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New coatings extend life and safety of hip implants

By Dick Meredith, London Press Service

New coatings have been developed for hip replacements to protect against post-operative infection and provide a barrier to minimise metal ion release into the body.

There has been concern about 'metal-on-metal' hip replacements – where a metal ball fits into a metal cup implanted into the pelvis – with problems reportedly occurring when friction between the ball and cup causes tiny metal filings to break off. These filings can seep into the bloodstream and cause inflammation, which can destroy muscle and bone.

The first metal-on-metal devices were introduced in the 1990s, when they were seen as offering better mobility than other materials, but most were withdrawn from the market in 2010.

Now SMART-HIP, a research project led by Cambridge coating specialist Tecvac Ltd and leading UK hip prosthesis manufacturer, Corin Ltd, has demonstrated the potential of new enhanced wear resistant coatings to increase implant longevity in Metal-on-Metal (MoM) hip prostheses and future potential applications to trunnions and tapers to prevent fretting and corrosion.

New PVD silver bearing coatings were developed which may protect against post-operative infection, and also provide a barrier to minimise metal ion release. Optimised coatings reduced the release of one important metal ion (Cobalt), implicated in chronic tissue inflammation, by 99 per cent.

Inherent wear resistance, anti-leaching and anti-microbial benefits have the potential to substantially reduce the lifetime cost of hip replacement implants to the UK National Health Service (NHS) and health organisations worldwide, while providing a much improved patient experience.

Overall the SMART-HIP project demonstrated the potential of the new family of coatings to reduce initial post-operative implant wear by a factor of 4 and eventually to double implant service life, according to the researchers.

The new Chromium Nitride (CrN) coatings reduce induced wear to negligible levels. The incorporation of silver in the outer layer of the coatings provided the planned and self-sustaining release of beneficial silver anti-microbial particles. This gives both self-lubrication of the bearing surfaces and short and long term protection against post-operative infections.

Funded by the government-backed Technology Strategy Board, the Engineering and Physical Sciences Research Council (EPSRC) and the industrial partners, the £1.6 million SMART-HIP project included teams from the Research Centre in Surface Engineering at the University of Sheffield, Queen Mary College, University of London (QMUL), Imperial College London and Charing Cross Hospital. The coatings and other elements of the research are protected by international patents and relevant patent applications.

SMART-HIP is the first major research project to approach biomedical coating design and development in a multi-disciplinary, integrated way, and recognised that a successful outcome of the project would depend on an effective cross-disciplinary approach to the technical challenges. Indeed a substantial part of the successful innovation was the direct result of the interdisciplinary nature of the group.

An important feature of the project was parallel evaluation of the toxicology and immunology as the coating formulation and orthopaedic work progressed. In particular, many of the immunological tests in this project had never previously been applied to the coating processes before. The work is particularly timely with the current concerns reported about metal-on-metal total hip prosthesis implant designs and the potential impact of trunnion wear rates on large diameter bearings.

During the three year project, Tecvac and the University of Sheffield developed a variety of new coating strategies and formulations, which were then applied by Tecvac to MoM implants designed and produced by Corin. Coated implants were then assessed by QMUL using a full-scale hip simulator which included eight simulation stations using bovine serum. Tests were also carried out on multiple samples to determine implant resistance to extreme and accidental loads. As the wear test progressed, endotoxin assays, ion release analysis, bio-compatibility, immunological and microbial evaluations were carried out by Imperial College, Charing Cross Hospital and QMUL.

The most successful coatings were based on a CrN PVD coating, including silver in the outermost layers and precision substrate pre-treatment to enhance wear properties and shock load performance. The bioactive coating provides controlled release of silver particles. These particles are generated in the normal wear process and have been shown to have effective anti-microbial properties. This controlled and engineered aspect of the wear process creates a benign environment around the implant reducing the post-operative chance of infection. Optimised coating formulations were shown to virtually eliminate cobalt ion leaching, achieving a reduction of 99 per cent in treated components compared with untreated components.

One in four uncoated hip components showed run-away wear and associated ion release whilst the coated surfaces show consistently low wear rates. Recent studies have shown that the cobalt ions in metal wear debris from MoM implants cause chronic inflammation, could result in genotoxic effects, and often lead to loss of mobility and other medical complications.

“This was of course a very complex multi-disciplinary project,” commented Dr Jonathan Housden, “involving investigation of engineering, metallurgical, biological and chemical processes… sometimes at a very fundamental level.”

“We were all very pleased the SMART-HIP was not only completed on time, but also on budget and with a very positive biomedical research outcome, including the development of methods to successfully monitor the evolution of ion release.

“Papers have been published in nine scientific publications covering the project. In addition, not only has the SMART-HIP project resulted in a proven new family of coatings, but they also include features that reduce structural metal ion release to almost negligible levels, while using the very same inevitable and unavoidable wear processes to impart new self-lubrication and beneficial anti-microbial properties to the implant surface,” he added.