The Influence of Probe Curvature on Articular Cartilage Friction Measurements

Abstract The potential lubrication mechanisms for articular cartilage remain as complex as the material itself. Many studies have examined systematically the effects of sliding velocity on the frictional response of cartilage, but a systematic analysis of probe curvature and contact area remains largely uncharted. Here, we explore this potential effect in three ways: a constant force experiment, a constant pressure experiment, and a nonconstant pressure and force experiment. Hard, impermeable spheres are used as probes and with sizes varied between 1.6 mm and 25.4 mm, allowing a sweep of physiologically relevant pressures. The aim is to effectively isolate the influence of probe curvature on the friction measurements. Articular cartilage samples were collected from horses at necropsy from the left and right distal and medial aspects of the radius. Each test was run for 5 min at a reciprocating sliding speed of 1 mm/s. Smaller probe diameters (and curvature) consistently resulted in high measured coefficients of friction of cartilage. Two lubrication regimes are observed during testing: an area-dependent regime at lower areas and an area-independent regime at higher areas. A power law (−1/3 to −1/2) relationship between friction coefficient and probe diameter is observed. Therefore, measurements made using different probe sizes will most likely result in different measurements of the friction behavior, and great care should be given when comparing these results.