Episode 186 | The Role of the Screw Home Mechanism in Terminal Knee Extension + ACL Rehab

What is up guys and welcome back to another episode on the ACL Athlete Podcast. If you are new here, whether you are an ACLer, clinician, coach, thank you for joining us. If you have been here for a minute, I appreciate your support as always. And also there’s someone cutting some wood outside at a neighbor’s house. We’ve got a saw in the background, but that is just on par. Let’s get into the episode today. Right before that, I did want to mention that we are going to be closing off our cohort three enrollment next Monday, that is going to be August 26th at midnight. If you’re listening to this and it is beyond August 24th, then this does not even apply. It doesn’t even exist anymore. But if you’re listening to it before this, and you’re a clinician or a coach, we are enrolling our third cohort of the ACL Athlete Mentorship. We were diving into all the things, ACL rehab, from injury to full performance. We were talking through program design, how to work with complex cases, the framework that I’ve created, that is the 4R roadmap. This is something that we use with all of our athletes to help guide them through this process, but allowing it to be individualized to each and every athlete, because we are basing this all on principles. It’s a principles–based approach to ACL rehab. 

A lot of physical therapy and what is done in this space is based on protocols and all these exercises. We’re flipping the script here and we’re using this foundation of principles to make sure that you can work with any ACLers, no matter of it’s the single injury to multiple injuries to all the complex cases that come along in this process, taking them to full return to sport and understanding that progression and process and being able to break down movements. This is something that we are super pumped. We already have such an incredible cohort of mentees. If you’re listening to this prior to August 26, then hop on the waitlist. Even if you’re interested in future cohorts, then this is something that you will see in the show notes below. If you have any questions, you can always email us ravi@theaclathlete.com and we will be happy to answer any questions you might have related to this mentorship. And we are so pumped for this last cohort of this year, and to just continue to move the needle for ACL rehab. 

Now on to the episode for today, and that is the role of the screw home mechanism in terminal, knee extension in ACL rehab. The screw home mechanism is a very interesting concept and I hope y’all are ready to get geeky on this one. I’m sure I got more clinicians and coaches who are listening in on this because we are talking a little bit more about some biomechanics, some background pieces, but I know my ACLers who are also loving this type of information and education. You guys get geeky with us as well. We’re going to talk about how this concept and this phenomenon plays a role within ACL rehab, especially for range of motion. And I’ve been thinking about this a lot with our ACLers and just the ACL rehab space in general. Because it’s very unique in terms of the mechanics of the screw home mechanism because there’s so much nuance to it. But with that said, it’s a huge component to being able to fully extend the leg and to unlock and bend the knee. It’s something that gets often overlooked. We learned about it in physical therapy school, and then we just don’t think about it. We’re just like, cool, we do flexion and extension exercises and hope it tackles it, which it does. But there are nuances to this that I think are important. And being able to implement this with some of our athletes and ACLers that we’re working with has actually been a big game changer and just something that I’ve been thinking more and more about with people who are dealing with range of motion issues because there can be a number of factors. But with that said, this is one of those things that often doesn’t get tapped into. I want to talk about it because it is such a foundational element of range of motion for knee extension and flexion. And so that way we can get the most out of our range of motion work and maybe think about another avenue that might not have been visited if you are dealing with some sort of limitation. 

First, let’s talk about some quick anatomy and some few terms to get out of the way to get this set up really well for this conversation. The knee joint itself is comprised of the femur (thigh bone), the tibia (shin bone) and patella (kneecap).Now for today’s purposes, the kneecap, the patella isn’t really factored into the screw home mechanism. It’s mostly the interaction between the femur (thighbone) and your tibia (shin bone).  This is going to be important as we talk about the movement of the knee joint related to, especially these two bones. And the other thing that I want to make sure is clear is that the knee is not just a hinge joint. If you think about like a door hinge, that’s what people often think about the hinge joint. You can almost think about it as like your elbow, for the most part, it bends and extends. But there’s actually some what we call pronation and supination of the elbow. And so that’s essentially think about it is like rotation. There’s a little bit of that at the knee. There’s some rotation, tibial rotation, internal and external rotation. This is something that again is like a nuance to the joint and most people don’t actually know this, unless they have really dove into the anatomy and the physiology of how the knee functions and the biomechanics of it. It’s an important factor whenever the knee is bending and extending, it is going through some relative rotation alongside it, especially in the end ranges of extension and initiating flexion. 

Some other terms, extension is straightening the knee. Flexion is bending the knee. OKC (open kinetic chain movements) this is often used in things related to knee extensions, especially with stretching the graft or people thinking the graft will stretch. You’ll see open kinetic chain versus closed kinetic chain. So there’s arguments around it, but really open kinetic chain. All this really means is that the lower limb is not fixed to the ground. Basically, think about something like a knee extension where your foot or your lower leg is floating. The straight leg raise, swinging your leg through the swing phase during a gait cycle. When it’s not on the ground, those are technically open kinetic chain movements. Closed kinetic chain is where your limb is fixed to the ground. Think about it’s a squat or a lunge, and of course, part of a lunge, you are free in the air, but whenever it’s fixed to the ground, that is technically a closed kinetic chain. A better example of this would be a split squat where there’s no lifting of the leg, is just up and down. Terminal knee extension is a very good example of this. It’s a closed kinetic chain, knee extension based movement. 

And then think about like hitting the ground with your foot or your leg during the stance phase of the gait cycle. Anytime that heel comes in contact rolls through, basically foot is in contact with a ground. That means it’s close kinetic chain. With open kinetic chain, the foot is free and your tibia is moving. With the closed kinetic chain, a lot of times your tibia is a little bit more fixed if you will. It’s just connected to the ground more so, and your femur might be doing a lot more movement. I hope that’s helpful. I know those terms can be confusing, especially for people not familiar with this. But it plays into a lot of what we will talk about today in terms of the movement of the screw home mechanism and what’s going on. 

And the last term I’ll mention is terminal knee extension. I feel like it’s thrown out there and sometimes people are like, well, what? It’s just extension, right? What’s terminal? It’s literally just end range extension. It’s literally locking out your knee. It’s called the closed pack position of the knee, if we’re talking about the knee joint and the technical terms of it. But it’s terminal knee extension is just literally full knee extension. 

What I want to talk about in terms of the screw home mechanism to break it down is basically describing the biomechanics at the knee joint when it goes into the last little bit of extension and then when it initiates flexion again. Think about your knee bent at 90 degrees as it is moving up into getting straighter, that’s going into full extension. As we get to that last 20 degrees from zero, the tibia starts to actually rotate a little bit and it rotates externally. I’ll get into these phases here in a second. It’s going into this locked position. And then whenever I go out of extension in that out of that locked position, it’s going to unlock and is going to rotate internally. This is where it’s going to be important to understand the screw home mechanism is literally just in those last little bits of where the knee is getting straightened and unlocking. There’s that rotation that occurs at the tibia (shin bone) relative to the femur (thigh bone) to lock and unlock the knee joint, helping with joint stability. 

Now, don’t take these for literal. It’s not like the knee joint is locked locks. It’s just like getting into this optimal stable place. And there are certain things that are getting taught. The bones are coming together. It’s a more stable joint in that knee extended position because of the congruency of all the bones, the joints, the ligaments, etc. It’s locking into that position and then it will unlock as it starts to bend again and flex. Let’s go through the phases here. The locking phase, or as we are approaching that terminal knee extension in the last 20 degrees of extension from bending to extending. This is approaching that full knee extension. As I had mentioned, the tibia will externally rotate or rotate outward relative to the femur. This is the screw home mechanism where the knee locks to help stabilize the joint. Unlocking is when it goes from that full extension and starts to bend or flex and the tibia will internally rotate or rotate inward to unlock the knee joint. 

Now, why does this motion happen? There are a few factors in place. Number one, and most importantly, why this happens is because the shape of our knee anatomy. Your femural condyles, which are the two bumps at the end of your femur, essentially, where it starts to sit on your tibia are not equal. The length of them are not equal. The medial condyle is actually a little bit longer, about a half inch longer and shaped a little different. Therefore, when the knee gets to near that extension, there’s a rotation that occurs at the joint because it’s not just this like perfect even joint that just like approximates as soon as it extends. It extends, but then there’s a little bit of rotation to make sure that joint is congruent and the femur, both condials are sitting on those menisci appropriately. The shape of the knee anatomy is the main reason why that screw home mechanism happens. 

Some other reasons for that is because of the pull of the quads. Believe it or not, when the knee goes to full knee extension because of the lineup pull of the quads, there’s a slight lateral pole of the quads, in terms of getting into that knee extension. Then therefore that helps to reinforce that rotation a little bit. The third point I’ll make is the tension of most of your cruciate ligaments, that being your ACL, your MCL and your LCL. The only thing that is not taught at full extension is your PCL. Given the ACL’s origin and insertions, that’s going to add some pull for that rotation of the tibia to move to external rotation going into extension. 

Now, the muscles responsible for these motions, if we’re talking about for extension, that’s going to be your quads. You have four quad muscles, and that is going to help extend the quads along with your tendons, quad tendon, you got your patella and then you got your patellar tendon that is attached to your tibial tuberosity. So that runs from the upper thigh, from the hip joint all the way down to the tibial tuberosity, to be able to extend the knee. We call that the knee extensor mechanism. 

Now with the muscles responsible for flexion, the popliteus helps to initiate and then the hamstrings take care of the rest. The popliteus is this small little muscle at the back of the knee that helps to—basically it’s known to be the muscle that unlocks the knee. It helps to go from extending the knee to being able to bend the knee, initiating that movement from that screw home mechanism. Other muscles that could contribute to this are sartorious and gracilis. Think about the groin muscles in there pull internally, which will help to assist on the tibia to rotate internally as we go to flex the knee. Those are not the only muscles, we’ve got other muscles going on that are helping to assist and think about any of the muscles that cross the knee joint so those can help with flexion and extension. But with that said, these are the primary muscles. You’re talking about the quads, the hamstrings and the popliteus being the main players. Quad for extension, flexion is going to the popliteus, initially, alone with the hamstrings. 

Now, the biomechanics. This is where having some rule of thumbs are going to be helpful in order to remember what’s going on. When we’re going into knee extension, that is going to be coupled with tibial external rotation. What I will do is often I will take my fist and I will take my left arm and my right arm and I’ll bring my fist together. They’re giving each other a fist bump. I’ll bring them together and then I’m just going to use my right leg as an example. My right arm and my right forearm, if you will, will represent my femur (my thigh bone) and my left forearm is going to represent my tibia or my shinbone. This is something that is going to help with understanding these mechanics. 

What I want you to do is think about first, bending your knuckles to where you have a gap there and I guess like the other part of your knuckles are meeting each other. And you’re slowly going to close that gap, where these are going to eventually start to fist bump each other. I’m going from flexion into flattening into extension. And as that extension happens, my right forearm is actually going to stay put here and what’s going to happen is my left forearm is actually going to rotate externally a little bit. What that’s going to do is that’s going to create some external rotation of the tibia to help with that screw home mechanism. Technically, the femur is going to have a little bit of internal rotation as well. And so then whenever I go to unlock or bend my arms and break this fist bump, I’m actually going to externally rotate the femur a little bit, and the tibia is going to internally rotate to unlock and then going to bend the knee. 

Now, it’s a very subtle nuance of this, but it’s something that’s really important with the knee joint and the biomechanics that does get overlooked, as I had mentioned earlier. And so that’s going to be really key for us to understand the extension is coupled with external rotation of the tibia knee, knee flexion is coupled with tibial internal rotation. And the only other factor that I want to plug into this is we do need to determine if it is a movement that is open chain and closed kinetic chain. Because that’s what I had mentioned earlier, breaking this down, because that will impact what bone is essentially moving. 

For example, if we’re talking about a screw home mechanism where we’re going into knee extension, the knee is going to lock, therefore we need to know if it is open kinetic chain. Think about whenever you’re doing a knee extension, for example, like a knee extension machine, that’s open kinetic chain. Then therefore, when the knee goes to lock or to extend, if you will, the tibia will rotate externally, relative to the fixed femur. Your femur is not moving, but your tibia is going to rotate externally to do that screw home mechanism and go into extension. Now, if this is a closed kinetic chain, think about when you’re coming up from a squat movement. Your tibia is fixed to the ground and your foot is fixed to the ground, then more so your femur is the bone that is moving. When it goes from that deep squat position, and you’re going to extend from that squat coming up, your femur is actually going to internally rotate and relative to that, your tibia is actually going to externally rotate. It’s the same motion; it’s just which bones moving. If it’s the tibia and going into extension is going to externally rotate. If it is going to be the femur is going to internally rotate. I know that we are talking about a lot of very specifics and biomechanical things. If you’re still with me, I appreciate it. You’re probably a physical therapist or a coach who’s geeking out on this and maybe some of my ACLers. The rest of you are just like, what is Ravi talking about? This is nothing about mindset or the other things that he talks about. But this is one that I wanted to touch on because it’s something that I see a lot and it’s one thing that we think about with our ACLer and it’s something that’s just untapped. My goal is always sharing information that either gets missed or forgotten or things that are just principles of this process, which is anatomy and the biomechanics of the knee joint; something that helps our ACL ERs. I want to pay this forward and make sure that you guys are familiar with this. 

Now, most importantly, why do we care about it? Why do we care? I shared a little bit about range of motion. But let me just share some reasons why we care about it. Range of motion is such a big goal after ACL injury and surgery and for really any knee injury at all. This applies to any knee injury, but especially for post ACL injury and surgery, it’s huge. We see a lot of athletes struggling with range of motion issues, especially that last little bit of extension. This is something that there can be a lot of factors related to this. Just because you don’t have extension doesn’t mean it’s because of the screw home mechanism so don’t go running to your PT and saying, this is the problem. It could be. But there are a lot of other factors that play into this, which is where having a good guide and good physical therapist and coach with you. We can roll this stuff in or out. This is something that we do see a lot of athletes struggling with range of motion issues, especially extension. This is one component that can get missed. It’s a small detail, but sometimes these small details can really make the difference for that athlete who has been trying everything and anything. And guess what? They’ve just been having a little bit of stiff tibial rotation, or maybe their knee just doesn’t want to get into that position. Maybe they had a meniscus repair or maybe there is just some fluid in the joint and some things that were really impacting For some reason that last little bit of rotation, maybe their ankle has been having some issues impacting this. Then therefore, we can make sure that we untap everything. That way we can make sure that we can explore all of our options, especially for those athletes who might be having these issues. If we can’t get full knee extension, there are a lot of implications of it. 

You guys know that I have a podcast on this, where I talk all about regaining knee extensions and what are the problems with that. But let’s dive into a few of these. Not being able to contract the quad fully. We need the quads to contract, and just feel that engagement. I know my athletes who can’t fully extend their legs, you kind of almost feel like it’s hard to really recruit that quad. So quad engagement recruitment is really helpful in that terminal extended position. More stress on the tendons and patellofemoral joint. When we even sit out of that extended position that is arrested position, we’re not engaging the muscles actually when you’re in full knee extension, really. But when you’re slightly bent, let’s say you have a minus five degrees of extension, but you’re sitting in plus three or four. There’s always this chronic contraction of the quad that’s activated, as well as the tendons are always engaged, if you will. Athen there’s also pressure on the patellofemoral (kneecap) within that groove that is not in normal resting place. It was throwing off the tension and the stresses that are being applied to different structures in your knee and the different tissues. 

Another point here is that it is not the best energy transfer. It’s not. Because whenever we think about different joints moving together, we want to make sure that the knee can extend with it and flex with it. If it doesn’t do it well, then therefore there’s going to be some leakage of energy or just inefficient energy transfer and force transfer. Another factor that plays into this is going to be the role of movement and performance. Think about strengthening for range to dynamic movements where the hip knee and ankle need to work together. I’m going back to the last point of knee energy transfer and force transfer. But think about all of these working in sync together through different ranges of motion at each joint with their muscles to create different movement options for our athletes. Think about jumping in different directions, think about a quick cut and a change of direction. There’s going to be a lot of flexing and bending that occurs at the knee, hip and the ankle. This is going to have some rotation also assisting with it when that knee is bending and extending. These are some factors that play into some of the implications around if we don’t get that full terminal knee extension.

Now for my ACLers who might be listening, or for clinicians and coaches, the ones who don’t get all this full knee extension. Don’t think that is the end of the world, if you’re listening. And you’re just like, man, I’ve been sitting in like plus two or plus five or something like that. Sure, these are factors to consider, but there are also other considerations with the knee joint and things that can continue to be worked on in order to improve that range of motion. There are some athletes we know who sit in these positions that do perfectly fine. Therefore, we do need to understand the knee has been impacted through injury and surgery, and therefore we are going to work as hard as we can with this new knee joint, and especially with the ACL and with the changes. It’s not necessarily you’re going to deal with all these bad things forever because there are athletes who sit in this and they’re okay. But with that said, for majority of folks, we want to get that full knee extension back, and that’s why we stress it so much because of these factors and so much more I didn’t even mention. 

Now post-ACL injury and surgery, the knee can have difficulty getting this knee extension and flexion back, especially knee extension. People often think that flexion is the thing they really need to push. When in reality post-op, knee extension is number one, along with getting our quads going by far. The flexion, yeah, it’s important to work on, but it will come with time typically. The knee extension, in my opinion, probably has more detriment, in terms of the performance of the quad and the knee joint itself than it does, if you’re lacking the last 20 degrees of flexion. Because you can do most things you need to once you hit that 120-marker with flexion. But with that said, the difficulty in getting this back, the knee can get really stiff feeling and sometimes rotation is really not worked on or restored. Thinking about that joint, it’s gone through a lot of stuff. And so then therefore, especially with this surgery, a new graft in place has been anchored and the anchoring technique will play a role in this as well, was an LET done,  a lot of factors play into this. But then sometimes that rotation can feel stuck or kind of pinchy. Therefore, we want to make sure that that tibial rotation has some action and some movement that we restore that along with the mobility of the joint, in order to make sure that athletes can get the most out of their screw home mechanism and more importantly, extension and flexion. 

Now don’t go off and go to a ton of tibial rotation work. As some of this stuff comes natural with some of the exercises and movements that are programmed into what you’re doing. If you do think that this is a concern, talk with your PT about it. If you have any questions, you can always reach out to us. It’s not always something that needs to be directly addressed. I wouldn’t say that most athletes we work with, we need to address this because it just comes naturally, and they restore the joint movement and they get their extension back. Therefore, the joint is doing its thing. But there is a subset of ACLer and knee injured athletes. This could be a huge game-changer for to free up some of that extra mobility and be able to reinforce that screw home mechanism, being such a key player for the knee joint, especially into extension. Of course, the rotation of the tibia is going to be important for flexion as well, but here very much an important issue for especially extension to get that external rotation of the tibia slightly in order to really hit home, if you will, that screw home mechanism at full knee extension. 

Now, what can we do about it? You’re like, all right, cool. I’ve talked about this for a while—way too accessibly. What can we do about it? When doing a knee extension mobilization so that’s where we’re kind of moving the joint manually a little bit. You want to add some tibial external rotation, especially if we’re in an open kinetic chain, or you can do some femoral internal rotation relative to the tibia. So that would be some closed kinetic chain to get the most out of the screw home mechanism in terminal knee extension. 

Some techniques to specifically improve it. You could think about isolated tibial rotation. If you can free up that isolated movement and therefore, hopefully that can help with the integrated movements of adding that into flexion and extension, and that’s a progression of this. But first isolated tibial rotations, you can do it open chain. That could be something like knee cars where you’re focused on freedom of joint with rotation, extension and flexion of the knee joint itself. There are different techniques where you can do this, but basically putting your elbow underneath your knee joint and just making sure you’re kind of locking it in to where it’s only movement at the tibia. There’s closed kinetic chain options where you’re kind of on a slider. We’ll put like a yoga block between the knees and then someone will have a slider on their foot and on a slick ground, if you will. And then they could rotate the tibia in and out. The way you’ll know is rotating is based on the tibial tuberosity movement. You can even put a pen mark or a Sharpie on it and see is the tibia rotating and seeing if that… is moving the tibial tuberosity, being where your patella tendon attaches on your shin bone. That will move, but you want to make sure you keep the hips still. If your knee cap is moving with it, or you notice your knee is moving in and out, then you’re really just moving your hip, instead of just your shinbone or your tibia. That’s where we’ll see people compensate here.

Also, just know that there’s more tibial external rotation than internal rotation. No, it won’t be equal joint motion. In a sense, you will notice your right foot turn outward, if you’re doing this correctly. But the ankle shouldn’t be moving. And there’s going to be more outward external rotation than there is internal rotation and that’s just basic human anatomy, just like there’s more flexion than there is extension of your knee joint. Therefore, that’s important to note. So then that way you’re not trying to match them up equally or saying, oh, well, one is different than the other. There’s normative values for every joint range of motion; and IR has less than ER. 

What we’ll do is we’ll take that isolated piece, working on it, and then we will work on it with a combined flexion and extension. Therefore, there could be a manual assist of IRR of the femur with external rotation, ER, of the tibia with oscillating pressure. Now, this is very rare that we do this with the athletes, but there are certain situations where this can happen. We also teach some self mobilizations with this for our athletes, to be able to do this pretty easily. And then half-kneeling manual tibial rotation. This is something where you work on internal rotation, which will help with the flexion or unlocking, if you will, which is something that’s going to be important along with restoring the external rotation. And so that’s going to be where their shin is rotate taking towards the midline. You’re just making sure that you are not letting your knee collapse in. You’re going to just use your hands to help rotate the shin inward while making sure you’re rocking forward, putting weight onto your front foot. 

So that’s going to be the half-kneeling manual tibial rotation, and then there’s a 3D step down. I learned from my buddy, Andy, and this is basically a single-leg balance. And then you’re working on opening up the hip to where you’re almost thinking if I’m standing on my right leg, I’m going to allow my arch to collapse and then not let my knee dive too far inward. But we’re going to focus on trying to open up, think about increasing the distance from your right knee to your left knee, to where you’re almost going to be turning your whole torso like an hip airplane in a sense to the side, you’re opening to. 

These are different ways to help work on rotation combined with flexion and extension. There is, of course, the TKE but we need to make sure we use proper mechanics of letting the foot supinate and pronate. That means collapsing versus being able to come back on us arch because that is going to help unlock and lock the foot, which is going to therefore free up motion at the tibia, which is going to impact the rotation of the tibia and that screw home mechanism. It’s a whole kinetic chain type sweet sequence. 

I hope you guys are exhausted at this one. I know that this was a lot of technical nuances of the screw home mechanism. I hope that this was helpful to break it down. You might have to listen to this like 10, 15, 20, and 30 times in order to grasp some of these pieces. 

I know that this is a visual thing so you can always go on to Google and look it up and just look at the screw home mechanism and what’s going on. You should notice that the tibia externally rotates whenever it extends. And it’ll slowly internally rotate as it goes to initiate flexing. This is really key, really important, just as a basic foundational element of knee joint range of motion, the biomechanics of it, for it to lock and unlock and how this screw home mechanism really does play an important role in how our knee joint interacts. This might be something, if you are someone who is dealing with potentially some issues with joint range of motion, it could be flexion with trying to work on some internal rotation because those are coupled together. And then it could be extension with some external rotation of the tibia.. 

And for a small subset of athletes we have worked with, this has been really helpful. It’s actually that last a little bit. They needed them to unlock a little bit of that extension, especially end range extension. One that we’re chasing a lot and trying to get that he’ll pop. 

I hope that this was helpful for you guys. If you have any questions about this, I know it was really technical, please do reach out. I am happy to chat about it. Answer any questions if you’re a rehab professional or a coach, or if you are an ACLer who’s like, I don’t really know, does this apply to me or not? Please reach out to us. And we are happy to help in any way that we can. Until next time team, this is your host, Ravi Patel, signing off.