Experimentelle und numerische Untersuchung des mechanischen Verhaltens von Bandscheiben am Beispiel gesunder und geschädigter Wirbelsäulenfunktionseinheiten

Strampe, Malte; Stoffel, Marcus (Thesis advisor); Büchs, Jochen (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2015)
Dissertation / PhD Thesis

Abstract

Due to the increasing amount of spinal diseases and afflictions in the population of industrial countries, there is immense interest to obtain increasing knowledge about the functionality of the spine and its components, as well as the relationships between mechanical stress and its effects on the biological tissue. The aim of this work is the experimental investigation and numerical simulation of the relaxation behavior of the intervertebral discs, which occurs for example during lifting, holding and again putting down loads. A comparison is drawn between the reactions of a healthy disc and those of a damaged disc with removed core. The examination of the occurring differences relates both to the external reactions, as well as the intrinsic stress distributions of both configurations. The experimental investigation of motion segments of sheep is used to detect the relaxation effects at constant deformation states and to verify the characteristic stress-strain behavior of specimens with existing and removed nucleus. The analysis of the relaxation tests shows, as expected, a significantly higher load carrying capacity in the intact intervertebral discs. With increasing damage the bearing capacity of the slices reduces. In all investigated specimens relaxation occurs during the holding processes. The relaxation effect and the maximum force absorption decrease at ongoing cyclic loading until a quasi-stationary state is reached. Furthermore the experiments allow the determination of material parameters of a user-defined material law in the subsequent numerical modeling. Hence, it enables the reproduction of the results of experimental investigations of the intervertebral discs by means of finite element simulations. This success enables the user to perform changes such as the resection of the core of the model and therefore to examine the effects of this intrusion. The structural models, which allow generating the finite element meshes, defining constraints, realizing the lamellar structure of the fiber ring with orthotropic, circumferential material orientation and prescribing deformation, are based on CT-images of the investigated compounds. Thus, it is possible to relate the experimentally determined non-linear viscoelastic, diffusion-dependent mechanical reactions of the intervertebral disc to their correct physiological geometries. Using the simulations, the changes in the mechanical behavior of the spine functional units can now be shown and predicted due to damage of the intervertebral disc. The external reaction of the core removal consists of a reduction of the maximum reaction force and thus the bearing capacity of the disc. The examination of the intrinsic relations clearly shows the influence of interactions between the individual components. The resection of the nucleus alters the mechanism upon which the high resistance to axial loads of the entire spine is based. At axial stress the ring is not anymore loaded in tension in circumferential direction by the core, so that the high stiffness of the fibers cannot contribute to the bearing capacity of the disc.

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