Diseases and Conditions
Swollen knee
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A swollen knee occurs when excess fluid accumulates in or around your knee joint. Your doctor might refer to this condition as an effusion in your knee joint. Some people call this condition "water on the knee."

A swollen knee may be the result of trauma, overuse injuries, or an underlying disease or condition. To determine the cause of the swelling, your doctor might 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 the swelling. Once your doctor determines the underlying cause of your swollen knee, appropriate treatment can begin.
Symptoms
Signs and symptoms typically include:

Swelling. The skin around your kneecap can puff up noticeably, especially when you compare the affected knee to the normal one.
Stiffness. When your knee joint contains excess fluid, you might not be able to bend or straighten your leg completely.
Pain. Depending on the cause of the fluid buildup, the knee might be very painful — to the point that it's difficult or impossible to bear weight on it.
When to see a doctor

See your doctor if:

Self-care measures or prescribed medications don't relieve the pain and swelling
One knee becomes red and feels warm to the touch compared to your other knee