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<v ->Okay, everybody we're back.

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We're ready to go.

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And we have so many knowledgeable people

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that are listening here that we've learned a few things

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in the break and I just would tell you

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some of the comments we've gotten.

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Segmentation of the sesamoid apparently is random,

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so we can't use the size of the two fragments

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that help us in that regard.

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Fractures, apparently statistically

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are in the middle of the sesamoid.

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Maybe that'll help, I'm not sure.

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And that owing to the vascularity

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of the sesamoid osteonecrosis would affect

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more often the proximal portion.

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Now that's interesting, 'cause I will spend time not today

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but we'll be talking about osteonecrosis later on,

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in this course and I'll be talking quite a bit

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about vascularity to bones such as scaphoid.

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But let's move on now.

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We're gonna move on and talk briefly

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about a joint effusion.

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One of the things that I would suggest

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that you should never put in your report

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and I'll explain why is that there is no joint effusion.

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And the reason I say that is orthopedic surgeons called me

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and showed me some bright signal

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and say, "You're wrong, there is fluid in the joint."

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So my expression is

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that there is a physiological amount of joint fluid

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but I don't use the term that there is no joint effusion.

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There is physiologic amount of fluids in all of the joints.

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There are some where you don't see any bright signal

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on the fluid sensitive

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but usually you can see at least a drop.

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And there's some very classic articles that have looked at,

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how one a radiologist can diagnose joint effusions

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based simply on conventional radiography.

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So I wanna go back to that for a moment

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because it has some significance with regard to MR.

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There was a kind of a fat pad sign

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that was described many many years ago by Ferris Hall

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in which he utilized the lateral radiograph of the knee

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and he found this kind of obliquely oriented area

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not quite as lucent as the areas around it.

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And he said that this

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in fact was the collapse suprapatellar recess

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or pouch of the knee joint.

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And if you measured the width of that transversely

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shown here by the arrows that typically the width of that,

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between the two fat pads would be

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less than or equal to five millimeters.

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Anything beyond that would indicate pathologic amount

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of joint fluid or a joint effusion.

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So just to give you an idea of what that looks like on MR,

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here's some sections that we have done

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and here is that suprapatellar recess, the black arrow.

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This is what it looks like and we can see here the fat pad

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in front of that recess

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and the prefemoral fat behind it, okay?

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And this is a normal width.

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So that is a normal amount,

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physiologic amount a joint fluid.

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Now the important thing to realize is

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if you look at an image like that in the transverse plane

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this is what it looks like.

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You have this fat anteriorly the suprapatellar fat.

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Number 2 is the prefemoral.

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That is not plicae and it is often misdiagnosed as a plicae.

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That is a collapsed suprapatellar recess of the knee joint.

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If there is plicae it's in that gray area

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but you gotta distant the area in order to find the plicae

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that is lying within it.

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So don't miss state that for a plicae.

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Now here, we can see what a joint effusion would look like

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and it becomes easier to pick these up,

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when indeed there's intracapsular fat in the joint.

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So here we measure that distance

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with conventional radiography,

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it's greater than five or 10 millimeters, it's abnormal

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and you can see it here on the MR as that fluid.

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One interesting thing and I don't have proof on this case,

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if you have this fluid that ends sharply like this,

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one of the differential diagnostic possibilities is

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that you have a complete suprapatellar plicae.

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I'll be talking more about that a little bit later.

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Here's another example, showing you a joint effusion.

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Here we've injected some gel within the joint.

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This is a plicae we'll be talking about,

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but I just wanted to show you how the fat is separated

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by the fluid within the joint, all right?

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Here on the transverse image,

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the joint effusion separating two areas of fat.

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So pretty easy to pick up joint effusions in the knee.

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Now I just wanna return to something Rodrigo

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had talked about very nicely, 'cause I learned something new

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about this probably about 10 years ago.

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I'm embarrassed to say it's been so long in my career

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but I too grew up in the age of conventional radiography.

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I too learned about the positive fat pad sign

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that we would see on the radiographs

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and of course a positive fat pad sign was an elevation

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of the anterior fat that looked like a cell

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and any visualization of the posterior fat.

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So what I assume because of that is that,

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there were two fat pads one located anteriorly

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and one located posteriorly.

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The importance of finding that,

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of course was usually not always there was a fracture.

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Well, there are three fat pads

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and I just wanted you to realize that,

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there two anterior fat pads and one posterior fat pad

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and all three of them sit in depressions

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on the distal surface of the humorous.

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So anteriorly, we can find a fat pad here,

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which sits in a coronoid fossa related to olecranon

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and there's a radial fossa.

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And so there are two fat pads in that area.

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Posterior the largest excavation, of course,

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is that olecranon fossa so there's a posterior fat pad.

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So although it seems like there's only two

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there's actually three.

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You're looking at an example here

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of juvenile idiopathic arthritis.

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We used to call it juvenile rheumatoid arthritis

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but here we can see a sagittal section

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on the (mumles) aspect

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the joint showing you the coronoid fat pad

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and olecranon fat pad that are elevated.

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One of things we looked at early on and here are images

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from an article we did well, it's 27, 28 years ago,

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but it just seems like yesterday.

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We were kinda interested in knowing,

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could we tell a bland diffusion

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from a more aggressive effusion

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by looking at something that is known

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as the intra interval of the knee.

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It is the deep part of Hoffa's fat pad.

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And we thought that if we looked at it

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that if it looked smooth and maybe displaced

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but otherwise intact, it would indicate bland effusion.

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But if it looked invaded as I...

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So here more often was an aggressive effusion.

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It works but it's not a great rule, all right?

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But we wrote it up, we said it was a great rule.

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It's a good rule, it's not a great rule

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for separating bland from somewhat aggressive effusions.