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

OBJECTIVE: To analyze the stability of lateral lumbar interbody fusion (LLIF) and compare various methods of supplemental fixation in adjacent-segment disease. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.