The Velocity and Compressive Strain Dependence of the Cartilage Equilibrium Friction Coefficient

Understanding the lubrication of diarthrodial joints has been an important objective in the field of biomechanics since the early part of this century. Many experiments have been performed to measure the cartilage friction coefficient, either on entire joints or on tissue explants. Some of these studies have demonstrated that the friction coefficient of cartilage is time-dependent following a step change in the load applied across the articular surfaces (e.g., McCutchen, 1962; Malcom, 1976; Wright and Dowson, 1976). It has been proposed that this time-dependence is directly related to the exudation of cartilage interstitial water during tissue creep (McCutchen, 1962; Malcom, 1976). When the tissue has reached its equilibrium creep deformation, i.e., when the interstitial fluid pressure has reduced to zero (Mow et al., 1980), the friction coefficient has been shown experimentally to also achieve an equilibrium value (Malcom, 1976; Wright and Dowson, 1976). The objective of the current study was to determine the dependence of this equilibrium friction coefficient on sliding velocity and tissue compressive strain under steady frictional shearing conditions, in order to better understand the frictional response of cartilage in the absence of interstitial fluid pressurization.