Computational simulation study on ilio-sacral screw fixations for pelvic ring injuries and implications in Asian sacrum

Objectives: Despite a high possibility of technique-related complications, ilio-sacral (IS) screw fixation is the mainstay of operative management in posterior pelvic ring injuries. We aimed to make IS screw trajectory with fully intraosseous path that was optimal and consistent, and confirm the possibility of transiliac–transsacral (TITS) screw fixation in Asian sacrum. Methods: Eighty-two cadaveric sacra (42 males and 40 females) were enrolled and underwent continuous 1.0-mm slice computed tomography (CT) scans. CT images were imported into Mimics^® software to reconstruct three-dimensional model of the pelvis. To simulate IS screws, we inserted 7.0-mm-sized TITS cylinder for first (S₁) and second (S₂) sacral segment and 7.0-mm oblique cylinder for S₁. TITS cylinder could not be inserted into S₁ of 14 models (sacral variation models) but could be inserted into the S₂ of all models. The actual length of virtual IS screws was measured, and anatomic features of safe zone (SZ_S2) including the area, horizontal distance (HD_S2), and vertical distance (VD_S2) were evaluated by the possibility of TITS screw fixation in the S₁. Results: When the oblique cylinder was directed toward the opposite upper corner of S₁ at the level of the first foramen, there was no cortical violation regardless of sacral variation. The average length of TITS cylinder was 152.3 mm (range 127.9–178.2 mm) in S₁ and 136.0 mm (range 97.8–164.1 mm) in S₂, and for oblique cylinder it was 99.2 mm (range 82.4–132.2 mm). The average VD_S2, HD_S2, and the area of SZ_S2 were 15.5 mm (range 8.7–24.4 mm), 18.3 mm (range 12.7–26.6 mm), and 221.1 mm² (range 91.1–386.7 mm²), respectively. The VD_S2 and SZ_S2 of sacral variation were significantly higher than those of normal (both p = 0.001). Conclusions: Considering the high variability of the S₁, it is better to direct the IS screw trajectory toward the opposite upper corner of the S₁ at the level of first sacral foramen. If a TITS screw is needed, the transverse fixation for the S₂ could be performed alternatively due to its sufficient osseous site even in Asian sacrum.