Calculation of CT ideal screw path and safety angle before percutaneous sacroiliac screw placement.

Introduction: Percutaneous sacroiliac screw placement is the main surgical approach to treat unstable posterior pelvic ring injuries. Intraoperative fluoroscopic imaging is an important part of safe sacroiliac screw placement. However, how to determine the right perspective view is challenging. We developed a trigonometric algorithm to verify how preoperative pelvic computed tomography (CT) can be used to predict ideal screw path and safety angle.

Materials And Methods: The normal pelvic CT data of 30 volunteers from our hospital between September 2021 and June 2023 were collected, and analyzed and reconstructed using Materialise Mimics 21.0. The angle between the cross-sectional ideal screw path and the horizontal plane (∠α), the angles of the pelvic inlet and outlet on the sagittal plane (∠1, ∠2), and the insertion angles of virtual screws at the inlet and outlet (∠a, ∠b) were measured. The ideal insertion angles (∠A, ∠B) and safety angles of the screws at the pelvic inlet and outlet were calculated using trigonometric functions.

Results: The virtual screw insertion angle ∠a of 30 pelvises measured at the inlet was 18.57 ± 4.33°, and the ideal screw angle ∠A calculated using trigonometric functions was 18.72 ± 4.71° (range, 13.84 ± 0.75°-23.36 ± 0.98°). The difference between the measurement and calculation was only 0.15 ± 0.19° and not significant. The angle ∠b of the virtual screw insertion measured at the outlet was 25.37 ± 5.13°, and the ideal screw angle ∠B calculated using trigonometric functions was 25.58 ± 4.93 ° (range, 19.02 ± 0.88°-31.31 ± 1.01°). The difference between the measurement and calculation was only 0.20 ± 0.13° and not significant. The distance e from the optimal screw insertion point to the vertical line through the anterior superior iliac spine is 32.34 ± 1.76 mm, and the distance f to the horizontal line through the posterior superior iliac spine is 28.61 ± 0.81 mm.

Conclusions: During preoperative planning, trigonometric calculations were used to determine the ideal screw placement angles and safety angle at the inlet and outlet of the screw path. Combined with intraoperative C-arm fluoroscopy, individualized screw insertion can help orthopedic surgeons quickly and accurately obtain intraoperative images and accurately determine the direction of screw insertion.