Knee swelling, sometimes called knee effusion or water on the knee, signals a problem with the knee. When mild knee osteoarthritis, non-septic knee bursitis, or a minor injury causes swelling, it usually can be treated at home with over-the-counter medication and the R.I.C.E. formula. Persistent knee swelling, or knee swelling accompanied by severe pain or other serious symptoms, warrants medical attention.

When to Contact a Doctor

In many cases, a mild to moderately swollen knee may be treated at home. A doctor should be contacted if:

  • The knee is severely swollen or has a pronounced abnormality
  • The knee cannot fully straighten or flex
  • The knee is severely painful (e.g. the pain is not adequately treated with over-the-counter medication)
  • The person cannot bear weight on the knee, or feels as if the knee is going to "give out"
  • The skin over the knee turns hot or red
  • The person has a fever of 100.4 or higher
  • Swelling has been present for 3 or more days

A doctor will examine the patient's knee and ask the patient several questions. The physical examination and patient interview may provide enough information to make an accurate diagnosis. If more information is needed, the doctor may recommend imaging (e.g., X-ray) or arthrocentesis, which can yield additional clues. (Arthrocentesis is described at the bottom of this page.)

If the patient is unsure whether to seek professional medical treatment, a phone call to a doctor or nurse can help determine whether an office visit is necessary.

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R.I.C.E. Formula

When home care is warranted, doctors often recommend treating a swollen knee with R.I.C.E.: Rest, Ice, Compression, and Elevation.

R.I.C.E. may be used in combination with medication, such as non-steroidal anti-inflammatory drugs, or NSAIDs, including ibuprofen, naproxen, and COX-2 inhibitors. Acetaminophen (e.g., Tylenol) will not relieve swelling but may be taken to relieve associated pain.

    Rest. Refraining from sports and other activities for 24 hours or longer will give the joint time to repair and recover. The joint should not necessarily be immobile; people with knee swelling should try to gently flex and straighten the knee several times a day to maintain range of motion.

    Ice. Applying a cold compress to the knee for 20 minutes 3 to 4 times each day can decrease swelling and promote healing. Ice should not be applied directly to the skin.

    Compression. Wrapping the affected joint in an elastic bandage (e.g. Ace bandage) may help limit or reduce swelling.

    Elevation. Sitting down with the leg elevated on a stool or lying down with the foot elevated on a pillow can help reduce blood flow to the area, thereby reducing inflammation.

Using the R.I.C.E. formula, swelling often goes down in 1 to 3 days. If swelling does not go down within a few days of starting R.I.C.E., or if swelling and pain worsen, contact a doctor.

Arthrocentesis (Joint Aspiration)

When a patient has a swollen knee, a doctor may want to verify or rule out certain diagnoses by analyzing the accumulated fluid. To do this, the doctor will remove fluid from the swollen knee joint or bursa using a needle and syringe. This process is called arthrocentesis, or joint aspiration.

Following arthrocentesis, the doctor will take note of the fluid's color and viscosity and may send it to a lab for further analysis. Determining the contents of the fluid can lead to an accurate diagnosis. For example, uric acid crystals in the fluid indicate gout, and bacteria in the fluid indicate infection.

Arthrocentesis is an important diagnostic tool because the underlying cause of knee swelling will determine the appropriate treatment.

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