A Biomechanical Analysis of Lateral Interbody Construct and Supplemental Fixation in Adjacent-Segment Disease of the Lumbar Spine

Abstract

To analyze the stability of lateral lumbar interbody fusion (LLIF) and compare various methods of supplemental fixation in adjacent-segment disease. Four fresh-frozen human cadaveric lumbar spines (L1 to sacrum) were used for motion analysis in extension, flexion, and lateral bending. The L4-L5 level was secured with a lateral interbody cage and pedicle screws to simulate a fused segment. The adjacent segment (L3-L4) was evaluated with flexibility testing sequentially under the following conditions: native disc (control), LLIF cage, cage with lateral plate, pedicle screws with z-rod, and single-rod construct. The difference in mean displacement (millimeters) between groups was studied by the analysis of variance and post-hoc Tukey test. Mean displacement (millimeters) on averaging motion in all planes was 0.741 for native disc, 0.273 for cage, 0.183 for cage with plate, 0.086 for pedicle screws and z-rod, and 0.106 for the single-rod construct. All 4 constructs led to a significant reduction (P < 0.001) in displacement in extension and flexion, as compared with native disc. There was no demonstrable superiority between the 4 constructs as the mean displacements were not significantly different from each other. LLIF with and without supplemental fixation reduced motion significantly at the adjacent segment as compared with intact disc. There was a trend toward increasing rigidity with supplemental fixation (plate and pedicle screw constructs). Further biomechanical studies with larger sample sizes are needed to confirm these initial findings.