Some knee joint anatomy

The first diagram below illustrates a typical normal knee joint looking from the side.

What is a Baker's cyst?

A Baker's cyst is a fluid filled swelling that can develop behind the knee. It is called a Baker's cyst as it is named after a doctor called William Baker who first described this condition in 1877. It is also sometimes called a popliteal cyst as the medical term for the area behind the knee is the popliteal fossa.

The cyst can vary in size from a very small cyst to a large cyst that is a number of centimetres across. Rarely, a Baker's cyst can develop behind both knees at the same time.

What causes a Baker's cyst?

There are two ways in which a Baker's cyst may form:

A 'primary' Baker's cyst

A Baker's cyst may develop just behind an otherwise healthy knee joint. This type of cyst is sometimes referred to as a 'primary' or 'idiopathic' Baker's cyst. It usually develops in younger people and children.

It is thought that in this type of Baker's cyst there is a connection between the knee joint and the popliteal bursa behind the knee. This means that synovial fluid from inside the joint can pass into the popliteal bursa and a Baker's cyst can form.

A 'secondary' Baker's cyst

Sometimes a Baker's cyst can develop if there is an underlying problem within the knee such as arthritis (including osteoarthritis and rheumatoid arthritis), or a tear in the meniscus cartilage that lines the inside of the knee joint. This type of Baker's cyst is the most common. It is sometimes referred to as a 'secondary' Baker's cyst.

In a secondary Baker's cyst, the underlying problem within the knee joint causes too much synovial fluid to be produced within the joint. As a result of this, the pressure inside the knee increases. This has the effect of stretching the joint capsule. The joint capsule bulges out into the back of the knee, forming the Baker's cyst that is filled with synovial fluid.

Who gets a Baker's cyst?

A Baker's cyst most commonly occurs in children aged 4 to 7 years and in adults aged 35 to 70 years. However, Baker's cysts are much more common in adults than children. You are more likely to develop a Baker's cyst if you have an underlying problem with your knee.

Arthritis is the most common condition associated with Baker's cysts. This can include various different types of arthritis such as osteoarthritis (most common), rheumatoid arthritis, psoriatic arthritis and gout.

Baker's cysts may also develop if you have had a tear to the meniscus or one of the ligaments within the knee or if you have had infection within the knee joint.

Does a Baker's cyst cause any symptoms?

Some people with a Baker's cyst do not have any symptoms. Also, small cysts may not always be found when a doctor examines your knee. The cyst may be found incidentally when having an investigation on your knee such as an MRI scan for some other reason.

In general, the larger the Baker's cyst, the more likely it is to produce symptoms. You may be able to see or feel the swelling behind your knee. Sometimes you may also notice that the knee joint itself is swollen. Some people feel an ache around the knee area. It may be difficult to bend your knee if you have a large Baker's cyst and the area behind your knee may feel 'tight', especially when you are standing up. Less commonly you may feel a sensation of clicking or locking of your knee.

If you have an underlying knee problem such as arthritis, you may also have symptoms related to that, such as knee pain.

Are there any complications that can develop with a Baker's cyst?

The most common complication of a Baker's cyst is for the cyst to rupture (split open). If this happens, the fluid from inside the cyst can leak out into the calf muscle. This can cause swelling of the calf. You may also develop itching and redness of the skin of your calf because of irritation caused by the fluid that leaks out from the cyst. About 1 or 2 in 20 Baker's cysts are thought to rupture.

If a Baker's cyst ruptures, it can be quite difficult to tell the difference between a ruptured Baker's cyst and a deep vein thrombosis in the leg. A deep vein thrombosis (DVT) is a blood clot that forms in a leg vein. In these cases, it is important that investigations are carried out to exclude a deep vein thrombosis because it can be a serious condition that needs treatment.

Having a Baker's cyst can also increase your risk of developing a deep vein thrombosis at the same time, even if the cyst does not rupture. For this reason, anyone who is found to have a Baker's cyst should also have a DVT excluded. Also, anyone who is found to have a DVT should be examined and investigated for a possible Baker's cyst. See separate leaflet called 'Deep Vein Thrombosis (DVT)' for more detail.

Very rarely, a Baker's cyst may become infected.

How is a Baker's cyst diagnosed?

Your doctor may suspect a Baker's cyst when they examine your knee. The area behind the knee may be swollen. Your doctor may shine a light through the swelling which can help to demonstrate that the swelling is full of fluid. That is, the swelling is a cyst.

An ultrasound scan is a good investigation to help diagnose a Baker's cyst as it can show a Baker's cyst and also help to exclude a DVT at the same time. Sometimes a CT or an MRI scan is used to help diagnose a Baker's cyst.

What is the treatment for a Baker's cyst?

A Baker's cyst often gets better and disappears by itself over time. However, it may persist for months or even years before it goes. In a lot of people it causes little in the way of symptoms and no specific treatment is needed.

However, there are various treatment options that may help if you do have symptoms associated with a Baker's cyst. These include:

Treatment of any underlying knee problem

It is important that any underlying knee problem is treated if you have a Baker's cyst. This may help to reduce the size of a Baker's cyst and any swelling or pain that it causes.

For example, if you have osteoarthritis, a steroid injection into the knee may help to relieve pain and inflammation. But note: this does not always stop the cyst from coming back again. See separate leaflet on osteoarthritis and its treatment, called 'Osteoarthritis'.

If you have an injury to the knee such as a meniscal tear, treatment of this may help to treat the Baker's cyst as well. A separate leaflet discusses knee injury and meniscal tears in more detail.

Treatment to help relieve symptoms

If you have pain and discomfort because of your Baker's cyst, one or more of the following may be helpful:

• Support bandaging (sometimes called a Tubigrip®) - This provides compression and can help to reduce the swelling.
• Non-steroidal anti-inflammatory drugs (NSAIDs) - These can help to relieve pain and may also limit inflammation and swelling. There are many types and brands. You can buy two types (aspirin and ibuprofen) at pharmacies without a prescription. You need a prescription for the others. Side-effects sometimes occur with NSAIDs. Stomach pain, and bleeding from the stomach, are the most serious. Some people with asthma, high blood pressure, kidney failure, and heart failure may not be able to take NSAIDs. So, check with your doctor or pharmacist before taking them to make sure they are suitable for you.
• Ice - This may also help to reduce swelling and pain. Make an ice pack by wrapping ice cubes in a plastic bag or towel. (Do not put ice directly next to skin as it may cause 'ice-burn'.) A bag of frozen peas is an alternative. Apply the ice pack for 10-30 minutes. Less than 10 minutes has little effect. More than 30 minutes may damage the skin.
• Crutches - It may be necessary to use crutches to get about until your symptoms ease. They help to take the weight off the affected leg while you are walking.
• Physiotherapy - Keeping your knee joint moving and using strengthening exercises to help the muscles around your knee may be helpful.

Other treatments

There are some other treatment options that are sometimes used:

• Fluid drainage - Sometimes your doctor may use a needle to drain excess fluid from your knee joint to help relieve your symptoms. However, it is often the case that the Baker's cyst can reform over time.
• Surgery to remove the cyst - This is sometimes done, especially if a cyst is very large or painful and/or other treatments have not worked. Sometimes a 'keyhole' method may be used to close off the connection between the Baker's cyst and the knee joint. The cyst is also sometimes removed using 'open' surgery. Surgery may also be done to treat an underlying problem at the same time. For example, repairing a meniscal tear.