Date of Award

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Laible, Jeffrey

Abstract

The intervertebral disc (IVD) is a complex orthopaedic tissue that is located between the vertebrae in the spine. Degeneration of the IVD is thought to be a contributor to low back pain (LBP), which affects up to 80% of the population at enormous economic cost. The role of the intervertebral disc in supporting and resisting applied loading to the spine, along with the observation of disorders associated with abnormal spinal loading, provide support to the theory that applied mechanical loading is crucial in maintaining the health of the intervertebral disc. The encompassing goal of this work was to examine the biological response of the intervertebral disc to changes in the surrounding mechanical environment in a large animal model. Aim 1 utilized an organ culture model to explore the relationship between disc mechanics and biology in needle puncture injury, a commonly used model of experimentally induced disc degeneration, thus providing a possible mechanism for in vivo injury induced disc degeneration models. Aim 2 was to explore the interaction between the amplitude of applied mechanical loading and intervertebral disc cell signaling, also performed in an organ culture model to include cell-matrix signal transduction. Aim 3 addressed frequency and age effects on the IVD response to mechanical stimulation, performed in vitro to control for the effects of varying matrix compositions between old and young animals. Finally, Aim 4 utilized kmeans and fuzzy c-means clustering techniques to reveal patterns in experimental phenotype (determined by gene expression data) and gene response to experimental conditions. The application of biclustering, where the gene responses within experimental phenotypes are clustered to elucidate possible mechanisms for different gene level-responses to experimental conditions, was also accomplished. Finally, the ability for the model to predict the behavior of other genes critical to IVD mechanobiology, or in determining the membership of an unexamined experimental phenotype was explored. Overall, applied dynamic compression was not found to significantly alter disc mechanics, while a disruption in the annulus through needle puncture rapidly decreased the compressive modulus. Changes in disc mechanics may precede biological remodeling, with little evidence of remodeling present without mechanical alteration. Aging, however, crucially impacts disc cell biology, particularly in the nucleus pulposus, and will interact with applied loading to further impact the ability for the intervertebral disc cells to maintain a healthy extracellular matrix.

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