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Knee anatomy on MRI.

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The posterolateral corner could be the greatest show on earth or on turf.

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We've got a couple of bunny ears looking

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from the back on a diagram, showing two sets of bunny ears.

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The bunny ear to your left consists

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of the arcuate, the lateral limb and the medial limb.

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The bunny ear to your right consists

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of the fibular collateral ligament and the biceps femoris.

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We're going to ignore those right now.

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The medial limb is very obliquely oriented

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and covers the myotendinous junction and sometimes covers, or sits posterior

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and superficial too. When you look from the back,

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the popliteus tendon. The more lateral limb has a more vertical

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upright consistency or configuration as we'll see in a minute.

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Here's a magnified view from the front.

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Pay no attention to the red popliteus tendon, or the insertion of the biceps

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femoris, or the fibular collateral ligament.

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Let's focus on the vertical limb

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of the arcuate and the arc-shaped medial limb, or curvilinear medial limb

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of the arcuate, that come to sit on the fibular styloid.

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So when you fracture this and get a little

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flake sign, often the arcuate has come off with that piece of bone.

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Now, the arcuate and fabellofibular ligaments are inversely related.

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The arcuate forms a V or Y shape as you've already seen.

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And you've already seen how it attaches to the apex of the fibula.

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But it's right next to another critical structure, the popliteofibular ligament.

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Kind of the order of things, as we've discussed in the past, is LCL goes,

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then the popliteofibular ligament goes, then the popliteus goes.

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So here's a map of the insertions,

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the key structures on the fibular head, here is the arcuate.

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Behind it is the fabellofibular ligament.

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This will be big, this will be small. This will be big,

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that'll be small. And right next to it,

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right next to it labeled in pink, is going to be the popliteofibular ligament.

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So these are very important loci

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to isolate, both in the axial, coronal, and sagittal projections.

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Now, the lateral or upright limb

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runs superiorly along the joint capsule on its way to the lateral femoral condyle.

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The medial, or what we call the arcuate

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limb, as you've seen, courses in a superior medial direction.

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So it goes from inferolateral, sweeping superior medially across the back,

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over the popliteus tendon, and it merges with the posterior capsule and the OPL,

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the oblique popliteal ligament,

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which you may remember, is part of the posteromedial corner.

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So the posteromedial corner sends some of its regimen over to the lateral side

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and has some bearing on the lateral side stability and function.

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So here's another look at the posterior knee.

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And this time I've taken the arcuate off

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so you can actually see the popliteus tendon, the myotendinous unit.

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Now that you know, at the myotendinous unit and muscle, the arcuate medial limb is

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going to course, you could draw it in on your own because you're smart.

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So here we go.

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Right from the fibular tip, right over the myotendinous unit of

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the popliteus, goes the medial limb of the arcuate.

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More vertical is the upright limb,

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fusing with the capsule, would be the lateral limb of the arcuate.

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Let's keep going.

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So the lateral limb is more commonly seen in the absence of a fabella.

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If there is a fabella, chances are you won't have the lateral limb.

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The converse is true for the medial limb.

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The fabellofibular ligament appears as a focal thickening of the distal edge

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of the capsular arm of the short head of the biceps femoris muscle.

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So it's not that frequently identified.

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But what is important is, don't confuse the fabella with the cyamella.

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The cyamella is a piece of bone that is located inside the popliteus tendon.

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So those should be pretty easy to distinguish for you.

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The fabellofibular ligament originates

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from the lateral margin of the fabella, or in the absence of a fabella,

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from the posterior aspect of the supracondylar process of the femur,

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and it has really less importance when there's no fabella present.

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Let's take a look at the arcuate on MRI.