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 ¹: 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 lipohemarthrosis

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.