The geometry, friction and lubrication of normal human ankle
joints have been investigated. The joints exhibited converging diverging surfaces in the direction of motion. The cylindrical form of the measured surface contours indicated that a reduced radius of about 0.35 m gave a good representation of the ankle joint geometry.
Human ankle joint specimens were tested in a joint simulator. Although considerable difficulties were encountered in the measurement of the very small coefficient of friction between the cartilage surfaces, an upper limit of about 0.01 was identified for this important tribological feature of synovial joints.
An equivalent bearing to represent the ankle joint was proposed which consisted of a rigid cylinder covered with a compliant layer sliding on a rigid plane. The dimensions for this geometry were based on the measurements of the present study. Theoretical models were developed to estimate the cyclic variation in elastohydrodynamic
film thickness and coefficient of friction for the
ankle during walking.
Theoretical minimum film thicknesses of about 1 um were
estimated along with coefficients of friction up to 0.001. The theoretical predictions of the cyclic variation of film thickness remained small compared with the magnitude of the film thickness itself. Furthermore, the theoretical film thicknesses were smaller than the measured Ra roughnesses for cartilage which appear in the literature. When a very considerable increase in the bulk viscosity of the lubricant was introduced into the calculations film thicknesses of about 18 um and coefficients of friction up
to 0.01 were estimated. This value for film thickness was
sufficient to separate the surface asperities of healthy articular cartilage.
Unless thin film mechanisms, such as an increased lubricant
viscosity or micro-elastohydrodynamic lubrication act, the present study indicated that full fluid film lubrication cannot be sustained. However, the predicted film thicknesses were not much smaller than the surface roughness of cartilage and the ability to generate and preserve fluid films was found to be greatly enhanced by the entraining and squeeze film action. Thus, the modes of lubrication for normal human ankle joints must include a significant contribution from elastohydrodynamic
lubrication.