The metabolic environment of disc cells is governed by the avascular nature of the tissue. Because cellular energy metabolism occurs mainly through glycolysis, the disc cells require glucose for survival and produce lactic acid at high rates. Oxygen is also necessary for cellular activity, although not for survival; its pathway of utilization is unclear. Because the tissues are avascular, disc cells depend on the blood supply at the margins of the discs for their nutrients. The nucleus and inner anulus of the disc are supplied by capillaries that arise in the vertebral bodies, penetrate the subchondral bone, and terminate at the bone-disc junction. Small molecules such as glucose and oxygen then reach the cells by diffusion under gradients established by the balance between the rate of transport through the tissue to the cells and the rate of cellular demand. Metabolites such as lactic acid are removed by the reverse pathway. The concentrations of nutrients farthest from the source of supply can thus be low; oxygen concentrations as low as 1% have been measured in the discs of healthy animals. Although gradients cannot be measured easily in humans, they can be calculated. Measured concentrations in surgical patients are in agreement with calculated values.

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The transport of oxygen and lactate (i.e., lactic acid) in the human intervertebral disc was investigated accounting for the measured coupling between species via the pH level in the tissue. Uncoupled cases were also analyzed to identify the extent of the effect of such coupling on the solute gradients across the disc. Moreover, nonlinear lactic production rate versus lactic concentration and oxygen consumption rate versus oxygen concentration were considered. The nonlinear coupled diffusion equations were solved using an in-house finite element program and an axisymmetric model of the disc with distinct nucleus and anulus regions. A pseudotransient approach with a backward integration scheme was employed to improve convergence. Coupled simulations influenced the oxygen concentration and lactic acid concentration throughout the disc, in particular the gradient of concentrations along the disc mid-height to the nucleus-anulus boundary where the solutes reached their most critical values; minimum for the oxygen tension and maximum for the lactate. Results suggest that for realistic estimates of nutrient and metabolite gradients across the disc, it could be important to take into account the coupling between the rates of synthesis and overall local metabolite/nutrient concentration.

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As the disc is the largest avascular structure in the body, disc cells depend for their normal function on an adequate supply of nutrients (oxygen and glucose) and the removal of metabolic by-products (lactic acid) via blood vessels at the cartilaginous endplates and annulus periphery. Concentration gradients develop depending on the balance between the rates of transport and rates of cellular activity. Since consumption and production rates are coupled via extracellular pH, the gradients are interdependent. This is a novel model study which takes into account the realistic 3D geometry of a L5-S1 lumbar disc in solving the nonlinear coupled diffusion equations. Effects of perturbations (calcification, sclerosis) in endplates, increases in cell metabolic rates following growth factor injection and changes in lumbar posture (kyphotic or lordotic) on extreme values of nutrient and metabolite concentrations and their spatial locations are investigated. Solute concentrations, particularly those of glucose, substantially diminish as a consequence of disturbances in supply at the endplates, increases in cell metabolic rate and more lordotic postures. Results, when compared to those from simplified axisymmetric models, demonstrate the importance of consideration of realistic 3D disc geometry.