713368 - Biomechanical Effects of a Novel Posteriorly Placed SI Joint Fusion Device Integrated with Traditional Lumbo-Pelvic Long Construct Instrumentation

Introduction: S2 alar-iliac (S2AI) screw fixation is an effective method used to ensure stability and enhance fusion across the lumbosacral junction in long segment constructs. Nevertheless, pelvic fixation is associated with a high rate of mechanical failure. Because of the transarticular nature of the S2AI screw, adding a second point of fixation on the joint, ideally through the same posterior approach, may provide additional stability and attenuate rod and screw strains. The objectives of the study were to evaluate changes in stability and pedicle screw and rod strain with integration of a standalone SI joint fusion device (iFuse-3D), implanted through a novel posterior SAI approach, supplemental to posterior L2 to S2AI lumbosacral fixation.

Methods: Seven human cadaveric L1-Pelvis specimens were tested intact and in the following instrumented conditions 1) L2-S1 pedicle screw and rod instrumentation with L5-S1 anterior lumbar interbody fusion (ALIF) 2) posterior fixation extended to include S2AI screws 3) added posterior bilateral standalone SI joint fixation (SIJF)(iFuse-3D). Nondestructive 7.5 Nm pure moment loads were applied in flexion/extension (F/E), lateral bending (LB) and axial rotation (AR) followed by 400N of compression (C). Angular ranges of motion (ROM) were analyzed using optical tracking. Screws at L5, S1, and S2AI, as well as rods at L5-S1 and S1-S2AI were instrumented with strain gages to compare peak resultant screw bending moments and rods strain (RS). Data were analyzed using RM-ANOVA followed by paired post-hoc Holm- Šidák (p<0.05).

Results: Distal fixation ending at S2AI significantly decreased global L2-S1 ROM in combined F/E (p≤0.044), L5-S1 ROM in combined F/E and C (p≤0.044), and SI joint ROM during F/E and LB (p<0.033) compared to S1 fixation. SI ROM with SIJF in place was significantly less than intact, S1, and S2AI fixation in combined F/E and LB (p≤0.013). S1 screw bending moment decreased following S2AI by as much as 78% in extension but with significance only in right AR (p=0.029). SIJF did not significantly change the S1 or S2AI screw bending moment in any direction of loading; however, S2AI moments were substantially reduced in F/E, LB, and C. Extending fixation to S2AI significantly increased the RS at L5-S during F, AR, and C (p≤0.048) and reduced the same strains during left LB (p=0.048). With SIJF there was a slight but significant increase in RS versus that for S2AI at L5-S during left LB (p=0.048). Compared to S1, fixation to S2AI increased the mean RS at L5-S during C (p=0.048). This difference became not statistically different with SIJF (p=0.124).

Conclusion: Extension of distal fixation from S1 to S2AI tends to create a more stable construct that reduces S1 screw bending loads at the expense of increased L5-S1 RS. Supplementing S2AI fixation with standalone SIJF was feasible through a single posterior surgical approach. Supplemental SIJF resulted in a small but significant stabilizing effect at the SI joint with protective effect on S2AI screw bending moments though the latter observation was not significant likely due to high variability.