Knee effusion

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Knee effusion
Other names	Swelling of the knee, water on the knee
Traumatic effusion of the right knee, with swelling lateral to the kneecap marked by an arrow
Prevention	ice bath

Knee effusion, informally known as water on the knee, occurs when excess synovial fluid accumulates in or around the knee joint. It has many common causes, including arthritis, injury to the ligaments or meniscus, or fluid collecting in the bursa, a condition known as prepatellar bursitis.