Knee joint effusion
Knee joint effusions are common and can occur in a variety of settings (e.g. trauma, degenerative change, infection or inflammation).
Knee joint effusions are only reliably seen on lateral projections. The following signs have been reflected as the most sensitive:
- rounded soft-tissue density in the suprapatellar recess
- loss of normal posterior fat plane of the quadriceps tendon
- fat pad separation sign: reflects the base of the suprapatellar bursa, which sits between peri-articular fat pads
- >10 mm is diagnostic
- 5-10 mm is equivocal and other signs of knee joint effusion are needed
- <5 mm means no joint effusion
- presence of lipohaemarthrosis
There are certainly other signs of knee joint effusions such as anterior displacement of the patella, but these are only reliably seen on large (>20 mL) joint effusions.