Baker's Cyst Behind the Knee: What Is It and When Should You Worry?

A Baker's cyst is a fluid-filled lump at the back of the knee, also called a popliteal cyst. It forms when joint fluid builds up and collects in a pocket behind the knee, usually because of another knee problem such as arthritis or a meniscus tear. Many cysts cause few symptoms and settle with time or when the underlying problem is treated.

The American Academy of Orthopaedic Surgeons (OrthoInfo) describes it as one of the most common knee disorders. The NHS notes that it is more common in people aged 35 to 70, and that in children it often has no clear cause and may go away on its own. Any new lump should still be checked by a doctor.

A Baker's cyst can burst, letting fluid leak into the calf. This can look and feel very like a deep vein thrombosis (DVT), a blood clot in a leg vein, which can be dangerous. Because the two cannot be told apart reliably by appearance alone, these symptoms need prompt medical assessment.

The NHS advises asking for an urgent doctor's appointment or contacting your local urgent advice service if you have:

• Sudden or worsening pain or swelling in the calf or leg.
• Swelling, bruising or warm skin in the leg.
• Red, blue or darkened skin around the painful area, which can be harder to see on brown or black skin.

Call emergency services or go to an emergency department if leg pain or swelling comes with shortness of breath or chest pain. These can be signs that a clot has travelled to the lungs (pulmonary embolism). Do not drive yourself.

MedlinePlus also advises seeking care if the swelling behind the knee becomes large or painful, or if a lump grows quickly or comes with fever, severe pain or pain at night, as these need further assessment.

When the knee joint is irritated or damaged, it can make extra joint (synovial) fluid. OrthoInfo explains that this fluid can be pushed through a one-way valve-like opening at the back of the knee and collect there, forming the cyst.

Knee problems commonly linked with a Baker's cyst in adults include:

• Osteoarthritis of the knee.
• A tear of the meniscus or other cartilage injury.
• Rheumatoid arthritis or other inflammatory arthritis.
• Gout.
• Other knee injuries, such as a torn anterior cruciate ligament (ACL).

Many people have no symptoms, and the cyst is found during an examination or scan for something else. When symptoms happen, the main sign is a soft swelling behind the knee, often more noticeable when standing. MedlinePlus says a larger cyst can feel like a water-filled balloon, and symptoms often come and go.

Other symptoms can include aching or tightness behind the knee, stiffness, difficulty bending the knee fully, and swelling in the knee. The NHS notes that symptoms may be worse after walking or being active. A very large cyst can occasionally press on nearby veins or nerves.

A clinician will usually examine both knees and compare them. OrthoInfo notes that the cyst often feels firmer when the knee is straight and softer when it is bent.

Ultrasound can show whether a lump is filled with fluid or solid. When a DVT is a concern, a clinician may arrange tests to check for a clot. Our guide on a high D-dimer result explains one of those tests and why it cannot confirm a clot on its own.

X-rays do not show the cyst but can show arthritis. MRI can show the cyst and possible causes such as a meniscus tear. If your knee hurts but a scan found little to explain it, see our guide on knee pain with a normal MRI.

Treatment is not always needed. A cyst that causes no trouble may simply be watched. The NHS says a cyst may get better on its own or when the condition causing it is treated, and that recovery can take months or sometimes years.

Self-care suggested by the NHS includes resting the knee, using a wrapped ice pack for short periods and raising the leg. A pharmacist or doctor can advise on pain relief that is suitable for you. Physiotherapy may also help.

Other options a clinician may discuss include a steroid injection into the knee, draining fluid from the cyst with a needle, and treating the underlying knee problem. MedlinePlus notes that a drained cyst often comes back. Surgery to remove the cyst itself is rare; surgery is more often aimed at the knee problem behind it, such as a meniscus tear.

If a cyst bursts, the NHS says symptoms can be worse for a few weeks until the swelling settles. This should only be assumed after a clinician has assessed you, because a clot can look the same.

These questions can help you understand the cyst and the knee problem behind it.

• What do you think is causing my Baker's cyst, and does that underlying problem need treatment?
• Has a blood clot been ruled out, and how?
• Do I need a scan, and what would it change?
• Which activities or exercises are safe for my knee right now?
• What should I do if my calf suddenly swells or becomes painful?
• If the cyst is drained or injected, how likely is it to come back?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with a Baker's cyst described their knee symptoms, linked arthritis or meniscus problems, and the tests they discussed can help you organise your own timeline and decide which questions to ask.

Similar cases cannot tell you whether your swelling is a cyst, a clot or something else. Only an assessment can do that. If your calf suddenly swells or you feel short of breath, seek care straight away rather than waiting for online replies.