Considering articular cartilage to be composed of an elastic organic matrix and interstitial water, compressional stiffness of canine knee (stifle) joint cartilage was determined in the form of shear moduli. Different stresses and indenters were used to reveal the suitability of the indentation measurement with a step-load. We were able to demonstrate the linearity of instant and equilibrium responses of cartilage. Using a theoretical solution for indentation of an elastic layer, the effect of the finite thickness of cartilage on the instant and equilibrium responses was eliminated. Using the intrinsic (equilibrium) shear modulus of the cartilage matrix, the stiffest cartilage was found to be located in the patellar surface of the femur. The thick, proteoglycan-rich tibial cartilage, which was uncovered by menisci, was softer than the opposing femoral cartilage. This finding casts doubt on the suggestion that the concentration of proteoglycans unambiguously determines the stiffness of cartilage, but emphasizes the importance of the collagen network also in the compressional behaviour of the cartilage.
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