Last Updated: May 24, 2018
“Water on the knee” is common term for knee effusion, a condition in which excess fluid accumulates around the knee joint. Such effusions happen for many reasons. The most common ones are injury, overuse, and chronic health conditions like osteoarthritis.
Your knee joint connects your femur (upper leg bone) to the tibia and fibula (lower leg bones) with three large ligaments. On each side of the joint is a meniscus, a crescent-shaped wedge of cartilage (soft bony tissue). The meniscus is slippery and provides cushioning and lubrication to help the ends of the bones move smoothly over one another.
Like most joints, the knee is contained within a “capsule” - a membrane that normally contains less than an ounce of lubricating joint fluid. However, an injury or illness can cause extra fluid to build up around the joint. The results are swelling, pain and restricted range of motion. In severe cases, it may be impossible to walk or even put weight on the affected knee.
Water on the knee usually isn’t dangerous in itself, but it can be a sign of a more serious medical condition. In addition, prolonged swelling can cause the leg muscles to weaken. This is especially dangerous in older people, because it increases the risk of falls. Another potential complication is Baker’s cyst, a fluid filled sac that forms at the back of the knee. Occasionally these cysts get so large or bothersome that a physician must drain them.
Water on the knee is more common in people who have significant “wear and tear” on their joints. Age, previous injury, and obesity are all risk factors. People who play sports or practice very physical occupations are also more likely to experience a knee effusion.
Osteoarthritis is the most common cause of water on the knee. This condition develops as the meniscus wears away due to repetitive stress and injury over time. Without the lubricating effect of cartilage, friction increases, and the knee becomes swollen and painful to use.
Trauma is another common cause of effusion. Twisting, hyperextension, a sudden stop or a direct blow to the knee can damage the bones, ligaments and meniscus, resulting in pain and swelling.
Interestingly, many diseases can cause knee problems, including some that may have origins elsewhere in the body. Some examples:
You might have water on the knee if you notice:
Swelling that appears in the absence of an injury may be caused by an infection. Additional symptoms include fever, chills, redness, or hotness.
Recent research suggests that a swollen knee can sometimes be the first sign of Lyme disease, a potentially serious bacterial infection spread by tick bites. Knee swelling due to Lyme disease is usually pronounced and develops quickly, but is not typically painful. It can occur in the absence of other common Lyme disease symptoms like rash or fever. Left untreated, Lyme disease can lead to permanent joint damage.
Tumor-related swelling may be accompanied by fever, night pain and sweating, and unexplained weight loss.
Mild knee swelling due to an injury or previously diagnosed condition may not require medical attention. Instead:
See a doctor if your knee is very swollen or doesn’t improve with home care. Also see a doctor if you notice infection symptoms like fever, redness or hotness. It’s a good idea to seek help if you don’t know what caused the swelling (in other words, it’s not related to an injury or diagnosed medical condition).
Primary care providers can diagnose and treat many straightforward knee problems. Depending on the findings, your provider may refer you to an orthopedist (skeletal system specialist) for further treatment.
A workup for water on the knee may include:
Next steps will vary depending on the underlying issue causing your knee effusion. Your provider’s goals include reducing swelling, relieving pain, improving function, and treating any underlying illnesses.
The course of water on the knee depends on its cause.
Osteoarthritis is incurable once it develops, but can often be managed with activity, physical therapy, and medication. In severe cases, surgery can greatly improve function.
People who have had a severe knee sprain or fracture are at increased risk for chronic (long-term) pain, stiffness, and joint instability. They may also be more likely to develop osteoarthritis as they get older.
To reduce your risk of knee pain and swelling: