Different designs of knee prosthesis use different materials, usually metal, plastic and ceramic.
Metal-on-plastic knee replacement
There are different types of knee replacement. The metal-on-plastic is the most common type of knee replacement. The metals used vary from manufacturer to manufacturer but may include cobalt-chromium, titanium, zirconium or nickel. The plastic is ultra-high molecular-weight polyethylene, and it is this polyethylene part of the common total knee replacement that tends to wear, depositing microscopic particles of plastic in the joint space.
Ceramic-on-plastic knee replacement
Metal-grade ceramic implants, for example of zirconium or alumina, have been widely used in hip replacements, but are less commonly used for the knee. The materials have good biocompatibility and friction characteristics, and surgical revision due to aseptic loosening, wear, or component fracture is rare, with outcomes comparable to metallic implants. The polyethylene spacer, however, is still prone to wear, although it appears to be less than with the metal-on-plastic replacements.
Ceramic implants have the advantage over metal ones in patients with metal allergies, as the ceramics are biologically inert.
Ceramic-on-ceramic knee replacement
Ceramic bearings (spacers) are not commonly used for knee replacements, although they are widely available for hip replacement.
Metal-on-metal knee replacement
Metal-on-metal implants may be associated with elevated levels of metal ions and adverse reactions to metal debris.