One of the aims of modeling articular cartilage is to determine its ability to sustain its physiological loading environment and the mechanisms by which this functional response might be compromised. It has long been hypothesized that excessive stresses in cartilage might initiate osteoarthritis, thus a determination of the state of stress within the tissue has been an important objective. It has also been hypothesized that the interstitial water of articular cartilage plays a primary role in producing low friction and wear as well as shielding the collagen-proteoglycan solid matrix from a significant portion of the loads applied across joints [1]. Furthermore, cartilage exhibits inhomogeneity through its depth, both in its tensile and compressive properties, though the significance of this inhomogeneity on the functional response of cartilage remains to be elucidated. The specific aims of the current study are to (a) experimentally determine the depth-dependent tensile and compressive properties of human patellar articular cartilage; (b) determine the response of cartilage to loading under a contact configuration using finite element models which employ these experimentally determined material properties; and (c) compare the response of the tissue to a hypothetical homogeneous distribution of material properties through the depth. The first hypothesis is that the inhomogeneity of articular cartilage acts to maximize the interstitial fluid load support at the articular surface. The second hypothesis is that the depth-dependent inhomogeneous distribution of cartilage properties acts to produce a more homogeneous state of stress than would be achieved had the properties been constant through the depth. This study extends our previous contact analyses of homogeneous cartilage layers [2,3].

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