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)
Preclinical Wear Tests 
Wear evaluations of the ACCIS implants, using hip simulators were performed by independent investigators at the Institut für Materialforschung und Anwendungstechnik, IMA in Dresden, Germany (52 ,53), and at the Atomic Energy Authority in Harwell, UK (62 ): ACCIS®ceramic coated MoM hip implants with femoral head components of 42 and 46 mm were compared to prostheses with bearing surfaces of metal-on-polyethylene, ceramic-on-polyethylene and metal-on-metal.
These studies resulted in the following conclusions:
the wear of the TiNbN-coated ACCIS® prostheses was
limited to the running in phase: partial removal of the ceramic
coating occurred during the initial 1- 2 million cycles after
which removal was neglible (54).
during the running-in phase TiNbN-coated prostheses
showed a 6-10 fold reduction of wear compared to M-on-M
articulations with 32 mm femoral head components(54).
the ceramic layer was intact after 5 million cycles
following the running-in phase.
hip simulator tests comparing ACCIS®ceramic coated
and ACCIS®non-ceramic coated components confirmed
the beneficial effect of the ceramic coating.
shed particles were shown to be of sub-micron size: < 0.1µ