Incorporating corrosion measurement in hip wear simulators: An added complication or a necessity?

Corrosion is not routinely considered in the assessment of the degradation or the lifetime of total hip replacement bearing surfaces. Biomechanical simulations are becoming ever more complex and are taking into account motion cycles that represent activities beyond a simple walking gait at 1 Hz, marking a departure from the standard ISO BS 14242. However, the degradation is still very often referred to as wear, even though the material loss occurs due to a combination of tribological and corrosion processes and their interactions.

Predictive wear modeling of the articulating metal-on-metal hip replacements.

The lubrication regime in which artificial hip joints operate adds complexity to the prediction of wear, as the joint operates in both the full fluid film regime-specifically the elastohydrodynamic lubrication (EHL) regime-and the mixed or boundary lubrication regimes, where contact between the bearing surfaces results in wear. In this work, a wear model is developed which considers lubrication for the first time via a transient EHL model of metal-on-metal hip replacements. This is a framework to investigate how the change in film thickness influences the wear, which is important to further investigation of the complex wear procedure, including tribocorrosion, in the lubricated hip implants.

The composition of tribofilms produced on metal-on-metal hip bearings.

Following wear testing in a hip simulator, the bearing surfaces of 36 mm metal on metal total hip replacements showed the formation of tribochemical layers. These layers were investigated in a transmission electron microscope, and analysis was performed using electron energy loss spectroscopy, energy dispersive x-rays and selected area electron diffraction. The tribofilm formed at the edge of the wear scar was 100 s of nanometres thick and contained cobalt sulphide particles embedded within.

Bio-tribology.

It is now forty six years since the separate topics of friction, lubrication, wear and bearing design were integrated under the title 'Tribology' [Department of Education and Science, Lubrication (Tribology) Education and Research. A Report on the Present Position and Industry's Needs, HMSO, London, 1966]. Significant developments have been reported in many established and new aspects of tribology during this period.