The diffusivity of water in cartilage can yield information regarding the structure of the tissue. In young bovine cartilage, the diffusivity of water in cartilage was found to be approximately 0.6 that of water in free solution. The diffusivity increased after trypsinization of the tissue, and decreased with tissue compression. These results indicate that the diffusion coefficient is sensitive to tissue hydration, a result directly observed in further compression studies of cartilage.

Furthermore, the diffusion coefficient of water in cartilage was not affected by going to pH 2 in tissue, which would titrate the charge groups to decrease the fixed charge in the tissue. In addition, the relative diffusivity of solute in cartilage relative to saline was similar for a number of anions and cations studies, consistent with the view that charge is not an important determinant of the intratissue diffusivity of small solutes in cartilage.

The results just described were obtained with an effective diffusion time of 13 ms, in other words, the water molecules were probed as to how far they moved within 13 ms. Further studies demonstrated that the diffusion coefficient was dependent upon the time allotted for diffusion (Figure 1). This implies that the diffusion of water is restricted by structures within the tissue on the time scale of the measurement. The shape of the curve of diffusivity versus diffusion time was not affected by the removal of GAG by trypsinization of cartilage, implying that the restrictions are due to collagen.

diffusivity
Figure 1: The diffusivity of water in cartilage is dependent upon the time allotted for diffusion to occur. This behavior implies that the diffusivity is "restricted" by structures within cartilage; the invariance of the curve with trypsinization implies that the restriction is due to the collagen component of tissue.

In these young cartilage samples, we did not note a dependence of the diffusivity with the direction of the diffusion encoding, in other words the diffusivity was isotropic. Further studies need to be done to further investigate the restriction to diffusion imposed by collagen, and whether diffusivity in adult cartilage is isotropic or anisotropic due to collagen orientation.

For more details, please refer to:

Burstein D, Gray ML, Hartman AL, Gipe R, Foy BD. Diffusion of small solutes in cartilage as measured by nuclear magnetic resonance (NMR) spectroscopy and imaging. J Orthop Res 1993; 11:465-478.