A cortical discontinuity (arrow) is noted on the lateral side of the distal femoral metaphysis on the AP view. No other findings were visible on this film.
A cortical discontinuity is seen posteriorly representing the posteriormost extent of this unicondylar fracture. The cross table lateral film (arrows) shows a straight line in the area of the suprapatellar bursa. Straight lines are rarely present in nature, unless a fluid level of some kind is present. In this case, the fluid level is between blood and fat floating on top of it in the knee joint. The presence of blood is relatively non-specific, but the presence of fat in the joint means that we are dealing with a fracture until proven otherwise.
Where did this free fat in the jont space come from? There are basically two choices: the subcutaneous fat or the marrow fat. Subcutaneous fat is not terribly mobile. This is unfortunate, since it would be nice to mold unwanted body fat into some more appealing shape. In fact, it takes liposuction to really do much with it. Marrow fat is much more mobile, and can easily enter the joint space from even a small fracture.
It is theoretically possible to see fat-fluid levels in any joint, but in practice, one only sees it in the knee. The main reason for this is that one needs to have the X-ray beam parallel to the fluid level. This is easy to do with the knee (cross-table lateral view), but not with most other joints. Of course, one can see a fluid-fluid level very easily with CT or MR, even in small joints.