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Biomechanics for Postgraduate Orthopaedic Exams
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Biomechanics for Postgraduate Orthopaedic Exams
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Language: EN.
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Thank you. Thank you, Jane. Good evening, everyone. Welcome to this teaching webinar presented to you today by orthopedic Research UK and orthopedic Academy. This is one of our series for FRCS exam. The present the presenter evening is Professor Emad Saweeres.. He is a consultant and a Professor in the teaching hospital and Good Shepherd hospitals in Cairo, Egypt.
He had his training in the UK and has done his exams in the UK, but he's very active educator. He's very active in teaching with us orthopedic Academy for a long time. He presented several times and active faculty on our courses, but in addition, he is very active internationally in teaching all over the world, but particularly in the Arab world and in Egypt. He's very active with the other board, educational board there and with the Egyptian board of orthopedics.
He's one of the founding members and main educational pillars in the Arabic. So very, very privileged that he is with us tonight, and he accepted our invitation. So I'm feeling slap modulating, and with me today. We got from our UK team, Hannah and Imogene, and I've got my colleagues as well. Tamir Carmel and Sian hinari helping us running the session today.
So the session, as usual, will include the lecture the lecture is about orthopedic biomechanics focused on the FRC as exam. It will run for about 30 to 35 minutes. We normally do queues at the end, but today to keep it a bit more interactive, we will do the emcee cues in between just to make sure you are guys all still awake and with us. Yes, we'll do them emcee queues in between.
After that, we will do the Viva session Habsi driver session. We have limited places only three. So if any of you is interested, please express that either send me a message or to hand our emma-jane or write on the chat box or raise their hand symbol. Just indicate to us you are interested as soon as you can. Yeah, we understand it's very stressful and we will wait. However, we encourage you to actively take part in this Viva practice.
Be very useful for you, for confidence, for your technique, as well as getting the feedback from the faculty. If you missed any part of this session, don't worry it will. It's been recorded and it will be posted later on the Australian Academy YouTube channel. And it just before I hand over to Professor civiles, I just want to share with you this our upcoming courses with our UK we have a mock exam courses running throughout the year.
We have one in January and we have again in March and in April. So please go to the our UK website for details. This is a mock exam course where we replicate the exact exam scenarios with IVUS and clinicals and try to put you the same exam situation time in exam situations with feedbacks. We also run our case discussion courses and we have multiple courses running throughout the year.
Please visit the summit Academy website if you're interested. The case discussion courses are more interactive and a lot more teaching and feedback is given to candidates, so it's very useful at any stage of your exam preparation. And it just happened today. We had someone canceled the course on 11th of December, which is Saturday is of anyone interested. Please go with the Academy and book as soon as you can.
So without further ado, we leave you now with Professor queries over to you, prof. Thank you for us. Thank you very much for the very kind of invitation and for the very generous presentation and introduction. So good evening, everyone. The orthopedic Academy have truly managed to save the best topic until the last webinar of the year.
My name is imad sawiris and I am one of the orthopedic doctors at a teaching hospital in Cairo. When I did my a-levels, I was not sure whether I wanted to go into engineering or medicine. So as a compromise, I decided to take up medicine and marry an engineer. Incidentally, I did also my M.D. thesis at Kean University with both an orthopedic and an engineering supervisors.
So by the end of this lecture, you should all be able to appreciate the importance of biomechanics for our practice. You should be able to use the Newton's three laws to analyze forces and women's draw and solve free body diagrams and be able to describe mechanical properties of different materials. Mechanics is the study of machines and when this.
Which you are not talking about, biomechanics critic biomechanics is particularly interested in facial joints, function and the mechanics of bone and soft tissue mechanics can be divided into statics, which is concerned with studying objects that are at rest or moving at a constant velocity. And dynamics, which is concerned with objects changing their velocity.
Kinematics is only concerned with description of motion without taking into account forces which cause such motion. Such forces are studied under kinetics. It all started when Sir Isaac Newton was attacked by a small Apple. This made him realize that the apple, which has a certain mass, was moved by the force of gravity. This force is equal to gravity, which on planet Earth is roughly equal to 9.81 meters per second squared.
And is measured in newtons. If one small Apple has a mass of 100 grams, its weight is approximately 1 Newton. He adequately described those three laws, which you all studied at school. Every Apple remains on the tree unless acted upon by an external force. Once free to move, it accelerates at a rate proportionate to the applied load and for every action, there is an equal and opposite reaction.
Remember inertia, acceleration, reactions, eye air as opposed to IRA. A potential question in the exam is what is a force? This is defined as a load acting on a structure or an object. In the human body, forces could either be external to the body, like the ground reaction force and the force of gravity or internal to the body like joint reaction forces, muscle tensile forces ligaments, constraint forces remember that forces are vectors, so they have both a magnitude and a direction.
They could be resolved into their components in the x, y and z-axis using Pythagoras' theorem. Quantities in life in general are either scalar, which can be fully defined by the number of units. So if I say I have 10 donkeys on one side and 10 donkeys on the other side, I could add them together and say I have 20 donkeys. However, a vector cannot be fully defined by its magnitude only, so I cannot add the force of the man who is pulling in one direction to the force of the donkey, which is pulling in the opposite direction.
So a force like all vectors have a magnitude, a direction and the point of application. They can be added together by drawing them to scale and using the parallelogram of force, giving a resultant force. Conversely, a single falls could be resolved into its components along different coordinates. Given two vectors, V1 and V2, the vector, some V1 plus V2 can be calculated graphically by transposing the tail of the second vector V2 onto the Arrowhead of the first vector V1.
The resulting vector is obtained by joining the tail of the first vector V1 to the Arrowhead of the second vector V2 to calculate the vector difference V1 minus V1 two. It is necessary to invert the orientation of the second vector and repeat the procedure described for the addition. So in this example, if I knew the magnitude of the body react body weight, which acts directly down at the center opposite as to sexual segment and the magnitude and direction of the hip abductor muscle pull, I can either draw the vector diagram or I could resolve the abductor force to its X and y components.
Both methods give the same magnitude and direction for the joint reaction force. A force acting on an object could either result in translation in which all points in the body are subject to the same displacement vector. He earmarked as the rotation refers to. So for that rotation refers to a type of motion where different. Into the two different displacement vectors like B and S in this illustration.
A moment then results when a force away from a pivot points. An example is a muscle which attaches some distance from a joint acting, either to keep the joint in equilibrium on where all movements, all movements balance out, all generate rotation about that joint. Remember that the moment is the perpendicular distance from the axis to the line of action of the force, the rotational effect is called turning moment or torque, which is equal to the force times the moment after.
A lever, therefore, is a reset bar that turns about an axis of rotation as a result of a force, which acts against a resistance. The thought letters IRF depending on what component is in the middle, livers can be used either for force amplification when the liver arm for the force is longer than that of the resistance or motion amplification to obtain higher velocity or longer range at the expense of energy efficiency.
Examples of the three classes in life and in the musculoskeletal system are given here. Almost every candidate in the Air Force says all the exam is going to be asked to draw a free body diagram. This is a simplified to the representation that isolates a part of the body in equilibrium in order to determine forces and movements acting on it. In order to draw a free body diagram, certain assumptions are made.
You will have to remember those by heart. So, for example, bones are considered as rigid rods. And they do not bend when in life. They sometimes deform. Joints are frictionless hinges, and the line of action of the muscle is acting in the center of the muscle. And joint reaction forces, for example, are assumed to be only compressed.
You need to practice drawing and solving free body diagram, problems of common parts of the body. Elbow flexion is a common example. Since we have two unknown forces, you start by calculating moments about the axis of rotation. This will remove the joint reaction force from the equation, since it's never arm is zero. Remember to convert all measurements to meters and kilograms in order to calculate the force in newtons.
Assuming that the weight of the forearm is 22 kilograms, which is 20 newtons, the biceps muscle pull can be calculated at 50 to newtons. The next step is to sum the forces in the X-axis. Positive forces acting upwards should equal those acting downwards, substituting for the biceps muscle fold, which we calculated at the previous step. You get a joint reaction force, which is equal to 1 and 1/2 times the weight of the forearm.
If you were a fan of mathematics, you may wish to consider adding a 1 kilogram ball in the subject's hand and repeat the calculations. Remember to some of the moments up first and then sum the forces in the y-axis. Those calculations will give us the biceps force, which here is 10 times the weight of the object and as joint reaction force in the elbow, which is 8 times the carried weight, you can do the mathematics on your own.
Another joint, which is commonly asked as a free body diagram is the hip joint. In addition to the general assumptions, one assumes that the subject is standing on one leg, which weighs one sixth of his total body weight, and that the center of mass is at the symphysis. Pubis always practiced beforehand, drawing your own simplified diagram and rehearse as you go. Here is an example of one simplified diagram.
Let us solve it together in an ideal and imaginary 70 kilograms person, the first step is to stop the moments about the center of rotation. The calculated abductor force is estimated at about 2 times the body weight. One mechanism of reducing the resultant load instead of reducing the resultant load on the femoral head is to use a walking stick in the opposite hand since it acts at a distance from the hip joint center of rotation.
A cane in the opposite hand, decreases the joint reaction force. We have previously demonstrated how to calculate joint reaction force using vector diagrams. This here is another method using free body diagram and resolving abductor muscle forces in the X-axis. With this in mind, how could you decrease the joint reaction force at the hip? Can we please share the poll?
Right, please vote. Well, keep the voting open for 30 seconds. OK, guys, this is anonymized, so feel free. Give us your best. Give it this bill best shot and see what you think is the right answer here. I don't know, might as well.
I encourage everyone to take part. So absolutely you've got most of you have got that right. You may decrease the joint reaction force in a hip replacement, for example, by idealizing the acetabular cup, using a long neck prosthesis or lateral lacing or the greeter to counsel. A patient, however, manages to do that by shifting the body weight over the hip, like in a trendy Limberg gait and Trent strindberg test, or using a cane in the contralateral head.
Moving on to a different topic, which is objects and their mechanical properties, we know that material properties are the fundamental behaviors of the substance independent of its geometry. While structural properties are the ability of an object to resist deformation under bending torsion or axial loads, it is a function of its shape and the distribution of the material around its cross-section. So force applied to a body either causes it to accelerate or results in its deformation.
If one plots a graph of displacement due to the force applied to a structure, the slope of the curve represents the object's stiffness. A typical curve has a linear region, which is the elastic region where the object returns to its resting state. If the force is removed like a rubber band, for example, stiffness again, is the slope of the elastic portion of the force displacement curve as more force is applied.
The object's behavior becomes plastic and permanent deformation occurs. The object will not return to its original state after the force is removed. Like when you bend a plastic ice cream spoon too far, it stays bent, largely absorbed into the object during the deformation process. This is irrespective of the dimensions of the object. What device the force applied by the area to calculate the stress, which is false over area strain is the change in a materials length due to an applied stress relative to its original length.
So its delta L over L. If what plots the resulting strain due to the applied stress, the slope of the resulting stress strain curve is the elastic modulus or the Young's modulus. Here are the definitions are described once more. So this is a curve that you follow. Let us see if you could, for example, identify the point number two, which is of proportionality, where the poor.
What is the point number two? Is it failure point, fatigue, strength, ultimate strength or yield for it? And then again, what about point number 3 on the curve? Can you identify that you've got the same options, but obviously it's not the same correct answer. A showing.
And obviously in the first question, which is number two, most of the people have got it absolutely correct. It's the yield point, which is the limit of proportionality when the material transforms from an elastic to a plastic region. But for some reason, the ultimate strength is not very clear. So let us define them again. He is the elastic modulus.
The steeper the slope, the stiffer the material. A material with a flat slope is flexible. The point on the stress strain curve where the material changes from elastic, which is the linear region to plastic deformation, is the point. As you correctly pointed out, at some point, the material will break. This point is called the material's failure point.
Ultimate strength is the highest point on the curve, which is the maximum stress that the material can take before it breaks. Here are the definitions once more, and we will have to remember these for the sake of the exam. Therefore, a material which experiences little plastic deformation before it fails is set to be brittle, like glass, for example. A material with a large plastic deformation region before it fails is set to be ductile like copper, while a material which can absorb more energy prior to failure indicated by a large area under the curve is set to be tough.
A material which can only absorb little energy prior to failure indicated by a small area under the curve is said to be weak. So let us go into some examples. This is the typical curves demonstrated some material properties. All the materials illustrated in those curves shown here could be described as stiff. However, they differ in other properties material.
A, for example, is stiff, brittle and strong, while material B is stiff, ductile, tough and strong because the area under the curve is the maximum in all the four curves materials. C is stiff, brittle and weak, while material D is stiff, tactile and weak. Similarly, materials shown in those curves could be described as flexible. Other properties are as follows material and a is flexible, brittle and strong.
B is flexible, tactile, tough and strong. C is flexible, brittle at weak. These flexible, tactile and weak. These are typical elastic moduli of common material encountered in orthopedic surgery relative to each other. It's a picture, and we all have been very vague. You know that the stiffest material we use in orthopedics is ceramic, so no one would be ceramic.
And if I tell you that the next three materials, which are number two to number four, are metals commonly used in orthopedic implants, could you arrange them from the higher to the lower stiffness, please? Let's vote on that. So almost 62 thirds of you have got that right, which is the top end cobalt chrome, followed by stainless steel, followed by titanium, let's reveal the answer.
That's correct. So we know that cobalt Chrome has got the higher stiffness, and because it's quite stiff, we do not commonly use it in femoral stems, for example, to prevent stress shielding. It has superb or the best of the methods wear and corrosion and friction properties. So this is why they're used for articulating components. Why titanium, for example, has got a stiffness, which is close to the cortical bone.
This is why it's used mostly now to prevent stress shielding. That was absolutely right. So far, we have been considering material properties at constant loading rate. Many biological materials, including bone, reveal different stress strain curves depending upon the speed at which the force is applied. This is called viscoelasticity. Some materials, like bone, have different mechanical properties dependent upon the direction of the applied load, whether it's transfers, longitudinal or shear.
Those are described as anisotropic. The two most commonly described physical elastic principles are concrete and stress relaxation in creep, a constant force causes increasing deformation without loss of material while in stress relaxation. There is a decrease in the stress at the core at a constant strain by done by intermolecular rearrangement. In addition to being viscoelastic, Bowen is anisotropic, its modulus depends upon the direction of load, Bowen is weakest in shear, then tension, then compression.
Bowen is therefore strong, meaning that it absorbs a lot of energy before it fractures stiff because of its calcium content, the stiffness depends on the direction of loading, hence it is described as being anisotropic. Remember that the material properties of bone vary with age. There are two types of failure modes or failure testing in lots of failure, a continuous force is applied until the material brakes.
Failure point occurs at the ultimate load bone, usually brakes by load failure to failure. Fatigue failure describes failure due to cyclic loading below this threshold of failure of the material. So load to failure testing is often reported in orthopedic literature. However, clinically failure of fixation fracture fixation constructs or implants is often due to fatigue failure.
In fatigue testing, the material is cyclically loaded at a stress below its ultimate strength. The number of cycles is plotted against on the X-axis, while the stress is plotted. What is that the X-axis is logarithmic material properties is either defined by fatigue strength, which is the stress required to cause failure of a material at any number of cycles, or by fatigue life, which is the number of cycles needed to cause.
Material to fail at certain stress, some materials exhibit endurance limit, which is the stress below which the material never fails, irrespective of the number of loading cycles. Tendons and ligaments are interesting structures. They have a considerably high ultimate tensile strength and exhibit physical elastic behaviors. The typical curve for tendons and ligaments show a toe lesion with a slow elongation due to straightening of the coalition.
By the ocean and elastic behavior, followed by early sequential failure as the failure of some stretched coalition fibers occur, the ultimate strength or stress is the maximum load or stress before the ligament fails completely. So in summary, we know now how important biomechanics is to our practice. We have reviewed the Newton's three laws of motion and the concepts of forces and moments.
We can draw free body diagrams and soul free body diagram problems using the examples shown, and we have discussed the structural and material properties of different orthopedic materials. Thank you very much, and I'll be ready for any questions. If you wish. Thank you very much, Professor Willis, for this presentation. It was very educational. We learnt a lot and thank you for the effort you put on this.
It clearly required long hours to prepare. You've covered the whole biomechanics chapter. Oh, we've got the typology to cover next. Yeah, the typology would be another one. But but oh, thank you very much, and I must say your talks are always inspirational to me and to other people interested in education and people like yourself, encourage us to keep going. Thank you.
Very, very pleased you are with us tonight. Prof so. It was the very kind of topic that's also close to my heart a lot. Just enjoy it because before exams, I could not understand it and then I just got to spend long time on it and then I loved it. Are you married to an engineer as well?
No, no. You've got the engineers in the family. So, yeah, not as much as you attach to that, but that. So they'll look there'll be, you know, each topic he discussed today, we could spend a long time going into details and examples and this is no end to this trial is very interesting and applies to everyday life as well as orthopedics.
But, you know, from exams questions and what we get asked by candidates, just ask you a few questions and think things like a little bit ambiguous to some people, like one of the questions that we frequently get asked is when you're drawing the free body diagram of the hip joint, y is the vector of the force of the abductor going downwards or most people in their head, it should be going upwards? I struggle with that.
Yeah, I struggled with that as well. Right if you're standing on one leg, like when you're doing a big test, for example, your body weight tends to pull your pelvis and the whole of your body down on the opposite side. In order to keep you balanced, the abductors will have to pull on your heavy pelvis on the Eliot crest on your hips. Lateral side downwards in order to keep you balanced.
So that is the explanation of why the force vector of the adductors is going down. We always think of the adductors as putting the greater Toronto towards the crest. Now you have to think because the foot is on the ground, the abductors are pulling the inner crest towards the greater to counter in order to keep you balanced. Otherwise, your sound side sags like what happens in the trend in buses.
Exactly, that's where assumptions come in, isn't it? So you have to be clear in our assumption, this is a person who is standing on one leg that we are examining or we are drawing the body diagram. So it's all depends where the assumptions come in. So also a lot of these would. The examiners these days interested of about the clinical implications of this basic sciences principles and people get asked a lot in the exam, which you hinted to quickly, but they get people ask about a principle of idealizing the cop and the examiners ask.
One of the question is, how does this explain? They ask, can you explain to us the child is low friction principles? And how does it apply to hip replacement? Why does it reduce joint action forces? Is there any sort of right answer to that? Your question has got two components. One of them are postponed, which is the LFA low friction arthroplasty because we are not talking about friction and wear now.
I'll leave that to the next talk, which is next year, probably on trabelsi. But what is it that mid-evening the cup? I have the chance of working for somebody at Wolverhampton who worked for John Charnley earlier on, and he's got the original set. And you do it in a supine position and have the set. You're sitting down and you've got the set in front of you, one tray for the acetabulum, one tray for the femoral component.
And the first step you do is you do this holding the middle of the acetabulum and then put the expanding reamer in order to expand the acetabulum. And what that does is it brings the cup as close as possible to the center of the body. Remember, the weight is working opposite s to in the middle of the body and you want to decrease the lever arm of the body weight.
So what if you bring the center of rotation closer to the center of the body? You're decreasing the bodyweight movement arm and you want to increase the abductor forces or the abductor muscle lever arm. So when you might realize that and use a slightly longer neck to restore the hip, then you're increasing as well the abductor lever arm. This is why John Charnley used to do the osteotomy because you can reattach to cancer whatever you want in order to balance the tension on the abductors and possibly increase the tension on the muscle.
So I think this is why we have that. An important application is when you've got that central posture fight inside the acetabulum, and some of the trainees do not recognize that there is an osteopath. And what they have reached is not the true flaw of the acetabulum, but it is the osteopathy itself. You will have to train yourself to recognize that on the X ray, when the hip is slightly subluxation and the bottom or the head is not opposite the teardrop, and you have to be able to get down to the tear drop down to the medial wall of the acetabular because this is the true flaw of the acetabulum.
So this is where you want to put the cup correctly. Thank you very much. I think we could talk about this forever. It's very interesting. We had a question from one of the delegates asking about. Young difference in young modulus between a osteoporotic bone and the bone with Austria Malaysia. So first of all, Austrian Malaysian osteoporosis, osteoporosis is not a bone quality problem.
It's a bone density problem. So quantity problem versus Austria Malaysia is a bone quality problem, which is the adult version of rickets. So essentially your problem with bone deposition of minerals, calcium and so on. So what happens in these scenarios is the if you think about normal adult bone. My apologies.
Sorry, my son is at the age where he can run into the room when he wants to. When, when you have a normal adult bone, you have a more rigid, more stiff construct, and therefore your modulus of plasticity is more vertical on the diagram. While if you have an osteoporotic bone, the bone is still rigid but is brittle and therefore will fail earlier than an adult bone would an adult middle aged man or whatever.
While an Austrian person, the bone is less rigid because it's more collagen and less mineral density, and therefore is more likely to, it will become more elastic. I forgive my bad drawings, but hopefully that does come across and you can see the different lines. So I'm exaggerating the adult bone. The standard adult bone there, and you can see the accumulation is much more elastic and will bend more before it fails.
The osteoporotic is more rigid than the Austrian Malaysia, but will is brittle and therefore will break. While the pediatric child has a very strong elastic component to it and therefore is more elastic than an adult bone and also will have a plastic, a bigger plastic deformity version of it, I hope I've explained that well. I do apologize if it's not coming across clearly. You have perfect qe1. I knew you were going to know the answer.
Stop, stop, stop. This is not an easy question at all, but as you think about it, it becomes quite obvious. Yeah, prof. Anything to add to this, I think, is a very interesting question. And Omeish, he's asked the question, and he said it has come in the exam, so I think it will be useful to everyone.
I think I think it's a very interesting question and a very Precice answer as well because the question was Young's modulus. And if you remember, Young's modulus describes material properties, so it's irrespective of the shape of the bone itself. And to elaborate on that, because your Young's modulus goes or is the same, whether your osteoporotic or not. So why on Earth is osteoporotic bone weaker?
Because it's the mass, which is reduced. So it's the structure itself, which is weakened. And one way we compensate for that is by increasing the diameter of the bone in order to bring the second or the polar moment of inertia. So that you can resist more dependent forces. So because the mass or the whatever mass you have left in, your osteoporotic bone is distributed on the periphery.
It does resist a little bit more the bending moments, but the problem is once it starts to fail because the cortex is thin, it starts to crumple quicker. So this is why Young's modulus, you remember we are talking about material properties and not about the structure of the bone, not the structural properties. And because we have compensatory mechanisms of increasing the diameter, then we compensate for the thinness cortex or for the weaker bone.
That's that's really very interesting, yes, so that's another that shows how the thinking of how you answer these questions and you're describing, as I said, the material, not the structure. Yeah and obviously, when it comes to young modulus young modules, Tony describes the elastic phase of the curve, as well. It doesn't go into the plastic.
So these are all answers the next question, which the same person is added on the set? Yeah does the bones show necking on Young's modulus or it? It's not exactly what. The question is not correct, exactly, but necking happens in the plastic phase close to your failure. So because bone is brittle or doesn't show a lot of plastic deformation, it wouldn't show.
Now with the caveat that, for example, pediatrics and. Oh, Yes. Exactly, exactly. I was very interesting. I could keep listening for this forever, if you guys. It's very, very interesting, very thought provoking this. Thank you for question. I think that's very important, and I'm sure it deepens our understanding of us because that's what's important about this.
Young modulus and stress strain curve is how to apply it in for the biomaterials as well as for. You know, pathological and deadly conditions, even though it still confuses me, you see, because. Sometimes they tell you this, these are. The this is a living material, isn't it? It's not synthetic material, and it's so we should be more thinking about creep and born and creep and hysteresis and stuff like that.
But it's they still have they still follow the stress strain curve anyway since any material will. Yeah so we get the typical questions Now on the free body diagram, as everyone wants to know. It's like two questions we have to stick. And the suitcase. Right that's a typical one. Write the stick is in the contralateral side when you push on the stick.
The ground reaction force acts upwards to push your pelvis or the whole of your body on the opposite side upwards. And because the stick is further away from the stern or pages or hips side, it's got a longer lever arm. So it eases off the weight of the body on this side and it decreases the contraction, which is necessary by the abductors, so it decreases your reaction force.
Remember, joint reaction force is a function of the weight, which is something you cannot control unless you go on a diet. And the abductor muscle pole and the abductor muscle pull is contracting whatever loads you are putting on it. If you carry a suit on the ipsilateral side, this acts as synergistic to the abductor muscle pull because it pulls this, this part of your body towards the hip and it counteracts the body weight.
So by decreasing that, you decrease the joint reaction force. It's like when you shift your whole weight onto this hip like Bloomberg test, so you're moving the center of rotation to the one side. In this case, you're just putting another weight on the opposite side of the lever arm, so you're balancing it off. I do not know if this is clear.
Something explained by diagrams, actually. Yeah so I think people get confused as is an extra weight, the suitcase is an extra weight. So how come how on Earth does that help? And I think it's not just, you know, it doesn't matter if it's extra weight, there are bridges. That way, tons and tons, and they still balance very well, and they don't suffer, and that's all following this biomechanical principles.
So just look at the body as you're facing the body, just as direction of forces going clockwise or anti-clockwise, yeah, so clockwise will be for a hip joint person. Standing one leg clockwise will be the body weight and anti-clockwise will be the abductors. And the suitcase? Yeah so if you have a suitcase, that will help the abductor. And will help balance the body better.
Yeah yeah, because it's working with the abductor. Yes, it's an extra weight, but it's working with the abductor, so it's balances the body with less force on abductors, so therefore the abductors will produce less force. They have to work less, less force on the abductors and less joint reaction forces. And just a reminder to everyone that we find a lot of the diagrams in the textbooks are inaccurate in terms of moments and how they draw them, especially the ones around the hip.
Prof purposely put this Millers diagram, which shows the moment is actually inaccurately drawn. Moment is forced multiplied by the perpendicular distance from the fulcrum, what at the time. Instead of doing that, they're drawing just a straight line across to where it interacts with the force, which is wrong. It's a perpendicular distance, and the police take the advantage of pointing that out because it then demonstrates you understand what a moment is.
Again, this was deliberately added there, because if you did not know the direction of the abductor muscle pull, it is not a fixed angle. Then if you have the straight line, the other trick you can do is analyze the force on of the y-axis and only the ones on the X-axis because these are the ones balancing the body weight. So you don't have the whole of the abductor muscle pull, but you only have the component acting on the X-axis.
So there are so many ways of skinning a cat. Yes absolutely, I think that's one of the mistakes, I don't know if people know about it, you know? Miller book is considered one of the bibles of Orthopedics exams, but it has mistakes, and as we work through these diagrams and stuff we discovered for you, this is one of them. And we have sort of tried to overcome this.
Obviously, in the concise orthopedic notes with the help of Joann and the other authors of the basic sciences chapter, we studied all of these and we reproduce a lot of these diagrams correctly. You guys, if you want to take photo of this, so this is the correct drawing of the diagrams. Are you happy with this one? Yeah, absolutely.
I'm one of the little tricks in the exam is just to draw a little rectangle at the point where they intersect, and that indicates automatically that is 90 degree, 90 degrees. Yeah so and obviously consultants to be diagnosed, we've covered all the free body diagrams of all. Of all major joints in a simplified manner, as we can, so that's great.
So if I think that's all the questions we have, so we'll move on now to the next stage of this session, which is the Viva the Viva.
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Thank you. Thank you, Jane. Good evening, everyone. Welcome to this teaching webinar presented to you today by orthopedic Research UK and orthopedic Academy. This is one of our series for FRCS exam. The present the presenter evening is Professor Emad Saweeres.. He is a consultant and a Professor in the teaching hospital and Good Shepherd hospitals in Cairo, Egypt.
He had his training in the UK and has done his exams in the UK, but he's very active educator. He's very active in teaching with us orthopedic Academy for a long time. He presented several times and active faculty on our courses, but in addition, he is very active internationally in teaching all over the world, but particularly in the Arab world and in Egypt. He's very active with the other board, educational board there and with the Egyptian board of orthopedics.
He's one of the founding members and main educational pillars in the Arabic. So very, very privileged that he is with us tonight, and he accepted our invitation. So I'm feeling slap modulating, and with me today. We got from our UK team, Hannah and Imogene, and I've got my colleagues as well. Tamir Carmel and Sian hinari helping us running the session today.
So the session, as usual, will include the lecture the lecture is about orthopedic biomechanics focused on the FRC as exam. It will run for about 30 to 35 minutes. We normally do queues at the end, but today to keep it a bit more interactive, we will do the emcee cues in between just to make sure you are guys all still awake and with us. Yes, we'll do them emcee queues in between.
After that, we will do the Viva session Habsi driver session. We have limited places only three. So if any of you is interested, please express that either send me a message or to hand our emma-jane or write on the chat box or raise their hand symbol. Just indicate to us you are interested as soon as you can. Yeah, we understand it's very stressful and we will wait. However, we encourage you to actively take part in this Viva practice.
Be very useful for you, for confidence, for your technique, as well as getting the feedback from the faculty. If you missed any part of this session, don't worry it will. It's been recorded and it will be posted later on the Australian Academy YouTube channel. And it just before I hand over to Professor civiles, I just want to share with you this our upcoming courses with our UK we have a mock exam courses running throughout the year.
We have one in January and we have again in March and in April. So please go to the our UK website for details. This is a mock exam course where we replicate the exact exam scenarios with IVUS and clinicals and try to put you the same exam situation time in exam situations with feedbacks. We also run our case discussion courses and we have multiple courses running throughout the year.
Please visit the summit Academy website if you're interested. The case discussion courses are more interactive and a lot more teaching and feedback is given to candidates, so it's very useful at any stage of your exam preparation. And it just happened today. We had someone canceled the course on 11th of December, which is Saturday is of anyone interested. Please go with the Academy and book as soon as you can.
So without further ado, we leave you now with Professor queries over to you, prof. Thank you for us. Thank you very much for the very kind of invitation and for the very generous presentation and introduction. So good evening, everyone. The orthopedic Academy have truly managed to save the best topic until the last webinar of the year.
My name is imad sawiris and I am one of the orthopedic doctors at a teaching hospital in Cairo. When I did my a-levels, I was not sure whether I wanted to go into engineering or medicine. So as a compromise, I decided to take up medicine and marry an engineer. Incidentally, I did also my M.D. thesis at Kean University with both an orthopedic and an engineering supervisors.
So by the end of this lecture, you should all be able to appreciate the importance of biomechanics for our practice. You should be able to use the Newton's three laws to analyze forces and women's draw and solve free body diagrams and be able to describe mechanical properties of different materials. Mechanics is the study of machines and when this.
Which you are not talking about, biomechanics critic biomechanics is particularly interested in facial joints, function and the mechanics of bone and soft tissue mechanics can be divided into statics, which is concerned with studying objects that are at rest or moving at a constant velocity. And dynamics, which is concerned with objects changing their velocity.
Kinematics is only concerned with description of motion without taking into account forces which cause such motion. Such forces are studied under kinetics. It all started when Sir Isaac Newton was attacked by a small Apple. This made him realize that the apple, which has a certain mass, was moved by the force of gravity. This force is equal to gravity, which on planet Earth is roughly equal to 9.81 meters per second squared.
And is measured in newtons. If one small Apple has a mass of 100 grams, its weight is approximately 1 Newton. He adequately described those three laws, which you all studied at school. Every Apple remains on the tree unless acted upon by an external force. Once free to move, it accelerates at a rate proportionate to the applied load and for every action, there is an equal and opposite reaction.
Remember inertia, acceleration, reactions, eye air as opposed to IRA. A potential question in the exam is what is a force? This is defined as a load acting on a structure or an object. In the human body, forces could either be external to the body, like the ground reaction force and the force of gravity or internal to the body like joint reaction forces, muscle tensile forces ligaments, constraint forces remember that forces are vectors, so they have both a magnitude and a direction.
They could be resolved into their components in the x, y and z-axis using Pythagoras' theorem. Quantities in life in general are either scalar, which can be fully defined by the number of units. So if I say I have 10 donkeys on one side and 10 donkeys on the other side, I could add them together and say I have 20 donkeys. However, a vector cannot be fully defined by its magnitude only, so I cannot add the force of the man who is pulling in one direction to the force of the donkey, which is pulling in the opposite direction.
So a force like all vectors have a magnitude, a direction and the point of application. They can be added together by drawing them to scale and using the parallelogram of force, giving a resultant force. Conversely, a single falls could be resolved into its components along different coordinates. Given two vectors, V1 and V2, the vector, some V1 plus V2 can be calculated graphically by transposing the tail of the second vector V2 onto the Arrowhead of the first vector V1.
The resulting vector is obtained by joining the tail of the first vector V1 to the Arrowhead of the second vector V2 to calculate the vector difference V1 minus V1 two. It is necessary to invert the orientation of the second vector and repeat the procedure described for the addition. So in this example, if I knew the magnitude of the body react body weight, which acts directly down at the center opposite as to sexual segment and the magnitude and direction of the hip abductor muscle pull, I can either draw the vector diagram or I could resolve the abductor force to its X and y components.
Both methods give the same magnitude and direction for the joint reaction force. A force acting on an object could either result in translation in which all points in the body are subject to the same displacement vector. He earmarked as the rotation refers to. So for that rotation refers to a type of motion where different. Into the two different displacement vectors like B and S in this illustration.
A moment then results when a force away from a pivot points. An example is a muscle which attaches some distance from a joint acting, either to keep the joint in equilibrium on where all movements, all movements balance out, all generate rotation about that joint. Remember that the moment is the perpendicular distance from the axis to the line of action of the force, the rotational effect is called turning moment or torque, which is equal to the force times the moment after.
A lever, therefore, is a reset bar that turns about an axis of rotation as a result of a force, which acts against a resistance. The thought letters IRF depending on what component is in the middle, livers can be used either for force amplification when the liver arm for the force is longer than that of the resistance or motion amplification to obtain higher velocity or longer range at the expense of energy efficiency.
Examples of the three classes in life and in the musculoskeletal system are given here. Almost every candidate in the Air Force says all the exam is going to be asked to draw a free body diagram. This is a simplified to the representation that isolates a part of the body in equilibrium in order to determine forces and movements acting on it. In order to draw a free body diagram, certain assumptions are made.
You will have to remember those by heart. So, for example, bones are considered as rigid rods. And they do not bend when in life. They sometimes deform. Joints are frictionless hinges, and the line of action of the muscle is acting in the center of the muscle. And joint reaction forces, for example, are assumed to be only compressed.
You need to practice drawing and solving free body diagram, problems of common parts of the body. Elbow flexion is a common example. Since we have two unknown forces, you start by calculating moments about the axis of rotation. This will remove the joint reaction force from the equation, since it's never arm is zero. Remember to convert all measurements to meters and kilograms in order to calculate the force in newtons.
Assuming that the weight of the forearm is 22 kilograms, which is 20 newtons, the biceps muscle pull can be calculated at 50 to newtons. The next step is to sum the forces in the X-axis. Positive forces acting upwards should equal those acting downwards, substituting for the biceps muscle fold, which we calculated at the previous step. You get a joint reaction force, which is equal to 1 and 1/2 times the weight of the forearm.
If you were a fan of mathematics, you may wish to consider adding a 1 kilogram ball in the subject's hand and repeat the calculations. Remember to some of the moments up first and then sum the forces in the y-axis. Those calculations will give us the biceps force, which here is 10 times the weight of the object and as joint reaction force in the elbow, which is 8 times the carried weight, you can do the mathematics on your own.
Another joint, which is commonly asked as a free body diagram is the hip joint. In addition to the general assumptions, one assumes that the subject is standing on one leg, which weighs one sixth of his total body weight, and that the center of mass is at the symphysis. Pubis always practiced beforehand, drawing your own simplified diagram and rehearse as you go. Here is an example of one simplified diagram.
Let us solve it together in an ideal and imaginary 70 kilograms person, the first step is to stop the moments about the center of rotation. The calculated abductor force is estimated at about 2 times the body weight. One mechanism of reducing the resultant load instead of reducing the resultant load on the femoral head is to use a walking stick in the opposite hand since it acts at a distance from the hip joint center of rotation.
A cane in the opposite hand, decreases the joint reaction force. We have previously demonstrated how to calculate joint reaction force using vector diagrams. This here is another method using free body diagram and resolving abductor muscle forces in the X-axis. With this in mind, how could you decrease the joint reaction force at the hip? Can we please share the poll?
Right, please vote. Well, keep the voting open for 30 seconds. OK, guys, this is anonymized, so feel free. Give us your best. Give it this bill best shot and see what you think is the right answer here. I don't know, might as well.
I encourage everyone to take part. So absolutely you've got most of you have got that right. You may decrease the joint reaction force in a hip replacement, for example, by idealizing the acetabular cup, using a long neck prosthesis or lateral lacing or the greeter to counsel. A patient, however, manages to do that by shifting the body weight over the hip, like in a trendy Limberg gait and Trent strindberg test, or using a cane in the contralateral head.
Moving on to a different topic, which is objects and their mechanical properties, we know that material properties are the fundamental behaviors of the substance independent of its geometry. While structural properties are the ability of an object to resist deformation under bending torsion or axial loads, it is a function of its shape and the distribution of the material around its cross-section. So force applied to a body either causes it to accelerate or results in its deformation.
If one plots a graph of displacement due to the force applied to a structure, the slope of the curve represents the object's stiffness. A typical curve has a linear region, which is the elastic region where the object returns to its resting state. If the force is removed like a rubber band, for example, stiffness again, is the slope of the elastic portion of the force displacement curve as more force is applied.
The object's behavior becomes plastic and permanent deformation occurs. The object will not return to its original state after the force is removed. Like when you bend a plastic ice cream spoon too far, it stays bent, largely absorbed into the object during the deformation process. This is irrespective of the dimensions of the object. What device the force applied by the area to calculate the stress, which is false over area strain is the change in a materials length due to an applied stress relative to its original length.
So its delta L over L. If what plots the resulting strain due to the applied stress, the slope of the resulting stress strain curve is the elastic modulus or the Young's modulus. Here are the definitions are described once more. So this is a curve that you follow. Let us see if you could, for example, identify the point number two, which is of proportionality, where the poor.
What is the point number two? Is it failure point, fatigue, strength, ultimate strength or yield for it? And then again, what about point number 3 on the curve? Can you identify that you've got the same options, but obviously it's not the same correct answer. A showing.
And obviously in the first question, which is number two, most of the people have got it absolutely correct. It's the yield point, which is the limit of proportionality when the material transforms from an elastic to a plastic region. But for some reason, the ultimate strength is not very clear. So let us define them again. He is the elastic modulus.
The steeper the slope, the stiffer the material. A material with a flat slope is flexible. The point on the stress strain curve where the material changes from elastic, which is the linear region to plastic deformation, is the point. As you correctly pointed out, at some point, the material will break. This point is called the material's failure point.
Ultimate strength is the highest point on the curve, which is the maximum stress that the material can take before it breaks. Here are the definitions once more, and we will have to remember these for the sake of the exam. Therefore, a material which experiences little plastic deformation before it fails is set to be brittle, like glass, for example. A material with a large plastic deformation region before it fails is set to be ductile like copper, while a material which can absorb more energy prior to failure indicated by a large area under the curve is set to be tough.
A material which can only absorb little energy prior to failure indicated by a small area under the curve is said to be weak. So let us go into some examples. This is the typical curves demonstrated some material properties. All the materials illustrated in those curves shown here could be described as stiff. However, they differ in other properties material.
A, for example, is stiff, brittle and strong, while material B is stiff, ductile, tough and strong because the area under the curve is the maximum in all the four curves materials. C is stiff, brittle and weak, while material D is stiff, tactile and weak. Similarly, materials shown in those curves could be described as flexible. Other properties are as follows material and a is flexible, brittle and strong.
B is flexible, tactile, tough and strong. C is flexible, brittle at weak. These flexible, tactile and weak. These are typical elastic moduli of common material encountered in orthopedic surgery relative to each other. It's a picture, and we all have been very vague. You know that the stiffest material we use in orthopedics is ceramic, so no one would be ceramic.
And if I tell you that the next three materials, which are number two to number four, are metals commonly used in orthopedic implants, could you arrange them from the higher to the lower stiffness, please? Let's vote on that. So almost 62 thirds of you have got that right, which is the top end cobalt chrome, followed by stainless steel, followed by titanium, let's reveal the answer.
That's correct. So we know that cobalt Chrome has got the higher stiffness, and because it's quite stiff, we do not commonly use it in femoral stems, for example, to prevent stress shielding. It has superb or the best of the methods wear and corrosion and friction properties. So this is why they're used for articulating components. Why titanium, for example, has got a stiffness, which is close to the cortical bone.
This is why it's used mostly now to prevent stress shielding. That was absolutely right. So far, we have been considering material properties at constant loading rate. Many biological materials, including bone, reveal different stress strain curves depending upon the speed at which the force is applied. This is called viscoelasticity. Some materials, like bone, have different mechanical properties dependent upon the direction of the applied load, whether it's transfers, longitudinal or shear.
Those are described as anisotropic. The two most commonly described physical elastic principles are concrete and stress relaxation in creep, a constant force causes increasing deformation without loss of material while in stress relaxation. There is a decrease in the stress at the core at a constant strain by done by intermolecular rearrangement. In addition to being viscoelastic, Bowen is anisotropic, its modulus depends upon the direction of load, Bowen is weakest in shear, then tension, then compression.
Bowen is therefore strong, meaning that it absorbs a lot of energy before it fractures stiff because of its calcium content, the stiffness depends on the direction of loading, hence it is described as being anisotropic. Remember that the material properties of bone vary with age. There are two types of failure modes or failure testing in lots of failure, a continuous force is applied until the material brakes.
Failure point occurs at the ultimate load bone, usually brakes by load failure to failure. Fatigue failure describes failure due to cyclic loading below this threshold of failure of the material. So load to failure testing is often reported in orthopedic literature. However, clinically failure of fixation fracture fixation constructs or implants is often due to fatigue failure.
In fatigue testing, the material is cyclically loaded at a stress below its ultimate strength. The number of cycles is plotted against on the X-axis, while the stress is plotted. What is that the X-axis is logarithmic material properties is either defined by fatigue strength, which is the stress required to cause failure of a material at any number of cycles, or by fatigue life, which is the number of cycles needed to cause.
Material to fail at certain stress, some materials exhibit endurance limit, which is the stress below which the material never fails, irrespective of the number of loading cycles. Tendons and ligaments are interesting structures. They have a considerably high ultimate tensile strength and exhibit physical elastic behaviors. The typical curve for tendons and ligaments show a toe lesion with a slow elongation due to straightening of the coalition.
By the ocean and elastic behavior, followed by early sequential failure as the failure of some stretched coalition fibers occur, the ultimate strength or stress is the maximum load or stress before the ligament fails completely. So in summary, we know now how important biomechanics is to our practice. We have reviewed the Newton's three laws of motion and the concepts of forces and moments.
We can draw free body diagrams and soul free body diagram problems using the examples shown, and we have discussed the structural and material properties of different orthopedic materials. Thank you very much, and I'll be ready for any questions. If you wish. Thank you very much, Professor Willis, for this presentation. It was very educational. We learnt a lot and thank you for the effort you put on this.
It clearly required long hours to prepare. You've covered the whole biomechanics chapter. Oh, we've got the typology to cover next. Yeah, the typology would be another one. But but oh, thank you very much, and I must say your talks are always inspirational to me and to other people interested in education and people like yourself, encourage us to keep going. Thank you.
Very, very pleased you are with us tonight. Prof so. It was the very kind of topic that's also close to my heart a lot. Just enjoy it because before exams, I could not understand it and then I just got to spend long time on it and then I loved it. Are you married to an engineer as well?
No, no. You've got the engineers in the family. So, yeah, not as much as you attach to that, but that. So they'll look there'll be, you know, each topic he discussed today, we could spend a long time going into details and examples and this is no end to this trial is very interesting and applies to everyday life as well as orthopedics.
But, you know, from exams questions and what we get asked by candidates, just ask you a few questions and think things like a little bit ambiguous to some people, like one of the questions that we frequently get asked is when you're drawing the free body diagram of the hip joint, y is the vector of the force of the abductor going downwards or most people in their head, it should be going upwards? I struggle with that.
Yeah, I struggled with that as well. Right if you're standing on one leg, like when you're doing a big test, for example, your body weight tends to pull your pelvis and the whole of your body down on the opposite side. In order to keep you balanced, the abductors will have to pull on your heavy pelvis on the Eliot crest on your hips. Lateral side downwards in order to keep you balanced.
So that is the explanation of why the force vector of the adductors is going down. We always think of the adductors as putting the greater Toronto towards the crest. Now you have to think because the foot is on the ground, the abductors are pulling the inner crest towards the greater to counter in order to keep you balanced. Otherwise, your sound side sags like what happens in the trend in buses.
Exactly, that's where assumptions come in, isn't it? So you have to be clear in our assumption, this is a person who is standing on one leg that we are examining or we are drawing the body diagram. So it's all depends where the assumptions come in. So also a lot of these would. The examiners these days interested of about the clinical implications of this basic sciences principles and people get asked a lot in the exam, which you hinted to quickly, but they get people ask about a principle of idealizing the cop and the examiners ask.
One of the question is, how does this explain? They ask, can you explain to us the child is low friction principles? And how does it apply to hip replacement? Why does it reduce joint action forces? Is there any sort of right answer to that? Your question has got two components. One of them are postponed, which is the LFA low friction arthroplasty because we are not talking about friction and wear now.
I'll leave that to the next talk, which is next year, probably on trabelsi. But what is it that mid-evening the cup? I have the chance of working for somebody at Wolverhampton who worked for John Charnley earlier on, and he's got the original set. And you do it in a supine position and have the set. You're sitting down and you've got the set in front of you, one tray for the acetabulum, one tray for the femoral component.
And the first step you do is you do this holding the middle of the acetabulum and then put the expanding reamer in order to expand the acetabulum. And what that does is it brings the cup as close as possible to the center of the body. Remember, the weight is working opposite s to in the middle of the body and you want to decrease the lever arm of the body weight.
So what if you bring the center of rotation closer to the center of the body? You're decreasing the bodyweight movement arm and you want to increase the abductor forces or the abductor muscle lever arm. So when you might realize that and use a slightly longer neck to restore the hip, then you're increasing as well the abductor lever arm. This is why John Charnley used to do the osteotomy because you can reattach to cancer whatever you want in order to balance the tension on the abductors and possibly increase the tension on the muscle.
So I think this is why we have that. An important application is when you've got that central posture fight inside the acetabulum, and some of the trainees do not recognize that there is an osteopath. And what they have reached is not the true flaw of the acetabulum, but it is the osteopathy itself. You will have to train yourself to recognize that on the X ray, when the hip is slightly subluxation and the bottom or the head is not opposite the teardrop, and you have to be able to get down to the tear drop down to the medial wall of the acetabular because this is the true flaw of the acetabulum.
So this is where you want to put the cup correctly. Thank you very much. I think we could talk about this forever. It's very interesting. We had a question from one of the delegates asking about. Young difference in young modulus between a osteoporotic bone and the bone with Austria Malaysia. So first of all, Austrian Malaysian osteoporosis, osteoporosis is not a bone quality problem.
It's a bone density problem. So quantity problem versus Austria Malaysia is a bone quality problem, which is the adult version of rickets. So essentially your problem with bone deposition of minerals, calcium and so on. So what happens in these scenarios is the if you think about normal adult bone. My apologies.
Sorry, my son is at the age where he can run into the room when he wants to. When, when you have a normal adult bone, you have a more rigid, more stiff construct, and therefore your modulus of plasticity is more vertical on the diagram. While if you have an osteoporotic bone, the bone is still rigid but is brittle and therefore will fail earlier than an adult bone would an adult middle aged man or whatever.
While an Austrian person, the bone is less rigid because it's more collagen and less mineral density, and therefore is more likely to, it will become more elastic. I forgive my bad drawings, but hopefully that does come across and you can see the different lines. So I'm exaggerating the adult bone. The standard adult bone there, and you can see the accumulation is much more elastic and will bend more before it fails.
The osteoporotic is more rigid than the Austrian Malaysia, but will is brittle and therefore will break. While the pediatric child has a very strong elastic component to it and therefore is more elastic than an adult bone and also will have a plastic, a bigger plastic deformity version of it, I hope I've explained that well. I do apologize if it's not coming across clearly. You have perfect qe1. I knew you were going to know the answer.
Stop, stop, stop. This is not an easy question at all, but as you think about it, it becomes quite obvious. Yeah, prof. Anything to add to this, I think, is a very interesting question. And Omeish, he's asked the question, and he said it has come in the exam, so I think it will be useful to everyone.
I think I think it's a very interesting question and a very Precice answer as well because the question was Young's modulus. And if you remember, Young's modulus describes material properties, so it's irrespective of the shape of the bone itself. And to elaborate on that, because your Young's modulus goes or is the same, whether your osteoporotic or not. So why on Earth is osteoporotic bone weaker?
Because it's the mass, which is reduced. So it's the structure itself, which is weakened. And one way we compensate for that is by increasing the diameter of the bone in order to bring the second or the polar moment of inertia. So that you can resist more dependent forces. So because the mass or the whatever mass you have left in, your osteoporotic bone is distributed on the periphery.
It does resist a little bit more the bending moments, but the problem is once it starts to fail because the cortex is thin, it starts to crumple quicker. So this is why Young's modulus, you remember we are talking about material properties and not about the structure of the bone, not the structural properties. And because we have compensatory mechanisms of increasing the diameter, then we compensate for the thinness cortex or for the weaker bone.
That's that's really very interesting, yes, so that's another that shows how the thinking of how you answer these questions and you're describing, as I said, the material, not the structure. Yeah and obviously, when it comes to young modulus young modules, Tony describes the elastic phase of the curve, as well. It doesn't go into the plastic.
So these are all answers the next question, which the same person is added on the set? Yeah does the bones show necking on Young's modulus or it? It's not exactly what. The question is not correct, exactly, but necking happens in the plastic phase close to your failure. So because bone is brittle or doesn't show a lot of plastic deformation, it wouldn't show.
Now with the caveat that, for example, pediatrics and. Oh, Yes. Exactly, exactly. I was very interesting. I could keep listening for this forever, if you guys. It's very, very interesting, very thought provoking this. Thank you for question. I think that's very important, and I'm sure it deepens our understanding of us because that's what's important about this.
Young modulus and stress strain curve is how to apply it in for the biomaterials as well as for. You know, pathological and deadly conditions, even though it still confuses me, you see, because. Sometimes they tell you this, these are. The this is a living material, isn't it? It's not synthetic material, and it's so we should be more thinking about creep and born and creep and hysteresis and stuff like that.
But it's they still have they still follow the stress strain curve anyway since any material will. Yeah so we get the typical questions Now on the free body diagram, as everyone wants to know. It's like two questions we have to stick. And the suitcase. Right that's a typical one. Write the stick is in the contralateral side when you push on the stick.
The ground reaction force acts upwards to push your pelvis or the whole of your body on the opposite side upwards. And because the stick is further away from the stern or pages or hips side, it's got a longer lever arm. So it eases off the weight of the body on this side and it decreases the contraction, which is necessary by the abductors, so it decreases your reaction force.
Remember, joint reaction force is a function of the weight, which is something you cannot control unless you go on a diet. And the abductor muscle pole and the abductor muscle pull is contracting whatever loads you are putting on it. If you carry a suit on the ipsilateral side, this acts as synergistic to the abductor muscle pull because it pulls this, this part of your body towards the hip and it counteracts the body weight.
So by decreasing that, you decrease the joint reaction force. It's like when you shift your whole weight onto this hip like Bloomberg test, so you're moving the center of rotation to the one side. In this case, you're just putting another weight on the opposite side of the lever arm, so you're balancing it off. I do not know if this is clear.
Something explained by diagrams, actually. Yeah so I think people get confused as is an extra weight, the suitcase is an extra weight. So how come how on Earth does that help? And I think it's not just, you know, it doesn't matter if it's extra weight, there are bridges. That way, tons and tons, and they still balance very well, and they don't suffer, and that's all following this biomechanical principles.
So just look at the body as you're facing the body, just as direction of forces going clockwise or anti-clockwise, yeah, so clockwise will be for a hip joint person. Standing one leg clockwise will be the body weight and anti-clockwise will be the abductors. And the suitcase? Yeah so if you have a suitcase, that will help the abductor. And will help balance the body better.
Yeah yeah, because it's working with the abductor. Yes, it's an extra weight, but it's working with the abductor, so it's balances the body with less force on abductors, so therefore the abductors will produce less force. They have to work less, less force on the abductors and less joint reaction forces. And just a reminder to everyone that we find a lot of the diagrams in the textbooks are inaccurate in terms of moments and how they draw them, especially the ones around the hip.
Prof purposely put this Millers diagram, which shows the moment is actually inaccurately drawn. Moment is forced multiplied by the perpendicular distance from the fulcrum, what at the time. Instead of doing that, they're drawing just a straight line across to where it interacts with the force, which is wrong. It's a perpendicular distance, and the police take the advantage of pointing that out because it then demonstrates you understand what a moment is.
Again, this was deliberately added there, because if you did not know the direction of the abductor muscle pull, it is not a fixed angle. Then if you have the straight line, the other trick you can do is analyze the force on of the y-axis and only the ones on the X-axis because these are the ones balancing the body weight. So you don't have the whole of the abductor muscle pull, but you only have the component acting on the X-axis.
So there are so many ways of skinning a cat. Yes absolutely, I think that's one of the mistakes, I don't know if people know about it, you know? Miller book is considered one of the bibles of Orthopedics exams, but it has mistakes, and as we work through these diagrams and stuff we discovered for you, this is one of them. And we have sort of tried to overcome this.
Obviously, in the concise orthopedic notes with the help of Joann and the other authors of the basic sciences chapter, we studied all of these and we reproduce a lot of these diagrams correctly. You guys, if you want to take photo of this, so this is the correct drawing of the diagrams. Are you happy with this one? Yeah, absolutely.
I'm one of the little tricks in the exam is just to draw a little rectangle at the point where they intersect, and that indicates automatically that is 90 degree, 90 degrees. Yeah so and obviously consultants to be diagnosed, we've covered all the free body diagrams of all. Of all major joints in a simplified manner, as we can, so that's great.
So if I think that's all the questions we have, so we'll move on now to the next stage of this session, which is the Viva the Viva.
But
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