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Knee joint

Sagittal view of the knee joint showing the patellofemoral and tibiofemoral joints.

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When we go jogging on a beautiful Saturday morning, most of us tend to spend our time running thinking about the week that’s gone by or maybe the weekend that’s ahead or whatever drama is going on in our lives. Right? Probably the last thing on your mind are the joints which are actually making it possible for you to run in the first place. You know – joints like your hip, knee, and ankle.

Running can take quite a toll on these joints, so although they need to be built for movement, they also need their share of reinforcements to protect them from the wear and tear of locomotion. Perhaps the most complex and susceptible to injury of these is the knee. But why is that? Perhaps, its anatomy has something to do with it? Let’s find out more as we explore the ins and outs of the knee joint.

Let’s quickly begin by previewing some of the exciting things that this tutorial will cover. We’ll start off by looking at the mainframe or the bones that make up this joint. We’ll learn about the distal femur, the proximal tibia, and the patella as well as any landmarks associated with the knee joint. We’ll then look at the articulations within the joint. This sagittal section will allow us to see the deepest structures of the knee joint, more specifically, structures which make up the articular surface of the knee joint as well as some surrounding ligaments, tendons, and soft tissues.

We’ll then see how the soft tissues support the stability of the joint. We’ll then move on to see which muscles move the knee joint and what movements the joint is actually capable of. From here, we’ll take a quick look at the innervation and vascular supply to the joint before we finish up this tutorial by looking at some clinical notes on the disease and trauma commonly affecting the joint.

So, can I count on you to join us in this tutorial while we explore the knee joint? Great! Let’s jump right in.

I want to start our examination of the knee joint by first looking at the three bones that contribute to the formation of the structure. We have the femur, the tibia, and the patella situated here. I’m sure you’ve also noticed this little guy down here – this is the fibula – however, it doesn’t actually contribute to the knee joint. So, we’ll not be looking at it in much detail. However, we’ll come across it again when we look at the muscles which move the knee joint as it’s an important site for muscle attachment.

Now let’s take a closer look at the bones which do articulate at the knee joint.

Let’s start with the femur. You can see here that the proximal end of the femur forms an articulation with the acetabulum of the hip bone. But let’s focus on why we’re gathered here today and let’s zoom in on the distal end of the femur so we can see the landmarks important to the knee joint.

Let’s start with the obvious. This longer, thinner section is the distal portion of the shaft or the body of the femur. You’ll notice raised lines or ridges on the posterior surface. These are the medial and lateral supracondylar lines. Where these lines converge proximally, we find the linea aspera. The distal end of the femur widens for better weight distribution to the tibia as it is a weightbearing surface. It is formed by two rounded parts called the medial and lateral condyles. You’ll notice that anteriorly, the two condyles are continuous but are defined by a slight groove called the intercondylar groove. On the posterior surface, they’re separated by the intercondylar fossa. Let’s take a closer look at the condyles.

On the medial condyle, you’ll find a bony protrusion called the medial epicondyle. There’s a further small projection on it called the adductor tubercle, which is an attachment site for the ischiocondylar part of the adductor magnus muscle. On the lateral condyle, there’s a similar but smaller protrusion called – you guessed it – the lateral epicondyle. It’s slightly smaller than the medial epicondyle and it’s an attachment site for the fibular collateral ligament. Finally, we have the articular surface divided into the patella and tibial surfaces.

We’re moving on now to the second longest bone in the body – the tibia. Let’s look right at its proximal end because we know that’s the part involved in the knee joint. Similarly to the femur, the tibia has a narrower shaft and a wider weightbearing proximal end. Just like the femur, the proximal end of the tibia is formed by two condyles. You can see the medial condyle over here and now I’ve highlighted the lateral condyle.

On each of these condyles, you have an articular area which articulates with the corresponding condyle of the femur, and these are also collectively known as the tibial plateau especially in the clinical setting. The two areas are separated by a non-articular, irregular intercondylar area with the rugged raised area called the intercondylar eminence.

Just below the condyles on the anterior surface, you’ll find the bony projection called the tibial tuberosity. It’s an important attachment site which we’ll learn more about a little later. We got to keep a bit of suspense going, you know?

We’ll, we’re onto our last bone of the knee joint, and it is the patella, also commonly known as the kneecap. And what a funny little bone this one is. It’s the largest sesamoid bone of the body, which means that it’s embedded in a tendon. In this case, it’s the quadriceps femoris tendon, which places the patella just anterior to the femoral condyles. Its superior edge is curved whereas the inferior aspect converges onto an apex giving it a kind of upside down teardrop shape.

The posterior surface of the patella contains an oval articular surface close to its superior edge. It’s roughly divided into two articular facets by a vertical ridge which sits on the intercondylar groove on the anterior surface of the distal femur. The two facets each articulate with the patellar articular surface on each of the femoral condyles.

So we’ve met the bones which make up the knee joint, but how do they move? How are they able to support the weight of the whole body? It might be easier to answer these questions if we cut through the joint and see what’s on the inside. Oof, that was really hard work.

First of all, let’s point out the familiar bony structures. Here, we have our femur, the tibia with its tibial tuberosity, and finally, our little patella. Now where I think the sagittal section will come in real handy is learning about the articular surfaces, and I’ve highlighted all the articular surfaces involved with the knee joint in this image.

So I mentioned that the femoral articular surface is divided into the patella and the tibial parts and I think this is where things can get a bit confusing if you’re only looking at the anterior and posterior images of the knee. The knee is a compound joint, so essentially we have two separate joints here. We’ve highlighted the femoropatellar joint in green and the femorotibial joint in blue to help distinguish between the two. Let’s talk about the femoropatellar joint first.

The femoropatellar joint is just one of those slightly odd things in the body. It’s a gliding or a plane joint, which means it just kind of glides up and down the femoral articular surface. The patellar articular surface is faceted and slightly irregular, which results in different degrees of contact during the movement at the knee joint. When the knee is extended, the medial patellar facet will be in contact with the lower part of the medial femoral condyle whereas in flexion, the lateral facet will be in contact with the higher portion of the lateral femoral condyle which produces the highest degree of contact. And this is indicative of the forces acting on the joint. It’s under the most pressure during flexion and so the most contact is needed for equal dispersion of this pressure. In fact, the pressure exerted on the patella in the femoropatellar joint is so great that to compensate, the patellar articular surface has the thickest layer of hyaline cartilage in the whole body.

Why you might be thinking while I’m telling you all of this is why? Why do we need the patella and its articulation with the distal femur? Well, it’s there to aid extension of the knee joint. It increases the angle at which the quadriceps tendon acts on the knee joint which increases the leverage or mechanical advantage of the joint, and therefore, less force is needed for the muscle to extend the knee than it would be to do the same job with smaller leverage. Who knew that we’d have a little physics lesson while learning anatomy?

So we’re moving on to the next element of our compound joint – the femorotibial joint. It’s ever so slightly more complicated than the femoropatellar joint, but together, we can conquer it.

So, first things first, this joint is actually a two-parter. There are two separate articulations between the medial and the lateral parts of the tibial articular surfaces and the corresponding femoral condyles. Its main characteristic is its lack of congruity, or in human terms, it’s poor fit.

If you have a look at the sagittal section, you’ll notice that the contact area of the articular surfaces is not really large at all. If you compare it to some other joints in the body, for example, the elbow joints where articular surfaces fit almost like two pieces of a puzzle, you’ll realize just how unstable this joint really is. It’s classed as a hinged joint, and like other hinged joints in the body, it moves along one axis. That means that the main movements at this joint are flexion and extension. But due to its shape, a small degree of internal and external rotation also occurs, which means rotation of the tibia against the femur when the knee is flexed. About a five to ten degree terminal rotation occurs to reach full extension and we’ll be coming back to it later on.

Right, so, how are all these parts of the joint able to move around and against each other with such ease? The answer is simple – it’s a synovial joint. As you probably already know, a synovial joint consists of hyaline cartilage covered articular surfaces with a small articular cavity between them. This space is filled with synovial fluid which lubricates the joint and it’s enclosed by a synovial membrane, parts of which I’m highlighting right now.

Okay, so you must be wondering how such an unstable joint just doesn’t collapse? Well, of course, it’s all because of ligaments.

For convenience, we’re going to divide the ligaments of the knee into internal and external ligaments, and this is based on where they are located in relation to the fibrous articular capsule of the knee joint. Due to that relation, they’re often referred to as either intracapsular or extracapsular ligaments. So it make sense to take a closer look at the articular capsule before we continue looking at the ligaments of the knee joint.

The articular capsule fairly loosely encloses the knee joint so we’ll come up with pretty much every external ligament that we’ll look at. The capsule actually consists of two parts. We’ve already seen the synovial membrane, which encloses the articular space, and now, we’re looking at the fibrous outer capsule which you can see highlighted in the image.

Now you can see the articular capsule from the posterior aspect of the knee. Posteriorly, the capsule stretches from the articular margins of the femoral condyles and the intercondylar fossa proximally to the tibial condyles distally and covers the whole posterior aspect of the joint. It blends to an extent with the oblique and arcuate popliteal ligaments, which we’ll see in a moment.

There’s a small opening at the posterior lateral tibial condyle which allows the tendon of the popliteus muscle. Anteriorly, it’s a slightly different story because the capsule is formed by the quadriceps tendon, the patella, the patellar ligament, and the patellar retinacula formed by the vastus medialis and lateralis expansions.

You might be wondering what all these structures are. Well, why don’t we find out?

So let’s start by looking at the patellar ligament. This strong flat band is actually just a continuation of the quadriceps femoris tendon. It stretches from the apex of the patella to the tibial tuberosity. It’s joined medially by the medial patellar retinaculum and laterally by the lateral patellar retinaculum. This is what we’d be looking at if we strip off the patellar ligament and all the muscles. The retinaculum is formed by aponeurotic expansions of the vastus medialis and lateralis muscles overlying the deep fascia.

Moving laterally, we have the fibular collateral ligament. It’s sometimes also called the lateral collateral ligament, or the FCL, and you’ll notice that abbreviating is a running theme with ligaments of the knee joint. The fibular collateral ligament is a strong cord-like ligament stretching from the lateral epicondyle of the femur to the fibular head, and it’s separated from the joint capsule.

Now moving on to the medial aspect of the knee joint, we find the tibial collateral ligament. This ligament is also known as the medial collateral ligament, or the TCL. Did I not tell you that we like abbreviating? The tibial collateral ligament is also a strong flat ligament, but weaker than its lateral counterpart and so is more prone to damage. It extends from the medial epicondyle of the femur to the medial condyle and the proximal medial part of the tibial surface. It’s actually continuous with the joint capsule, and at its midpoint, its deep fibers are firmly attached to the medial meniscus. The two collateral ligaments together limits excessive sideways movement at the joint.

We’re now moving on to the posterior surface of the joint to look at the oblique popliteal ligament. It’s a recurrent expansion of the semimembranosus tendon. Recurrent means that instead of carrying down distally from the semimembranosus insertion at the posterior aspect of the medial tibial condyle, it travels superolaterally. It obliquely crosses the intercondylar fossa and inserts into the lateral femoral condyle. It blends with the central part of the posterior articular capsule and its main function is to give the capsule additional support.

Another ligament strengthening the articular capsule posterolaterally is the arcuate popliteal ligament. It is a Y-shaped ligament which means that it arises as a single band at the fibular head and then goes two separate ways. The medial part passes superomedially over the popliteus muscle tendon and blends with the posterior capsule and the oblique popliteal ligament. The lateral part passes superiorly over the knee joint and inserts into the capsule close to the gastrocnemius lateral head roughly at the lateral femoral condyle.

So, I have a little fun fact for you. The arcuate popliteal ligament is only present in about sixty five percent of individuals. It is usually absent if a little ossicle called the fabella is present in the gastrocnemius muscle strengthening the posterolateral capsule that way, which simply makes the arcuate popliteal ligament unnecessary.

So we’ve seen how the knee joint is secured exteriorly, but that still doesn’t fully explain how such an unstable joint is able to stay intact under so much weight and still achieve such range of motion. Well, what really anchors everything in place is the internal ligaments. Let’s see what they are.

The most important and perhaps best known intracapsular ligaments are the cruciate ligaments. These two very strong bands – the anterior cruciate ligament and the posterior cruciate ligament – cross over close to the articular center of the knee. The crossing over is what gives these ligaments their name. Let’s start with the anterior one.

The anterior cruciate ligament, commonly known as the ACL, attaches at the anterior intercondylar area of the tibia and ascends posterolaterally within the joint to insert into the posteromedial aspect of the lateral femoral condyle. When the knee is extended, the anterior cruciate ligament is under tension which allows it to prevent hyperextension.

The posterior cruciate ligament, often referred to as the PCL, does the opposite job. It is under tension when the knee is flexed. This is achieved by its attachments to the lateral surface of the medial femoral condyle and the posterior intercondylar area of the tibia. It’s actually the stronger of the two ligaments and prevents hyperflexion of the knee joint. Together, these strong ligaments prevent the femur and the tibia from slipping off each other or dislocating.

No matter what position the knee is in, one or parts of both ligaments will be under tension ensuring a stable articulation at the joint at all times. They also play a role in the rotation of the knee joint. In internal rotation, the ligaments wrap around each other, whereas in external rotation, they are positioned as two parallel bands. Quite interestingly, the two ligaments are contained in the fibrous articular capsule, but not within the synovial membrane.

Okay, guys, the next structures are not technically ligaments, but they lie within the knee joint and are extremely important for its movement. We’re looking at the menisci. So, the menisci are semilunar fibrocartilaginous structures found on the tibial articular surfaces. They serve to broaden and deepen the area of articulation between the femoral condyles and the tibial articular surfaces. They’re thicker around the outer edge and thin out as we move inwards to create two nice rounded hollows which receive the femoral condyles. This way, the weight is distributed over a much larger area of the tibial articular surfaces. They also stabilize the joint and provide cushioning at the extremes of the ranges of movement.

The medial meniscus is almost semicircular in shape and covers a slightly smaller area of the two menisci. Its anterior horn attaches to the anterior tibial intercondylar area. The posterior horn is fixed to the posterior tibial intercondylar area and its fibers are continuous with the transverse ligament of the knee when present. Peripherally, it’s attached to the fibrous articular capsule and deep fibers of the tibial collateral ligament which allows for very little movement of the meniscus.

The lateral meniscus covers a larger area and is more circular in shape. Its anterior horn attaches in front of the intercondylar eminence posterolateral to the attachment of the anterior cruciate ligament. The fibers of the anterior horn and the anterior cruciate ligament actually blends to an extent. The posterior horn attaches posterior to the same eminence anteriorly to the posterior horn of the medial meniscus. The posterior horn is often anchored down by two meniscofemoral ligaments and more laterally by fibers from the popliteus tendon which control the mobility of the meniscus.

We encountered a couple of unfamiliar ligaments while we were looking at the menisci, so let’s talk about them. Why don’t we start with the transverse ligament of the knee. It connects the anterior horn of the medial meniscus to the anterior margin of the lateral meniscus and keeps the menisci in place during extension. The fun thing about this ligament is that it’s of varying thickness and is often absent altogether.

Moving on, the next structures we’ll look at are the meniscofemoral ligaments and they connect the posterior horn of the lateral meniscus to the lateral or inner aspect of the medial femoral condyle. The anterior meniscofemoral ligament passes anterior to the posterior cruciate ligament while the posterior meniscofemoral ligament passes posterior to it and attaches close to the attachment of the posterior cruciate ligament.

We’ve seen the movements at the knee joint and we’ve seen the support from the ligaments. Time to move on now to the structures of the knee joint, which both creates movement and provides support – the muscles.

Let’s jump right in to see which muscles extend the knee joint. The chief extensor is the quadriceps femoris muscle with its four heads. The rectus femoris, the vastus intermedius, the vastus medialis, and the vastus lateralis muscles, with some weak assistance from the tensor fascia latae. Moving on to the flexors of the knee joint, we have the posterior thigh muscles or the hamstrings as the chief flexors. This group of muscles include the semitendinosus, the semimembranosus which also help with the internal rotation of the knee, and the biceps femoris muscle. All of the posterior thigh muscles work to extend the hip joint and flex the knee.

The weak flexors of the knee are the gracilis, the sartorius, and the gastrocnemius. The gracilis and the sartorius muscles also weakly contribute to the medial rotation of the joint. The tensor fascia latae muscle may also help to flex the knee when it is already flexed by at least twenty degrees as well as to externally rotate it together with the biceps femoris muscle.

There’s one more assisting muscle called the popliteus muscle that I wanted to talk about separately because it does a very important job. When the knee is fully extended, the femur makes a small medial rotation called the terminal rotation, which means that a joint is locked in place. That makes the whole lower limb into a solid weightbearing structure. It also means that lower limb muscles can relax briefly without the lower limb collapsing, however, that poses a problem when flexion at the knee is needed again. This is where the popliteus muscle comes into play.

When the knee is locked, the popliteus’ insertions allow it to laterally rotate the femur on the tibia to unlock the knee joint to allow flexion. When the knee joint is already flexed, the popliteus can medially rotate the tibia against the femur and it also acts as a weak flexor.

To learn more about the functions of the muscles that move the knee, check out our muscle functions video showing all the movements on a 3D model.

And we’re jumping back into the sagittal section to wrap up some loose ends. Or more specifically, to learn about the bursae and fat pads surrounding the knee joint.

So you might be wondering why we didn’t do this right at the start. That’s because bursae and fat pads are often named after or related to other structures, so I wanted to make sure we knew these first to avoid confusion.

Let’s start by identifying some structures which we’ve learned about since last looking at the sagittal section.

So here we have the quadriceps femoris muscle, the chief extensor of the knee joint, and its tendon is highlighted now, and you can see it continuing over the surfaces of the patella to become the patellar ligament.

Just around here, you’ll find the gastrocnemius tendon and this is the lateral meniscus.

Now onto the actual fat pads. So we’re now looking at one of the two main fat pads associated with the knee joint called the infrapatellar fat pad. Found below the patella, it’s conveniently named by its location. Also named after its location is the suprapatellar fat pad which can be further subdivided into a suprapatellar prefemoral fat pad and the suprapatellar quadriceps fat pad, named after their adjacent structures.

These fat pads – just like everything in the body – have their own function. They serve as a reserve of cells for tendon and ligament repair and cells involved in the inflammatory response.

The bursae of the knee are a tad more complex. There are at least twelve of them around the knee joint, but we’ll only talk about the most important ones. So just to jog your memory, bursae are synovial fluid-filled pouches around a joint that create a sort of cushion between the joint and the surrounding structures – for example, tendons of muscles.

The structure you see highlighted is the suprapatellar bursa, which is situated above the patella, between the femur and the quadriceps tendon. The next bursa is the deep infrapatellar bursa, and it is sandwiched between the patellar ligament and the tibia as you can see.

Now if we have a deep infrapatellar bursa, then, yep, that’s right, there must be a superficial one – the superficial infrapatellar bursa. You can see it sitting between the patellar ligament and the skin, making it a quite superficial structure indeed. And there’s also a bursa sitting right on the anterior surface of the patella called the prepatellar bursa. You may come across this bursa also being referred to as the subcutaneous prepatellar bursa.

So, posteriorly, we have a medial and a lateral bursa between the gastrocnemius tendon and the femur called the medial and lateral subtendinous bursae of the gastrocnemius – a pretty self-explanatory name.

Laterally, we have the popliteus bursa between the popliteus muscle and the lateral condyle of the tibia. Medially, you’ll find the semimembranosus bursa between the semimembranosus and the gastrocnemius tendons, and lastly, the anserine bursa.

The anserine bursa pats the area between the pes anserinus and the medial tibia. A few bursae are actually continuous with the synovial cavity and that means that if there is an infection in one of the bursae, especially the large suprapatellar bursa, it can spread into the knee joint.

Okay, so we’re almost right at the end of the gross anatomy part of this tutorial, but just so you have the full picture, I want to quickly tell you about the vascular supply and the innervation to the knee joint.

In the traditional model, blood supply to the knee joint comes from many small arterial branches forming what’s known as the genicular anastomosis around the joint. The word anastomosis just means that two branches are joining together. The two main contributors are the popliteal artery which gives six genicular branches – and you can see these on the screen right now – and the femoral artery.

There are small contributions from the anterior tibial recurrent and circumflex fibular arteries. However, it’s useful to bear in mind that the model has been challenged suggesting that it may not be an accurate representation of the actual blood supply and it is in fact much more variable. But let’s not go there today.

The innervation to the knee joint is quite easy to remember because the nerves are named after the bones located in that general area. That means the majority of innervation comes from the articular branches of the femoral, the tibial, and the common fibular nerves, as well as the saphenous nerve and the obturator nerves. The structure that you see highlighted on the screen is the tibial nerve.

Okay, well done, guys! We’ve finished up the gross anatomy for today, so now we get to move on to the really exciting stuff and look at some clinical notes.

So, in terms of trauma, I want to talk about damage to the ligaments today as an ACL tear is one of the most common knee injuries. So, generally speaking, most injuries occur due to nonphysiological movement at the knee joint; in other words, when the knee joint moves in a way that it really shouldn’t move. The cruciate ligament injuries occur due to a sudden change in direction such as rotation at the knee joint or excessive pressure; for example, when incorrectly landing from a jump.

PCL tears also occur, but are not as common, and it’s important to note that trauma in cruciate ligaments rarely occurs in isolation. Usually one of the collateral ligaments or the articular capsules will be damaged, too. So quite interesting is the test to check for cruciate ligament damage. The physician would flex the patient’s lower limb with suspected trauma and would try to move the tibia anteriorly and posteriorly.

Excessive posterior movement would indicate rupture of the posterior cruciate ligament whereas excessive anterior movement would suggest a torn ACL.

Alright, that’s a wrap on this tutorial. But let’s quickly recap what we learned today before we part ways.

So we started by looking at the distal femur, the proximal tibia, the patella, and a bunch of bony landmarks important to the knee joint. We saw the inside of the joint by looking at the sagittal section, and that allowed us to learn that the knee is a composite joint consisting of the femoropatellar joint and the femorotibial joint. Here, we also had a look at the elements which contribute to making the synovial knee joint.

We looked at the fibrous joint capsule and the ligaments which help stabilize the generally unstable knee joint, the notable extracapsular ligaments where the fibula and the tibial collateral ligaments, and the patellar ligament. The two main intracapsular ligaments were the anterior and posterior cruciate ligaments. We also had a look at the medial and lateral menisci which provide better weight distribution and stability on the tibial articular surface.

We moved on to muscles, which are another contributor to stabilizing the knee joint and are, of course, responsible for the movement at the joint. We learned that quadriceps femoris is the main extensor, and semitendinosus and semimembranosus and biceps femoris muscles are the main flexors of the knee joint. We also saw that the knee joint was capable of a small degree of medial and lateral rotation and that the popliteus muscle unlocks the knee joint when it is in full extension to allow flexion.

We went back to the sagittal section to investigate the fat pad and bursae around the knee joint which create padding between the muscle tendon, the skin and bone to reduce friction. We saw that some bursae were continuous with the synovial space which could result in knee joint infection if one of those bursae were infected. And, finally, we took a peek at the vascular supply and innervation to the knee joint and had a look at some clinical notes especially in relation to the knee ligament injuries.

That’s it folks! Thanks for sticking with me to the end of this tutorial. See you next time and happy studying!

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