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Posterolateral corner injury

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in a 16-year-old male.

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Soccer player, question mark.

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Patient has posterior knee pain.

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We're going to stay with our same approach.

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We're going right to the axial.

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This time, we have more than one axial.

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We have a beautiful 3D gradient echo,

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fat suppressed axial,

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which demonstrates the internal

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architecture of the cartilage.

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What you're seeing here are the struts of the

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chondral matrix, going from deep to superficial,

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producing this radial component.

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So, you're actually seeing the radial orientation

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of the proteoglycan milieu in the cartilage.

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This little, thin,

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dark line here is called the lamina splendens.

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And then you can see the cartilage is a little darker,

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deeper, and it's a little brighter,

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superficial in an organized way from medial to lateral.

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So, that's just gorgeous.

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That's not what the case is about.

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We also have a T2 weighted image.

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Let's scroll and look at

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our Medial Patellofemoral Ligament,

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back to the adductor tubercle.

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It's normal.

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Lateral retinaculum,

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normal.

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Shape of the patellar,

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Patellar architecture, normal.

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Trochlear groove, normal.

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So, there's no dysplasia.

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On the medial side,

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we've got our MCL.

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We don't see our POL very well.

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Here's the superficial layer, the MCL.

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Here's the tibial collateral ligament.

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Here's the OPL coming around.

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And here is our semimembranosus tendon.

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On the opposite side,

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let's have a look on the more

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water weighted image.

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Not a lot can be gleaned posterolaterally.

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So, let's turn our attention now to the sagittal.

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We're going to take this sagittal

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proton density fat suppression,

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sagittal T1.

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This is a thick linear scar at the growth plate.

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That is not a fracture.

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It might have been a fracture at some point,

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but it is a scar that is bland.

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There's absolutely, positively no edema around it.

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But there is edema in the neighborhood.

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There's Mr. Rogers neighborhood, right there.

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There is a little bit of edema in the

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posterolateral tibia.

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Completely unrelated to this.

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And then, we also have a little edema and swelling

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in the region of the popliteus muscle

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and in the tib fib articulation.

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So let's take a look at the posterolateral corner,

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because we were asked to do so in the history.

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That wasn't something I made up.

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That was the history that was given.

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There should be a body

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and a nice, smooth,

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black tail that goes right here.

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And when you lose that tail,

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it will curl up in a little squiggly ball.

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And that's known as the mermaid sign.

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Now look at this T1.

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Let's take away our lines and scroll around.

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We should have a fairly black structure

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that has a tail right to the fibular head.

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We don't have that.

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Now, here's the squiggle.

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Let me draw over the squiggle for you.

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Here's the mermaid sign.

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There's the mermaid's body.

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And now let's draw the tail.

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There's the tail.

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Now, I'll take it away,

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and I think you'll see it.

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There it is.

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The mermaid sign of popliteofibular ligament tear

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in a posterolateral corner injury.

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The meniscopopliteal attachments of the lateral

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meniscus are still there.

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The arcuate,

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which is behind the popliteus tendon, is fine.

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It's still there.

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Let's check out the coronal projection

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and evaluate the fibular collateral ligament,

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which is still there.

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Now, when you have complex posterolateral injuries,

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especially if you lose the fibular

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collateral ligament with it.

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And if you have a cruciate injury,

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you can end up with,

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especially with the PCl,

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PLRI, posterolateral recurrent instability.

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What happens is

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the lateral aspect of the knee opens.

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And then, the tibia will rotate posteriorly

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as it opens.

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And every time you walk, you'll get this.

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And you'll get this rotation

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of the tibia rotating externally.

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So when that happens,

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the patient will grab their leg and

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try and push it in with each step.

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It looks like the patient has a neurologic injury,

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but they don't.

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They have intermittent instability of the knee

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from this combination of posterolateral corner,

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perhaps PCL,

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and perhaps fibular collateral ligament injury.

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So, it's a very important finding to recognize

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when present with other injuries.

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Otherwise, it goes unnoticed,

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and the patient ends up with this

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chronic instability syndrome.

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So, this is an example of

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posterolateral corner injury,

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conclusion with popliteofibular ligament tear

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and the mermaid sign.