported into Mimics^® software to produce a three-dimensional pelvis model. The anterior height (BH) and superior width (BW) of the elevated sacral segment was measured, followed by verification of the safe zone (SZ_S1 and SZ_S2) in a true lateral view. Their vertical (VD_S1 and VD_S2) and horizontal (HD_S1 and HD_S2) distances were measured. VD_S1 less than 7mm was classified as impossible sacrum, since the transverse fixation of 7.0 mm-sized IS screw could not be done safely. Fourteen models (16.7%; six females, eight males) were assigned as the impossible sacrum. There was no statistical significance regarding gender (p=0.626) and height (p=0.419). The average values were as follows: BW, 31.4mm (SD 2.9); BH, 16.7mm (SD 6.8); VD_S1, 13.4mm (SD 6.1); HD_S1, 22.5mm (SD 4.5); SZ_S1, 239.5mm² (SD 137.1); VD_S2, 15.5mm (SD 3.0); HD_S2, 18.3mm (SD 2.9); and SZ_S2, 221.1mm² (SD 68.5). Logistic regression analysis identified BH (p=0.001) and HD_S1 (p=0.02) as the only statistically significant variables to predict the possibility. Receiver operating characteristic curve analysis established a cut-off value for BH and HD_S1 of impossible sacrum of 20.6mm and 18.6mm, respectively. BH and HD_S1 could be used to predict the possibility of TIS screw fixation. If the BH exceeds 20.6mm or HD_S1 is less than 18.6mm, TIS screw fixation for S₁ should not be undertaken because of narrowed SZ.