COMPRESSIVE BEHAVIOR OF ARTICULAR-CARTILAGE IS NOT COMPLETELY EXPLAINED BY PROTEOGLYCAN OSMOTIC-PRESSURE

It has been hypothesized that applied mechanical or osmotic loads whic h decrease cartilage volume by 5% or more are sufficient to relieve al l collagen tensile forces, and that further changes in the applied loa d are completely supported by changes in proteoglycan osmotic pressure . In this view. cartilage should behave mechanically like a concentrat ed solution of proteoglycans. We tested this hypothesis by measuring t he equilibrium axial and radial stresses in bovine articular cartilage during uniaxial confined compression. If the hypothesis is correct, t he observed changes in the radial and axial stresses in confined compr ession should be equal for compression greater than 5%. However, the o bserved change in axial stress was always substantially greater than t he change in radial stress over the range of strains (5-26%) and salin e concentrations (0.05-0.15 M) tested. This indicates that the mechani cal behavior of cartilage in confined compression cannot solely be exp lained by changes in proteoglycan osmotic pressure even for strains as large as 26%. A linear isotropic model was found to describe the obse rved equilibrium behavior adequately. In addition, the inferred shear modulus was found to be independent of saline concentration and simila r to measurements by others of the flow-independent shear modulus. Our results have implications regarding the relative contribution of the proteoglycans and collagen to the mechanical properties of the tissue in compression, and suggest that tensile forces in the collagen networ k may play an important role in determining tissue behavior in confine d compression even for relatively large volume changes. (C) 1997 Elsev ier Science Ltd.