Iliosacral screw placement: are uniplanar changes realistic based on standard fluoroscopic imaging?

Background: With the aim of improving the understanding of iliosacral screw placement, two hypotheses were tested: (1) standard intraoperative inlet and outlet images are not based on orthogonal coordinates, and (2) therefore making starting point and aim changes by moving perpendicular to the c-arm beam will displace the guide wire on the other intraoperative radiographic view.

Methods: This is a prospective case series with review of intraoperative data from consecutive patients treated at a University Level I trauma center. The study group included ten consecutive patients with nondysmorphic upper sacral segments and unstable posterior pelvic ring injuries that required surgical treatment. Posterior surgical stabilization included iliosacral screw placement using a standardized three- view technique in the supine position. The main outcome measurement included the angles from the perpendicular required to achieve what have been considered the ideal inlet and outlet views intraoperatively. The angle arc for each patient created by the recorded angles was then determined.

Results: The average sagittal plane tilt required to achieve the ideal inlet view was 25 degrees (range, 21-33 degrees). The average sagittal plane tilt required to achieve the ideal outlet view was 42 degrees (range, 30-50 degrees). The average arc between the ideal inlet and outlet views was 67 degrees (range, 62-76 degrees). These views never created an orthogonal system.

Conclusion: We commonly work in orthogonal systems. Within these systems, it is possible to make a uniplanar correction by moving perpendicular to one plane or radiographic view. The ideal views to image the safe zone for iliosacral screw placement do not create an orthogonal system. When this average angle arc is placed on a graphic model of the pelvis, it becomes clear that the plane of the radiographic beam of the ideal inlet view is collinear with the anterior aspect of the upper two sacral bodies. The outlet view is oblique to the upper sacral bodies. Surgeons must keep this in mind when using fluoroscopic views to insert iliosacral screws.