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. This article evaluates how, by incorporating real-time corrosion measurements in total hip replacement simulations, pre-clinical evaluations and research studies can both yield much more information and accelerate the process towards improved 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. The film itself was rich in carbon, and appeared to contain no long range graphitic ordering when analysed with electron energy loss spectroscopy, and the spectra gathered from the tribofilm closely resembled those collected from amorphous carbon. The location at which the most substantial tribological layers formed may be explained by the formation of a blunt wedge at the edge of the wear scar following conformal changes to the bearing surfaces.