Balance between swelling pressure and collagen tension in normal and degenerate cartilage

ARTICULAR cartilage contains a high concentration of acid glycosaminoglycans (GAG), reaching 6% by wet weight and associated with fixed charge densities up to 0.2 mEq g−1. This leads to considerable swelling pressure within cartilage, due to, first, the strongly non-ideal osmotic pressure, characteristic of polymer solutions, which increases sharply with concentration and, second, the ionic contribution, in accordance with the Gibbs-Donnan equilibrium. Ogston and Wells1–3 have estimated the values of these two components and I have calculated them from my experimental data on cartilage4,5. The two components of swelling pressure are approximately of the same order of magnitude and can reach values as high as 1.7kgcm−2 (refs 4 and 5). Since normal cartilage does not swell in solution, even when it is removed from the joint and cut into thin (250 µm) slices (lowest curve, Fig. 1), this implies that its high swelling pressure must be counteracted by considerable elastic forces within the collagen fibre network. It has been known for some time that the concentration of GAG gradually increases from the articular surface to the deep zone (for example, refs 6 and 7). A typical variation of total GAG content with depth, measured as fixed charge density, is shown in Table 1. I now suggest that this particular profile is adapted to the physiological function and mechanical properties of cartilage.