Introduction: As an alternative to iliac screws, S2‐alar screws provide supplemental sacropelvic fixation, without violating the SI joint. The purpose of this study was to defermine the anatomic safe zone, the proximity to vital structures, and the standard screw flajectory for this technique.
Methods: 10 cadaveric torsos (5M/5F) undervern bicortical S2‐alar screw fixation in the prone position with a standard insertion technique (lateral and cephalad towards the arcuate line under fluoroscopic guidance) using 6.5 mm multi‐axial pedicle screws (ave 50 mm). The pelvises were then dissected and anatomic structures and screw tip distances to critical neurovascular structures were measured with a fine caliper (mm). Measures included: transverse sacral width, S1 width, R/L lateral S1 foramen to lateral ala, superior S1 foramen to sacral promontory, R/L lateral common iliac artery and vein to lateral ala; and R/L screw to S1 joint, L5 and S1 nerve roots, and iliac arteries and veins. Medial‐lateral trajectory was measured relative to midline and cephalad trajectory relative to superior S1 endplate. A “safe trajectory arc” was calculated. (Table).
Results: Neurovascular structures measured: left common iliac vein to SI joint 24.8 mm (22–28) and right 22.3 (20–26), left common iliac artery to SI joint 22.1 mm (19–26) and right 20.4 (18–23). Screws averaged 11.8 mm (range, 5–20) and 12.5 mm (range, 7–17) respectively from the right iliac artery and left iliac vein (the closest vascular structures). The L5 roots were closer to the screws than S1, 11.8±3.2 (7–17) and 12.5±2.4 (10–16) respectively. Screws averaged 8.8 mm (5–12) from the right S1 joint and 8.4 mm (3–13) from the left SI joint. The trajectory averaged 29–45° in the sagittal plane and 25–40° in the coronal plane.
Conclusion: Bicortical S2‐alar screw plarenent is reliable and safe. Specifically, there is a 15 × 25 mm “safe zone” on the lateral sacral ala which projects at a 25–40° lateral arc projected onto the arcuate line of the ala. The average screw trajectory for the right screw was medial to lateral 31.9°±4.8 (25–40) and caudad to cephalad 36.9°±5.2 (29–45). For the left screw, the average screw trajectory was medial to lateral 32.9°±3.2 (27–37) and caudad to cephalad 37.2°±3.8 (33–43).