The negatively charged proteoglycans (PG) provide compressive resistance to articular cartilage by means of their fixed charge density (FCD) and high osmotic pressure (pi(PG)), and the collagen network (CN) provides the restraining forces to counterbalance pi(PG). Our objectives in this work were to: 1), account for collagen intrafibrillar water when transforming biochemical measurements into a FCD-pi(PG) relationship; 2), compute pi(PG) and CN contributions to the compressive behavior of full-thickness cartilage during bovine growth (fetal, calf, and adult) and human adult aging (young and old); and 3), predict the effect of depth from the articular surface on pi(PG) in human aging. Extrafibrillar FCD (FCD(EF)) and pi(PG) increased with bovine growth due to an increase in CN concentration, whereas PG concentration was steady. This maturation-related increase was amplified by compression. With normal human aging, FCD(EF) and pi(PG) decreased. The pi(PG)-values were close to equilibrium stress (sigma(EQ)) in all bovine and young human cartilage, but were only approximately half of sigma(EQ) in old human cartilage. Depth-related variations in the strain, FCD(EF), pi(PG), and CN stress profiles in human cartilage suggested a functional deterioration of the superficial layer with aging. These results suggest the utility of the FCD-pi(PG) relationship for elucidating the contribution of matrix macromolecules to the biomechanical properties of cartilage.
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