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Modelling human hip joint lubrication subject to walking cycle

This paper deals with the study of a hip joint prosthesis subject to a walking cycle. Human joints are lubricated by synovial fluid (SF). To properly model joint performance, one must take into account the behaviour of this fluid, which is highly viscoelastic as confirmed in early experimental work. The Phan‐Thien and Tanner (PTT) model is one of the most widely used rheological models. It can properly describe the common characteristics of viscoelastic non‐Newtonian fluids. Based on a microstructural description of the fluid, it is possible to establish laws of rheological behaviour to reproduce shock or the sudden inception of shear, taking into account large deformation and viscoelastic effects. Modelling the geometry of the total hip replacement, the PTT model is applied in spherical coordinates to a thin confined fluid film. Using a modified Reynolds equation developed for this specific geometry, we present a numerical simulation of an artificial hip joint subject to a walking cycle. The equilibrium position depends on the load magnitude and also depends strongly on the load direction. The non‐linear response of the SF leads to some unanticipated complexities. In our case, the dominant effect is the squeeze action between the surfaces. The contact behaviour depends not only on the geometry and kinematics but also on the fluid properties. The minimum film thickness remains higher than likely roughness height during these ‘normal’ conditions. When considering the design of hip joint prostheses, attention should be paid to the rheological properties of SF. Copyright © 2008 John Wiley & Sons, Ltd.