Successful Use of Fluoroscopic Inlet and Outlet Views to Verify Safe Screw Trajectories when Placing Percutaneous Iliosacral Screws

Purpose: The purpose of this study was to introduce practical landmarks for the successful use of the fluoroscopic inlet and outlet views to verify safe screw trajectories when placing percutaneous iliosacral (IS) screws. Materials and Methods: A total of 10 sacra (19 hemi-sacra) from cadavers without gross deformity or previous injury were included in this study. The upper boundaries and S1 were marked with 1 mm lead wire. The marked sacra were positioned on a radiolucent operative table similar to the operative supine position and projected into the pelvic inlet and outlet views and the true lateral view of the pelvis via an image intensifier. Using image editing software, fluoroscopic images were analyzed to identify the fluoroscopic landmarks and ideal entry points. Results: The posterior-superior corner of the lateral articular surface was constantly projected as a curve point in the pelvic inlet view, and the imaginary connecting line between two points did not violate the posterior walls of S1 in any of the sacra with a little space. Based on the curve points, screws had to be directed either straight or anteriorly (range: 18.3°-29.6°) on inlet view. On outlet view, the ideal screw trajectory was the imaginary oblique line from the height of the S1 foramen to the opposite upper corner of S1 (range: 15.5°-24.4°) and the sacral ala was not violated. Conclusion: For successful fluoroscopy, two simple landmarks could be utilized, the imaginary connecting line between the two curve points in the pelvic inlet view and the oblique screw trajectory toward the opposite corner of S1 in the pelvic outlet view, using the standard technique and irrespective of sacral dysplasia.