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Joint ventures: Hip replacements are pretty commonplace operations these days. Yet there are still a lot of unknowns.

You're probably attached to your hips. But a whole posse of engineers, medics, biologists, materials scientists and biochemists are also attached to them. In a rather less physical way than you are, of course.

Hips and hip implants have been the cornerstone of bioengineering for the past 50 or so years, generating cross-disciplinary collaborations that give rise to new disciplines and creating new specialisms within existing disciplines.

And creating endless debate and research work: put the words "hip" and "implant" into the search engine of Sage Publications, the leading academic publisher in this area, and you find more than 2,000 proper, refereed academic papers have been written on aspects of the subject in the past 20 years in engineering journals alone.

Hips are fascinating to many disciplines and the engineering problems that they throw up just keep on coming, says Anne Neville, professor of engineering at the University of Leeds and a leading researcher at what has become the UK's, and perhaps the world's, strongest engineering department dealing with the topic.

Neville's own background as an engineer is in tribology, the science of rubbing and friction and the effects on surfaces, originally in her case with particular application to oil and gas industry structures. But hip implants, she says, are fairly similar: "If you take a subsea valve in the oil and gas industry the kinds of things that you're worried about are fairly similar to the things if you put a hip joint into a body," she says.

Putting a hip implant, a new ball and socket joint, into a human body starts out with a passive film between the two surfaces and a very close fit. "But then you get movement and that passive film gets removed and it's very similar to what happens in the oil and gas industry if you get sand attending the surface," she says. "Ifs a process called tribocorrosion: tribology is the movement that extends the corrosion. Sometimes because you're in a biological environment it can help, because the proteins may give you extra protection, but in some cases the proteins augment the corrosion process.

"The thing is that you're dealing with a very complex environment and because of that it is very difficult to predict exactly what will happen."

The difficulties are reflected in changes to the common practices of hip replacement, where there has been a constant succession of innovations in terms of the materials used, the ways the two parts of the prosthesis interact with each other and the way in which the whole structure is then integrated into the body and the bone, muscle and cartilage.

Further complexity has been brought in because of the sheer success of the procedure. Back when the early hip replacements were done, there was an expectation that the patient would get an extra 10 to 15 years of walking and working life out of the implant: now the archetypal late-middle-age candidate for a new hip wants maybe 30 years of activity. With this kind of complexity, there is a heavy burden on both modelling and testing, and a lot of national and international standards for testing hip joints in simulators. Leeds University, as a centre of the study of tribology, has some of the best facilities in the world for this. But, says Anne Neville, there are limitations.

"Essentially what you do is you take a hip joint and then you do say a million cycles on that joint and that would perhaps be equivalent to one month of patient use," she says. "But it's accelerating a test so that we can get the data back quickly and in the real process it's rather more complicated than just taking a million steps and working out what that means per step. We have to look at what a hip joint actually does: it doesn't just walk, and there are different loadings on it when people get out of bed or get out of a car. Knowing what the damage is that's associated with each of these cycles is very important too.

"So the testing has been based very much on a walking cycle but we're now starting to realise that we need to factor in movement cycles that are really quite different and very complex. We're trying to develop more complex mechanical models so that we're working on what are the forces, what is the tribology doing at the interface and then using this inside a simulator to get a realistic view of what the damage actually would be."

An extra complication in this, of course, is that unlike a subsea valve you're dealing here with biology and with a "mechanical" system that interacts with a biological system that will, if it isn't perfect, say "Ouch". Or worse.

"Some of the most spectacular failures in this area have been tested in the standard way and have followed all the standards, and they've then had very high failure rates that simply weren't picked up in simulation," she says. "So I think at the moment no one is in any doubt that simulation has limitations. It could be that we're not simulating the motion properly or that we can't compress five years into one month of testing. There's a whole combination of things we might not be getting right. There's a lot of people trying to work out what's happening."

This is one of the applications where the move of material science into the nano and even atomic scale may bring benefits. "If you start looking at the microscopic and nano level you start seeing the interaction of the implant material with the proteins in the body and in some cases that's positive and in others it's negative," she says. "What it comes down to is that if we don't understand on the nano scale what is happening then we don't really stand a chance to stop high wear and high friction."

One of the big difficulties of this whole area, Neville says, is that body tissue isn't like the kind of engineering materials that implants are made of. "With a mechanical material or a conventional material, you know its properties." With implants and with other devices introduced into the body - surgical instruments or temporary fixtures, for example - the body tissue is the unknown.

"With all the engineering materials, we can characterise them properly and so if I'm asked how it reacts to a five-newton force then I can look it up," she says. "But if it's, say, the abdominal wall, it's very difficult to predict and you have to go back to basic science and start to build up that knowledge."

This is perhaps less of an issue in hip implants and other kinds of surgery where there is now long experience and where the mechanical loads and stresses are fairly familiar. But a different part of the work that Neville and her colleagues investigate is the scope for robotic, remote and minimally invasive surgery, using robotic devices that need to attach themselves to parts of the body and then detach themselves easily when the surgery is done.

"With the abdominal wall, there's really nothing that been published on what the material properties are," she says. There are possibilities in some adhesive formulations, but the stuff that sticks things together with biological tissue tends often to be less good at the other necessary - unsticking them when the surgery is done.

This work of implants, innovative surgical techniques and introducing new items into the human body on a long-term or a temporary basis is absolutely at the crossroads of many different disciplines. Neville is working alongside biologists, surgeons, chemists and many others: she calls herself mostly, she says, a corrosion engineer or a tribologist. Each of the disciplines has a different take on the problems of the area, and ideas can come from almost any direction: even nature, I where lots of tree frogs and insects find no problem attaching and detaching I themselves from surfaces, oblivious of gravity.

But hips and, to a lesser extent, knees are very much at the heart of this area of medical engineering. You and I may well be attached to our own ones, but a lot of i engineers and scientists are also attached. In a slightly different way.
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Author:Puffin, John
Publication:Environmental Engineering
Date:Apr 1, 2014
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