Baker's cyst behind the knee seen by chance on a scan
A knee MRI or ultrasound done for something else may report a "Baker's cyst" or "popliteal cyst". This is a fluid-filled pocket at the back of the knee. It is common, especially with age, and most are found by chance. (NHS Baker's cyst)(Hasan 2025) This page covers what the report means and when another test is suggested, not treatment.
What the report words mean
- Baker's cyst or popliteal cyst. Joint fluid that has collected in a small sac (the semimembranosus-gastrocnemius bursa) behind the inner side of the knee. It is not a true cyst. (Kim 2016)(Hasan 2025)
- Communicates with the joint. The sac is linked to the knee joint by a small opening. A review describes a valve-like effect that lets fluid in but not back out. (Hasan 2025)
- Small, medium or large. MRI reports may grade the size. One study scored cysts from small to large. (Kim 2016)(Cao 2014)
- Fluid-filled, anechoic or hypoechoic. Ultrasound words for a pocket that looks dark because it holds fluid. (Hasan 2025)
- Ruptured or leaking. Fluid has escaped into the calf. This can cause sudden calf pain and swelling. (Hasan 2025)(NHS Baker's cyst)
- Other findings in the same knee. Reports often also mention joint wear, cartilage damage or a meniscus tear, because these often come with a Baker's cyst. (Hasan 2025)(Cao 2014) See hip or knee joint wear seen by chance.
How common it is by age
Most Baker's cysts cause no symptoms and are found on imaging done for other knee problems. A review puts them at 4.7% to 37% of adults without symptoms. (Hasan 2025) They are less common in children. (Neubauer 2011)(Hasan 2025)
| Group | Share with a Baker's cyst | Source |
|---|---|---|
| Children (screening and knee pain studies) | 2.4% to 6.3% | Neubauer 2011 |
| Adults aged 50 to 79, average 63 (900 people, MRI) | 11.7% | Cao 2014 |
| Adults aged 50 and over, average 72 (348 people, MRI) | 27.9% | Kim 2016 |
| Same group, with knee pain in the last month | 36.3% | Kim 2016 |
| Same group, without knee pain | 21.8% | Kim 2016 |
Both adult studies picked people at random from the community and scanned them whether or not they had knee pain. (Cao 2014)(Kim 2016) In one, age and sex were not linked to having a cyst. (Cao 2014) Numbers vary with the scan and the group studied.
When it matters
- Often nothing new. The NHS says a Baker's cyst does not always need treatment, and sometimes, particularly in children, it gets better on its own. (NHS Baker's cyst) A review says cysts in children usually go away within 12 to 24 months. (Hasan 2025)
- It may point to the knee joint. Because the cyst fills with joint fluid, reviews link it to knee osteoarthritis, rheumatoid arthritis and meniscus tears. (Hasan 2025) Whether anything else is looked at depends on the rest of the report and your symptoms. Your doctor decides.
- Calf pain or swelling. A burst cyst and a blood clot in the leg (deep vein thrombosis) can look alike, with calf pain and swelling. A clot is the most common look-alike, and an ultrasound is usually used to tell them apart. (Nanduri 2021)(Hasan 2025) A review notes other look-alikes, such as a widened artery behind the knee. (Hasan 2025)
- Which test. The NHS says a GP may send you for an ultrasound or MRI scan, or refer you for more tests to rule out serious causes. (NHS Baker's cyst) A review of 13 studies found ultrasound picked up Baker's cysts about as well as MRI. (Liu 2022)
- Getting help. The NHS says to see a GP about any lump you do not know the cause of. Ask for an urgent GP appointment or use NHS 111 for leg pain or swelling that suddenly gets worse, swelling with bruising and warm skin, red or darkened skin on the leg, or swelling that gets harder or softer. Call 999 or go to A&E for leg pain and swelling with breathlessness or chest pain. (NHS Baker's cyst)
- Worry is real. See after an abnormal screening result.
Other things a scan may show
The same scan may mention other findings, each with its own page: joint wear in the knee and a fatty lump (lipoma).
Who it applies to
Adults told that a knee MRI or ultrasound, done for something else, shows a Baker's or popliteal cyst. A scan done because of a new lump, sudden calf pain or swelling, people already being treated for a knee condition, and children are outside this page.
Routine age-based screening
A finding on a knee scan does not replace the routine screenings for your age. See screenings by age and gender.
Where they differ:
- How common it is. One community MRI study of adults aged 50 to 79 found 11.7%, another of adults aged 50 and over found 27.9%. (Cao 2014)(Kim 2016) Reviews give 4.7% to 37% in adults without symptoms and 10% to 41% across adult imaging studies. (Hasan 2025)(Neubauer 2011)
- The link to knee wear. One study found Baker's cysts went with more bone spurs and cartilage damage. (Cao 2014) Another found no clear difference between people with and without knee osteoarthritis on X-ray, and says earlier studies disagree. (Kim 2016)
- The link to pain. Both community studies linked Baker's cysts to more knee pain. (Kim 2016)(Cao 2014) One notes that other reports found no such link. (Kim 2016)
Your doctor decides what your result means for you.
Questions people ask
What is a Baker's cyst?
A Baker's cyst, also called a popliteal cyst, is a fluid-filled pocket at the back of the knee. It is not a true cyst but joint fluid that collects in a small sac (bursa) between muscles and tendons there. (NHS Baker's cyst) (Kim 2016)
How common is a Baker's cyst on a knee scan?
It is the most common fluid pocket seen on knee MRI. Studies of adults aged 50 and over found it in 11.7% and 27.9% of people scanned, and reviews give ranges up to about 40%. (Cao 2014) (Kim 2016) (Neubauer 2011)
Is a Baker's cyst linked to knee wear?
Often. Reviews link it to knee osteoarthritis, rheumatoid arthritis and meniscus tears, and one study found it alongside more bone spurs and cartilage damage. Another found no clear difference by knee osteoarthritis on X-ray. (Hasan 2025) (Cao 2014) (Kim 2016)
Can a Baker's cyst be confused with a blood clot?
Yes. A burst cyst can cause sudden calf pain and swelling that looks like a deep vein clot, so an ultrasound is often used to tell them apart. Your doctor decides what your result means for you. (NHS Baker's cyst) (Nanduri 2021) (Hasan 2025)
When should I get help?
The NHS says to ask for an urgent GP appointment or use NHS 111 for leg pain or swelling that suddenly gets worse, or swelling with bruising and warm or red skin, and to call 999 for leg pain or swelling with breathlessness or chest pain. (NHS Baker's cyst)