Changes in pore morphology and fluid transport in compressed articular cartilage and the implications for joint lubrication.
ABSTRACT Cartilage sections were cut from the middle zone of pig knee articular cartilage and attached to substrates in two different kinds of newly designed 'pressure cells', one for fluorescence the other for NMR measurements. The fluorescence cell was filled with buffer solution containing fluorescently marked 70 kDa dextran which was allowed to diffuse into the cartilage pores. A second glass surface was then pressed down onto the thin cartilage sample under different loads (pressures), and the resulting compression (strain) and change in pore volume were measured as a function of time, simultaneously with measurements of the lateral diffusion and flow pattern of the dextran molecules using Fluorescence Recovery After Photobleaching (FRAP). Complementary experiments were made on the normal diffusion coefficients of pure electrolyte solutions (no dextran) in thicker cartilage sections with pulse-gradient NMR using a new pressure cell suitable for such measurements. Taken together our results show that the highly anisotropic structure of cartilage has a strong effect on the way fluid diffuses laterally and normally at different stages of compression. Our results also show how geometric constraints on a cartilage network and trapped high MW polymer such as HA during normal compressions are likely to affect both the normal and the lateral mobilities of polyelectrolytes and water.
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ABSTRACT: Nuclear magnetic resonance (NMR) spin-lattice relaxation rates were measured in bovine and porcine articular cartilage as a function of water content. Water content was varied by freeze-drying samples for short periods of time (up to 15 min). The samples were weighed at all stages of drying so that water content could be quantified. Spin-lattice relaxation rates were measured using magnetic resonance imaging (MRI). Linear correlations were observed between relaxation rate and two measures of inverse water content: (1) solid-to-water ratio (ρ), expressed as a ratio of the mass of the solid component of the cartilage (m(s)) and the mass of water at each freeze-drying time point (m(w)), and (2) a ratio of the total mass of the fully-hydrated cartilage and m(w) (1/w). These correlations did not appear significantly different for the bovine and porcine data. However, fitting the data to a piecewise-linear model revealed differences between these two species. We interpret the first two segments of the piecewise model as the depletion of different water phases but conjecture that the third segment is partially caused by changes in relaxation rates as a result of a reduction in macromolecular mobilities. Whilst we can produce linear correlations which broadly describe the dependence of the measured spin-lattice relaxation rate on (inverse) water content, the linear model seems to obscure a more complicated relationship which potentially provides us with more information about the structure of articular cartilage and its extracellular water.Osteoarthritis and Cartilage 12/2011; 20(2):184-90. · 3.90 Impact Factor