The intervertebral disc is the largest avascular structure in the human body, with very low cellularity and largely without sensory innervation. It has a slow rate of tissue turnover and its viability depends primarily on transport of dissolved nutrients and metabolites, through long diffusion pathways. The disc has been implicated in painful conditions that affect a large proportion of the population. Epidemiological data suggest that long-term degenerative changes are more likely to produce these painful conditions than are acute overload injuries. Therefore, here we propose to combine existing and new information on disc tissue properties and structure into a coordinated theory of the whole disc behavior that can subsequently be applied to explain its healthy and pathological behavior. We will perform experiments to document intervertebral disc mechanical behavior, and compare these data with a new combined theory of mechanical, fluid, and chemical behavior of the disc's tissues and structure. This theory will be incorporated in a finite element model of the whole disc that will be refined based on the experimental data. Specifically: (1) Mechanical behavior will be documented by measurements of time-dependent load-displacement behavior of intervertebral discs and internal displacements. (2) Intervertebral disc swelling behavior will be documented over time by placing semi-constrained discs in baths of differing ionic concentrations and measuring volumetric increase, constraining force and intradiscal pressure. (3) Fluid flow and diffusion in the intervertebral disc will be measured under conditions of different end-plate permeability by recording the concentration of fluorescent dyes of different molecular weights. (4) Electrical potentials will be mapped at known positions in intervertebral discs subjected to time-varying displacements of the specimen with controlled boundary conditions (displacements and porosity of the end fittings). This theory will, in turn, provide a way to understand the relationships between the physical environment of the disc (mechanical, nutritional, etc.) and the local conditions that can influence its metabolism, eventual composition and function in three-dimensions.
