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Format:
Print
Author:
MacLean, Jeffrey J.
Dept./Program:
Biomedical Engineering Program
Year:
2004
Degree:
M.S.
Abstract:
Despite significant advances in our understanding of the spine, back pain remains the most common cause of disability in individuals 20-50 years of age. While the specific conditions responsible for this epidemic are not well understood, discogenic back pain, related to injury and/or degeneration of the intervertebral disc and subsequent pathology, is a significant contributor. Disc degeneration refers to the progressive remodeling that leads to altered composition, structure and mechanical properties. Disc degeneration is an aging related event, although epidemiology and mechano-biology studies have suggested that mechanical loading has the propensity to accelerate the degenerative process. To date, few studies have evaluated the in vivo biological response of the disc to dynamic loading. The goal of this thesis was to investigate the dose dependent response of the disc to in vivo dynamic compression, and to determine if specific combinations of load magnitude, frequency and duration initiate tissue remodeling.
An animal model was used to apply loads to rat caudal discs. This system was chosen because in vivo models most closely represent the complex mechanisms involved in disc biosynthesis. The tail model also provided an easily accessible system that allows application of carefully controlled dynamic loading. Loads were applied to carbon fiber rings that were surgically attached to the 8th and 9th caudal vertebrae. The effects of loading were determined by measuring changes in gene expression levels using the technique of real time quantitative reverse transcription polymerase chain reaction (real time RT-PCR). This method allowed us to quantify relative changes in the expression levels of genes coding for both structurally important matrix proteins and the enzymes responsible for matrix degradation in response to short-term dynamic compression.
Three separate investigations were carried out. In the first study, the dynamic loading model was refined, and the effects of immobilization and a single dynamic compression condition were evaluated. Following this, a detailed investigation of the independent and interactive effects of six combinations of load magnitude and frequency were conducted. And finally a third study evaluated the time dependent response of the disc by investigating the independent variable of load duration using a single compression magnitude and frequency. The results demonstrated a dose dependent, and region specific response of the disc to dynamic compression. Immobilization had largest catabolic effect on the disc, in which decreases in matrix gene expression and increases in enzyme expression were observed.
Under dynamic loading, the nucleus response appeared to be primarily dependent on the frequency of loading with peak gene expression after 0.5 or 2 hours, while annulus gene expression depended more on the magnitude of loading with peak expression levels at 4 hours of loading. Specific trends were observed with metabolic patterns indicative of matrix turnover, matrix degradation and homeostasis. Furthermore, these trends were related to the magnitude, frequency and duration of loading. In addition to providing the first detailed characterization of the metabolic response of the disc to dynamic compression, this thesis has evaluated those dynamic compression loading conditions that will most likely lead to biochemical remodeling in future chronic investigations. Finally, it is hoped that the results of these studies will help to define more precisely those loading conditions that may lead to accelerated degeneration as well as repair with implications for prevention of disc injury as well as disc tissue engineering.