Overview

A Baker's cyst, also called a popliteal cyst, is a fluid-filled swelling that develops at the back of the knee.

It's caused when the tissue behind the knee joint becomes swollen and inflamed. The swelling and inflammation can cause:

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

However, sometimes a Baker's cyst may not cause any symptoms other than the fluid-filled swelling at the back of the knee.

A Baker's cyst can sometimes burst (rupture), resulting in fluid leaking down into your calf. This can cause sharp pain, swelling and redness in your calf, but redness can be harder to see on brown and black skin.

What causes a Baker's cyst?

Knee damage caused by a sports injury or a blow to the knee can lead to a Baker's cyst developing.

A Baker's cyst can also sometimes occur if you have a health condition such as:

• osteoarthritis – usually caused by age-related "wear and tear" of joints; it particularly affects the knees, hips, hands and big toe
• inflammatory arthritis – including rheumatoid arthritis which is a less common type of arthritis and is caused by the immune system attacking the joints
• gout – a type of arthritis that usually affects the big toe and is caused by a build-up of the waste product uric acid in the blood

Baker's cysts usually develop in people aged 30 to 70, although they can affect people of any age, including children.

When to see your GP

See your GP if you have a lump behind your knee that's causing problems and does not clear up on its own. They'll usually be able to diagnose a Baker's cyst by examining the back of your knee and asking about your symptoms.

Your GP will ask you whether you have any associated health conditions, such as arthritis.

Tests may be recommended to rule out other more serious conditions, such as a tumour, an aneurysm (bulge in a section of a blood vessel) or DVT (deep vein thrombosis) (a blood clot in one of the deep veins of the body). You may need an ultrasound scan or an MRI scan.

Treating a Baker's cyst

Treatment will not usually be necessary if you have a Baker's cyst that isn't causing any symptoms.

Painkillers such as parcetamol and ibuprofen can be used to reduce the swelling and relieve any pain. Bandages or an ice pack may also help. A bag of frozen peas wrapped in a tea towel works well as an ice pack.

If you have an underlying condition that's causing your cyst, it's important that the condition is properly managed. The cyst may disappear when the condition causing it has been treated.

In some cases, it may be possible to drain the cyst. Surgery may also be needed to repair any significant damage around the knee joint.

Treatment

Treatment for a Baker's cyst isn't needed if you don't have any symptoms. Any symptoms you have can be treated at home.

Further treatment will only be needed if the cyst stops you using your knee or causes persistent pain.

Self care

To treat a Baker's cyst you can:

• take non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, to reduce swelling and pain in the affected knee
• hold an ice pack to your knee for 10-20 minutes to reduce any swelling – try a bag of frozen peas wrapped in a tea towel (never put ice directly on your skin)
• rest your knee joint
• wear a knee support – you can buy these from a pharmacy

Further treatment

See your GP for further treatment if your cyst still causes problems after you've tried the self care treatments.

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

Treating a ruptured cyst

A Baker's cyst can sometimes rupture (burst), resulting in fluid leaking down into your calf. This causes a sharp pain in your calf, which becomes swollen, tight and red, but redness can be harder to see on brown and black skin.

The fluid will gradually be reabsorbed into the body within a few weeks. The recommended treatment for a ruptured cyst is rest and elevation (keeping the affected calf raised).

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

Surgery to repair knee damage

In some cases, it may be possible to drain (aspirate) a Baker’s cyst. However, it may not be possible to aspirate long-term (chronic) cysts because they’re made of jelly.

Surgically removing a Baker’s cyst is not easy because, unlike other types of cyst, they do not have a lining.

Surgery may be needed to repair your knee joint if it's significantly damaged as a result of an injury or a condition such as osteoarthritis.

A type of keyhole surgery called arthroscopy, is usually used. The surgeon will use an instrument called an arthroscope to look inside your knee joint. Small surgical instruments can be used alongside the arthroscope to repair any damage.