Biomechanical Evaluation of the Monoaxial Pedicle Screw and Plate Construct for Minimally Invasive Transforaminal Lumbar Interbody Fusion after Decompressive Surgery

Abstract

Introduction Minimally invasive transforaminal lumbar interbody fusion (MI-TLIF) is one popular posterior lumbar fusion surgery. The immediately biomechanical stability after unilateral or bilateral facetectomy of direct decompression is dependent on the type of supplement pedicle screw fixation used. The monoaxial pedicle screw and plate (PSP) has been approved for clinical use for added stabilization after cage instrumentation, especially for axial rotation stabilization. However, little biomechanical data exist comparing PSP fixation with bilateral multiaxial pedicle screw and rod (PSR) fixation. Materials and Methods Eight L3-L6 calf spines were instrumented with posterior cages at L4-L5 after intact kinematic analysis. The TLIF model was generated by clinical procedure and fixed with multiaxial PSR or monoaxial PSP by randomized sequence. A pure moment (±5.0 Nm) were applied to specimens to induce flexion/extension, lateral bending (LB) (left + right), and axial rotation (AR) (left + right) while recording intact versus instrumented range of motion (ROM). Two monoaxial strain gauges were instrumented at the surface of L4 and L5 vertebrae body each, to record the strain distribution under the flexion and lateral bending motion. Destructive tests of monoaxial PSR, multiaxial PSR, and monoaxial PSP construct were used to record the modes of failure, yield strength, and yield stiffness in a static compression bending test. Results Monoaxial PSP fixation significantly reduced ROM ( p < 0.001) at the lumbar level (flexion/extension: 88.7%; LB: 94.7%; AR: 53.8%) relative to the intact condition. Multiaxial PSR fixation afforded the similar ROM reductions ( p < 0.001) relative to the intact condition (flexion/extension: 83.1%; LB: 92.6%; AR: 42.3%). The strain distribution with monoaxial PSP fixation is not significantly larger than that with multiaxial PSR fixation when different loading conditions were applied (flexion vs. lateral bending). The strain distribution of L4 in both fixation conditions was significantly less than that in intact condition in lateral bending. The yield stiffness of the monoaxial PSP construct is significantly less than that of the monoaxial PSR construct ( p < 0.001), while the yield strength of the monoaxial PSP construct is significantly larger than that of the monoaxial PSR construct ( p < 0.001). Conclusion Based on these biomechanical findings, monoaxial PSP fixation and multiaxial PSR fixation are equal to afford the immediate stability after TLIF surgery. The property of low yield stiffness in monoaxial PSP construct has the potential to reduce the stress-shielding effect of the fusion segment. Disclosure of Interest None declared