Baker’s Cyst (Popliteal Cyst)

Baker’s Cyst (Popliteal Cyst)

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Baker’s Cyst (Popliteal Cyst)

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

Understanding 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). There are several small bursae around the knee joint. These are small sacs filled with a special fluid (synovial fluid) that cushions the rest of the joint. Between the bones is a space that also contains this fluid.

What causes a Baker’s cyst?

A small bursa sits just below the crease at the back of your knee. When this sac fills with too much fluid, it’s called a Baker’s cyst. This might happen when an injury or disease causes extra synovial fluid to leak into the bursa from the joint space.

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.

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 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.

Diagnosing a Baker’s cyst

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 look at 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

Treatment for a Baker’s cyst

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 medicine for rheumatoid arthritis. Or you may need physical therapy for osteoarthritis.

Other treatments for a Baker’s cyst can include:

  • Over-the-counter pain medicines

  • Arthrocentesis to remove extra fluid from the joint space

  • Steroid injection into the joint to reduce cyst size

  • Surgery to remove the cyst

Possible complications of a Baker’s cyst

In rare cases, a Baker’s cyst may cause complications. The cyst may get larger, 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 to call the health care provider

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.