The dramatic influence of loading velocity on the compressive response of articular cartilage.

Experiments were carried out to investigate the influence of loading velocity on the stiffness of the articular cartilage matrix. Compression tests were conducted on cartilage alone and cartilage-on-bone at strain-rates ranging from 10(-5)sec-1 to 10(3)sec-1 and it was established that matrix stiffness increased progressively in the "low" and "medium" strain-rate regimens and assumes a limiting value at "high" rates of loading up to impact. Analysis of the strain field characteristics associated with the compression process, both at low and high velocities, suggests that two fundamentally different mechanisms of deformation control the development of cartilage matrix stiffness. At low strain-rates a consolidation-dependent stiffness occurs while at high strain rates the high stiffness results from a classical elastic deformation process. This bifurcation in the tissue's response to loading is likely to affect the redistribution of joint contact stresses being transmitted into the subchondral bone.