Knee effusion, also known as water on the knee, is a condition where excess synovial fluid accumulates in or around the knee joint. Knee effusion can be caused by a variety of factors, including arthritis, injury to the ligaments or meniscus, or fluid collecting in the bursa. Knee effusion can also be caused by underlying diseases or conditions such as knee osteoarthritis, rheumatoid arthritis, and gout. Knee effusion can affect both big and small joints, and symptoms include swelling, stiffness, pain, and difficulty moving the joint.

To diagnose knee effusion, a doctor may perform a joint aspiration, which involves withdrawing fluid from inside the knee for analysis such as cell count, culture for bacteria, and examination for crystals, such as uric acid or calcium pyrophosphate dihydrate crystals found in gout or pseudogout. A thorough history and physical exam are also essential to make an accurate diagnosis. Treatment for knee effusion depends on the underlying cause and may include aspiration or drainage of the fluid, corticosteroid injections, and managing symptoms. Preventative measures include maintaining a moderate weight, staying active with low-impact sports, avoiding sudden jolting movements and rough running surfaces, and seeking medical attention when necessary.