Complex nature of stress inside loaded articular cartilage.

We show that in the early stages of loading of the cartilage matrix extensive water exudation and related physicochemical and structural changes give rise to a distinctly consolidatable system. By enzymatically modifying the pre-existing osmotic condition of the normal matrix and measuring its hydrostatic excess pore pressure, we have studied the exact influence of physicochemistry on the consolidation of cartilage. We argue that the attainment of a certain minimum level of swelling stiffness of the solid skeleton, which is developed at the maximum hydrostatic excess pore pressure of the fluid, controls the effective consolidation of articular cartilage. Three related but distinct stresses are developed during cartilage deformation, namely (1) the swelling stress in the coupled proteoglycan/collagen skeleton in the early stages of deformation, (2) the hydrostatic excess pore pressure carried by the fluid component, and (3) the effective stress generated on top of the minimum value of the swelling stress in the consolidation stages following the attainment of the fluid's maximum pore pressure. The minimum value of the swelling pressure is in turn generated over and above the intrinsic osmotic pressure in the unloaded matrix. The response of the hyaluronidase-digested matrix relative to its intact state again highlights the important influence of the osmotic pressure and the coefficient of permeability, both of which are related to the volume fraction of proteoglycans on cartilage deformation, and therefore its ability to function as an effective stress-redistributing layer above the subchondral bone.