Estimating the size of knee effusions

• Small effusions (5 to 10 mL) will fill the peripatellar dimples with the knees extended and quadriceps relaxed.
• The ballottement sign is positive when there is at least 10 to 15 mL of intraarticular fluid.
• Large effusions (20 to 30 mL) fill the suprapatellar space. 

While this size range is typically easily detectable on exam. This may not apply to patients who are either very muscular or obese.

If the detection of a small to moderate sized effusion would change patient management 

• For example, ones confidence to successfully drain a knee effusion knee based on a physical exam

Consider ultrasound: 

As compared to MRI (sensitivity of 81.3 % and a specificity of 100 %)

References

Draghi F, Urciuoli L, Alessandrino F, Corti R, Scudeller L, Grassi R. Joint effusion of the knee: potentialities and limitations of ultrasonography. J Ultrasound. 2015 Sep 18;18(4):361-71.