Improving hip replacements

20 August 2019

Interview with 

Sophie Williams, University of Leeds

WALKING-COUPLE

Couple walking down street

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If we did the recommended 10,000 steps a day - that equates to 3.65 million steps every year, a quarter of a billion in a lifetime... It’s not really surprising that, as we age, the cartilage that provides the slippery shock-absorbing surface inside our joints begins to wear and can become painful. Eventually, the only option may be a hip replacement. This is probably one of the most successful surgeries - in terms of quality of life improvement - ever invented; Even so, Sophie Williams, from the University of Leeds, is trying to make things even better, as she explained to Katie Haylor.

Sophie - The hip is a ball-and-socket joint. So you have the socket in your pelvis, we call that an acetabulum. And with that in the top of your thigh bone there’s like a ball, and that sits inside this hollow of your acetabulum. And both of those are covered in cartilage, and that's really smooth and that is lubricated really well. So as you walk along it all sort of glides, very low friction.

Katie - Considering this design then, why might someone need a hip implant?

Sophie - In the UK it's really because of osteoarthritis. So that's when your cartilage is effectively worn away, so you don't have that lovely soft cartilage layer there that makes it frictionless. You end up with bone against bone and that starts to cause pain.

Katie - So we know what a hip joint looks like. What does an implant look like?

Sophie - Very much like that ball and socket. So the head is just going to be a round sphere, a ball. Think about a very smooth golf ball. They tend to be smaller than the natural hip: so the natural hip may be around five centimetres in diameter; hip replacements tend to be smaller, so fairly common is something that is 28 millimetres. And then the socket most likely will be a white plastic that is then also fitted into a metal shaft, so you have a plastic liner and then a metal outer and then that all fits into the socket of the hip.

Katie - Why metal and plastic?

Sophie - That was sort of a combination that was found by chance back in the 1960s and we haven't hugely deviated from that. Those materials are quite low-wearing; we've been able to choose metal-on-plastics that don't have a huge reaction from the body; and very much looked at more natural alternatives, but actually not found anything that is wear-resistant that would meet the demands of the job.

Katie - So by wear are you talking about, if you get any two surfaces and rub them together over time, that's going to become worn right? You might get bits breaking off and causing... well I guess in the body do they cause trouble?

Sophie - They can do. And this is one of the things, we're always looking at materials that wear less. So in the body, as you walk along, then you will get that metal head rubs against the plastic and you get very, very tiny plastic wear particles.

Katie - What happens then if they get into your bloodstream? Do they get into your bloodstream?

Sophie - The particles do tend to stay fairly locally. While I say that they're really tiny, they're not tiny enough to start flowing around the body. But cells will come along - and they're kind of around the size of bacteria - so cells will come along and see them as something foreign and try to eat them up, and will cause a reaction. Those cells want to get rid of those particles because they don't think they should be there. So they'll send out some chemical messengers, and those chemical messengers then change the behaviour of the bone cells in that area. So you start to get a wearing away of the bone, so actually the hip replacement can become loose. This is quite a long-term process, this tends to happen sort of 15 years after the joint was put in the body. Our plastics have changed; while we still use metal and plastic like we did back in the 1960s, we've changed our plastics so we get even less wear with them, so you’re even more unlikely to have issues with wear.

Katie - So what things do we still need to work on when it comes to hip implants? Because that sounds like it could be quite a complex issue to solve. So how can you go about getting less wear?

Sophie - So we've already done quite a lot. It's a polyethylene that’s used, and that's what your plastic carrier bags are also made of, but it's ultra-high-molecular-weight. That means it's really, really big, long polymer chains that are in that material and then really closely packed together. We zap these implants with gamma radiation that causes cross-links in the material, so chemical linkages between those chains, and that actually reduces the wear of the material quite significantly. So you have that hip head going backwards and forwards over it. You need to put in a lot more energy, if you like, into that system to get those chains to break down and the particle to be released.

The problem is it also makes the material less ductile, so that's less stretchy. Now it's not massively stretchy anyway, but it becomes much more brittle. So let's think about a bar of chocolate on a hot day, and it has a bit of give to it, you could bend a little bit. That's a good thing in a hip replacement: just a tiny bit of bend in that plastic socket. If you put it in the freezer it’s then going to be really brittle. So if you put lots and lots of gamma radiation into your polymer it will become more brittle. So as material scientists we do need to think about what that offset is; that you want to reduce the wear but you don't want to cause the material to become too brittle. So there is a compromise between those two. Lots of research has been done and we've found the region that we need to be in so the materials aren't too brittle but that they will be low-wearing.

Katie - Gamma rays… this doesn't mean that a hip implant becomes radioactive, does it?

Sophie - It doesn't, no. Not radioactive at all. And in fact gamma radiation is something that's used to sterilise all sorts of medical implants and instruments before surgery.

Katie - And it's part of your job to try and help the surgeon to reduce the chances of anything going wrong then?

Sophie - Yeah. You know, we work with surgeons and we also work with implant manufacturers. We have simulators in our lab, so these act like pseudo-patients, someone walking along for millions of millions of steps, and we see how those materials wear. And then we'll also change things a little bit, like maybe how the head in the socket has been positioned, and see if that changes the amount of wear we get. So we can really understand how those materials are working before we put them into patients.

Katie - How good are these metal-on-plastic hip implants? Are they as good as the real deal?

Sophie - No, it is always going to be a replaced part of your body. But the vast majority of patients really do talk about their forgotten joint that they don't really know it’s there. They can certainly continue to have the kind of lifestyle that they want to be having.

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