The temporal response of the friction coefficient of articular cartilage depends on the contact area.

The hypothesis of this study is that the time constant for the transient increase in friction coefficient of articular cartilage under a constant load is proportional to the size of the contact area, as predicated by the dependence of the frictional response on interstitial fluid pressurization. This hypothesis is verified experimentally from measurements of the frictional response of bovine articular cartilage disks of three different diameters (4, 6 and 8mm) against glass. At two different applied stresses (0.127 and 0.254 MPa), the coefficient of determination of a linear regression of the time constant versus the contact area yielded R(2) = 0.892 and R(2) = 0.979 (p < 0.001). The results of this study provide a cogent explanation for the expectation that the friction coefficient in situ will not achieve the elevated equilibrium values observed under common testing conditions.