What is a Baker's cyst?
Baker's cysts are fluid-filled sacs that form behind the knee. They don’t often cause any problems or need treatment. Sometimes, if they are large or painful, they’ll need treatment, which can be surgical or non-surgical. There’s some danger of complications if Baker's cysts go untreated.
Baker's cysts are named after Dr. William Morrant Baker, who described them in the 1800s. They may be called synovial cysts because they are filled with synovial fluid, which is a liquid that lubricates the knee joint. They are also called popliteal cysts because they occur in an area of the knee called the popliteal fossa.
Signs of a Baker's cyst
A Baker's cyst may not cause any problems. You may not realize it’s there until it shows up on an imaging test. At other times, it will cause symptoms, which may include:
Lump behind the knee
A lump behind the knee is the primary symptom of a Baker's cyst. If it’s large enough to feel, it’s usually soft when you bend your knee and firm when you stretch out your leg. This characteristic, called Foucher's sign, helps distinguish Baker's cyst from other masses.
Pain or achiness
Knee pain is one of the most common complaints of those who have a Baker's cyst. In one study, around one-third of those with Baker's cysts complained of achiness. Sometimes pain occurs when you fully extend your leg.
Swelling is the most common complaint of those with Baker's cysts. Of course, the cyst itself can swell if it’s large enough. Baker’s cysts can also cause swelling in the leg.
Baker's cysts can cause stiffness, including an inability to bend the leg entirely and comfortably. If the cyst is large, it can limit your mobility.
Causes of a Baker's cyst
A knee injury, especially one that damages cartilage, can lead to a Baker's cyst. The injury triggers the knee to produce extra fluid, some of which flows out of the joint and forms the cyst behind the knee. A valve-like structure prevents the fluid from flowing back into the knee. This is most likely to happen after a tear in the meniscus, a cartilage structure in the knee that acts as a shock absorber.
A cyst may form if you have osteoarthritis, rheumatoid arthritis, or an infection in the knee joint known as infectious or septic arthritis. The resulting inflammation can cause the knee joint to make more fluid. Some experts believe the cyst may act as a safety valve, draining off excess fluid and preventing harmful pressure in the knee joint.
When to see the doctor for a Baker's cyst
Although many Baker's cysts go away on their own, you should see a doctor to confirm that your swelling is a Baker's cyst and not something more serious. Several other conditions can present as a lump behind the knee, including:
- Strain or tear of the calf muscle
- Soft tissue tumor
- Infection in the knee
- Inflammatory arthritis, such as gout or rheumatoid arthritis
- Aneurysm, or a bulge in a blood vessel
If you have pain, swelling, heat, and redness in your lower leg, you should see a doctor right away. These are signs of thrombophlebitis, a condition in which a blood clot blocks a vein. When the clot occurs deep in a muscle, it’s called deep vein thrombosis (DVT), and it can have serious effects.
It’s also possible that a Baker's cyst has ruptured, releasing its fluid into the leg. This is a less-serious condition called pseudothrombophlebitis. Sometimes, however, it can be serious, causing compartment syndrome, when pressure builds inside and between muscles. Compartment syndrome is more likely if you are taking blood thinners.
Diagnosis for a Baker's cyst
A doctor can often diagnose a Baker's cyst from a physical examination. If the cyst is large or is causing problems, the doctor will probably perform one or more tests to be sure of the diagnosis. These tests may include:
Treatments for a Baker's cyst
Doctors often recommend conservative treatment of Baker's cysts, starting with the RICE method:
- Rest the leg
- Ice the knee
- Compress the knee with a wrap to decrease swelling
- Elevate the leg when possible
Your doctor may recommend ibuprofen or another over-the-counter medication to reduce pain and inflammation. If your cyst is a result of arthritis, exercises for the muscles around the knee can help. Start with non-weight-bearing exercises, or see a physical therapist for an appropriate regimen. You may need to cut back on activities that stress the knee, such as jogging. Maintaining a healthy weight will also reduce stress on the knee.
At some point, your doctor may decide that it isn't safe for your Baker's cyst to go untreated. A steroid injection in the knee is often the first step in treatment, or your doctor may use a needle to drain fluid from the cyst. The next step may be arthroscopic knee surgery, which allows the doctor to see inside the knee and correct some problems.
Doctors don’t often recommend surgical removal of Baker's cysts because it usually requires a large incision, time in the hospital, and significant recovery time. Also, the cyst is more likely to recur. If your Baker's cyst comes from a problem within the knee that can't be repaired with arthroscopy, a surgeon may open up the knee to repair the damage and deal with the cyst at the same time.
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Harvard Health Publishing: "Ask the doctor: How do you treat a Baker's cyst?"
International Journal of Critical Illness and Injury Science: "Compartment syndrome secondary to Baker's cyst rupture: A case report and up-to-date review."
Journal of Orthopaedic Surgery and Research: "A comparison of clinical efficacy between different surgical approaches for popliteal cyst."
Leib, A. et al. StatPearls, StatPearls Publishing, 2020.
Mayo Clinic: "Thrombophlebitis."
Medicine: "The pseudothrombophlebitis syndrome."
Sports Health: "Baker's Cyst: Diagnostical and Surgical Considerations."
University of Rochester Medical Center: "Baker Cyst."
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