On the lateral view of this elbow, you can see large post-traumatic joint effusion. The posterior fat-pad is faintly visible and the anterior fat-pad is elevated and anteriorly displaced (Sail Sign). These findings indicate a fracture about the elbow that in an adult should be in the radial head and in a child usually means a supracondylar fracture.
Source: Dr. M.T. Niknejad (radRounds Radiology Network Member)