Degradation of modular hip joint implants : a corrosion and tribocorrosion approach

Total hip joint arthroplasty is a medical procedure to replace a natural failed joint by an artificial joint to improve the quality of life of the patient. In spite of its broad success, there is a concern about the relation between the presence of modular hip joint implants and health complications. In parallel, several degradation mechanisms such as are pitting, galvanic corrosion, hydrogen embrittlement, crevice-corrosion and fretting-corrosion have been proposed to explain what occurs in the modular joints. However, no consensus except for the recognized role of fretting-corrosion has been reached. Based on the required conditions for these mechanisms to happen, the two more likely occurring phenomena are fretting-corrosion and crevice-corrosion. Even though, several contradictions subsist about the effective incidence of crevice-corrosion on biomedical Ti and CoCr alloys. Accordingly, the general goal of this work is to clarify and shed more light on the mechanisms responsible for modular implant degradation, identifying the occurrence of crevice-corrosion in modular hip joint configuration and to rationalize through a predictive model the different mechanical and chemical mechanisms responsible for fretting corrosion degradation. The strategy taken here consists in the combination of an experimental and theoretical approach. The study of crevice-corrosion of Ti6Al4V and CoCr alloy was carried out in a configuration mimicking the geometrical crevice found in modular hip trunnions in contact with ceramic heads, under different pH conditions and several potential applied in NaCl 0.9% wt. No potential drop or evidence of degradation was found when testing CoCr and Ti biomedical alloys up to a duration of one month. In addition, an existing IR-Voltage model was used to predict the occurrence of crevice-corrosion based on the experimental results and data from the literature. Results from the experiments and the model indicate that crevice corrosion can occur only under the unrealistic condition that the electrical conductivity of the body fluid surrounding the implant approaches the one of drinking water. According to these results, the attention was addressed towards fretting-corrosion phenomena as the main responsible for modular hip joint implants deterioration. A predictive model of fretting corrosion wear integrating mechanical and chemical phenomena has been developed. A set of experiments was carried out in NaCl 0.9%wt under cathodic and anodic potential; in order to study the degradation mechanisms of the modular hip joint implants under different electrochemical and mechanical conditions. The properties of the third body formed depend on the prevailing electrochemical conditions in the contact, thus determining the wear rate. Considering these observations and taking into account mechanical and chemical parameters, this work proposes a fretting wear model inspired from an existing tribocorrosion model based on the mass balances betwe