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<v ->Okay, we've covered the cartilage.

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We're now gonna move on to the subchondral bone

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and explain what happens there in an injury.

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In order to understand the subchondral bone,

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you have to know something about Wolff's law.

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I hope most of you in the audience

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have heard that term "Wolff's law".

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It's named through, because of the investigations

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of Julius Wolff, who was a German anatomist and surgeon.

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In a series of articles,

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he came up with a philosophy that has been translated

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in a variety of ways, but I like what I read in "Wikipedia"

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as the best translation, bone in a healthy person

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or in an animal will adapt to the loads

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under which it is placed.

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Now, for those of you who have read any of the articles

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that myself or the other bone radiologists

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have produced at UCSD, you will know that we have utilized

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a lot of cadaveric work in our studies,

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and we like to compare imaging and pathologic findings.

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So we use a lot of cadavers,

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and I can tell you that every time I section a cadaver,

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I see Wolff's law at work.

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Let me show you what I mean.

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Here are two sagittal sections in two different knees.

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Let's look at the left image first,

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here, we're looking at the anterior cruciate ligament

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at its foot print and the bone beneath it.

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These are the collagen fibers, longitudinally oriented

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within anterior cruciate ligament.

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And you'll note the orientation of the trabeculae

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along that same axis.

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The ligament transmits tensile force,

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the bone lines up along the axis of that tensile force,

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that my friends is Wolff's law at work.

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And if you go ahead and look on the right side

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and look at a sagittal section,

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through the distal quadriceps tendon,

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attaching to the anterior surface of the patella,

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here are the collagen fibers within the quadriceps tendon,

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and look at the orientation of the trabeculae

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along that same access of tensile force.

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Yes, again, we're looking at Wolff's law at work.

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And even in this particular section

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where we're looking at the tibial plateau

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and tibial condyle, compressive forces apply to the surface

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of articular cartilage, reaching the subchondral bone

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and the trabeculae oriented along the axis

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of those compressive forces.

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Wolff's law at work again here in the proximal tibia.

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So let's go ahead and apply compression and sheer forces

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to the surface of articular cartilage.

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Those forces are transmitted

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through the articular cartilage by the collagen fibers.

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We've already talked about that,

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they reach the subchondral bone plate

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and go beyond that reaching the subchondral bone.

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In the subchondral bone, they are resisted in two ways.

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The first, they are resisted by the vertical

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and horizontal trabeculae located

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within the subchondral bone.

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The second, they are resisted by the marrow contents

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in each of these trabeculae chambers.

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And as I'd mentioned a couple times during this course

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in the adult, typically the marrow at the end of a bone

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is going to be mainly yellow marrow.

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So the force applied to cartilage resisted

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by the trabeculae as well as by hydrostatic pressure

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within the fatty marrow in each of these chambers lined

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by horizontal and vertical trabeculae.

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I mean, in a way, think of a automobile tire, right?

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When you think of a compressed automobile tire,

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you think of a load that is applied to the top.

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It's applied to the bottom of the tire.

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It pressurizes the air within,

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that pressure produces tensile force

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on the side walls of the tire.

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The exact same thing is going on in these marrow chambers.

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If in fact, the tensile forces placed on the walls

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of the chambers greater than the tensile strength

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of the trabeculae micro fractures develop,

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we have a name for them,

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we call them bone contusions or bone bruises.

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Diagrammatically here in a specimen,

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you can see what they look like,

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hyperemic, within those trabeculae chambers.

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When we look at this with MR, we see kind of an appearance

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that looks like the stars in the sky, right?

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A sparkly appearance, areas of right signal areas

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of intermediate signal in the background.

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This is classic for a bone contusion or a bone bruise.