Baker’s cyst – causes, symptoms and treatment

What is a Baker’s cyst?

A Baker’s cyst is a kind of ‘mucous sac’ in the back of the knee, filled with a gelatinous fluid. The cyst is connected to the structures of the knee joint – often by a narrow stalk. Small cysts cause little or no discomfort, but larger ones can be felt in the back of the knee as a firm swelling and may restrict movement in the knee joint. They can also be uncomfortable or painful. The cyst often disappears if the knee is kept still for a while, but usually reappears afterwards. Sometimes the cyst can rupture. In this case, the leg also becomes very sensitive to pain. The condition is most common in adults and becomes more frequent with age. Such cysts are rare in children, but when they do occur, they often develop in both knees at the same time. In very rare cases, a Baker’s cyst is congenital.

What causes a Baker’s cyst?

Like any joint, the knee is surrounded by a connective-tissue-like sheath known as the joint capsule. On the one hand, this helps to stabilise the joint; on the other, its inner layer produces what is known as synovial fluid (synovia). This acts as a kind of ‘joint lubricant’, reducing friction between the joint surfaces. The synovial fluid also supplies the joint cartilage with nutrients and helps to cushion the joint. If the knee joint is damaged or inflamed, the body reacts by producing more synovial fluid. However, this also increases the pressure within the joint capsule. If this pressure becomes too high, the capsule may thin at a weak point and bulge out like a sac. Such a weak spot is located at the back of the knee joint capsule and manifests as a cyst in the back of the knee. Typically, a Baker’s cyst develops on the inside of the back of the knee, between the insertions of a calf muscle and a large posterior thigh muscle.

Illustration of a Baker’s cyst

What are the most common symptoms of a Baker’s cyst?

The symptoms of a Baker’s cyst depend to a large extent on its size. Smaller cysts often cause no symptoms, but the larger the Baker’s cyst, the more likely it is to cause discomfort. The size of such a cyst depends, on the one hand, on how long it has had to develop and, on the other hand, varies with the mechanical stress placed on the affected knee. The latter can be explained by the fact that the body reacts to heavy strain on an already damaged joint with an increased inflammatory response and, consequently, increased production of synovial fluid. Consequently, a Baker’s cyst swells further under strain – for example, during sport or physical work – and shrinks again when the patient rests the knee for a few days. However, it is important to treat the underlying condition. If this is not done, the Baker’s cyst usually continues to grow in size and will eventually cause symptoms. These symptoms may include:

• a palpable swelling in the back of the knee once the Baker’s cyst has reached a certain size (from around two centimetres)
• a noticeable movement of fluid under the skin of the back of the knee
• a vague sensation of pressure at the back of the knee. The back of the knee and upper calf may also become increasingly painful.
• Circulatory problems and numbness, or even paralysis, in the lower leg and foot (this occurs when the Baker’s cyst presses on blood vessels and nerves in the back of the knee).

What happens if a Baker’s cyst bursts?

A large Baker’s cyst that is distended with fluid can rupture. This causes synovial fluid to leak into the surrounding tissue. The mechanical pressure exerted by the fluid causes severe pain. It also leads to severe inflammation in the tissue. This results in further swelling, increased blood flow and more pain. Movement of the knee joint is severely restricted; the pain is intense and diffuse and may also be felt in the calf region. Due to the general swelling and the additional pressure exerted by the synovial fluid within the tissue, a condition known as compartment syndrome may develop. This means that high pressure builds up, which can cause permanent damage to the surrounding nerves, blood vessels and muscles. Furthermore, the tissue that should normally be supplied by the blood vessels is under-supplied. The person affected experiences severe pain. If the tissue remains under-supplied over the long term, it may die off. Numbness or paralysis in the area supplied by the compressed nerves is also possible. Prolonged compression can lead to irreversible nerve damage.

How is it diagnosed?

The diagnostic process begins with taking a medical history and investigating any previous conditions or symptoms affecting the knee. During the subsequent physical examination, the swelling is palpated and, as a rule, an ultrasound scan is carried out to confirm whether it is indeed a Baker’s cyst and to determine its stage. Another method for diagnosing a Baker’s cyst is magnetic resonance imaging (MRI). Fluid accumulations in the tissue are clearly visible on MRI scans. Furthermore, MRI often reveals the cause of the Baker’s cyst, such as meniscal damage or joint wear and tear (osteoarthritis).

What complications can arise with Baker’s cysts?

A Baker’s cyst can lead to various complications, particularly if it remains untreated or grows in size. Here are some of the possible complications:

• Rupture of the cyst: One of the most common complications is the cyst bursting. This can lead to sudden, sharp pain in the back of the knee, followed by pain and swelling along the calf. The leakage of fluid into the surrounding tissue can also trigger an inflammatory reaction.
• Oedema: If the cyst is large enough, it can press on blood vessels, leading to a build-up of fluid (oedema) that causes the calf to swell.
• Limited range of motion: A large Baker’s cyst can restrict the mobility of the knee, making it painful or difficult to bend or straighten the knee.
• Inflammation: If the Baker’s cyst bursts, the surrounding tissue may become inflamed. However, complications are generally very rare.

How is a Baker’s cyst in the knee treated?

The treatment used for a Baker’s cyst depends on the stage of the cyst and the individual circumstances of the patient. If the Baker’s cyst does not cause any symptoms, it does not necessarily need to be treated, as cysts can sometimes resolve on their own. The primary aim is to treat the symptoms – particularly any pain. Various painkillers and anti-inflammatory drugs are used for this purpose, either taken orally or administered by a doctor via injection. If necessary, cortisone can be injected directly into the affected knee joint to stop the inflammatory process there. If severely damaged joint cartilage is the cause of the Baker’s cyst, additional injections of hyaluronic acid can also provide long-term relief for the knee joint. As regards the treatment of the underlying cause of a Baker’s cyst, the knee injury, joint wear and tear or inflammation must be treated. If this is not done, it is very likely that the Baker’s cyst will repeatedly fill with fluid and cause symptoms. Medication is available to treat inflammation. Damage to the joint can also be treated surgically, for example through minimally invasive arthroscopy. The aim is always to repair the joint damage causing the Baker’s cyst and to stabilise the knee. Physiotherapy or occupational therapy can also be helpful. If the cause of the Baker’s cyst can be resolved by treating the knee, the cyst will often resolve on its own. If this does not happen, the doctor can drain it. This involves aspirating the accumulated synovial fluid using a syringe. The now empty cavity can then be flushed with a cortisone solution to prevent any inflammation. As regards surgery for a Baker’s cyst, the size of the cyst and the patient’s level of discomfort are decisive factors. Many experts advise exhausting all options for conservative treatment before surgery, as the procedure usually requires an incision in the back of the knee. Newer techniques utilise bipolar current, which is applied to the cyst wall via electrodes after the cyst’s contents have been drained. The heat generated by this causes the cyst wall to shrink and seal, thereby preventing further flow of synovial fluid. A ruptured Baker’s cyst must be treated without delay. As much synovial fluid as possible is aspirated, and further measures may be taken. The affected leg must be rested and kept elevated. Painkillers and anti-inflammatory medication can be taken to relieve pain and reduce inflammation. Thrombosis prophylaxis is also recommended.

Which doctor treats a Baker’s cyst?

If a Baker’s cyst is suspected, your GP may be the first point of contact. Subsequently, and as the symptoms affect the knee joint, orthopaedic surgeons are the specialists for Baker’s cysts. If a Baker’s cyst has ruptured, you should contact the nearest hospital immediately.