INTRODUCTION: Long posterior fixations have shown significant screw loosening and fracture at the lumbosacral junction. S1 screws are often protected from loosening with supplemental distal or anterior fixation. The objective of this study was to compare the effect of various surgical strategies on L5‐S1 ROM and S1 screw bending moment utilizing a finite element (FE) model.
METHODS: A validated nonlinear FE model was used to simulate four surgical techniques: L2‐S1 posterior fixation (pedicle screws); pedicle screws with distal fixation; pedicle screws with ALIF at L4‐L5 and L5‐S1; pedicle screws with AxiaLIF 2L+ at L4‐S1. The disc space was not distracted and for ALIF the anterior annulus was compromised simulating surgical procedures. Strain gages were modeled on four orthogonal surfaces of the screw to measure bending moments. An unconstrained moment of 7.5Nm without preload was applied in all directions.
RESULTS: Compared to pedicle screws, distal fixation and AxiaLIF 2L+ reduced ROM in flexion‐extension and torsion by 45‐91%, while ALIF decreased ROM by 77% in flexion but increased ROM by 97% in extension and 11% in torsion. AxiaLIF 2L+ and distal fixation showed a reduction in S1 screw bending moments compared to pedicle screws of 93% and 86%, respectively, in flexion/extension; 45% and 31% in torsion. ALIF reduced the bending moment by 54% in flexion, increased it by 53% in extension, and had no effect in torsion. ROM and the S1 screw bending moments were smallest in lateral bending for all constructs.
DISCUSSION: Compared to iliac screws and ALIF, the presacral access of AxiaLIF minimizes the surgical exposure, requires less operative time and does not require patient repositioning. The results of this FE model indicate that AxiaLIF 2L+ may be beneficial as supplementary support in long posterior fixations to protect the S1 screws due to greater strain reductions versus common surgical techniques.