A Baker's cyst, also called a popliteal cyst, is a pocket of fluid at the back of the knee, on the inner side. In adults it usually forms when extra fluid from inside the knee joint fills a small sac there (a bursa), so it is mostly a sign of what is happening inside the joint, such as arthritis or a meniscus tear. Small cysts also turn up in knees that do not hurt. One safety point: new calf pain or swelling needs a same-day medical check, because a ruptured cyst and a blood clot can look alike.
Assessing and treating knee pain at the Burlington clinic. Convenient for Waterdown, Oakville, Hamilton, Flamborough, and Carlisle residents.
When to see a doctor firstBroader Knee Pain GuideWhere it sits. Behind the knee, on the inner side, the inner head of the calf muscle (medial gastrocnemius) and one of the inner hamstring tendons (semimembranosus) run close together, with a small sac between them, a bursa, that helps them glide. This bursa can connect with the knee joint through an opening in the joint capsule, and the opening is thought to act like a one-way valve, letting fluid in more easily than out. When the joint makes extra fluid, the bursa can fill and stay full. That is what the report calls a Baker's cyst, and in one large series of knee scans every cyst connected with the joint (Sansone et al., 1995).
Ultrasound or MRI. Both show a Baker's cyst well. On ultrasound, the sign a radiologist looks for is a neck of fluid between the semimembranosus tendon and the tendon of the inner calf muscle, joining the cyst to the joint; in a small study, that sign identified every Baker's cyst correctly (Ward et al., 2001). An MRI also shows the inside of the joint, which is usually where the explanation lies.
The words that come with it. These are the terms you are most likely to see around it.
| On the report | What it means |
|---|---|
| Baker's cyst, popliteal cyst | The same finding. Popliteal refers to the hollow at the back of the knee, and Baker is the surgeon the cyst is named after. |
| Distended semimembranosus-gastrocnemius bursa | The anatomical name for the same thing. Distended means stretched with fluid. Some reports put the muscle names the other way round (gastrocnemio-semimembranosus); it is the same sac. |
| Communicating with the joint, neck | The radiologist can see the channel between the cyst and the joint. It marks the lump as a Baker's cyst rather than another kind of swelling. |
| Simple or anechoic | Clear fluid with nothing floating in it. Anechoic is the ultrasound word for fluid that shows up black. |
| Complex, septations, debris, loose bodies | Thin walls inside the cyst (septations), material floating in the fluid (debris), or small fragments of cartilage or bone that have collected in it (loose bodies). They describe the contents; whether they matter depends on your symptoms and the rest of the report. |
| Ruptured, leaking or dissecting | Fluid has escaped into the calf, or the cyst has stretched down between the calf muscles. New calf pain or swelling with this needs a same-day medical check, explained in the section on calf swelling below. |
| Joint effusion | Extra fluid inside the knee joint, the usual source of the fluid in the cyst. The fluid on the knee guide explains the size words. |
| Tear of the posterior horn of the medial meniscus | A tear at the back of the inner meniscus, one of the tears most often found with a cyst (Sansone et al., 1995). The meniscus tear page covers what it means for you. |
| Osteoarthritis, cartilage loss, osteophytes | Worn cartilage and bony spurs. Cysts are found more often in arthritic knees (Hill et al., 2001), and the knee osteoarthritis page covers the plan. |
| Size in mm or cm | How big the cyst was on the day of the scan, often in three directions. In painless knees, most cysts measured 3 cm or less at their largest (Tschirch et al., 2003). |
Chest pain, shortness of breath, coughing up blood or feeling faint, especially with a painful or swollen calf
Go to emergency now, or call 911. A clot that has travelled to the lungs (pulmonary embolism) needs to be ruled out.
Severe calf pain that keeps building with a calf that feels very tight and hard, or a foot that turns cold, pale or blue
Go to emergency now. Pressure building inside the calf muscles (compartment syndrome) or a blocked blood supply needs urgent treatment.
A hot, red or swollen knee and you feel unwell
Go to emergency.
New calf pain, swelling, warmth or redness, whether or not you know you have a cyst
Same-day medical assessment to rule out a blood clot (deep vein thrombosis). A ruptured cyst can look the same and the two can occur together, so do not assume it is the cyst.
A newly hot, red or swollen knee, even without a fever
Same-day medical assessment to rule out infection. Infection is possible even without a fever.
A knee that locks and will not straighten, especially after a twist
A displaced meniscus tear needs to be ruled out. Get it assessed the same day by your family doctor, urgent care or emergency; it needs an urgent orthopaedic referral.
A lump behind the knee that pulses, feels hard or fixed, or keeps growing
See your family doctor promptly. Not every lump behind the knee is a cyst: a widened artery (popliteal aneurysm) or, rarely, a growth in the soft tissues can look similar and needs a medical diagnosis.
It usually points to the inside of the knee. In a series of 1,001 knee scans, 94 percent of the cysts came with at least one problem inside the joint, the commonest being meniscus tears, often at the back of the inner meniscus, and cartilage damage (Sansone et al., 1995). In another series, extra joint fluid, a meniscus tear and arthritis were each linked with cysts independently of one another, and the more of them a knee had, the more likely a cyst was: 38 percent of knees with all three had one (Miller et al., 1996).
It can turn up in knees that do not hurt. On MRI, about one in five painless knees had a cyst in one study, most of them 3 cm or smaller (Tschirch et al., 2003). In older adults, cysts were found in 28 percent of painless knees with arthritis on X-ray and 9 percent of painless knees with a normal X-ray (Hill et al., 2001). The figures vary with age, with who is scanned and with the type of scan: in another study, none of 100 people without knee complaints had a cyst on ultrasound (Rupp et al., 2002).
It is usually not the whole explanation for the pain. Studies differ on how much a cyst adds to knee pain. In those older adults, once the severity of arthritis was taken into account, painful knees did not have cysts more often than painless ones, though they did have more joint fluid and a thicker joint lining (Hill et al., 2001). On the other hand, among people with arthritis bad enough for a knee replacement, 71 percent had symptoms from the cyst (Hommel et al., 2016). A large cyst can give a feeling of fullness or tightness behind the knee, particularly when the knee bends fully.
What keeps it filled matters more. The report names the cyst because it can be seen. What matters is what keeps it filled, and that is usually in the other lines of the same report, about the menisci, the cartilage and the joint fluid, and in how your knee behaves.
What a rupture is. A cyst can split or leak, letting joint fluid run down between the calf muscles, or it can stretch down into the calf without bursting, which reports call dissecting. Either can cause pain, tightness and swelling in the calf, sometimes suddenly.
Why it is not assumed to be the cyst. A Baker's cyst, especially a ruptured one, and a blood clot in a deep vein of the leg (deep vein thrombosis) can be hard to tell apart by examination. Among 95 people found to have a Baker's cyst on leg vein ultrasounds done for a suspected clot, 7 also had a clot, 10 had a ruptured cyst, and in 6 the cyst was pressing on the main vein behind the knee (Langsfeld et al., 1997). In more than 350,000 leg vein scans, a cyst was not linked with a higher chance of a clot being found, but 8 percent of the scans that showed a clot also showed a cyst (Daniels et al., 2022). A cyst already named on your report does not rule out a clot.
What to do. New calf pain, swelling, warmth or redness needs a same-day medical assessment, whether or not you know about a cyst. The usual test is an ultrasound of the leg veins, arranged by the doctor who sees you. Chest pain, shortness of breath or coughing up blood, especially with a painful or swollen calf, means go to emergency now, because part of a clot can travel to the lungs.
Other lumps behind the knee. Not every lump behind the knee is a Baker's cyst. In one study, a cyst from the meniscus and a rare soft tissue growth had both been reported as Baker's cysts on ultrasound, and on review neither had the neck of fluid that marks a true one (Ward et al., 2001). A widened artery (popliteal aneurysm) can also sit there. A lump that pulses, feels hard or fixed, or keeps growing is one for your family doctor.
Treatment is aimed first at the knee problem that keeps the cyst filled. The cyst is treated directly only when it keeps causing symptoms.
I start with your calf. If it is newly painful, swollen, warm or red, it needs a same-day medical assessment to rule out a blood clot before anything else, whatever the report says about the cyst. Then I ask how the knee has been behaving: whether it swells after activity, catches or locks, whether a twist or injury started it, and what it stops you doing.
On examination I check for fluid in the joint and fullness behind the knee, measure how far the knee bends and straightens, feel along the joint line, and test the strength of the quadriceps and hip. I screen the lower back as well, because pain behind the knee can come from there. I read the report alongside that, with more weight on the lines about the menisci, the cartilage and the joint fluid than on the cyst itself.
The plan follows what keeps the cyst filled. For an arthritic knee, that means strengthening, particularly the quadriceps and hip, with walking or cycling and changes to the loads that make the knee swell; for a meniscus tear, it is the rehabilitation for that tear. If the back of the knee feels tight in a full bend, deep bending is built back gradually. If the cyst keeps causing symptoms, or something on the report or in the examination does not fit, the next step is your family doctor. I cannot order imaging; when further imaging or a procedure such as drainage would change the plan, I flag it and refer you.
Usually not on its own. In adults it is mostly a sign of extra fluid from a problem inside the knee, such as arthritis or a meniscus tear, and small cysts turn up in knees that do not hurt. New calf pain, swelling, warmth or redness needs a same-day medical check to rule out a blood clot, and chest pain or breathlessness with it means go to emergency now. A lump that pulses, feels hard or keeps growing needs your family doctor.
In adults, usually a problem inside the knee joint that makes extra fluid, most often osteoarthritis or a meniscus tear; inflammatory arthritis such as rheumatoid arthritis can do the same. The fluid passes into a sac at the back of the knee that connects with the joint, and the opening is thought to let fluid in more easily than out. In children, cysts here usually appear without a problem inside the joint, and the family doctor is the place to start.
Sometimes. A cyst can disappear once the knee problem behind it is treated, though less often in arthritic knees, where it often stays visible on later scans. A year after a knee replacement, most cysts were still there, while far fewer people had symptoms from them. The plan is judged by how the knee feels and works, not by whether the cyst has gone.
Often a sudden pain or tightness in the calf, followed by swelling, as fluid leaks down between the calf muscles. A blood clot in the leg can feel the same, and the two can occur together, so it needs a same-day medical assessment, usually with an ultrasound of the leg veins. Chest pain or breathlessness with it means go to emergency now.
In more than 350,000 leg vein scans, a cyst was not linked with a higher chance of a clot being found. A cyst can occasionally press on the main vein behind the knee, though, and a cyst and a clot can be present together. New calf symptoms need a same-day check whether or not you have a cyst.
Usually that a little joint fluid has collected in the sac behind the knee. Small cysts are found in painless knees as well: in one study about one in five painless knees had a cyst, most of them 3 cm or smaller. On its own it rarely changes the plan; the rest of the report and how the knee behaves matter more.
Not usually as a first step. A steroid injection, sometimes with the cyst drained, can ease a cyst that keeps causing symptoms, but cysts can refill when the knee keeps making extra fluid, which is why the knee is treated first. Keyhole surgery is considered for a cyst that keeps causing problems despite that, and the decision is made with an orthopaedic surgeon.
Usually, yes, guided by the knee problem behind it. Strengthening and regular activity are central to the plan for an arthritic knee and for many meniscus tears, and deep bending may need building back gradually if the back of the knee feels tight. New calf pain or swelling is the exception: stop and get a same-day medical check.
The main sources behind this guide. Other studies are named in the text where they are used.
1995 Sansone V et al., International Orthopaedics
1,001 people having knee MRI scans. A cyst was seen in 4.7 percent, and 94 percent of those came with at least one problem inside the joint; the commonest were meniscus tears (83 percent), often at the back of the inner meniscus, and cartilage damage (43 percent). Every cyst connected with the joint.
1996 Miller TT et al., Radiology
Reports of 400 knee MRI scans in a general orthopaedic population. Cysts were linked with joint fluid, meniscus tears and arthritis, each independently of the others, but not with anterior cruciate or medial collateral ligament injuries. The chance of a cyst was 8 to 10 percent with one of those three findings, about 20 percent with two and 38 percent with all three.
2002 Rupp S et al., American Journal of Sports Medicine
100 people booked for knee arthroscopy and 100 people without knee complaints, checked by ultrasound. Cysts were found in 20 percent of the surgical group and none of the others, and those with a cyst had more inner meniscus tears (70 versus 19 percent) and cartilage damage (85 versus 28 percent). After surgery that treated the joint and left the cyst alone, 11 of the 16 cysts followed up remained at one to three years, all in knees with deep cartilage damage.
2003 Tschirch FT et al., American Journal of Roentgenology
MRI of 102 knees without symptoms, the other knee of people being scanned for a suspected meniscus tear (average age 43). A Baker's cyst was found in 19 knees, about one in five, and 92 percent of the cysts measured 3 cm or less.
2001 Hill CL et al., Journal of Rheumatology
MRI of one knee in 458 older adults (average age 67). Cysts were found in 33 percent of painful knees with arthritis on X-ray, 28 percent of painless knees with arthritis and 9 percent of painless knees with a normal X-ray. After allowing for the severity of arthritis, cysts were not more common in painful knees; larger effusions and a thickened joint lining were.
2001 Ward EE et al., American Journal of Roentgenology
36 knees scanned with both ultrasound and MRI. On review, fluid between the semimembranosus tendon and the inner head of the calf muscle, joined to the cyst, identified every Baker's cyst correctly. On the original ultrasound reports, a meniscal cyst and a rare soft tissue growth had been called Baker's cysts.
1997 Langsfeld M et al., Journal of Vascular Surgery
3,072 people had a leg vein ultrasound for a suspected clot, and 95 (3.1 percent) had a Baker's cyst. Seven of those 95 also had a clot, 10 had a ruptured cyst, and in 6 the cyst pressed on the main vein behind the knee. The authors found the two hard to tell apart by examination.
2022 Daniels P et al., Journal of Thrombosis and Thrombolysis
357,703 leg vein ultrasounds in 237,052 adults at the Mayo Clinic. Clots and cysts were each seen on 9.1 percent of scans. A cyst was not linked with a higher chance of a clot, but 8 percent of the scans that showed a clot also showed a cyst.
2016 Hommel H et al., Bone & Joint Journal
102 people with osteoarthritis and a Baker's cyst on MRI, followed for a year after a knee replacement. The cyst was still visible on ultrasound in 85 percent, while the share with symptoms from it fell from 71 percent to 31 percent.
2020 Van Nest DS et al., JBJS Reviews
30 treatment studies in people aged 16 and over. Eight of the nine non-surgical studies used steroid injections, more recently into the cyst under ultrasound, sometimes with the cyst wall punctured; most surgical studies widened the cyst's connection with the joint through keyhole surgery. Both reported improvement, cysts coming back has long been a problem, and the authors called for further studies to show whether surgery is better than other methods.
Condition pages for the knee problems most often behind a Baker's cyst.
I see patients for knee pain at Endorphins Health & Wellness Centre in Burlington. The clinic serves people coming in from Burlington, Waterdown, Oakville, Hamilton, Flamborough, and Carlisle, with free parking on site and a ground-floor entrance.
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