| Discussion: |
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.