Knee Meniskus

7/29/2019 Knee Meniskus

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Knee - Meniscal pathologyby David Rubin and Robin Smithuis

Radiology department of the Washington University School of Medicine, St. Louis, USA and the Rijnland hospital in Leiderdorp, the Netherlands

Normal Meniscal Anatomy

Medial meniscus: The posterior horn is always larger than the

anterior horn.

Medial meniscus

Both horns are triangular in shape and have very sharppoints.

The posterior horn is always larger than the anterior horn

(figure).

If this is not the case than the shape is abnormal, which can

be a sign of a meniscal tear or a partial meniscectomy.

LEFT: normal medial meniscal root immediately anterior to theposterior cruciate ligament.

RIGHT: missing posterior root due to meniscal root tear.

The posterior root is immediately anterior to the posterior

cruciate ligament.

If it is missing on the sagittal images, then there is a meniscal

root tear (figure).The anterior horn has an insertion on the tibia and a secondportion that travels from medial to lateral to connect to the

anterior horn of the lateral meniscus ( intermeniscal or

transverse ligament).

Lateral meniscus. Both horns are about the same size.

Lateral meniscus

On sagittal images the posterior horn is higher in position

than the anterior horn.

Both horns are about the same size.

Lateral meniscus: posterior horn and posterior meniscal root.

The lateral meniscus posteriorly comes up higher over thetibial spine to insert near the posterior cruciate ligament.

This upward position of the posterior horn may be the reason

for the higher signal intensity of the posterior horn in all

planes due to magic angle effect.

Meniscal tears


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Criteria for tears

The two most important criteria for meniscal tears are an

abnormal shape of the meniscus and high signal intensityon

PD-images unequivocally contacting the surface .

High signal intensity not unequivocally contacting surface. Smallblack line on inferior margin of the meniscus. At arthroscopy the

meniscus was normal.

It is a misunderstanding that menisci should be

homogeneously low in signal intensity on proton-density

images.

The meniscus does not have to be black.

Only when the high signal unequivocally reaches the surface

of the meniscus you can make the diagnosis of a tear.

If there is doubt whether the high signal touches the surface,

look at all the adjacent images and if there still is doubt than

do not diagnose a tear.

If you have a questionmark in your head, say meniscus is

normal. (figure)

Basic shapes: Longitudinal, Horizontal and Radial.

Nomenclature of Meniscal TearsShapes. There are 3 basic shapes of meniscal tears:

longitudinal, horizontal and radial .

Complex tears are a combination of these basic shapes.

Bucket handle, Horizontal Flap tear and Parrot beak.

Displaced Tears

Bucket-handle tear = displaced longitudinal tear.

Flap tear = displaced horizontal tear.

Parrot beak = displaced radial tear.

Longitudinal, horizontal and radial tears

Longitudinal tears

Longitudinal tears parallel the long axis of the meniscus

dividing the meniscus in an inner and outer part.

So the distance between the tear and the outer margin of themeniscus is always the same (figure).

The tear never touches the inner margin.
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Three sagittal images of a longitudinal tear

Longitudinal tears follow the collagen bundles that parallel the

contour of the meniscus.

If a longitudinal tear has other components (horizontal or

radial) than it is a complex tear violating the collagen

bundles.

This requires a higher energy trauma.

LEFT: abnormal shape of posterior horn. a piece is missing. RIGHT:

displaced fragment in the intercondylar fossa.

Longitudinal tear (2)

Bucket handle tear

is a displaced longitudinal

LEFT: meniscus is abnormal in shape and there is a displaced

fragment. RIGHT: Three structures in intercondylar fossa: post

cruciate lig (1), ant cruciate lig (2) and displaced fragment (3).

On coronal images bucket handle tears are easier to

recognize.

Normally there are only two structures in the intercondylar

fossa: the anterior and posterior cruciate ligament.

Any other structure in the intercondylar fossa is abnormal

and a displaced meniscal fragment is the most likely

possibility.

Flipped meniscus: posterior horn is missing because it is flipped over

and located on top of the anterior horn.

Longitudinal tear (3)

Flipped meniscus is a form of bucket handle tear.

There is a capsular detachment or peripheral tear of the

meniscus, usually the posterior horn.

The posterior horn flippes over onto the anterior horn.
http://www.radiologyassistant.nl/images/42dc029d15860Bucket-cor2.jpghttp://www.radiologyassistant.nl/images/42dc029d15860Bucket-cor2.jpghttp://www.radiologyassistant.nl/images/42dc029d15860Bucket-cor2.jpghttp://www.radiologyassistant.nl/images/42dc029d15860Bucket-cor2.jpg


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Horizontal tear with a meniscal cyst

Horizontal tears

Horizontal tears divide the meniscus in a top and bottom part

(pitta bread).

If horizontal tears go all the way from the apex to the outer

margin of the meniscus they may result in the formation of a

meniscal cyst.

The synovial fluid runs through the horizontal tear and

accumulates periferally to the meniscus.

The connection with the joint space is often lost, so they will

not fill with contrast on MR-arthrography.

The synovial fluid is absorbed and is replaced by a gelatinous

substance.

There are 3 criteria for the diagnosis of a meniscal cyst:

1. Horizontal tear.

2. Fluid accumulation bright on T2.

3. Flat lining against the outside margin of the meniscus.

The diagnosis of a meniscal cyst is important to the surgeon

because it takes one operation on the outside of the knee toremove the cyst and another operation on the inside for the

meniscus.

Radial tears

Radial tears are perpendicular to the long axis of the

meniscus.

They violate the collagen bundles that parallel the long axis of

the meniscus.

These are high energy tears. They start at the inner margin

and go either partial or all the way through the meniscusdividing the meniscus into a front and a back piece.

Radial tears are difficult to recognize. You have to combine

the findings on sagittal and coronal images to make the

diagnosis.

LEFT: triangle missing the tip.

RIGHT: disrupted bow tie.

The following combination of findings is diagnostic:

In one plane: triangle missing the tip and in the other plane:a disrupted bow tie.

Disrupted bow tie indicating a small radial tear.

Small radial tears are difficult to diagnose.

Sometimes the only sign is a disrupted bow tie.


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LEFT: Absent or empty meniscus on sagittal image.

RIGHT: Axial image shows complete radial tear leading to a defect in

the meniscus.

If you image a complete radial tear directly along the length

of the tear you will see an absent or empty meniscus.

These complete radial tears open up and give the impression

that there is a part missing.

However you will not find a displaced meniscal fragment. It is

simply separation of the meniscal parts.

Meniscal root tear: on sagittal images there is an absent or emptymeniscus-sign adjacent to the posteior cruciate ligament where the

meniscal root should be. On coronal images a meniscal root tear is

confirmed.

Meniscal root tear

A meniscal root tear is a radial tear located at the meniscal

root.

Normally when you image the posterior cruciate ligament on

sagittal images you should see a considerable posterior horn

of the meniscus on that image or the image adjacent to it.If this is not the case it is an absent or empty meniscus-sign

indicating a radial tear.

Post-operative Menisci

Post-operative Menisci are harder to evaluate because the

two most important criteria, i.e. abnormal signal andabnormal shape, do not work any more.

Abnormal signal is not anymore a reliable sign of a tear,

because if there has been a suture repair, this will heal with

scar tissue, which also has high signal on PD-images (figure).

However if there is also high signal on T2-weighted imagesthan you can make the diagnosis of a tear as as this is the

result of synovial fluid leaking into a tear.This however is an uncommon finding.

Abnormal shape can be the result of partial meniscectomy.

So you need to know what procedure was performed during

arthroscopy.

Only when you can compare with prior postoperative images,

you can say if an abnormal shape is a new finding indicativeof a new tear.

Sometimes differation between normal post-op findings and

a re-tear is not possible on conventional MR-images.In these cases MR-arthrography with 40cc diluted

Gadolineum helps to make the distinction because evensmall amounts of Gadolineum that leak into a tear are readily

visible on fat saturated T1 images.


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PD and T2W images. Prior partial meniscectomy and suture repair.

At arthroscopy no tear.

Post-operative Meniscus 1

The case on the left shows a meniscus with an abnormal

shape aswell as abnormal signal touching the surface on PD

but not on T2W-images.

This patient had a prior partial meniscectomy and a suture

repair.

On the basis of these imaging findings it's impossible to tell if

this is a tear or normal postoperative finding.

This patient had another operation for ACL reconstruction.

They also looked at the meniscus and the meniscus was

found to be normal i.e. no tear.

LEFT: Old MR exam with tear. Patient had a suture repair.

RIGHT: On new exam there is a new tear (yellow arrow). Notpossible to tell if old tear has healed.


This patient had a suture repair for meniscal tear. There was

a new injury.

On the new MR impossible to determine if the old tear had

healed.

However a new tear is seen, so this case ia easy.

MR-arthrogram: In the new tear the signal is as bright as in thesynovial fluid (yellow arrows). In the healed tear the signal is not as

bright.

On a MR-arthrogram there was very high signal intensity in

the new tear comparable with the synovial fluid, but only

moderate signal intensity at the healed old tear.

So comparison with the old films was diagnostic for the new

tear, while the arthrogram showed that the old tear has

healed.

PD and MR-arthrogram after suture repair for meniscal tear: healed

tear.


This patient also had a suture repair for meniscal tear.

After a new injury the PD-images show high signal

unequivocally reaching the surface of the meniscus (seen on

the original films, but not clearly seen on the compressed

image on the left.

On this image it is not possible to tell if the tear has healed.

So a MR-arthrogram was performed which showed that the

tear has healed.

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