Intervertebral disc (IVD) degeneration and associated back pain place a significant burden on the population. IVD degeneration is a progressive cascade of cellular, compositional and structural changes, which results in a loss of disc height, disorganization of extracellular matrix architecture, herniation of the nucleus pulposus (NP), tears in the annulus fibrosus (AF), and remodeling of the boney and cartilaginous endplates. These changes often occur concomitantly, making it difficult to determine which factor initiates degeneration. Furthermore, assessment of the subcomponents and interfacial regions of the IVD have been largely qualitative to date, and the changes that occur at this length scale with degeneration are not yet well understood. This poor understanding of disease etiology hinders the development of therapies to restore native structure and function of the IVD, and so, current clinical treatments are focused on alleviation of symptoms rather than focused regeneration. As such, tissue engineering provides a promising treatment option and has the potential to restore both the structure and function of the native disc.

The overall goal of this thesis is to establish quantitative microscale and macroscale outcomes that define the spectrum of degeneration and to inform regeneration. In Aim 1, we developed a quantitative dataset of the structural and functional features of degeneration over time in an in vivo rabbit model and identified the primary contributors to disc degeneration using a machine learning approach. We found that puncture acutely compromised disc macro and microscale mechanics, followed by progressive AF stiffening and remodeling. These dynamic changes were accompanied by increases in endplate bone volume fraction, increases in microscale stiffness of the soft tissue interfaces between the disc and vertebral bone, and reductions in endplate vascularity and small molecule transport into the disc as a function of degenerative state. Notably, our neural network model identified changes in diffusion into the disc (a measurement of disc nutrition) as the most significant predictor of disc degeneration. In order to extend the translatability of this animal model, we employed similar multimodal analyses in Aim 2, using human cadaveric spines of differing degenerative states. We observed widespread multiscale alterations to the IVD, endplates, bone, and whole motion segment. Interestingly, and in contrast to the rabbit model, we observed motion segment softening and endplate resorption with degeneration. We discerned several correlations between the quantitative variables, particularly between IVD and endplate measurements, suggesting crosstalk between these tissues during degeneration.

To address a potential treatment strategy for end-stage degeneration, we sought to advance and optimize the composition and mechanics of a tissue-engineered whole disc replacement (DAPS). Our findings from Aims 1 & 2 indicated that AF structural integrity and micromechanical properties are severely affected during degeneration, and as such, in Aim 3, we included a soluble polymer into the AF region of the DAPS to promote cell ingress and improve matrix distribution. This modification better approximated native micro- and macro-mechanical properties of the DAPS, cellular colonization, and matrix elaboration, in the in vitro and vivo environments.

Overall, the results from this Thesis identify the most significant contributors to disc degeneration, and advance new treatment strategies towards clinical use.