A Baker's cyst is a swelling that can develop behind the knee. It is filled with synovial fluid which is the lubricating fluid that is usually found inside the knee joint. It most commonly occurs if there is an underlying problem with your knee such as osteoarthritis. Symptoms can include pain, swelling and tightness behind the knee. Rarely, a Baker's cyst can split open (rupture) and cause similar symptoms to a deep vein thrombosis (DVT). A Baker's cyst often gets better and disappears by itself over time. However, there are various treatments that may help if you do have symptoms associated with it.
Some knee joint anatomy
The first diagram below illustrates a typical normal knee joint looking from the side.
The joint capsule is a thick structure that surrounds your whole knee and gives it some support. It is lined by a special membrane called the synovium. The synovium produces a fluid called synovial fluid. This fluid acts as a lubricant within your knee joint and helps to cushion it during movement.
There are also various tissue pouches called bursae next to the knee. A bursa is a small sac of synovial fluid with a thin lining. Bursae are normally found around joints and in places where ligaments and tendons pass over bones. They help to reduce friction and allow maximal range of motion around joints. The bursa at the back of your knee is called the popliteal bursa.
Each knee joint also contains a medial and a lateral meniscus. These are thick rubber-like pads of cartilage tissue. The menisci cartilage sit on top of, and are in addition to, the usual thin layer of cartilage which covers the top of one of the bones of the lower leg, called the tibia. They act as shock absorbers to absorb the impact of the upper leg on the lower leg. They also help to improve smooth movement and stability of the knee.
What is a Baker's cyst?
A Baker's cyst is a fluid-filled swelling that can develop behind the knee. It is one cause of knee pain.
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 your 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 in 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 or a tear in the meniscal 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 in 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.
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 you are 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?
The most common complication of a Baker's cyst is for it to split open (rupture). If this happens, the fluid from inside the cyst can leak out into your calf muscle. This can cause swelling of your 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 the ruptured cyst and a deep vein thrombosis (DVT) in the leg. A 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 DVT because it can be a serious condition that needs treatment. See separate leaflet called Deep Vein Thrombosis 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 your knee may be swollen. Your doctor may shine a light through the swelling. If the light passes through it, the swelling is full of fluid. The swelling is therefore a cyst.
An ultrasound scan is a good investigation to show a Baker's cyst and to help to exclude a DVT at the same time. Sometimes a magnetic resonance imaging (MRI) scan is used to confirm the diagnosis.
What is the treatment for a Baker's cyst?
A Baker's cyst often gets better and disappears by itself over time. Primary Baker's cysts in children all usually disappear with time. However, the cyst 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.
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. However, this does not always stop the cyst from coming back again.
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. See separate leaflets called Knee Ligament Injuries and Meniscal Tears (Knee Cartilage Injuries) for 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:
- 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 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.
- Stronger pain relief. If the cyst ruptures, the fluid from inside the cyst may leak into the calf and cause worse pain. In this situation, stronger medication may be needed.
- Ice 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.
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 to relieve your symptoms. However, it is common for the Baker's cyst to re-form over time.
- Cortisone (steroid) injection - this is sometimes used following fluid drainage, to reduce the pain and inflammation caused by the cyst. It does not prevent it from coming back again.
- 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 is 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 be carried out to treat an underlying problem at the same time - for example, repairing a meniscal tear.
Further reading and references
Herman AM, Marzo JM; Popliteal cysts: a current review. Orthopedics. 2014 Aug37(8):e678-84. doi: 10.3928/01477447-20140728-52.
Frush TJ, Noyes FR; Baker's Cyst: Diagnostic and Surgical Considerations. Sports Health. 2015 Jul7(4):359-65. doi: 10.1177/1941738113520130.
Zhou XN, Li B, Wang JS, et al; Surgical treatment of popliteal cyst: a systematic review and meta-analysis. J Orthop Surg Res. 2016 Feb 1511:22. doi: 10.1186/s13018-016-0356-3.
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