A 16yo F presented for 1 week of right knee pain and 2 days of knee swelling. POCUS of the knee was performed which revealed a knee effusion.

Knee effusions will present as fluid under the tendon. If fluid is visualized above the tendon, then other diagnoses such as cellulitis, abscess, hypoproteinemia, hematoma, etc should be considered. Fluid will usually be seen superiorly as in this patient.