A physical model for the time-dependent deformation of articular cartilage.

A physical analogue was developed to simulate the time-dependent deformation of articular cartilage. The analogue was constructed from a matrix of water-saturated sponge material whose permeability could be varied, and was constrained so as to allow one-dimensional deformation under both static and dynamic compressive loading. Simultaneous measurements were made of the applied stress, matrix excess pore pressure and matrix strain. The results obtained reinforce the view that under static and low strain-rate loading conditions, a consolidatable system like cartilage sustains the applied stress through a stress-sharing mechanism between matrix water and the solid skeleton. However, at high strain-rates load-bearing is dominated by a mechanism in which the matrix water is immobilized and the excess pore pressure rises to almost that of the applied stress, thus suggesting that the constituents of the matrix act as a single functional entity to support the applied load. The model supports the description of cartilage as a poro-visco-hyperelastic material.