Baker’s cysts

When a Baker’s cyst occurs, fluid builds up at the back of the knee, causing a swelling.

The upper and lower leg bones meet at the knee joint. Between them is the narrow joint cavity. To keep the joint working smoothly, the cavity is filled with a fluid known as synovial fluid. The joint and joint cavity are surrounded by the joint capsule, which also contains cushioning sacs of fluid that are called bursae.

When a Baker’s cyst occurs, excess synovial fluid builds up in a bursa. The fluid often breaks through into the joint capsule and appears as a swelling at the back of the knee.

In adults, Baker’s cysts usually occur due to an injury or an inflammation in the knee joint. Depending on the size of the cyst, it may frequently be accompanied by a feeling of tightness or pain in the knee.

Small Baker’s cysts often go unnoticed and clear up on their own without treatment. Treatment is necessary if symptoms develop.

Baker’s cysts occur most frequently in women and men aged between 35 and 70.

Small Baker’s cysts often go unnoticed and cause no pain.

Larger cysts may cause the following symptoms:

• feelings of stiffness and tightness behind the knee
• pain in and behind the knee
• stiffness in the knee joint
• swelling at the back of the knee
• swollen calf

In addition, pain is often worse if the knee is moved a lot.

In adults, Baker’s cysts often develop after a knee injury, such as a torn meniscus, or as a result of a chronic joint disease, such as rheumatoid arthritis, gout or osteoarthritis.

Fluid from the joint cavity can flow or be squeezed through fine cracks into a bursa. The enlarged bursa becomes distended and appears as a swelling at the back of the knee.

The stability of the knee capsule declines with age. Frequent exertion or excessive strain can cause tiny cracks in the capsule. This, in turn, enables an exchange of fluid between the joint cavity and bursae in the knee capsule – increasing the risk of Baker’s cysts developing.

Older adults are also more likely to have suffered a knee injury at some point in their lives. Chronic joint diseases are also more likely to occur with age. Both of these factors increase the risk of developing a Baker’s cyst.

If a large volume of fluid builds up in a Baker’s cyst, it presses on the surrounding blood vessels. The cysts may also extend into the calf. In both cases, redness, fluid retention and swelling in the legs may occur.

Baker’s cysts occasionally burst. If the synovial fluid from the cyst flows into the surrounding tissue, it may cause inflammation. This causes sharp pain, swelling and skin discoloration. If nerves in the legs are affected, this may cause feelings of numbness and calf pain.

While the pain generally disappears after a few days, the swelling and restricted movement in the knee may last for longer.

A Baker’s cyst is most clearly visible when the affected leg is stretched out. The doctor can then see and feel the solid bulge where the fluid has accumulated. When the knee is bent, pressure is taken off the cyst and so it becomes softer or may even disappear entirely.

Further tests are required if the diagnosis is not clear. In these cases, doctors will use imaging techniques, such as:

• ultrasound
• magnetic resonance imaging (MRI)
• X-ray

These techniques make it easier to detect changes in joints or tissue, for example. Small cysts are also easier to see on an ultrasound scan.

Baker’s cysts are only treated if symptoms develop. It is important to avoid placing stress on the knee for a while.

Anti-inflammatory pain medication and physiotherapy may also help. The goal of physiotherapy is to keep th knee mobile and to stabilize it. This is achieved through targeted training of the leg muscles.