**3. Coating technologies in total knee arthroplasty**

Coating of metallic components, mostly applied by the physical vaporing deposition (PVD) process, was first conceived as a solution for patients with hypersensitivity reactions to cobalt and nickel, as it prevented the release of such ions from the substrate material. There are currently different coating technologies for TKA implants in use. Historically, single-layer coatings, also known as monolayer coatings, came first in clinical application. Monolayers are in clinical use in two versions: composed of titanium nitride (TiN) and titanium niobium nitride (TiNbN). TiN and TiNbN monolayer coatings are in clinical use for hip, knee, and ankle arthroplasty since the early 1990s. They are applied by means of PVD on typical orthopedic implant materials such as titanium alloy (Ti6Al4V) or cobalt chromium molybdenum alloy (CoCrMo).

Another option is the oxidized zirconium (OxZr), which is not considered as a coating technology but more a surface modification of a ZrNb2 base material [28] driven by thermal oxidation at 500°C [33]. The resulting zirconium oxide (ZrO2) is a ceramic surface with good wear characteristics and reduced release of metal ions. This material has been in clinical use for knee arthroplasty since the 1980s [34]**.**

Further development is represented by a zirconium nitride multilayer coating, which is composed of seven layers applied via PVD and has been in clinical use since 2006. The ceramic surface is composed of zirconium nitride as the top layer, chromium nitride and chromium carbon nitride as transition layers and a bonding layer of chromium, which integrates with the base material of the implant.
