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Dr. P here.

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3 00:00:01,480 --> 00:00:04,510 This is a 12-year-old girl with ankle pain,

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and we incidentally discovered this lesion.

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Let's scroll through the axial projection, and

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the lesion is bright on the water-weighted image.

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On your right, the fat-suppressed proton density.

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In the middle, the T2 without fat suppression,

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and on the left, the T1 spin echo.

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The lesion has a sharp zone of transition.

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It is internal, very homogeneous, although

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this diagnosis, this lesion, is often

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associated with very thick septations.

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It's eccentric, although this lesion

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tends to be more central, so the

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eccentricity is a bit atypical.

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And then you can see it's very close

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intimacy with the cortex of the calcaneus.

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It may scallop it just a hair, but

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it certainly doesn't go through it.

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So now I think it's time to put up the

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sagittal, or so-called lateral, projection.

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And we've got a T1 on the left, we've

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got a proton density on the right.

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Once again, you can see the intimacy,

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but no violation of the cortex.

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The sharp zone of transition

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is demonstrated again.

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And this lesion, like one I've shown you

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previously, will show up as a lucency on X-ray.

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So often the differential diagnosis is between

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lipoma, which causes pain by virtue of infarction,

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and the unicameral bone cyst, or bone cyst of

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the calcaneus, which may cause pain because it's

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thinking about fracturing or it has fractured.

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Now we know that unicameral bone cysts or bone

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cysts occur more frequently in males, about three

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to one, usually in the first two decades of life.

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And this patient is in the first two decades.

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It likes the central part of the bone,

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especially when it's in the humerus,

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you know, it's right smack dab in the

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middle, it's not that often eccentric.

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And it may yield a pathologic fracture.

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Now, when you have that pathologic

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fracture, let's draw it, here's our cyst.

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And you get a fracture.

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And that fracture, fragment,

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layers down at the bottom.

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Maybe we'll make it a different color.

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So our fragment is down here.

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And this is known as the fallen fragment sign,

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for the residents that are listening out there.

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There's also something called the trap door

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sign, where you have a piece of bone that has

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fractured off, but it hangs on and now it's

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floating in the sea of fluid, kind of flopping

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back and forth, and it looks a little bit

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like an opening and closing trapdoor, making

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a little arrow there just to illustrate it.

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If you see these, the Hounsfield units

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are usually gonna run about 15 or 20,

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so the final dispensation or diagnosis

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in this case is unicameral bone cyst.

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Dr. P out.