MyPACS.net: Radiology Teaching Files > Case 834865

never visited KNEE PAIN FOLLOWING TRAUMA.
Contributed by: Michael Richardson, Radiologist, University of Washington, Washington, USA.
History: Knee pain following trauma.
Images:[small]larger

Fig. 1: AP Radiograph of the Knee

Fig. 2: Cross-table lateral

Fig. 3: 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.

Fig. 4: 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. Cross-table lateral 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.
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.

References: Source: UW Radiology Main Online Teaching File: Case No. 16
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Additional Details:

Case Number: 834865Last Updated: 11-03-2004
Anatomy: Skeletal System   Pathology: Trauma
Exam Date: Access Level: Readable by all users

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