Baker's cyst: a swelling appears behind the knee, often more obvious with the leg straight. The technical name is popliteal cyst, a sac of synovial fluid that collects in the back of the knee. It was first described by the surgeon William Baker in 1877.
The most useful thing to know is that in adults the cyst is almost never the disease itself: it is the visible sign of something happening inside the joint. For this reason, the guide starts from a case that should not be managed with ice and patience, but with an urgent assessment.
⚠ Important: read this before continuing
The exercises and advice in this article are general indications and may not suit your situation. Every person has a unique clinical history: what works for others could be counterproductive for you.
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Swollen, painful calf: this is an emergency, not a nuisance
If the calf suddenly becomes swollen, painful, warm or red, whether or not you know you have a cyst, go to the emergency department the same day. No ice, no rest, no "let's see whether it's better tomorrow".
The reason is that rupture of a popliteal cyst and deep vein thrombosis produce the same clinical picture, and thrombosis can cause a pulmonary embolism. Calf pain with swelling is one of the presentations in which thrombosis is the first diagnosis to rule out, and the distinction is made with venous duplex ultrasound and ultrasonography, not with the examination (Leow et al., 2019).
Two findings make the matter less straightforward than it is usually presented.
Having a cyst does not rule out thrombosis. In a series of 3,072 venous duplex scans performed precisely to exclude thrombosis, 95 patients had a popliteal cyst: 7 of them also had a deep vein thrombosis, and in 6 the cyst was compressing the popliteal vein. The authors conclude that the two presentations are similar enough that they cannot be told apart by clinical examination (Langsfeld et al., 1997). Finding the cyst does not close the question: thrombosis has to be looked for separately.
The opposite error also does harm. Seven consecutive patients are described who were treated for thrombosis, with therapeutic-dose low molecular weight heparin and without any imaging, when in fact they had a ruptured cyst: all of them worsened abruptly to compartment syndrome of the leg, and 4 needed an emergency fasciotomy (Tejero et al., 2018). This is not an argument against treating thrombosis: if a thrombosis is there, it must be treated, and the treatment is decided by the doctor. It is an argument for ultrasound before deciding, and against any initiative taken on your own.
Swelling of the whole leg, breathlessness or chest pain, fever with a hot and red knee, a knee that locks after an injury, and loss of strength or sensation in the foot also need to be assessed straight away.
A remote consultation, ours included, does not replace that assessment: thrombosis cannot be ruled out on a video call.
Why it forms
Behind the knee, between the medial head of the gastrocnemius and the semimembranosus, there is a bursa that often communicates with the joint. When the knee produces excess fluid, that opening works like a one-way valve: the fluid enters the bursa and does not flow back, the bursa stretches and becomes the cyst you can see and feel. In some cases there is no communication, and the cyst arises as a bursitis of the same bursa (Handy, 2001).
It follows that the cyst is downstream of the problem, not upstream. In adults there is almost always a joint condition feeding the effusion:
- Meniscal tear. In 382 knee MRI scans, 38% showed a popliteal cyst, and its presence and size were significantly linked to the amount of joint fluid and to the presence and type of meniscal tear, while they were not associated with cruciate or cartilage lesions (Martí-Bonmatí et al., 2000). An ultrasound study of 131 knees found the association with a medial meniscal tear independently of the other intra-articular changes (Artul et al., 2015).
- Knee osteoarthritis and effusion. In 399 patients with knee pain studied with ultrasound, the cyst was present in 25.8% of cases and was associated with ultrasound signs of osteoarthritis and with the presence of effusion (Picerno et al., 2014).
- Inflammatory arthritis, such as rheumatoid arthritis, and in general any condition that produces synovitis and fluid.
Depending on the population studied and the imaging technique, the cyst appears in 5–32% of knees with problems, with two peaks of age: 4–7 years and 35–70 years (Handy, 2001).
In children the picture is different: the cyst is usually primary, produces little in the way of symptoms and is not linked to internal damage to the joint (Akagi et al., 2013).
How it shows up
The typical sign is a swelling in the popliteal fossa, more obvious with the knee straight, of very variable size. It is often accompanied by a feeling of tension or fullness behind the knee, especially on full flexion, and by difficulty bending the leg fully.
Two clarifications that change the approach. The first: many cysts are asymptomatic and are found by chance, and an asymptomatic cyst discovered incidentally does not need treatment. The second: the physical examination misses about half of them, which is why the diagnostic method of choice is ultrasound (Handy, 2001). If there is swelling but you cannot feel it, that does not mean there is nothing there; and if you can feel it, ultrasound is still needed to see what is inside the joint.
Pain, when present, can come from the cyst itself or, often, from the knee problem that produced it: in many cases it is the latter that dominates the picture.
What physiotherapy can do, and what it cannot
Let us be clear: there are no studies showing that exercises make the sac disappear. Anyone promising that is promising something the literature does not support. What can be worked on is the engine of the cyst and the symptoms that come from it.
When the engine is osteoarthritis, exercise has evidence in its favour, but it is worth reading that evidence for what it says. The Cochrane review updated to 2024 included 139 studies and 12,468 participants: compared with an attention or placebo control, exercise improves pain by about 8.7 points out of 100 with low certainty and function by about 11 points out of 100 with moderate certainty, and most studies were at unclear or high risk of bias (Cochrane, 2024). These are real improvements at low risk, not a cure.
In practice the work consists of reducing the load that irritates the joint, recovering the extension and flexion you tolerate, strengthening the quadriceps and knee stabilisers, and gradually rebuilding tolerance to movement. If the cyst tends to shrink, it is because the inflammation producing the fluid has eased.
Medical treatments and their limits
A systematic review of 30 studies published between 1970 and 2019 assessed all the treatments proposed for popliteal cysts. On the non-surgical side, the option with the most literature behind it is the intracystic corticosteroid injection under ultrasound guidance, possibly with fenestration of the cyst: 8 of the 9 non-surgical studies included used cortisone. On the surgical side, the arthroscopic approach that widens the communication with the joint is the most studied, with positive results. But the overall level of evidence of the review is IV, and the authors state explicitly that further studies are needed to confirm the superiority of one technique over the others (Van Nest et al., 2020).
A useful detail for calibrating expectations comes from an Italian randomised trial of 60 patients with knee osteoarthritis and a cyst: in the two groups treated with ultrasound-guided aspiration and corticosteroid injection, pain at four weeks stayed lower than at baseline, but the maximum axial area of the cyst did not change in any of the three groups (only the sagittal measurement varied over time) (Di Sante et al., 2012). It is a small study, at a single centre and with a one-month follow-up, so it does not decide anything on its own; but it points in the same direction as everything else: you gain on the symptom, not necessarily on the sac.
Surgery remains rarely necessary, reserved for large, symptomatic cysts that do not respond to everything else (Handy, 2001).
In children
In a series of 20 knees followed on average for more than four years, 17 cysts (85%) shrank or disappeared spontaneously, with MRI confirming complete disappearance in some of the cases re-evaluated. The authors conclude that in the paediatric population complete resolution without treatment can be expected (Akagi et al., 2013). It is a case series, so the evidence is of low level, but it points towards watchful waiting rather than surgery.
What to expect
The prognosis depends on the cause. If the joint problem producing the fluid is managed, the cyst tends to shrink; if it is not addressed, recurrence is frequent whatever is done to the cyst itself. And it is reasonable to live with a cyst that causes no symptoms, rather than chasing its disappearance.
The overall picture of the evidence on this condition is of low quality: many case series, few randomised trials, small samples. Anyone offering you a resolving protocol with certainty is going beyond what we know.
How we work remotely
On a video call a physiotherapist can reconstruct the history of the knee, assess movement, strength and pain, work out which joint problem is most likely feeding the cyst, and set the load and exercises accordingly, as well as directing you to an ultrasound or a specialist visit when needed. What cannot be done remotely is rule out thrombosis or palpate the popliteal fossa: that requires an in-person examination, and if the calf is acutely swollen, it is needed straight away.
With Ri-Hub an online physiotherapy session costs 29,90 €.
In summary
- A suddenly swollen, warm or painful calf needs assessment the same day: a ruptured cyst and deep vein thrombosis look too alike, and having a cyst does not protect you from getting both.
- In adults the cyst is almost always secondary: meniscus, osteoarthritis, effusion, arthritis.
- If it causes no symptoms, it often does not need treatment.
- Physiotherapy acts on the cause and on the symptoms; it has not been shown to eliminate the sac.
- Most of the available evidence is of low quality: realistic expectations, not promises.
Sources
- Handy JR. Popliteal cysts in adults: a review. Seminars in Arthritis and Rheumatism, 2001. PubMed
- Martí-Bonmatí L, et al. MR imaging of Baker cysts: prevalence and relation to internal derangements of the knee. MAGMA, 2000. PubMed
- Picerno V, et al. Prevalence of Baker's cyst in patients with knee pain: an ultrasonographic study. Reumatismo, 2014. PubMed
- Artul S, et al. The association between Baker's cyst and medial meniscal tear in patients with symptomatic knee using ultrasonography. Clinical Imaging, 2015. PubMed
- Langsfeld M, et al. Baker's cysts mimicking the symptoms of deep vein thrombosis: diagnosis with venous duplex scanning. Journal of Vascular Surgery, 1997. PubMed
- Tejero S, et al. Ruptured Baker's cyst: complications due to misdiagnosis. Emergencias, 2018. PubMed
- Leow KS, et al. Sonographic assessment of musculoskeletal causes of calf pain and swelling. Emergency Radiology, 2019. PubMed
- Van Nest DS, et al. Popliteal cysts: a systematic review of nonoperative and operative treatment. JBJS Reviews, 2020. PubMed
- Di Sante L, et al. Ultrasound-guided aspiration and corticosteroid injection compared to horizontal therapy for treatment of knee osteoarthritis complicated with Baker's cyst: a randomized, controlled trial. European Journal of Physical and Rehabilitation Medicine, 2012. PubMed
- Akagi R, et al. Natural history of popliteal cysts in the pediatric population. Journal of Pediatric Orthopaedics, 2013. PubMed
- Lawford BJ, et al. Exercise for osteoarthritis of the knee. Cochrane Database of Systematic Reviews, 2024. PubMed
Sources last checked: August 2026.
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