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37-year-old male,

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left knee MRI complex instability.

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Let's focus on the good old fashioned,

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fat weighted, T1 sagittal,

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lateral view, spin echo image.

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Immediately,

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I'm sure you all notice the aberrant

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position of the tibia.

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It is translated forward with the patient.

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Not even weight bearing.

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The patient is just lying on their back,

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and the femur is flopping backwards.

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This is known as the passive anterior

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tibial translation sign,

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telling us that the acl is no more.

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We can't find it. It is transected.

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But there's so much more.

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There are the typical fractures of a pivot shift

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injury involving the femoral terminal sulcus.

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This one's slightly depressed.

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There is a fracture of the posterolateral tibia.

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And there is also a fracture of the

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postural medial tibia, a big one,

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at a very important locus,

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the locus where the semimembranosus,

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one of the most important structures of the

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posteromedial corner, inserts. Now,

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when the semimembrnosis blows,

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it can either blow as an avulsion fracture

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or it can blow as a tendon tear.

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In my experience,

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it's more common to see it with an avulsion

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fracture like we have here,

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and this one is somewhat commonuted.

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So let's turn our attention now to

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our water weighted sagittal.

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Let's go right to that meniscus.

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The inferior aspect of the meniscus and its

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attachment to the capsule is blunted

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at the site of the fracture.

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It's also blunted in the suproposte apex,

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where the tibial arm of the pol reflects.

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So that is injured. The distal aspect of the pol.

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What else makes up the posteromedial corner?

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Well, the menisco capsule attachments do.

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Those are torn.

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We have a large globular area of blood and some

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fibrillated fibers. The meniscus itself does.

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Intrinsically, the meniscus itself is okay.

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The semimembranosus, it is not okay.

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It has five components, or five arms to it.

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A capsuler, an anterior,

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an inferior popliteal arm, a distal arm,

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and a direct arm. The direct is the most dominant,

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followed by the anterior arm,

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which is the second most dominant.

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And you can read about these and hear about

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them in some of our other vignettes.

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So here is a little bit of the semimembrnosis

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anterior reflected arm remaining.

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The rest of it is completely off and separated.

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So we have a semimembranosus,

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nearly complete avulsion of all the

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arms of the semimembrnosis,

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along with a meniscocapsular ligament injury,

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or meniscocapsular reflection tear.

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So two important components.

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The posteromedial corner, gone.

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We've also said the distal pol is gone.

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The posterior oblique ligament of the knee.

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How about the OPL,

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which is basically fused with the capsule?

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The capsule should be a nice straight structure,

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and it's not.

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It's actually ill defined and gray.

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It should look more like this.

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So the inferior capsule,

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which is fused with the OPL,

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the oblique papillateal ligament, it's torn.

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So now we have three components,

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perhaps four if you include the pol of the

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posteromedial corner that are gone.

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The only one that's really spared is the intrinsic

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tissue of the medial meniscus.

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We're not done yet, though.

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Let's focus back on the pol for a minute.

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We said the distal pol reflection right there.

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That gray structure is injured.

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But what about the rest of the pol?

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Well, let's go to this sagittal projection,

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which is right along the barrel,

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tangent to the tibial collateral ligament.

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Let's draw,

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the tibial collateral ligament has a middle layer.

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Well,

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the tibial collateral ligament is the middle

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layer. Sorry. It's layer number two of the MCl.

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Here's layer one right there.

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This is layer two, then the capsule.

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And meniscocapsular attachments

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make up layer number three.

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So I want to focus again on layer number two,

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the middle layer. There it is vertically oriented.

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I'm going to draw over it one more time in red.

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There's your tibial collateral ligament,

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middle layer of the MCl.

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Now I want to draw the pol.

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I'm going to make my line a little bit thinner,

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and I'm going to go for something like pink.

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Now there are upper fibers above the joint.

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They're very hard to see,

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but you actually can see them.

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They're a little wispy. They go like that.

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Those are the capsuler fibers of the pol.

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But these fibers,

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you can see these oblique ones right here,

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which form the tibial arm, the main arm,

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that goes towards that postro superior apex of the

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meniscus, the proximal portion. Looks good.

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Let's take it away so you can see it.

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There it is right there.

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That is the tibial arm of the pol.

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That's the main arm,

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also known as the central arm.

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Then there is a more distal component.

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You can kind of see it right here.

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If you really squint hard,

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kind of courses the other way, courses like this.

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And that would be the distal superficial arm,

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which is less important to us.

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So we see that the anterior component of the pol,

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while swollen, is intact.

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It's only the distal aspect

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of the central tibial arm,

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which we've identified at the postro superior

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corner of the meniscus. Let's go back to it again.

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Blunted posturo superior corner of the meniscus

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where the distal pol attaches the central tibial

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arm. That's torn. Semimembranosus,

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torn meniscocapsular reflection, torn capsule.

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Oblique popliteal ligament, torn.

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Four out of five,

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sparing only the meniscus intrinsic tissue itself.

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That's a nasty one. And by the way,

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the posterilateral corner was also torn.