Water on the knee is a general term for excess fluid accumulation in or around your knee joint. Your doctor may refer to this condition as an effusion (uh-FU-zhun) in your knee joint.
Water on the knee may be the result of trauma, overuse injuries, or an underlying disease or condition. To determine the cause of water on the knee, your doctor may need to obtain a sample of the fluid to test for infection, disease or injury.
Removing some of the fluid also helps reduce the pain and stiffness associated with water on the knee. Once your doctor determines the underlying cause of your water on the knee, appropriate treatment can begin.