Baker's cyst is a buildup of joint fluid (synovial fluid) that forms behind the knee.
Popliteal cyst; Bulge-knee
Causes, incidence, and risk factors
A Baker's cyst is caused by swelling in the knee. The swelling is due to an increase in synovial fluid - the fluid that lubricates the knee joint. When pressure builds up, fluid bulges into the back of the knee.
Baker's cyst commonly occurs with:
- A tear in the meniscal cartilage of the knee
- Knee arthritis (in older adults)
- Rheumatoid arthritis
- Other knee problems
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.
Signs and tests
During a physical exam, the doctor will look for a soft mass 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 limitation in range of motion caused by pain or by the size of the cyst. In some cases there will be signs and symptoms of a meniscal tear.
Transillumination, or shining a light through the cyst, can show that the growth is fluid filled.
If the mass 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, such as arthritis or a meniscus tear. The cyst is usually not removed because it can come back. The surgery may also damage nearby blood vessels and nerves.
Sometimes, a cyst can be drained (aspirated) or, in rare cases, removed with surgery if it becomes very large or causes symptoms.
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.
Complications are unusual, but may include:
- Long-term pain and swelling
- Complications from related injuries, like meniscal tears
Calling your health care provider
Call for an appointment with your health care provider if you have swelling behind the knee that becomes large or painful. Pain could be a sign of infection.
Foley BA, Christopher TA. Injection therapy of bursitis and tendinitis. In: Roberts JR, Hedges JR, eds. Clinical Proceedings in Emergency Medicine. 5th ed. Philadelphia, Pa: Saunders Elsevier;2009:chap 52.
Huddleston JI, Goodman SB. Hip and knee pain. In: Firestein GS, Budd RC, Harris ED Jr., et al., eds. Kelley's Textbook of Rheumatology. 8th ed. Philadelphia, Pa: Saunders Elsevier;2008:chap 42.
Reviewed By: Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington, School of Medicine; and C. Benjamin Ma, MD, Assistant Professor, Chief, Sports Medicine and Shoulder Service, UCSF Department of Orthopaedic Surgery. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.