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Biomaterial Behaviour in Arthroplasty for Orthopaedic Exams
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Biomaterial Behaviour in Arthroplasty for Orthopaedic Exams
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Language: EN.
Segment:0 .
Webinar, this is organized jointly by our UK and the FRCS mentor group. The presenter this evening is Mr. Allen Norris. He's a senior fellow at the senior research fellow at the University of Nottingham and a senior clinical fellow at the University Hospitals of Nottingham.
And I know is a PhD degree in basic sciences. I'm from University of Cambridge. And he's very well known to present internationally on subjects related to basic sciences in orthopedics. He's also very active, doing other projects related to orthopedics and patient safety and clinical governance.
He's particularly clinically interested in hip and knee surgery, as well as in orthopedic trauma surgery, with emphasis on limb reconstruction and bone infection. And he has over 40 publications cited for his knee. So we are very pleased that he's a kindly accepted to be our guest tonight, and I'm certain that all of us will learn a lot from him. I'm first of note, I will be moderating this session in association with Ruth from our UK, as well as my fellow mentors will be ready to support you during the Viva part and also during their questions.
So we will start by having a lecture. This will be followed by answering any questions you have. We will try to answer as much as you can in the chat box for you and we will leave a couple of interesting questions at the end to ask Allen. And there will be questions after the lecture, so we encourage you all to listen very carefully so you can able you can answer the questions, but you will be anonymous, so you can.
Please, I encourage you all to take part. After that, there will be a case based discussions where Allen will going to present us with three very interesting cases, and he's going to present us with a modern answer on how to answer these scenarios in the exam. Afterwards, we will start our vital practice on the hot seat Viva as usual, and we understand the stress you are under and we will support you and allow you time.
And to answer questions and try to help you out with reduced the stress and help you to manage. Basically talking to examine us, we'll give you feedback on this. So let there be only three questions. So if anyone is interested, please express your interest as soon as possible. We only can have three candidates for the hot seat tonight.
This will be followed by and that's the reason why is because then there's an additional short talk afterwards about 15 minutes. This will be focusing from the two candidates who successfully passed the exam a couple of weeks ago, and they were going to present us with how the new format of the clinical part is structured because now the clinical components of exams changed.
It's not there are no patients anymore, so it's all examiners based and case based discussion based. So they were going to let us know how did that go for them and what they learned and to allow the future candidates to prepare better? So thank you very much for log in without further ado. We leave you with Mr norrish, our guest tonight. Thank you so much for us, that's very helpful, and I'm just going to share my screen with you if that's OK.
And then I'll start my talk. So as you mentioned, I'm an orthopedic surgeon, but I've got an interest in basic science and I can see so many of you have logged on tonight, so you must also have that same interest in learning more. And I remember when I was studying for my exam, lots of the things that we were studying seemed to make quite a lot of intuitive sense.
I'm pretty comfortable with some of the basic science, like the anatomy. But then suddenly there was this whole subject about materials and material science and material behavior that we've never covered in medical school. We've never covered really in great detail anywhere else, and I was very intimidated by it. And I sort of made it a point of interest to try and learn as much as I could.
And I've been teaching this particular area. Now for probably about eight years. And this little talk, which I hope will get you through any it's designed just to get you through mainly the questions about the materials that you might use in orthopedics and to get you through any questions about the stress strain graph. So we have a limited objective.
But on the other hand, I hope by the end of this sort of half an hour, you feel pretty comfortable with some of those things. So this is just before we start. This is just to give you an idea. This is taken from ortho bullets about the high yield basic science topics that come up in the mix, and you can see that orthopedic implants and material properties of implants unusually high compared to some of the other things that you think come up.
And so I do think this is a pretty important subject for us to get to grips with. So I'm going to talk to you about some of the definitions, some of the terms that are used in material science and just try and make them clear for us all. And then I'm going to really focus in on the stress strain graph. And what we can learn from it and where we can get some of those definitions.
And then just towards the end of the talk, I want to talk specifically about hip and knee replacement because that's the kind of question you might get asked in Aviva. And that'll be some of my case presentations along the lines of, oh, you know what? Hip replacement do you use? And you think it's going to be a question about the evidence base for a certain prosthesis, but then they suddenly say, and what's that made out of and why have you chosen that particular type of material and what are the material properties?
And I hope to be able to give you some good ideas on how to answer that. So let's start with a few definitions within material science. And so material science is really about exploring the properties of a material, but we can't just explore them by looking at the material. We actually have to apply a load to a material and see how that material behaves when a load is applied.
And that that's one of the key things about understanding material science that it's how a material responds to a load which is applied to it. And of course, it can imagine a load as an externally applied force, and it can be applied in different directions like compression or tension, shear or torsion. And as soon as we apply a load to a body, there's going to be some internal effects or deformations of that particular kind of body.
And those are the kind of effects that we're going to be studying in material science to give us a list of properties, mechanical properties or material properties of a certain type of material. And those are the kind of terms that we use those deformations that I mentioned that happened to a body when a load is applied to it can either be elastic deformations, which are temporary. In other words, when you remove the load, the shape of that material go back to what it was before.
And plastic definitions deformations are permanent changes in length or shape and in general, and we'll see that on the stress strain graph in a minute. The degree of deformation is proportional to the amount of load that is applied to material. And then we also have a concept of elasticity, and that's important for us to understand because it's the ability of the material to return to its original resting length after a load has been applied and then removed.
So well, again, we'll see that on the stress strain graph as well. But these are just some of the terms that we're going to be looking at what is stress in a material. So it's actually the intensity of an internal force. It's very similar to pressure. And in fact, it has the same units as pressure, which is Newton per meter squared or foot.
Area otherwise known as pascals. So whilst we can understand if a pressure is applied externally, we can understand that actually when we apply a pressure, when we're sorry, when we apply a force to a body, we may not see any obvious defamation, but there will be an intensity of the internal force within the body. And that's really what stresses.
It's used to analyze the internal resistance of a body to a load, and normally something actually has to go on in there because if you apply Newton's third law as every action has an equal and opposite reaction. So if you're applying a force onto a body, something is going to be happening within that body to resist the force that is applied on it, even if it doesn't move.
So those are some of the principles. So stress basically comes in two forms, you can have a normal stress, which is a load, which is when a stress or force is applied perpendicular to the body that they act on. So that would be a typical example, would be a tensile or compressive stress. Or you can have shear stresses, which you can see in this diagram down at the bottom, where the forces are parallel to each other and they act in a different way because of that.
So we've mentioned stress, and that has the units of Newton per meter squared or Pascal's now strain is measured as the change in length over the original length of an object. In other words, the length is measured in meters, so it's meters over meters, and therefore they cancel each other out. And so strain has no units. I've seen that come up in multiple choice questions, occasionally in a.
What? what are the units of stress? Are the Newton meter squares. What are the units of strain? Answer it has no units. It's a proportion. It's the change in length over the original length of a material. And what's interesting about strain is stress and strain are proportional to each other on a graph up to a certain limit.
And I'm going to show you that in a second. But that is called Hooke's law. So those two are going to give you a straight line for an orthopedic material up to a certain limit. That means that they're proportional and that limit, although it does have different names, which I'll go through, is generally referred to as the proportional limit.
So I want to come on to the stress strain graph. But before I do, I just want to talk to you a moment about the Young's modulus, because the Young's modulus is actually a measure in the stress strain graph of the slope of that proportional zone within the elastic zone and the Young's modulus, which I'll show you on the graph in a second, is a measure of stiffness of a material.
And quite often there are questions about stiffness of the material because it does have a relation to the biology, to the effects within a system like, for instance, stress shielding and things like that. So we'll talk about that in a second as well. And the e, which is the measure of Young's modulus of elasticity, is unique for every material. So sometimes we want to know where it is in relation to each other.
So different relative Young's modulus and the higher the e, the greater the forces that are material can withstand. So let me show you this. This is quite an important graph to try and remember some of the details from. So it's the first time we've seen a stress strain graph, so you can see the stress here on this axis. You can see the strain down here on this axis, and what you can see is a straight line for all of these different materials, which I've drawn here not to scale, but showing you the relative stiffness.
Because remember, it's the slope that dictates the stiffness of a material that is the Young's modulus and their straight lines. Because, according to Hooke's law, the stress will be proportional to strain up until the elastic limit, and I've drawn them all for each case up to the elastic limit. So what we can see here on this stress strain graph is that some materials like ceramics, are very stiff.
Then we've got cobalt chrome, then stainless steel and titanium. Then we've got cortical bone. And that's important because a commonly asked question is, you know, which metal is most similar to cortical bone in its stiffness or in its Young's modulus? And the answer there would be titanium alloy. You can see that of the three main metals that we use in orthopedics, cobalt Chrome is the most stiff, whereas stainless steel sits between cobalt, Chrome and titanium.
You can see some of these other materials coming down here, like polymer Thelma sacrilege. You can see high density polyethylene there cancellous bone and then coming down to some of the biological tissues that really don't have much strength. So that's a pretty useful graph to remember, especially up at the top end, as those do tend to come up in multiple choice questions occasionally if you haven't done that yet.
So when it comes to the stress strain graph, the way that they're actually measured in metals is they put a set piece of metal within a special kind of vice, which I've shown here, and they pull that vise. They pull that metal apart and they measure the stresses and the change in length within it. And so. Just coming onto the next one, so here is the stress strain graph.
This is the kind of graph that you may be expected to draw in a Viva. They might say, can you draw me a stress strain graph? And they're going to ask you about certain points of it. So we have to remember stresses here on the y-axis. Strain on the X-axis. And the first thing we could draw is a straight line for our representative material because we know that stress is proportional to strain.
Up until the proportional limit and the slope of that is going to give us the Young's modulus. And that if we apply a C tensile force to this material. Before we get to the proportional limit and we remove that stress at any point, the strain will be such that it returns exactly to the same length that it had at the beginning. And what that means is that the material is in the elastic zone.
It is the elastic zone because it has the ability. If you remove the load at any point during that element of the stress strain graph, it will return to its original length. If you go beyond the proportional limit, then new things start to happen, which I'll show you. Unfortunately for us, the proportional limit has a number of different names, which you can read in different books like proportional limit yields points, elastic limit, but within the context, you just have to realize that this is absolutely critical point on the stress strain graph where you move out of the elastic zone into the plastic zone, and that is something like yield point or proportional limit, depending on what the particular question is asking.
So if we carry on, then along the stress strain graph, what we're going to see is that this is exactly what we've looked up before, so we get to the proportional limit. So the yield point, if we carry on applying this tensile strength, what immediately happens as you begin to get defamation within the material is that actually releases some of the stress in the material.
So you can see the stress going down. If, for instance, you took away the load at that particular point, what would happen is that you would get some elasticity, but you would not return to the same length that you had when you first started. And so you would have a defamation and we call that defamation if a load is removed, the plastic stream defamation.
In other words, you've reached the elastic zone, you've gone through the elastic zone. You've had permanent defamation, but then you've released it and that's the final plastic stream defamation that you're left with. If you don't remove the load and you carry on applying increasing load to that material. You find that the stress begins to build up again. The stress builds up and up and up until it gets to a point where no more stress can be held within that particular material.
And we call that the ultimate strength in this case that the tensile test. So we would call it the ultimate tensile strength, but it can be the ultimate strength. Sometimes we talk about, is it material strong? Well, that all depends on how much stress, what is the maximum stress that material can cope with in it? And what then happens if you carry on applying force, even a load?
Sorry, you carry on applying load even after you've reached your ultimate strength. The actual material itself begins to thin down in diameter until it gets to the point where it snaps and breaks. And that's why you get this. This necking, it's called just a release of some of the stress just before the break point. It's because you're changing the shape of the piece of material that you're testing as it begins to really begin to pull apart.
So and that point where it pulls apart, we call the breakpoint. So if we were to look at the whole of the graph, we would see that we have an elastic zone that's absolutely critical. Nice straight line. Remember, hooks for low stress is proportional to strain within the elastic zone and that if we were to remove the load, it would come back to its original length.
But if we go past the yield points or proportional limit, then we enter into this plastic deformation that ultimately goes up to the ultimate strength and then down to the break point. And all of that within that is called the plastic zone. Because if we remove the load at any point during this, we will have some plastic string defamation. And that is the really these are the really key points of the stress strain curve that we need to understand, because from that, we get a lot of information.
So the other thing that we can tell when we look at two different materials on a stressed train graph is whether they're brittle and the opposite of brittle is ductile and brittle basically means that the distance between the proportional limit and the breakpoint is very close. Whereas a ductile material is able to undergo a lot of plastic strain deformation or plastic deformation.
And so the distance between the proportional limit and the breakpoint is far. And in this particular example, we would be looking at these two materials if I asked you which was the strongest material you could potentially say, and you'd be right that the brittle material here is stronger because it's able to eat it reach a slightly higher ultimate tensile strength than this more ductile material.
So it's interesting, isn't it? So we have to get the terms precisely. But strength really relates to how much is the maximum amount of stress that can be contained within a material. Because the other element that we need to talk about is this concept of toughness. And what's important about material properties is to recognize a difference between the idea of strength, which is the ultimate tensile strength or toughness, which is actually a measure of the total strain energy and the Total Strength energy is the area underneath the stress strain graph.
So what you can immediately tell is that when you have a more ductile material, it's going to have an increased range of this plastic deformation. There's going to be a long distance between the proportional limits there and the breakpoint there. So a ductile material will have a lot of area under the graph. That is a measure of the total stream energy. And if you have a very high total straight energy, that's the amount of energy that a material is able to absorb before it breaks, then it will be a tough material.
So perhaps you can see now the difference between strength and toughness in this have come back one slide. But in this side, you can see that this material is strong, but this material is tough. So that gives you some more things that you can talk about if you're looking at a stress strain graph with somebody, a couple of other things that do come up that I want to mention about stress is that in terms of stress, there are two other sort of viscoelastic properties that we make use of in orthopedics.
I haven't talked a lot about this girl elastic properties in this talk, but elastic. We have talked about it. Do you remember the elastic zone, the plastic zone when you add visco? It's a measure of what fluids do. And so when you have biological tissues like bone, tendon, cartilage, they actually behave slightly like liquids and slightly like solids.
And together they have this thing called viscoelastic properties. So when we're talking about bone now, we have these two other material properties that I would like to mention. One of them is called crepe and the other stress relaxation crepe actually don't see that. So much with bone, but you see it with bone cement. And it's the ability of a material under a constant load to begin to flow like a liquid ever so slowly when it begins to do that.
A good example of that would be in a very old building, like a cathedral that's 1,000 years old. If you look at the stained glass windows in that, you see that although the glass is a solid, it's actually dripping over 1,000 years and it's collecting at the bottom of the pane. And it's much thinner at the top of the pain. And we see that also in bone cement, where gradually over a 20 year period that polished tapered stems inside the hip, for instance, it's actually creeping into the interstices of the bone over time because of the constant load applied to it because of the polished, tapered stem.
So that's something that can sometimes come up and stress. Relaxation is a very practical thing that's worth knowing if you're sticking in an interim medullary nail or perhaps an unscented acetabular cup. But wherever you have around area of bone and you are push something in to get a pressed fit and it's very tight, what's happening is you're creating a stress within the bone because you're applying a load.
But one of the wonderful things about viscoelastic materials is that stress will dissipate. That's because stress relaxation, so it gets a peak of stress. And then because it also behaves slightly like a liquid, it gradually dissipates. And that means that if you are hitting your acetabular cup in and you're hitting it really hard and fast but not giving time for the stress, the hoops stresses that are building up in the acetabulum to dissipate, then you can get a fracture.
I've seen that with intramedullary nails going down a slightly unredeemed, you know, isthmus of a bone. If you hit it too hard and fast, you're not giving time for the bone to relax. And as a consequence, you can end up with fractures because you're not allowing stress or relaxation to occur. So just coming on just to time, but I think we're OK for a few more minutes. I wanted to talk about some of the materials in particular that we use in arthroplasty in orthopedics when we talk about arthroplasty and some of the complications that we get.
We're mainly looking at in this case, some of the complications of metal. And I want to mention fatigue failure. Now, if you take a paper clip and you just move it gradually up and down, up and down, eventually it will snap and that is called fatigue failure. It's defined as repetitive loading cycles that are below the ultimate strength of material. But because there's so many of them, it actually ends up causing a fracture and we can see that we often see it, for instance, in plates when a fracture is slow to heal, we end up getting movement of the plate and then the plate will snap.
They don't tend to snap because their bad plates, they tend to snap because the fracture hasn't healed before. You've got fatigue failure. But in some situations, an example would be an airplane wing. You don't, you know, airplane wings do move a little bit when you take off and land. You don't ever want the airplane wing to fall off. So there's this concept of an endurance limit, and that means no matter how many millions and millions and millions of times you put a material through a load, it will never reach the stress that will allow it to get fatigued failure.
So you can repeatedly cycle it without fear of getting complications like fatigue failure. The problem is that endurance limit often means that things have to be very big. For instance, a plate would have to be massively bulky and not very practical. So in the end, sometimes you just can't use the endurance limits in orthopedic situations because it would be impractical.
So what about the metals we use, and I'm just going to talk really about three main metals. Stainless steel, cobalt Chrome and titanium alloy and stainless steel, as you probably know, is surgical grade stainless steel L standing for low carbon. That does tell you that it has some carbon in it. And so sometimes you can find questions that ask you about what kind of stainless steel that is.
And it's surprising how many things are found in the alloy of stainless steel and iron is about 2/3. But you've also got look how much chromium you've got and how much nickel. Quite a lot of nickel in there. And of course, some people have nickel allergies, so that's worth thinking about. So stainless steel is an alloy that's made up about 2/3 iron.
But then it's also got a lot of chromium, quite a lot of nickel, and it's got molybdenum. There are two metals that have molybdenum, stainless steel and cobalt Chrome. There's no molybdenum in titanium. So if you get a question that's talking about that particular element, then it's either one or one of these. And of course, there is carbon in stainless steel, but in this case, it's very low carbon.
And you can imagine if somebody was to say to you, if someone was to see this, someone's to give you an exit, total hip replacement and you'll buy it and say, you know, what is this X the total hip replacement native? You might say, oh, that is a great surgical grade stainless steel 3.6l. It's an alloy of iron, chromium, molybdenum, carbon and the material properties.
Well, it's strong. It's tough because it's ductile. You know, it has a Young's modulus because to talk about that, that's not too far away from cortical bone. But some of the other good things are that it's biocompatible. So it doesn't cause a lot of reaction within the body, except for people that have a nickel allergy.
So that's one thing to think about. Many of those things are true about cobalt, Chrome as well. The key thing about cobalt and Chrome is that those are the main elements of it cobalt, Chrome. And then just like stainless steel, it's got this molybdenum in it. It's you could use all of the same terms to describe it. But if you were talking about the Young's modulus, which I think would be a good idea, you would say that cobalt Chrome has the highest Young's modulus of the metals that we use in orthopedics.
It's higher than stainless steel, and it's higher than titanium. And therefore, whilst it has some good properties like you know, where properties, we have to be a little bit careful of things like stress, shielding of bone. And in fact, some implants like the CPT implants, which is a hip implant, which is a polished, tapered stem, is made out of cobalt chrome, and it's said to have a slightly higher periprosthetic fracture rate than the Exeter, which looks very similar to.
But they're made of different materials, and the stiffness of the cobalt Chrome could be playing a factor in that. And the other key thing that gets asked about cobalt Chrome is what if you use it in something like a metal or metal hip replacement? Obviously, you get a lot of chromium and cobalt ions, and that causes necrosis of the tissues around and can cause systemic problems as well.
So the other matter that I wanted to mention was the titanium. We use titanium a lot. If you can remember that it's mainly made out of titanium, but it's actually a titanium alloy and it has some aluminum and some vanadium in it as well. And the lovely thing is, a couple of really good things to remember about titanium. One is that it's Young's modulus.
That measure of stiffness is similar of as the most similar to cortical bone of any of the metals that we've mentioned. It, just like the other metals it's ductile, which makes it very tough but also strong and it's biocompatible. And one of the nice things is this particular property called self preservation. So normally if you take iron and it and it gets exposed to oxygen, it rusts that iron oxide.
But if you take aluminum and you expose it to oxygen, it degrades into aluminum oxide, which is actually a ceramic. So you end up getting this layer of ceramic over the metal that protects it from further sort of corrosion. That and titanium does that as well. So it creates a titanium oxide layer, which is actually a kind of ceramic, which stops it from undergoing any further corrosion. And that process is called self preservation.
It creates that layer that's resistant to corrosion. But you think titanium would be the perfect metal, but unfortunately it's got a weak area, and that is, it's very poorly resistant to wear. I took I've not talked so much about where because it's also a big subject, but in general, we don't want to use titanium in a bearing surface because it's poorly resistant to wear.
A couple of other things, just to mention a polyethylene. It's a plastic. So if you wanted to talk about its material properties, you could definitely talk about it being ductile. So has a long range within that plastic zone. It's also got some good where properties, but it has a low Young's modulus. We, where we use it. One two of things about the material properties of polyethylene that sometimes come up and that they can be modified by the radiation that's used to sterilize it.
So actually, radiation can improve its air characteristics, but it has a negative effect on its Young's modulus, its activity and its strength. And historically, although they don't do it any more, of course, they sometimes used to irradiate an air, but that creates a free radicals, which led to very poor where characteristics we bring on bone cement. We know that it's made of methane.
The fact relate it's not an adhesive, rather a Grout, and it helps to give us even lower distribution. And one of the wonderful things about it, which we've mentioned already, is this ability for it to undergo creep. I've mentioned this word anisotropic, and that means isotropic means that it has the same mechanical properties. Doesn't matter which direction that it is applied, but almost all materials, at least in the body, are anisotropic, and they will have different abilities to resist forces depending on whether their intention or compression.
And people love to talk about vacuum mixing as well. You might have a view on that. So coming up towards the end now, but I just wanted to mention ceramics, ceramics. If you were to talk about their material properties, they are brittle. They have a very short distance between the end of the elastic zone, the proportional limit and the break point.
They have a very high Young's modulus. It's even higher than cobalt Chrome. They have this property called wet ability, although this is another one of those where properties its ability for a drop of water, put on a ceramic head to spread over the ceramic head. And it does that in a much better way than if you were to put the same drop of water, for instance, on a stainless steel head.
And you can Polish them to get a very smooth surface, allowing you To have very low where. So sorry for the speed at which we've rushed through some of the mechanical properties, but we've looked at that stress strain graph and I would love it if by the end of this evening you felt that you could draw a stress strain graph and mention some of the key things on it, like the elastic zone, the plastic zone, the proportional limit.
You could explain what Hooke's law was. You could explain what tactility, brittleness, strength, toughness the area under the graph. That would be really good, and those things would get you through any Viber question that you'd be asked on that kind of thing. We've talked a little bit about viscoelastic materials. In particular, I've mentioned creep and stress relaxation. We've talked about some of the particular materials we've talked about three metals cobalt, chrome, stainless steel and titanium.
And we've looked at one or two of the other materials that we use in orthopedics as well. So thank you so much and happy to answer any of your questions. That's lovely, Allen. Many Thanks for this presentation. You've managed in half an hour to squeeze a lot of information in this presentation. It's wonderful and very useful and very interesting as well.
Lots of. I think you pitched it exactly at the level of the exam. What can it need to know? And I wouldn't expect that kind of need to know any more than this at all. If you if you know what's in this lecture about principles of biomaterials, you'll get through the stations. So thank you very much, Allen.
I enjoyed that and I'm sure everyone did. It was lovely. Thank you. So there have been lots of questions which myself and the other mentors try to answer a lot of questions. I think like, you know, you, maybe you can help us with that. But I think one of the candidates was there's a bit of confusion about the utility of titanium and its notch sensitivity.
I know this obviously described completely to different parts of the titanium, so not describe the surface. And often an implant as an articulating surface, but the activity described the actual material from the inside and how it behaves. So I think the exam is probably need to be careful what you're describing in and just use the terminology right.
So not sensitivity. You can correct me, Allen, but it doesn't come into the equation of. The activity at all, isn't it? Yeah, it's just describing the surface of the implant. And it's kind of the microscopic surface in a way, and it just means that actually it's easy to scratch it. Yeah, it's easy to scratch it. And when you get the scratches, they, for instance, if they're rubbing up against ultra high molecular weight polyethylene, they're going to scratch the ultra high molecular weight polyethylene, giving you increased where debris.
So it's very, very small scratches, but it's just not scratch resistant. So when we talk about duck utility, that's true for all metals, and that is a slightly different thing. That's its ability to undergo plastic strain. Defamation is that long distance between the proportional limit and the breakpoint. Yeah, I think in the example where I had it in my head is that when describing any material is described as ask about materials, slap the manufacturing of it, what is it made of?
What are the surface properties and what are the actual material properties? So I think and if you just divide this into those components, you'll be a lot easier in the exam to go systematically. So there was some other questions. I think candidates wanted to know a lot about the difference between proportional limit, elastic limit and the yield point.
The third question is, do we need for the iraqis? Do they need to worry about the difference between them? Or can we just described them S1 term as transitional points between the elastic and the plastic zones? Exactly so I mean, I think if you're in a vyver situation, you can just describe it any way that you want. And most likely somebody will give you. I don't know about covid, but often they used to just give you a pencil and a piece of paper and say, show me what you mean.
The problem comes when you are answering multiple choice questions because they're the same point could be coming up in different sort of with different sort of names. All of those three things that you mentioned the yield point, proportional limit and the elastic limit are exactly the same thing. But sometimes in North America, they like to use yield points more than proportional limit. So there's just a sort of cultural difference in different parts of the world as to which term people prefer.
And so I guess it depends which multiple choice that you're practicing on. But I would think for the vyver, it's no problem. You just have to explain whichever term you use. I'm sure the examiners will be familiar with it, so you could just go ahead and use it. That's lovely. I think I agree with you, when I was doing the exam, I didn't really so complex and I say different references speak about them differently.
So it was one interesting point you mentioned about that titanium, the self preservation of civilization, self preservation of the titanium. Now does that mean that titanium can be mixed with other metals? For example, can you put the titanium stem and cobalt Chrome here? OK, so great. So it's a question about galvanic corrosion here. So I haven't really gone into the different types of corrosion, but galvanic corrosion means that you're going to get accelerated corrosion because of mixing two different metals, and they're swapping electrodes and the two metals.
By the way, just before I answer your question, I feel like a politician, but I just get this in because the two metals that get the most galvanic corrosion that you should almost always avoid mixing together a stainless steel and cobalt Chrome. Actually, probably the correct answer for the exam is don't mix your metals because you're going to get galvanic corrosion.
In reality, if you're going to have to mix one metal in, then titanium because it's able to self isolate, it actually is more resistant to galvanic corrosion than any of the other metals. So for the exam, avoid mixing metals. Always avoid mixing cobalt, Chrome and stainless steel, which are highly sensitive to galvanic corrosion. Remember that there's some resistance to galvanic corrosion in titanium because of the process of self preservation.
Just lovely, Allen, I think you're very good at using the buzzwords which they can describe a complex concept, in few words. I think we all the candidates need to learn to use these buzzwords galvanic corrosion. And so on. And, you know, Hooke's law, all these things, know, they can't, you can't, they can't meet all up in the exam situation and they can redirect the station, according to that.
So many times Allen for that. Unfortunately for you, because of this interesting lecture, now everyone asking more, they want to know about other parts of the material they want to know more about try biology part, but his thesis and so on. So but I think this maybe if you could kindly find this time in the future again for another lecture, I think that would be very popular for everyone.
But obviously, you cannot cover all aspects of the biomaterials in a short lecture like that. But this is very stimulating this one. So thank you very much, Allen. Now I know you have a lot more still. So we'll get on with that. So next part will be the part. So if Ruth could kindly put the questions on this, please everyone answer these questions.
Three questions and you have just about 3 minutes to answer all anonymized. I encourage you all to attempt, please. At least you will know if you've answered correctly. And, you know, if you have a deficiency that you need to cover. And also, We know how much you benefit from the lecture today. Allen will take us through the answers afterwards.
Now with some explanation. At questions, please. Great here, so I can see that there's a lot of concordance on that first question. I'm just going to pull it back up to. So which of the following materials has a Young's modulus of elasticity most similar to cortical bone?
So this is really the one you may remember that one graph of the relative Young's modulus, that's one that's worth putting in the memory somewhere at least the first part of it, because titanium is the correct answer. So you guys have done so well on that because it's the metal that has the closest Young's modulus to cortical bone. Zirconium is a type of ceramic used, particularly in femoral heads, but actually there's one type of knee replacement that's also using that in the femoral part.
And so it's being a ceramic. It actually has a very high Young's modulus. Stainless steel is actually the next one up from titanium. And then the third one up is the cobalt Chrome and then the last one. And the most difficult is the ceramic of which we have to represented here. The zirconia and the stainless steel. And I'm just going to show you this slide again.
So you can see there that that's basically what we're looking at. We can see the different ones that I mentioned. So the most important thing to remember there is really the order that you've got here. And even if you can't remember that, just remember titanium equals critical. I think you've done that pretty well because you all got that right.
More or less. OK next one, which of the following defines the stress of which a material begins to undergo plastic deformation. This is a question about the proportional limit, but they're probably not going to use that term. So they said toughness. Now you remember that toughness is the area under the stress strain graph and materials that are tough have high dexterity because there's a long distance between the yield points and the breakpoint.
Ultimate strength is actually the highest point in the stress strain graph, where it gets the maximum stress that it can hold. So it's not that one. The yield strength, this is yet another term. I would have been happier. This is taken straight from the author Bullock's UK kite exam, so they put the yield strength now the strength. Remember, it's a measure of the stress that a material can hold, and it's measured by the amount of stress that can be measured on the y-axis.
And it is actually a certain strength at which point it yields and moves from the elastic to the plastic zone. And so that is the correct answer yield, strength, fatigue, strength. That's something different. Remember, that's when it's below the level, which is the breaking point stress and but it's multiple cycles which cause it to fail and endurance limit is related to fatigue strength.
But it's the maximum stress that you can do unlimited amounts of cycling and not get fatigue strength. So yield strength is the correct one there. And the key word there really is yield yield is the term that describes going from the elastic to plastic. And then the last question the elements of chromium, molybdenum, cobalt are the basic components of which following implant materials.
So aluminum oxide? No, that isn't the correct one that is basically made of aluminum and oxygen cobalt cobalt, Colbert said. Cobalt chromium or cobalt alloy is the correct answer. So remember, it's mainly chromium, but it has cobalt molybdenum in there as well. And then stainless steel has molybdenum, but it doesn't have any cobalt or Chrome.
It's iron, molybdenum, nickel and a little bit of carbon. And pmma, that's probably me. Third with that. Nobody thought that tantalum. I've mentioned tantalum, but tantalum is a rare Earth metal, and it's used by particular companies because you can get very nice sort of modeling microarchitecture of tantalum. But it's but it's not that commonly used, although it is probably something that we ought to start thinking about in the future, including as a material.
But it itself is an element. It's a metal element. So that is that's the answer to the polls, and I don't know if you want to follow up for us. No, I think that's fantastic. It's very good to see that most candidates have answered the correct one. And thank you for the explaining. Obviously, you know, there's some confusion about the stainless steel resistance.
It doesn't contain chromium and cobalt, so. So this very, very I think it's very typical of our case exam questions. And it's good to get used to this question, so they're very suitable, thank you very much and. And so we move on now next to Allen has prepared the three case discussions typical Kessler exam case discussions, and you cannot tell us the perfect answer or a modern answer for these three, and that's to prepare you guys for the vivo components afterwards.
Thank you so much, sir. I think when I was thinking about case discussion and material properties, I was trying to think, well, how does this actually come up in my experience? And I think the way that it most commonly comes up is people ask you some simple questions to start off with and then suddenly you find yourself talking about material properties.
And one of the ways I just get my slides up for that. One of the ways that they do that is looking at the hip replacement. So in my experience, they'll say something like, oh, Allen, have you ever seen a hip replacement? Yep and then they'll say, well, what hip replacement do you use in your practice? And then you say, well, I use the, you know, the Exeter total hip replacement and cemented stem in a cemented cup.
And then the very next question that they might follow on with is say, oh, use that. So what? What is the Exeter hip replacement made out of? And then it can become a little bit embarrassing. If you've said that you use that one in your practice, but you don't really know what it's made out of. But hopefully, whichever one you pick, then I think you should be able to talk a little bit about it.
So for instance, I think if you pick a cemented femoral stem, you need to be able to talk about what it's made of. And so I mean, you could pick whichever one you want, but if you pick the Exeter total hip replacement, then you could talk about the fact that it's made of surgical grade stainless steel. That's the 3 1 six L low carbon stainless steel. And you think that is a good choice for a stem because it's going to have a Young's modulus, which is not too far off the cortical bone, but.
It's better than titanium, because titanium has very low resistance, scratch resistance, and when you're talking about putting it inside the polished, you know, having a polished, tapered stem inside cement, you're going to end up getting scratching of the metal on the cement, which would create a lot of titanium where debris. And that's why stainless steel is much better than titanium, and it's the next one that's closest to cortical bone in the metals that we use in its Young's modulus.
Obviously, if they say, oh, what is Young's modulus? You can say it's a measure of stiffness of the material, and they can say, oh, well, if Young's modulus is important, what if we have too high a Young's modulus and you could talk about some implant companies make a Polish tapered stem with cobalt chrome? And that would be the Zimmer prosthesis, which uses it's called the CPT. And that particular prosthesis uses cobalt chrome, so it has a higher Young's modulus.
But there is some concern with that prosthesis that it has a higher periprosthetic fracture rate because it's potentially a very stiff area in an elderly patient going into a less stiff area at the end of the prosthesis, creating this sudden step off and a place where stress can concentrate, resulting in prosthetic fractures, although of course, it's hotly debated. But you you could talk then about the material principles why stainless steel has the advantage over cobalt Chrome.
So when it comes to the stem, you could also talk about the advantage of using polymer with acrylic. You could talk about the way that the cement creeps over time and allowing a sort of repeated micro lock because of the constant load that the polished, tapered stem applies on the cement as the bone remodels around the cement. It allows that cement to continue to creep and create the micro lock over time, minimizing the chances of loosening.
Of course, you must say that in order to get that constant load, you can't have end loading of the prosthesis. And that's why you have that small sort of air pocket at the end of the prosthesis, which is created by the central visor to allow there to be space there to give load along the sides of the polished, tapered stem. And you could also mention how important it was to have a kind of uniform thickness of cement around the polished, deepest system so that you could allow that load to dissipate evenly.
And 2 millimeters is thought to be the ideal amount when it comes to the acetabulum. Most of the bearing surfaces, or the bearing surface that I would choose if they were asking me what I used in my practice will be ultra high molecular weight polyethylene and I would use cross-linked polyethylene because it gives better where properties and I would use cement in this situation, or I would use an unscented cup and a hybrid, and you could talk about whether you would modify your practice dependent on the patient that you were treating.
For instance, a young patient might be able to tolerate greater hoop stresses that would be created from a press fit and cemented stem. And they might ask you about hoop stresses, and you could mention about stress relaxation being important to leave time between impact to allow the stress to dissipate so that you don't end up fracturing the bone. And you can talk about that.
Then what I would also say is that you, if you like the second part of my case discussion is about what if you want to use an unprecedented femoral stem, I know that they're becoming more popular, especially for younger patients where you have a good, you know, morphology of your proximal femur. And if you're going to use an unsympathetic femoral stem, or if they ask you about that, you could change your material. And in this case, titanium would probably be the preferable material.
You're not so much worried about the scratch of the titanium against the bone inside the femur, but here what you want is to have a Young's modulus of your metal that's as close as possible to that of cortical bone to minimize the effect of stress shielding. I can tell you more about stress shielding if you want. They might ask you about that, but basically, stress shielding is where all of the load is being carried by the implant to the distal bone.
And the consequence of that is that according to wolf's law, which is a form follows function. If the bone is not being put under stress, then the bone will begin to gradually dissolve away and you have a reduced bone density in those places. So that stress shielding some advantages of titanium are that great. It has a great Young's modulus, but it also allows you to get different kinds of coating on it.
You can either actually change the shape of the metal to create a better lock into the bone, or you can blast it with this plasma spray to put a ceramic like hydroxyapatite onto the outside of the titanium and bonds. Really worried about 20 degrees centigrade, and that then creates a very good ability for the titanium to bond into the bone. But they might say, would you use a titanium head with your titanium stem?
Answer no, because titanium is not scratch resistant, and so it makes a very poor surface for bearing. And so perhaps a much better bearing surface would be something like a ceramic head. And then again, in the same way as we talked about for the previous case of cemented stem, you could talk about whichever acetabular cup you wanted, most likely unless you're doing a reverse hybrid. Most likely, if you have an unscented femoral stem, you'll use an unscented acetabular component as well.
And the best acetabular component would probably either be tantalum, which you did mention earlier. You could talk about that. That would be something like continuum cup, or you could talk about using titanium again, minimizing stress shielding. And often they have special coatings on them like a hydroxyapatite.
And inside that cup, you could either have ceramic or polyethylene cup. We could talk about that. So the last bit of this case discussion that I wanted to talk about was about knee replacements, because that's another way that this particular question. Out material properties can come up there say something like, oh, you know, what type of knee replacements do you use in your practice?
And then you say the name of it. In my practice, I would use a Genesis two made by Smith and nephew. It doesn't really matter what you pick. To be honest with you, as long as I'm not crazy but pick anything that's within the top five on the National Joint Registry. But take a moment to find out what the materials are in those so that if anyone asks you about them, or if you get the opportunity to talk about it, you can talk about it with knowledge.
So most knee replacements actually use cobalt Chrome for the distal female for coating the distal femur. And the reason for that is that the reason for that is that it actually isn't such a worry about stress shielding right down there like it would be in the proximal part of the femur. And you can really Polish cobalt Chrome so that you can get great where properties.
The tibial tray, of course, is made of ultra high molecular weight polyethylene, often cross-linked. But interestingly, the base plate is normally made of titanium alloy, which is a little strange because actually it's often cemented in place. And like I mentioned in the proximal femur, we don't like to mix cement and titanium.
And yet in the proximal, in the proximal part of the tibia, it seems to be OK to mix those two together. Perhaps there's much less movement. Obviously, we're not relying on a polished, tapered STEM type model in general. Those are the three main components of a knee replacement. Cobalt Chrome for the femoral components, ultra high molecular weight polyethylene for the inserts and also for the patella button, and then a titanium base plate, which is cemented in both sides often cemented in.
Although of course, there are plenty of unselected total knee replacements as well. I've got this material to see if it comes up on the screen, but there's also another material which you might see. One particular company makes a material called axilo. It's the only one that really looks Black like this, and that's a ceramic sized metal. It's a kind of hybrid of ceramic on the outside and metal on the inside, so that it's less brittle and less likely to fracture as total knee replacement femoral components made of ceramica.
But because it behaves like a metal, it's less likely to fracture. But the very outer part of the metal is actually oxidized to create a smooth ceramic layer that can be polished. That's that particular proprietary brand called obsidian. So really, that I think brings me to the end of those sort of case discussions.
And I just I just wanted to give you an answer to the kind of question that you might get. What hip replacement do you use? What does it made of? What are the material properties of those particular things that it's made of? So hopefully, if you get asked that you'll have some ideas on how to answer it. Yeah, that's wonderful, and that covers it, really, I think these are most commonly asked scenarios when they so be prepared when someone asks you what happened with knee replacement use.
They're not just interested to know the brand. You could throw in any brand and then say, that's. It's a well-established one, but they just want to take it further and why you use this, so prepare one good answer and be ready that everyone talks about Exeter, so everyone will be prepared and seminars and everyone. So if you can say Exeter. The exam, you have to know all about the ins and outs of Exeter and how it functions and the whole Exeter philosophy.
So thank you very much, Allen. So now we will move on to the next part of this teaching session, which is hot seat driver. And we have three candidates. Obviously first come, first serve basis and sorry for anyone who didn't get an opportunity today. Obviously, as you know, we run some vyver courses now and you'll find the next course advertised on the UK website.
So today is we have three candidates, so we will time you guys to five minutes for questions and then we will give you feedback for five minutes. There will be two examiners. One of them is Allen. And we'll have the second examiner will be one of the mentors, so.
Segment:0 .
Webinar, this is organized jointly by our UK and the FRCS mentor group. The presenter this evening is Mr. Allen Norris. He's a senior fellow at the senior research fellow at the University of Nottingham and a senior clinical fellow at the University Hospitals of Nottingham.
And I know is a PhD degree in basic sciences. I'm from University of Cambridge. And he's very well known to present internationally on subjects related to basic sciences in orthopedics. He's also very active, doing other projects related to orthopedics and patient safety and clinical governance.
He's particularly clinically interested in hip and knee surgery, as well as in orthopedic trauma surgery, with emphasis on limb reconstruction and bone infection. And he has over 40 publications cited for his knee. So we are very pleased that he's a kindly accepted to be our guest tonight, and I'm certain that all of us will learn a lot from him. I'm first of note, I will be moderating this session in association with Ruth from our UK, as well as my fellow mentors will be ready to support you during the Viva part and also during their questions.
So we will start by having a lecture. This will be followed by answering any questions you have. We will try to answer as much as you can in the chat box for you and we will leave a couple of interesting questions at the end to ask Allen. And there will be questions after the lecture, so we encourage you all to listen very carefully so you can able you can answer the questions, but you will be anonymous, so you can.
Please, I encourage you all to take part. After that, there will be a case based discussions where Allen will going to present us with three very interesting cases, and he's going to present us with a modern answer on how to answer these scenarios in the exam. Afterwards, we will start our vital practice on the hot seat Viva as usual, and we understand the stress you are under and we will support you and allow you time.
And to answer questions and try to help you out with reduced the stress and help you to manage. Basically talking to examine us, we'll give you feedback on this. So let there be only three questions. So if anyone is interested, please express your interest as soon as possible. We only can have three candidates for the hot seat tonight.
This will be followed by and that's the reason why is because then there's an additional short talk afterwards about 15 minutes. This will be focusing from the two candidates who successfully passed the exam a couple of weeks ago, and they were going to present us with how the new format of the clinical part is structured because now the clinical components of exams changed.
It's not there are no patients anymore, so it's all examiners based and case based discussion based. So they were going to let us know how did that go for them and what they learned and to allow the future candidates to prepare better? So thank you very much for log in without further ado. We leave you with Mr norrish, our guest tonight. Thank you so much for us, that's very helpful, and I'm just going to share my screen with you if that's OK.
And then I'll start my talk. So as you mentioned, I'm an orthopedic surgeon, but I've got an interest in basic science and I can see so many of you have logged on tonight, so you must also have that same interest in learning more. And I remember when I was studying for my exam, lots of the things that we were studying seemed to make quite a lot of intuitive sense.
I'm pretty comfortable with some of the basic science, like the anatomy. But then suddenly there was this whole subject about materials and material science and material behavior that we've never covered in medical school. We've never covered really in great detail anywhere else, and I was very intimidated by it. And I sort of made it a point of interest to try and learn as much as I could.
And I've been teaching this particular area. Now for probably about eight years. And this little talk, which I hope will get you through any it's designed just to get you through mainly the questions about the materials that you might use in orthopedics and to get you through any questions about the stress strain graph. So we have a limited objective.
But on the other hand, I hope by the end of this sort of half an hour, you feel pretty comfortable with some of those things. So this is just before we start. This is just to give you an idea. This is taken from ortho bullets about the high yield basic science topics that come up in the mix, and you can see that orthopedic implants and material properties of implants unusually high compared to some of the other things that you think come up.
And so I do think this is a pretty important subject for us to get to grips with. So I'm going to talk to you about some of the definitions, some of the terms that are used in material science and just try and make them clear for us all. And then I'm going to really focus in on the stress strain graph. And what we can learn from it and where we can get some of those definitions.
And then just towards the end of the talk, I want to talk specifically about hip and knee replacement because that's the kind of question you might get asked in Aviva. And that'll be some of my case presentations along the lines of, oh, you know what? Hip replacement do you use? And you think it's going to be a question about the evidence base for a certain prosthesis, but then they suddenly say, and what's that made out of and why have you chosen that particular type of material and what are the material properties?
And I hope to be able to give you some good ideas on how to answer that. So let's start with a few definitions within material science. And so material science is really about exploring the properties of a material, but we can't just explore them by looking at the material. We actually have to apply a load to a material and see how that material behaves when a load is applied.
And that that's one of the key things about understanding material science that it's how a material responds to a load which is applied to it. And of course, it can imagine a load as an externally applied force, and it can be applied in different directions like compression or tension, shear or torsion. And as soon as we apply a load to a body, there's going to be some internal effects or deformations of that particular kind of body.
And those are the kind of effects that we're going to be studying in material science to give us a list of properties, mechanical properties or material properties of a certain type of material. And those are the kind of terms that we use those deformations that I mentioned that happened to a body when a load is applied to it can either be elastic deformations, which are temporary. In other words, when you remove the load, the shape of that material go back to what it was before.
And plastic definitions deformations are permanent changes in length or shape and in general, and we'll see that on the stress strain graph in a minute. The degree of deformation is proportional to the amount of load that is applied to material. And then we also have a concept of elasticity, and that's important for us to understand because it's the ability of the material to return to its original resting length after a load has been applied and then removed.
So well, again, we'll see that on the stress strain graph as well. But these are just some of the terms that we're going to be looking at what is stress in a material. So it's actually the intensity of an internal force. It's very similar to pressure. And in fact, it has the same units as pressure, which is Newton per meter squared or foot.
Area otherwise known as pascals. So whilst we can understand if a pressure is applied externally, we can understand that actually when we apply a pressure, when we're sorry, when we apply a force to a body, we may not see any obvious defamation, but there will be an intensity of the internal force within the body. And that's really what stresses.
It's used to analyze the internal resistance of a body to a load, and normally something actually has to go on in there because if you apply Newton's third law as every action has an equal and opposite reaction. So if you're applying a force onto a body, something is going to be happening within that body to resist the force that is applied on it, even if it doesn't move.
So those are some of the principles. So stress basically comes in two forms, you can have a normal stress, which is a load, which is when a stress or force is applied perpendicular to the body that they act on. So that would be a typical example, would be a tensile or compressive stress. Or you can have shear stresses, which you can see in this diagram down at the bottom, where the forces are parallel to each other and they act in a different way because of that.
So we've mentioned stress, and that has the units of Newton per meter squared or Pascal's now strain is measured as the change in length over the original length of an object. In other words, the length is measured in meters, so it's meters over meters, and therefore they cancel each other out. And so strain has no units. I've seen that come up in multiple choice questions, occasionally in a.
What? what are the units of stress? Are the Newton meter squares. What are the units of strain? Answer it has no units. It's a proportion. It's the change in length over the original length of a material. And what's interesting about strain is stress and strain are proportional to each other on a graph up to a certain limit.
And I'm going to show you that in a second. But that is called Hooke's law. So those two are going to give you a straight line for an orthopedic material up to a certain limit. That means that they're proportional and that limit, although it does have different names, which I'll go through, is generally referred to as the proportional limit.
So I want to come on to the stress strain graph. But before I do, I just want to talk to you a moment about the Young's modulus, because the Young's modulus is actually a measure in the stress strain graph of the slope of that proportional zone within the elastic zone and the Young's modulus, which I'll show you on the graph in a second, is a measure of stiffness of a material.
And quite often there are questions about stiffness of the material because it does have a relation to the biology, to the effects within a system like, for instance, stress shielding and things like that. So we'll talk about that in a second as well. And the e, which is the measure of Young's modulus of elasticity, is unique for every material. So sometimes we want to know where it is in relation to each other.
So different relative Young's modulus and the higher the e, the greater the forces that are material can withstand. So let me show you this. This is quite an important graph to try and remember some of the details from. So it's the first time we've seen a stress strain graph, so you can see the stress here on this axis. You can see the strain down here on this axis, and what you can see is a straight line for all of these different materials, which I've drawn here not to scale, but showing you the relative stiffness.
Because remember, it's the slope that dictates the stiffness of a material that is the Young's modulus and their straight lines. Because, according to Hooke's law, the stress will be proportional to strain up until the elastic limit, and I've drawn them all for each case up to the elastic limit. So what we can see here on this stress strain graph is that some materials like ceramics, are very stiff.
Then we've got cobalt chrome, then stainless steel and titanium. Then we've got cortical bone. And that's important because a commonly asked question is, you know, which metal is most similar to cortical bone in its stiffness or in its Young's modulus? And the answer there would be titanium alloy. You can see that of the three main metals that we use in orthopedics, cobalt Chrome is the most stiff, whereas stainless steel sits between cobalt, Chrome and titanium.
You can see some of these other materials coming down here, like polymer Thelma sacrilege. You can see high density polyethylene there cancellous bone and then coming down to some of the biological tissues that really don't have much strength. So that's a pretty useful graph to remember, especially up at the top end, as those do tend to come up in multiple choice questions occasionally if you haven't done that yet.
So when it comes to the stress strain graph, the way that they're actually measured in metals is they put a set piece of metal within a special kind of vice, which I've shown here, and they pull that vise. They pull that metal apart and they measure the stresses and the change in length within it. And so. Just coming onto the next one, so here is the stress strain graph.
This is the kind of graph that you may be expected to draw in a Viva. They might say, can you draw me a stress strain graph? And they're going to ask you about certain points of it. So we have to remember stresses here on the y-axis. Strain on the X-axis. And the first thing we could draw is a straight line for our representative material because we know that stress is proportional to strain.
Up until the proportional limit and the slope of that is going to give us the Young's modulus. And that if we apply a C tensile force to this material. Before we get to the proportional limit and we remove that stress at any point, the strain will be such that it returns exactly to the same length that it had at the beginning. And what that means is that the material is in the elastic zone.
It is the elastic zone because it has the ability. If you remove the load at any point during that element of the stress strain graph, it will return to its original length. If you go beyond the proportional limit, then new things start to happen, which I'll show you. Unfortunately for us, the proportional limit has a number of different names, which you can read in different books like proportional limit yields points, elastic limit, but within the context, you just have to realize that this is absolutely critical point on the stress strain graph where you move out of the elastic zone into the plastic zone, and that is something like yield point or proportional limit, depending on what the particular question is asking.
So if we carry on, then along the stress strain graph, what we're going to see is that this is exactly what we've looked up before, so we get to the proportional limit. So the yield point, if we carry on applying this tensile strength, what immediately happens as you begin to get defamation within the material is that actually releases some of the stress in the material.
So you can see the stress going down. If, for instance, you took away the load at that particular point, what would happen is that you would get some elasticity, but you would not return to the same length that you had when you first started. And so you would have a defamation and we call that defamation if a load is removed, the plastic stream defamation.
In other words, you've reached the elastic zone, you've gone through the elastic zone. You've had permanent defamation, but then you've released it and that's the final plastic stream defamation that you're left with. If you don't remove the load and you carry on applying increasing load to that material. You find that the stress begins to build up again. The stress builds up and up and up until it gets to a point where no more stress can be held within that particular material.
And we call that the ultimate strength in this case that the tensile test. So we would call it the ultimate tensile strength, but it can be the ultimate strength. Sometimes we talk about, is it material strong? Well, that all depends on how much stress, what is the maximum stress that material can cope with in it? And what then happens if you carry on applying force, even a load?
Sorry, you carry on applying load even after you've reached your ultimate strength. The actual material itself begins to thin down in diameter until it gets to the point where it snaps and breaks. And that's why you get this. This necking, it's called just a release of some of the stress just before the break point. It's because you're changing the shape of the piece of material that you're testing as it begins to really begin to pull apart.
So and that point where it pulls apart, we call the breakpoint. So if we were to look at the whole of the graph, we would see that we have an elastic zone that's absolutely critical. Nice straight line. Remember, hooks for low stress is proportional to strain within the elastic zone and that if we were to remove the load, it would come back to its original length.
But if we go past the yield points or proportional limit, then we enter into this plastic deformation that ultimately goes up to the ultimate strength and then down to the break point. And all of that within that is called the plastic zone. Because if we remove the load at any point during this, we will have some plastic string defamation. And that is the really these are the really key points of the stress strain curve that we need to understand, because from that, we get a lot of information.
So the other thing that we can tell when we look at two different materials on a stressed train graph is whether they're brittle and the opposite of brittle is ductile and brittle basically means that the distance between the proportional limit and the breakpoint is very close. Whereas a ductile material is able to undergo a lot of plastic strain deformation or plastic deformation.
And so the distance between the proportional limit and the breakpoint is far. And in this particular example, we would be looking at these two materials if I asked you which was the strongest material you could potentially say, and you'd be right that the brittle material here is stronger because it's able to eat it reach a slightly higher ultimate tensile strength than this more ductile material.
So it's interesting, isn't it? So we have to get the terms precisely. But strength really relates to how much is the maximum amount of stress that can be contained within a material. Because the other element that we need to talk about is this concept of toughness. And what's important about material properties is to recognize a difference between the idea of strength, which is the ultimate tensile strength or toughness, which is actually a measure of the total strain energy and the Total Strength energy is the area underneath the stress strain graph.
So what you can immediately tell is that when you have a more ductile material, it's going to have an increased range of this plastic deformation. There's going to be a long distance between the proportional limits there and the breakpoint there. So a ductile material will have a lot of area under the graph. That is a measure of the total stream energy. And if you have a very high total straight energy, that's the amount of energy that a material is able to absorb before it breaks, then it will be a tough material.
So perhaps you can see now the difference between strength and toughness in this have come back one slide. But in this side, you can see that this material is strong, but this material is tough. So that gives you some more things that you can talk about if you're looking at a stress strain graph with somebody, a couple of other things that do come up that I want to mention about stress is that in terms of stress, there are two other sort of viscoelastic properties that we make use of in orthopedics.
I haven't talked a lot about this girl elastic properties in this talk, but elastic. We have talked about it. Do you remember the elastic zone, the plastic zone when you add visco? It's a measure of what fluids do. And so when you have biological tissues like bone, tendon, cartilage, they actually behave slightly like liquids and slightly like solids.
And together they have this thing called viscoelastic properties. So when we're talking about bone now, we have these two other material properties that I would like to mention. One of them is called crepe and the other stress relaxation crepe actually don't see that. So much with bone, but you see it with bone cement. And it's the ability of a material under a constant load to begin to flow like a liquid ever so slowly when it begins to do that.
A good example of that would be in a very old building, like a cathedral that's 1,000 years old. If you look at the stained glass windows in that, you see that although the glass is a solid, it's actually dripping over 1,000 years and it's collecting at the bottom of the pane. And it's much thinner at the top of the pain. And we see that also in bone cement, where gradually over a 20 year period that polished tapered stems inside the hip, for instance, it's actually creeping into the interstices of the bone over time because of the constant load applied to it because of the polished, tapered stem.
So that's something that can sometimes come up and stress. Relaxation is a very practical thing that's worth knowing if you're sticking in an interim medullary nail or perhaps an unscented acetabular cup. But wherever you have around area of bone and you are push something in to get a pressed fit and it's very tight, what's happening is you're creating a stress within the bone because you're applying a load.
But one of the wonderful things about viscoelastic materials is that stress will dissipate. That's because stress relaxation, so it gets a peak of stress. And then because it also behaves slightly like a liquid, it gradually dissipates. And that means that if you are hitting your acetabular cup in and you're hitting it really hard and fast but not giving time for the stress, the hoops stresses that are building up in the acetabulum to dissipate, then you can get a fracture.
I've seen that with intramedullary nails going down a slightly unredeemed, you know, isthmus of a bone. If you hit it too hard and fast, you're not giving time for the bone to relax. And as a consequence, you can end up with fractures because you're not allowing stress or relaxation to occur. So just coming on just to time, but I think we're OK for a few more minutes. I wanted to talk about some of the materials in particular that we use in arthroplasty in orthopedics when we talk about arthroplasty and some of the complications that we get.
We're mainly looking at in this case, some of the complications of metal. And I want to mention fatigue failure. Now, if you take a paper clip and you just move it gradually up and down, up and down, eventually it will snap and that is called fatigue failure. It's defined as repetitive loading cycles that are below the ultimate strength of material. But because there's so many of them, it actually ends up causing a fracture and we can see that we often see it, for instance, in plates when a fracture is slow to heal, we end up getting movement of the plate and then the plate will snap.
They don't tend to snap because their bad plates, they tend to snap because the fracture hasn't healed before. You've got fatigue failure. But in some situations, an example would be an airplane wing. You don't, you know, airplane wings do move a little bit when you take off and land. You don't ever want the airplane wing to fall off. So there's this concept of an endurance limit, and that means no matter how many millions and millions and millions of times you put a material through a load, it will never reach the stress that will allow it to get fatigued failure.
So you can repeatedly cycle it without fear of getting complications like fatigue failure. The problem is that endurance limit often means that things have to be very big. For instance, a plate would have to be massively bulky and not very practical. So in the end, sometimes you just can't use the endurance limits in orthopedic situations because it would be impractical.
So what about the metals we use, and I'm just going to talk really about three main metals. Stainless steel, cobalt Chrome and titanium alloy and stainless steel, as you probably know, is surgical grade stainless steel L standing for low carbon. That does tell you that it has some carbon in it. And so sometimes you can find questions that ask you about what kind of stainless steel that is.
And it's surprising how many things are found in the alloy of stainless steel and iron is about 2/3. But you've also got look how much chromium you've got and how much nickel. Quite a lot of nickel in there. And of course, some people have nickel allergies, so that's worth thinking about. So stainless steel is an alloy that's made up about 2/3 iron.
But then it's also got a lot of chromium, quite a lot of nickel, and it's got molybdenum. There are two metals that have molybdenum, stainless steel and cobalt Chrome. There's no molybdenum in titanium. So if you get a question that's talking about that particular element, then it's either one or one of these. And of course, there is carbon in stainless steel, but in this case, it's very low carbon.
And you can imagine if somebody was to say to you, if someone was to see this, someone's to give you an exit, total hip replacement and you'll buy it and say, you know, what is this X the total hip replacement native? You might say, oh, that is a great surgical grade stainless steel 3.6l. It's an alloy of iron, chromium, molybdenum, carbon and the material properties.
Well, it's strong. It's tough because it's ductile. You know, it has a Young's modulus because to talk about that, that's not too far away from cortical bone. But some of the other good things are that it's biocompatible. So it doesn't cause a lot of reaction within the body, except for people that have a nickel allergy.
So that's one thing to think about. Many of those things are true about cobalt, Chrome as well. The key thing about cobalt and Chrome is that those are the main elements of it cobalt, Chrome. And then just like stainless steel, it's got this molybdenum in it. It's you could use all of the same terms to describe it. But if you were talking about the Young's modulus, which I think would be a good idea, you would say that cobalt Chrome has the highest Young's modulus of the metals that we use in orthopedics.
It's higher than stainless steel, and it's higher than titanium. And therefore, whilst it has some good properties like you know, where properties, we have to be a little bit careful of things like stress, shielding of bone. And in fact, some implants like the CPT implants, which is a hip implant, which is a polished, tapered stem, is made out of cobalt chrome, and it's said to have a slightly higher periprosthetic fracture rate than the Exeter, which looks very similar to.
But they're made of different materials, and the stiffness of the cobalt Chrome could be playing a factor in that. And the other key thing that gets asked about cobalt Chrome is what if you use it in something like a metal or metal hip replacement? Obviously, you get a lot of chromium and cobalt ions, and that causes necrosis of the tissues around and can cause systemic problems as well.
So the other matter that I wanted to mention was the titanium. We use titanium a lot. If you can remember that it's mainly made out of titanium, but it's actually a titanium alloy and it has some aluminum and some vanadium in it as well. And the lovely thing is, a couple of really good things to remember about titanium. One is that it's Young's modulus.
That measure of stiffness is similar of as the most similar to cortical bone of any of the metals that we've mentioned. It, just like the other metals it's ductile, which makes it very tough but also strong and it's biocompatible. And one of the nice things is this particular property called self preservation. So normally if you take iron and it and it gets exposed to oxygen, it rusts that iron oxide.
But if you take aluminum and you expose it to oxygen, it degrades into aluminum oxide, which is actually a ceramic. So you end up getting this layer of ceramic over the metal that protects it from further sort of corrosion. That and titanium does that as well. So it creates a titanium oxide layer, which is actually a kind of ceramic, which stops it from undergoing any further corrosion. And that process is called self preservation.
It creates that layer that's resistant to corrosion. But you think titanium would be the perfect metal, but unfortunately it's got a weak area, and that is, it's very poorly resistant to wear. I took I've not talked so much about where because it's also a big subject, but in general, we don't want to use titanium in a bearing surface because it's poorly resistant to wear.
A couple of other things, just to mention a polyethylene. It's a plastic. So if you wanted to talk about its material properties, you could definitely talk about it being ductile. So has a long range within that plastic zone. It's also got some good where properties, but it has a low Young's modulus. We, where we use it. One two of things about the material properties of polyethylene that sometimes come up and that they can be modified by the radiation that's used to sterilize it.
So actually, radiation can improve its air characteristics, but it has a negative effect on its Young's modulus, its activity and its strength. And historically, although they don't do it any more, of course, they sometimes used to irradiate an air, but that creates a free radicals, which led to very poor where characteristics we bring on bone cement. We know that it's made of methane.
The fact relate it's not an adhesive, rather a Grout, and it helps to give us even lower distribution. And one of the wonderful things about it, which we've mentioned already, is this ability for it to undergo creep. I've mentioned this word anisotropic, and that means isotropic means that it has the same mechanical properties. Doesn't matter which direction that it is applied, but almost all materials, at least in the body, are anisotropic, and they will have different abilities to resist forces depending on whether their intention or compression.
And people love to talk about vacuum mixing as well. You might have a view on that. So coming up towards the end now, but I just wanted to mention ceramics, ceramics. If you were to talk about their material properties, they are brittle. They have a very short distance between the end of the elastic zone, the proportional limit and the break point.
They have a very high Young's modulus. It's even higher than cobalt Chrome. They have this property called wet ability, although this is another one of those where properties its ability for a drop of water, put on a ceramic head to spread over the ceramic head. And it does that in a much better way than if you were to put the same drop of water, for instance, on a stainless steel head.
And you can Polish them to get a very smooth surface, allowing you To have very low where. So sorry for the speed at which we've rushed through some of the mechanical properties, but we've looked at that stress strain graph and I would love it if by the end of this evening you felt that you could draw a stress strain graph and mention some of the key things on it, like the elastic zone, the plastic zone, the proportional limit.
You could explain what Hooke's law was. You could explain what tactility, brittleness, strength, toughness the area under the graph. That would be really good, and those things would get you through any Viber question that you'd be asked on that kind of thing. We've talked a little bit about viscoelastic materials. In particular, I've mentioned creep and stress relaxation. We've talked about some of the particular materials we've talked about three metals cobalt, chrome, stainless steel and titanium.
And we've looked at one or two of the other materials that we use in orthopedics as well. So thank you so much and happy to answer any of your questions. That's lovely, Allen. Many Thanks for this presentation. You've managed in half an hour to squeeze a lot of information in this presentation. It's wonderful and very useful and very interesting as well.
Lots of. I think you pitched it exactly at the level of the exam. What can it need to know? And I wouldn't expect that kind of need to know any more than this at all. If you if you know what's in this lecture about principles of biomaterials, you'll get through the stations. So thank you very much, Allen.
I enjoyed that and I'm sure everyone did. It was lovely. Thank you. So there have been lots of questions which myself and the other mentors try to answer a lot of questions. I think like, you know, you, maybe you can help us with that. But I think one of the candidates was there's a bit of confusion about the utility of titanium and its notch sensitivity.
I know this obviously described completely to different parts of the titanium, so not describe the surface. And often an implant as an articulating surface, but the activity described the actual material from the inside and how it behaves. So I think the exam is probably need to be careful what you're describing in and just use the terminology right.
So not sensitivity. You can correct me, Allen, but it doesn't come into the equation of. The activity at all, isn't it? Yeah, it's just describing the surface of the implant. And it's kind of the microscopic surface in a way, and it just means that actually it's easy to scratch it. Yeah, it's easy to scratch it. And when you get the scratches, they, for instance, if they're rubbing up against ultra high molecular weight polyethylene, they're going to scratch the ultra high molecular weight polyethylene, giving you increased where debris.
So it's very, very small scratches, but it's just not scratch resistant. So when we talk about duck utility, that's true for all metals, and that is a slightly different thing. That's its ability to undergo plastic strain. Defamation is that long distance between the proportional limit and the breakpoint. Yeah, I think in the example where I had it in my head is that when describing any material is described as ask about materials, slap the manufacturing of it, what is it made of?
What are the surface properties and what are the actual material properties? So I think and if you just divide this into those components, you'll be a lot easier in the exam to go systematically. So there was some other questions. I think candidates wanted to know a lot about the difference between proportional limit, elastic limit and the yield point.
The third question is, do we need for the iraqis? Do they need to worry about the difference between them? Or can we just described them S1 term as transitional points between the elastic and the plastic zones? Exactly so I mean, I think if you're in a vyver situation, you can just describe it any way that you want. And most likely somebody will give you. I don't know about covid, but often they used to just give you a pencil and a piece of paper and say, show me what you mean.
The problem comes when you are answering multiple choice questions because they're the same point could be coming up in different sort of with different sort of names. All of those three things that you mentioned the yield point, proportional limit and the elastic limit are exactly the same thing. But sometimes in North America, they like to use yield points more than proportional limit. So there's just a sort of cultural difference in different parts of the world as to which term people prefer.
And so I guess it depends which multiple choice that you're practicing on. But I would think for the vyver, it's no problem. You just have to explain whichever term you use. I'm sure the examiners will be familiar with it, so you could just go ahead and use it. That's lovely. I think I agree with you, when I was doing the exam, I didn't really so complex and I say different references speak about them differently.
So it was one interesting point you mentioned about that titanium, the self preservation of civilization, self preservation of the titanium. Now does that mean that titanium can be mixed with other metals? For example, can you put the titanium stem and cobalt Chrome here? OK, so great. So it's a question about galvanic corrosion here. So I haven't really gone into the different types of corrosion, but galvanic corrosion means that you're going to get accelerated corrosion because of mixing two different metals, and they're swapping electrodes and the two metals.
By the way, just before I answer your question, I feel like a politician, but I just get this in because the two metals that get the most galvanic corrosion that you should almost always avoid mixing together a stainless steel and cobalt Chrome. Actually, probably the correct answer for the exam is don't mix your metals because you're going to get galvanic corrosion.
In reality, if you're going to have to mix one metal in, then titanium because it's able to self isolate, it actually is more resistant to galvanic corrosion than any of the other metals. So for the exam, avoid mixing metals. Always avoid mixing cobalt, Chrome and stainless steel, which are highly sensitive to galvanic corrosion. Remember that there's some resistance to galvanic corrosion in titanium because of the process of self preservation.
Just lovely, Allen, I think you're very good at using the buzzwords which they can describe a complex concept, in few words. I think we all the candidates need to learn to use these buzzwords galvanic corrosion. And so on. And, you know, Hooke's law, all these things, know, they can't, you can't, they can't meet all up in the exam situation and they can redirect the station, according to that.
So many times Allen for that. Unfortunately for you, because of this interesting lecture, now everyone asking more, they want to know about other parts of the material they want to know more about try biology part, but his thesis and so on. So but I think this maybe if you could kindly find this time in the future again for another lecture, I think that would be very popular for everyone.
But obviously, you cannot cover all aspects of the biomaterials in a short lecture like that. But this is very stimulating this one. So thank you very much, Allen. Now I know you have a lot more still. So we'll get on with that. So next part will be the part. So if Ruth could kindly put the questions on this, please everyone answer these questions.
Three questions and you have just about 3 minutes to answer all anonymized. I encourage you all to attempt, please. At least you will know if you've answered correctly. And, you know, if you have a deficiency that you need to cover. And also, We know how much you benefit from the lecture today. Allen will take us through the answers afterwards.
Now with some explanation. At questions, please. Great here, so I can see that there's a lot of concordance on that first question. I'm just going to pull it back up to. So which of the following materials has a Young's modulus of elasticity most similar to cortical bone?
So this is really the one you may remember that one graph of the relative Young's modulus, that's one that's worth putting in the memory somewhere at least the first part of it, because titanium is the correct answer. So you guys have done so well on that because it's the metal that has the closest Young's modulus to cortical bone. Zirconium is a type of ceramic used, particularly in femoral heads, but actually there's one type of knee replacement that's also using that in the femoral part.
And so it's being a ceramic. It actually has a very high Young's modulus. Stainless steel is actually the next one up from titanium. And then the third one up is the cobalt Chrome and then the last one. And the most difficult is the ceramic of which we have to represented here. The zirconia and the stainless steel. And I'm just going to show you this slide again.
So you can see there that that's basically what we're looking at. We can see the different ones that I mentioned. So the most important thing to remember there is really the order that you've got here. And even if you can't remember that, just remember titanium equals critical. I think you've done that pretty well because you all got that right.
More or less. OK next one, which of the following defines the stress of which a material begins to undergo plastic deformation. This is a question about the proportional limit, but they're probably not going to use that term. So they said toughness. Now you remember that toughness is the area under the stress strain graph and materials that are tough have high dexterity because there's a long distance between the yield points and the breakpoint.
Ultimate strength is actually the highest point in the stress strain graph, where it gets the maximum stress that it can hold. So it's not that one. The yield strength, this is yet another term. I would have been happier. This is taken straight from the author Bullock's UK kite exam, so they put the yield strength now the strength. Remember, it's a measure of the stress that a material can hold, and it's measured by the amount of stress that can be measured on the y-axis.
And it is actually a certain strength at which point it yields and moves from the elastic to the plastic zone. And so that is the correct answer yield, strength, fatigue, strength. That's something different. Remember, that's when it's below the level, which is the breaking point stress and but it's multiple cycles which cause it to fail and endurance limit is related to fatigue strength.
But it's the maximum stress that you can do unlimited amounts of cycling and not get fatigue strength. So yield strength is the correct one there. And the key word there really is yield yield is the term that describes going from the elastic to plastic. And then the last question the elements of chromium, molybdenum, cobalt are the basic components of which following implant materials.
So aluminum oxide? No, that isn't the correct one that is basically made of aluminum and oxygen cobalt cobalt, Colbert said. Cobalt chromium or cobalt alloy is the correct answer. So remember, it's mainly chromium, but it has cobalt molybdenum in there as well. And then stainless steel has molybdenum, but it doesn't have any cobalt or Chrome.
It's iron, molybdenum, nickel and a little bit of carbon. And pmma, that's probably me. Third with that. Nobody thought that tantalum. I've mentioned tantalum, but tantalum is a rare Earth metal, and it's used by particular companies because you can get very nice sort of modeling microarchitecture of tantalum. But it's but it's not that commonly used, although it is probably something that we ought to start thinking about in the future, including as a material.
But it itself is an element. It's a metal element. So that is that's the answer to the polls, and I don't know if you want to follow up for us. No, I think that's fantastic. It's very good to see that most candidates have answered the correct one. And thank you for the explaining. Obviously, you know, there's some confusion about the stainless steel resistance.
It doesn't contain chromium and cobalt, so. So this very, very I think it's very typical of our case exam questions. And it's good to get used to this question, so they're very suitable, thank you very much and. And so we move on now next to Allen has prepared the three case discussions typical Kessler exam case discussions, and you cannot tell us the perfect answer or a modern answer for these three, and that's to prepare you guys for the vivo components afterwards.
Thank you so much, sir. I think when I was thinking about case discussion and material properties, I was trying to think, well, how does this actually come up in my experience? And I think the way that it most commonly comes up is people ask you some simple questions to start off with and then suddenly you find yourself talking about material properties.
And one of the ways I just get my slides up for that. One of the ways that they do that is looking at the hip replacement. So in my experience, they'll say something like, oh, Allen, have you ever seen a hip replacement? Yep and then they'll say, well, what hip replacement do you use in your practice? And then you say, well, I use the, you know, the Exeter total hip replacement and cemented stem in a cemented cup.
And then the very next question that they might follow on with is say, oh, use that. So what? What is the Exeter hip replacement made out of? And then it can become a little bit embarrassing. If you've said that you use that one in your practice, but you don't really know what it's made out of. But hopefully, whichever one you pick, then I think you should be able to talk a little bit about it.
So for instance, I think if you pick a cemented femoral stem, you need to be able to talk about what it's made of. And so I mean, you could pick whichever one you want, but if you pick the Exeter total hip replacement, then you could talk about the fact that it's made of surgical grade stainless steel. That's the 3 1 six L low carbon stainless steel. And you think that is a good choice for a stem because it's going to have a Young's modulus, which is not too far off the cortical bone, but.
It's better than titanium, because titanium has very low resistance, scratch resistance, and when you're talking about putting it inside the polished, you know, having a polished, tapered stem inside cement, you're going to end up getting scratching of the metal on the cement, which would create a lot of titanium where debris. And that's why stainless steel is much better than titanium, and it's the next one that's closest to cortical bone in the metals that we use in its Young's modulus.
Obviously, if they say, oh, what is Young's modulus? You can say it's a measure of stiffness of the material, and they can say, oh, well, if Young's modulus is important, what if we have too high a Young's modulus and you could talk about some implant companies make a Polish tapered stem with cobalt chrome? And that would be the Zimmer prosthesis, which uses it's called the CPT. And that particular prosthesis uses cobalt chrome, so it has a higher Young's modulus.
But there is some concern with that prosthesis that it has a higher periprosthetic fracture rate because it's potentially a very stiff area in an elderly patient going into a less stiff area at the end of the prosthesis, creating this sudden step off and a place where stress can concentrate, resulting in prosthetic fractures, although of course, it's hotly debated. But you you could talk then about the material principles why stainless steel has the advantage over cobalt Chrome.
So when it comes to the stem, you could also talk about the advantage of using polymer with acrylic. You could talk about the way that the cement creeps over time and allowing a sort of repeated micro lock because of the constant load that the polished, tapered stem applies on the cement as the bone remodels around the cement. It allows that cement to continue to creep and create the micro lock over time, minimizing the chances of loosening.
Of course, you must say that in order to get that constant load, you can't have end loading of the prosthesis. And that's why you have that small sort of air pocket at the end of the prosthesis, which is created by the central visor to allow there to be space there to give load along the sides of the polished, tapered stem. And you could also mention how important it was to have a kind of uniform thickness of cement around the polished, deepest system so that you could allow that load to dissipate evenly.
And 2 millimeters is thought to be the ideal amount when it comes to the acetabulum. Most of the bearing surfaces, or the bearing surface that I would choose if they were asking me what I used in my practice will be ultra high molecular weight polyethylene and I would use cross-linked polyethylene because it gives better where properties and I would use cement in this situation, or I would use an unscented cup and a hybrid, and you could talk about whether you would modify your practice dependent on the patient that you were treating.
For instance, a young patient might be able to tolerate greater hoop stresses that would be created from a press fit and cemented stem. And they might ask you about hoop stresses, and you could mention about stress relaxation being important to leave time between impact to allow the stress to dissipate so that you don't end up fracturing the bone. And you can talk about that.
Then what I would also say is that you, if you like the second part of my case discussion is about what if you want to use an unprecedented femoral stem, I know that they're becoming more popular, especially for younger patients where you have a good, you know, morphology of your proximal femur. And if you're going to use an unsympathetic femoral stem, or if they ask you about that, you could change your material. And in this case, titanium would probably be the preferable material.
You're not so much worried about the scratch of the titanium against the bone inside the femur, but here what you want is to have a Young's modulus of your metal that's as close as possible to that of cortical bone to minimize the effect of stress shielding. I can tell you more about stress shielding if you want. They might ask you about that, but basically, stress shielding is where all of the load is being carried by the implant to the distal bone.
And the consequence of that is that according to wolf's law, which is a form follows function. If the bone is not being put under stress, then the bone will begin to gradually dissolve away and you have a reduced bone density in those places. So that stress shielding some advantages of titanium are that great. It has a great Young's modulus, but it also allows you to get different kinds of coating on it.
You can either actually change the shape of the metal to create a better lock into the bone, or you can blast it with this plasma spray to put a ceramic like hydroxyapatite onto the outside of the titanium and bonds. Really worried about 20 degrees centigrade, and that then creates a very good ability for the titanium to bond into the bone. But they might say, would you use a titanium head with your titanium stem?
Answer no, because titanium is not scratch resistant, and so it makes a very poor surface for bearing. And so perhaps a much better bearing surface would be something like a ceramic head. And then again, in the same way as we talked about for the previous case of cemented stem, you could talk about whichever acetabular cup you wanted, most likely unless you're doing a reverse hybrid. Most likely, if you have an unscented femoral stem, you'll use an unscented acetabular component as well.
And the best acetabular component would probably either be tantalum, which you did mention earlier. You could talk about that. That would be something like continuum cup, or you could talk about using titanium again, minimizing stress shielding. And often they have special coatings on them like a hydroxyapatite.
And inside that cup, you could either have ceramic or polyethylene cup. We could talk about that. So the last bit of this case discussion that I wanted to talk about was about knee replacements, because that's another way that this particular question. Out material properties can come up there say something like, oh, you know, what type of knee replacements do you use in your practice?
And then you say the name of it. In my practice, I would use a Genesis two made by Smith and nephew. It doesn't really matter what you pick. To be honest with you, as long as I'm not crazy but pick anything that's within the top five on the National Joint Registry. But take a moment to find out what the materials are in those so that if anyone asks you about them, or if you get the opportunity to talk about it, you can talk about it with knowledge.
So most knee replacements actually use cobalt Chrome for the distal female for coating the distal femur. And the reason for that is that the reason for that is that it actually isn't such a worry about stress shielding right down there like it would be in the proximal part of the femur. And you can really Polish cobalt Chrome so that you can get great where properties.
The tibial tray, of course, is made of ultra high molecular weight polyethylene, often cross-linked. But interestingly, the base plate is normally made of titanium alloy, which is a little strange because actually it's often cemented in place. And like I mentioned in the proximal femur, we don't like to mix cement and titanium.
And yet in the proximal, in the proximal part of the tibia, it seems to be OK to mix those two together. Perhaps there's much less movement. Obviously, we're not relying on a polished, tapered STEM type model in general. Those are the three main components of a knee replacement. Cobalt Chrome for the femoral components, ultra high molecular weight polyethylene for the inserts and also for the patella button, and then a titanium base plate, which is cemented in both sides often cemented in.
Although of course, there are plenty of unselected total knee replacements as well. I've got this material to see if it comes up on the screen, but there's also another material which you might see. One particular company makes a material called axilo. It's the only one that really looks Black like this, and that's a ceramic sized metal. It's a kind of hybrid of ceramic on the outside and metal on the inside, so that it's less brittle and less likely to fracture as total knee replacement femoral components made of ceramica.
But because it behaves like a metal, it's less likely to fracture. But the very outer part of the metal is actually oxidized to create a smooth ceramic layer that can be polished. That's that particular proprietary brand called obsidian. So really, that I think brings me to the end of those sort of case discussions.
And I just I just wanted to give you an answer to the kind of question that you might get. What hip replacement do you use? What does it made of? What are the material properties of those particular things that it's made of? So hopefully, if you get asked that you'll have some ideas on how to answer it. Yeah, that's wonderful, and that covers it, really, I think these are most commonly asked scenarios when they so be prepared when someone asks you what happened with knee replacement use.
They're not just interested to know the brand. You could throw in any brand and then say, that's. It's a well-established one, but they just want to take it further and why you use this, so prepare one good answer and be ready that everyone talks about Exeter, so everyone will be prepared and seminars and everyone. So if you can say Exeter. The exam, you have to know all about the ins and outs of Exeter and how it functions and the whole Exeter philosophy.
So thank you very much, Allen. So now we will move on to the next part of this teaching session, which is hot seat driver. And we have three candidates. Obviously first come, first serve basis and sorry for anyone who didn't get an opportunity today. Obviously, as you know, we run some vyver courses now and you'll find the next course advertised on the UK website.
So today is we have three candidates, so we will time you guys to five minutes for questions and then we will give you feedback for five minutes. There will be two examiners. One of them is Allen. And we'll have the second examiner will be one of the mentors, so.
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