What is a Baker's cyst?

A Baker's cyst, or popliteal cyst, is a fluid-filled lump or swelling behind the knee. It is usually caused by excess fluid that comes from the knee joint.

People with a Baker's cyst might also have arthritis or an injury such as a torn cartilage in their knee.

What are the symptoms of a Baker's cyst?

If the cyst is small, there may not be any symptoms and you might not notice it.

Other symptoms may include:

• pain
• swelling
• a lump behind your knee
• pressure when you straighten your leg
• stiffness or tightness in your knee

What are the causes of a Baker's cyst?

Common causes are:

• injury to the knee
• torn cartilage
• arthritis especially rheumatoid arthritis and osteoarthritis
• infection

Baker's cysts can sometimes develop for no known reason — they can sometimes just happen.

How is a Baker's cyst diagnosed?

A doctor can usually diagnose a Baker's cyst by examining the knee and shining a torch through the lump to see whether it is filled with fluid. They might order an x-ray to check for arthritis in the knee, which may be causing the problem.

You might also need an ultrasound or MRI scan.

How is a Baker's cyst treated?

In adults, Baker's cysts often get better and go away by themselves, so they may not need any treatment. If a child has a painless Baker's cyst, it can usually be left alone. In children, most Baker's cysts disappear eventually without treatment.

You can ease any pain or swelling from a Baker's cyst with cold packs, or by taking paracetamol or anti-inflammatory medicines and using crutches to take weight off the knee.

The pain may also ease with light physical activities such as walking or swimming, that do not aggravate the knee join.

Maintaining or reaching a healthy body weight may help.

If your Baker's cyst does not go away or is causing you a lot of pain and discomfort, your doctor may suggest treatment. It can be drained under ultrasound guidance or injected with corticosteroids. This has been shown to improve symptoms and there is a low chance that the cyst will come back.

If the cyst is associated with arthritis or a knee injury, then treating these conditions can help. Another treatment is physiotherapy using hot or cold packs and exercises to keep the knee strong and mobile.

For some people, surgery is needed to fix the cause. For example, repairing a torn cartilage might help. In this situation, removing the cyst alone is not helpful, as it usually comes back unless the underlying cause is addressed.

Your doctor can advise you on the treatment options that are best for you.

What are the possible complications of a Baker's cyst?

Occasionally Baker's cysts become infected or grow bigger.

Sometimes a Baker's cyst bursts beneath the skin, causing pain and swelling in the calf. In this case, your doctor may want to rule out a serious problem such as a blood clot (deep vein thrombosis).

Can I prevent Baker's cysts?

Sporting activities can lead to knee injuries, which can cause Baker's cysts or having a Baker's cyst return.

Here are some tips to prevent knee injuries:

• Before and after exercise or sports, make sure to warm up and cool down.
• Wear supportive shoes.
• When moving, turn on the balls of your feet instead of at your knees.
• If you hurt your knee, stop what you are doing and use ice packs to reduce swelling, and speak with a doctor.

Resources and support

If you think you may have a Baker's cyst speak to your doctor, a sports medicine professional or physiotherapist.

You can read the Musculoskeletal Australia Baker's cyst fact sheet, or call their helpline and speak to one of their nurses on 1800 263 265.

The Royal Children's Hospital also has a Baker's Cyst fact sheet and explains how they form in children.