Another sign you kneed to know

This elderly patient had fallen and was struggling to bear weight.  What acute sign can you see on this Xray?  And what does it mean?

There is a lipohaemarthrosis, which occurs when fat floats on the aqueous fluid in the joint (blood and/or normal synovial fluid).  Just to the right of the patella is very distinct perfectly straight line - at right angles to the arrow which points up - telling you that it is truly horizontal; the surface of a liquid layer. 

The fat can only have come from Marrow, which under normal circumstances is contained within the bony cortex.  So it means there is almost certainly a fracture there somewhere .  The ones we tend to miss are tibial plateau fractures (as indeed this was).

You can get a similar appearance from a simple haemarthrosis (as the cells in the fluid settle to the bottom giving a denser layer) but it's generally better to assume there's a break until proven otherwise.

They're surprisingly common, and are mostly missed because we don't look for them.

​Si.