A middle aged man presents with right knee pain and swelling. You are concerned it might be gout and would like to tap the joint. Is there an effusion?
There is an insufficient evidence base on this topic, but a review of the classic teachings on joint effusion is helpful.
- Inspection – Assess the medial and lateral peripatellar dimples, comparing to the contralateral knee. Loss of a dimple can happen with as little as 10mL of joint effusion. This is best assessed in slight flexion (perhaps 15-20 degrees). The knee with an effusion is often spontaneously in this position anyway, with the patient unable to hold extreme ranges of motion.
- Wave sign – milk the fluid to one side or the other, quickly compress it, and feel for an impulse on the opposite side. This tends to work better if the joint fluid is thick.
- Ballottement – milk the fluid to the center and then tap on the patella. You will feel a clunk with 10-15 mL of fluid.
- Suprapatellar space – palpate the lateral and medial suprapatellar spaces. A large effusion (>60mL) will fill this area. Remember that the purpose of the articular capsule of the knee is to allow extreme degrees of flexion. This happens through redundant synovial lining of the suprapatellar bursa which slides to lubricate the lining between the joint and patella even through extreme ranges of motion. The articularis genu muscle retracts this back during extension. The reason this is important is that if you elevate the knee when examining it, all the fluid might track up into this large suprapatellar cavity. So keep the knee dependent, and in slight (15-20 degrees) flexion during the examination. This also is why we aim upward during arthrocentesis. There is nothing inferiorly, all the capacity of the joint capsule is superiorly oriented.
Finally, the purpose of the physical examination isn’t to give you the answer, though it is nice when it does. It just has to raise or lower your probability of disease enough to justify or eliminate the need for further testing. Use ultrasound for confirmation if needed.