Baker’s Cyst

What is a Baker’s cyst?

A Baker’s cyst (popliteal cyst) is a fluid-filled sac that forms behind the knee.

The knee is a complex joint that has many parts. The lower end of the thighbone (femur) rotates on the upper end of the shinbone (tibia). The knee joint is filled with a special fluid (synovial fluid) that cushions the joint. 

A Baker's cyst forms when an injury or disease causes extra synovial fluid to leak into the extra space behind the knee.

Baker’s cysts are common in both adults and children. But they’re more common as a person gets older.

What causes a Baker’s cyst?

In adults, other problems with the knee joint often cause the Baker’s cyst. Injury or a knee disorder can change the normal structure of the knee joint. This can cause a cyst to form.

The synovial fluid inside the joint space may build up as a result of injury or disease. As the pressure builds up, the fluid may bulge into the back of the knee. This can cause the cyst.

What are the risks for a Baker’s cyst?

You may be more likely to get a Baker’s cyst if you have other problems of the knee joint, such as:

  • Osteoarthritis of the knee
  • Rheumatoid arthritis of the knee
  • Tearing of the pads of cartilage (menisci) inside the knee
  • Infectious arthritis

What are the symptoms of a Baker’s cyst?

A Baker’s cyst often doesn’t cause symptoms. A cyst will more often be seen on an imaging test, like magnetic resonance imaging (MRI), done for other reasons. If you do have symptoms, they may include:

  • Pain in the back of the knee
  • Knee stiffness
  • Sense of swelling or fullness behind the knee, especially when you straighten your leg
  • A swelling behind the knee that goes away when you bend your knee

These symptoms tend to get worse when standing for a long time, or being active.

If you have another problem with your knee, you might have more symptoms. For example, if you have osteoarthritis, you may also have joint pain. Complications of a Baker’s cyst can cause additional symptoms, such as warmth, redness, or numbness.

How is a Baker’s cyst diagnosed?

Your health care provider will ask you about your medical history and your symptoms. He or she will give you a physical exam, which will include a careful exam of your knee. It’s important to make sure your symptoms are caused by a Baker’s cyst and not a tumor or a blood clot.

If the cause of your symptoms is not clear, you may have imaging tests, such as:

  • Ultrasound, to view the cyst in more detail
  • X-ray, to get more information about the bones of the joint
  • MRI, if the diagnosis is still unclear after ultrasound, or if your health care provider is considering surgery

How is a Baker’s cyst treated?

You likely won’t need any treatment if you don’t have any symptoms from your Baker’s cyst. Some Baker’s cysts go away without any treatment. If your cyst starts causing symptoms, you might need treatment at that time.

If you do have symptoms, you may be treated depending on the cause of your cyst. For example, you may need medication for rheumatoid arthritis. Or you may need physical therapy for osteoarthritis.

Other treatments for a Baker’s cyst can include:

  • Over-the-counter pain medications
  • Arthrocentesis to removes excess fluid from the joint space
  • Steroid injection into the joint to reduce cyst size
  • Surgery to remove the cyst

Most people’s Baker’s cysts go away without surgery. Health care providers only rarely advise surgery. You might need surgery if your Baker’s cyst is causing you severe symptoms and no other treatments have worked. Your health care provider will check you carefully for other knee problems to treat before advising surgery. In many cases, a Baker’s cyst will come back after surgery. This is most often true when it’s caused by a problem that hasn’t gone away.

What are the complications of a Baker’s cyst?

In rare cases, a Baker’s cyst may cause complications. The cyst may enlarge, which may cause redness and swelling. The cyst may also rupture, causing warmth, redness, and pain in your calf.

The symptoms may be the same as a blood clot in the veins of the legs. Your health care provider may need imaging tests of your leg to make sure you don’t have a clot. Rupture can also lead to its own complications, such as:

  • Trapping of a tibial nerve. This cases calf pain and numbness behind the leg. It can be treated with arthrocentesis and steroid injections.
  • Blockage of the popliteal artery. This causes pain and lack of blood flow to the leg. It can also be treated with arthrocentesis and steroid injections.
  • Compartment syndrome. This causes intense pain and problems moving the foot or toes. Compartment syndrome is a medical emergency. It needs immediate surgery. It can lead to permanent muscle damage if not treated right away.

When should I call the doctor?

If your cyst starts causing mild symptoms, plan to see your health care provider soon. See him or her right away if you have symptoms such as redness and swelling of your leg. These symptoms may mean your Baker’s cyst has ruptured.

Key points

  • A Baker’s cyst is a fluid-filled sac that forms behind the knee. They usually do not cause major problems.
  • A Baker’s cyst is usually the result of some other problem with the knee. It may be caused by osteoarthritis or a tear of the knee’s cartilage.
  • Many people with Baker’s cysts don’t have any symptoms. You might have some pain behind the knee.
  • Your health care provider will try to treat any underlying conditions. You may also need fluid removed from the knee joint space.
  • Surgery is not usually needed for a Baker’s cyst.
  • In rare cases, a Baker’s cyst can rupture. This can cause serious complications. See your health care provider right away if your leg is red and swollen.

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