Substrate Concentration Influences Effective Radial Diffusion Coefficient in Canine Cortical Bone

Transport of nutrients and waste across osseous tissue is dependent on the dynamic micro and macrostructure of the tissue; however little quantitative data exists examining how this transport occurs across the entire tissue. Here we investigate in vitro radial diffusion across a section of canine tissue, at dimensions of several hundred microns to millimeters, specifically between several osteons connected through a porous microstructure of Volkmann’s canals and canaliculi. The effective diffusion coefficient is measured by a “sample immersion” technique presented here, in which the tissue sample was immersed in solution for 18–30 h, image analysis software was used to quantify the solute concentration profile in the tissue, and the data were fit to a mathematical model of diffusion in the tissue. Measurements of the effective diffusivity of sodium fluorescein using this technique were confirmed using a standard two-chamber diffusion system. As the solute concentration increased, the effective diffusivity decreased, ranging from 1.6 × 10⁻⁷ ± 3.2 × 10⁻⁸ cm²/s at 0.3 μM to 1.4 × 10⁻⁸ ± 1.9 × 10⁻⁹ cm²/s at 300 μM. The results show that there is no significant difference in mean diffusivity obtained using the two measurement techniques on the same sample, 3.3 × 10⁻⁸ ± 3.3 × 10⁻⁹ cm²/s (sample immersion), compared to 4.4 × 10⁻⁸ ± 1.1 × 10⁻⁸ cm²/s (diffusion chamber).