Fluid film lubrication in natural hip joints

The film thickness in the normal human hip joint has been predicted under steady-state entraining motion, pure dynamic squeeze-film motion and the transient condition involving both the entraining and squeeze-film motions. The simplified method adopted to predict the film thickness has been compared with the full numerical method under both steady-state entraining and transient conditions for the normal human ankle joint and good agreement has been found. The transient film thickness predicted for the hip joint is almost constant throughout one walking cycle due to the combined effect of entraining during the swing phase, generating a sufficiently large lubricant film and squeeze-film action preserving this lubricant film during the stance phase. However, the lubricant film thickness predicted is only of the order of 1.5 (μm) and thus it seems clear that micro-elastohydrodynamic action also plays as important a part in the lubrication of hip joints as it was found in a previous study to have in the ankle joint.