The deformation occurring in the articular cartilage covering the human femoral head has been measured both when the femoral head is loaded in its natural acetabulum and when the cartilage is loaded with a small indentor. The results indicate that the material response is substantially different in these two situations. In the intact joint the cartilage deformation is substantially greater in older joints, but the response of cartilage to loading with an indentor does not change significantly with age. Theoretical elastic models of the cartilage behavior in these two situations were analyzed. For old cartilage which is idealized as an elastic material the increased deformation which is observed in the intact joint can be attributed to changes in Poisson’s ratio, though in the real material increased fluid flux under load is the more probable cause.

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