UVM Theses and Dissertations
Format:
Print
Author:
Barbir, Ana
Dept./Program:
Mechanical Engineering
Year:
2010
Degree:
PhD
Abstract:
Low back pain is a public health issue, affecting up to 80% of the population in the US at some point during their lives and resulting in significant socioeconomic costs. Degeneration of the intervertebral disc (IVD) has been implicated in pathogenesis of back pain. Yet current therapy for discogenic back pain is mainly aimed at relieving symptoms rather than the underlying disease mechanisms. Understanding the mechanical properties of native tissue as well as being able to control and modulate mechanical stimuli will be necessary to develop tissue engineering approaches and integrate them within the body. The IVD is a heterogeneous structure which separates the vertebrae and provides flexibility to the spine. Like all biological tissues, it is a complex system balancing input to the tissue with the resulting output by the cells. This work aims to provide insight into the interconnected relationship between tissue level perturbation, cellular level response and the resulting alterations in tissue properties in the IVD through the following three studies.
In study 1 chemical alterations to collagen and elastin structure and content were induced in order to measure resulting changes in compressive properties of the IVD. Rat caudal motion segments were treated with a collagen crosslinker, collagenase and elastase and compared to control under five force-controlled loading stages. Crosslinking was found to have the greatest effect on IVD properties at resting stress. Elastin's role was greatest in tension and at higher force conditions, where proteoglycan content was also a contributing factor. Collagenase treatment caused tissue compaction, which impacted mechanical properties at both high and low force conditions.
In study 2 effects of torsion on biosynthesis and injury in rat caudal IVDs were explored in vivo and in vitro enzymatically treated motion segments were assessed for the contribution of their sturcture to torsional properties. Elastin was found to be important in torsion: elastase treatment significantly decreased torsional motion segment stiffness in vitro and cyclic torsion stimulated elastin expression in vivo. IVD cells were shown to have distinct responses to torsion and compression with cyclic torsion loading stimulating elastin expression and having small effect on other genes while cyclic compression resulting in a more general increase in metabolic response. Increased torsion amplitudes caused no structural changes detectable by biomechanical testing during the duration of the experiment, yet some increases in proinflammatory cytokines were present, suggesting minor injury was detectable biologically before biomechanically.
In study 3 differences in diffusivity due to alterations in the structure of the extracellular matrix were measured. Micro-CT (micro-computed tomography) was found to be a viable method of measuring changes in diffusivity, showing that diffusivity increases with elastase treatment while decreasing with collagen crosslinking. Diffusivity was also shown to vary in the radial direction. Taken together, these findings elucidate the relationship of important structural proteins in both IVD biomechanics and transport. Results provide baseline information important for tissue engineering approaches as well as improved understanding of the interaction of changes in IVDs with degeneration.
In study 1 chemical alterations to collagen and elastin structure and content were induced in order to measure resulting changes in compressive properties of the IVD. Rat caudal motion segments were treated with a collagen crosslinker, collagenase and elastase and compared to control under five force-controlled loading stages. Crosslinking was found to have the greatest effect on IVD properties at resting stress. Elastin's role was greatest in tension and at higher force conditions, where proteoglycan content was also a contributing factor. Collagenase treatment caused tissue compaction, which impacted mechanical properties at both high and low force conditions.
In study 2 effects of torsion on biosynthesis and injury in rat caudal IVDs were explored in vivo and in vitro enzymatically treated motion segments were assessed for the contribution of their sturcture to torsional properties. Elastin was found to be important in torsion: elastase treatment significantly decreased torsional motion segment stiffness in vitro and cyclic torsion stimulated elastin expression in vivo. IVD cells were shown to have distinct responses to torsion and compression with cyclic torsion loading stimulating elastin expression and having small effect on other genes while cyclic compression resulting in a more general increase in metabolic response. Increased torsion amplitudes caused no structural changes detectable by biomechanical testing during the duration of the experiment, yet some increases in proinflammatory cytokines were present, suggesting minor injury was detectable biologically before biomechanically.
In study 3 differences in diffusivity due to alterations in the structure of the extracellular matrix were measured. Micro-CT (micro-computed tomography) was found to be a viable method of measuring changes in diffusivity, showing that diffusivity increases with elastase treatment while decreasing with collagen crosslinking. Diffusivity was also shown to vary in the radial direction. Taken together, these findings elucidate the relationship of important structural proteins in both IVD biomechanics and transport. Results provide baseline information important for tissue engineering approaches as well as improved understanding of the interaction of changes in IVDs with degeneration.