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2 International Journal of Applied Mechanics and Engineering

2 2002

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Lubrication analysis of metal-on-metal hip joint prostheses in a free pendulum machine

Udofia I. J., Jin Z. M.

International Journal of Applied Mechanics and Engineering

2002

Vol. 7, no 3

1015-1024

Total joint arthroplasty is probably the most effective treatment for severe degenerative bone diseases such as osteo-arthritis. There has been renewed interest in the use of hard bearings, particularly the metal-on-metal combination for hip joint replacements due to the osteolysis problem associated with current metal-on-polyethylene prostheses. The potential for full fluid film lubrication has been shown to exist during part of the walking cycle in these hard bearing joints. In this study, theoretical and numerical analyses were carried out to characterise the loading and velocity and the corresponding lubrication regime in a typical metal-on-metal hip joint replacement tested in a free pendulum machine. It was shown that the loading in the joint remains relatively constant throughout the duration of swing. The maximum angular velocity achieved under frictionless conditions with an initial angular displacement of 5o, was found to be about 0.387 rad/s, corresponding to a sliding velocity of 5.412 mm/s for a femoral head radius of 14mm. The lubricating film thickness due to the entraining action is only 0.011 micro m, which implies at best a mixed lubrication regime is experienced for the metal-on-metal hip joint tested in a pendulum machine with an average surface roughness of about 0.01 micro m. Under squeeze-film motion, the lubricating film thickness decreases to only about 0.01 micro m after the first few swings, assuming an initial separation between the head and the cup by the radial clearance. The theoretical predictions are consistent with experimental observations reported in literature. The lubrication regime under more realistic walking conditions cannot be deduced in the pendulum machine, largely due to the inappropriate loading and velocity conditions experienced in the pendulum.

Contact mechanics of a graded layered bearing surface for knee prosthesis

Virdee S. S., Jin Z. M.

International Journal of Applied Mechanics and Engineering

2002

Vol. 7, no 3

1025-1032

Recent studies have shown that a full fluid film lubrication regime can be developed in artificial knee joint replacements under simulated steady-state walking conditions by employing a thin compliant bearing surface with an elastic modulus similar to that of natural articular cartilage (Unsworth et al., 1988); Auger et al., 1993). The compliant bearing surface is usually bonded to a relatively stiff substrate in order to provide the structure support. Due to the abrupt change in stiffness, high interfacial shear strain is developed between the compliant layer and the stiff substrate and de-bonding usually occurs. The de-bonding problem can be expected to be alleviated to a certain extent by using functionally graded materials with variable stiffness. The purpose of this study was to analyze the contact mechanics of these functionally graded materials, with particular reference to knee prostheses. The finite element method was used to predict the contact parameters including the predicted contact pressure at the bearing surfaces and the interfacial shear strain between the compliant layer and the polymeric substrate.