Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Virginia L Ferguson

Second Advisor

Stephanie J Bryant

Third Advisor

Yifu Ding

Fourth Advisor

Wei Tan

Fifth Advisor

Evalina Burger

Abstract

The objective of this dissertation is to understand the influences of material structure on the properties, function and failure of biological connective tissues. Biological interfaces are becoming an increasingly studied system within mechanics and tissue engineering as a model for attaching dissimilar materials. The elastic modulus of bone (≈ 20 GPa) and cartilage (≈ 0.1-1 MPa) differ over orders of magnitude, which should intuitively create high stress concentrations and failure at the interface. Yet, these natural interface systems rarely fail in vivo, and the mechanism by which loads are transferred between tissues has not yet been established. Tissue quality is one major contributor to the mechanical behavior of bone and cartilage, and is defined by properties such as collagen orientation, mineral volume fraction, porosity and tissue geometry. These properties have yet to be established at the bone-cartilage interface in the spine, and the lack of quantitative data on material microstructure and behavior limits treatments and tissue engineering construct design.

In this dissertation, second harmonic generation imaging, quantitative backscattered scanning electron imaging and nanoindentation are combined to characterize micrometer scale tissue quality and modulus in both bone and calcified cartilage. These techniques are utilized to: 1) determine the hierarchical micrometer to millimeter scale properties of lamellar bone, 2) quantify changes throughout development and aging at the human intervertebral disc-vertebral body junction, and 3) explore compressive fractures at this interface. This work is the first to provide quantitative data on the mineral volume fraction, collagen orientation and modulus from the same, undecalcified sections of tissue to corroborate tissue structure and mineralization and describe quantitative parameters of the interface.

The principal findings from this work indicate that the underlying matrix, or collagen, organization in mineralized biological tissues and at the bone-cartilage interface plays an important mechanical role. Nanoindentation measurements in osteonal bone are affected by location within the lamellar structure, even though mineral volume fraction within a single osteon is relatively consistent compared to the differences observed between bone and calcified cartilage. While increasing mineral volume fraction contributes to increases in modulus in the calcified cartilage layer of the vertebral body-intervertebral disc interface, significant scatter remains. The collagenous matrix structure and type of collagen appear to have a significant influence on modulus as well. Collagen fibers of the disc mineralize adjacent to the bone of the vertebral body, and the persistence of this attachment zone from adolescence through senescence indicates that it likely serves a mechanical function. Fiber insertions into thick calcified cartilage regions likely create mechanically robust anchor points at the osteochondral interface.

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