Nature of fretting corrosion products in CoCrMo hip implants from in-vivo study to in-vitro simulation

CoCrMo alloys have been widely used in total hip implants because of their excellent mechanical properties and corrosion resistance. Unfortunately, in the past decade, adverse local tissue reactions (ALTRs) arising from fretting corrosion products of CoCrMo alloys used in hip implants affected a significant number of patients. Despite their importance to the corrosion process and ALTRs, the chemical nature and formation mechanisms of these fretting corrosion particles have not been clearly identified. In this study, we first analyzed the chromium rich corrosion particles retrieved from patients with failed CoCrMo hip implants. We further carried out in vitro fretting experiments to study their nature and formation process. The results showed that the fretting corrosion particles in vivo were mainly amorphous nanoparticles with varying chromium and phosphate contents. Similar particles could be generated in the laboratory fretting tests between CoCrMo and Ti alloy only in simulated body fluid containing P and Ca. Volume loss of CoCrMo during fretting tests also strongly depended on the type of the testing fluid. Volume loss of CoCrMo was almost ten time higher in 0.9% saline solution than phosphate buffered solution and the simulated body fluid. However, such a big difference was not observed in the absence of fretting based on corrosion current densities derived from polarization dynamics. This study clarified the formation process of the fretting particles from CoCrMo hip implants and established proper particle processing protocols for future mechanistic studies at both cellular and tissue levels.