Metal articles are often used in contact with internal human or animal body tissue for a wide range of uses. However care must be taken, particularly if such articles are implanted into the human or animal body, that they are long-lasting, non-corrosive and do not leach metal ions into the body.
Orthopaedic implants (such as a replacement hip) conventionally employ a metal-on-polymer arrangement, but suffer from the disadvantage of wear-induced osteolysis associated with polyethylene debris, even with improved modern polyethylenes. Metal-on-metal prostheses offer an alternative without these disadvantages, potentially providing greatly increased dislocation resistance and a longer service life, opening up the technique for the young active category. However, because of the stresses encountered in orthopaedic implants, particularly in articulating orthopaedic implants, it would be beneficial to use coatings to reduce wear.
Additionally, the use of metal-on-metal orthopaedic implants can result in undesirable wear debris, at levels such that they are detectable in blood. For example, cobalt/chromium has been shown to provide a ten-fold increase in these potentially carcinogenic metal ions in the blood of a patient, compared to normal healthy subjects.
A coating for an articulating orthopaedic implant, must satisfy a number of additional demanding requirements, e.g. it must be hard, have low surface friction and also be tough enough to withstand potentially high impact forces over a long period of time. Coatings which are not tough enough (i.e. too brittle) or hard wearing enough may have a short service life or may even become detached, which could be catastrophic. It would be additionally advantageous if such coatings had anti-microbial properties.
It is becoming generally accepted that silver in ionic form has antimicrobial properties. Also very fine particles of silver e.g. nano-particles, have a surface area to volume ratio such that they can readily oxidise to form the ionic compound silver oxide which provides an antimicrobial effect. However, larger silver structures tend to be ineffective antimicrobials. It is therefore not simply a matter of using silver as a coating in order to provide an antimicrobial effect. In any case, silver is too soft for use as a coating in applications where a significant amount of stress is encountered, e.g. in articulating orthopaedic implants.
U.S. Pat. No. 6,361,567 discloses a process of forming an anti-microbial coating on a surface of a medical implant, which involves incorporating silver into a diamond-like carbon coating. However, whilst diamond-like carbon is very hard and has a low friction, it is often a brittle material. It is therefore not ideal for use as a coating especially in articulated prosthetic implants.