Your knee joint is able to move smoothly and friction free due to the constant presence of a substance known as synovial fluid. Sometimes due to injury or disease, your body will produce an excess of this fluid, or blood, pus or a crystalline substance may be introduced into the synovial fluid, aggravating the joint and causing your knee to swell. This is commonly referred to as "water on the knee," but is also known by its medical term: joint effusion.
One common way to get a joint effusion is through trauma, such as sports injuries, car accidents or falls. Such traumas include fractures into the knee, torn cartilage or ligaments and patellar injuries.
Water on the knee may also result from overuse injuries, such as repetitive squatting or hopping, done on the job or at play. People who are overweight are especially at risk for this syndrome.
Infection is another cause of excess fluid in the knee joint, and other underlying causes include osteoarthritis, gout and pseudogout, tumors or cysts and rheumatoid arthritis.
Signs and Symptoms
In the case of knee trauma or overuse syndromes, you may experience both edema and joint effusion. Effusion refers to increased swelling in or around the joint, while edema is swelling elsewhere---often appearing in the lower leg in the case of knee injuries.
The main symptoms of knee joint effusion are swelling along the joint line, warmth, redness and limited range of motion, meaning you may not be able to straighten or bend the affected knee as much as you can the opposite knee. If it resulted from an acute trauma, you may also see some bruising. If your joint effusion is due to osteoarthritis, the pain will be greater with weight-bearing. If it's due to a fracture into the joint, you may not be able to put any weight at all on your leg.
Joint effusions from trauma may include blood that has been introduced into the joint at the time of the injury; effusions due to infection, gout or pseudogout may have other substances in them. You would need to have an arthrocentesis, in which fluid is extracted from the knee, to discover which, if any, of these substances are present. Other tests that can help identify the degree and cause of the joint effusion include x-rays, manual tests of the ligaments and cartilage and blood tests.
Prevention & Treatment
If you're very active, you may not be able to completely avoid water on the knee, but with care you can minimize the risk. Keep your weight down, avoid repetitive tasks that stress your knees, and ask your doctor or physical therapist about the use of a knee brace for athletics, especially if you've injured your knees in the past. Keep your leg muscles strong and flexible, and don't skip medications your doctor has prescribed for chronic conditions, such as gout.
If your joint effusion is not due to a significant acute trauma, such as a fracture, you may be able to treat it conservatively, using the RICE principle: rest, ice, compression and elevation. Pain medications and bracing may also be recommended. If the fluid in your knee doesn't seem to be going away, you need to follow up with your doctor, as some underlying causes of joint effusion can be quite serious. You may need to have the fluid drawn off your knee, an injection to reduce the inflammation, or possibly even surgery to correct the underlying cause.