Development of a Micro-Finite Element Model of the Interlamellar Matrix of the Intervertebral Disc

Author: Iska Pretorius

Pretorius, Iska, 2017 Development of a Micro-Finite Element Model of the Interlamellar Matrix of the Intervertebral Disc, Flinders University, School of Computer Science, Engineering and Mathematics

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Abstract

The annulus fibrosis (AF), within the intervertebral disc (IVD), is a heterogenous structure made up of various substructures. One such substructure is the inter-lamellar matrix (ILM), which is located between adjacent lamellae. It is theorized that the ILM plays a significant role in preventing delamination. However, limited literature exists on the exact function and mechanical properties of the ILM, in isolation. Therefore, micro-finite element models were developed representing the ILM, that were validated using experimental results. Three model types were developed: an ideal model, ideal model with an additional diagonal fibre, and a model of a partially digested ovine sample. The aim of the study was to initially develop a model of the ILM, validate the model, and then gain an understanding of how the fibre orientations within the ILM contribute to its mechanics. Mechanical testing (unpublished) was conducted in shear and tension, on partially digested ovine ILM samples (Tavakoli et al., 2017a). The experiment was displacement controlled, during which the applied load and mechanical properties of the tested sample were measured. The experimentally applied load was used to validate the developed ideal model, after which the parameters used, such as the geometry and mechanical properties, were applied to the two remaining models. The ideal model was the reference model, from which the effects of fibre orientation variation, between models, could be studied. It was determined that fibres orientated closer to the direction of the applied displacement, resisted the deformation more than the other fibres in the system. Under tension, along the x-direction, fibres orientated closer to the x-axis had a higher computational applied load, indicating that the fibres supported the applied displacement better in the x-direction. In shear, along the y-direction, fibres orientated further from the x-axis had a higher computational applied load, indicating that the fibres supported the applied displacement better in the y-direction. However, an increase in the computational load also increased the stress within the system and could therefore be a potential cause of initial fibre failure, which leads to failure propagation and finally delamination. It was also determined that an increase in fibre concentration attachment points, within an area, caused peak stresses within the system – with constant applied strain. Therefore, an increase in the amount of fibres attached to an area on the lamellae, could potentially cause failure such as fibre pull-out.

Keywords: Interlamellar Matrix, Intervertebral Disc, Annulus Fibrosis, Elastic Fibres, Lamellae
Subject: Engineering thesis

Thesis type: Masters
Completed: 2017
School: School of Computer Science, Engineering and Mathematics
Supervisor: Associate Professor John Costi