Abstract
BACKGROUND CONTEXT Variations in load distribution based on surface strain on instrumentation spanning the lumbosacral junction during in vitro loading are often used to evaluate biomechanical effects of different PSR configurations. The gold standard for biomechanical comparisons of spinal instrumentation involves the application of pure moment loads to induce bending of the instrumented spine. However spinal constructs in vivo are subjected to substantial vertical compressive forces due to gravitational loading of upper body mass. The effects of added compressive loads during in vitro bending of the PSR instrumented lumbar spine, as determined by surface strains at the lumbosacral level, are not well understood. PURPOSE The purpose of this study was to analyze the biomechanical differences in lumbosacral rod strain, and sacral screw bending moments during sagittal plane bending, with and without a compressive preload. STUDY DESIGN/SETTING In vitro biomechanical study using human cadaveric specimens. PATIENT SAMPLE A total of 14 (L1-Pelvis) cadaveric spines (5F/9M, 51.6±7.4 years, DEXA 0.837g/cm2). OUTCOME MEASURES L5-S rod strains and resultant sacral screw bending loads. METHODS A total of 14 cadaveric specimens were tested in a 6DOF robot under continuous loading in two PSR configurations (L2-S and L2-IL) using five types of loads: 7.5Nm flexion (FL), extension (EX), 400 N compression (C), combined C+FL and combined C+EX. Strain gauges were used to measure rod strains (RS) at L5-S and net sacral screw bending moments (SS). Data were analyzed using One-way RM-ANOVA followed by Holms-Sidak paired analysis (p RESULTS RS significantly increased with L2IL versus L2S (FL: +77%; EX: +65%; C+FL: +46%; C: +40%; p .2]). CONCLUSIONS The relative effects of L2IL versus L2S (increased RS and reduced SS) were consistent among all types of loads. However, the additive effects of compression on RS and SS varied with type of instrumentation (L2S vs. L2IL) and direction of bending (FL vs. EX). Compression loading significantly influences instrumentation performance and can help provide improved understanding of lumbosacral junction load distributions with PSR constructs.
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