The popliteal cyst: a pocket of fluid in the knee
A popliteal cyst, often called a ‘Baker's cyst’, is a pocket filled with synovial fluid (the same fluid found in the joints) that forms at the back of the knee, in the popliteal fossa. This pocket, which is generally benign, often develops in response to another knee condition, such as osteoarthritis, meniscus damage or inflammation of the joint.
How does a popliteal cyst form?
A popliteal cyst forms when synovial fluid, normally present in the knee joint, seeps through a small opening in the joint capsule and forms a pocket at the back of the knee. This pocket can gradually increase in size and sometimes rupture, causing sudden pain and inflammation in the calf.
The symptoms of a popliteal cyst can vary depending on its size and location. The most common are
- A palpable mass at the back of the knee, often painless.
- A feeling of tension or swelling in the calf.
- Pain in the knee or calf, which may worsen with physical activity.
- A feeling of heaviness in the leg.
What causes popliteal cysts?
The main cause of a popliteal cyst is an increase in pressure inside the knee joint, often due to :
- Osteoarthritis: Wear and tear of the cartilage leads to excessive production of synovial fluid.
- A meniscus lesion: A torn meniscus can cause inflammation and an increase in the production of synovial fluid.
- Inflammation of the joint: Rheumatoid arthritis, for example, can lead to the formation of popliteal cysts.
How is a popliteal cyst diagnosed?
A popliteal cyst is usually diagnosed by a doctor during a clinical examination. The doctor will palpate the mass and may order additional tests such as:
- Ultrasound: to confirm the presence of the cyst and assess its size.
- MRI: to give a more detailed view of the structures in the knee and look for any associated lesions.
What treatments are available?
Treatment of a popliteal cyst depends on the size of the cyst, the symptoms and the underlying cause. Treatments may include
- Rest: To relieve pain and reduce inflammation.
- Ice: To reduce swelling.
- Anti-inflammatory medication: To relieve pain.
- Puncture: To aspirate the fluid contained in the cyst.
- Surgery: In the most severe cases or in cases of frequent recurrence, surgery may be considered to remove the cyst.
