The Knee Joint

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Author: Dr Peter de Souza
Last modified: 28 December 2020


Part 1

This is a tutorial on the knee joint. The knee joint is the largest synovial joint in the body and it’s these articulations between the femur and the tibia and also between the patella and the femur.


It’s a hinge joint and the main movements you get at this joint are flexion and extension.   In this position here, which we’re looking at the model, this is in the fully extended position, so you get flexion and extension. Flexion is back this way and extension is up this way.

You also get a little bit of rotation at this joint.   When the joint is flexed, so when the femur is pulled up this way, you get a little bit of medial and lateral rotation. In full extension, which is what we’re looking at here (so in a standing position), the femur medially rotates on the tibia and the joint is locked into position and you’ve got ligaments on either side, the collateral ligaments, which tighten to lock this joint into position so it’s stable in this position. And then to unlock the joint, the femur is laterally rotated on the tibia.


First, we’ll just take a look at the articular surfaces of this joint.   The articular surfaces are covered by hyaline cartilage and you’ve got the femoral condyles – I’ll just rotate it back. You’ve got these two femoral condyles articulating with the tibial condyles of the tibia, so the superior aspect of the tibial condyles.


We’ve got a medial and a lateral femoral condyle here. We’ve got the same for the tibia.   We’ve got medial and lateral tibial condyles. The articular surface of the tibial condyle is the superior surface, which I’ll show you in a moment.


The two femoral condyles here are separated by this region, which sits between them. This is the intercondylar fossa.   This fossa, this intercondylar fossa is the area where the anterior and posterior cruciate ligaments have that proximal attachment.   I’ll come on to talk about that in a moment.


And just above the condyles, you’ve got epicondyles.   This is the site of proximal attachment for the collateral ligaments, the medial and lateral collateral ligaments or the tibial and fibular collateral ligaments.   The epicondyles are actually not articular. You’ve got these epicondyles here where the collateral ligaments attach.


And then anteriorly if I just remove the patella bone, you’ve got this little sort of groove in the femur where the patella articulates.


We’ll just take a look at the tibia as well. We’ll look at the proximal tibia. You’ve got the tibial condyles which articulate with the femoral condyles.   You’ve got a medial tibial condyle and a lateral tibial condyle.


I’ll just remove the other bones so we can look at the superior aspect of the tibial condyle. I’m just going to rotate the model so we’re looking at the superior aspect. You can see the sort of faded fibula, so this side is lateral and this side is medial and we’ve got anterior up here and posterior down here.   I’ll just fade out those bones now and we’ll look at the superior surface of this bone.


This is the articular surface of the tibial condyles. We’re looking at the superior surface of the medial condyle. Here’s the superior surface of the lateral tibial condyle. In between these two condyles, you’ve got these regions, the intercondylar region. This region is important because you get the menisci of the knee joint attaching in this region and also the anterior and posterior cruciate ligaments attaching in this region. I’ll show you that in this tutorial.


This whole region, this whole superior surface is referred to as the tibial plateau.   It consists of the articular surfaces of the tibial condyles and this intercondylar region.   That’s the tibial plateau.


If I just rotate the model around a little bit, you can see that there’s eminence, this protuberance of bone. This is the intercondylar eminence. Laterally, we’ve got the lateral intercondylar tubercle and medially, we’ve got the medial intercondylar tubercle.


Just while we’re looking at this view, I’ll just talk a little bit about the menisci.   These are fibrocartilaginous structures which sit on either side and they’re crescent-shaped.


The menisci are also called semilunar cartilages because of their half moon shape. They function really to act as shock absorbers and they accommodate changes in the movements of the bones at this joint.


I’ll just switch over to another diagram. We’re looking at the exact same view. We’re looking superiorly at the tibia, the tibial plateau. This is anterior up here, posterior down this side. You can see these crescent-shaped menisci. You’ve got a medial meniscus and a lateral meniscus. You can see how they attach in this central intercondylar region. You’ve also got the anterior cruciate and posterior cruciate ligaments attaching in this intercondylar region.


You can see anteriorly that there’s this ligaments which connects the two menisci. This is called the transverse ligament.


Some other things about the medial meniscus. It’s actually attached medially to the tibial collateral ligament or sort of blended with the fibers of the tibial collateral ligament and also to the capsule of the joint of the knee.   It’s actually not very mobile because of these attachments, whereas the lateral meniscus doesn’t have these attachments to the joint capsule, so it’s a lot more mobile.


Part 2

Now we’ll just take a look at some of the ligaments of the knee. What you need to remember about the ligaments is that you have collateral ligaments, cruciate ligament and a patella ligament.


We’ll start off by looking at the collateral ligaments.   We’ve got two collateral ligaments which reinforce the joint on either side. We’ve got a medial collateral ligament, which is also referred to as the tibial collateral and we’ve got a lateral collateral ligament or a fibular collateral ligament.


We’ll take a look at the medial collateral ligament here. You can see its proximal attachment on the medial epicondyle of the femur and you can see its distal attachment on the tibia.


It attaches medially on the tibia just a little bit above the insertion points of the sartorius, gracilis and semitendinosus. If I just rotate the model around, you can see the tibial collateral ligament and the three tendons of the sartorius, gracilis and the semitendinosus muscle.   These three tendons, the tendinous insertion of these three muscles is collectively referred to as the pes anserinus. These three insertions could be remembered with the mnemonic sergeant, SGT – sartorius, gracilis and semitendinosus.


Laterally, we’ve got the fibular collateral ligament or lateral collateral ligament. This attaches on the lateral epicondyle proximally and distally, it attaches onto the lateral part of the head of the fibula.


Next we’ve got the cruciate ligaments. I’ve just rotated the model around so we’re looking posteriorly at the knee. You can see these two ligaments which cross over each other.   This is why they’re called cruciate ligaments. The word ‘cruciate’ comes from the Latin ‘crux’, which means cross. You get other words like ‘crucify’, ‘crucifix’, which just comes from the Latin cross.   You can see how these ligaments form this cross.   This is the posterior cruciate and the anterior cruciate. It crosses over like this.


You can see their attachment in the intercondylar fossa.   This ligament here is the posterior cruciate ligament and its proximal attachment is on the medial wall of this fossa.   You can see this wall of the fossa. It’s medially on the intercondylar fossa. And the anterior cruciate ligament has its proximal attachment on the lateral wall of this intercondylar fossa. This attaches medially on the lateral condyle and the posterior cruciate ligament attaches laterally on the medial condyle.


The posterior cruciate ligament is called ‘posterior’ because it attaches posteriorly on the intercondylar region of the tibial plateau. And if I rotate the model anteriorly, the anterior ligament attaches anteriorly in the intercondylar region of the tibial plateau.


I’ve just isolated the ligament so you can see them a bit more clearly now. You can see how they cross over at the back here.


The function of the ligaments is to prevent anterior and posterior displacement of the tibia and the femur.   You can see because the anterior cruciate ligament attaches anteriorly on the tibia, it prevents the tibia from sliding forward in this direction. And because of the attachment of the posterior cruciate ligament, it prevents the tibia from sliding backwards.


Then we’ve got this ligament here, the patella ligament.   This is continuous with this tendon above the quadriceps femoris tendon. This attaches around the sides of the patella and at the apex of the patella and it attaches distally to the tibial tuberosity.   You can see the tuberosity of the tibia.


We’ve got a medial collateral, lateral collateral, anterior cruciate, posterior cruciate and a patella ligament that you need to know about.


Lastly, I’ll just talk a little bit about the joint capsule of the knee. The joint capsule of the knee has a fibrous membrane and a synovial membrane.   The fibrous membrane is reinforced by various ligaments.   Medially, it’s reinforced by the medial collateral ligament. It blends with fibers of this ligament. And it also attaches to the medial meniscus, which is why the medial meniscus isn’t very mobile. Anteriorly, it blends with the patella ligament and attaches to the margins of the patella.


Either side, you’ve got extensions from these two muscles, the vastus medialis and vastus lateralis, which contribute fibers to this fibrous joint capsule which are called retinacula.   That strengthens the capsule anteriorly.


And laterally, we’ve got the lateral collateral ligament, so the fibular collateral ligament, but there’s a space between this in the joint capsule. There’s a bursa underneath the fibular collateral ligament called the fibular bursa, which separates it from the joint capsule. But it is reinforced by this tract of fibers. The iliotibial tract reinforces the capsule laterally.  


And if we rotate around to the back, you can see this ligament, which arches over the back of the joint capsule. This is called the oblique popliteal ligament. This is actually an extension from this tendon, the semimembranosus tendon, which attaches onto the tibia. It gives off this ligament called the oblique popliteal ligament, which reinforces the posterior aspect of the joint capsule.


Lining the fibrous membrane, you’ve got the synovial membrane. This attaches to the margins of the articular surfaces and the outer aspects of the menisci.


It’s important to note that the cruciate ligaments aren’t actually kept contained within the articular cavity, so the synovial membrane kind of reflects back and it doesn’t enclose the cruciate ligaments.


The synovial membrane folds in various places to form bursi. We’ve got the suprapatella bursa, which as the name suggests is a reflection of the synovial membrane, which forms a bursa above the patella. It’s supra- meaning ‘above’.


We’re looking laterally at the knee here. You can see the patella bone here. This is the quadriceps femoris tendon here. The suprapatella bursa lies in this region just above the patella. This is a fold of the synovial membrane.


It also folds to form the subpopliteal recess. This fold lies between the popliteus tendon and the lateral meniscus. If I just fade away the muscles, we might be able to – so you can see this muscle here, this is the popliteus muscle. And if I rotate it around, you can see the lateral meniscus here.   This bursa sits under the popliteus tendon between the lateral meniscus.


Another thing to point out is that just underneath the patella ligament, we have a pad of fat, which is called the infrapatellar fat pad. This separates the patella from the synovial membrane.   It sits underneath the patellar ligament and separates the ligament from the synovial membrane.


A few other bursi worth mentioning. You’ve got this bursa here which sits over the knee cap. It’s called the prepatellar bursa, the subcutaneous prepatellar bursa. And then you’ve got this bursa down here, which sits underneath the patellar ligament. This is called the deep infrapatella bursa. You’ve also got one that sits on top of the patellar ligament and that’s more superficial. This is called the superficial infrapatella bursa.


And then you’ve got loads of other little bursa which sit over the tendons and ligaments, but I won’t go into details with all of those.   That’s the knee joint.