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The posterior cruciate on MRI.

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Let's show you some pathology.

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The mechanism of posterior cruciate

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injury is familiar to most of you.

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If you think about the aviator injury

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or the dashboard injury,

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where the front of the tibia takes a blow

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and drives the tibia backwards,

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that's all it takes,

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especially in a position of slight flexion,

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which you're in when you're sitting in an

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airplane or you're sitting in a car.

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So your knee may be jammed up against

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the dashboard or the steering wheel,

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but it's usually a blow to the upper tibia or

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the bottom of the patella that does it.

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Now, another very common mechanism is a fall,

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and when the patient falls,

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their knee is flexed and their toe,

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which I don't have displayed here, is pointed.

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So the first structure striking the ground is

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usually going to be the tibial tubercle

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right here, where my ring finger is.

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So as that tubercle strikes the ground in flexion,

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the tibia is driven backwards.

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And as it's driven backwards, the PCL ruptures.

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You're going to see that most PCL tears

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are pretty innocuous. Initially,

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the patient may return to their activity

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within a matter of minutes,

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feeling a little bit unsteady or having some

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discomfort or soreness in the knee.

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And then a few minutes later,

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as they try to run or cut,

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they realize something just isn't right.

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And then, surprisingly, if it's an isolated PCL,

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two or three weeks later,

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they're back out doing their activity.

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On the other hand,

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if a PCL is associated with another injury, say,

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to the posteromedial or lateral corner,

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you have a much different situation to deal with.

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So once you see a PCL,

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you better be darn sure that the other ligaments,

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especially the corners, are normal.

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Now,

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I like to break PCL tears into two basic types,

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the ones that involve bone and the ones that

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don't. The ones that don't involve bone,

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as long as they're isolated,

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are frequently treated conservatively.

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Whereas if they involve bone,

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usually at the level of the tibia,

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in the deep posterior cruciate notch

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below the tibial plateau,

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those may require an external approach from

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the back with a screw.

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And we've got one.

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LEt's take a look at it.

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Very simple case. We'll start out with.

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There's your pcl. The pcl has two bundles,

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an anterolateral and a postural medial.

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And unlike the ACL,

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where the bundles tend to be more parallel,

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they tend to twist a little more,

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almost like a braid. So when you tear the PCL,

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it often does not distract.

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If you see distraction,

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it's usually a more violent injury.

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Or you have to be concerned about a fracture.

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If you see a blood fluid level with a pcl,

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you have to be concerned about a fracture.

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Now, this one's obvious.

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A big chunk of bone has come off.

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The pcl remains ATtached to that chunk.

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Now, if the chunk is in close opposition,

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this will be a judgment call

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by the orthopedic surgeon.

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Then you can get granulation tissue and bridging.

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But if there's separation or

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diastasis of the fragment,

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then you're going to have to come in from the

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back and screw that piece of bone back on.

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So here is a coronal T1 weighted image.

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This is a pd in the midDle,

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so you don't see the edema as clear as you

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would on a standard T1 It's a long tr,

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short te sequence, not very good for bone,

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but good for morphology.

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I frankly don't use it very much

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without fat suppression.

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But the T1 shows the fracture defect

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and the associated edema with it,

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making your diagnosis of suspected pcl avulsion

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fracture injury all the more strong.

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We'll scroll a little bit.

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We'll look inside the pcl.

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We see the two bundles.

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There is the lateral bundle,

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there's the medial bundle,

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and there's always a little grayest

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signal intensity in the PCL.

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LEt's look at it on the water WEIGhted image.

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And when we look inside the ligament,

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nothing focal, nothing bright,

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nothing razor edge sharp, nothing of the ilk of,

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say, fluid, which is seen right here.

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In other words, this gray signal intensity,

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that is just some interdigitation of sheath

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material and the separation of the two bundles

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of the PCL, that is not a tear.

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So we're going to show you in SOme OTHER vignettes

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what a high grade, functional,

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full thickness interstitial tear really looks

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like. But you do have to look inside.

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It's like intel.

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Look inside the PCL when you are in a cross

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sectional position or perpendicular to the axis

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of the PCL. Let's take a look at our axial.

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Our axial for the ligament itself isn't very

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helpful because the ligament itself isn't torn.

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It's the fracture that shows up in

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the axial projection right there.

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So now let's put up the sagittals

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together and let's scroll them.

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And now you see just how easy it is to spot that

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fracture with the PCL still

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attached to the fracture.

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Now look at the signal of the PCL every time the

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PCL makes an angle turn gray, another angle black,

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another angle gray, another slight angle black,

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and then gray and then black.

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This banding phenomenon,

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as the pcl waves is very typical

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of collagenous structures.

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This should not be confused with a tear.

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This is normal. If you have a true T2 spin echo,

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it goes away.

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And this is called the magic angle effect,

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also known as the 55 degree artifact or

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the anisotropic artifact. You know,

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it's a fake because it occurs at sites of

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waviness. It's bandlike or horizontally oriented.

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And if you have a true T2 spin echo,

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it will absolutely go away.

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But when you have curved structures like the

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posterior crucial ligament, which is curved,

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the more curved they are, the more wavy they are,

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the more magic angle you're going to get.

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You're going to see some examples of interstitial

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injuries of the PCl that are far bright,

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far more etched,

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far more well defined as you look inside

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the ligament. Let's move on, shall we?