Microscale properties of the annulus fibrosus in the disc: effect of degeneration

Author: Abraham Jacob

Jacob, Abraham, 2023 Microscale properties of the annulus fibrosus in the disc: effect of degeneration, Flinders University, College of Science and Engineering

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The main region of focus in this study is the interlamellar matrix (ILM), which is situated between neighbouring lamellae of the annulus fibrosus (AF) and has a complicated pattern of elastic fibres, while elastic fibres of the intra-lamellar region are primarily parallel to the collagen fibres. Researchers have focused on the fundamental biomechanical functions of the interlamellar matrix, and its role in load distribution, shock absorption, and spinal flexibility are discussed, emphasising the importance of maintaining their integrity, thus establishing their clinical relevance. Studying the nature of the ILM can also help in tissue engineering methods to develop biomaterials that can help to alleviate pain and promote tissue healing. The thesis investigates the impacts of intervertebral disc (IVD) degeneration, a multidimensional phenomenon driven by genetic, mechanical, and environmental variables, on the mechanical properties of the IVD. This was done by isolating and testing the non-destructive mechanical properties of the circumferential regions of healthy and degenerated discs and comparing them. Samples from various circumferential (anterior, anterolateral, lateral, posterolateral, and posterior) regions within the disc were used to assess variations, and the viscoelastic properties (stiffness and hysteresis loss coefficient) was evaluated by applying the load at three different strain rates. While similar studies to determine the delamination thresholds have been done before, this is the first time it has been done on human discs and analyzing all the regions. Future research into the numerous molecular and cellular mechanisms involved in degeneration will provide insight into potential treatment targets for reducing or stopping the degenerative cascade. Additionally, failure mechanics were studied using ultimate failure load and energy absorbed analysis. Prior to working with human tissue, extensive practice testing was done on ovine specimens to ensure the procedure's repeatability, and then the results were confirmed by pilot testing on human specimen. Higher modulus and strength were found in the anterior regions, as opposed to the posterior and lateral regions, while degeneration grade had a noticeable effect on the ultimate strength of some regions but not all. A strong strain rate dependency supported the human ILM's viscoelastic characteristics. The modulus and ultimate stress were also influenced by the loading direction. These findings offer a new perspective on human ILM mechanics and serve as a solid foundation for further research in the area. The findings of this study hold great promise for furthering our understanding of intervertebral disc health and aiding the development of novel treatment strategies, potentially transforming spinal disease therapy and improving patients' quality of life.

Keywords: Annulus Fibrosis, Interlamellar Matrix, Cell Scale, Intervertebral disc, degenerated disc, modulus, functional lamellae unit

Subject: Engineering thesis

Thesis type: Masters
Completed: 2023
School: College of Science and Engineering
Supervisor: Dr. John Costi