Effect of compression and anisotropy on the diffusion of glucose in annulus fibrosus

Study Design. Investigation of the effect of static compression and anisotropy on the apparent diffusivity of glucose in bovine annulus fibrosus (AF). Objective. To determine the apparent glucose diffusivity in 2 directions (axial and radial) of bovine AF under 3 levels of compressive strain (0%, 10%, and 20%). Summary of Background Data. Knowledge of diffusivity of small molecules is important for understanding nutritional supply in intervertebral discs and the mechanisms of disc degeneration. However, little is known regarding the strain-dependent and anisotropic behavior of glucose diffusivity in intervertebral discs. Methods. Apparent glucose diffusivity measurements were performed on 10 axial and 10 radial AF specimens from bovine coccygeal discs. The dependence of diffusivity on compression was determined using 3 levels of strain (0%, 10%, and 20%). Results. The apparent glucose diffusivity (mean +/- standard deviation) of the bovine AF in the axial direction was 1.38 +/- 0.015 x 10(-6) cm(2)/s (n = 10) at 0%, 1.00 +/- 0.070 x 10(-6) cm(2)/s (n = 10) at 10%, and 7.65 +/- 0.552 x 10(-7) cm2/s (n = 10) at 20% compression. For radial specimens, the apparent glucose diffusivity was determined to be 9.17 +/- 1.12 x 10(-7) cm(2)/s (n = 10), 7.29 +/- 0.863 x 10(-7) cm(2)/s (n = 10), and 5.43 +/- 1.16 x 10(-7) cm(2)/s (n = 10) for 0%, 10%, and 20% compressions, respectively. A significant decrease in diffusivity with increasing strain was found for both axial and radial specimens [analysis of variance (ANOVA), P < 0.05]. Diffusion in the radial direction was determined to be significantly less than that in the axial direction (ANOVA, P < 0.05). A significant interaction was found between the level of strain and the direction of diffusion (ANOVA, P < 0.05). Conclusion. Diffusion of glucose in bovine AF is dependent on strain and the direction of diffusion.