A popliteal cyst, also known as a baker’s cyst, is a fluid-filled swelling that causes a lump at the back of the knee, leading to tightness and restricted movement. The cyst can be painful when you bend or extend your knee.
Usually, this condition is caused by a problem that affects the knee joint, such as arthritis or a cartilage injury. Treating the underlying cause can often alleviate the problem. Though a popliteal cyst does not cause any log-term damage, it can be very uncomfortable.
Severe swelling of the knee causes a popliteal cyst to form. The swelling is a result of an excess of synovial fluid (the fluid that keeps the knee joint lubricated).
Synovial fluid is a clear liquid that normally circulates though the cavities in your knee joint. Sometimes, the knee produces too much of this fluid. As the pressure increases, the fluid is forced to the back of the knee, where it creates a bulge.
The most common causes of a popliteal cyst are:
- damage to the knee’s cartilage (meniscus)
- arthritis of the knee
- rheumatoid arthritis
- other knee conditions that cause inflammation of the joint
Since the knee is a complicated joint, it can be injured easily. According to the American Academy of Orthopedic Surgeons, about 19.4 million Americans saw their doctors about a knee problem in 2003, making it the most common reason for seeing an orthopedic specialist (AAOS). Such injuries may cause the inflammation that leads to a popliteal cyst.
You may not feel any pain with a popliteal cyst. In some cases, you may not notice it at all. If you do experience symptoms, they might include:
- mild to severe pain
- limited range of motion
- swelling behind the knee and calf
- bruising on the knee and calf
- rupturing of the cyst
A blood clot can also cause bruising and swelling behind the knee and on the back of the calf. It is important that your doctor examines the swelling to determine if the cause is a cyst or a clot. An MRI or ultrasound might be used to help make a diagnosis.
Your doctor will examine your knee and feel the swelling. If the cyst is small, he or she may compare the affected knee to the healthy one and check your range of motion. By shining a light through the cyst, your doctor can tell if there is fluid inside the swelling.
If the cyst rapidly increases in size or causes severe pain or fever, your doctor may recommend more tests. These will determine if some other form of growth, such as a tumor, is causing the swelling. These noninvasive tests include MRIs (magnetic resonance imaging) and ultrasounds.
Although the cyst will not show up on an X-ray, one might be used as well to check for other problems, such as inflammation or arthritis.
An MRI will enable your doctor to see the cyst clearly and to determine if you have any damage to the cartilage.
Very often a popliteal cyst will require no treatment and it will go away on its own. However, if the swelling becomes large and causes severe pain, your doctor may recommend one of the following treatments:
Your doctor will insert a needle into the swelling. He or she will use an ultrasound to help guide the needle to the correct place and then draw the fluid from the joint.
Regular, gentle exercises may help to increase your range of motion and strengthen the muscles around your knee. Crutches may help to alleviate the pain. You can also help reduce pain by using a compression wrap or placing ice on the joint.
Your doctor may recommend a corticosteroid medication for the cyst, such as cortisone. This drug will be injected into the cyst. Though it may help relieve the pain, it does not always prevent a popliteal cyst from recurring.
Usually, your doctor will treat the underlying cause of the cyst rather than the cyst itself. For example, should your doctor determine that you have damage to the cartilage, he or she may recommend surgery to repair or remove it.
If you have arthritis, the cyst may persist even after your doctor has treated the underlying cause. If the cyst is causing you pain and limiting your range of motion, your doctor may recommend surgery to remove it.
A popliteal cyst will not cause any long-term damage, although it can be uncomfortable and annoying. The symptoms may come and go. In most cases, the condition will improve over time or with surgery. Long-term disability due to a popliteal cyst is very rare.
Complications are rare, but may include:
- prolonged swelling
- severe pain
- complications from related injuries, such as torn cartilage