Multi-segment FEA of the human lumbar spine including the heterogeneity of the annulus fibrosus

Abstract

This study pursues the numerical validation of human lumbar spine segments. By means of the finite element (FE) method, computational analyses are carried out of various load cases. In particular Flexion-Extension, Lateral Bending and Axial Torque are considered. By means of a literature review the underlying constitutive data is verified. In this context, the heterogeneity of the annulus fibrosus, the transversely isotropic stress response of the spinal ligaments and aspects of the FE discretization are particularly emphasized. The numerical results show good agreement with experimental investigations for Extension and Axial Torque for a FE model that accounts for intact human lumbar spine response. In Flexion and Lateral Bending, however, the results of the intact FEmodel do not properly account for the experimental data. A good correlation for these load cases can be found by taking disc degeneration into account in the FE-model. This fact shows that tissue degeneration plays a key role in the current validation process and must be accounted for if the lumbar spine specimen is employed for spinal implant evaluation. A degenerated FE-model that represents the stage of degeneration of the specimen and fits the experimental data for all load cases could not be found in this study and warrants further work in this area. Introduction and motivationVirtual design of (new) implants is a major task in orthopedic industries today. Numerical simulation techniques have the potential to significantly reduce costs during the development and prototyping phase and therefore improve time to market. This study addresses the need for a reliable numerical tool that accounts for an accurate three-dimensional biomechanical response of the human lumbar spine. By means of the finite element (FE) method a validation of the numerical model is pursued against experimental test data. It is a major task to firstly obtain an accurate FE model that accounts for the response of the intact or possibly degenerated human lumbar spine. Such a FE model can later be employed in order to quantify the impact of a spinal implant on the natural biomechanics of the human spine.This study is organized in five chapters. Chapter 2 contains a literature review on multi-segment analyses of the human lumbar spine and aspects of spinal ligament modeling. It is carried out in order to confirm the constitutive parameters particularly chosen for the intervertebral discs (IVDs) and the spinal ligaments. These tissues are mainly responsible for the kinematics and compliance of the spinal column. Spinal muscle forces are neglected in this study, because the lumbar spine specimen used for experimental testing and validation does not represent muscle tissue.In chapter 3 the FE modeling of the experimentally analyzed L2-S1 segments is explained in detail. It comprises aspects of the CAD data transformation, the FE model alignment according to the underlying experimental set-up and outlines the constitutive equations for the annulus fibrosus and the spinal ligaments, ...

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