Ultrasound of the Knee - Intraarticular Knee - SD

Introduction to Ultrasound of the Knee

Greetings and welcome to the ultrasound of the knee.

This is a modular approach on ultrasonography of a joint
wherein we start showing first the anatomy
and how to routinely explore the knee.

And after that we go and talk about pathology
and this part will combine some of the anatomy
and pathology of the knee,
but in particular we're going
to look at the intraarticular portion of the knee.

Why Use Ultrasound for Intraarticular Knee

You might be wondering why,
you might be wondering why we're gonna use ultrasound on the
intraarticular segments of the knee.

Try to remember
that ultrasound is a great screening tool
and while you're exploring the more reference
or more requested extra articular structures of the knee,
you and I know that you'll be passing through some
of the intraarticular components of this joint.

One of the approaches
to looking at a joint is looking at the regions.

In the case of the knee you've got the four regions.

Starting with the anterior, we could go
through the medial part of the knee, then the lateral,
and finally the posterior area.

Suprapatellar Region and Intraarticular Lesions

Let's look at probably the window to the knee
or in the patella bursa is where changes occur
that you could more or less look at the knee superficially,
but what is happening to the inside part
of the knee is also very important.

So effusion, crystal depositions, disease,
infection, and maybe even loose bodies,
are intraarticular lesions,
but they may express themselves in the suprapatella region.

The suprapatella region is most famous of course for detecting
and localizing effusion.

Here in a cone down view of Netter's illustration,
you'll notice the fibrillar pattern on the distal quadriceps
tendon inserting on the patella and behind it, right
after you cross the suprapatella fat pad,
you see fluid in the suprapatella recess in front
of the peripheral fat pad.

In gray scale imaging on an extended field of view,
just concentrating on the superior part of the knee,
you'll see the bone acoustic landmark of the base
of the patella, the fibrillar pattern
of the distal quadriceps tendon, the triangular hyperechoic,
suprapatella fat pad.

And between this triangular suprapatella fat pad
and the fat pad in front of the femur, also known
as the prefemoral fat pad, you see a small amount of fluid.

So this is the area that usually may
and should give you some physiologic fluid,
but once you have localized topographically where
to localize exactly where some fluid may pool, then
that's where you apply your transducer
in the long axis view.

Again, this time with a little bit of zoom effect,
you see the bone acoustic landmark of the base
of the patella, the fibrillar pattern
of the distal quadriceps tendon,
the triangular hyperechoic suprapatella fat pad,
and the more L articulate shape prefemoral fat pad.

In
between the two, you can now detect
and appreciate the marked separation
between these two fat pads for
what is essentially an uncomplicated
intraarticular and joint fluid.

Suprapatellar Effusion

When you look at suprapatella effusion in the long axis
view, again identify your bone acoustic landmark
and once you've done so, now you begin to appreciate
that you can have an extended field of view going both
superiorly towards the patient's hip
or inferiorly towards the patient's leg and ankle.

But it is the suprapatella recess that we like
to concentrate on this example in
between the distal quadriceps tendon
and the cortex of the femur.

You'll notice that instead of just plain fluid,
we now have some rind forming and an
otherwise clearly anechoic fluid.

This rind represents chronic synovial proliferation
and thickening of these lining in a patient
with osteoarthrosis.

And so aside from characterizing
the changes within the fluid of these patients,
you also begin to appreciate the fact that you can detect
synovial changes with ultrasound.

Try to remember that unless you use contrast in both CT
and MRI, you will not be able to see synovial proliferation.

Once in a while a case that will come across your workshop
or workstation is going to be something like this
wherein now instead of just simply having
anechoic fluid behind the distal quadriceps tendon,
you have something that is more intermediate echogenicity
and in this case, again, some intraluminal debris
in an individual that would actually might even
represent some subacute blood.

We do not see the lipohemarthrosis of an acute fracture,
but here you could see the hemarthrosis in this individual
and so don't forget
that aside from characterizing the intraluminal portion
of the distended suprapatella bursa also try to assess
what's happening to the synovium,
which usually should be imperceptible.

In this case we could see that it is indeed imperceptible,
so it's more an effusion hemarthrosis rather than a
peripheral proliferation around the suprapatella bursa.

Don't forget,
but remember now that it's best
to look at anything in long axis and in short axis.

Recall that we always warn you that to look
around the entire volume of the suprapatellar area
and here in this otherwise unremarkably distended bursa
with near complete anechoic fluid, all
of a sudden you have something growing out from the premoral
area of the suprapatella recess.

A closer look will show you that there are some frond-like
or arborization that arises from this mass.

It is entirely a benign mass known as lipoma arborescens.

Because of the chronicity of the effusion
and its individual there was formed this benign mass
because of the chronic hypertrophy
of the fat itself rather than the synovium.

Often when you look at a swollen knee
and you get a history of inflammatory arthritis,
not only will this intraluminal pannus jump out at you
as you could see in the long axis view,
but if you care to look around, you'll also notice what used
to be imperceptible.

Synovium now shows you a moderate thickening
of the synovial lining compatible with pannus
and synovitis in this individual with rheumatoid arthritis,
not only in long axis but also in short axis.

Loose Bodies

Loose bodies on the other hand, will appear
as higher level echoes and depending on their density
and concentration you can have a
posterior acoustic shadowing.

And this individuals of course, orthogonal planes,
not only in long axis
and short axis will confirm the configuration
and volumetric dimensions of this loose body.

But remember that ultrasound is a dynamic type of modality
and so changing the patient's position will show you
that this loose body can translate over time and distance.

Therefore, its true nature as a loose body.

It is not uncommon that within a distended suprapatella bursa,
you might see this innumerable punctate
or at least oval like high level echoes would hardly any
posterior acoustic shadowing.

In gray scale imaging already you have the idea
that this is going to be some form of rice bodies.

These are rice bodies secondary
to synovial osteochondromatosis.

This finding is not specific to this disease
because it could be seen in rheumatoid arthritis
and in tuberculosis of the joint.

Osteochondral Defect and Loose Body

In the frontal radiograph of the knee.
It's very difficult to appreciate any abnormality
immediately on the lateral radiograph of the knee.

You'll appreciate that there are two
extra cortical densities.

The first one is this oval density in the region
of the fundus of the suprapatella recess.

And the second one is the one in the posterior recess
representing a normal variant fabella.

It is the one that is in front of the knee
that is the abnormality.

Once you've seen this, one
of the differential diagnosis is going
to be a loose body upon seeing this.

Therefore, the job of the imager is to see
where the donor is.

And as you trawl along the knee, you'll notice
that the medial femoral epicondyle is intact throughout its
entire convexity.

The lateral femoral epicondyle, along
with the more posterior fibula for confirmation,
will show you that there is a divot on its femoral
condyle retrospectively.

When you jump back to the frontal view,
you'll see the same radiolucent defect on the
lateral femoral condyle.

This osteochondral defect now
for confirmation might necessitate
because of its intraarticular nature
and MRI here in the fluid sensitive T2 sequence
with fat saturation around the fundus
of the suprapatella recess,
you'll see this osteochondral defect,
but it's actually the loose body of the osteochondral defect.

You notice that there are at least by laminar nature
of this loose body here,
you notice the hyaline cartilage itself
and then underneath it, the subchondral plate.

This is the slice selection
to be exactly over the loose body in the suprapatella recess.

Let's go to another slice selection when using the same fluid
sensitive sequence and here confirming
what we saw on the radiograph is this large divot on the
lateral femoral condyle.

Once you've detected this on MRI, ultrasound can
play a role in this because now you do have an acoustic window,
you can approach the knee posteriorly
and again, confirm the same changes
that you have appreciated on MRI.

Here on the left image,
you see the morphologic sequence representing the divot,
and then you've already seen the fluid sensitive
sequence showing you the same defect
and on ultrasound again with a posterior acoustic window.

You now appreciate the fact that while on MRI
you could not completely characterize the cartilage cap over
this defect on ultrasound, you begin to appreciate that
the hypoechoic cartilage cap
over the subchondral condyle defect is now trying to regenerate.

Not only that, but you also see the fact
that there may be a residual fissure across the hypoechoic
regenerating cartilage.

Again, you could always take a look at it in a single
tomographic slice on ultrasound
or employ the fact that you can move around, try
to see the rest of the cartilage on the more superior image.

Now you begin to appreciate that the undulation
of the cartilage cap says
that this hyaline structure is relatively abnormal.

Underneath that the subchondral deficiency
also represents some amorphic fissures telling us that
not only was there a crack across the cartilage,
but that there also is some nitrogen gas in
the subchondral plate.

It is essential, however, to say
that we do see the cartilage cap
and that it is indeed trying to regenerate.

So this may at least avoid an early surgical intervention.

Menisci Visualization and Pathology

MRI is the modality of choice to look at the menisci.

However, when you're looking at the knee
with ultrasound, you can use it as a screening tool.

Now remember, the craniocaudal orientation
of the transducer across the femur
and tibia cuts across the interarticular line.

The joint line is
where you'll see this hyperechoic homogenous triangle
in this case of the medial meniscus.

Note that it has a base of a triangle
and the apex of a triangle.

The base of the triangle represents the external portion
of the meniscus while the apex is the free edge.

So now you can appreciate the fact that yes,
we can visualize the menisci with ultrasound on gray scale
and exactly what Netter showed it in his illustration.

Remember that the definition in a short axis view
of the meniscus was a homogenous triangle.

Here you see that there are hypoechoic fissures cutting right
across the entire swath of the meniscus in this case.

Therefore, you could see clearly that
what was intrasubstance hypoechoic defect surfaces on the
femoral level, clearly suggesting
that this is a meniscal tear
or at least marked intrasubstance
changes of this structure.

It's not uncommon though if you had too many
triangles, that's too many menisci, then
that this is a fragmented tear.

But it is always with MRI that we're going
to confirm what's going on.

But every hyperechoic fragment
that you see on the ultrasound image on your left is
represented by the same fragments
of the MRI coronal T2 image on your right hand side.

Depending on the slice that you choose, is where you might
or might not see a meniscal tear here case from Dr.
Tony Wong from Canada.

You'll notice that anteriorly
the meniscus is relatively intact,
but as we go a little bit more posteriorly, you begin
to appreciate the loss of the hyperechoic triangle
that you're expecting to see in a meniscus.

Now of course, just turning your transducer around
and from the short axis views.

Now on your right hand side you see the long
axis view of a meniscus.

You begin to appreciate
that the anterior portion is relatively intact as seen
by this red line
and that when I go to the posterior portion
with a yellow line, you got to see
what we saw earlier on the short axis view,
which is the lower left hand image for confirmation here.

You can appreciate that the triangle
of the meniscus on this fluid
sensitive sequence coronal MRI image shows you
that there is fluid going into the tear
both on coronal and on sagittal.

Again, one more time, that the black triangle of the fibro-
cartilage of the meniscus is interrupted
by this hyperintense line.

It is with ultrasound
that this lesion probably is better diagnosed than with MRI.

That is because this case from Dr.
Vu from Ontario Canada shows us that like an ice cream cone,
you have a hypoechoic collection of fluid over
representing the scoop of the ice cream over the triangle
of the wafer cone.

And notice that with dynamic imaging we're able to create
and confirm that there is indeed a tear
of this meniscus creating this cyst opposing and above it.

So remember that the advantage of ultrasound is being able
to use stress maneuvers here applying various stress
we can shear and help create
and accentuate the tear of the meniscal defect.

Here you could see a varus stress applied
by one of our fellows, Dr.
Sung Moon Lee, using his elbow as control note
where his transducer is for a varus valgus stress.

And here now you could create
a stress maneuver without the aid of an assistant.

Another defect that could be probably
appreciated better on ultrasound than MRI is meniscocapsular
separation because the fluid over this area can
mask out any changes of the meniscus.

But with ultrasound you could see that instead
of an ice cream cone, you have whole pancake
across the entire posteromedial post medial,
post medial part of the knee
where you see this fluid collection coming from all the way
from the femur to the tibia.

So for meniscocapsular separation, it is a larger amount
of fluid often associated with a tear, not only
of the capsule from the meniscus,
but also through the intrasubstance portion of the knee.

On this fluid sensitive coronal image cone down,
you'll notice that the black triangle
of the meniscus shows a
complete tear across the medial meniscus
and sometimes you may need to employ different type
of orientations in order
to accentuate the pancake appearance of the meniscal cyst.

And again, for the more general,
generally accepted sagittal oblique view, you begin
to appreciate the interruption
of the signal abnormality across the posterior meniscus.

Posterior Knee and Extraarticular Structures

Let's finish by looking at the extra articular knee
by cruising through the rhomboid fossa
in the posterior knee, there'll be several lesions
that you'll be looking for.

This would be the Baker cyst, aneurysms, venous thrombosis,
hematomas, and posterior cruciate ligament tears.

Of course, in this region you may have some masses
as ganglion cyst.

The rhomboid fossa, as you could see,
contains the neurovascular bundles,
but also the posterior portion of the knee joint.

The normal bursa on the medial aspect
of the knee is the semimembranosus gastrocnemius bursa.

So with the transducer across the medial aspect of the knee,
try to identify on ultrasound three landmarks.

The three landmarks are going to be the hyperechoic convexity
of the posterior condyle of the femur.

The second one is going to be the fibrillar pattern
of a short axis view of the semimembranosus.

And then slightly laterally you'll see the striated
appearance of a transverse
or short axis view of the medial head of the gastrocnemius.

In between this triad exists the normal semimembranosus-
gastrocnemius bursa on short axis on long axis.

Most of us would like to park ourself
between the pennate structure of the medial head
of the gastrocnemius
and the fibrillar pattern of the semimembranosus in long axis
as it inserts on the posterior tibial condyle.

Here you see also another landmark,
which is the posterior femoral condyle.

And in between the two bones of the femur
and tibia is the hyperechoic triangle
of the posterior horn of the meniscus.

Baker's Cyst

A Baker cyst when seen in the long axis view could exist
with one limb in between the gastrocnemius
and semimembranosus
or sometimes all the way posterior to the medial head
of the gastrocnemius.

Here you see a schematic gross drawing of the position
of the Baker cyst
and it's always a medial structure or lesion.

And again, when the semimembranosus gastrocnemius
bursa becomes distended, it is indeed now known
as a popliteal Baker cyst or a popliteal cyst.

So therefore the Baker cyst can only exist
on the medial aspect.

Now, when it distends, it has this horseshoe pattern
or boomerang shape as you can see on MRI
and a reverse image of the Baker cyst.

As we try to mimic the position of the MRI here again
between the triad of the cortex of the femur,
the tendon of the semimembranosus
and the muscle of the gastrocnemius,
you've got this horseshoe lesion of a Baker cyst.

So in short axis,
therefore what is supposed
to be a small non distended bursa can balloon out
to the epicenter Baker cyst on the medial aspect of the knee.

And in the long axis,
coming from a short axis view, note that it is now in front
of the gastrocnemius and behind the semimembranosus,
but it's the contour of the cul de sac of the Baker cyst.

That's important because note that it's got a rounded
contour or shape to it.

The minute the round cul-de-sac turns
into an amorphic type of fluid collection
or elongates, then you have a leaking
or ruptured Baker cyst.

Sometimes it ruptures all the way down to the ankle,
as in this case in a patient with rheumatoid arthritis,
you've got pannus collecting well within this Baker cyst
for a dissecting type of lesion in this patient.

Posterior Cruciate Ligament

We're going to finish the intraarticular knee
by talking about the posterior cruciate ligament
on the downsloping part of the posterior tibia,
the intercondylar notch as known as the intercondylar area.

You'll notice the fibrillar
or partly anisotropic fibrillar pattern
of the posterior cruciate ligament.

Any change in the contour size
and echogenicity of the posterior cruciate ligament should
raise the suspicion for a tear case in point.

In the same individual showing the contralateral knee,
you'll notice that the contour has changed from something
like a triangle to a convex upward contour.

Also look at the difference in size.

It has now become almost twice the size of
what the normal asymptomatic posterior cruciate ligament
was for confirmation.

It is always helpful to look for MRI here instead
of having a black tendon
or ligament representing normal here.

Instead of a black ligament representing the posterior
cruciate ligament, you see
that it's got increased intrasubstance signal abnormalities
on a fluid sensitive sequence.

You could see comparing it
to intact anterior cruciate ligament that the interruption
of the posterior cruciate ligament shows this increased
signal abnormality representing the hematoma.

Conclusion

I'm gonna skip this and go to conclusion.

So finally,
for the ultrasonography of the knee, remember
that you are going to be crossing some
of the intraarticular structures
and when you do so, then most of this will be
because of abnormal fluid, especially collection
of fluid in the suprapatella recess.

And the most obvious indication
and the most common indication you'll be using this is
to confirm the presence of a Baker cyst,
which can only exist on the medial portion of the knee.

Last but not least,
although maybe we still continue to use MRI
for meniscal lesions, it is so much easier.

It is easier to detect meniscal cyst
and meniscocapsular.

Last but not least,
it is the most common indication to
examine the knee.

The most common indication that is requested to
examine the knee is because of a Baker cyst.

Remember that it can only exist on the medial
posterior portion of the knee.

Also, remember that MRI is what we're going to be using
for the knee, but let me conclude by telling you
that it is so much easier to visualize a meniscal cyst
or meniscocapsular separation with ultrasound
because of its more
or less extra articular position in the knee.

Thank you so much for your attention.