A Baker’s cyst is a localized accumulation of synovial fluid (joint fluid) behind the knee. It results from abnormal swelling of a bursa in that location. A bursa is a fluid-filled sac that normally decreases friction (resulting from the movement of joints) between tendons, muscles, and bones.
This condition, named after William Morrant Baker, a British surgeon who first described it, is usually not serious. It does not cause any long-term damage and usually improves on its own with time. In some cases, however, surgery may be necessary.
The exact cause of some Baker’s cysts is unknown. Half of incidences occur in children, appearing as painless masses behind the knee that are more obvious when the knee is fully extended. This type may be formed by the connection of a normal bursa with the knee joint.
Other Baker’s cysts come from many conditions that cause swelling and/or damage to the knee joint, including:
- Arthritis—Osteoarthritis is the most common type associated with Baker’s cysts
- Cartilage tears, such as a torn meniscus.
- Injury or accidents
- Joint effusion
The following factors increase your chances of developing a Baker’s cyst. If you have any of these risk factors, tell your doctor:
- Rheumatoid arthritis
- Past knee injuries or cartilage tears
A Baker’s cyst may exhibit no symptoms at all. The following are common symptoms of a Baker’s cysts. If you experience any of these symptoms, do not assume it is due to a Baker’s cyst. These symptoms may be caused by other, less serious health conditions. If you experience any of them, see your doctor:
- Rounded swelling behind the knee about the size of a golf ball
- Pain or pressure in the back of the knee joint, which can travel to the calf muscle.
- Aching or tenderness after exercise and bending the knee
Your doctor will ask about your symptoms and medical history. During a physical exam, the doctor will look for a soft mass in the back of the knee, and compare range of motion (and limitations) of the affected knee to the normal knee. The following tests or procedures may be done:
- Shining a light through the cyst to show the mass is fluid-filled. This is called “trans-illumination.”
- Ultrasound and CT scans—tests useful in sorting out other causes of knee swelling
- MRI scan—a test that uses magnetic waves to make pictures of structures inside the body. This can be helpful to visualize the cyst and show any meniscal injury.
- X-rays may be taken. A Baker’s cyst will not appear on X-rays but they can show arthritis or other knee abnormalities.
If the cyst is growing rapidly and accompanied by fever, blood samples may be taken to rule out more serious tumors that could occur behind the knee.
Often, no treatment is necessary, since many Baker’s cysts resolve on their own within a two-year period. Talk with your doctor about the best treatment plan for you. Treatment options include:
- Draining excess knee fluid
- Medications to relieve pain and inflammation such as nonsteroidal anti-inflammatory drugs (NASIDs)
- Resting and elevating the leg
- Physical therapy
- Pain medications
- Wearing an elastic bandage (ie, knee brace)
- Surgery is rarely required to remove cysts
There is no way to prevent a Baker’s cyst.