SET T0027 -
Emergency Department -
Knee Radiographs.

 No evidence of a fracture.

There is an abnormality of the patella - but this is a classic normal variant. A bi-partite patella. This is a clinically unimpotant developmental anomaly.

Note: the fragment in the supero-lateral aspect of the patella has smooth and well corticated margins; this, and the lateral position, are the classic diagnostic features indicating a bi-partite patella.

This normal variant is invariably best shown on the AP radiograph; in this particular case the abnormality is visible on the lateral view as well.

1. These abnormal features are evident: a) a lipo-haemarthrosis (ie a fat/fluid level or FFL) in the suprapatellar bursa on the lateral radiograph; and, b) an apparent "double articular surface" of the lateral tibial plateau on the AP radiograph as indicated by the arrow.

2. The conclusions: the FFL indicates that an intra-articular fracture is present; the features shown on the AP radiograph indicate that the injury that has been sustained is a lateral tibial plateau fracture.

FFL . . .   See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Pages 248-249

Plateau fractures . . .   See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Pages 250-253

Normal radiographs.

No evidence of a Fat-Fluid level.

No evidence of a fracture.

Normal anatomy . . .   See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Pages 244-249

• No bone abnormality.
• A large effusion is present in the knee joint.
• Note: a fat fluid level (FFL) is not present. 

The key radiograph to examine - in terms of the soft tisue evaluation - is the lateral projection.

To recapitulate in relation to a lateral radiograph obtained with a horizontal X-ray beam:

1. A distended supra-patellar bursa can be detected (as in this particular patient). See the explanation under suggested reading.
2. If a fat-fluid level (ie a FFL or a lipohaemarthrosis) in the supra-patellar bursa is detected on the lateral radiograph - then this would be indicative of an intra-articular fracture.
3. Fat is less dense than synovial fluid, and it floats on the top of synovial fluid - as in dirty washing-up water. Fat appears darker (ie as a black area) than does synovial fluid. 
4. Remember that soft tissue, and a synovial effusion, and blood are all very similar in terms of their density on a radiograph; they appear grey.
5. When fat is present in the supra-patellar bursa, the fat has come from the bone marrow of the fractured bone.  Hence, the finding of a fat-fluid level connotes the presence of an intra-articular fracture.
6. Be careful. Only some - by no means all - intra-articular fractures will show a fat-fluid level. Yes, it is a variable finding, but a FFL does provide an important message when it is present on the lateral radiograph.

See images below - as follows:

Top - Normal.

Middle - Effusion. Increased supra-patellar soft tissue density and increased distance (red arrows). Forward angulated patella (green lines). No evidence of a Fat-fluid level.

Bottom - Lipohaemarthrosis (ie a Fat - Fluid level).

 Suggested reading . . .  See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Pages 245-249