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Knee effusion, sometimes called water on the knee, occurs when excess fluid accumulates in or around the knee joint. Common causes include arthritis and injury to the ligaments or meniscus, which is cartilage in the knee.

A small amount of fluid exists in normal joints. In some cases, however — for example, with rheumatoid arthritis — excess fluid can build up, and the knee becomes puffy and swollen. Statistics suggest that 27% of people will experience fluid on the knee at some time.

In this article, we will discuss the treatments, symptoms, and causes of water on the knee and some ways to prevent it.

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Most treatments for knee joint effusion depend on the underlying cause, and treatment will vary between individuals.

Options include:

Excess fluid will cause puffiness around the bony parts of the knee. One knee may look and feel bigger than the other. It may be difficult to bend or straighten the knee.

Typical symptoms include:

  • swelling
  • stiffness
  • a reduced range of motion

The following symptoms need urgent medical attention:

  • inability to put weight on the joint
  • no pulse in the feet
  • loss of feeling below the knee
  • fever

Other symptoms will depend on the underlying cause. For example:

  • Osteoarthritis: Pain and stiffness tend to be worse first thing in the morning but often improve within 30 minutes of getting up.
  • Rheumatoid arthritis: Pain and swelling often affects several joints and often symmetrically. The joint may feel hot to the touch. Pain persists longer than 30 minutes after waking.
  • Traumatic injury: There may be bruising and severe pain, and it may be hard to put weight on the knee.
  • Infection: The knee may be hot, and there may also be fever.

Along with medical care, self-care measures can also help relieve knee joint effusion.

They include:

  • resting the leg
  • raising the leg
  • placing ice wrapped in a towel on the knee
  • using a knee support
  • using a mobility aid, such as a cane
  • following weight management strategies to reduce strain on the knee joint for those with arthritis
  • doing low-impact exercise, such as tai chi, swimming, and yoga, especially for people with arthritis

Ice packs are available for purchase online.

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A traumatic knee effusion may feature puffiness or swelling of the joint and surrounding area. Photo credit: James Heilman, MD.

Inflammation and swelling can result from a traumatic injury, arthritis, or an infection. Fluid on the knee can cause discomfort and is usually a sign that something is wrong.

The type of fluid that accumulates around the knee depends on the underlying disease, condition, or type of traumatic injury that caused the excess fluid. A doctor may remove some of the fluid and send it for testing in the lab. The results will show them what caused the problem and how to treat it.

Tests on fluid may show that one of the following has occurred or is present:

  • a traumatic injury, leading to a fracture or ligament damage
  • rheumatoid arthritis
  • osteoarthritis
  • gout
  • pseudogout
  • septic arthritis
  • tuberculous arthritis
  • juvenile rheumatoid arthritis
  • overuse, due to certain physical activities or sports
  • infections, such as Lyme disease or syphilis
  • reactive arthritis
  • inflammatory bowel disease
  • hemarthrosis, or bleeding into the joint

Excess weight and obesity may also increase the risk by placing strain on the knee.

The time it takes to recover from knee effusion depends on the reason for the problem and recommended treatment course. Experts suggest starting treatment within 24–48 hours of its onset to prevent complications, but the treatment and recovery time will depend on the cause and other factors, such as the person’s age and overall health.

With treatment, many types of knee effusion will resolve. However, they may come back if they result from a chronic condition, such as arthritis.

Tips for preventing water on the knee include:

  • staying active with low-impact sports, such as walking and swimming
  • avoiding sudden jolting movements and rough running surfaces
  • avoiding repetitive movements, when possible
  • maintaining a moderate weight
  • seeking medical attention for chronic conditions such as arthritis

Knee effusion, commonly known as water or fluid on the knee, can occur for various reasons, ranging from a traumatic injury to a systemic or chronic disease such as rheumatoid arthritis.

Anyone who experiences new, sudden, or worsening swelling in the knee should seek medical advice, as leaving it may result in complications.

A person needs immediate medical attention if they cannot put weight on the knee, if there is no feeling or pulse below the knee, or if they have a fever.