Relationship between the conformity and the lubricating ability of synovial joints

Objective. To relate conformity of sliding surfaces with the lubricating ability of synovial joints. Design. Measurement of start-up friction in the stifles of various animals. Assessment of conformity by Hertzian contact area. Background. Past studies showed that the start-up friction in synovial joints sharply increased with the loading duration. The reasons why the friction increased and why the increasing rate is different in different joints were, however, not found. Methods. Nine stifle joints from various animals were used. A robotic arm was used to give the compressive force and the sliding motion to the joint. Start-up friction was measured by a universal force sensor. The principal curvatures of the sliding surfaces were directly measured by a radius-gauge. Hertzian contact area was calculated from the principal curvatures of the sliding surfaces. Results. The duration until the frictional coefficient reached 0.1 was related to the Hertzian contact area. Conclusion. The conformity of sliding surfaces is related to the lubrication ability in synovial joints. The squeeze-film mechanism plays an important role in joint lubrication.