In Vivo Corrosion of Acetabular Modular Tapers Reduces Local Corrosion Resistance: Near‐Field Electrochemical Impedance Spectroscopy as an Indicator of the Type and Severity of Corrosion in Retrieved Hip Implants

ABSTRACT Retrieved hip implant components experience a range of corrosion types and severity, often on small surface areas of the taper region. Tribocorrosion‐related damage within modular taper regions and resulting changes of the oxide‐covered surface can alter the local impedance of the interface. We hypothesize that near‐field electrochemical impedance spectroscopy (nfEIS), where local area impedance measurements can be made, can serve as an excellent method to distinguish different forms and severity of tribocorrosion degradation in localized regions of the implant surface. Impedance measurements can also help determine subsequent corrosion susceptibility. Recent studies have shown that EIS on a localized, or global basis, is a useful tool to ascertain the corrosion resistance and extent of oxide alteration in retrieved hip implant components. Hence, the goal of this study was to systematically assess different types of corrosion modes found on the modular taper surfaces of retrieved acetabular liners using nfEIS and to correlate the resulting local impedance response with visually identified different corrosion modes. Utilizing an easy‐to‐manufacture microelectrode on wrought CoCrMo liners, we found that local impedance measurements are a good indicator of differences it forms of corrosion‐related damage. nfEIS measurements captured damage‐specific differences in the impedance response that were unique to specific types and severity of corrosion. We could describe the surfaces in terms of equivalent circuit models consisting of resistive and capacitive (or constant phase) elements. Intergranular corrosion (IGC) and oxide deposits were found to match a coated model behavior, with a characteristic double‐hump phase angle response. Phase boundary corrosion (PBC) and control (polished CoCrMo disk) were found to match a constant‐phase‐element Randle's circuit model with a mostly intact surface and minimal to no material loss. We found that polarization resistance, where higher R p indicates greater corrosion resistance, correlated with the intactness of the surface: greater material loss on the surface translated to lower R p values (IGC sample R p = 8.46E6 Ω/cm 2 compared to PBC sample R p = 5.79E7 Ω/cm 2 ). Our findings demonstrate the versatility of this technique to analyze different types of retrieved device surfaces to make objective determinations in terms of the nature of the corrosion damage and the surface's continued corrosion susceptibility. Further developing this technique and testing it on a variety of tribocorrosion features and different biomedical alloys will help ascertain its applications in retrieval analysis and may assist in determining the extent of device damage at revision surgery.