A Baker's cyst is a fluid-filled sac that forms behind the knee. It's also known as a popliteal cyst or popliteal synovial cyst.

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 lubricates 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 in older people.

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.

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.

A Baker's cyst often doesn't cause symptoms. A cyst will more often be seen on an imaging test, like MRI or ultrasound, 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 or bend your leg.
• A swelling behind the 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.

Your doctor will ask you about your medical history and your symptoms. They 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 isn't clear, you may have imaging tests, such as an:

• 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 an ultrasound or your doctor is considering surgery.

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 or anti inflammatory medicines.
• Needle aspiration of fluid from the cyst.
• Arthrocentesis (inserting a needle into the knee joint) to remove excess fluid from the joint space.
• Steroid injection into the joint to reduce inflammation in the joint.
• Surgery to remove the cyst.

Most Baker's cysts go away without surgery. Doctors 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 doctor 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.

In rare cases, a Baker's cyst may cause other problems. 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 doctor may need imaging tests of your leg to make sure you don't have a clot.

If your cyst starts causing mild symptoms, plan to see your doctor soon. See them right away if your leg is red and swollen. These symptoms may mean your Baker's cyst has ruptured.

• A Baker's cyst is a fluid-filled sac that forms behind the knee. They usually don't 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 or meniscus.
• Many people with a Baker's cyst don't have any symptoms. You might have some pain behind the knee.
• Your doctor will try to treat any underlying conditions. You may also need fluid removed from the knee joint space or the cyst.
• Surgery isn't usually needed for a Baker's cyst.
• In rare cases, a Baker's cyst can rupture. This can cause serious problems. See your doctor right away if your leg is red and swollen.

Tips to help you get the most from a visit to your doctor:

• Know the reason for your visit and what you want to happen.
• Before your visit, write down questions you want answered.
• Bring someone with you to help you ask questions and remember what your doctor tells you.
• At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also write down any new instructions your doctor gives you.
• Know why a new medicine or treatment is prescribed, and how it will help you. Also know what the side effects are.
• Ask if your condition can be treated in other ways.
• Know why a test or procedure is recommended and what the results could mean.
• Know what to expect if you do not take the medicine or have the test or procedure.
• If you have a follow-up appointment, write down the date, time, and purpose for that visit.
• Know how you can contact your doctor if you have questions, especially after office hours or on weekends.