a: © Polizeiinspektion Stade b: © Fa. Schaeffler d:  ©RWTH Aachen c und e: © Max-Planck Institut für Eisenforschung GmbH

Atomic level insights of chemical and structural mechanisms of tribological failure in hip implants

The interplay of mechanical loads and body fluids leads to local decomposition and surface alloying effects in the modular taper joints of hip implants.  We are investigating this engineering problem with state-of-the art correlative atom probe tomography and electron microscopy techniques.

Such an approach is necessary to understand the near atomic scale phenomena leading to chemical and structural evolution not only in hip implants, but also in dental and knee joints as well. Failure of implants in the body necessitate revision surgeries to replace prostheses, which carry a higher risk of complications and are a costly burden to healthcare services.

Analysis of an in-vitro Ti-6Al-4V/Co-Cr-Mo alloy couple which had undergone fretting damage shows a high degree of chemical redistribution of elements from both alloys, nano-scale substructures, microtexture and amorphous layers.  We discover that although the cobalt alloy is harder (and hence comparatively more wear resistant) than the titanium alloy, the cobalt alloy degrades due to tribocorrosion. This occurs due to a folding mechanism taking place on the titanium alloy surface during the fretting, which leads to the formation of raised shelves on its surface. These shelves microplough the cobalt alloy surface, thus accelerating tribocorrosion in the presence of biological fluids.

Figure: The titanium alloy hip stem used in total hip replacements, when coupled with a cobalt alloy femoral head, can degrade in the body due to fretting corrosion. From the atom probe tomography reconstruction of the subsurface of the titanium alloy (on the right), the MPIE scientists discovered that surface and subsurface oxides, fragmented titanium particles, the constituent elements of the cobalt alloy (cobalt, chromium) and the interfacial medium (chlorine) are incorporated.
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