Water on the Knee (Effusion) Causes and Treatments

Why Your Knee May be Swollen and What to Do About It

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Water on the knee is when fluid collects around and inside the knee joint, causing pain and swelling. Also known as knee effusion or fluid on the knee, it can occur whenever there's damage to the joint due to injury or underlying disease, such as arthritis.

This article explores the common causes and treatments of water on the knee and when you should get medical care.

Home Remedies to Get Rid of Fluid on the Knee - Illustration by Jessica Olah

Verywell / Jessica Olah

Causes of Water on the Knee

The knee joint is a synovial joint, meaning it is lined with a type of tissue that produces synovial fluid. This fluid provides nutrition to the cartilage lining the joint, lubricates it, reduces friction, and helps with joint movement.

Excess fluid around the joint can cause swelling, pain, and stiffness.

Common causes of fluid build-up on the knee include:

What Is Cartilage?

Cartilage is a flexible tissue on the ends of bones that allows bones to glide smoothly over each other.


Injury is the most common cause of fluid on the knee in active, healthy people. This can include:

  • Anterior cruciate ligament (ACL) injury: This may be a sprain or tear of the ACL—the ligament that helps connect your thigh and shinbones, as well as stabilize your knee.
  • Meniscus tear: This is a rip in the C-shaped cartilage within your knee. It compromises the cushion the meniscus normally provides the shinbones and thigh bones where they enter the knee joint.
  • Contusion: A tissue injury such as a bruise

Repetitive movements from sports such as running or from squatting and lifting often cause knee pain and sometimes contribute to swelling.

Arthritis and Other Inflammatory Diseases

Underlying diseases can lead to an abnormal inflammatory response that causes excess fluid on the knee as your body tries to protect your joint.

These conditions include:

  • Osteoarthritis: A common type of arthritis caused by cartilage breakdown
  • Gout: Also called gouty arthritis; a buildup of uric acid crystals in the joint causes sudden, intense pain
  • Rheumatoid arthritis: An autoimmune disease in which the body attacks the synovial lining of joints and other tissues
  • Prepatellar bursitis: Occurs when thin, fluid-filled sacs that cushion and lubricate the knee (bursae) become inflamed and fill with fluid, causing swelling at the front of the knee


Sometimes, osteoarthritis or tears can cause cysts called Baker’s cysts, which can cause fluid on the knee.

Baker's cysts are fluid-filled lumps that form behind the knee when the joint is inflamed from injury or disease. Fluid flows toward the back of the knee and forms the cyst, and the cyst may contribute to swelling.


Infections can also cause fluid and pain. They can happen as a complication from surgery or inflammation, and can be due to dangerous bacteria.

Infections in the joint can be extremely painful and come on rapidly. They require prompt medical care.

When to Go to the ER

If your knee is tender, warm, red, and if you have a fever, chills, or feel ill, get immediate medical care.


To diagnose fluid on the knee, your healthcare provider will do a physical exam and check for these symptoms:

  • Swelling
  • Stiffness
  • Decreased range of motion
  • Fever
  • Loss of sensation
  • Difficulty walking or bearing weight on the affected leg
  • Warmth and redness

Your healthcare provider may order a procedure called joint aspiration, where a sample of the fluid is removed with a needle. The fluid is then analyzed for white blood cells that indicate inflammation, bacteria that reveal infection, or uric acid crystals that signal gout.

Imaging tests such as an X-ray or MRI may be ordered to help hone in on a diagnosis as well, especially if your healthcare provider suspects a tear or other injury.

Treatment for Water on the Knee

The right treatment for water on the knee depends on the cause.

In mild cases, fluid could go away on its own or with the help of some at-home treatments, such as:

  • R.I.C.E. protocol: Rest, ice, compression, and elevation can help relieve minor pain directly after an injury.
  • Compression: Try gently wrapping the knee with an elastic bandages to reduce swelling.
  • Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs), such as Advil (ibuprofen) or Aleve (naproxen), which can reduce inflammation and pain
  • Physical therapy exercises, as directed by a healthcare provider
  • Wearing a knee brace: A healthcare professional can determine if this is right for you and, if so, what type you should use.

If you do not notice improvement after a few days, you may need medical treatment.

Medical Treatment

If at-home treatments prove insufficient, your healthcare provider may drain fluid from the knee (joint aspiration) to provide temporary relief.

Does Draining Fluid From Knee Hurt?

Local anesthetic (one applied directly to the knee) should keep joint aspiration from being very painful. You may have some discomfort for a few days afterward, but it shouldn't be severe.

Injections of corticosteroids into the joint can help reduce pain and inflammation from injury or arthritic joint damage.

If fluid on the knee is caused by an infection, your provider will prescribe antibiotics to kill the bacteria. Taking oral (by mouth) antibiotics for 14 days usually takes care of it.

If the infection is due to drug-resistant bacteria, you may be given up to four weeks of intravenous (IV, through a vein) antibiotics.

For underlying conditions such as rheumatoid arthritis, typical treatment includes medications that suppress an overactive immune system.

For severe cases of fluid on the knee, you may require joint surgery (arthroplasty) that may include joint replacement. These methods are only used as a last resort if all other medical interventions fail.

If at-home treatment methods or any prescribed medications don't work, tell your healthcare provider right away.

Preventing Knee Effusion

You may be able to prevent water on the knee with certain lifestyle changes and by taking some precautions during physical activity. For example:

  • Loosing weight
  • Discussing arthritis treatment with your healthcare provider
  • Choosing lower-impact sports like walking instead of running
  • Avoiding activities that require repetitive motions 


Water on the knee is a painful condition that occurs when synovial fluid builds up in and around the knee joint. Injuries, infections, and medical conditions such as arthritis can cause this.

A healthcare provider can recommend treatments, such as medications and physical therapy, based on the cause of the fluid buildup.

Frequently Asked Questions

  • Is it common to have fluid on the knee after knee replacement?

    Yes, fluid on the knee is common after knee replacement surgery. Expect your healthcare provider to explore various causes, though, including rheumatoid arthritis and infection.

  • Which is better for knee pain, heat or ice?

    It depends on the injury. Ice reduces inflammation, which is helpful for a recent sprain or meniscus tear. Heat relieves chronic (long-lasting) joint or muscle pain and improves flexibility.

  • What kind of exercise reduces water on the knee?

    It depends on what's causing it. Once you have a diagnosis, ask about physical therapy so you can learn the right exercises. Common ones are stretches to improve range of motion and strength-building exercises.

  • Can fluid on the knee get worse?

    Yes, it can get worse without treatment. A bacterial infection can spread and cause permanent damage. An untreated meniscus tear can cause debilitating pain and loss of mobility. See a healthcare provider right away.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Rachel Macpherson
Rachel MacPherson is a health writer, certified personal trainer, and exercise nutrition coach based in Montreal.