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 x 10(-7) +/- A 3.2 x 10(-8) cm(2)/s at 0.3 mu M to 1.4 x 10(-8) +/- A 1.9 x 10(-9) cm(2)/s at 300 mu M. The results show that there is no significant difference in mean diffusivity obtained using the two measurement techniques on the same sample, 3.3 x 10(-8) +/- A 3.3 x 10(-9) cm(2)/s (sample immersion), compared to 4.4 x 10(-8) +/- A 1.1 x 10(-8) cm(2)/s (diffusion chamber).