Articular cartilage is a highly hydrated fibre composite material that provides a resilient, lowfriction bearing surface covering bones where they articulate. The literature suggests that the tissue becomes increasingly elastic, less viscoelastic, as the loading rate increases, i.e. that hysteresis, the energy lost between loading and unloading, will decrease with increasing strain-rate. Here we show, using a controlled impact, that hysteresis increases with strain rate. No fluid was lost during the deformation and the ratio of the radial to the axial strains, Poisson's ratio, measured using highspeed video recording, increased as the tissue was deformed, starting close to zero and tending towards that for an isovolumetric deformation. The decreasing coefficient of restitution, a measure of the hysteresis, was modelled using a non-linear viscoelastic element, as a first approximation. These results indicate that the tissue remains viscoelastic with increasing strain rate, dissipating energy which might otherwise generate cracks in the matrix.

The mechanical properties of articular cartilage vary enormously with loading rate, and how these properties derive from the composition and structure of the tissue is still unclear. This study investigates the mechanical properties of human articular cartilage at rapid rates of loading, compares these with measurements at slow rates of loading and explores how they relate to the gross composition of the tissue. Full-depth femoral head cartilage biopsies were subjected to a slow, unconfined compression test followed by an impact at an energy of 78.5mJ and velocity 1.25ms(-1). The modulus was calculated from the slope of the loading curve and the coefficient of restitution from the areas under the loading and unloading curves. Tissue composition was measured as water, collagen and glycosaminoglycan contents. The maximum dynamic modulus ranged from 25 to 150MPa. These values compared with 1-3MPa measured during quasi-static loading. The coefficient of restitution was 0.502 (0.066) (mean (standard deviation)) and showed no site variation. Water loss was not detectable. Composition was not strongly associated with modulus; water and collagen contents together predicted about 25% of the variance in modulus.

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