Baker's Cyst: Swelling Behind the Knee, Causes and Treatment

Baker's Cyst: Swelling Behind the Knee, Causes and Treatment

What is a Baker's cyst and why does swelling appear behind the knee? Symptoms, why a burst cyst must be told apart from a blood clot, diagnosis, home care and medical treatment options.

A Baker's cyst (popliteal cyst) is a soft, usually harmless swelling behind the knee caused by a build-up of joint fluid. In adults it is rarely a condition on its own; it usually results from a problem inside the knee, such as a meniscus tear or osteoarthritis, making the joint produce excess fluid. Many Baker's cysts cause no symptoms, or only a feeling of fullness and tightness behind the knee. Treatment usually targets the underlying knee problem rather than the cyst itself. However, sudden pain and swelling spreading into the calf can mean the cyst has burst or, more importantly, that there is a blood clot in the leg, so it needs prompt assessment.

This article explains how a Baker's cyst forms, its symptoms, why not every lump behind the knee is a Baker's cyst, how it is diagnosed, what you can do at home and when medical treatment is considered.

What is a Baker's cyst and why does it form?

The knee joint is surrounded by a capsule containing a small amount of lubricating fluid. Behind the knee, between the calf muscle and a hamstring tendon, there is a small sac (bursa) that in many people connects to the joint through a narrow opening. When inflammation or damage increases the fluid in the joint, it passes through this opening into the sac. Because the opening often works like a one-way valve, the fluid cannot flow back and the sac gradually swells, forming a Baker's cyst.

Common knee problems associated with Baker's cysts in adults include:

• Meniscus tears: particularly tears at the back of the inner meniscus
• Knee osteoarthritis: one of the most common causes in middle and older age
• Inflammatory arthritis: such as rheumatoid arthritis and gout
• Cartilage damage and other joint injuries

In children, Baker's cysts usually occur without any problem inside the joint and often shrink on their own over time. Our article on fluid accumulation in the knee covers the causes of joint fluid build-up in more detail.

What are the symptoms?

Small cysts are often found by chance on an ultrasound or MRI done for another reason. When they do cause symptoms, the most common are:

• A soft swelling behind the knee, usually on the inner side, more noticeable when standing
• Fullness, tightness or pressure behind the knee
• A squeezing feeling behind the knee when fully bending or straightening it
• Aching that increases after standing for long periods or after activity
• Symptoms of the underlying problem, such as catching with a meniscus tear or morning stiffness with arthritis

The size of the cyst can vary through the day and with activity. A cyst that softens when the knee is bent and becomes firmer when the leg is straight is typical.

Is every lump behind the knee a Baker's cyst?

No. Most swellings behind the knee are Baker's cysts, but some important conditions can look similar and need to be ruled out:

• Deep vein thrombosis: a clot in a leg vein can cause calf swelling, pain and tightness very similar to a burst cyst, and needs different, urgent treatment.
• Popliteal artery aneurysm: a widening of the artery behind the knee that may feel like a pulsating lump.
• Soft tissue masses: rarely, other growths can occur behind the knee.

That is why a new, rapidly growing, hard, pulsating or painful lump behind the knee should not simply be assumed to be a Baker's cyst; it should be checked by a doctor.

What happens if a Baker's cyst bursts?

Sometimes the cyst ruptures and the fluid tracks down between the calf muscles. This typically causes sudden calf pain, sometimes described as a feeling of water running down the leg, calf swelling, tightness and redness, and bruising around the ankle after a few days.

A burst cyst is usually not dangerous and the fluid is reabsorbed over time. But these symptoms are almost identical to those of a blood clot, and the two often cannot be told apart by examination alone. If you develop sudden calf pain and swelling, seek medical care the same day so that a clot can be ruled out, usually with a Doppler ultrasound.

How is it diagnosed?

• Examination: the doctor assesses the location and feel of the swelling, how it changes as the knee bends and straightens, and the knee as a whole.
• Ultrasound: quickly shows the cyst, its size and contents without radiation; the veins can be checked if a clot is suspected.
• MRI: shows the cyst and any underlying meniscus tear or cartilage damage in detail, which is valuable for planning treatment.
• X-ray: does not show the cyst but may be used to assess arthritis.

Baker's cyst treatment: what can you do at home?

Small cysts that cause no symptoms often need no treatment and can simply be monitored. For mild symptoms, these steps may help:

1. Adjust your activity: cut back for a while on deep squats, long periods of standing and intense running if they worsen swelling and pain.
2. Cold therapy: 15–20 minutes with an ice pack wrapped in a towel.
3. Elevation: raise the leg above heart level when resting.
4. Gentle exercise: strengthening exercises for the muscles around the knee and low-impact activity that does not increase pain.
5. Painkillers: short-term use can help; check with a doctor or pharmacist before using them regularly.

Do not try to rub, squeeze or drain the cyst yourself; this does not help and carries risks such as infection. Because home measures do not remove the underlying cause, the cyst may grow again from time to time.

Medical treatment and when surgery is needed

If symptoms persist, treatment is planned according to the cause:

• Treating the underlying problem: when arthritis, a meniscus tear or inflammatory arthritis is treated appropriately, joint fluid decreases and the cyst often shrinks.
• Physiotherapy: helps maintain range of motion and muscle strength.
• Aspiration and injection: for troublesome cysts, fluid can be drained, usually under ultrasound guidance, sometimes with medication injected into the joint. If the cause remains, the cyst may refill.
• Arthroscopic surgery: if there is a problem inside the joint such as a meniscus tear, treating it arthroscopically, and addressing the valve between the cyst and the joint where appropriate, may be considered.
• Open removal: less often, for large symptomatic cysts that have not responded to other treatment; recurrence is possible if the cause is not addressed.

For cysts linked to knee arthritis, our article on knee osteoarthritis and its treatment may also help.

When should you see an orthopedic surgeon?

• Same day: sudden calf pain, swelling, tightness or redness, so that a clot can be ruled out; go to the emergency department if you have shortness of breath or chest pain.
• A new, rapidly growing, hard or pulsating lump behind the knee
• Locking, catching or giving way that suggest a problem inside the joint
• Swelling behind the knee with marked warmth, redness or fever
• Symptoms that last for weeks despite home measures or limit daily life

In Prof. Dr. Gökhan Polat's practice, a suspected Baker's cyst is assessed together with possible underlying causes such as meniscus, cartilage or arthritic problems through examination and imaging, and the treatment plan is based on these findings and your symptoms. Responses vary from person to person. For more on diagnosis and treatment, see our page on knee joint diseases.

In summary

• A Baker's cyst is a build-up of joint fluid behind the knee, usually linked in adults to a knee problem such as a meniscus tear or osteoarthritis.
• Small cysts without symptoms are often just monitored; treatment mainly targets the underlying cause.
• Not every lump behind the knee is a Baker's cyst; new, hard or pulsating lumps should be checked.
• Sudden calf pain and swelling need same-day assessment because of the possibility of a blood clot.

This article is for general information only; diagnosis and treatment decisions are made individually after examination and the necessary tests.