A Baker’s cyst is a swelling on the back of the knee caused by the build-up of fluid inside sacs called bursae between the two heads of the gastrocnemius (calf muscle). The symptoms are mild unless the cyst bursts or extends down into the calf muscles. Common causes of Baker’s cyst include arthritis, infection, torn knee cartilage and other knee injuries. Baker’s cyst is also known as a popliteal cyst.
The knee is a hinge joint, situated between the thigh bone and shin bones. The entire joint is enclosed inside a tough capsule lined with a membrane and filled with lubricating synovial fluid. Extra capsules or sacs of fluid, known as bursae, cushion the joint and help reduce the friction between tissues caused by movement.
A large cyst may cause some discomfort or stiffness, but there are often no symptoms. There may be a painless or painful swelling behind the knee.
The cyst may feel like a water-filled balloon. Sometimes, the cyst may break open (rupture), causing pain, swelling, and bruising on the back of the knee and calf.
It is important to know whether pain or swelling is caused by a Baker’s cyst or a blood clot. A blood clot (deep venous thrombosis) can also cause pain, swelling, and bruising on the back of the knee and calf. A blood clot may be dangerous and requires immediate medical attention.
Causes of Baker’s cyst
Some of the causes of Baker’s cyst include:
- injury – trauma or injury to the knee can cause a build-up of fluid (effusion), which triggers baker’s cyst
- torn cartilage – usually affecting the cartilages (known as menisci) that bolster the knee joint on both sides
- arthritis – particularly rheumatoid arthritis and osteoarthritis of the knee joint
- infection – local infection can cause fluid retention around the knee joint
- unknown causes – baker’s cysts can sometimes develop in children for no apparent reason.
Complications of Baker’s cyst
A person may be less inclined to seek medical help for Baker’s cyst if the symptoms are mild, which they generally are. However, if left untreated, complications can include:
- The cyst continues to grow, causing the symptoms to worsen.
- The cyst may extend down into the calf muscles (dissection).
- The cyst can burst and cause bruising on the ankle of the affected leg, due to leaked fluid.
The symptoms of calf dissection and cyst rupture are similar to those caused by inflammation of veins, which may make diagnosis difficult and delay treatment. It is important to seek medical advice, as more serious but less common problems may present in a similar way. These may include a tumour or popliteal artery aneurysm.
Exams and Tests
During a physical exam, the health care provider will look for a soft lump in the back of the knee.
- If the cyst is small, comparing the affected knee to the normal knee can be helpful.
- There may be a decrease in range of motion caused by pain or by the size of the cyst.
- In some cases there will be catching, locking, pain, or other signs and symptoms of a meniscal tear.
Shining a light through the cyst (transillumination) can show that the growth is fluid filled.
If the lump grows quickly, or you have night pain, severe pain, or fever, you will need more tests to make sure you do not have other types of tumors.
X-rays will not show the cyst or a meniscal tear, but they will show other problems that may be present, including arthritis.
MRIs can help the health care provider see the cyst and look for any meniscal injury.
Often no treatment is needed. The health care provider can watch the cyst over time.
If the cyst is painful, the goal of treatment is to correct the problem that is causing the cyst, such as arthritis or a meniscus tear.
Sometimes, a cyst can be drained (aspirated) or, in rare cases, removed with surgery if it becomes very large or causes symptoms.
- The cyst has a high chance of returning if the cause is not addressed.
- The surgery may also damage nearby blood vessels and nerves.
A Baker’s cyst will not cause any long-term harm, but it can be annoying and painful. The symptoms of Baker’s cysts usually come and go.
Long-term disability is rare. Most people improve with time or arthroscopic surgery.