Biomechanical Stability Before and After Graft Fusion with Unilateral and Bilateral Pedicle Screw Fixation: Finite Element Study

BACKGROUND: Minimally invasive transformational lumbar interbody fusion (MI-TLIF) with unilateral pedicle screw (UPS) fixation was controversial. The aim of this study was to compare the stability between UPS and bilateral pedicle screw (BPS) fixation before and after graft fusion. METHODS: An L3-L5 finite element model was modified to simulate L4/5 MI-TLIF. Five different statuses of posterior instrumentation were simulated: UPS fixation or BPS fixation before and after graft fusion and removal of posterior instrumentation after graft fusion. Range of motion and Von Mises stress were evaluated for intact and instrumentation models in all loading planes. RESULTS: Range of motion of the L4/5 segment with UPS fixation was 2.1, 1.3, and 1.7 times greater than those with BPS fixation before fusion in flexion-extension, lateral bending, and axial rotation, respectively, while it was 1.3, 1.1, and 1.4 times greater after fusion. The peak Von Mises stresses on posterior instrumentations with UPS fixation ranged from 1.0 to 1.7 times greater than those in BPS fixation before fusion, while it ranged from 1.0 to 1.4 times greater after fusion. The peak Von Mises stresses on intervertebral graft with UPS fixation ranged from 1.9 to 3.5 times greater than those with BPS fixation before fusion, while it ranged from 0.9 to 1.2 times greater after fusion. CONCLUSIONS: Fusion of graft improved the fixation effect of posterior instrumentation system. Unilateral pedicle screw fixation could provide similar biomechanical stability to bilateral pedicle screw fixation in 1-level MI-TLIF after fusion.