Compressive and tensile properties of articular cartilage in axial loading are modulated differently by osmotic environment

Aims of the present study were to test the hypotheses that (1) the compressive properties of articular cartilage are affected more by changes in the medium ionic concentration than the tensile properties, (2) collagen network controls the compression–tension nonlinearity of articular cartilage, and (3) proteoglycan (PG) and collagen contents are primary determinants of the compressive and tensile properties of cartilage, respectively. These hypotheses were experimentally tested by axial compressive and tensile tests (perpendicular to the cartilage surface) of bovine articular cartilage samples immersed in 0.005 M (n = 6), 0.15 M (n = 12) and 1.0 M (n = 6) saline solutions. Compressive and tensile behaviour was analyzed by a nonlinear fibril-reinforced poroelastic model. Tissue PG and collagen contents were measured using Fourier transform infrared imaging spectroscopy (FT-IRIS). The compressive modulus of cartilage varied significantly (n = 6, p < 0.05) as the medium concentration changed. The tensile modulus changed significantly only as the medium concentration was reduced from 0.15 to 0.005 M (n = 6, p < 0.05). The fibril-reinforced poroelastic model with stiff, nonlinear collagen fibrils predicted the experimentally measured compression–tension nonlinearity of cartilage. Tissue PG and collagen contents accounted for the compressive and tensile properties of cartilage.