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<v ->Now, when we look at osteonecrosis

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involving the femoral head

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we see a regional distribution with five zones

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that I've color coded in this particular slide.

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The first zone shown in white is the articular cartilage.

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And as you know, it derives most of its nutrition

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from the synovial fluid.

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So even with severe osteonecrosis of the femoral head,

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the joint space may be preserved until later on

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with significant collapse

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and secondary mechanical osteoarthrosis.

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The dark blue circle is showing you

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the necrotic region of the femoral head.

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And as you know, within it, we may see a fracture.

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That's the red dot that we can see here.

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I show it, as well, within this drawing that I have made.

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Now as we go to the periphery of the necrotic zone,

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we come across what is known as the reactive interface.

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That reactive interface has two components.

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There is granulation tissue.

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I'm showing that by the green dot

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which is radiolucent on the radiograph.

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It's this area in my drawing.

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And then peripheral to that,

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shown by the gray dot,

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is the sclerotic bone.

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So the granulation tissue, okay,

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and the sclerotic bone together make up

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what is known as the reactive interface.

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It's not surprising then

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that one of the earliest findings of osteonecrosis

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related to change is in the reactive interface.

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And what was described was a double line sign

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in cases of osteonecrosis.

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When you looked at the T1 weighted image

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and looked at the granulation tissue and sclerotic bone,

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it was mainly of low signal as you can see here.

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But when you looked at the fluid sensitive sequence

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the granulation tissue on the inside became brighter,

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and the area of low signal became narrower.

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And it was that particular finding

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that was known as the double line sign.

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Here on the T1, this is what it would look like.

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Here on T2,

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We see a line of high signal and a line of low signal.

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That was known as the double line sign.

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Now I use that, it's valuable, but it's not always visible,

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but it's something that you ought to think about.

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Now, I can tell you that reactive interface can be single

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or multiple, and it can be small or large.

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And when it gets to be multiple and large,

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that reactive interface

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on the T1 weighted images will present

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over portions of the femoral neck.

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And therefore, in some cases

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it is so prominent as in this example

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that it simulates a fracture involving the femoral neck.

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So beware of that diagnostic pitfall.

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These are just reactive interfaces,

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sometimes with a double line sign

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that are projected over the femoral neck.

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Now let's talk about the crescent sign.

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This is a fracture that occurs beneath,

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and I want to emphasize this,

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beneath or within the subchondral bone plate.

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Now remember that.

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It can occur beneath the subchondral bone plate

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or within the subchondral bone plate

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or in both places

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and then can extend into the cartilage.

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Here, a nice specimen from Fiona Bonar

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that shows you this fracture

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that is located within the subchondral bone,

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extending up toward the subchondral bone plate.

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All right, now, this becomes important

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in terms of differential from an insufficiency fracture

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as I will show you in a moment.

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Here are beautiful images showing you,

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in fact, the crescent sign, the fracture,

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with delamination now of the subchondral bone plate

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and penetration here of the deep portion

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of the articular cartilage.

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So again, the crescent sign that we see radiographically

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is a fracture through necrotic bone

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that may be present within the subchondral bone,

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the subchondral bone plate,

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and may extend into the articular cartilage.

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We can see that beautifully here

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in the specimen radiographs.

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When, in fact, that fracture extends up

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here in a comminuted fashion

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into and then breaking that subchondral bone plate.

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It is at that time

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that collapse of the subchondral bone plate

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and deformity of the articular cartilage may occur.