A Knee Jerk Reaction

Viet Tran

They say that in exams, your first answer is usually the right answer. This knee jerk reaction sometimes needs a little reassurance - just like how surrogate signs can reassure or bring your attention to a problem on an x-ray...

Here's the deal. 23yo male gets knocked over by a magpie. Falls on his left knee. Unable to bear weight, he turns up in your ed. X-rays follow:

What are your findings?

AP and lateral of the left knee with missing patient details. 
The proximal tibia appears to have a disrupted cortex around the intercondylar eminence, extending laterally towards the head of the fibula. This is in keeping with a tibial plateau fracture. 

If the AP wasn't as obvious, what else could help confirm or support the diagnosis

Known as the "FBI" sign, the Fat Blood Interface is pathognomonic for a tibial plateau or distal femoral fracture. The FBI is made by the combination of fat and blood leaking out of the marrow and then settling in their defined densities. Be reminded of the forces of gravity, the physics of fluid in a compartment and the interpretation of this finding in anything but a supine, still patient.

So what do you do now?

Tibial Plateau fractures are very modest on x-ray, which is why ortho ALWAYS wants a CT (see the patients below). 

Other things to know about tibial plateau fractures:

• typically an axial loading injury e.g. fall from height
• young people split it vs old people depress it
• lateral is weaker (i.e. more common)
• ergo medial fractures are high mechanism injuries, almost always with other fractures - so look!