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Knee anatomy and MRI. The dreaded and oft-forgotten posteromedial corner.

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Let's start out with a diagram.

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We've got the medial patellofemoral ligament.

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But remember that the quadriceps sends

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a few fibers over to the medial side, anteriorly.

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And then, not displayed here at this time, are some superficial structures

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that include the medial retinaculum, what's called the deep fascia.

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And this forms layer one of the MCL anteriorly.

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Anteriorly, there's also contributions from the capsule, which would be layer three.

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So you'd have layer three, layer two, layer one.

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Now, as you go back, this layer two, the MPFL, or Medial Patellofemoral Ligament,

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will merge imperceptibly with this sort of thickened structure

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right here, the tibial collateral ligament, which doesn't have a lot

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of length from front to back and has a lot of verticality.

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In other words, it runs into the screen or up and down.

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But as the tibial collateral ligament comes backwards,

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not shown on this particular slice, it's going to merge with another layer,

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a deeper layer right here, which would be layer number three,

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the POL, or Posterior Oblique Ligament of the knee.

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Let's take another slice.

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This one, not at the level of the patella,

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and we can see the medial patellofemoral ligament

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blending with the tibial collateral ligament.

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These are components of layer two.

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The tibial collateral ligament is also known as the deep component

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of the superficial MCL, a rather confusing name for it.

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And then as it comes back, now we see some additional tissue posteriorly

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that represents the POL, or Posterior Oblique Ligament of the knee,

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and that gets us back around the back

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of the knee where we have other supporting structures that support the posteromedial corner,

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including the POL, which is our introduction to this area.

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So let's talk a little bit about this area, the posteromedial corner, because

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failure to address this area may result in a failed surgical situation.

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Let's drill a little deeper with this view of the semimembranosus corner or

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posteromedial corner of the knee, which is more or less a subset of the MCL.

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But it's in the back,

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so the abnormalities that accompany it are a little bit different.

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Let's look at a side view of the knee for a minute.

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Here's the femur, and here's the tibia with my rudimentary drawing skills.

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And I'm going to make for you the vertical portion of the MCL known as the tibial

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collateral ligament, also known as the deep portion

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of the superficial MCL, previously known as the tibial collateral ligament.

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And then posterior to that, I'll maybe use blue, with oblique fibers is the POL.

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So these two blend together and we have

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that drawn here except in the opposite direction.

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So this would be anterior, this would be posterior.

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And you're actually looking from the back of the knee.

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So this is a posterior view from back to front.

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So here is your tibial collateral ligament or deep portion of superficial MCL,

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analogous to this orange area, just flipped over.

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And here's your POL.

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And we have it more or less looking straight, but we've already said

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that the POL, more or less, has sort of triangular shape fibers.

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So we might have drawn it a little bit

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differently here, but I think you get the picture.

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So tibial collateral ligament or

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superficial MCL, as you move posteriorly, becomes POL.

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Now, loss of the semimembranosus corner, with any of these others, may result

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in a condition known as Anteromedial Rotational Instability or AMRI.

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And this leads to excessive anterior translation.

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In other words, the tibia will move anteriorly,

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and there'll be excessive rotation this way.

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In other words, in a counterclockwise orientation or excessive anterior, sorry,

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excessive external rotation of the medial tibial plateau relative to the femur.

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And this will also be accompanied by a valgus form of joint space opening.

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So this joint will open as well.

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In our next vignette, I'm going to take you

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through the semimembranosus corner and its individual expansions.