0:00

Now we have to look at injury mechanisms, some

0:04

of the basic concepts.

0:05

So I'm gonna use this particular drawing

0:07

where I show you the tibia in white.

0:10

I show you the outline of the femur above it.

0:13

And I'm showing you that in normal situations,

0:17

the vertical axis

0:18

of the knee passes near the center, okay?

0:22

The tibia is divided into four quadrants, anterolateral,

0:27

anter, medial posterolateral, and postal medial.

0:31

When we have ligament disruption, there is a shift

0:36

in the vertical axis,

0:38

and that shift may occur in a straight line shown

0:41

by the brown arrows

0:43

or in a curve line related to rotational

0:48

instability shown by these kind of bright green arrows.

0:52

So those are the two patterns of instability

0:55

that we may have.

0:57

Straight instability or rotary or rotational instability.

1:02

And in general, we talk about the way the tibia moves

1:07

with respect to the femur.

1:08

Even though the femur may be the bone that is moving.

1:12

We talk about the way the tibia moves

1:14

with respect to the femur.

1:16

We end up with two patterns of instability of the knee.

1:20

The first of these is straight instability related

1:24

to translation in one of four basic directions,

1:27

and obviously you know those directions.

1:30

And posterior medial

1:32

or lateral, I'll show you

1:34

what anterior translation looks like.

1:37

No mystery here.

1:38

The tibia is moving anteriorly with respect to the femur.

1:43

The second is rotary

1:44

or rotational instability, where we gonna have rotation

1:48

and translation combined.

1:51

There are four basic types and you can see them listed here.

1:55

The two that we see most commonly,

1:57

and the two I will concentrate on in this lecture

2:00

are anterolateral and anter medial rotary instability.

2:05

Let me show you an example

2:06

of anterolateral rotary instability, which occurs

2:10

with anterior translation of the tibia

2:13

and internal rotation of the tibia.

2:17

You can see the movement

2:18

and why this is called antral lateral.

2:21

That's where in fact, the arrow ends up.

2:25

Now let's get to the footprints

2:27

because following an injury to ligaments

2:30

and also perhaps to the menisci,

2:32

there are certain footprints that are left behind.

2:36

Now on Friday, we will talk more detail about these

2:40

footprints in terms of confusion, uh, of contusions

2:44

or in terms of osteochondral fractures, chondral fractures,

2:48

and subcon fractures.

2:50

But in any case, these are the footprints left behind

2:53

and it's my belief if you find the footprints, identify

2:57

where they are, you can figure out the movement of the knee.

3:02

In many cases what's happened, the mechanism of injury,

3:05

and you learn to look at specific regions.

3:09

For tho, those are the ligaments that should be injured.

3:13

Now, during an injury, there are two potential forces

3:17

that may be applied to the knee or to any joint.

3:20

I'm gonna call the first one, the external force.

3:23

And the second one we'll talk about is the internal force.

3:27

They may occur separately or they may occur together.

3:32

In terms of the external force,

3:34

this is the environmental force originating outside

3:39

of the patient that is applied to the surface of the body

3:43

and hence the surface of the bone.

3:45

Right? It may be a car striking the knee, it could be a bus,

3:49

it could be the helmet on a team of the opponent's helmet,

3:54

typically, but not invariably.

3:56

External force rarely involves the end

3:59

of a long tubular bone.

4:01

So about the knee, we tend to see the footprints,

4:04

not at the end of the bone, but away from it.

4:07

To show you one example of an external force applied here

4:12

to the antral medial aspect of the distal femur,

4:16

you can see here the bone contusion,

4:19

not reaching the subc chondral bone.

4:21

This is an external force.

4:24

When we talk about mechanisms of injury

4:26

that are often not invariably related to external force,

4:31

there are three that I place here as important, two

4:35

of which are fairly common valgus

4:38

where the external force is applied

4:40

to the lateral aspect of the knee.

4:42

We've kind of talked about that, where we see tensile injury

4:46

to the medial supporting structures

4:49

ligament failure on the opposite side of the knee.

4:53

Okay, this is what it would look like in a person.

4:55

Here's my diagram to kind of show you what's going on.

4:59

And note here, 'cause you're gonna see this diagram again,

5:02

this is one condyle, this is another.

5:04

Here's the tibia, located it kind of a coronal picture.

5:10

Here's a, uh, another uh, type

5:13

of mechanism often related

5:15

to an external force hyperextension.

5:18

Again, the image comes from the internet,

5:21

but this is a history for this particular patient.

5:24

So typically with a hyperextension,

5:27

there is an external force applied

5:29

to the anter aspect of a knee.

5:31

A number of things can happen, but one

5:34

or more of the cruciate ligaments can be torn.

5:37

Here we see the contusion related to the external force.

5:41

We can see tearing of both cruciates raising the possibility

5:45

that there was in fact a knee dislocation.

5:49

In any case, this is the external force

5:53

related to a hyperextension mechanism.

5:57

Now, one of the problems we have that

5:59

although we may identify an external force based upon the

6:02

footprint left behind, sometimes finding the vector of

6:06

that force may be difficult.

6:09

Here we find an external force, but this was the vector

6:12

and hence, in this particular case,

6:15

the posterior cruciate ligament was torn.