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 due to 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 doesn’t cause any long-term damage, it can be very uncomfortable.
Synovial fluid is a clear liquid that normally circulates through the cavities in your knee joint. Sometimes the knee produces too much of this fluid. The increasing pressure forces the fluid to the back of the knee, where it creates a bulge. This severe swelling of the knee causes a popliteal cyst to form.
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 joint inflammation
Since the knee is a complicated joint, it can be injured easily. According to the American Academy of Orthopedic Surgeons (AAOS), about 10.4 million Americans saw their doctors about a knee problem in 2010, making it the most common reason for seeing an orthopedic specialist. Such injuries may cause the inflammation that leads to a popliteal cyst.
A blood clot can also cause bruising and swelling behind the knee and on the back of the calf. It’s 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.
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
Your doctor will examine your knee and feel the swelling. If the cyst is small, they may compare the affected knee to the healthy one and check your range of motion.
Your doctor may recommend more tests if the cyst rapidly increases in size or causes severe pain or fever. These will determine if some other form of growth, such as a tumor, is causing the swelling. These noninvasive tests include MRI and ultrasounds.
Although the cyst won’t show up on an X-ray, your doctor may use one 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.
A popliteal cyst often doesn’t need treatment and 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 and use an ultrasound to help guide the needle to the correct place. They’ll then draw the fluid from the joint.
Regular, gentle exercises may help increase your range of motion and strengthen the muscles around your knee. Crutches may help 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, such as cortisone, for the cyst. Your doctor will inject this drug into the cyst. Though it may help relieve the pain, it doesn’t always prevent a popliteal cyst from recurring.
Treating the cause of the cyst is very important to prevent the cyst from returning. As a general rule, if the cyst is left alone it will go away once the underlying cause is treated. Should your doctor determine that you have damage to the cartilage, they may recommend surgery to repair or remove it.
If you have arthritis, the cyst may persist even after your doctor treats the underlying cause. If the cyst causes you pain and limits your range of motion, your doctor may recommend surgery to remove it.
Complications are rare, but they may include:
- prolonged swelling
- severe pain
- complications from related injuries, such as torn cartilage
A popliteal cyst won’t cause any long-term damage, but 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.