The Role of the Nucleus Pulposus in Human Intervertebral Disc Mechanical Function Quantified by Mechanical Loading and Non-Invasive Imaging

The intervertebral disc performs the mechanical roles of supporting loads, permitting motion, and dissipating energy. Disc degeneration affects a large portion of the population, reduces the jointâ??s effectiveness, and is strongly implicated as a cause of low back pain. Degeneration is an irreversible process that manifests early within the centralized nucleus pulposus and subsequently affects other disc components. An incomplete understanding of the role of the nucleus pulposus and how alterations in nucleus function affect disc mechanics has hindered successful development of disc degeneration treatment. The objective of this dissertation was to evaluate the mechanical contributions of the nucleus pulposus to intervertebral disc function in compressive loading. In cyclic loading experiments, it was determined that removal of the nucleus pulposus via nucleotomy caused acute changes to the discâ??s mechanical response such as a decrease in compressive stiffness with an accompanying increase in compressive strain. These changes correspond to hypermobility, which alters overall spinal mechanics and may impact low back pain via altered motion throughout the spinal column. In addition to these acute changes, nucleotomy also decreased the fluid-flow related effects of cyclic compressive loading. Filing the void left by nucleotomy with a hydrogel implant preserved the creep response of the discs. A procedure for creating disc strain templates, which describe average disc strain, from MR images taken before and after loading was developed to non-invasively measure internal disc strains that result from compression loading. In mildly-degenerate human discs, removal of the nucleus increased axial strain throughout the annulus fibrosus, consistent with the existing literature stating that the nucleus plays a significant role in supporting compressive loads. Removal of the nucleus also unevenly altered the distribution of circumferential and radial strains within the annulus. Nucleotomy caused substantially higher circumferential strain in the posterolateral region, which may increase the risk of annular tears. The novel tools developed in this work and the experimental results can be utilized to further understand the mechanical role of the nucleus pulposus on intervertebral disc function, how that role changes with degeneration, and to design and treatments that restore disc mechanics