Materials used for the manufacturing of orthopaedic implants are known to be "inert" or "biocompatible". This means that the human body accepts the material under normal circumstancel. Materials used are Chromium Cobalt a
lloys (already more than 50 years) Titanium-alloys (since about the 1970's) and Polyethylene (since the 1950's). Stainless steel is used for plates and screws for fracture treatment. Some other metals, especially Nickel, are known to cause bodily reactions.
Debris generated from articulation of orthopaedic materials however may cause physical reactions. As polyethylene can not ingested by the normal human immune system, the reaction tiwards poleyhtlene debris is local. Thickening of the capsule and surrounding tissues can be seen and if the number and voilume of wear partcile passes a critical point it may lead to implant loosening and failure . Reaction to metal wear debris can be tofold: local and systemic.
Titanium, that is weka to be used as articualting material, but often used in hip stem, does not seem to generate local nor systemic reaction.
Chromium and Cobalt wear particles used in the alloy for articulation against polyethylene, or against itself seems to be able to cause both type of reactions. With the reemegring of the metal-on-metal articualtion and the large scale of application in hip resiurfacving world wide, a very small percentage of patients seem to develop a local reaction on the wear particles. (15,16,17)
The clinical follow-up of implants of the metal-on-metal type implanted in the 1950's and 1960's were published and although the worru remains over the long term effect of larger amounts of metal debris, no publication have shown any local ill effect of these long term implantations .
Local reactions known as "hypersensitivity reaction" or 
"allergy" reactions have been described as well It is believed that about 10 % of the population was a hypersensitivy for one of the materials used Chromium of Cobalt. . Under normal circumstances the contact between a solid implant and the human body does not trigger any reaction
An exception is this picture from the 1970's seen on the left where an implant containing Noickel caused a local reaction. Nickel is no longer used in implants these day !. When wear particle are generated in a metal-on-metal articulation, these wear particle ase much smaller than those from polyethylen on metal. However they are much more numerous and they are smaller. All these small particles together probably form an large enough surface area to cause a local sensitivty reaction.
Reducing wear reduces also the chance of hypersentivity reactions. From hip simulatior studies can be concluded that teh use of a ceramic engineered implant reduces the wear dramatically
Aonther aspect of wear is the probability that these small metal wear particles are ingested into the body in the form of metal ions Please continue to read here
BIOLOGICAL PRE-CLINICAL TESTING
Biocompatibility testing of the ACCIS materials was performed at the LPT Laboratory of Pharmacology and Toxicology, Prof.Dr.med F. Leuschner, in Hamburg Germany in order to investigate possible biological reactions to Titanium Nipbium Nitride Ceramic when implanted into the body.
implantation test according to the USP XXII following the LPT protocol 7362/92. Result: No difference in tissue reaction was found between Titanium-Niobium-Nitride ceramic implanted in rabbits and negative control material. The effect observed was limited to a non-specific tissue reaction (56).
examination for cytotoxic properties of plates made of CoCrMo coated with TiNbN in a cell culture test according to USP 28 and EN 30993-5. Result: no signs of cytotoxicity were revealed, while the positive control revealed severe cytotoxicity (57).