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Implant modularity has become a primary implant design concept used in total hip replacement procedures. However, over the last 5 years evidence of corrosion attack in taper crevices formed at the junction of these components has come to light from retrieval analyses. The goals of this paper are to summarize the results of retrieval studies and to discuss the material properties and the mechanical and electrochemical processes which are hypothesized to govern the corrosion attack observed. This paper will review several test methodologies developed by the authors to evaluate aspects of the proposed mechanism of mechanically assisted crevice corrosion. Retrieval studies have shown that both mixed-alloy (Ti-6Al-4V stems and Co-Cr-Mo heads) and similar-alloy (Co-Cr-Mo stems and Co-Cr-Mo heads, and Ti-6Al-4V stems and Ti-6Al-4V heads) couples demonstrate corrosion attack in-vivo. These results indicate that this corrosion process is not due solely to the mixing of dissimilar metals but may occur as a result of mechanical-electrochemical interactions in the taper crevice. Oxide film fracture due to mechanical fretting and the restricted crevice environment of the taper combine to cause changes in the solution chemistry of the fluid inside the taper including pH drops and increases in chloride concentrations. Furthermore, the potential of the implant can become significantly more negative causing the oxide film which reforms to be thinner than the original film and moves the implant's potential toward the active range for corrosion attack. The role of solution chemistry, sample potential and the mechanical tenacity of the oxide films and the test methods used to evaluate these factors will be discussed as they effect the corrosion attack observed.
Modularity, corrosion, crevice, fretting, oxide films, Titanium, Cobalt, Chromium
Associate Professor, Northwestern University, Chicago, IL
Associate Professor, Rush Presbyterian St. Lukes Medical Center, Chicago, IL