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Signs and symptoms of water on the knee typically include:

• Swelling. The flesh around your kneecap may puff up noticeably. This swelling is most apparent when you compare the affected knee to the normal one.
• Stiffness. When your knee joint contains excess fluid, you may not be able to bend or straighten your leg completely.
• Pain. Depending on the cause of the fluid buildup, the knee may be very painful — to the point that it's impossible to bear weight on it.

When to see a doctor
Seek prompt medical attention if:

• Self-care measures or prescribed medications don't relieve the pain and swelling
• One knee becomes red and feels warm to the touch as compared to your other knee

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Water on the knee can be caused by many different types of problems, ranging from traumatic injuries to diseases and conditions.

Injuries
Damage to any part of your knee can cause the painful buildup of excess joint fluid. Examples of traumatic injuries that cause fluid buildup in and around the knee joint are:

• Broken bones
• Meniscus tear
• Ligament tear
• Overuse injuries

Diseases and conditions
Underlying diseases and conditions that may produce fluid buildup in and around the knee joint include:

• Osteoarthritis
• Rheumatoid arthritis
• Infection
• Gout
• Pseudogout
• Bursitis
• Cysts
• Bleeding disorders
• Tumors

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• Age. Water on the knee tends to happen more often in people older than age 55.
• Sports participation. People who participate in sports that involve sudden changes of direction, such as basketball, are more likely to experience the types of knee injuries that cause water on the knee.
• Obesity. Excess weight puts added stress on the knee joint, contributing to the wear-and-tear damage that may lead to water on the knee. This increases your risk of osteoarthritis, one of the more frequent causes of water on the knee.

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If left untreated, water on the knee can severely limit your mobility. If the swelling is caused by an infection, it can destroy the joint.

©1998-2013 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
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While you may initially bring your symptoms to the attention of your family doctor, he or she may refer you to a doctor specializing in conditions related to arthritis (rheumatologist), in sports medicine or in orthopedic surgery.

What you can do
Before your appointment, you may want to write a list that answers the following questions:

• Have you injured your knee recently? If so, describe the injury in detail.
• Do you play recreational sports? If so, what sports?
• Do you have any type of arthritis?
• Has your knee felt warm or looked red? Do you have a fever?
• Do you have a family history of autoimmune disease?

What to expect from your doctor
During the physical exam, your doctor will need to press on different parts of your knee to check for pain and swelling. He or she will also ask you to try to move your knee into different positions to check your range of motion.

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Water on the knee is a sign of an underlying knee problem, usually arthritis, an infection or an injury. Your doctor may order some of the following tests to determine the underlying cause.

Imaging tests
These studies show the inside of your knee joint. They can help determine whether there is fluid within the joint or in a bursa and may provide information about what's causing fluid to accumulate.

• X-ray. Your doctor may request an X-ray to make sure you haven't broken or dislocated any bones or to determine if you have arthritis.
• Ultrasound. This noninvasive imaging test is widely used in Europe and gaining popularity in the U.S. to evaluate joint pain and swelling. An ultrasound exam is less expensive than an MRI. The test can be used to diagnose arthritis or tendon/ligament disorders.
• Magnetic resonance imaging (MRI). If your history, examination and X-rays don't identify the cause of your swollen knee, you may need an MRI. This test can detect tendon and ligament injuries that aren't visible on X-rays.

Blood tests
A sample of blood taken from your arm can be tested for evidence of:

• Infection
• Inflammation
• Rheumatoid arthritis
• Lyme disease
• Bleeding disorders
• Gout

Joint aspiration (arthrocentesis)
During this procedure, your doctor withdraws fluid from inside your knee to check for the presence of:

• Blood, which may stem from injuries or bleeding disorders
• Bacteria, which may be causing an infection
• The crystals common to gout or to pseudogout

Arthroscopy
An orthopedic surgeon inserts a small, lighted tube with a magnifying lens (arthroscope) through a small incision in your knee and examines the inside of your knee joint.

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Pain medications and procedures to remove fluid from the knee joint reduce the symptoms of water on the knee. Treatment varies for diseases and conditions causing water on the knee, but depending on the severity of the swelling and your medical history, options may include:

Medications
Drugs used to treat the symptoms or underlying causes of water on the knee include:

• Pain medications. If over-the-counter pain medications don't work well enough, your doctor may prescribe stronger drugs to help control your pain.
• Antibiotics. If your symptoms are being caused by an infection, your doctor may prescribe antibiotics to fight the infection.
• Corticosteroids. These strong anti-inflammatory drugs, such as prednisone, can be taken orally or may be injected directly into your knee joint. Corticosteroids can cause serious side effects, so you shouldn't use them too often or for a very long period of time.

Surgical and other procedures
Minimally invasive surgery and other methods of treating water on the knee and its most common underlying causes include:

• Joint aspiration (arthrocentesis). Removing fluid from your knee joint can help relieve the pressure of joint fluid buildup. After aspirating joint fluid, your doctor may inject a corticosteroid into the joint to treat inflammation.
• Arthroscopy. During this procedure, the surgeon makes a small incision in the skin over your knee joint, then inserts a small, lighted tube (arthroscope) that sends real-time images of the inside of your knee to a video terminal. Your surgeon may also use small, precise tools — sometimes placed in the joint through an attachment to the arthroscope — to remove loose tissue or repair damage in your knee.
• Joint replacement. If bearing weight on your knee joint becomes intolerable, your doctor may refer you to an orthopedic surgeon for knee replacement.

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Taking care of yourself when you have water on the knee includes:

• Rest. Avoid weight-bearing activities as much as possible when your knee is painful and swollen.
• Ice and elevation. Cold therapy can help control pain and swelling. Apply ice to your knee for 15 to 20 minutes every two to four hours. You may use a bag of ice, frozen vegetables or an iced towel cooled down in your freezer. When you ice your knee, raise your knee higher than the level of your heart, using pillows for comfort.
• Pain medication. Over-the-counter drugs such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin, others) may help relieve your knee pain. If you need something stronger, ask your doctor about prescription medications.

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Water on the knee is typically the result of a chronic health condition or traumatic injury. To prevent water on the knee, work with your doctor and other members of your health care team to manage the condition or traumatic injury that causes it. For example:

• See your doctor regularly. See your doctor regularly if you have a chronic joint condition, such as osteoarthritis, rheumatoid arthritis or gout.
• Follow through on your doctor's orders. Take the medications your doctor prescribes to treat the underlying disease or condition that causes water on the knee. If your doctor, physical therapist or sports trainer recommends you wear a knee brace because of a prior injury, be sure to follow through.
• Strengthen your thigh muscles. If your thigh muscles are weak, your doctor may refer you to a physical therapist to learn how to strengthen these muscles to better support your knee.
• Be gentle with your knees. If you're overweight and plan to start exercising as part of a weight-loss program, select an activity that doesn't place continuous weight-bearing stress on your knee joints — such as water aerobics or swimming.

©1998-2013 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
Read this article on Mayoclinic.com.