Baker's Cyst (Popliteal Cyst)

How a Baker's cyst forms

The knee joint is lined with a membrane (synovium) that makes lubricating fluid. When the knee is inflamed or damaged, it produces extra fluid. Behind the knee there is a natural pocket (bursa) between the calf muscle and a hamstring tendon that connects to the joint through a small opening. Fluid squeezed into this pocket during movement can get trapped by a one-way valve effect, so the bursa swells into a cyst.

In children, Baker's cysts often occur without any knee problem and usually disappear on their own.

Causes

• Knee osteoarthritis: the most common cause in adults over 50
• Meniscus tear: especially tears at the back of the medial meniscus
• Rheumatoid arthritis and other inflammatory arthritis
• Gout and pseudogout
• cartilage injuries and other knee trauma

Symptoms

• a soft, rounded swelling behind the knee, more noticeable when standing
• tightness or fullness behind the knee
• stiffness and difficulty bending the knee fully
• aching behind the knee, worse after activity
• symptoms of the underlying problem, such as knee pain, swelling or locking

Many Baker's cysts cause no symptoms and are found by chance on scans.

Ruptured Baker's cyst

If the cyst bursts or leaks, fluid tracks down into the calf, causing:

• sudden sharp pain in the back of the knee and calf
• calf swelling and tightness
• redness, and sometimes bruising around the inner ankle a few days later (the "crescent sign")

These symptoms are very similar to deep vein thrombosis (DVT), and a cyst can also press on veins and contribute to a clot. An ultrasound is needed to tell them apart. A ruptured cyst settles over several weeks with rest, elevation and pain relief.

Diagnosis

A doctor examines the back of the knee with the knee straight and bent; Baker's cysts often become softer or smaller when the knee is bent (Foucher's sign). Tests:

• ultrasound: confirms a fluid-filled cyst and checks for DVT
• MRI: shows the cyst and the underlying cause, such as a meniscus tear
• X-ray: shows arthritis
• joint fluid aspiration: if infection or gout is suspected

A solid lump, or one that does not behave like a cyst, needs further imaging to rule out other masses such as a popliteal artery aneurysm or a tumor.

Treatment

Treat the knee problem

Because the cyst is fed by joint fluid, controlling the cause often makes it shrink:

• osteoarthritis: exercise therapy, weight loss, pain relief and steroid injections into the knee
• inflammatory arthritis: disease-modifying medicines
• meniscus tears: physical therapy, and arthroscopic surgery when there are mechanical symptoms such as locking

Symptom relief

• Ice for 15 to 20 minutes, a few times a day
• Pain relievers: acetaminophen (paracetamol) or NSAIDs such as ibuprofen; follow the label dose
• Activity modification: reduce activities that cause pain, without total rest
• Compression and elevation for swelling
• Physical therapy: to restore movement and strengthen the quadriceps and hamstrings

Procedures

• Steroid injection into the knee joint: reduces inflammation and fluid production.
• Aspiration: draining the cyst with a needle under ultrasound, sometimes with a steroid injection. Cysts can refill if the cause persists.
• Surgery: removal of the cyst is rarely needed and has a high recurrence rate unless the knee problem is also fixed.

Outlook

Many Baker's cysts shrink or disappear when the underlying knee condition is managed. They can recur. In children, they usually resolve without treatment within a year or two.

Frequently asked questions

What causes a Baker's cyst?

A Baker's cyst forms when extra fluid from the knee joint collects in a pocket behind the knee. The extra fluid usually comes from knee osteoarthritis, a meniscus tear, rheumatoid arthritis or gout. In children it can occur without an obvious knee problem.

What does a Baker's cyst feel like?

It feels like a soft, rounded swelling or tightness behind the knee, more obvious when standing with the leg straight. It may ache or limit bending. If it ruptures, it causes sudden calf pain and swelling.

Will a Baker's cyst go away on its own?

Sometimes. Many shrink when the underlying knee problem is treated, and in children they often disappear without treatment. Cysts linked to ongoing arthritis may persist or come back.

Is a Baker's cyst dangerous?

Usually not. The main concerns are a rupture, which causes calf pain and swelling that can mimic a blood clot, and pressure on nearby veins. Any new swollen, painful calf should be checked with an ultrasound to rule out deep vein thrombosis.

How is a Baker's cyst treated?

Treatment targets the knee problem causing the extra fluid, along with ice, pain relievers and physical therapy. A steroid injection into the knee or draining the cyst with a needle can help. Surgery to remove the cyst is rarely needed.