Effects of proteins and pH on tribocorrosion performance of cast CoCrMo – a combined electrochemical and tribological study

Abstract

Cast CoCrMo (F-75) based metal-on-metal (MOM) hip resurfacing implants are an increasing option for younger and more active patients as they are less invasive to the human body. However, symptoms such as inflammation, tissue necrosis and metal sensitivity have been reported for some patients with MOM joints, which may be associated with the liberated metal debris and the elevated metal ion levels from such joints. Therefore, there is a requirement to assess the tribocorrosion performance of the CoCrMo alloy in physiological environments. In this study, the wear corrosion of cast CoCrMo, more specifically, sliding wear corrosion and abrasive wear corrosion (using ∼4 μm SiC abrasive particles) have been investigated using a modified ball cratering rig with a three-electrode electrochemical cell. The concentration effects of proteinaceous material (25 and 50% bovine serum) and pH levels, i.e. pH 4·0 (the pH for an infected joint area) and pH 7·4 (the normal pH for a healthy joint) on the tribo-corrosion performance of the alloy are explored. Electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy have been deployed to study the protein adsorption/corrosion of the alloy under static, non-abrasive conditions, whereas in situ electrochemical current noise measurements were employed to quantify the mechanoelectrochemical mechanisms present in the tribocorrosion of the alloy under dynamic conditions. In general, protein adsorption was found to be dependent on pH and protein concentration. Protein adsorption in turn influenced the overall tribocorrosion performance of the CoCrMo alloy, including the in situ electrochemical current noise, the specific wear rates, as well as the wear scar morphologies. The presence of bovine serum enhanced the separation of the bearing surfaces and for the sliding wear corrosion the wear rate was markedly reduced. However, the bovine serum appeared to increase the abrasive particle entrainment and lead to accelerated wear in the presence of abrasive particles (abrasive wear). For the more acidic physiological environment, in comparison to pH 7·4, higher corrosion rates occurred under wear corrosion conditions leading to increased metal ion release. Consequently, the proteins and the pH level of hip joint environments play a significant role in the tribocorrosion performance of the cast CoCrMo and a discussion of the combined wear and corrosion mechanisms will be presented.