Computational simulation of a novel surgical screw guide system to determine the optimal trajectory for S2-alar-iliac screw fixation in minimally invasive pelvic and spine surgery

Abstract

Introduction: The ideal trajectory for S2-alar-iliac (S2AI) screw insertion remains difficult to determine intraoperatively. Using a novel surgical guide which references the greater sciatic notch and outer pelvic surface, in conjunction with computational simulation, we identified a point through which an S2AI screw may be passed that optimises screw length, while minimising perforation hazards. Methodology: Computed tomography scans of 87 adult hemipelvises were segmented and imported for 3D manipulation. A simulated array of screws was passed from the sacral entry point through nine target points with distances from the greater sciatic notch (Y) and outer pelvic surface (X) varying in 1-cm intervals. At each point, the maximum allowable screw length and incidence of critical perforations of the hip joint and inner pelvic cortex were recorded. Results: Target points (X,Y) = (1,1) (1,2) and (2,1) allowed for the longest screw lengths, with mean 104.5 mm (95% CI=101.1107.9) vs 101.84 mm (95% CI=97.7-106) vs 105.71 mm (95% CI=99.4-112.1). (1,1) has significantly lower risk for complete inner cortex perforation versus (1,2) and (2,1), 1% vs 8% vs 31% (p<0.001) and partial inner cortex perforation, 2% vs 8% vs 28% (p<0.001). However (1,1) has higher risk of hip perforation than (1,2) 18% vs 2% (p<0.001). Conclusion: We determined that target point (1,1) is optimal for S2AI screw insertion. The surgeon should use fluoroscopy to monitor for potential hip perforation.