Glossary
Joint effusion
Quick answer
A joint effusion is extra fluid inside the knee joint, what many people call water on the knee. It is a sign that something is irritating the joint, such as an injury, arthritis, gout or infection, rather than a diagnosis in itself, explains Doha knee surgeon Dr. Pablo Gelber at Aman Hospital.
What the report means
A healthy knee contains only a small amount of lubricating fluid. MRI and ultrasound reports often grade any increase as small, moderate or large. A small effusion is common and may be of little significance; a large one usually means the joint is clearly inflamed or injured.
The fluid can be clear joint fluid, blood after an injury, crystals in gout, or pus in an infection.
Why it matters
The cause and speed matter. Swelling within an hour or two of a twisting injury often means bleeding into the joint, which is linked to ACL tears and fractures. Swelling that builds over a day after activity is more typical of a meniscus or cartilage problem. Gradual, recurring swelling is common in osteoarthritis.
A hot, red, very painful knee with fever needs same-day medical attention, because infection can damage cartilage quickly.
How it is assessed and treated
Examination confirms the fluid and looks for the cause. Sometimes fluid is drawn off with a needle, both to relieve pressure and to test it. Treatment is aimed at the underlying problem, with rest, ice, compression and elevation helping in the short term.
Frequently asked questions
Should water on the knee be drained?
Not always. Draining helps when the knee is tense and painful, or when the fluid needs testing. If the cause is not treated, fluid often returns.
When is a swollen knee an emergency?
Seek urgent care if the knee is hot and red, you have a fever, you cannot bear weight, or it swelled rapidly after a significant injury.
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