Knee joint effusion

Case contributed by Henry Knipe
Diagnosis certain


Fall with twisting injury.

Patient Data

Age: 50 years
Gender: Male

No fracture or malalignment. Small knee joint effusion. 

Annotated image

Annotated image demonstrating signs of knee joint effusion:

Red lines highlights the position of peri-articular fat pads.

Yellow line indicates the base of the suprapatellar bursa (i.e. fat pad separation sign - see discussion for further detail). 

Red arrow indicates loss of normal crisp outline of the posterior aspect of the quadriceps tendon. 

Case Discussion

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. 

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