Fluid, or water, on the knee is also called knee effusion. Joints naturally contain some fluid to help lubricate and cushion the joint while moving. However, certain situations 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. A physician can order x-rays, blood tests and aspirate some of the fluid, to help determine the cause of fluid on the knee.
Injuries
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, states MayoClinic.com. 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 effusion from injuries. In more severe cases, it may be necessary to drain the knee of fluid, or have surgery to correct the underlying problem.
Infections
Any kind of infection that affects the knee 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, warns the American Academy of Family Physicians. 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 can help to confirm a diagnosis.
Arthritis
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, so that 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 and can also 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 what type of arthritis is present.
Overuse
Training or exercising too hard, too fast or beyond personal limits can cause the knee to swell and become painful. The National Institutes of Health, recommends that in addition to training properly, it is important to perform a proper warm-up and cool-down to help prevent knee swelling. Bursitis, an inflammation of the bursa sacs in the joint; tendonitis, which is an inflammation of a tendon in the knee; knee sprains and strains, can all cause fluid on the knee due to overuse.
