Knee effusion
（重定向自Water on the knee）
![](/Images/godic/202502/20/Kneeffusion3105.JPG")
![Skyline view of the patella demonstrating a large joint effusion as marked by the arrow](/Images/godic/202502/20/Knee_effusion3105.jpg")
Knee effusion or swelling of the knee (colloquially known as water on the knee) occurs when excess synovial fluid accumulates in or around the knee joint. There are many common causes for the swelling, including arthritis, injury to the ligaments or meniscus, or fluid collecting in the bursa, a condition known as prepatellar bursitis.