Baker’s cyst: swelling behind the knee and what helps

Quick answer, as of October 2026: A Baker's cyst is a pocket of fluid behind the knee. In adults it often develops because another knee problem is producing extra fluid. Treatment starts by confirming what the lump is and addressing the knee problem, rather than assuming that every swelling needs an injection.

Get urgent help for sudden or worsening calf pain, swelling, warmth or a change in skin colour. A ruptured cyst and a blood clot can look similar. Call 999 if leg symptoms are accompanied by chest pain or breathlessness.

What is a Baker's cyst?

A Baker's cyst, also called a popliteal cyst, is a fluid-filled swelling at the back of the knee. It can feel like fullness or a soft lump, with stiffness, aching or difficulty bending the knee. Some people notice it more when standing or after walking.

The NHS guide advises getting an unexplained lump checked. Do not assume that the position of a lump establishes its cause.

Why does a cyst form behind the knee?

The knee normally contains fluid that helps the joint move smoothly. Arthritis or a meniscus tear can cause the joint to produce extra fluid. The American Academy of Orthopaedic Surgeons explains that this fluid can pass through a one-way opening towards the back of the knee and collect there.

That is why a cyst and knee pain often belong to the same problem. Draining the swelling may not deal with what is making the joint produce fluid in the first place.

For a broader view of knee swelling, read our water on the knee guide. This article focuses on a lump behind the knee rather than swelling throughout the joint.

How can I tell a Baker's cyst from a blood clot?

You cannot reliably make that distinction at home. A cyst can rupture and cause calf pain and swelling, but a deep vein thrombosis can cause similar symptoms. The NHS advises urgent assessment for leg pain or swelling that suddenly gets worse, warmth, bruising or red or darkened skin.

If you also have chest pain or breathlessness, call 999. Do not wait for a private knee appointment or a reply to a message.

Once urgent causes have been excluded, a planned assessment can focus on the knee and the reason the swelling developed.

Will I need an ultrasound or an MRI?

A clinician first asks about the symptoms and examines the leg. An ultrasound or MRI may help establish the diagnosis or investigate the knee problem behind it. Mayo Clinic notes that symptoms can resemble those of a clot, aneurysm or tumour, so imaging is sometimes needed to clarify the cause.

Not everyone needs every scan. The right investigation depends on what the examination shows and the question that needs answering.

Ask about a non-urgent knee assessment

What treatment helps a Baker's cyst?

Some cysts settle without a procedure. If the swelling comes from another knee condition, treating that condition can help. The NHS lists physiotherapy, pain relief, drainage and, in selected cases, surgery to repair a problem within the joint. Surgery to remove the cyst itself is uncommon.

After diagnosis, useful options may include:

• Adjusting activities that repeatedly aggravate the knee.
• A physiotherapy plan to support movement and strength.
• Short periods of rest, a wrapped ice pack and raising the leg to ease symptoms.
• A clinician-led discussion about whether drainage or another procedure has a useful role.

Mayo Clinic notes that aspiration is usually ultrasound-guided. It can reduce the swelling, but the assessment still needs to address the underlying joint condition. Ask what the proposed procedure is intended to improve and what happens if the cyst returns.

Can PRP treat a Baker's cyst?

PRP should not be presented as a proven way to remove a Baker's cyst. The evidence discussed in our knee service concerns diagnosed joint conditions, particularly osteoarthritis, rather than a guarantee that an injection will resolve a cyst.

For knee osteoarthritis, NICE guidance says PRP raises no major safety concerns, but evidence of benefit is limited in quality. It recommends special arrangements for clinical governance, consent and audit or research.

If osteoarthritis is contributing to the swelling, a doctor can discuss the treatment choices and whether PRP belongs among them. The decision should follow a diagnosis, an explanation of the uncertainty and a clear account of the alternatives. PRP does not replace urgent assessment of a newly hot or swollen leg.

Explore our joint treatment services or ask the team which assessment comes first.

How much does treatment cost?

As of October 2026, the clinic's published knee PRP treatment is £750. That is not a quote for cyst drainage, an ultrasound or surgery. The team will confirm the appropriate assessment, any investigations and their fees before you book. An injection is only considered if it suits the diagnosed problem.

What should I ask at my appointment?

Bring a short history of when the lump appeared, whether it changes with activity, any knee injury and any previous scans. Useful questions include:

• Is this definitely a cyst, and have urgent causes been excluded?
• What is making the knee produce extra fluid?
• What can I do while it settles?
• Would a scan or procedure change the treatment plan?
• If an injection is suggested, which condition is it treating and what benefit is realistic?

A clear diagnosis is the most useful first step. It helps you choose treatment for the actual problem rather than paying for a procedure because a lump is visible.

Discuss a non-urgent knee assessment · Call 020 3951 3429 · Email the team

General information, not a diagnosis or a substitute for individual medical advice.