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Principles of TKR Designs For Postgraduate Orthopaedic Exams
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Principles of TKR Designs For Postgraduate Orthopaedic Exams
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Segment:0 .
Good evening everyone, and welcome to this teaching session that jointly organized by the orthopedic Research UK and the orthopedic Academy.
Our invited guest this evening is Mr Ahmad zada, who is a consultant, orthopedic surgeon from Cambridge. His special interest is in knee surgery. He is finisher trained in this speciality and he's been finishing his training in 2020. It's all fresh. But he's done all the fellowships and everything, so it's really wonderful. That's what's he going to bring to us tonight.
So Ahmad is going to bring us today. His experience from working with the high volume arthroplasty surgeons and from his traveling fellowships both in the UK and in the United States. He has done his fellowship in Philadelphia, so it's wonderful to have that also that kind of input from across the Atlantic. So we're very pleased that RMAN has accepted our invitation to join us this evening, and I'm sure we'll all learn from him.
My name is Shiraz. And now I'll be moderating the session. I'll be taking your questions. And we'll put them to RMAN at the end. So please, guys, if anyone has any question, don't leave tonight with any doubts. Please put on all your questions and we will put them through and we will endeavor to answer all your questions. At the end of the session there will be an MSC, so please all pay attention so you can answer the questions correctly.
But the answers are all anonymous. So no worries, no pressure. If anyone is interested in taking part in a seat, Viva practice at the end. Please let Hannah or Lydia know that you are interested. So without further ado, I will leave you now with RMAN. Over to you, Mr Ambassador. Thank you for us for that very kind introduction.
I'm just going to share my screen. Here we are. OK good evening, everyone. Thanks for having me. I hope you get something useful out of this session. I'm going to be talking about my experience of this. Really? so if you don't agree with some of it, that's fine. This is just my view of it all.
I'm going to talk about total knee arthroplasty biomechanics and implant design. I am a consultant in Cambridge hospital, Cambridge University Hospitals with a specialist interest in knee surgery. So my talk is going to cover three areas, three main topics, really. History of totally nasty design. Then I'm going to talk about components and then we'll talk about mechanical alignment and balancing a total knee replacement.
So let's talk about the history and also how our understanding of knee kinematics has changed over this period. So I'm going to take you back, right back to 1891 to Themistocles Gluck. What a name he designed, the first hinged total knee arthroplasty using gypsum cement that's basically plaster, Paris and ivory. And how do you think the results were?
Well, they were a disaster. Lots of lots of soft tissue reaction and lots of loosening and then nothing very. So at this stage, the knee was thought to be a simple hinge. That's the biomechanics of this stage. So then nothing very much happens until all this. Designs this hinged implant made up of a metallic implant and uncemented and a simple hinge which only does not to 90.
It also doesn't have a chocolate roof. So unsurprisingly, this implant was plagued by migration, impingement and stiffness. So that also didn't work out very well. And you can see that implants are cutting out of the bone there. And then we've got 19. In the 1960s, Professor Swanson and Freeman challenged this idea of a simple hinge, and they decided to do more research into knee biomechanics.
Excuse me for a moment while I just remove the participants. It's just distracting these, like. So they designed the knee laboratory in Imperial and they realized that there's more than just a simple hinge. There is also sliding involved. So as the knee goes into early flexion, it's definitely rolling the fingers rolling on the tibia. And as it's going in, as the knee goes into further, deeper flexion, you can see that the contact points change and that there is also sliding of the femur on top of the tibia.
So it's more complex, complex biomechanics here. So they came up with the idea of this four bar linkage model whereby the anterior and posture cruciate ligament control that femoral slide and roll back. And it does seem to work in a very simplistic way if you just consider the lateral or the sagittal plane. So then with this idea, they designed the first condyle metal on metal on poly, totally arthroplasty.
Now prior to this, there were all hinges, whereas this implant trying to recreate that sort of four bar linkage model. Oh, and that also didn't have a chocolate groove. And then in the seventies, a chocolate groove was added and a midline gap between the condyles was created. And the reason for that actually was just to be able to remove the cement more easily. And this surprisingly had really good results.
10 year survival of 96% That's pretty good by today's standards. But this is a very small group. And then in the eighties, there was this massive. In the 70s there was this massive explosion of different total knee arthroplasty designs. And some of them went down the route of mechanical alignment and some of them went down the route of anatomic alignment. And that's why there are so many different needs on the market.
Now, what's the difference between them? So let's talk about mechanical alignment and anatomical alignment. So we've got. The mechanical alignment Cam which which work to create a perpendicular axis of the tibia. So that if completely perpendicular to the mechanical axis of the mechanical alignment axis.
And the idea behind that is that that, that way of implanting the tibial tree. Balances the forces on the medial, medial and lateral side and therefore allows it to stay implanted for longer. Whereas in the anatomical alignment Cam you've got natural three degrees of variance of the tibia. And therefore the idea was to try and recreate that and implant the tibial tree at three degrees of varus.
To try and recreate the anatomical alignment. It's important to say this alignment method was 3 degrees for everybody, so it was the same 9 degrees of hallux in distal femur and the three degrees of varus proximal tibia. Now, if we look at the results of that, there was actually catastrophic wear at five years in the anatomical alignment group. You can see there was lots of wear on the medial and actual side of the polys there.
And so anatomical alignment was abandoned in favor of mechanical alignment. And that's the way of doing our philosophy that we're all very familiar with. So in the 90s. So so that was the dominant way of doing knee replacements for decades and then in the 90s and 2000. A few more names into the fray here Freeman, Pinsker and Nakagawa.
They did some more research into natural native knee kinematics, and they discovered actually there are differences between the medial and lateral side. So the first thing to say is that the medial meniscus is excuse me, the medial meniscus is pretty fixed, whereas the lateral meniscus does is highly mobile and can move. Especially in deep flexion there is.
The medial, the medial tibial, the medial tibial plateau is dished, so it's concave, whereas the lateral Tibetan plateau is convex and that again allows the medial side to be more stable and the lateral side to be more mobile, not unstable, just mobile. And then we've got and then more research into this discovered that the MCL is a tough, thick band, which is isometric, whereas the lateral collateral ligament is a relatively thin band and it's quite lax in inflection.
So these are the differences. And this is a transverse or axial view of the tibial plateaus. The left side here, you can see that's the medial side and the right, the right of that is a lateral. And this is mapping the contact points of the femur on the tibia. And as you can see, as you go into deeper flexion, the contact point on the medial side stays relatively stable, relatively static, whereas on the right side, the deeper you go into flexion, the more posterior that contact point becomes, which indicates that the medial side stays current more or less in the same place.
So it's more of a balling socket, whereas the lateral side definitely has this posterior translation. And you can see on that flexion MRI scan, the tibia. So far, anterior femurs practically falling off the tibia. So to summarize that this has sort of proven that the native knee, at least is a medial pivot joint where the medial compartment, the medial plateau or medial femoral condyle, they stay static and they rotate about that axis, whereas a lateral compartment is mobile and is also lacks inflection.
So now take a deep breath there. That's native knee kinematics. So how do we design totally our plants to do the same thing? Well, it is complicated because knee knee totally outclassed. E kinematics relies on so many different things. You've got femoral geometry, got polyethylene shape or tibial geometry. You've got the constraints, the manner constraint. Patella obviously plays a role.
We haven't even talked about it. Alignment of the implants, ligament balancing. If it talks about balancing and of course, the dynamic stabilizes. Everything else can be really great wherever. If you don't have quads at work, it's not going to function. Also, pretty much all knee arthroplasty implants require the cutting of at least one of the cruciate. So it's quite difficult to recreate the native knee kinematics.
So let's talk about I'm just going to talk about a few of those points. I'm not going to go through all of it, but femoral geometry, first of all. So there are different implants on the market. Some have variable radius. Some have single radius. A single radius implants include the strange triathlon and GM k sphere by the doctor.
And the variable variable radius are Zimmer persona Gen 2 and more of the implants have the variable radius. So this if you look at the lateral view of an X of a knee, you can see that the shape of the femoral condyle is kind of oval. Yes so it can be forgiven for thinking that a variable radius is required here. But if you were to map just the posterior condyles, which is actually the axis about which the tibia tibia rotates, you could, if you are so inclined, model this on a circular or a cylindrical basis.
So you can decide which of the camps you go into. Now, I've got to say the variable radius and a single radius designs haven't shown a difference in terms of outcomes or in terms of survivorship, whether that's just down to the radius or whether there are other factors at play. I'll leave you to decide. But I think I would say if you believe that the MKL is an isometric structure and you use a variable radius, you have to be prepared to accept certain level of ankle laxity.
Inflection, whether that's clinically relevant or not, is a different point. OK then we've got conformity. Now conformity is defined as the degree to which the radii of the femur and the polyethylene conform to each other. So the difference between the radii now you can see on the right, this is just to orientate you. This is a sagittal view of an implant.
The grays metal and the White is or cream's poly. So the implant on the right is a highly conforming implant. And the one on the left is, is a highly non-conforming implant. It's the opposite. And the one in the middle is halfway now in a, in a, in a non conforming implant. What you get is a smaller area of smaller contact area and therefore higher contact stresses. Whereas in a very highly conforming design, what you get is a much wider contact area and therefore much lower contact stresses.
So that in theory is really good in terms of reducing linear wear on your polyethylene because you're not concentrating all the forces as you walk on a tiny area. Now, that was a real issue with older polyethylene, especially if they were stored in EXOGEN because that oxidized the top layer and it reduced its structural integrity. Conversely, if you're going for a highly conforming design such as the one on the right, what you get is a much higher contact area, which then in turn means that you are creating higher volumetric wear, more, more volumetric wear.
So higher generation of particles. But again, that's in a highly cross-linked polyethylene. That's not such an issue. We all know the poly is that the polyethylene has improved vastly. So that's conformity. And you can also consider this the coronal plane. So how much conformity you have in the other plane is orthogonal plane.
I think most of the time people talk about it in the sagittal plane. Good now, the other point to say is you don't have to have symmetry between the medial and lateral compartments of the joint. So this is the Medyka implant sphere. And you can see the left is a medial compartment and that's highly conforming or highly congruent. And on the right you've got the lateral compartment where you have quite a flat poly and therefore low conformity, and that allows for more stability in the medial side and more mobility on the lateral side, trying to mimic natural kinematics like that.
OK, let's talk about the constraint ladder. So the amount of constraint constraint is a description of how much resistance that is imparted by the design of the implants. Sometimes that's considered only in a various plane, and sometimes that's considered both in sum, essentially sometimes as is included in that. And sometimes not. It just depends on the point of view of your examiners, I guess.
So I would start with a CR implant and what you've got there is no post. So you're relying so this doesn't really provide much in the way of constraint. The next step up from that is so posterior stabilized and essentially that big post stops your tibia from falling posteriorly against the femoral component. So it's providing some anterior posterior stability or constraint.
And then we can go one step. But the implant doesn't confer very much in the way of various well, this constraint, whereas you've got there are various types of constrained implants or non so whereby there is movement and the femoral and the tibial components are not linked non linked constraint or condyle constraint implants. And essentially the post is just bigger, thicker and wider and and that fits a bit more concretely into the Cam or into the box of the, the femoral component.
And so not only provides anterior posterior stability, also provides some medial lateral stability. You would go with an implant like that if you're concerned about the collaterals. However, I would say it's not this is not a substitute for the MKL. So if you're concerned about the MCL, you should be really using a hinge. And then we're moving on to the hinges, the rotating hinges.
So the it's important to say the higher up you go in a constraint ladder, the more force you are transmitting to the implant or cement bone interface. And that's why you can see that the CR and pieces don't have a stem. They do have a keel. And sometimes there's a stubby stem on the. The implant, but if you go for a constrained or higher, you should be using a stem on both of your components.
So then we've got hinges, you've got a rotating hinge and a simple hinge. So rotating hinge allows for a bit of rotation, as the name suggests. And a simple hinge is it is only movement in a single plane. And as you saw from the wall, this designs and a thermos Togolese Gluck designs a simple hinge transmits a lot of force to the implant bone interface and therefore, is at risk of earlier loosening.
So that's the constraint ladder. Good this is how I felt when I first started learning about all this stuff. Where do you. Where do you go? Which implant. Do you choose? Well, you kind of have to think about.
Just move back. You kind of have to think about which philosophies you agree with. What's what's most important for the patient. And that specific case and what you're comfortable using, what you've been trained in. And then you also have to consider how your implant does on the air, what's approved in your area, in your hospital.
So let's say you have now chosen your total knee implant. What have we got to do? Well, we have to make some cuts and size the implant appropriately and balance the ligaments. There's a little bit more to it than that. You have to expose the knee and you also have to implant it perfectly so that it lasts for the long term. But we'll focus on these three things in the context of mechanical alignment.
So let's plan our cuts. This is an illustration of a mechanical alignment versus anatomical alignment in this right low, in this right lower limb. And you can see the blue line there indicates mechanical alignment, center of the hip to the center of the ankle. And the red line there is the mechanical is the anatomical axis of the femur and also the tibia.
Now, it's important just to reiterate that, that in the majority of patients, the tibia is in 3 degrees of natural virus. That's the majority. Obviously, there's a standard distribution. If you if you're interested in learning a little bit more about that, look at the Belmont paper, seminal paper on alignment in total knee arthroplasty. But essentially three degrees of areas is what you've got in the tibia on average in the population.
So I'm just going to Zoom in on the tibia there, on the knee there and do what we all do, of course, pre-operative templating. So the Black line there indicates joint line obliquity. The blue line there is a mechanical alignment line, the mechanical axis. And what I've done there is drawn a line 90 degrees or perpendicular to that blue line because in mechanical alignment, what we want is tibial tibial implant to be perpendicular to the alignment to balance the forces on it.
Here we are. That's how we're going to make our tibial cut. Now, in order to be able to. To to create a rectangular space for our femur and tibia. What we have to do is cut the femur in extension 90 degrees or parallel to this line essentially, or 90 degrees to the mechanical axis. So that's where we're going to make our cut. And that gives us a nice rectangular box for our implants.
Now, that just happens to be in six degrees of balance in the majority of patients on average, should I say? Actually, it's not the majority of patients, but on average is 6 degrees valgus. Good and that's where the six degrees of valgus comes from. So now we have a nice rectangular space in extension. And what we have to do is match that inflection so that we've matched our flexion extension gaps so the patient isn't too tight in flexion or isn't too loose in flexion.
So I've got to draw those lines for you. This is the knee in flexion. You can see the femur end on the blue vertical blue line is the mechanical axis. You can see goes through the center of the femur. And the blue line there is the cut that we've already decided to make on the tibia, perpendicular to the mechanical axis. So now what we have to do is cut our tibia and flexion or the posterior cut of our femur perpendicular to the tibial cut.
So there that gives us a nice rectangular box, both in flexion and extension, and it should then they should match. Now you'll notice that the Black line there is the thick Black line is the natural obliquity of the joint. And we said that the tibia was in 3 degrees of Eris. This this makes it 3 degrees of external rotation to the posterior axis. OK, so that makes sense.
So that's when we are sizing the femur, once we've taken it, once we've done our distal cut and we're sizing it, always reaching for that three degrees of external rotation. This is the reason we choose three degrees of external rotation because on average, the knee is in 3 degrees of obliquity. And if we've cut the tibia in perpendicular to the mechanical axis, the posterior needs to be three degrees, extend, rotated.
Otherwise you get an oblique flexion gap. So you have some laxity in flexion on the medial or lateral side. Does that make sense? Now, I'll just go back to that because. A lot of people think that the three degrees is so that you bring it closer to the patella. There is that does happen. But that's not the primary reason.
That's a subsequent occurrence. The reason you do 3ds orientation is because you want to match the flexion, extension, and you've cut the tibia in perpendicular to the mechanical axis. Now, I hope that makes sense. We can go back. We can come back to that later. Now I'm going to go through posterior referencing and anterior referencing, but I'm just going to have a little drink.
Now I'm sure you've heard about posty referencing an anterior referencing essentially. We're onto the second part of our treatment algorithm. We need to sign the femur. So we want to get the right size implant for that patient. So essentially, we need to measure what the length of this femur is from anterior to posterior. Now, where you decide to measure from is what posterior anterior referencing means.
So this is a posterior referencing jig. So what you're doing is measuring from the posterior condyle upwards. Now, in the majority of cases, you get absolutely right and it's perfect and you don't have to worry about it. If you if, say, you were to have an off day and under size the femur because you have referenced it from the posterior femoral condyle, what you get is a smaller implant than what the native femur.
But you're taking that extra bit of bone cut in the anterior part. So therefore, that leads to notching. And we're all worried about notching because there's a fracture risk of the femur. On the other hand, if you were to oversize it, if you were to go one bigger because you're worried about notching, what you get is the right place posterior again because that's where you've measured it from.
But you do end up with a bigger chunk of metal in the anterior aspect and that's called overstuffing the joint. So you would use posterior referencing if what you want is a consistently well matched posterior cut and therefore flexion gap, but that is at the expense of potentially either notching the femur or overstuffing the telephone joint or you can just get it right and not have to worry about it. Now in contrast, you are anterior referencing where this is just reversed.
So what you're doing is measuring from the anterior cortex. So when you get it right, it's perfect. When it's small. If you are two undersized femur, you would never notch because you've actually referenced it from the frontal cortex. But what you get is a smaller recreation of that posterior condyle.
Therefore, you get reduced posterior offset. Which is a concept you can look into later, and therefore you get a looseness inflection. If you were to oversize it over stuff, you over stuff reflection gap and therefore it's difficult for the patient to flex their knee because you've created a bigger femur than was originally there. So without your referencing, you never notched a femur, so you prevent femoral notching.
But that's at the expense of the flexion gap. Now it's up to you to decide which of those methods you want to employ, but you just need to know what they mean. This is this was probably more of an issue when John insole originally came up with originally started popularizing the mechanical alignment philosophy because implants there weren't so many sizes of implants available. So you had to go within 5 millimeters.
The next size up would have been 5 millimeters bigger. It's less of an issue now, but the principle is something you need to be aware of. OK so then we get on to balancing, sagittal balancing. Essentially this means balancing the flexion and extension gap. Now there are certain things that will affect the flexion gap and extension gap. The tibial cut affects both the flexion and the extension gap, whereas all the other things affect only the flexion or the extension gap.
So the posterior femoral cut we've already talked about affects the flexion gap. The tibial slope affects the flexion gap and the PCL affects the flexion gap. The extension gap is the distal femoral cut or the posterior capsule. So I'm going to go through some scenarios. Let's say you have done your femoral and tibial cuts and you can't get the smallest polyethylene liner in flexion or extension.
But actually the gap that you have is fairly symmetrical. So that's an easy problem to answer. What you do is you take a little bit more of the tibia. And that allows you to put because the tibia, a tibial cut affects both the flexion accident and extension, you can create a bit more space and it's automatically symmetrical, which is where it was before. So that's an easy fix.
Let's say, conversely, you've done your cuts and you've got a good flexion gap, but your extension gap is too tight. Now, what can you do in that situation? The first thing you can do is you can release the posterior capsule because we've said that can affect the extension gap. And if that doesn't work, what you'd have to do is cut a bit more off the distal femur. We know the distal femur affects the extension gap.
If you were to do that, you'd have to recut the chamfers as well. Just be aware of that. So again, fairly easy problem to fix. Let's say the opposite is true and that you've got asymmetrical gaps and the extension gap is, is, is good, but you're tight inflection. Now, this is interesting because it can either be that you're tight in flexion or that you're losing extension so that problem can fall into both of these.
It's a slightly different, more difficult problem to fix. You can either cut a bit more off the tibia. You have to be aware of how much you cut it out before. Or you can downsize the femur and translate it anteriorly. So that depends on whether you've referenced it. You've done anterior fencing or posterior fencing. If you've gone for anterior reference, you can just downsize the femur. If you gone for posterior referencing, as you remember, downsizing, it doesn't affect the flexion gap.
So you would have to translate it as well. So a little bit more thinking involved here, maybe. So you can downsize. It gives you a bit more flexion gap or you can destabilize the femur. Now if you are going to use an augment, you're going to have to use a stem and use a revision or revision kit. So not very easy to do unless you have the kit available.
So it's slightly more difficult problem to fix. But these are all scenarios that you should be familiar with and should be the tips of your tongues when you're sitting in the exam. So just to summarize that. If the problem is symmetrical tightness, then you can't proximal tibia. If the problem is tied to an extension, you can release the posterior capsule or cut a bit more of the distal femur.
And if you're tight in flexion, then you can either cut more slope on the tibia. You can reassess the PCO, use distal femoral augment, or you can downsize the femur and shift. It just requires a little bit more thinking. So I think that's the end of those three things. I think we talked about the knee knee biomechanics that I believe is a medial pivot joint. We've talked a lot about total knee arthroplasty designs and also talk to talked a lot about mechanical alignment, different cuts, balancing and referencing that are involved.
I hope you find that useful. That's the end of that. Well, Thank you very much, Mr yamada, for this comprehensive and focused lecture. We certainly. From it. You took us through everything about knee replacement history, biomechanics, constraint, leather, balancing techniques and all of that.
Really very, very interesting and very educational. Thank you for all the time and effort you put into producing this. So if you don't mind, can I put through to you some of the audience questions? Yes as you can imagine, as much as it is straightforward, a lot of the terminology, some people get confused with the terminology. Obviously, using knee replacement, it is a complex joint.
So we have a question from it. Well, he's asking, what's the difference between GAAP balancing and major dissection? You obviously touched on that, but you use different terminologies there, isn't it? Yes, absolutely. So so it's exactly what mine have said before. But he will explain to us more. So when you're doing.
OK so in reality, we use we use both of those methods in most people use both of those methods in doing a knee replacement. Measured resection is where you try to figure out. How much your implant is the thickness of your implant, and then you measure how much you take off the bone, whether it's the distal femur or the proximal tibia. And you cut at that level because what you're doing is you're replacing the bone that you take off with the metal.
Now, what you have to consider there is cartilage loss, and what you have to consider is bone loss. So when you're, for example, when you're doing your distal femoral cut, you put your intramedullary alignment in the jig, usually has 9 millimeters or 10 millimeters, depending on your system that you use. And that's essentially measured resection. You just have to figure out whereabouts that that's referencing from.
And so you may have seen that people take off any large osteophytes if they're interfering with that, because essentially you're going to be overstuffing it. If you're measuring from an area that you're not supposed to be measuring from. Similarly, with your proximal tibial cut, if you use an external alignment or jig, you will use a stylus and reference from either the medial or lateral side.
And on one side, an area that you think would have been where the tibia was and you cut it at that level thinking that what you've taken off is 9 millimeters, 10 millimeters or whatever it is, the thickness of your implant is from the proximal tibia where it was. So you replacing that gap balancing is a different thing. And it's slightly different in that it's more of an American thing.
You kind of cut make your cuts on one of the bones and then what you do is you Jack it out and you try to find the soft tissue envelope to a level that you think is normal. So not too tight. Not too not too lax. And that is your reference point point for cutting your other side, which is a female of the tibia and.
There are various aids to help you with that. You can use kind of pressure monitors and that sort of stuff. But essentially, I think what we do is, is a blend of the two. We definitely use measures resection. And then through this, what you also do is you do you do balance the ligaments at the same time. So if a side is too tight, you either decide to release. For example, let's say the medial side is too tight in a various knee.
You either decide to release, release the MCL and go right around to the medial side or you cut the ends in a degree of aerosol. So I hope that answers it. So essentially measured resection. You measure, you measure how much you take off and you replace it with metal balancing. You try and get the top tissues right and then make your cuts. I think we use a blend of the two personally.
Thank you very much. So would you think that's the gut balancing technique goes in line with the kinematic kind of alignment because you're cutting where you think that soft tissue balance dictates? Yeah, yeah, definitely. I think so, especially when it comes to. So I believe the metal is isometric. So I think if you're going to balance anything, you've got to make sure that what you're doing is right for the ankle.
So if you're going to balance it, you've got to make sure that the big osteophytes on the medial side have gone to be able to reference it, because you guys will understand if there's a big osteophyte on the medial side, the MCL is being tented. Let's say it makes you over cut and therefore you lose quite a lot of once you've made your cuts, then the muscle becomes quite like. So you have to bear that in mind.
Thank you very much. Now we have one more question from Martinique is asking what is said in your lecture that you would use a stem if using constrained knee replacement. And I want to know why we're using stem. Very good question. So essentially, you may remember I said it was all implants transmit forces at the cement or implant bone interface.
And the more constraint you have, the less is the lesson. The more constraint you have, the more of that force goes through the implant bone interface. And if you have a small surface area in the case of a cementless implants, then that that's quite a lot of force concentrated on a small area. It's going to loosen really quickly. So to try and reduce that, what you use is a stem which then increases the contact area of the implant bone or cement bone area, and therefore that dissipates.
The force is a bit more hoping that the implant lasts for longer and loosens less quickly. Excellent that was very clear. Thank you very much. Now we're talking about the referencing anterior refreshing and thus you always used to confuse me. I spent years getting confused and no one could tell me the clear answer. You've done it very well today.
But you know, it used to always confused me. I think people don't know what is posterior as simple as it is again, but people think that style is always in the front, so they think it's an interior referencing all the time. Yeah, but the stylists will have to be in the front, but it doesn't mean that it's interior referencing. It it can be done on the device and on the actual design of the device. On what exactly when you put your.
But if you can just explain that to us again, just go through the posterior reference, the anterior referencing before us again. So that people will know exactly what does this anterior and posterior referencing. It's not where you put the stylus, guys, it's not just your measuring the size, but referencing depends on the implant you use and some implants have anterior referencing and posterior referencing.
So that's a very good point. And you do need to get to know your system. This is I'm just going to show you this slide. You've got a jig with both anterior and posterior referencing holes there. I know. Sorry, I've got the wrong one. So you've got a jig here. On the left side is anterior referencing.
And can you see this tiny little anterior referencing laser marking and on the right side, you've got posterior referencing. So it's quite subtle. If you don't know that this is difference, you could easily get it wrong. So you just need to know your system. This is clearly not one that I use, otherwise I wouldn't have known it from the beginning.
But essentially it determines where the two points are on your cutting jig, on your cutting block, so that moves up or down, depending on the size. And if I were to show you this one. This this is a jig with both anterior and posterior referencing pins. So you can do this with this jig. You can certainly get it wrong. You've got anterior and posterior and it doesn't even have a written on it, does it?
No so you just have to know your system. In this one. This is the attune kit. You just need to know where you are going with it, because if you go for the wrong hole, you could really notch or over stuff or cause some serious issues. So if I were to go back to making that point, when you got from posterior referencing, you're referencing from the posterior femoral condyle upwards.
So if you were to get it, if you were to size it, you would. Notch but the idea is that you always get consistent flexion gap. So that's really good if you want to not have mid flexion instability, if you always want your need to be well balanced, you just have to be aware that you can not the femur. Now personally, this is the system I like to use because I can manage the notching bit myself, try and go as small as possible.
And if it looks like I'm not cheating, I just take the jig off and try and blend the top. What I don't want to do is over stuff the telephone joint, but I really want to make sure that consistently I've got the flexion gap right. Whereas if you are concerned about if you're using anterior referencing, you'll never notch the femur. But it's much harder to blend the posterior femoral cut if you get it too small, too big.
So that's what I would say. But get to know your system again. I wish I listened to this lecture a few years ago, gave me a lot of embarrassment. So we have one more question now with more complex questions. So Sinan asking how would you use the posterior anterior referencing system if you had posterior medial control loss from previous fractures, for example? Yeah, good point.
So there are various ways around it. Essentially what you're doing is you've got a plan in your mind and what you have to do is get that right in the femur. You plan your cuts and that's where you want to make the cuts. Now, you can either be really good at judging where those lines are, so you can try and imagine where the old femur used to be. And that's probably plan a, so you're trying to figure out what you're referencing off now, the posterior femoral posterior condyle.
That line isn't the only line you have available as referencing. What you've got is a consular access. You got Whiteside's line, you can use either of any of those references. Really, if you think that there's too much, too much bone loss for that sort of referencing, you can use you can use patient specific instrumentation, you can use robots, you can use navigation.
So there are various ways of getting referencing it. What you have to decide is whether you can do that yourself or do you need some extra help. Now, I would say that in a majority of cases there is a bit of if there is a bit of posterior loss and that happens to be usually in there or people talk about hyperplastic lateral femoral condyle, that's probably the most common, common thing you see. And people try and surgeons try to align their implants with their axis, which isn't affected by the posterior condyle where or perpendicular to white size line, which is the deepest part of the chocolate groove.
Now, if you feel that you can, you can use those over those references, then great. And I think in most cases, you can if there is quite a lot of where that and you need to use SCI or robots or navigation, then I, then you may also need to use an augment. So you probably need to go up in terms of your constraint. I hope that's helpful. That's great.
Thank you very much. Yeah so we have one more question from Hamid. He's asking about the kinematic concept in take care. You touched on that. But they I think they just wanted to know what is dangling that little worm and they bought it perfect. So kinematic alignment. Very interesting lots of research being going into this at the moment.
Quite a divisive, divisive subject because if you look at kinematic and mechanical alignment, there isn't a lot to choose between them. And kinematic alignment is a bit early in terms of its development. It doesn't have the decades of outcomes that mechanical alignment has. Essentially what you're doing is matching. The alignment of your implants to that patient's anatomy.
Now, if you look at the Benjamins curve, there's a standard distribution, there's a normal distribution of alignments in the population, with the average being around 3 degrees of varus. But that only really matches about 15% of the population. The rest of the population, they sit either more virus or more vagus. So if you go for that three degrees of virus, a mechanical alignment, you will get that.
Absolutely right. What you're doing is doing kinematic alignment for 15% of your patients and the rest of them less. So And you have to do various releases to cut to balance the me what I'm not answering very well here is essentially kinematic in kinematic alignment. You make your cuts according to the soft tissue tension so you don't have to release any ligaments.
It's quite complicated. And I would suggest reading a little bit more. There are some really good review articles about kinematic alignment. A pundit from Leeds has written a good review article. Charles Riviere has written some good articles about it. And look at, look at, look at the Benjamins paper and remember the name of the Californian guy. There's lots of good articles on there, but I'd start with the review article first.
Thank you very much. I mean, it's quite a complex concept. It's not that straightforward. Yeah and a lot of it a lot of us would use combination as well, isn't it? Right yeah, absolutely. There's no point doing perfect cups and then the balance is all off.
You've got to check your balance and vice versa. If you do balance techniques and you're one has to be born, cuts have to be well measured as well. So it's a combination. Most the most knee arthroplasty surgeons will use a combination in real life, but there are implants, which are particularly designed to be kinematic alignment implants, isn't it? You touch on the doctor and all of these.
So there are implants specially made for kinematic alignment. That's great. Thank you very much. Wonderful so I think we can now move on to the M6 OK please analogy if you can start chairing the M6. So k guys, these are three M6. We have related to the lecture. Mr Dave prepared for us.
He will go through the answers and discuss this with you shortly. So please, everyone, we have 84 participants tonight. So please, everyone answers. These answers completely anonymized. So you can safely give it a go. But please all go. We'll give you a couple of questions. Sorry a couple of minutes to answer.
If any one wants to take part in a diverted practice after this, please let Hanna know. Please you'll find Hanna. She's the host. Let her know. And any further questions about the lecture? Please write it in the chat.
Well, then, guys, you got. Yeah We need more people to answer. Only 10% We need 100% Everyone give it a go. You won't lose anything. You will learn whether you got if you got to try it. You learn that you have learned something new.
If you got it wrong, it will. You will remember it next time. So please give it a go. Everyone knee replacement.
The more you think about it, the more tricky it gets into, the more difficult. Definitely yeah, I'm doing those because I, I had to do a lot of thinking to make sure that. Right now let me know when.
How many minutes we've been down. Three I've got the timer. 3 minutes. So I think that's. That's enough, guys. I think that's what you get in your exams. I think you get just over 1 minute question the real exam. So I think. That's it.
So we'll end the poll here. Thank you, everyone. Guys who try to answer that will take us through the questions and the answers now, please. OK so question number one, native kinematics most closely, most closely resembles medial pivot model. We talked about this. We've got the medial compartment is very static and rotates, whereas the lateral component is highly mobile because of those differences that you saw the convex lateral tibial plateau and the contact point moving posteriorly.
So it's a medial pivot. Question number two, very confusing, which is, which of the following statements about total implants is false? So reducing conformity increases contact stresses on the polyethylene varying. It does, doesn't it, because it makes the contact area smaller, which concentrates the stresses. Reducing conformity increases the amount of volumetric wear on polyethylene bearing.
Well, it doesn't because it increases the amount of linear wear because you've got a smaller contact area, you have less volumetric wear, the more kind of boring, more of that linear wear. So that's false, which is why that's the right answer. Reducing conformity increases mobility of range of movement. Yes so if you have a smaller if you have less conforming implants that allows for more mobility and reduce conformity, increases the amount of linear where we already.
We already discussed that on the other bit. So the final final question, which of the following statements regarding flexion extension gap balancing is true? Well done to most of you. The cutting tibia will affect both flexion and extension. So distal resection. The femur will increase the extension gap when using the posterior fencing method downsizing the femoral component doesn't do anything to the flexion gap when using the anterior referencing method, downsizing the femur will reduce the.
It will increase the collection gap because you're making a smaller. So I hope that was useful. Well done to those who got it right. And it's all a learning opportunity. It was brilliant. Yeah Thank you very much. That was great.
Again, on behalf of all UK and the Academy would like to Thank you for joining us. And as I said before, we rely on generous general and generous educational consultants like yourself to join us and keep this program going. And without you, we cannot continue. So thank you very much again for joining us and we hope to see you again. Thank you very much.
And the. And that. Enjoy the evening. Thank you. Good luck to all of you. Goodbye Thank you very much.