Nutrient transport in human annulus fibrosus is affected by compressive strain and anisotropy

Abstract

The avascular intervertebral disc (IVD) receives nutrition via transport from surrounding vasculature; poor nutrition is believed to be a main cause of disc degeneration. In this study, we investigated the effects of mechanical deformation and anisotropy on the transport of two important nutrients--oxygen and glucose--in human annulus fibrosus (AF). The diffusivities of oxygen and glucose were measured under three levels of uniaxial confined compression--0, 10, and 20%--and in three directions--axial, circumferential, and radial. The glucose partition coefficient was also measured at three compression levels. Results for glucose and oxygen diffusivity in AF ranged from 4.46 × 10(-7) to 9.77 × 10(-6) cm(2)/s and were comparable to previous studies; the glucose partition coefficient ranged from 0.71 to 0.82 and was also similar to previous results. Transport properties were found to decrease with increasing deformation, likely caused by fluid exudation during tissue compression and reduction in pore size. Furthermore, diffusivity in the radial direction was lower than in the axial or circumferential directions, indicating that nutrient transport in human AF is anisotropic. This behavior is likely a consequence of the layered structure and unique collagen architecture of AF tissue. These findings are important for better understanding nutritional supply in IVD and related disc degeneration.

Figure 1

Schematic diagram showing the structure of the IVD as well as the size and orientation of AF specimens for diffusivity and partitioning experiments. This diagram does not represent the specific locations within the AF region from which specimens were harvested for the particular experiments; for all experiments, specimens were taken from both the anterior and posterior AF.

Figure 2

Schematic diagram of the custom-designed chambers for measuring (a) diffusivity and (b) partition coefficient. For oxygen measurements, the diffusivity chamber includes an optical oxygen sensor and air-tight sealing cap, as shown in (a); this sensor is not present for glucose diffusivity measurements. The metal spacer between the two chamber halves is used to control the amount of uniaxial confined compression on the specimen; that is, the spacer matches the desired compressed height of the tissue (i.e., for a 0.5 mm thick specimen at 0% strain, the spacer is 0.5 mm, while at 10% strain the spacer is changed to a 0.45 mm thickness).

Figure 3

Results for glucose partition coefficient, K, versus tissue porosity in human AF tissue. Results shown are pooled data from both axial and circumferential directions at all three strain levels. There was no statistical difference between these two groups (i.e., partitioning is isotropic) in preliminary studies (data not shown), so no distinction was made between tissue orientations. Tissue porosity was directly calculated using the described buoyancy method.