Knee Joint Effusions
By MSK Teaching Cases
Last updated on: November 26, 2024
*Compiled from an educational email to University of Minnesota residents written by Scott Boeke, DO
Let's start with anatomy.
We'll use this normalish left knee in a 55 yo female (there is some mild DJD):
Looks pretty darn good. But what are we looking at...
On a lateral radiograph, the suprapatellar joint recess is going to being outlined by the Prefemoral Fat Pad (red outline) and the Suprapatellar Fat Pad (blue outline). That little line of fluid attenuation(black arrow) is a completely normal amount of joint fluid.
When an effusion forms, its going to expand the distance between the margins of the fat pads with fluid attenuation (similar to non fatty tissue), mostly appearing to depress the anterior margin of prefemoral fat pad. This is my squiggly-line pictorial gradient....
SMALL: This is upper limits of small for me. Anything between this and the sliver of fluid density gets put in this designation. Of note, this is not unexpected in the setting older adult degenerative change.
MODERATE: Appearance of fluid extends more superiorly and has some increased curvature along it's superior margin.
Quick example: 18 yo M s/p slip and fall with continued pain 2 weeks later. I called this mod/large and recommended an MR when I couldn't find a fracture.
LARGE: Fluid distends and rounds the superior joint recess and even pushes on the quad tendon. Usually straight badness (acute trauma, inflammatory arthritis, acute on chronic DJD).
Appreciate the contralateral right knee in the 55 yo female from earlier for a great example of a large effusion and also lipohemarthrosis (blue line at the fat/fluid line on this cross table lateral projection)
The effusion fully distends the superior joint recess and even has mass effect on the quad tendon silhouette. Appreciate the CT correlate where you can see quad tendon mass effect as well as the line separating the margins of the suprapatellar fat pad and intraarticular fat layer (blue line). Blue arrow points toward the tibial medullary fat that has escaped into the joint space. Red arrow points toward the layering blood density in the dependent portion of the effusion.
So what's the brass tacks on this whole effusion stuff:
Use the amount of fluid in the joint to gauge your level of concern for underlying trauma.
Small effusions are WNL for many adults with degenerative changes. Start looking for additional answers when you exceed that small effusion designation.
Effusions are always abnormal in children, teens, and young adults. Get especially nervous in the setting of trauma. Take a little extra time looking and if a bone injury isn't evident, recommend an MR.
Enjoy this Radiopaedia that goes further in depth about lipohemarthosis and cross table lateral projections.