0:00

So let's look at what happens when we apply shear

0:03

or axial loads to the articular cartilage.

0:07

We've already commented those loads are transmitted

0:10

through the articular cartilage by the collagen.

0:13

Many of the arcades we talked about,

0:16

they reach the subcon bone plate

0:18

and they are then transmitted to the subc chondral bone.

0:22

In the subc chondral bone.

0:24

They are resisted by two things,

0:27

the green arrow pointing out that there are vertical

0:30

and horizontal trabecula that form the walls

0:34

of these marrow chambers.

0:36

So those trabecular walls, vertical

0:39

and horizontal, are important in resisting the compression

0:43

applied to the articular surface.

0:46

And then within the chambers, particularly in adults,

0:50

we have fatty marrow, not hematopoietic marrow,

0:54

but generally fatty marrow.

0:56

And as you apply this pressure,

0:58

you have increased hydrostatic pressure

1:01

of the fatty marrow in each of the chambers.

1:04

Okay, here are the chambers,

1:06

and that too resists the force applied to the surface

1:10

of articular cartilage.

1:13

I mean, when you think about it,

1:15

it's a little bit like a compressed automobile tire.

1:19

Here are the green arrows showing you forces applied

1:22

to the top and bottom of the tire.

1:24

What does it do? It pressurizes the air within the tire

1:29

and that produces tensile force on the side walls

1:33

of the tire, very similar to what's going on

1:37

in the image on your left.

1:40

So if in fact, the tensile forces placed on the walls

1:45

of the marrow chambers is greater than the tensile strength

1:49

of those trabecula, we begin to see abnormality.

1:53

And the least severe is something we call bone contusions

1:58

or bone bruises, classically described as hemorrhage

2:02

and edema within the subchondral bone.

2:05

Now a little bit later on we're gonna add to

2:08

that a another component,

2:10

but let's go with a hemorrhage edema,

2:12

and this is what it looks like.

2:14

We often see this speckled appearance, right?

2:17

Looking a little bit like a starry night.

2:20

So I put a picture of a starry night at the top left.

2:24

This is hemorrhage and edema, belated to a bone contusion

2:29

or a bone bruise.

2:31

And this is what it might look like in a specimen,

2:34

perhaps here as well.