Cobalt/chromium alloys biocompatibility

Titanium alloys generally show a combination of strength and biocompatibility which makes them suitable for medical devices (prosthesis, surgical instruments). The high strength Ti-6Al-7Nb alloy has several orthodontic applications. Only a limited number of alloys have the necessary combinations of properties needed for successful use in the human body. Titanium and its alloys, stainless steels and cobalt-chromium alloys are the workhorse alloys in the medical device industry. 

Titanium is used in medicine mainly for its mechanical benefits in surgical and dental materials in a host of orthopedic and orthodontic appliances, with or without other metals (for example nickel, cobalt, chromium), and generally without serious adverse effects. Titanium and its alloys are in use as implants in bone surgery (1,2) and in dental materials (3,4). Research on the biocompatibility of metal and tissue continues (5). 

Biocompatible materials that have been successfully used for implantable medical device packaging include titanium and its alloys, noble metals and their alloys, biograde stainless steels, some cobalt-based alloys, tantalum, niobium, titanium-niobium alloys, Nitinol, MP35N (a nickel-cobalt-chromium-molybdenum alloy). 

Cr-Co alloys are known for their high strength and wear resistance. Despite these benefits, there is a concern about the adverse biocompatibility of chromium and cobalt ions, whereas other studies have indicated that chromium promotes the crystallization of HAp (Wakamura et al. 1997) and, therefore, might be an important contributing factor leading to good fixation in bone. 

Manufacturers of nitinol products are known to electropolish the surface of the nitinol wire to produce an oxidized surface layer that contains only titanium and oxygen. This reduces the potential for nickel allergy and toxicity problems. The alloy MP35N used in the CS/SF device is a quaternary of cobalt, nickel, chromium, and molybdenum, and is known to have high strength and corrosion resistance. Additionally, this metal has been found to be biocompatible and MR compatible. These are all desirable properties for an implantable device, but its nickel content should be noted and considered when dealing with patients who are allergic to nickel. 

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