Wear of Implants
The application of a ceramic surface finish articulating against itself in orthopaedic implants is relatievely new, but the application of ceramic to 
reduce wear and corrosion has been applied for longer time. Ceramic coatings are used to improve the performance, and durability and are known to reduce wear in the automotive industry. Pistons, bearings,cylinder blocks, valves and transmission components are now offered with ceramic coating. In the airplane industry often, both the combustion chamber and the turbine receive special ceramic coatings to improve the preformance. Ceramic Coated orthopaedic components articulating against polyethylene were showing increase longivity and reduced wear, have been clincially applied already for many years. Would it be possible to articulate Ceramic Coated Metal against Ceramic Coated Metal ? Since 1997 a full range of preclinical testing was performed (see Design Rationale) untill the implant become clincally available in 2000 as a Total Hip and in 2004 as a Resurfacing Hip.
How to reduce wear To understand the effect on wear of a TiNbN coating it is important to know how M-on-M prostheses wear. Most wear in metal-on-metal articulations occurs in the first few millions of cycles of weight bearing (47). During this running-in phase surface asperities are worn flat. These asperities on metal surfaces are formed by block carbides.
Block carbides are molecules formed from metal atoms and carbon; a typical structure is M7C3. They are formed during the cooling phase of the metal component and are particularly common at the surface. The molecules are relatively large:
20 – 30 microns. Their presence in the alloy makes the alloy less homogeneous and weaker, but they are more wear resistant. The presence of block carbides in 
the metal alloy thus imparts both advantages and disadvantages.
The block carbides are much harder than their housing alloy, and may cause abrasive wear on the other opposing prosthetic surface. Asperities may also break free from the surface and cause three-body abrasive wear.
To minimize the disadvantages it is possible to reduce the number and size of the carbides present. Heat treating the component after the cooling process leads to the presence of fewer carbides, with remaining molecules reduced in size to 5 – 10 microns. This process leads to retention of block carbide advantages, while reducing disadvantages.
A ceramic coating is helpful to further reduce the disadvantages of block-carbides. Ceramics have the advantage of being extremely hard. In the ACCIS®surface replacement prosthesis the ceramic titanium-niobium-nitride is used to reinforce the articulating surface and to reduce wear of the chromium-cobalt-molybdenum alloy. The titanium-niobium-nitride coating is very wear resistant, and being much harder than the block carbides is capable of slicing asperities off as small particles; this prevents the breaking away of carbides and the initial high wear of metal-on-metal articulations is to a large extent reduced. After the running in phase the opposing surfaces are completely flat and show minimal wear.(click on the image to enlarge)