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Knee anatomy.

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Postromedial corner on MRI, axial T2 fast spin echo

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without fat suppression.

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A coronal water-weighted image like an AP projection

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and a sagittal like a lateral projection, also water-weighted.

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Let's start out with the biggest,

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easiest structure, semimembranosus muscle and tendon.

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The tendon comes down and divides into two major components.

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An anterior component called the pars reflexa and a more

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distally coursing component called the direct component.

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Now, there is a distal component to the semimembranosus,

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but we often don't see it,

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although I am going to point it out for you

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in the coronal projection.

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The semimembranosus also has an attachment or reflection

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going up to this structure right here,

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which is the capsule.

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The capsule is merged with an indistinguishable

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from the OPL, the Oblique Popliteal Ligament.

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The meniscus and its attachments are stabilizers

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of the posteromedial corner.

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So the capsule of meniscus are included in that discussion.

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And this very slit-like structure in the upper corner of

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the meniscus is part of the posterior oblique ligament

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of the knee. So again, the semimembranosus,

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which is a dynamic stabilizer, in contraction,

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the semimembranosus flexes and internally rotates the tibia.

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It acts as a restraint to valgus motion

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when the knee is in extension.

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So it supports the knee in a valgus force or when

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there's a blow to the lateral aspect of the knee.

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It also acts as a restraint in external rotation

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when the knee is in flexion, not extension.

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So now let's look at the coronal projection.

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And a very common mistake is to dial into the

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semitendinosus and assign at the wrong anatomy.

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So the best way to handle that is to go up a little bit

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higher and find the semitendinosus muscle, and then find

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the semitendinosis tendon. So here's the muscle, there's the tendon.

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So semitendinosus tendon, which means that's the gracilis.

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And there is your sartorius.

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So now you've got your pes anserinus.

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How are we going to find our POL?

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We go to the middle layer of the MCL,

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also known as the superficial portion

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of the deep MCL, layer number two,

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previously called the tibial collateral ligament,

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mid coronal from front to back.

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And then we follow it back.

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Let's follow it back.

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And as it thins out along the meniscal attachments,

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it has now become the POL.

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And we can't separate it as a discrete structure with this

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resolution, with this particular pulsing sequence,

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but we will be able to.

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And then it'll continue around back,

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blending with the capsule and the OPL.

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Now, we have a number of linear structures here.

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We've already talked about the semitendinosus.

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The gracilis.

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What's this just deep to the gracilis.

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Well, this is the distal arm of the semimembranosus,

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which I promised you earlier.

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You're not going to see all the arms in every case.

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There are actually five of them.

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The two most important ones are the anterior going

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forward, and the big direct fat one coming down.

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But there's a capsular and inferior and a distal,

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and this one is the distal.

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So don't confuse it with the pes anserinus.

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Now, let's go to the axial for a moment and we see our fat,

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somewhat juicy, globular, tibial collateral ligament,

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as it was formerly known, the middle layer of the MCL.

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And if you go back immediately behind it,

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are the obliquely coursing fibers of the POL.

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That is the POL. Scroll it.

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There it is.

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So the POL will then swing around because it's oblique.

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It swings around obliquely and then merges

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with the posterior capsule and OPL.

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So let's go down a little bit towards the tibia.

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It's a little black down here because it's T2 weighted.

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Little easier to see right there.

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There it is.

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There's the posterior capsule,

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and that is formed by the capsular reflection of the

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POL, the capsular reflection of the semimembranosus,

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the OPL, and it goes all the way across.

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So the posteromedial corner has an important bearing on

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what happens at the posterolateral corner because

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this is going to swing all the way across and the two are connected.