Biomechanical analysis of lumbar interbody fusion with an anisotropic hyperelastic model for annulus fibrosis

Based on computed tomography scanning images, this paper developed a detailed finite element model for the human L2–L4 lumbar spine segment with or without L3–L4 fusion. The model included vertebrae, intervertebral disc, facet articulating surfaces and various ligaments. A previously developed hyperelastic fibre-reinforced constitutive model was used to characterize the material property of annulus fibrosus. Numerical results of L3–L4 motion unit such as load–displacement curves and nucleus pressure were compared with experimental data to validate the FE model. The normal and fused lumbar spine segments under various loading conditions, such as flexion, extension and axial rotation, were analysed. The motion range and stress distribution of the L2–L4 models under different loading conditions were then obtained to investigate the effect of lumbar fusion operation. It was shown that under the same loading condition, the fused model had a much smaller body motion range. Interbody fusion brought out obviously different stress distribution in adjacent intervertebral disc annulus fibrosus. And it also increased the intradiscal pressure of adjacent intervertebral disc significantly.