Lateral Knee Radiographs

Compiled from an educational email to University of Minnesota residents written by Scott Boeke, DO.

25 year old with right knee pain after fall/twisting injury.   Nothing striking about the AP and oblique so lets go straight to the lateral...

The #1 thing that is going to lead me toward an acute traumatic injury of the knee in most persons is the presence of a joint effusion, this is especially true in patients younger than a senior Radiology resident (sorry, it's all downhill after....).   The lateral radiograph is critical in my mental triage.  This person has a smudgy looking joint effusion but is there a bony injury?

This is a great example of a Lateral Femoral Notch Sign.  If we follow the lateral condyle and look for the normal shallow condylopatellar notch (AKA terminal sulcus) and see that it's conspicuously deeper than approx 2 mm, we should be worried for an impaction fracture related to pivot shift injury - and pivot shifts destroy ACLs.  There are a couple tricks to verify it's the lateral femoral condyle, I like using the adductor tubercule along the medial metaphysis if I can.  As the poorly drawn blue line shows, there should be a slight thickening of cortex and/or hump along the posterior aspect of the medial condyle metaphysis, the lateral condyle cortex is smooth/thin and flat.   

Lets check the MR....

Anddd here we see the ACL injury.

Lateral Femoral Notch Sign

https://radiopaedia.org/articles/lateral-femoral-notch-sign-knee?lang=us