Hello anatomy friends this is dr Alsup and in this video we will really get into one of my
all-time favorites the knee joint. So our goals here will be to understand the different joints
that fall under the category of the knee joint, it's actually three separate joints in this region,
and we will of course talk through the many associated structures so those ligaments the
tendons the menisci in this region. As with all joints we will discuss what movements occur at
the knee joint and it's a bit more complicated than just the typical flexion and extension. And
lastly we will pull in any clinical information and how that melds with the anatomy in this region,
so which structures are commonly injured in unison or which areas are more prone to injury.
And if you think about it you likely know someone who has had some form of issue
regarding the knee so either just generalized pain in the knees or a diagnosed injury.
And this has a lot to do with the fact that the knee is relatively weak mechanically because of
the incongruence of its articular surfaces. These surfaces are, I like this description, they're
sometimes described as two comparatively large balls which are the femoral condyles
so you have one here and you'll have one here and these are sitting on what sometimes is described
as like a warped table top which means really just these relatively shallow tibial condyles. And these
articulations between the the femoral condyles and the tibial condyles will form the medial
and the lateral tibiofemoral joints and this is these two joints are really what
creates that kind of hinge motion that we typically think of when we think of the knee.
But also within the same articular capsule is the patellofemoral joint,
excuse me, and this is where the patella or the kneecap which has been reflected
in this image is going to articulate with the patellar surface of the femur. And so this joint
formed by the patella and the femur is capable of very small sliding motions of the patella
that occur in conjunction sometimes with the typical hinge motions of the tibiofemoral joints.
So to reiterate, the knee joint is primarily a hinge joint but you often hear it referred to
as a modified hinge joint and we will talk about why that is more in a moment. But first let's focus
in on the hinge motion so here we're kind of looking at a lateral view of the knee region.
And recall a hinge joint moves in one plane it allows flexion and extension. Now like most
joints of the body flexion and extension at the knee joint occurs in the sagittal plane.
But unlike most joints in the body flexion does not move in an anterior direction in the knee
joint. Instead flexion will move the leg in a more posterior direction reducing the angle
at the knee joint so it moves the leg posteriorly not anteriorly.
Extension will be increasing the angle at the joint and it's really kind of moving the the leg
back towards anatomical position and you don't want it to go too far past anatomical position
at the knee joint so really avoiding over extension.
So again flexion is the leg moving posteriorly and that is a key difference here at the knee joint.
Now let's bring in the modified portion of the hinge modified hinge joint concept. The knee joint
is capable of small amounts of rotation as well so whereas flexion and extension are the
main movements there is a small amount of medial and lateral rotation that's allowed at the knee.
And these are small movements again I keep saying small but these are important movements.
So when the knee is fully extended with the foot kind of planted on the ground so say
I'm standing I'm just standing I don't want to fall down but I don't want to
move any more than that what the knee can do is kind of what's often referred to as lock
in position and it can lock in position with a slight medial rotation of the femoral condyles
on the tibial condyles, okay so you have a little bit of medial rotation occurring there. And this
slight medial rotation in the locking of the knee joints due to those condyles moving is
often referred to as the "screw-home" mechanism and so you'll hear this brought up a few times.
And this is important because it allows it allows for the lower limb to really bear weight
with this lock knee without forceful contraction of the muscles and without the knee joint
stability decreasing so it allows a allows for the surrounding muscles to relax a bit during standing
you don't have to have forceful contraction constantly due to this slight bit of movement here.
To unlock the knee, the popliteus muscle will contract allowing slight lateral rotation
and this will allow for flexion of the knee joint to occur so when you're ready to move
again you can unlock with a little bit of lateral rotation to allow flexion to occur. So
slight medial and lateral rotation are important in maintenance particularly in terms of medial
rotation of an extended standing position without forceful contraction of surrounding muscles.
Okay both ligaments and muscles as well as their tendons play important roles in stability of the
knee joint and that's that that needs to happen you don't have a particularly good fit between the
bones. But between ligaments and tendons it is the muscles that are key here although we will spend
quite a bit of time discussing the ligaments as well because they are more prone to injury
or damage. And many sports injuries are actually preventable through muscle conditioning.
Of the muscles and their tendons the most important are the quadriceps femoris muscles
and in particular their distal tendon so it's the quads that are key here.
The patella I hope you recall forms within this tendon and the portion of the tendon so here's
the patella forming within, the portion of the tendon that extends from the patella down to
the tibial tuberosity which would be just deep to this, this portion right here is often referred to
as the patellar ligament but it's really just an extension of the tendon of the quadriceps femoris.
It is this tendon complex that really is the anterior protection of the knee
as the articular capsule in this region is very thin and oftentimes incomplete and as
you might imagine it'll also play an important role in maintaining the alignment of the patella.
Okay so let's get back into ligaments so the ligaments that are outside of the articular
capsule even though one of these will actually be a thickening of the capsule. We just discussed the
patellar ligament in the previous slide so now let's focus in on the collateral ligaments, and
like all collateral ligaments these are found on the medial and lateral sides of the joint.
And these will be taut so kind of tight when the knee is fully extended but will slacken
some with flexion and will also check rotation at the knee joint to prevent over rotation.
The fibular often referred to as the lateral as well so the fibular or lateral collateral ligament
is as you might imagine lateral so it's going to play a role in protecting that lateral side.
It is completely extrinsic meaning it is not a thickening of the articular capsule
so it's completely separate from the articular capsule and it is shaded here in yellow so over
here on the lateral side on the same side as the fibula. On the opposite side is the tibial
or medial collateral ligament now this ligament is a thickening of the
articular capsule so it's just it looks like a thickened portion of the articular capsule
and it tends to be the weaker of the two between the two collateral ligaments and this is important
so I hope you put lots of little stars next to it. It is actually attached to the medial meniscus.
This attachment between the TCL MCL and the medial meniscus is an important one because
if one gets damaged or injured it is likely that the other will as well, and you can see
this over here that is highlighted in orange. Now moving deeper into the articular cavity of the
knee joint and you will find two relatively short ligaments crisscrossing one another
so you can see them crisscrossing right here and these are the cruciate ligaments that's
where they're getting their name from the crisscrossing. The point where these cross
serves as the pivot for the rotary or the rotation movements and will limit
the amount of rotation possible so let's talk about each one of these and we will start with
the anterior cruciate ligament for two reasons. First it's anterior we like to start anterior
and second you've likely heard of the ACL as it's one of the more commonly injured ligaments. The ACL
will arise from the anterior intercondylar area of the tibia and will attach to
the lateral condyle of the femur and it will prevent the tibial anterior displacement
meaning it will not let the tibia move too far forward. Additionally it will also prevent
femoral posterior displacement so it will not allow the femur to be displaced posteriorly.
So when the joint is flexed the tibia cannot be pulled anteriorly like it cannot be pulled
out like a drawer because it is held in place by the ACL. If it does move too far anteriorly
then that is known as the anterior drawer sign meaning there's likely an issue with the ACL
so not a good thing there you do not want that tibia to be able to go too far anteriorly. The ACL
is the weaker of the two cruciate ligaments and more prone to injury sprains or damage.
The posterior cruciate ligament is the stronger of the two so if anterior is the weaker of the two
then you have to have a stronger one and that is the PCL. And this ligament kind of does the
opposite of what the ACL does it will help prevent tibial posterior displacement or femoral anterior
displacement. So it if there's too much posterior tibial displacement allowed this would indicate
that there is likely damage to the PCL. So those drawer tests are really testing whether the tibia
moves too far anteriorly or too far posteriorly. In the weight-bearing flex knee such as when
you're walking downhill or stairs the PCL is hugely important in stabilization of the femur.
Okay and we can't talk about the knee joint without discussing the meniscus so menisci is
plural meniscus is singular. Recall that meniscus are fibrocartilaginous plates shapes similar to
crescent so they're not completely encircling the tibial condyles they're more of a
crescent shape and again these are located on the tibial condyles are kind of sitting right
on those tibial condyles and serve to kind of deepen that articular surface to allow for a
slightly better fit between the bones. It will also play a role in terms of shock absorption.
Again we we mentioned this before the medial meniscus is going to be firmly attached
you can see that here, to the tibial or medial collateral ligament. So I brought that up that
connection a few times that attachment between the medial meniscus and the tibial collateral ligament
a few times so make sure to find some space in your memory for that anatomical relationship.
So you can see the middle meniscus here in green and the lateral meniscus here in light blue.
And before we finish up let's discuss a specific clinical consideration and you will
get much more clinical guidance in this area in the knee clinical lecture. But this injury
of three anatomical structures called the unhappy or terrible triad and that sounds awful right
this is often brought up on board exams so I want to mention it here. So the 3 anatomical
structures include the anterior cruciate ligament or ACL which makes sense I said that the ACL is
the weaker of the two cruciate ligaments as well as the medial meniscus and tibial or
medial collateral ligament which makes sense that these two structures would be injured
together as they are attached to one another. This type of injury is more common in contact sports so
if you think of somebody tackling or hitting you on the lateral side of the extended knee
so you have a blow say to this lateral side that force will kind of travel through and disrupt the
tibial collateral ligament which will typically tear or even detach the medial meniscus. And just
due to the inherent weakness of the ACL with a powerful blow this is commonly injured as well
can you have injury to just one of these structures you most absolutely can but just how
the knee is organized it's not uncommon for these three structures to be injured at the same time.
All right excellent we have made our way to the summary slide, so much going on here but
the knee joint is one of the more common places for MSK complaints so it is important we get our
bearings regarding the anatomy of this area. Please take time to review and as always I
hope you reach out if you have any questions or clarifications. Thanks for your time and attention