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Baker's cyst

Introduction

A Baker’s cyst, also called a popliteal cyst, is a fluid-filled swelling that develops at the back of the knee. It is caused by a problem with the knee joint or the tissue behind it.

The swelling may cause:

  • pain in the knee and calf
  • a build-up of fluid around the knee
  • occasionally locking or clicking in the knee joint

However, it may cause no symptoms at all.

Causes of a Baker's cyst include:

You can reduce the swelling and relieve any pain using ice, over-the-counter painkillers and bandages. The cyst may go away when the condition causing it has been treated. Find out more about treating a Baker's cyst.

A Baker's cyst is more common in women than men, probably because women are more likely to develop osteoarthritis and rheumatoid arthritis. It usually develops in people aged over 40, although it can affect people of any age, including children.

When to see your GP

See your GP if your cyst causes you problems and does not go away. It's important that any condition that causes it, such as arthritis, is properly managed.

In some cases, particularly if it's caused by osteoarthritis, the cyst may not go away and surgery may be needed to drain or remove it.

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Symptoms

In some cases of Baker’s cyst, there are no symptoms. The size of the cyst and the amount of swelling varies but large cysts usually cause more problems.

Symptoms may include; discomfort or pain behind the knee and sometimes a tightness that increases when you are standing. You may also find it difficult to bend your knee.

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Causes

A Baker's cyst is a build-up of fluid caused by a problem with the knee joint or the tissue behind it.

The knee joint can become damaged after a sports-related injury or blow to the knee.

A Baker's cyst can also be caused by the following health conditions:

  • Osteoarthritis – the most common type of arthritis, usually caused by age-related "wear and tear" of joints. It particularly affects the knees, hips, hands and big toe.
  • Rheumatoid arthritis – a less common but crippling type of arthritis, caused by the immune system attacking the joints. This leads to the joint tissues to become swollen, red and warm.
  • Gout – a type of arthritis that usually affects the big toe. It is caused by a build-up of the waste product uric acid in the blood.
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Diagnosis

Your GP can usually diagnosis a Baker’s cyst by examining your knee and asking about your symptoms.

They will want to know if you have any associated health conditions, such as arthritis.

If your GP cannot diagnose it in this way, they may recommend further tests. These will be used to rule out very rare but more serious conditions, such as a tumour or an aneurysm (a bulge in a section of a blood vessel).

The tests may include:

  • an ultrasound – where a scanner that produces high-frequency sound waves is moved over your body to create an image of the inside of your body
  • a magnetic resonance imaging (MRI) scan – where you lie inside a large tube and strong magnetic fields and radio waves produce a detailed image of the inside of your body
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Treatment

You can treat a Baker’s cyst yourself at home. Further treatment is only needed if your cyst stops you using your knee properly or causes pain that doesn't go away.

To treat a Baker's cyst:

  • Take non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, to reduce swelling and pain in the affected knee.
  • Hold an ice pack to the knee to reduce any swelling. Try a bag of frozen peas wrapped in a cloth (never put ice directly on your skin). 
  • Rest your knee joint.
  • Use compression bandages to support your knee joint. You can buy these from a pharmacy.

Further treatment

See your GP for further treatment if your cyst still causes problems after you have tried the treatments above.

One treatment option is to inject corticosteroid medication directly into the affected knee. This helps reduce inflammation and swelling.

Treating a ruptured cyst

In rare cases, a Baker’s cyst can burst, causing fluid to leak down into your calf. This can cause sharp pain and swelling in your calf. The fluid will gradually be reabsorbed into the body over one to four weeks.

Prescription painkillers – usually a combination of paracetamol and codeine – can be used to control any pain. See your GP for a prescription.

Surgery to repair damage to the knee

If there is a lot of damage to the knee joint, due to a condition such as osteoarthritis or a physical injury – surgery may be needed to fix the problem with the joint. This is usually done using a surgical technique called arthroscopy.

Arthroscopy is a form of keyhole surgery that is used to look inside a joint and repair or remove any damage that has occurred.

An instrument called an arthroscope is passed through the skin to light up and magnify the joint, so the surgeon can assess the inside of the joint and deal with any damaged tissue or bone. An arthroscope is a narrow tube with a light and camera attached that sends pictures back to a television monitor.

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