0:00

So here we have a lateral projection

0:03

of the knee showing this very large supra

0:05

patellar joint effusion.

0:07

And on a cross table lateral we identify

0:11

that there is a fat fluid level.

0:13

And so this represents a lipo orthosis.

0:16

Here's a diagram showing how that finding is generated.

0:21

So when you have a joint effusion,

0:23

the knee joint actually has a very large capacity, can hold

0:27

probably an excess of 30 mls of uh, fluid.

0:30

So when a joint effusion occurs, it often works its way

0:34

into the supra patellar recess

0:36

and that's where it's best visualized

0:38

and detected on radiography.

0:41

So on the lateral projection, particularly if it's done

0:44

supine, the so-called cross table lateral,

0:47

we then can see in tangent that there is the fluid

0:51

or blood products which are less dense than fat.

0:55

So that will be in a dependent position.

0:58

And then fat which can be expressed into the joint related

1:02

to an intraarticular fracture, tends to be lighter

1:06

and it will aggregate in an anti dependent position

1:10

and create this interface that we can see on the radiograph.

1:15

So when we're looking at our cross table laterals

1:18

of the knee, we want to pay particular attention

1:21

to the supra patella recess

1:23

and identify whether there is

1:27

a fluid density collection in that area.

1:30

Typical criteria

1:31

that people use is in the pre femoral fat is that zero

1:34

to five would be a normal thickness, five

1:38

to 10 would be indeterminate or maybe a small effusion,

1:41

and then greater than 10 is

1:43

considered positive for a effusion.

1:44

And they can be small, medium,

1:46

or large, just like I showed you previously, a large one.

1:49

And so this represents a joint effusion.

1:52

However, if you see an interface that is a straight line

1:56

between that effusion rather than this rounded

1:59

or ovoid contour between the fat

2:02

and the fluid, then this becomes diagnostic

2:05

of a lipo orthosis

2:07

and you are very suspicious for intraarticular

2:11

fracture causing expression of the fat.

2:14

And in this case an MRI was done.

2:17

And we see that there is a posterior column,

2:21

tibial plateau fracture that extends the articular margin

2:25

and that's the reason that this person had the

2:27

lipo orthosis.

2:29

The fat is dark on this fat suppressed sequence.

2:33

Here the fluid is bright,

2:35

but one can visualize a straight line

2:38

on this supine acquired MRI.

2:41

So here's a, a case of a large joint effusion,

2:45

somewhat dense and there is a

2:46

differential diagnosis for that.

2:49

Most commonly in the emergency department,

2:52

it's gonna be related to an acute injury of an ACL tear.

2:56

So with ACL tears, you do get a substantial amount

2:58

Of hemorrhage.

2:59

And when that hemorrhage gets into the joint, in addition

3:03

to the joint fluid can produce a lot of distension to the

3:07

sup patella recess, but also look relatively radio opaque.

3:11

In a non-traumatic setting, uh,

3:13

or minor trauma, it might be an issue related

3:16

to a bleeding disorder.

3:18

And in a young male, such as in the case here,

3:21

could represent bleeding related to hemophilia.