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

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 (SZS1 and SZS2) in a true lateral view. Their vertical (VDS1 and VDS2) and horizontal (HDS1 and HDS2) distances were measured. VDS1 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); VDS1, 13.4mm (SD 6.1); HDS1, 22.5mm (SD 4.5); SZS1, 239.5mm2 (SD 137.1); VDS2, 15.5mm (SD 3.0); HDS2, 18.3mm (SD 2.9); and SZS2, 221.1mm2 (SD 68.5). Logistic regression analysis identified BH (p=0.001) and HDS1 (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 HDS1 of impossible sacrum of 20.6mm and 18.6mm, respectively. BH and HDS1 could be used to predict the possibility of TIS screw fixation. If the BH exceeds 20.6mm or HDS1 is less than 18.6mm, TIS screw fixation for S1 should not be undertaken because of narrowed SZ.