0:00

This is a 28-year-old with a fall.

0:02

He described a twisting element to his fall,

0:06

but in front of the tibia is an area of swelling,

0:09

the typical pretibial abrasion, site of impaction,

0:15

where his knee struck the ground,

0:17

probably with an element of flexion.

0:19

So he probably fell like this,

0:21

and struck the front of the tibia

0:24

as he went down.

0:25

This drives the tibia posteriorly.

0:29

And actually,

0:30

if you look at this image and the one next to it,

0:33

the back of the tibia looks like it's way far

0:36

posterior compared to the posterior femoral line.

0:39

This is, in fact,

0:40

a clinical sign called the sag sign

0:43

of a PCL injury or PCL deficiency.

0:47

We take our model and we lay a patient down

0:50

with severe PCL deficiency,

0:52

the tibial will actually passively sag back.

0:55

And you can push it back inordinately.

0:58

So, you can get the tibial sag sign on MR.

1:00

You can also get it clinically.

1:02

We've got a T1 fat weighted image on the left,

1:06

a water weighted image in the middle,

1:07

and a T2 weighted image on the far right,

1:10

also allegedly water weighted.

1:12

This is the most sensitive sequence.

1:14

So, here is our detection sequence.

1:17

On the left,

1:18

we use the T1 weighted image for morphology

1:20

and especially to trace the hypointense cortex at

1:25

the tibial locus of insertion on the tibia,

1:30

which lies, by the way, below the tibial plateau.

1:33

So, you can't see this area when the ACL is intact,

1:37

arthroscopically.

1:37

It's invisible to the surgeon,

1:40

and this is an MR diagnosis.

1:43

Now, one question that you always have to answer

1:46

when you see signal in the PCL is,

1:48

are you volume averaging,

1:50

because the PCL is surrounded by a sheath

1:53

that is common to the PCL and the ACL.

1:55

So you could injure the ACL, swell the sheath,

1:58

and volume average that swollen

2:01

sheath with the PCL,

2:02

or are you actually seeing an abnormality

2:07

in the ligament itself?

2:08

There are two bundles to the PCL,

2:11

but they tend to be more twisted.

2:13

So, interstitial but functional full thickness tears

2:16

of the PCL are common.

2:19

Interstitial but functional full thickness tears

2:22

of the ACL, much less common.

2:25

Distraction and separation of ACL tears, common.

2:29

Distraction and separation of PCL tears, uncommon.

2:33

Now, you could get distraction

2:35

and separation of a fracture,

2:36

which would change the therapy completely.

2:39

So, if you avulse a piece of cortex

2:41

or a piece of bone and it's distracted,

2:44

that changes the therapy dramatically.

2:45

And the tip off there is a hemarthrosis

2:48

and losing this cortical outline.

2:51

So, what's the T2 weighted image for?

2:53

Because the T2 weighted image certainly dulls

2:57

the signal intensity within the ligament.

2:59

Does that mean it's not a tear?

3:00

Absolutely not.

3:02

Because what's the signal of

3:03

acute blood on a T2?

3:05

Dark.

3:06

What's the signal of hemosiderin?

3:09

Dark.

3:10

What's the signal of fibrous tissue?

3:12

Dark.

3:13

What's the signal of a normal tendon or ligament?

3:17

Dark.

3:18

So, there are many ways and many tissue types

3:21

that you can get fooled on

3:23

on a T2 weighted image

3:25

where you actually can miss tears,

3:27

both acutely with acute blood on a T2

3:30

and chronically with hemosiderin scar

3:33

and fibrous tissue on a T2.

3:34

So, do not rely on the T2

3:37

to exclude a PCL tear.

3:39

What should you rely on?

3:40

You should look at the

3:42

posterior cruciate ligament perpendicularly.

3:45

Now, ideally,

3:46

I'd like to go in this direction,

3:48

but most of us have orthogonal images.

3:51

So, go to your orthogonal images

3:53

and inspect the PCL.

3:56

You can see we've got two bundles,

3:59

a medial bundle and a lateral bundle.

4:01

Let's follow them.

4:02

I want to see a well defined,

4:05

high etched signal intensity inside my PCL.

4:09

And you bet, I've got it.

4:10

Right there.

4:11

I'm going to blow it up for you.

4:12

There's my PCL.

4:14

There's all that signal inside the PCL.

4:17

It's like intel inside.

4:19

It's inside, not around,

4:21

although there is swelling around the PCL,

4:23

and we continue to follow it forward,

4:25

and eventually, it disappears.

4:27

So, how would I describe this abnormality?

4:30

As a high grade, functional,

4:33

full thickness interstitial PCL tear

4:37

without evidence of fracture

4:39

and with posterior tibial translation

4:41

consistent with PCL deficiency.

4:44

One last caveat.

4:45

I've got a T2 axial.

4:46

Why not show it?

4:48

Look at how deficient the T2 weighted image is

4:52

in really highlighting the PCL tear.

4:55

Let's put it up against

4:56

our coronal fat suppression image.

4:59

Look inside the PCL.

5:00

Easy.

5:01

Look inside the PCL,

5:03

it's gray, it's dull, it's amorphous,

5:07

it's ill defined.

5:08

Yes, it's abnormal.

5:10

You'd like to see a nice black ball,

5:12

a nice black pendant, and you don't.

5:14

But it's not as easy for the novice

5:17

or beginner to see

5:18

as this water weighted fat suppressed image.

5:22

So again, I caution you,

5:25

don't rely on the T2 weighted image

5:27

to exclude cruciate ligament tears.