The influence of loading time and lubricant on the friction of articular cartilage.

Friction of cartilage on metal, metal on cartilage and cartilage on cartilage contact configurations, within a mixed lubrication regime, was measured using synovial fluid, Ringer's solution or with no lubricant present. The main test variable was the period of stationary loading which ranged from 5 s to 45 min, prior to sliding and consequently measuring friction. The coefficient of friction rose gradually with increasing stationary loading time, up to a value of approximately 0.3 at 45 min for all the contact configurations. Following the re-application of load, after short periods of load removal, friction was also found to drop sharply. The flow of liquid in the biphasic cartilage and load carriage by the fluid phase was highlighted as being an important factor in reducing friction within the mixed or boundary lubrication regime. Movement of the contact zone over the cartilage counterface ensured very low friction as the slider moved over fully hydrated cartilage. For the cartilage--cartilage contacts synovial fluid significantly reduced friction compared to Ringer's solution. This was attributed to an effective boundary lubrication action, which was not as effective for the cartilage--metal contacts.