Analysis of the electrochemical behaviour of CoCrMo alloy hip heads after revision and subsequent solution annealing

Bojan Mitevski, Universität Duisburg-Essen, ITM, Lehrstuhl für Werkstofftechnik, Duisburg, Germany
Alfons Fischer, Universität Duisburg-Essen, ITM, Lehrstuhl für Werkstofftechnik, Duisburg, Germany

Corrosion of CoCrMo alloy in modular junctions of total hip replacements (THR) was identified as a major contributor to premature implant failure. Metal ions and debris were shown to cause adverse local tissue reactions in many cases which can cause pain to the patient and most often lead to revision surgery. Thus, it is important to find ways to minimize ion release due to the corrosive process in the modular junction. It is well known that optimal corrosion properties can be achieved by a controlled heat treatment solution annealing of the CoCrMo alloy. The microstructure of CoCrMo alloys is characterized by its grain size and the amount and type of hard phases (carbides, intermetallics). THRs can exhibit at least four different hard phase types: (A) intermetallic phases, (B) coarse mixed hard phases (MHP) along the grain boundaries, (C) localized fine MHP on the grain boundaries and (D) interdendritic MHPs. It was the hypothesis of this study that the corrosion behaviour of CoCrMo is strongly dependent on the type of hard phases. This is rationalized by the introduction of additional phase boundaries which are the preferred location of electrochemical attack. In order to test this hypothesis, this study compares the corrosion behaviour of CoCrMo alloy from retrieved THR femoral heads (condition 1) with samples from the same implants that underwent additional solution annealing treatment post retrieval (condition 2).

For this study 20 femoral heads taken from total hip replacements were available and have been subdivided in 4 hard phase type groups. For the corrosion tests, two cross-section samples were made to provide a previously unexposed and flat surface. One sample was kept in the as retrieved condition (condition 1), whereas the other was solution annealed at 1050°C for 1 hour (condition 2). An open circuit potential (OCP) and electro-chemical impedance spectroscopy tests were conducted prior to the polarization tests. The polarization tests were performed according to ASTM G5-94-(2011) with an anodic scan from -0.8 V to 1.8 V at a scan rate of 1mV/s and followed by a corresponding cathodic scan from 1.8V to -0.8V to investigate the repassivation behaviour. All tests were conducted in newborn calf serum (protein content: 30 g/l) buffered to a basic pH 7.6 solution at 37°C. The morphology and microstructure of the corroded samples were analyzed with a scanning electron microscope.

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