Swollen knee

A swollen knee means there's a fluid on the knee. It’s sometimes called knee effusion or water on the knee.

By Wallace Health I Medically reviewed by Adrian Roberts.
Page last reviewed: October 2018 I Next review due: October 2021


Knees are very prone to swelling and pain because they're weight bearing. The most common causes are:

  • Injury or trauma
  • Overuse
  • Underlying disease

Knee inflammation (swelling) can cause these symptoms:

  • Difficulty bending or straightening your knee completely
  • Unable to put your weight on the knee
  • Knee may be red and warm
  • Knee may be tender
  • Occasionally, ‘locking’ of the joint can occur

Often you'll also experience knee pain. The onset of swelling and the type of pain varies depending on the cause. 

In many cases, swelling can be treated at home with painkillers and by elevating your leg. However, you should see your doctor if you think it’s infected, or if swelling doesn’t go down after a few days. Signs of an infection are:

  • Rapid onset of swelling
  • Tenderness and pain
  • Hot to touch

Causes of swollen knee

If your pain and swelling has come on suddenly, it may be due to an injury from playing sport or following an accident or fall. Common injuries include:

If your swelling has come on gradually without any obvious trigger, it could be due to overuse of the knee joint. This often happens if you play a lot of sport or through repeated bending or kneeling. Conditions include:

  • Bursitis knee (sometimes called housemaid’s knee)
  • Tendinopathy

Chronic (long-term) swelling – often with night-time pain - could be due to an underlying disease. These conditions are more likely as you age and include:

Many of these injuries and knee problems can also lead to something called a Baker’s cyst, which is when joint fluid leaks out into the back of the knee and causes it to swell.

Other less common causes of a swollen knee are:

  • Reactive arthritis
  • Septic arthritis
  • Tumours
  • Bleeding in the joint (haemarthrosis) caused by an injury – this needs urgent medical treatment
  • An infection

Talk to your doctor if you’re concerned about symptoms

Book an appointment with a Spire GP today

Getting a diagnosis for swollen knee

Chronic swelling can cause permanent damage to the joint tissue, cartilage and bone so it’s important to ask your doctor for advice if swelling doesn’t go down.

They'll discuss your symptoms and carry out a physical examination.

They may also arrange for you to have some tests. These may include:

They may also arrange for you to have joint aspiration. This is when a fine needle is inserted into the swollen area to check for blood, bacteria or crystals (which are often found in people who have gout or pseudogout).

Treatments for swollen knee

Your treatment will depend on what’s causing the swollen knee. It'll also depend on how painful it is. In most cases, you’ll be advised to take painkillers. You can also apply ice and elevate your knee to help reduce the swelling.

However, treating the cause of the inflammation may involve:

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