Knee swelling is sometimes referred to as "water on the knee." The medical term for this condition is knee effusion. Water on the knee can result from an injury, chronic overuse, or disease.

See Knee Pain and Arthritis

Sometimes a swollen knee can be caused by chronic conditions such as bursitis. Learn more: Knee (Prepatellar) Bursitis

Swelling in a knee joint may limit knee flexibility and function. For example, a person may find it difficult to fully bend or completely straighten a swollen knee, and the joint may naturally bend 15 to 25 degrees while the leg is at rest.1

Depending on the underlying condition, the swollen knee may exhibit no other symptoms or it may be painful, red, and/or difficult to put weight on.

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12 Potential Causes of Knee Swelling

This article describes 12 conditions that frequently cause knee swelling, or water on the knee. Included are the most common causes of knee swelling, such as injuries, osteoarthritis, and bursitis, as well as less common causes, such as Baker's cysts and reactive arthritis.

Whether water on the knee is mildly annoying or painfully debilitating, a person will want to identify the likely cause and treat the symptoms to help mitigate future problems. Chronic or long-standing swelling may lead to joint tissue damage, cartilage degradation, and bone softening, therefore treatment is usually recommended.

See How to Care for a Swollen Knee

    1. Injury to the Knee
    A trauma to the knee's bones, ligaments, tendons, bursae, meniscus, or articular cartilage can cause pain and swelling. Serious injury can cause blood to flood into the knee joint, leading to significant swelling, warmth, stiffness, and bruising.

    This condition is called "haemarthrosis" and warrants urgent medical care. A patient should also seek medical attention if knee pain is severe, if the affected leg cannot bear weight, or if the patient suspects a bone may be broken.

    2. Knee Osteoarthritis
    Degeneration of the cartilage of the knee joint can result in an overproduction of joint fluid, causing the knee to swell. A swollen knee due to knee osteoarthritis is typically accompanied by pain.

    Read more: What Is Knee Osteoarthritis?

In This Article:

Overuse and degeneration of the knee joint may cause water on the knee. Learn more: Knee Osteoarthritis Symptoms

    3. Septic and Non-Septic Bursitis
    Throughout the body are tiny, thin, fluid-filled sacs called bursa that normally protect joints. An inflamed knee bursa can fill with excess fluid, causing swelling, or water on the knee.

    See What Is a Bursa?

    The swollen knee may feel "squishy" and may or may not be painful. The most common types of knee bursitis are prepatellar bursitis and pes anserine bursitis.

    A bursa that has been infected with a microorganism can become inflamed and fill with pus. The swollen knee may appear red and feel hot. Patients should seek medical attention immediately if they suspect symptoms are caused by septic bursitis.

    Read more about Septic Bursitis

    4. Gout
    A painful accumulation of microscopic uric acid crystals in the joint defines a gout attack. Knee swelling may occur rapidly and be accompanied by excruciating pain, redness, and warmth.

    See Hyperuricemia - High Uric Acid Levels and Gout

    Approximately half of gout cases affect the big toe while other cases typically affect the knee, wrist or fingers.2

    See All About Gout - Symptoms, Diagnosis, Treatment

    5. Pseudogout
    Less common but similar to gout, pseudogout is an accumulation of calcium pyrophosphate crystals in a joint.

    The accumulation of crystals typically causes sudden, severe pain and swelling. Pseudogout occurs most frequently in the knee.

    See All About Pseudogout - Symptoms, Diagnosis, Treatment

    6. Rheumatoid Arthritis
    An autoimmune disease that affects the delicate lining of the joints, rheumatoid arthritis can cause knee swelling, stiffness, pain, tenderness, and redness. Symptoms often occur on both sides of the body, so if the right knee is affected it's likely the left knee is also affected.

    Although the knees can exhibit symptoms, the hands, wrists and feet are more often affected by rheumatoid arthritis.

    Read more: What Is Rheumatoid Arthritis?


  1. Johnson MW. Acute knee effusions: a systematic approach to diagnosis. Am Fam Physician. 2000 Apr 15;61(8):2391-400. Review. PubMed PMID: 10794580.
  2. Center for Disease Control and Prevention. Arthritis: Gout. Last reviewed: August 1, 2011.
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