Materials and Wear

# Hip Basic Sciences (2): Free body diagrams, Wear and Corrosion

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# Summary

Attend this on-demand teaching session with medical professional, Mr. Singh, to learn about the mechanics of 1st, 2nd and 3rd class levers as they relate to the body. Learn the assumptions, mechanics and equations of free body diagrams, such as the static moments of a hip joint and how the addition of a walking stick can affect a hip joint's force. Explore the implications of joint replacements and the resulting joint forces at play. Mr. Singh will be available for questions during the session.

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# Description

# Learning objectives

Learning Objectives:

- Identify the different types of levers and how they apply to the body
- Calculate joint reaction force using different tools such as a seesaw, force weighted diagram, or a vector diagram
- Analyze the effects of nonweightbearing on joint reaction force
- Understand the assumptions made when creating a free body diagram for the hip
- Analyze the effects of offset joint replacement on muscle force generation.

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#### Computer generated transcript

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The following transcript was generated automatically from the content and has not been checked or corrected manually.

Uh basically as you can see the first class lever like your hip joint, um a second class lever. It's like your foot and ankle. Um here where you have the fulcrum which is the tips of your toes, so standing on your tippy toes, is your fulcrum your load and your force versus your third class lever, which is in essence like your elbow, where you have your fulcrum, which is the elbow, the force which is your biceps tendon pulling up, and the load of whatever is in your hand. Um Those are the ones that are relevant to uh the body uh. The other ones where you can think of it are, I think it's a shovel, a wheelbarrow and a nutcracker uh and I'll leave you to figure out which ones of those are relevant to which, but each of those so picking up a load with a shovel in your hand, uh a nutcracker and a wheelbarrow are all examples of the 1st, 2nd, and 3rd past levers as well, so in order to talk about free body diagrams, inevitably one of the basic questions that comes up is that there are assumptions because most of what we're you know what we're assuming is not relevant entirely to real life, but you have to make assumptions in order to draw a simplistic diagram of what is otherwise a relatively complicated joint. Um So the assumptions are listed below and basically again, and you have to be able to pretty much real them off before you draw your free body diagram because inevitably one of the questions will be what are the assumptions and I've listed as many as I can think of so that bones are rigid levers, so two dimensional representation, representation of the three dimensional structure, hence the simplified nature of it, joints are frictionless. There is no antagonistic muscle axion muscles act only in tension. The line of the action of must sorry, the line of axion is along its cross sectional area, and joint forces are compressive. The other thing to consider which can help with the maths of it. All is that free body diagrams are static things um and that in essence um the sum of all moments is zero and what I basically mean by that is the moments of the clockwise and the anti clockwise um forces that we've just talked about, but if you add them all up, the joint must balance, I some of all of the moments must be zero and you'll see why that's perhaps relevant. So this is the most simplified version that I can draw of a hip and when you draw hip free body diagram, I would also urge you to draw it as perhaps as simply as you can, so you don't have to get all artistic about the trick antas and things like that I literally used to just draw as I've demonstrated here just the line of the femur representing the neck ahead an arbitrary acetabulum, and then these are the lines you end up drawing so the muscles that we talk about in the hip joint are the abductors, so you've got the force of the abductors and the perpendicular distance from the center from the center of the pivot, and then you've also got the body weight, the force of weight or the body weight, and the perpendicular distance um from the center of the head now for the sake of mass. Again, if you can, it is helpful to try and keep some figures in your mind, but the force of body weight there's always assumed to be 5/6 of your body weight. If you assume that it's a 60 kg person, which is 600 newtons, it just means your force of body weight becomes 500 it's just easier maths to deal with in the exam. The reason it's 56 of your body weight is because you take out the leg which is 1/6 of your body weight. I've suggested some figures there because it's in newton meters um that's why they're in decimals, so it's 50 a 500.15 for the hip for the uh for the center of your body, so 15 centimeters from the center of your femoral head to the middle of your body, or five centimeters or 50.5 m from the center of your uh femoral head to the abductors, so if we look at our seesaw down at the bottom right, you can see that the counter clockwise moments are the force of your abductors times b, and they must equal your counter your clockwise moments, which is your bodyweight times a. So by just the logic of doing that and hopefully as you'll see why the reason you have simple maths is that you can basically then perhaps do that math either memorized or on the table um and in essence, you're just dividing 0.15 by 0.5, which is three so 500 times three, is 1500 your force of your abductors, you can therefore calculate is 1500 newtons roughly the other way to rationalize this is something called a force vectored diagram. Now, as I mentioned to you, vectors have not only a magnitude so for example, um for the force of the abductors, it we've just said it was 1500 newtons, but it also has a direction so the arrow that you saw is the force in which the muscle will pull, so it points downwards and the body weight points down because of well gravity. So if we assume that f a. B. Is 1500 newtons, the length of the when you draw a free four specter diagram, the length of the arrow is in relevance to the size of the force so that when you then draw your force weight which is 500 newtons that's a third of f a. B, and as a result the arrow is a third of the length, and because I've mentioned to you that the sum of all forces have to be zero, the arrow has to go back to its home, so you can draw the two arrows in the direction in which they apply force, so you can see that the force of abductors is slightly on the wonk, The force of weight is directly down and then when you then join them back up, you can calculate your joint reaction force by just measuring the length of it if you do it on graph paper, and it also tells you the direction in which it pulls okay, so hence the reason why you can see the golden arrow pointing upwards, So the next inevitable question of this is what happens when you add in a walking stick and as a result which hand do you carry it in and I would hope that you know that when it comes to hips, you always carry the stick on the opposite side to your painful hip and the idea being that if you want to add in, if you think about where the center of your hip is, if you think about the seesaw and the bigger the counter clockwise moment, the lower the potential force of your abductors, it helps have another counterclockwise moment albeit on the other side of the body, and again if you can, if you were to think of it as the face of a clock and if you draw you know this pointing up, it would then just go round, like this because it would be a moment. It's always turning anti clockwise like that and that's why the force of the stick is counted as a anti clockwise moment. So now the equation changes where not only your anti clockwise moments are your force of your abductors times the distance, but it's also the force applied from the stick, times the distance, and so you can see here. The force of a stick is not much but where it gains its leverage is the fact that it's quite a distance so half a meter away and it still has to be balanced by the force of your weight and the distance from it, So again, the idea behind doing the joint reaction force is that as you can see, you add in your stick here and your force of your abductors suddenly goes down by a third, so 500 newtons versus 1500 without a stick and as a result of that, you also decrease your joint reaction force, so the questions of, when it's relevant uh and I'm really sorry, I would ask you more questions, but I don't think I can hear anyone. Um The question here is if this is the rock carrying his big bag if he had a painful right hip, would he be helping himself or hindering himself um and then I'm trying to see if I can vaguely hear anyone talking. Um I don't know that I can um and if I tell you that if you think of it as depending on which side your hip hurts, the stick is an anti clockwise moment, if you use it on the opposite side, because in essence, it pulls it up now. If you carry the big heavy bag, let's assume the rock has a painful right hip. If you carry the big heavy bag on your left side, it is adding another clockwise moment, so it will make things worse if he had a painful right hip and he carried his big heavy bag in his right hand, it would suddenly become an anti clockwise moment and it would then help you have to obviously the, depending on which hip you're talking about effects whether you talk about clockwise or anti clockwise moments. We traditionally talk about the right hip, where it also becomes relevant is for example in joint replacement surgery. Where again, depending on whether if this is the center of rotation now you see that the lateral offset has increased of this hip versus the native hip, so sorry, if you medialize the cup too much, you can, again you can potentially decrease the distance from your abductors, or indeed, if you increase the officer, a lateral offset here, what you're theoretically doing is also increasing, be which is where your abductors were So. In theory, you are advantage ing, uh the abductors if you increase the offset, but it has its own problems, and that's why we don't try and think of it too much, but it's just to be aware that where you put in hip replacements whether how much you medialis your couple whether you increase your offset. All of these can have effects on the eventual force that the abductors have to undergo obviously. I think we would all try and keep it as anatomical as possible okay. That's the end of the hip stuff, I'm sorry about the i t. Issues. Um I I don't know if anyone's got any questions and I'll try and hear them as best as I can any questions for mr singh guys. The chat box is empty. Can I ask something academically really can you hear me mr, sink. Um So guys, first of all uh can anyone hear me, does anyone have any questions uh. Maybe, if you can type it in the text and I'll reply to you. If you if I can what's your question panos uh get away now, Yeah just so uh so palaces asking about nonweightbearing um is nonweightbearing, therefore pointless. Uh interestingly um it's not pointless, but it's more the fact that you actually have a higher joint reaction force. If you're if you're nonweightbearing, then if your toe touch weightbearing because if you rest the leg to ground, even if it's just the toes, you allow the muscles to relax even temporarily. Whereas if you're truly nonweightbearing, actually, all your joints are pulling all your muscles are pulling that joint up into the hip. So theoretically it is better to toe touch weightbear than nonweightbearing. You have a lower joint reaction force uh perfect, perfect, I'm not sure that am rush can hear me, but thank you mr singh, I think we've got mr shock akani signed in. I saw him a minute ago, yes we do hi majeed uh you mean morning majid uh lovely um majid if mr chaka khan e, for those of you haven't met him yet as a consultant at the West Suffolk and is going to talk to you about uh wear and corrosion. Thank you, spare with me guys. It's the first time I used the middle and I'm trying to share, can share entire screen. It usually works a bit better interesting. I can't share my power point bear with me a minute. Again, i t issues, um While I'm doing this, I'm gonna try multitask while I'm trying she ate it and talk to you guys about basic science, it can't be any drier than this and I'm very sorry if we try and make it um uh interactive and enjoyable, but basic sciences, basic science uh share your screen and try and do that again working now not yet majid. When you click on present now, If you go to share your screen, there's then three tabs, you can go into window or entire screen window sometimes works well and then so the the two options share slides and share your screen, yeah good for share screen, and then you've got the option of chrome, tap window and entire screen, so either window or entire screen, whichever is easier, you'll be able to your presentation there and then just click on it. Thank you can you see my screen now. Mm no um is that did you go for a window or entire entire, I'm going to do. I'm going to defer to either Iggy or mike, might have me at it uh okay or can you say