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It's a 23-year-old with pain after a fall.

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Something familiar, something classic,

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so you ought to be comfortable with this case,

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with one minor exception.

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So, let's start out by scrolling the middle.

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And we are looking at a proton density,

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high quality fat suppression image.

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Although, the TE of 20,

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a little low for my taste.

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I like my TEs to be about 30, 35,

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or 40 for maximal contrast resolution of water

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to background.

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But that being said,

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we do have excellent fat suppression and ispy.

0:39

A pivot shift injury.

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I spy a posterolateral tibial impaction injury,

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an antrolateral femoral terminal sulcus injury,

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and a posteromedial tibial injury.

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So how does that happen?

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It happens with the classic pivot shift mechanism

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of injury. So the tibia is internally rotated,

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the femur is externally rotated.

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The patient may receive a blow to the outside

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of the knee, producing a vagus impact,

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and the tibia glides forward.

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As it glides forward, the femur slams down.

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So if you look from the side,

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the femoral terminal sulcus

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slams down on the tibia,

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and the back of the tibia takes the impact from

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the femur as it glides back on the tibia.

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So that impact can cause a fracture.

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It can tear the back of the meniscus,

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it can tear the meniscocapsular junction.

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And if it's really severe enough,

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the patient may even experience a knee

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dislocation. Usually when that happens,

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the downward impact is not as great.

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There's a little more distraction.

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And in those patients,

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the meniscai are actually normal.

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So if you've got some serious collateral ligament

1:59

and cruciate ligament injuries,

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in the absence of any meniscal pathology,

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but the back of the capsules and corners are

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abnormal. Should worry about a knee dislocation.

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But these are the typical pivot shift mechanism,

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bony injuries.

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And we also have typical meniscal injuries because

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we are taking the tibia and we are just driving

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it forward, the femur backwards,

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and the capsule just can't take the distraction,

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so the capsule bleeds.

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That's blood in the capsule.

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So there's a meniscocapsular sprain.

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That happens with almost every pivot shift injury.

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There's even a small vertical tear next to

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the sprain. I'll blow it up for you,

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in case you don't believe me.

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There it is.

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Now let's go over to the lateral side.

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And by the way,

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this is the insertion inflection point of the pol,

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or posterior oblique ligament,

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in which there is now a small area

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of blood occupying its position.

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Let's go over to the lateral side and take a look

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at the lateral meniscus. Oh, it's fine.

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But there is swelling of the meniscocapsular

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reflection. Granted,

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there's a little fracture here.

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The hiatus, the popliteus hiatus is normal.

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Upper fascicles normal. Fascicles still intact.

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Popliteofibular ligament still intact. Okay,

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our corners are all right.

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We said we have a pivot shift.

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We should have an acl tear.

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Oh, we do.

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Now,

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some of you might hedge around the fact that you

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have some fibers here, and you might say, well,

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high grade or intermediate to high grade.

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No, it's not intermediate to high grade.

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It's a full thickness tear.

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What are these fibers?

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They're shredded fibers that are waving in the

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breeze. They're attached to absolutely,

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positively nothing.

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It's a full thickness tear.

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Let's look at the sagittal T2 weighted image.

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There's no acl going back here.

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These are

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free fibers that have been completely detached.

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This is bone. That's bone.

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There is nothing here that attaches

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this acl to that femur.

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Now, that's not all.

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We have an oblique sagittal T2 on the far right.

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Let's have a look at it.

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Got a few really interesting findings here.

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One.

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What is this?

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Let's make it a little lighter.

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I think many of you, including myself,

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would say triangular shape. Must be a meniscus.

4:53

You'd be wrong. Me too.

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Bucket handle tear, maybe. Displaced meniscus.

5:01

You'd be wrong. Me too.

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Let's go back and follow it again.

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It goes right back to the acl.

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That is the Acl folded anteriorly on itself.

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The ACl is doing this,

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and it just happens in that projection,

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in that cross section to make a triangular

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appearance. Ooh, that's tricky.

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This is the kind of ACl that restricts the

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range of motion and can produce locking.

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Sometimes we call this a pseudocyclops lesion.

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You can get pseudocyclops lesions

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from other structures, too,

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from thickening of the ligamentum mucosum.

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In fact,

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there is some thickening of the

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ligamentum mucosum right here,

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and that also might contribute to decreased

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range of motion in this patient.

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The ligamentum mucosum,

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not well developed in everybody,

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is a fascia membrane that sits at the base of the

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acl, and there is a portion of it, again seen,

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it is often contiguous or continuous with the

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infrapatellar plica. So there is the wavy,

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injured ligamentum mucosum producing another

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pseudocyclops like phenomenon

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at the base of our torn ACl.

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Now,

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a couple of other take home points about looking

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at acls. The sagittal is prince, princess, king,

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queen.

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But it's best suited for mid substance ruptures.

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If you've got a tough case and you're trying

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to tease out an area of the ACL,

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my recommendation is pull down your

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coronal and with your coronal,

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use it for distal insertions in the tibial

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spines and intertibial spinous notch.

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Also try and follow the contour,

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which should look like a water slide.

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Should look a little bit like this.

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Should be a very consistent waterslide from top

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to bottom. And if your water slide is wavy,

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this one is goes this way.

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Then it's over here. Then it's over here.

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They're not connected to each other.

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Let's blow it up to prove it.

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That is not connected to that.

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The hip bone is not connected to

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the knee bone in this case.

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So our coronal projection helps us overall with

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the ACL, but particularly valuable distally.

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What about proximally? Axial.

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Let's pull down a couple of axials.

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Let's focus on this one right here in the center.

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It's an axial simple T2 I use the axial when

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I have isolated femoral wall avulsions.

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So very high tears, proximal tears,

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what I call origin tears.

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So here we are distally near the tibial spines.

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We follow the ACl up.

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Now,

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that's not what a proximal ACl would look like.

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Approximal ACl should be a ligament.

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It should be black. It should be linear.

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It should have a shape like this.

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Jet black.

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That's not jet black. That's gray.

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That's a ball.

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There's nothing there.

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So the femoral attachment of the ACL is off.

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Axial for the proximal end for the origin,

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coronal for the distal end for the attachment,

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sagittal as the end. All to be,

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all to assess most of the mid portion of the ACL.

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Let's move on, shall we?