Diffusion of small solutes in cartilage as measured by nuclear magnetic resonance (NMR) spectroscopy and imaging

Abstract

The ability of water and solutes to move through the cartilage matrix is important to the normal function of cartilage and is presumed to be altered in degenerative diseases of cartilage such as osteoarthritis and rheumatoid arthritis. Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) techniques were used to measure a self diffusion coefficient (D) for small solutes in samples of explanted cartilage for diffusion times ranging from 13 ms to 2s. With a diffusion time of 13 ms, the intratissue diffusivity of several small solutes (water, Na⁺, Li⁺, and CF₃CO₂⁻) was found consistently to be about 60% of the diffusivity of the same species in free solution. Equilibration of the samples at low pH (which titrates the charge groups so that the net matrix charge of −300 mM at pH 8 becomes approximately −50 mM at pH 2) did not affect the diffusivity of water or Na⁺. These data, and the similarity between the D in cartilage relative to free solution for water, anions, and cations, are consistent with the view that charge is not an important determinant of the intratissue diffusivity of small solutes in cartilage. With 35% compression, the diffusivity of water and Li⁺ dropped by 19 and 39%, respectively. In contrast, the diffusivity of water increased by 20% after treatment with trypsin (to remove the proteoglycans and noncollagenous proteins). These data and the lack of an effect of charge on diffusivity are consistent with D being dependent on the composition and density of the solid tissue matrix. A series of diffusion-weighted proton images demonstrated that D could be measured on a localized basis and that changes in D associated with an enzymatically depleted matrix could be clearly observed. Finally, evidence of restriction to diffusion within the tissue was found with studies in which D was measured as a function of diffusion time. The measured D for water in cartilage decreased with diffusion times ranging from 25 ms to 2 s, at which point the measured D was roughly 40% of the diffusivity in free solution. Although changes in matrix density by compression or digestion with trypsin led to a decrease or increase, respectively, in the measured D, the functional change in measured diffusivity with diffusion time remained essentially unchanged. In a different type of study, in which bulk transport could be observed over long periods of time, cartilage was submerged in 99% D₂O and MRI studies were performed to demonstrate the bulk movement of water out of the cartilage matrix. These studies yielded a diffusivity estimated to be approximately 40% of the diffusivity of water in solution, which is in agreement with the value obtained for the diffusivity at a diffusion time of 2 s. These NMR measurements of diffusion can be totally noninvasive; thus, the results reported here can be extended to in vivo situations.