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Corrosion associated failure mechanisms of CoCrMo alloys in total hip replacements Wang, Qiong

Abstract

Over one million hip replacements are performed worldwide to restore hip mobility every year. CoCrMo alloys are widely used in hip implants because of their excellent corrosion resistance and mechanical properties. However, some complications related to corrosion of CoCrMo alloys emerged, leading to unexpected clinical failures such as fracture and adverse local tissue reactions. The purpose of this thesis is thus to investigate the in vivo corrosion mechanisms of retrieved CoCrMo alloys used in total hip implants from two specific clinical failures- mechanical fracture and adverse local tissue reactions. In the mechanical failure study, systematic analyses on the clinically failed CoCrMo based implants revealed a multi-step fracture process. Multiple micro cracks were developed under the combined action of pitting corrosion and dynamic tensile stress on the lateral side of the CoCrMo connection taper, leading to the final catastrophic failure. Such a crack initiation process has not been previously reported on retrieved CoCrMo components. Our findings provide valuable information on the clinical performance of such implants, as well as the material selection and structural designs for future modular stems. In the biological failure study, increasing cases of adverse tissue reactions related to CoCrMo metal release have been observed in hip implants of metal-on-polyethylene articulation (MoP), the most commonly used hip system in recent years. Comprehensive studies were conducted on the origins of CoCrMo metal release in MoP hip system– fretting corrosion at head-neck junction and tribocorrosion at articulating surface. In fretting corrosion, two types of corrosion particles were observed at head-neck junction. Further laboratory tests revealed that the formation of these two types of corrosion particulates was associated with different species, especially the phosphate in electrolytes. This study indicated different electrochemical environments between the inside and opening site of head-neck junction during in vivo fretting corrosion. Meanwhile, retrieval studies revealed evidence of tribocorrosion associated CoCrMo metal release in patients with MoP hip implants, which was rarely reported to our knowledge. It raised the awareness about the risk of tribocorrosion on metal release and the consequent adverse tissue reactions in patients with a MoP hip system.

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