**1. Introduction**

Total knee arthroplasty (TKA) is a very effective treatment option for advanced osteoarthritis of the knee with overall low revision rates [1]. However, not all patients benefit from the surgery and not all problems have been solved. One issue is the biological reaction to implanted materials. Knee arthroplasty implants are usually made of cobalt-chromium alloys. Material from implants is always released after implantation, either as a result of mechanical (wear) or electrochemical processes (corrosion). Biological reactions in patients, caused by the release of metal particles and metal ions, have been reported (allergy, inflammatory response). Furthermore, cobalt, a relevant part of the implant alloy, has recently been listed as a potentially carcinogenic and mutagenic category 1B hazard by the European Commission. To overcome these potential issues, the surface of knee arthroplasty implants has been coated or ceramised. Besides a reduction in the release of metal ions and metallo-organic complexes, this results additionally in better wear performance. Unfortunately, there have been reports about less favorable results with such hypoallergenic materials [2–4]. In a retrieval study, 21% of the TKAs demonstrated coating delamination, which might affect the performance of the coating [4].

This chapter explains the background of metal ion release, current coating technologies being used in modern knee arthroplasty implants, their biomechanical properties, biological reactions and clinical results.
