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Coatings on knee replacements

I am an orthopaedic surgeon specialising in knee surgery. Some of the knee replacements I use are made of Titanium with a surface coating. Some of the manufacturers describe this as "Tinanium Nitrite" and some describe it as a "Titanium Nitride" coating. What is the difference? Is one likely to have better wear characteristics than the other?

Dr Glyn Evans
London Knee Clinic - London, UK
2005

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Titanium nitride (TiN) is a very hard coating initially developed for metal cutting tools. If by titanium nitrite they mean titanium nitrous oxide, then the coating is not as hard as titanium nitride. It's difficult to assess the relative wear resistance of the two coatings, however, without more information about them. Other features of the coating, such as adhesion, surface roughness, toughness, and residual stress often have greater impact on coating performance than hardness, especially in a wear situation where the opposing surface is much softer, such as is often the case for hip and knee joints, where the opposing surface is high density polyethylene.

Jim Treglio - scwineryreview.com
PVD Consultant & Wine Lover - San Diego,
California
2005

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While there is a coating that could be made of titanium nitrite, it appears in this case to be a typo, where it should read "titanium nitride". There are some vendor web sites on google that list the correct chemistry of TiN, but incorrectly refer to this compound as "titanium nitrite". When referring to TiN thin film, the description should be "titanium nitride".

Joe Kubinski
- Enfield, Connecticut
2005

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2005

Hi Glyn,

Greetings from the Colonies.

My background is ion implantation, and most of the knees I've ever heard about are done by ionizing nitrogen gas, which can then be accelerated by a voltage potential difference, and then either mass analyzed (or not), before kinetically implanting the nitrogen INTO the surface of the Ti ball. This results in an in situ growth of TiN in the first few atomic layers, rather than a coating which is a surface treatment. Coatings are sometimes followed by a heat treatment to diffuse the coating into the substrate.

As far as performance is concerned, you get what you pay for. Knees, etc., come in a variety of grades, (not a well known fact), and therefore, prices.
There can be any number of places to cut corners to shave price. The general thinking goes that you wouldn't need to do the same thing for an eighty-five year old woman, as you would for a twenty year old athlete. Use and lifetime are the determining metrics. The twenty year old, will wear out several sets of even the best replacements over his lifetime. SOA is about ten years, last I knew. There is a lot of excellent work going on in the area of ceramics instead of metals, which I could point you to.

My research has been in the area of harnessing boron clusters, (similar to carbon Buckyballs in structure), which have properties unlike elemental boron or any of the other known forms of boron compounds. They appear to have the potential for developing an entirely new chemistry for boron, and look extremely promising for all kinds of new surface treatments, because they form a brand new set of customizable compounds, so that "designer" materials will actually be able to do things unthinkable today. Look for this in the next few years. Knees will be one of the early applications.

Rick Becker
- Ipswich, Massachusetts