Dx: Knee Effusion by Ultrasound

The image shows the distal femur with the knee in maximal tolerated flexion. With the knee in this position, fluid from within the joint is squeezed forward into a palpable bulge just lateral to the patellar tendon. On physical exam this is called the “Flex-Bulge sign” or “Pregerson sign” (yes I made it up, by examining my own knee I might add.) This bulge of fluid, which is easily palpated in the non-obese knee, especially when you compare to the contralateral side, is demonstrated on the ultrasound image as a semi-circle of hypoechoic fluid pocket. Flexion is probably the best position to use in order to pick up a small effusion. Knee flexion will often be limited in the presence of an effusion, so of course only do what is tolerable to the patient. Joint fluid, like most other fluid, appears black or anechoic and is clearly seen as a prominent anterior bulge in the ultrasound image. Of course this may also be easier to appreciate in a patient with an athletic build like this, rather than one who is soft and doughy.


You show the image to your patient, even giving him a comparison view using his other knee, to make sure he trusts you. He signs the consent form for the nurse, albeit somewhat apprehensively and you make sure to give him a nice dose of hydromorphone subcutaneously as well as generous local anesthesia, before proceeding with you arthrocentesis. Fortunately, when the results return they show negatively birefringent needle-shaped crystals and no bacteria. You tell your patient, “I’m glad to say that you were right! It looks like gout. I’ll send you home with those prescriptions you requested, but if it gets any worse, or you get a fever, return immediately.” He answers with a smile, “You were right too doc. It’s better to be safe than sorry.”


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