Micromechanics of the annulus fibrosus: role of biomolecules in mechanical function

Jessica Lauren Isaacs

doi:10.17918/etd-3842

Lower back pain, caused by disc degeneration or injury, has a major effect on the United Stated economy, resulting in large medical costs - 2.5% of US health care expenditures (~50 billion dollars) annually [1]. A herniation is a common injury to the intervertebral disc that is characterized as the migration of the inner nucleus pulposus through the layers of the outer annulus fibrosus. There have been many studies quantifying the mechanical characteristics of the annulus fibrosus and modeling the response, both mathematically and computationally. There has been some work investigating the failure mechanisms of the annulus in a degenerative, micromechanical model, however the work for a larger injury model is lacking. Experimental work shows that repetitive, compressive and bending loads of the disc, causing the annulus to fail in tension, will result in catastrophic disc herniation. The goal of this work is to characterize the failure properties of annular lamellae using a micro-mechanical testing protocol with the long-term goal of developing a failure criterion for the annulus fibrosis. Single layered annular samples were obtained from isolated cadaveric lumbar intervertebral discs in one of four orientations: longitudinal, transverse, radial, and circumferential. Uniaxial tensile tests were performed to failure and the engineering constants and failure stresses and strains determined. Key findings showed different of properties between orientations. Failure stress, elastic modulus and Poisson's ratio were higher when tested in plane to the fibers or lamellae (longitudinal and circumferential) compared to the out-of-plane orientations (transverse and radial) with higher failure strain for out-of-plane than in-plane specimens. This was furthered by a study investigating the role of macromolecules in the intervertebral disc on the micromechanical behavior of the human cadaveric lumbar annulus fibrosus to determine the role these molecules play in annular mechanics. Using composite theory, a model of the annulus fibrosus was used to determine the stresses in each lamella at different loading conditions. Failure envelopes based on the Tsai-Hill criteria were created. The properties were used to create failure envelopes for the annulus which may predict catastrophic failure of the annulus that contribute to disc herniation and lower back pain. Full understanding of the mechanical properties and failure envelopes of the annulus could potentially lead to a failure model for disc tearing and herniation.

Micromechanics of the annulus fibrosus: role of biomolecules in mechanical function

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Micromechanics of the annulus fibrosus: role of biomolecules in mechanical function

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