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Dr. P here.

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3 00:00:01,630 --> 00:00:04,859 This is a 57-year-old woman with

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post-exercise pain in the foot.

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We've got an axial or short axis, uh, heavily

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fat-suppressed image on the left, proton density.

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In the middle, the T1 fat-weighted image.

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And on the far right, the standard simple

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T2 spin echo without fat suppression.

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And we've got a round, smooth, hyper-

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intense, homogeneous mass sitting

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next to the extensor tendon mechanism.

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Our differential diagnosis for lesions

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like this, especially as they relate to

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the extremities, either the hands or the

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feet, would include things like ganglion,

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pseudocyst, which, by the way, is a pseudocyst.

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It's not lined by epithelium.

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It is lined by fibrous tissue and is a

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microscopic dissection out of joints.

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It can be attached to tendons or

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come from tendons or the sheaths.

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And this one is very close to the

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extensor digitorum and peroneus tertius

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complex extensor tendon mechanism.

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So that would be a consideration.

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An epidermoid would have similar

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characteristics like this.

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Usually they're not round, but they can be.

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They are often inclusion

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cysts rather than congenital.

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It's pretty hard though to get an inclusion

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in the top of your foot without knowing

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that somebody stuck the top of your foot.

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So the history goes against that.

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A tenosynovial cyst.

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That likes to run along the tube

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of the sheath of the tendon.

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So, let's scroll it and

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see just how tubular it is.

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Not very, right?

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It goes away, and then it comes

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back, and then it goes away again.

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So we'll look at the sagittal to

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see how tubular it is, but not very.

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Myxomatous mass.

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That would still play into the

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differential diagnosis here,

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although not a very common location

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for a myxoid tumor, and we may have some other

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signs that help us go away from that diagnosis.

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Another very bright lesion, but not quite

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this homogeneous, is the glomus tumor.

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It's not a good place for a glomus tumor.

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It likes to be a bit more distal, you

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know, near the nail beds and the fingers.

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And certainly more distal in

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the foot and toes as well.

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Then we get into cystic schwannoma.

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Those can look exactly like this

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in terms of signal intensity.

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But one problem is it's not where the nerves live.

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I mean, the nerves are going

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to live here more centrally.

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There's the anterior tibial nerve right there.

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So, we've been able to separate the

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main nerve in the extensor compartment

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from this lesion.

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So, that doesn't necessarily exclude

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it, but it makes it highly unlikely.

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Something like a varix, wrong signal

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intensity, you know, I like to

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see a little bit of layered blood.

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Plus, I like to see it connected

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to some type of venous tube.

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And I don't see that.

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I see it intimate with the extensor complex.

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So right now, the leading candidate in the

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axial projection would be a ganglion pseudocyst

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perhaps emanating from the extensor mechanism.

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However, if you're observing very

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carefully, and you are the best

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observers in the world, what is that?

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You know, there's a funny

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looking tail to this lesion.

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Now, whenever you hear the word tail, the so-

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called tail sign, or tetherball sign, you've

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got to think about an attachment to something.

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And that's typical of a ganglion.

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It's attached to a capsule with a tail.

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It's attached to a tendon with a tail.

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So that further supports the

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diagnosis of ganglion pseudocyst.

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You can also have a tail that's attached

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to a vein in an eccentric varix.

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You could have a tail that's attached to

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a nerve structure if it's a neural tumor.

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So that doesn't help us all that much unless

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we can figure out where the tail goes.

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And I think we're going to

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need some other projections

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to figure that out.

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So let's do that together.

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Let's pull down the sagittal proton density,

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the sagittal T1 fat-weighted image, and the

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sagittal thin-section 3D gradient echo image.

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Now here is our mass, and here is our tail.

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And where is our tail going?

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Our tail is going to the

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infralateral subtalar space.

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There's no doubt about it.

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And what is that linear structure?

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That is the lateral retinaculum, the most

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lateral aspect of the subtalar stabilizers.

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The other ones being the cervical

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ligament and most medial, the

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talocalcaneal interosseous ligament.

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This is also known by the term stem

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ligament or frondiform ligament.

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So we refer to these as lateral stem

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or frondiform ligament, bursal cysts.

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And yes, there is a bursa that lives

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in the subtalar space, and there it is.

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And there it is pooching out.

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Now, sometimes these lesions may

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wrap around, like this one does.

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They come out, and they sort of

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wrap around the dorsum of the foot.

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This one isn't going too dorsally; it's going

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sort of straight out, but they may curve back

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around the top and even go up and over the ankle,

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and that can make them particularly confusing.

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This is a very important lesion because

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it's extremely common, especially in

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athletic individuals that are users

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or overusers of the lower extremity.

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The stem or frondiform ligament bursal

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cyst arising from the subtalar space.

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That's the diagnosis here.

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Dr. P out.