A Baker's cyst is a fluid-filled swelling at the back of the knee. In adults it nearly always forms because of another problem inside the knee, such as osteoarthritis or a meniscus tear, which makes the joint produce extra fluid that collects in a pouch behind it. It causes tightness or a lump behind the knee, and treatment is aimed at the underlying knee problem. A cyst can occasionally burst and cause calf pain and swelling that looks like a blood clot.
Also known as popliteal cyst, swelling behind the knee
Anatomy: where the problem is
Front view of the knee. Highlighted in the diagram: back of the knee.
Symptoms
- A soft lump or fullness behind the knee, most obvious when standing
- Tightness or aching behind the knee, especially when fully bending or straightening
- Stiffness and swelling of the knee
- Symptoms that vary as the knee flares and settles
- Sudden sharp pain with swelling and bruising in the calf if the cyst bursts
- Sometimes no symptoms at all
Causes and risk factors
- Knee osteoarthritis
- A meniscus (cartilage) tear
- Inflammatory arthritis such as rheumatoid arthritis or gout
- A previous knee injury
- In children, cysts can appear without any knee problem and often go away by themselves
What the evidence says helps
- In adults a Baker's cyst is almost always linked to a problem inside the knee, and treating that problem is the main way to settle the cyst.12
- Many cysts need no specific treatment and are simply monitored. Easing off aggravating activity, pain relief and exercise for the knee are the usual first steps.1
- Drawing off the fluid, with or without a steroid injection, can give relief, but the cyst often returns if the underlying knee problem remains.1
- Surgery to remove the cyst alone has a high rate of recurrence, so any operation is usually directed at the cause inside the knee.2
- A burst cyst can cause calf pain and swelling that cannot be told apart from a deep vein thrombosis without a scan, so it needs urgent assessment.1
Things you can try at home
- Ease off deep squatting, kneeling and high-impact activity while the knee is swollen
- Use ice for short spells and keep the leg raised when resting
- Keep the knee moving and the thigh muscles working with gentle exercise
- Follow the advice for the underlying knee problem, such as strengthening for osteoarthritis
- Do not try to massage or squeeze the lump away
When to get urgent help
Contact your GP urgently, call NHS 111, or call 999 in an emergency if you have:
- Sudden pain, swelling, warmth or redness in the calf: get same-day medical help, as a blood clot must be ruled out
- Calf pain or swelling with breathlessness or chest pain: call 999
- A hot, red, swollen knee with a fever or feeling unwell: get same-day medical help
- A lump behind the knee that is hard, pulsating, growing quickly or painful at night: see your GP
- A foot that turns cold, pale or numb: go to A&E
How Apollo can help
We examine the knee to work out what is driving the extra fluid, since the cyst itself is rarely the real problem. We also check the calf and, if there is any sign that might indicate a blood clot, send you for same-day medical assessment. Treatment focuses on the underlying knee condition: managing swelling, restoring movement and building strength with a graded exercise plan. If the cyst is large or persistent, we write to your GP about a scan or having it drained.
No GP referral needed. Clinics in Sheringham, Coltishall and Norwich.
Related reading
Frequently asked questions
Will a Baker's cyst go away on its own?
It can. Cysts often shrink as the underlying knee problem settles, and in children they frequently disappear without treatment. In adults they tend to come and go with the knee.
Is a Baker's cyst dangerous?
The cyst itself is harmless. The important point is that a burst cyst and a blood clot in the leg cause very similar symptoms, so sudden calf pain and swelling should always be checked urgently.
Should a Baker's cyst be drained?
Draining can relieve tightness, but the fluid often comes back if the cause inside the knee is not addressed. It is usually considered for large or uncomfortable cysts alongside treatment of the knee itself.
References
- Herman AM, Marzo JM (2014). Popliteal Cysts: A Current Review. Orthopedics 37. View source
- Frush TJ, Noyes FR (2014). Baker’s Cyst. Sports Health: A Multidisciplinary Approach 7:359-365. View source
This page is general information, not a diagnosis or a substitute for medical advice. If you are worried about your symptoms, speak to your GP or a registered healthcare professional. Content prepared from the sources above, November 2026.
