Solute transport in intervertebral disc: experiments and finite element modelling

The human intervertebral disc (IVD) is the largest avascular tissue in the body. Discs are made up of a gelatinous substance called the nucleus pulposus (NP), which acts as a shock absorber and provides flexibility during the diurnal cycle. The NP is surrounded by a fibrocartilaginous ring, the outer annulus (OA), which keeps the NP intact when forces are applied. Essential nutrients such as oxygen and glucose are supplied to the IVD by capillaries at the disc margins which may be up to 7 to 8 mm away. Nutrient gradients from the blood supply to the disc centre thus develop and the nutritional environment of the cells varies throughout the disc. Loss of nutrient supply to the disc cells is thought to be a major cause of disc degeneration. In this work, transport of molecules of different size have been analysed by a combination of experimental and modelling studies. Solute transport has been compared for steady state and transient diffusion of a number of different solutes with molecular weights (MW) in the range 3-70 kDa, injected into parts of the disc where degeneration is thought most likely to occur first and into the blood supply to the disc. This has been used to identify the most suitable method of getting drugs or growth factors to prevent or remedy disc degeneration to parts of the disc where degeneration is most likely to occur. Diffusion coefficients of fluorescently tagged Dextran molecules of different MW have been measured in vitro using the concentration gradient technique in thin specimens of disc OA and NP. Diffusion coefficients were found to decrease with molecular weight following a non-linear relationship. Diffusion coefficients changed more rapidly for solutes with molecular weights less than 10 kDa. The coefficients were higher in the nucleus than the annulus. Although unrealistic or painful, solutes injected directly into the disc achieve the largest disc coverage with concentrations that would be high enough to be of practical use. Although more practical, solutes injected into the blood supply do not penetrate to the central regions of the disc and their concentrations dissipate more rapidly. Injection into the disc would be the best method to get drugs or growth factors to regions of degeneration in IVDs quickly; else concentrations of solute must be kept at a high value for several hours in the blood supply to the discs.