Paper XII(ii) An analysis of micro-elasto-hydrodynamic lubrication in synovial joints considering cyclic loading and entraining velocities

Attention has been drawn to the powerful role of micro-elasto-hydrodynamic action in the lubrication of synovial joints under quaisi-static conditions in a previous paper (7). The findings from this earlier analysis did much to reconcile the conflicting indications of experimental and theoretical studies of synovial joint lubrication extending over many years. Experimental investigations increasingly suggested that natural synovial joints experienced fluid-film lubrication, whereas the theories failed to confirm the formation of such films. It has now become necessary to extend the micro-elastohydrodynamic analysis to conditions more representative of physiological motion and loading and this is the subject of the present paper. The results of this extended analysis confirm that, even under the dynamic conditions representative of the walking cycle, the effective film thickness remains remarkably constant and that micro-elasto-hydrodynamic action remains as a powerful and highly beneficial action in synovial joints. A new feature of the solutions for cyclic conditions is the partial re-emergence of the surface rugosities at instants when entraining action is very small and squeeze-film action predominates. This is evident in the vicinity of the central dimple which is known to form under elasto-hydrodynamic squeeze film action between smooth surfaces. The current study has provided further confirmation of the major role played by micro-elasto-hydrodynamic lubrication in synovial joints. The model adopted is still relatively simple and it may be necessary to give further attention to lubricant rheology in order to bring theory and experiment fully into accord in relation to joint friction. However, the imposition of fully dynamic conditions has not impaired the essential indications of quasi-static micro-elastohydrodynamic analysis of synovial joints reported earlier.