Fluid, or water, on the knee is also called knee effusion. Joints naturally contain some fluid to help lubricate and cushion the joint while you move. However, certain conditions can lead to an excess buildup of fluid. When this happens, the knee can swell, become painful and stiff, or there can be skin discoloration and bruising. See your doctor for an accurate diagnosis if you experience these symptoms. Your doctor may order x-rays, blood tests and aspirate some of the fluid to determine the cause of fluid on your knee.
Whenever the body is injured, the immune system responds to help protect the area. Part of this reaction often includes swelling. Knee injuries that cause swelling can lead to excess fluid buildup in the knee joint. This can include broken bones or tears to the ligaments, tendons and muscles that support the knee joint. Pain medication, bracing and anti inflammatory medications can treat knee swelling caused by injuries. In more severe cases, it may be necessary to drain the knee of fluid, or have surgery to correct the underlying problem.
Training or exercising too hard, too fast or beyond personal limits can cause the knee swelling and pain. In addition to training properly, it is important to perform a proper warm-up and cool-down to help prevent overuse of your knee. Bursitis -- an inflammation of the bursa sacs in the joint, tendonitis -- inflammation of a tendon in the knee, and knee sprains and strains can all cause fluid on the knee with overuse.
The two most common forms of arthritis, osteoarthritis and rheumatoid arthritis can both affect the knee and cause fluid buildup. In the case of osteoarthritis, the cartilage in the joint wears away, and eventually bone rubs on bone. Rheumatoid arthritis is an autoimmune disease in which the immune system attacks the synovial lining of the joint. Both of these conditions can lead to swelling, pain, stiffness and joint deformity. Gout, which is an abnormal buildup of uric acid in a joint, is another type of arthritis that can cause the knee to swell. X-rays, MRI's and blood tests can help to detect if arthritis is the cause of fluid on the knee and determine which type of arthritis is present.
Infection in your knee joint can lead to swelling and fluid buildup in the joint. Some infections that can cause knee effusion include Lyme disease, gonorrhea, tuberculosis, brucellosis and forms of infectious arthritis. If an infection is the cause, other symptoms may be present such as a fever, chills, nausea, stomach cramps and unexplained weight loss. Joint aspiration and blood tests are used to confirm the presence of infection.