“In vitro” studies of surface hardened cobalt chromium molybdenum alloys used for orthopaedic applications

Abstract

The objective of this thesis is to analyse the biocompatibility of low temperature carburised cobalt-chromium-molybdenum alloys, and to characterise the material surface in order to show the advantages gained by using the low temperature carburised material relative to the original untreated alloy, and other materials. Studies have been carried out on four main fronts including; material characterisation, “in vitro” corrosion testing, biotribocorrosion testing and solid state biological testing, in conformance to international standards including but not limited to BS EN ISO 10993-18:2009 - Biological evaluation of medical devices. Corrosion testing has been performed in phosphate buffered saline solution, Ringer's solution and dilute bovine serum solution mimicking the conditions in vivo as closely as possible. The bovine serum concentration was chosen such that the concentration of proteins in the final solution was similar to that recorded from freshly collected synovial fluid which is found within the joint region in vivo. In all solutions, the low temperature carburised Co-Cr-Mo alloy was seen to outperform the untreated alloy. In this regard the low temperature carburised alloy showed a consistently higher OCP value and a lower corrosion current. This was attributed mainly to the more resistive passive film developing on the surface of the carburised alloy. This then gave rise to a better performance in terms of in vitro elution biocompatibility testing performed on the carburised alloy. The low temperature carburised alloy was also found to perform as well as its untreated biocompatible counterpart when regarding direct "in vitro" contact biocompatibility carried out using a bone osteosarcoma (Saos-2) cell line, a mouse osteoblast (MC3T3-E1) cell line and human foetal osteoblast (hFOB 1.19) cell line as well as primary human osteoblast cells. It was noted that the combined effect of higher wettability and an increase in surface composition of the carburised alloy produced no effect on the biocompatibility of the alloy, or rather each parameter having an effect which is countered by the effects of another parameter. The carburised alloy increases in hardness by approximately 250% compared to the untreated alloy which was seen to lend itself well to the pure mechanical wear scenario as shown when the untreated and low temperature carburised tribopairs were tested in phosphate buffered saline solution and dilute bovine serum solution at cathodic potentials. However, when considering tribocorrosion testing at open circuit potentials and anodic potentials, the synergistic effects taking place on the surface of the carburised tribopair, mainly in the form of type 1 wear, were seen to have a prominent role both in bovine serum and also in PBS solution. This lead the carburised alloy to a reduction in overall tribocorrosion performance compared to the treated alloy. An average reduction in tribocorrosion properties of 140% and 50% were recorded when tribocorrosion testing was carried out at OCP conditions in phosphate buffered saline solution and dilute bovine serum solution respectively. This was echoed at anodic potentials where reductions in tribocorrosion performance of 60% and 10% were recorded in the same solutions respectively.