UC Berkeley

Back pain is a widespread economic and public health concern, affecting around 70% of the population and incurring annual costs over $100 billion in the US alone. Back pain is frequently associated with degenerative changes to intervertebral discs, including altered biochemical composition, rheology, and mechanics. However, due to the interdependent cascade of degenerative changes occurring in vivo, the relative contribution of individual tissue constituents to disc failure remains unclear. Elucidating the fundamental structure-function relations that drive tissue failure aids in the prevention of avoidable injuries, guides the design of protective therapies and tissue repair strategies, and facilitates the development of advanced computational models that may help predict tissue injury.This dissertation details the development, validation, and implementation an experimental framework to elucidate fundamental tissue-level structure function relations between degeneration- and disease-mediated biochemical changes and annulus fibrosus (AF) failure mechanics. Chapter 2 reports on the development and validation of a novel method for repeatable failure testing of soft tissues with fibers oriented off-axis from the applied loading, such as AF specimens tested in the circumferential-radial or circumferential-axial orientations. With minor subsequent modification, the method described in Chapter 2 serves as the basis for the reliable characterization of AF tensile failure mechanics in subsequent chapters. Chapter 3 describes the effects of enzymatic proteoglycan degradation and concomitant water loss on AF tensile failure mechanics in the circumferential-radial direction at low and high loading rates. Chapter 4 provides a brief empirical analysis of the relative contribution of the three main AF biochemical constituents to the tensile failure mechanics of AF tissue from a cross-sectional population of human donors The use of 0.15M phosphate-buffered saline in Chapters 3 and 4 resulted in hyper-physiologic tissue water contents, which are known to alter the tissue mechanical response. Thus, Chapter 5 provides a more robust method to target and maintain AF hydration levels, mechanics, and composition at fresh-tissue levels during in vitro testing. Chapter 6 utilizes these methods to provide discrete and continuous descriptions of the role of advanced-glycation end products in AF sub-failure, failure, and post-failure tensile mechanics at quasi-static and dynamic loading rates. Finally, Chapter 7 addresses the limitations of the current work and suggests possible directions for future investigations that might build on this dissertation work.