Sacral Screw Strain in a Long Posterior Spinal Fusion Construct with Sacral Alar-Iliac versus Iliac Fixation

Long instrumented posterior fusion constructs to the lumbosacral spine have a significant rate of pseudoarthrosis and S1 screw failure. With the increasing popularity of Sacral Alar-Iliac (S2AI) fixation with its purported advantages of (1) decreased implant profile and (2) obviating the need for a lateral offset connector, the biomechanical properties with respect to S1 screw strain remain unknown. We set out to compare the biomechanical effect of S2AI versus traditional iliac screw fixation on S1 screw strain.Methods Five fresh-frozen human cadaveric specimens were instrumented from L2–pelvis, maintaining all osteoligamentous structures, with bilateral titanium 6.0 × 40-mm pedicle screws and 5.5-mm cobalt–chromium rods. Bilateral S1 pedicles were instrumented with 8.0 × 50-mm screws that were centrally cored out and two uniaxial strain gauges inserted at 0 and 90 degrees. S2AI fixation and/or iliac fixation with 8.0 × 80-mm titanium pedicle screws was performed to evaluate the following four different constructs: (1) bilateral S1 screws (control); (2) bilateral S2AI; (3) bilateral iliac; (4) hybrid (S2AI with contralateral iliac). Bilateral S1 screw strain was measured (microstrain), and pure moment loads (12.0 Nm) were applied in axial rotation (AR), flexion extension (FE), and lateral bending (LB). One way repeated measure ANOVA was used to analyze the S1 screw strain data.Results Compared with S1 screws alone, both S2AI fixation and iliac fixation significantly reduced sacral screw strain in FE by 58 and 67%, respectively ( p   0.05). Hybrid constructs demonstrated a significant reduction in only FE, with reduction in screw strain by 56% for S2AI fixation and 59% for iliac fixation, with no difference in AR and LB moments. When S2AI fixation and iliac fixation were compared, there was no significant difference in screw strain for all bending moments ( p > 0.05). Similarly, hybrid constructs demonstrated no side-to-side significant difference between S2AI fixation and iliac fixation for all bending moments ( p > 0.05).Conclusion Both S2AI fixation and iliac fixation provide significant reduction in S1 sacral screw strain compared with sacral fixation alone. Bilateral S2AI fixation is a viable and biomechanically comparable to alternative traditional iliac fixation, and presents another option to achieve protection of the S1 sacral screws for long segment constructs to the pelvis.