Physiotherapy · Knee

Baker's cyst UPDATED 2026

Q: What is a Baker's cyst (popliteal cyst)?

Baker's cyst, or popliteal cyst, is a distension of the gastrocnemio-semimembranosus bursa, situated at the back of the knee between the semimembranosus and the medial head of gastrocnemius. This bursa is unusual because it communicates with the joint cavity of the knee through an opening in the capsule behind the medial femoral condyle. That opening creates a one-way valve mechanism: the passage opens in flexion and closes in extension, letting synovial fluid build up in the posterior region of the knee with no return possible. Its prevalence is about 5 % in adults and rises with age.

Q: Is Baker's cyst a disease in itself?

No. In adults, the popliteal cyst is not a disease in itself but a phenomenon secondary to intra-articular pathology of the knee: its formation is associated with an intra-articular disorder in up to 94 % of cases. Across 400 MRI scans, the probability of finding a cyst goes from 8 to 10 % in the presence of a single factor (effusion, meniscal lesion or degenerative arthropathy) to 38 % when all three are present. Management must therefore target the underlying knee pathology rather than the cyst itself.

Q: Which knee lesions accompany a Baker's cyst?

Most often meniscal and cartilage lesions. In an arthroscopic series of 100 patients, those carrying a cyst had a far higher prevalence of medial meniscal lesions (70 % against 19 %) and of cartilage lesions (85 % against 28 %) than controls. Meniscal lesions predominate at the medial meniscus (82 % of cases) ahead of the lateral meniscus (38 %). Across 1001 MRI scans, cysts were associated with one or more joint disorders in 94 % of cases, above all meniscal (83 %, often the posterior horn of the medial meniscus) and cartilaginous (43 %). The physiotherapist's attention must therefore turn to the meniscus and the effusion as the causes to treat.

Q: How is a Baker's cyst diagnosed?

Ultrasound is the investigation of choice. In a meta-analysis (13 studies, 1011 subjects), its pooled sensitivity was 0.97 and its specificity 1.00 against histopathological examination, with an area under the curve of 1.00. Ultrasound also detects cysts that are most often silent: in primary knee osteoarthritis, an ultrasound study found an incidence of 21.1 %, of which 89.28 % were asymptomatic cysts. In a patient with knee pain, finding a cyst should prompt a search for underlying knee osteoarthritis or effusion.

Q: Can a Baker's cyst be dangerous? Which red flag should be known?

Yes, through its rupture. A ruptured or dissecting Baker's cyst can closely mimic a deep vein thrombosis (pseudothrombophlebitis), with a painful, swollen calf, which leads to diagnostic uncertainty and delayed management. The distinction is essential because management differs from that of a phlebitis. Doppler (duplex) ultrasound is the first-line investigation: it reliably rules out deep vein thrombosis by showing normal venous compressibility and flow. Faced with a painful, swollen calf, the physiotherapist must take this possibility on board and direct towards imaging rather than conclude straight away.

Q: How is a Baker's cyst treated?

By treating the cause, not the cyst in isolation. In adults, management targets the underlying intra-articular pathology (osteoarthritis, meniscal lesion) through load management and strengthening, excision surgery remaining rare. For a symptomatic cyst on a background of knee osteoarthritis, the best improvement in pain, function and cyst size is obtained by combining physiotherapy with ultrasound-guided aspiration and corticosteroid injection, rather than either alone. Aspiration with corticosteroid injection under ultrasound guidance brings clinical improvement and a reduction in the volume of the cyst. In children, most cysts regress spontaneously or after treatment of the cause, which justifies a conservative and reassuring approach.

Clinical summary on Baker's cyst (popliteal cyst): understanding that it is most often the symptom of a joint problem, knowing that its rupture can mimic a phlebitis, and targeting management on the cause. Each reference has been checked individually on PubMed.

Popliteal cystA symptom, not a diseaseRupture vs phlebitisUltrasound

94%

Share of popliteal cysts in adults associated with one or more intra-articular disorders of the knee (series of 1001 MRI scans): the cyst is a secondary phenomenon

Sansone 1995 · International Orthopaedics

0.97sensitivity

Pooled sensitivity of ultrasound for the diagnosis of Baker's cyst against histopathology (specificity 1.00; 13 studies, 1011 subjects)

Liu 2022 · Journal of Orthopaedic Surgery and Research

20-40%

Prevalence of Baker's cyst in patients with knee osteoarthritis, rising with age, severity and duration of the disease

Abate 2021 · Medical Principles and Practice

📝 In brief: clinical summary

Baker's cyst (popliteal cyst) is a distension of the gastrocnemio-semimembranosus bursa, at the back of the knee between the semimembranosus and the medial head of gastrocnemius; it communicates with the joint cavity through a one-way valve mechanism that opens in flexion and closes in extension ¹⁴². Its prevalence is about 5 % in adults and rises with age ⁴.
In adults, it is not a disease in itself but a secondary phenomenon : the formation of the cyst is associated with an intra-articular disorder of the knee in up to 94 % of cases ; management must target the joint cause, not the cyst in isolation ⁵³⁶.
The associated lesions are above all meniscal and cartilaginous : in an arthroscopic series, medial meniscal lesions 70 % against 19 % and cartilage lesions 85 % against 28 % in cyst carriers versus controls ⁵⁴.
The cyst is frequent in knee osteoarthritis (prevalence 20 to 40 %, rising with age and severity) and most often silent: up to 89 % asymptomatic on ultrasound ¹¹¹².
The ultrasound is the investigation of choice : in meta-analysis (13 studies, 1011 subjects), pooled sensitivity 0.97 and specificity 1.00 against histopathology ²⁴.
Red flag: the ruptured cyst can mimic a deep vein thrombosis (pseudothrombophlebitis): a painful, swollen calf; the Doppler ultrasound is the first-line investigation to rule out a phlebitis ²⁰¹⁹²⁵.
Treatment targets the underlying intra-articular lesion ; in symptomatic knee osteoarthritis, combining rehabilitation with ultrasound-guided aspiration and corticosteroid injection gives the best results. In children, the cyst is most often primary and regresses spontaneously: a conservative approach ^2122214.

💧 What is a Baker's cyst?

🤫 Frequent in osteoarthritis, and most often silent

Finding a Baker's cyst on imaging does not mean it is the source of the pain: the great majority are asymptomatic.

An ultrasound study in primary knee osteoarthritis. Source: Shakya et al., 2024 (PMID 40457908).

Baker's cyst, or popliteal cyst, is one of the swellings most frequently met at the back of the knee. For the physiotherapist, the point is less to name it than to understand what it reveals: in adults it is rarely a disease in its own right, but rather the visible marker of an underlying joint disorder. That distinction directs the whole of management.

Definition and anatomy

Baker's cyst corresponds to a distension of the gastrocnemio-semimembranosus bursa, situated at the back of the knee between the semimembranosus tendon and the medial head of gastrocnemius ¹. It is therefore not a new growth, but the dilatation of a pre-existing bursa in the popliteal fossa ².

The decisive anatomical feature of this bursa is that it communicates with the joint cavity of the knee, unlike the other peri-articular bursae. That communication runs through an opening in the capsule behind the medial femoral condyle ². It is this continuity between the joint and the bursa that explains the formation of the cyst in adults.

The “valve” mechanism

Understanding the mechanism is essential to clinical reasoning. The communicating opening does not let fluid through in both directions: it works as a one-way valve. Synovial fluid runs from the knee towards the bursa, with no return possible. This has been shown experimentally: after injecting a radio-opaque medium into the cyst, the flow always went from the knee towards the cyst, never the other way ³.

The mechanism is modulated by the position of the knee: the “valve” opens in flexion and closes in extension, letting fluid build up gradually in the posterior region ⁴. As soon as a joint produces an effusion, whatever its cause, that surplus fluid tends to drain towards the bursa and distend it. The cyst thus becomes the reservoir of a knee producing too much fluid.

The cyst is not the disease: it is the trace of a knee producing too much synovial fluid.

A phenomenon that is most often secondary in adults

This is the central clinical message. In adults, the popliteal cyst is not a condition in itself but a secondary phenomenon : management must target the underlying intra-articular lesion rather than the cyst itself ⁵. The association with joint involvement is striking.

94 %of cysts associated with an intra-articular disorder ³

70 %medial meniscal lesions vs 19 % in controls ⁵

85 %cartilage lesions vs 28 % in controls ⁵

In an arthroscopic series of 100 patients, those carrying a cyst showed far more medial meniscal lesions (70 % against 19 %) and cartilage lesions (85 % against 28 %) than controls; the authors conclude that the cyst is a secondary phenomenon and that treatment should address the intra-articular lesions ⁵. Across 1001 MRI scans, cysts were associated with one or more joint disorders in 94 % of cases, mainly meniscal (83 %, often the posterior horn of the medial meniscus) and cartilaginous (43 %) ⁶.

Meniscal involvement comes back constantly: a medial meniscal lesion in 82 % of cases and a lateral meniscal lesion in 38 % ⁴. Across 400 MRI scans, the probability of finding a cyst went from 8-10 % with a single factor (effusion, meniscal lesion or degenerative arthropathy) to 38 % when all three were present ⁷. Other series confirm this link with meniscal lesions and effusion, but not systematically with cartilage or with the cruciate ligaments ⁸: the literature is therefore not entirely of one voice on the respective weight of each lesion, even if the direction is clear: look for the cause inside the joint.

Epidemiology: frequent, and rising with age

The prevalence of popliteal cyst is about 5 % in adults and rises with age ⁴⁹. It climbs markedly as soon as joint disease is present. In older people from the general population, a cross-sectional study of 900 adults (mean age 63 years) found a cyst in 11.7 % and a subgastrocnemius bursitis in 12.7 %, associated with cartilage defects in every compartment and with medial tibiofemoral bone marrow oedema ¹⁰.

Knee osteoarthritis is the favoured ground. In these patients, the prevalence of the cyst lies between 20 % and 40 % and rises with age, with the severity of the osteoarthritis and with the duration of the disease ¹¹. An observational ultrasound study in primary osteoarthritis found an incidence of 21.1 %, of which 89.28 % of the cysts were asymptomatic, hence the value of ultrasound in detecting them ¹². Among patients consulting for knee pain, a cyst was present in 25.8 % of 399 subjects, associated with the ultrasound signs of osteoarthritis and with effusion ¹³.

89 %of cysts in knee osteoarthritis are asymptomatic ¹²

The clinical corollary is direct: in adults, finding a popliteal cyst should prompt a search for underlying knee osteoarthritis or effusion, even with no posterior complaint. The cyst is often silent and found incidentally.

Child or adult: two opposite natural histories

The distinction between the child and the adult is probably the most important point to take on board, because it radically changes the prognosis and the approach.

Criterion	Child	Adult
Nature	Most often primary	Most often secondary
Intra-articular link	Usually absent	Strong (up to 94 %)
Symptoms	Barely symptomatic	Often asymptomatic, sometimes a nuisance
Course	Spontaneous regression frequent	Depends on the joint cause
Approach	Conservative, monitoring	Treat the joint cause

In children, the cyst is most often primary, barely symptomatic and self-limiting. The majority regress spontaneously or after treatment of the underlying cause, which justifies a conservative and reassuring approach ¹⁴. In a paediatric follow-up, 17 cysts (85 %) decreased in size or disappeared on clinical examination and/or ultrasound; these lesions are “usually minimally symptomatic and not related to” intra-articular morbidity in children ¹⁵. Asymptomatic cysts in children can therefore be managed conservatively with good results, surgery being reserved for a minority of cases ¹⁶. Simple monitoring is the rule.

In adults, conversely, the cyst is as a rule secondary, communicating with the joint, and associated with a meniscal lesion (most often medial), a cartilage lesion or an anterior cruciate ligament lesion ¹⁷. In older people, “there is usually coexisting joint pathology” ¹⁸. The approach therefore cannot be one of watchful waiting alone: it must address the cause.

A red flag never to be missed

Before even entering rehabilitation, the physiotherapist must know one major diagnostic trap. A ruptured or dissecting Baker's cyst can closely mimic a deep vein thrombosis (pseudothrombophlebitis), with an acute, painful, swollen calf ¹⁹²⁰. The distinction is essential because management differs entirely from that of a phlebitis.

Faced with a painful, swollen calf, Doppler (duplex) ultrasound is the first-line investigation, above all because it reliably rules out DVT by showing normal venous compressibility and flow ²⁰. One suggestive but rare clinical sign is the “crescent sign”: a dependent bruise below the medial malleolus, produced by the distal migration of synovial fluid along the fascial planes ²⁰. A physiotherapist meeting this picture should direct the patient towards imaging rather than concluding straight away that it is a simple muscle strain.

Key points

Anatomy: distension of the gastrocnemio-semimembranosus bursa, which communicates with the knee through a one-way valve (open in flexion, closed in extension).
In adults: a secondary phenomenon in up to 94 % of cases: look for and treat the joint lesion (medial meniscus, cartilage, effusion, osteoarthritis), not the cyst in isolation.
Epidemiology: ~5 % of the adult population, 20-40 % where there is knee osteoarthritis, very often asymptomatic.
In children: most often primary, barely symptomatic, spontaneous regression frequent: monitoring and reassurance.
Red flag: a ruptured cyst mimics a DVT; Doppler ultrasound first line to rule out a thrombus.

🔗 The real message: a symptom, not a disease

🔗 A Baker's cyst in an adult = a diseased knee underneath

Patients carrying a cyst have far more intra-articular lesions than others. The cyst is the symptom; the disease is in the joint.

An arthroscopic series of 100 patients. Source: Rupp et al., 2002 (PMID 11799006). Across 1,001 MRI scans, a cyst was associated with a joint disorder in 94 % of cases ⁶.

Faced with a swelling in the popliteal fossa, the reflex is often to fix attention on the cyst itself: its size, the trouble it causes, whether to aspirate it. That is the wrong target, however. In adults, Baker's cyst is almost never a disease in its own right: it is the visible consequence of a joint problem in the knee. Understanding that entirely changes the logic of the physiotherapist's management: you do not treat a lump, you treat a knee.

The anatomical reminder that underpins this reasoning: the cyst corresponds to distension of the gastrocnemio-semimembranosus bursa, at the back of the knee, between the semimembranosus and the medial head of gastrocnemius, a bursa that typically communicates with the joint cavity ¹². That communication is not passive: it works as a one-way valve. Synovial fluid passes from the joint towards the bursa in flexion, the opening closes in extension, and return is impossible ³⁴¹⁸. This one-way flow has in fact been confirmed by injecting an opaque medium into cysts: the flow goes from the knee towards the cyst, never the other way ³. In other words, a cyst that grows signals that there is, upstream, excess fluid in the joint, hence something to treat in the knee.

In adults, Baker's cyst is not the disease: it is the witness of a diseased knee.

A secondary phenomenon in the great majority of cases

The literature is remarkably consistent here. In adults, the popliteal cyst is described as a phenomenon secondary to intra-articular pathology, and not as an isolated entity ⁵¹⁷¹⁸. The scale of that association is striking: the formation of the cyst in adults is found in association with an intra-articular disorder in up to 94 % of cases ³. A large series of 1001 knee MRI scans reaches exactly the same figure: cysts were associated with one or more joint disorders in 94 % of cases ⁶.

94 %of Baker's cysts in adults associated with an intra-articular disorder of the knee ³⁶

This dependence on joint pathology can also be read probabilistically. Across 400 MRI scans, the presence of a cyst was significantly associated (P < 0.001) with effusion, with a meniscal lesion and with degenerative arthropathy, each factor weighing independently. The probability of having a cyst went from 8-10 % with only one of these factors to 38 % when all three were present ⁷. The more the knee is affected, the more probable the cyst becomes: that is the very language of a symptom, not of a disease arising by chance.

Meniscus and cartilage: the causes to look for first

When you look at what it is, in the knee, that comes with a cyst, two structures dominate: the medial meniscus and the cartilage. The reference arthroscopic series covering 100 patients is eloquent: those carrying a cyst had a far higher prevalence of medial meniscal lesions (70 % against 19 %) and of cartilage lesions (85 % against 28 %) than controls ⁵. The authors draw a directly workable conclusion: the cyst is a secondary phenomenon, and treatment should address the underlying intra-articular lesions.

Intra-articular lesion	With a popliteal cyst	Without a cyst (controls)
Medial meniscal lesion	70 %	19 %
Cartilage lesion	85 %	28 %

An arthroscopic series of 100 patients ⁵.

Other work confirms and refines this meniscal preference. Across 1001 MRI scans, the lesions most frequently associated with the cyst were meniscal (83 %, often the posterior horn of the medial meniscus) and cartilaginous (43 %) ⁶. In a series of 382 knees, 145 cysts (38 %) were found, with a statistically significant relationship between the cyst and the amount of synovial fluid (P = 0.002) as well as the presence and type of meniscal lesion (P = 0.01), but not with the cruciate ligaments or the cartilage in that particular study ⁸. Finally, an MRI series puts medial meniscal involvement at 82 % and lateral meniscal involvement at 38 % ⁴. The anterior cruciate ligament is not absent from the picture: a complete rupture was found in 13 % of cases in a series where the incidence of the cyst was around 5 % ⁹, and the ACL features among the associated lesions listed in adults ¹⁷.

An honest nuance for the reader : these series do not agree perfectly on the role of the cartilage. Rupp ⁵ and Sansone ⁶ find a strong association with chondral lesions, whereas Martí-Bonmatí ⁸ finds none in his sample. What they all share, on the other hand, is the constant link with the medial meniscus and with effusion. Clinically, that is enough to orient: finding a cyst in an adult means having to look actively for a meniscal lesion and an effusion.

Osteoarthritis and arthropathies: the ground of the older patient

Baker's cyst is also, very largely, a phenomenon of the ageing, osteoarthritic knee. Its prevalence is about 5 % in the general adult population and rises with age ⁴. In older people, there is usually coexisting joint pathology ¹⁸. A cross-sectional study of 900 adults (mean age 63 years) found a popliteal cyst in 11.7 % and a subgastrocnemius bursitis in 12.7 %, both significantly associated with cartilage defects in every compartment and with medial tibiofemoral bone marrow oedema ¹⁰. Here again, the cyst is not isolated: it marks a damaged joint structure.

20–40 %prevalence of Baker's cyst where there is knee osteoarthritis, rising with age and severity ¹¹

Where knee osteoarthritis is established, the prevalence climbs to 20-40 % and rises with age, with the severity of the osteoarthritis and with the duration of the disease ¹¹. Among patients consulting for knee pain, an ultrasound study of 399 subjects found a cyst in 25.8 % of them, with a positive association with the ultrasound signs of osteoarthritis and with effusion ¹³. The practical message is plain: in adults, demonstrating a cyst should prompt a search for underlying knee osteoarthritis or effusion.

One point is crucial if the therapeutic target is not to be mistaken: in primary knee osteoarthritis, the cyst is most often silent. An observational ultrasound study found an incidence of 21.1 %, of which 89.28 % were asymptomatic cysts ¹². Treating at all costs a cyst that does not hurt, when it is only the reflection of an osteoarthritis, amounts to confusing the warning light with the breakdown.

Aspirating the cyst without treating the osteoarthritis means switching off the light without repairing the engine.

What this changes for rehabilitation

If the cyst is a symptom, then the therapeutic logic reorganises around the joint cause. Several elements support this. First, conservative treatment is less durable when a cyst accompanies osteoarthritis: in one study, scores improved at 3 months in every case, but at 6 months they stayed stable without a cyst and deteriorated in patients carrying one, in the medium term, the effectiveness of the therapy declines in the presence of an associated cyst ¹¹. The cyst therefore signals a more affected knee, one to monitor more closely.

Next, when a local procedure is indicated on a symptomatic cyst complicating knee osteoarthritis, it gains from being combined with rehabilitation rather than set against it. A randomised controlled trial on 60 patients compared ultrasound-guided injection alone, physiotherapy alone and the combination of the two: patients receiving the injection (alone or combined), but not those treated with physiotherapy alone, maintained a lower level of pain at follow-up, the combined treatment giving the best results on pain, function and cyst dimensions ²¹. Ultrasound-guided aspiration with corticosteroid injection remains, besides, an option that reduces volume and pain ²²²³, but it never removes the need to treat the osteoarthritis or the meniscal lesion upstream. Excision surgery remains rare, and postoperative rehabilitation itself depends largely on the condition associated with the cyst ².

Key points

In adults, Baker's cyst is almost always secondary : associated with an intra-articular disorder in up to 94 % of cases ³⁶.
The causes to look for first are the medial meniscus (70 % vs 19 %), cartilage lesions (85 % vs 28 %) and effusion ⁵⁷.
It is frequent in knee osteoarthritis (20-40 %), rising with age and severity, but often asymptomatic ¹¹¹².
You treat the joint cause, not the cyst in isolation: load management, strengthening and management of the osteoarthritis or the meniscal lesion ⁵².
Nuance: the role of the cartilage is not found in every series (a strong association in Rupp/Sansone, absent in Martí-Bonmatí): the constant link remains medial meniscus + effusion.

In practice, the right question is therefore not “how do I make this cyst disappear?” but “what knee disorder is this cyst signalling to me?”. Asking the second is already to have grasped the essentials of the condition in adults.

🩻 How to diagnose it, and not confuse it with a phlebitis

🩻 Ultrasound: the investigation of choice, and it is excellent

Non-invasive, available, highly accurate: it is the first-line investigation for confirming a Baker's cyst.

A meta-analysis of 13 studies (1,011 subjects), compared with histopathological examination, area under the curve 1.00. Source: Liu et al., 2022 (PMID 36510299).

Faced with a swelling or discomfort in the popliteal fossa, the point of the assessment is not only to confirm the cyst: it is to understand why it is there and not to let its main trap slip through, the false phlebitis. Let us recall the mechanism, because it directs the whole approach: Baker's cyst arises from distension of the gastrocnemio-semimembranosus bursa, at the back of the knee between the semimembranosus and the medial head of gastrocnemius, which communicates with the joint cavity through an opening behind the medial femoral condyle ¹². That opening works as a valve: in flexion it opens, in extension it closes, so that fluid passes from the knee towards the bursa without being able to come back ⁴³. Diagnosing a cyst, in an adult, therefore almost always amounts to diagnosing a knee that produces too much fluid.

You do not diagnose a Baker's cyst: you diagnose the knee that manufactures it.

Ultrasound: the first-line investigation

Ultrasound is the investigation of choice for confirming a popliteal cyst. In a meta-analysis pooling 13 studies and 1011 subjects, its pooled sensitivity reached 0.97 and its specificity 1.00 against histopathological examination, with an area under the curve of 1.00 ²⁴. In other words: when ultrasound sees a cyst, it is one; and it misses very few.

0.97sensitivity of ultrasound for Baker's cyst ²⁴

1.00specificity and area under the curve of ultrasound vs histopathology ²⁴

This performance explains why ultrasound is also the screening tool for silent cysts. In primary knee osteoarthritis, an observational ultrasound study found an incidence of 21.1 %, of which 89.28 % were asymptomatic cysts ¹² : without ultrasound, most of these cysts would go unnoticed. In the same way, among 399 patients consulting for knee pain, a cyst was found on ultrasound in 25.8 % of cases, in positive association with the ultrasound signs of osteoarthritis and with joint effusion ¹³. For the physiotherapist the message is twofold: ultrasound confirms the cyst with near-certainty, and finding one should prompt a search for underlying knee osteoarthritis or effusion rather than close the reasoning.

MRI: for the cause, more than for the cyst

MRI is not needed to confirm a cyst, ultrasound is enough, but it becomes valuable when looking for the joint lesion responsible. Its strength is mapping the whole knee, and the imaging series show just how much the cyst is a marker of intra-articular involvement:

Across 400 knee MRI scans, the presence of a cyst was significantly associated (P < 0.001) with effusion, with a meniscal lesion and with degenerative arthropathy; the probability of having a cyst went from 8-10 % with only one of these factors to 38 % when all three coexisted ⁷.
Across 1001 MRI scans, popliteal cysts were associated with one or more joint disorders in 94 % of cases, the most frequent being meniscal (83 %, often the posterior horn of the medial meniscus) and cartilaginous (43 %) ⁶.
Across 382 knees, the cyst was present in 38.0 % and correlated significantly with the amount of synovial fluid (P = 0.002) and with the presence and type of meniscal lesion (P = 0.01), but not with the cruciate ligaments or the cartilage ⁸.
Across more than 1000 MRI scans, the incidence was about 5 %, strongly linked to medial meniscal tears, with a complete rupture of the anterior cruciate ligament in 13 % of cases ⁹.

One detail deserves honesty: these studies do not agree perfectly on the role of the cartilage and the cruciates: Sansone ⁶ and Miller ⁷ find a strong cartilaginous or degenerative association, whereas Martí-Bonmatí ⁸ finds no link with the cartilage or the cruciates. The solid common ground, for its part, does not vary: in adults, the cyst goes hand in hand with effusion and with a medial meniscal lesion. That is where MRI usefully directs the rehabilitation plan.

What clinical examination brings, and its limits

Clinically, the cyst presents as a swelling in the popliteal fossa, tenser in extension and softening in flexion, consistent with the valve mechanism described above ⁴. But clinical examination settles neither the exact nature of the mass nor, above all, its most formidable differential diagnosis. In adults, let us remember that the cyst is most often asymptomatic and secondary ¹²⁵ : palpating a popliteal mass is a starting point, not a conclusion. Imaging remains indispensable.

The major trap: pseudothrombophlebitis

Here is the red flag to know by heart. A ruptured or dissecting Baker's cyst releases its synovial contents into the planes of the calf and causes acute pain and swelling that closely mimic a deep vein thrombosis (DVT): this is “pseudothrombophlebitis” ¹⁹²⁰. The confusion is not theoretical: among patients with negative venography, 80 % of a selected group in fact had a dissecting or ruptured Baker's cyst ¹⁹. And in a prospective study comparing 100 ruptured cysts with 100 intact ones, the ruptured cysts were significantly more often discovered during the differential diagnosis with a thrombophlebitis (28 against 17; p = 0.04) ²⁵.

A calf that becomes abruptly painful and swollen is not a phlebitis until an ultrasound scan has said so.

The distinction is crucial because management differs radically: anticoagulation for a DVT, joint management for a cyst ¹⁹. The first-line investigation is the same under both hypotheses: ultrasound, Doppler / duplex, of the femoral vein and the popliteal fossa. It reliably rules out DVT by showing normal venous compressibility and flow, and it demonstrates the thrombus or the ruptured cyst ¹⁹²⁰. One suggestive clinical sign, rare but telling, can accompany the rupture: the “crescent sign”, a dependent bruise appearing below the medial malleolus, due to the distal migration of synovial fluid along the fascial planes ²⁰.

Feature	Ruptured Baker's cyst (pseudothrombophlebitis)	Deep vein thrombosis
Picture	An acute, painful, swollen calf, often after exertion or flexion; sometimes a crescent sign below the medial malleolus ²⁰	A painful, swollen calf, clinically indistinguishable on its own ¹⁹
Key investigation	Popliteal + femoral vein Doppler: vein compressible, normal flow, ruptured cyst visible ¹⁹²⁰	Doppler ultrasound: vein not compressible, thrombus visible ²⁰
What is at stake	Treat the joint cause; do not anticoagulate wrongly ¹⁹	Anticoagulation, a medical emergency ¹⁹

When to refer, and quickly

The physiotherapist is often the first professional to see a swollen, painful calf. Their responsibility is not to decide between a ruptured cyst and a phlebitis: it is to think of it and to direct the patient towards imaging rather than concluding straight away ²⁵. Any suspicion of DVT calls for venous ultrasound first line, without delay ¹⁹²⁰. Outside the emergency, finding a cyst on ultrasound in an adult should direct the workup towards the joint cause (effusion, medial meniscal lesion, osteoarthritis), since the cyst is almost always its symptom and not the disease ⁶⁷.

Key points

Ultrasound = investigation of choice to confirm the cyst ²⁴ and to screen for the silent cysts that are frequent in osteoarthritis ¹².
MRI = for the cause, not for the cyst: it reveals the associated intra-articular involvement in up to 94 % of cases ⁶⁷.
Red flag: an acute, painful, swollen calf may be a ruptured cyst simulating a phlebitis ¹⁹²⁰.
Reflex: suspicion of DVT → venous Doppler ultrasound first line, urgently; never conclude without imaging ²⁰²⁵.
In adults, the cyst is a secondary phenomenon : look for and treat the knee, not the cyst ⁵.

⚠️ Rupture and complications: what needs to be known

The great majority of popliteal cysts are silent and harmless: in knee osteoarthritis, an ultrasound study found 89.28 % of cysts to be asymptomatic ¹². But one development deserves the physiotherapist's full attention because it constitutes a genuine diagnostic trap: rupture of the cyst. It is not a frequent complication; it is above all a complication that must be recognised so as not to be confused with a vascular emergency.

Faced with a calf that becomes abruptly painful and swollen, the first question is not “where does the cyst come from?” but “is this a thrombosis?”.

Rupture: when the cyst empties into the calf

Baker's cyst fills through a one-way valve mechanism: synovial fluid passes from the joint towards the gastrocnemio-semimembranosus bursa, with no return possible ³⁴. When the internal pressure becomes too great, the wall can give way and the contents spread downwards, along the planes of the calf. The resulting picture is abrupt: acute pain and swelling of the calf, sometimes a sensation of “snapping” or tearing behind the knee.

The problem is that this picture resembles, almost feature for feature, that of a deep vein thrombosis (DVT). This is called pseudothrombophlebitis : a hot, tense, painful, oedematous calf. Rupture of a Baker's cyst is described as an under-recognised situation that “closely mimics” a DVT and leads to diagnostic uncertainty and delayed management ²⁰. Yet the two forms of management have nothing in common: urgent anticoagulation on one side, rest and local treatment on the other. Confusing them risks either treating a phlebitis that does not exist, or missing a genuine DVT.

The scale of this trap has long been documented. In an old but eloquent series, among patients with a painful, swollen calf whose venography was negative (no thrombus), 80 % in fact had a ruptured or dissecting Baker's cyst ¹⁹. More recently, a prospective study comparing 100 ruptured cysts with 100 intact ones showed that ruptured cysts were significantly more often discovered in the context of a differential diagnosis with a thrombophlebitis (28 against 17; p = 0.04) ²⁵.

80 %of painful calves with negative venography in a selected series: in fact a ruptured or dissecting cyst, not a phlebitis ¹⁹

Recognising the picture: calf, bruising, crescent sign

A few clinical features can orient, without ever being enough to settle matters. One suggestive, though rare, sign is the crescent sign : a dependent bruise appearing below the medial malleolus, produced by the distal migration of synovial fluid along the fascial planes of the calf ²⁰. In other words, a “bruise” at the inner ankle in a patient with a painful calf, a few days after symptoms begin, should suggest a ruptured cyst rather than a simple contusion.

But no clinical sign replaces imaging. The authors' message is constant: faced with this picture, the first-line investigation is Doppler (duplex) ultrasound, above all because it reliably rules out DVT by showing normal venous compressibility and blood flow ²⁰¹⁹. Ultrasound also visualises the cyst directly: in a meta-analysis (13 studies, 1011 subjects), its pooled sensitivity for the diagnosis of Baker's cyst was 0.97 and its specificity 1.00 ²⁴.

Feature	What the literature shows	Level of evidence
Rupture = DVT mimic	A picture of pseudothrombophlebitis: a painful, swollen calf, very close to a genuine phlebitis ¹⁹²⁰	High / consistent
Investigation to request first	Doppler ultrasound to rule out a DVT (normal venous compressibility and flow) then visualise the cyst ²⁰²⁴	High
Crescent sign	A dependent bruise below the medial malleolus, from fluid migrating along the fasciae: suggestive but rare ²⁰	Moderate (a rare sign)
Frequency in a context of vascular doubt	Ruptured cysts more often identified during the differential diagnosis with a thrombophlebitis: 28 vs 17 (p = 0.04) ²⁵	Moderate

And what about compression?

One sometimes reads that the cyst, by occupying the space of the popliteal fossa, would compress the vein or nerve structures. Let us be honest about the state of the evidence: the solid claims available to us document the rupture and its differential diagnosis, not a validated mechanism of compression. We shall therefore not settle the matter beyond what the literature selected allows us to assert. The practical principle remains the same: any calf or popliteal fossa that becomes abruptly painful and swollen, with vascular signs, falls outside the scope of physiotherapy reasoning on its own and calls for a medical opinion with Doppler ultrasound to rule out a DVT ²⁰. Caution takes precedence over a hasty label.

What the physiotherapist spots and what they do

The physiotherapist's role is not to make the diagnosis of rupture, but to think of the possibility and to refer rather than to conclude straight away ²⁵. In concrete terms:

Spot the red flag : sudden appearance of a painful, hot, swollen calf, especially in a patient already carrying knee osteoarthritis or a known cyst. Do not reassure too quickly: this picture must first prompt a DVT to be excluded.
Do not mobilise or massage aggressively an acute, tense calf as long as a thrombosis has not been ruled out: the step is logical out of vascular caution.
Direct towards imaging : refer for a Doppler ultrasound of the popliteal fossa and the femoral vein, the first-line investigation that rules out DVT and confirms the cyst ²⁰¹⁹²⁴.
Once the emergency has been ruled out, come back to the underlying logic: in adults, the cyst is a secondaryphenomenon, associated with intra-articular pathology in up to 94 % of cases ³⁶. Rehabilitation targets the cause (a medial meniscal lesion, osteoarthritis, effusion) and not the cyst in isolation ⁵².

Key points

The complication to know is rupture, which mimics a deep vein thrombosis: an acute painful, swollen calf = pseudothrombophlebitis ²⁰¹⁹.
The reflex is not to treat, but to rule out a DVT first : Doppler ultrasound first line (normal compressibility and flow), also highly accurate for seeing the cyst (sensitivity 0.97) ²⁰²⁴.
A suggestive but rare sign: the crescent sign, a bruise below the medial malleolus ²⁰.
On any possible vascular or nerve compression , the evidence selected remains limited: we direct towards a medical opinion rather than assert.
Emergency ruled out, you treat the joint cause, not the cyst alone ⁵².

🎯 What management, and what place for physiotherapy?

The principle that organises the whole of management fits in one sentence: in adults, Baker's cyst is not a disease in itself, but a secondary phenomenon. It arises from distension of the gastrocnemio-semimembranosus bursa, which communicates with the joint through a one-way valve mechanism: fluid passes from the knee towards the cyst with no return possible ³¹⁸². In other words, the cyst is the visible witness of a joint problem upstream, and the therapeutic logic consists in treating the cause, not the consequence.

In adults, you do not treat a Baker's cyst: you treat the knee that manufactured it.

Treat the joint cause before the cyst

The data converge with rare consistency. In an arthroscopic series of 100 patients, those carrying a cyst far more often had a medial meniscal lesion (70 % against 19 %) and cartilage lesions (85 % against 28 %) than controls; the authors conclude explicitly that the cyst is a secondary phenomenon and that treatment should target the underlying intra-articular lesion ⁵. Across 1001 MRI scans, a joint disorder was found in 94 % of cases, dominated by meniscal (83 %, often the posterior horn of the medial meniscus) and cartilaginous (43 %) lesions ⁶. Another MRI series confirms a statistically significant link between the cyst and effusion and meniscal lesion, but not with the cruciate ligaments or the cartilage ⁸.

94 %of Baker's cysts in adults associated with an intra-articular disorder of the knee ⁶

This association rises with the number of abnormalities present: across 400 MRI scans, the probability of having a cyst went from 8-10 % with a single factor (effusion, meniscal lesion or degenerative arthropathy) to 38 % when all three coexisted ⁷. In a patient consulting for knee pain, finding a cyst should therefore prompt a search for, and management of, knee osteoarthritis or effusion: in an ultrasound study of 399 patients in pain, a cyst was present in 25.8 % of them, in association with the signs of osteoarthritis and with effusion ¹³.

In concrete terms, physiotherapy management follows the pathology identified. Herman and Marzo ² put it plainly: rehabilitation, including after any procedure, depends largely on the condition associated with the cyst. The work therefore bears on load management and strengthening directed at the osteoarthritis or the meniscal lesion, rather than on the cyst itself. This section describes a line of reasoning; it does not replace individual clinical examination.

Key points

In adults, Baker's cyst is most often secondary : it reflects a knee pathology (up to 94 % of cases).
The target of treatment is the intra-articular lesion (medial meniscus, cartilage, effusion), not the cyst in isolation.
In knee osteoarthritis it is frequent (20-40 %) and often asymptomatic : finding one does not on its own justify a procedure.
Physiotherapy is directed at the cause (load management, strengthening); combined with an ultrasound-guided injection, it gives the best results on the symptomatic cyst.
Red flag: a ruptured cyst can mimic a phlebitis: Doppler ultrasound first line to rule out a DVT.

The asymptomatic cyst: often, do nothing

An important nuance tempers the interventionist impulse: in knee osteoarthritis the cyst is common but most often silent. An ultrasound study finds an incidence of 21.1 %, of which 89.3 % were asymptomatic cysts ¹². Its prevalence in osteoarthritis patients lies between 20 and 40 % and rises with age, severity and duration of the disease ¹¹. A cyst seen on imaging is therefore not, in itself, an indication for treatment: it is the functional trouble that guides, and the joint cause that structures management.

89 %of Baker's cysts on a background of knee osteoarthritis are asymptomatic ¹²

In children, the reasoning is more conservative still: the cyst is most often primary, barely symptomatic and self-limiting. The majority regress spontaneously or after treatment of the cause ¹⁴ ; in a paediatric follow-up, 85 % of cysts decreased or disappeared, these lesions usually being minimally symptomatic and without intra-articular morbidity ¹⁵. Simple monitoring is the rule, surgery an exception ¹⁶.

Ultrasound-guided aspiration and injection

When the cyst is genuinely symptomatic (large, painful, troublesome), the best documented local procedure is aspiration followed by a corticosteroid injection under ultrasound guidance. This approach brings clinical improvement and a reduction in volume in every subgroup of patients carrying a cyst secondary to knee osteoarthritis ²². As a stand-alone treatment, a technique combining fenestration and injection (triamcinolone + bupivacaine) brought the WOMAC score down from 48.55 to 17.15, with no infection and no complication, with a recurrence rate requiring a further aspiration of 12.7 % in the long run ²³.

12.7 %recurrences requiring a further aspiration after ultrasound-guided aspiration and injection ²³

The most useful point for the physiotherapist comes from a randomised controlled trial (60 patients, knee osteoarthritis + cyst) comparing three arms: ultrasound-guided injection alone, physiotherapy alone, and the combination. Patients receiving the injection, alone or combined, maintained a lower level of pain over time, unlike those treated with physiotherapy alone; and it was the combined treatment that gave the best results on pain, function and cyst dimensions ²¹. The argument is therefore not “local procedure or rehabilitation”, but precisely the articulation of the two.

Strategy for a symptomatic cyst (knee osteoarthritis)	Effect observed	Evidence
Ultrasound-guided injection + physiotherapy	Best results on pain, function and cyst size	RCT ²¹
Ultrasound-guided aspiration and injection alone	Clinical improvement, reduction in volume; recurrence ~13 %	Cohorts ²²²³
Physiotherapy alone	Does not maintain the fall in pain over time; effectiveness declining at 6 months if a cyst is associated	RCT / observ. ²¹¹¹

Rehabilitation: directed at the cause, to be qualified

One observation calls for clear-sightedness: conservative treatment, rehabilitation included, appears less durable when a cyst accompanies knee osteoarthritis. At 3 months, scores improve with or without a cyst; but at 6 months they stay stable without a cyst and deteriorate in cyst carriers: the effectiveness of the therapy declines in the medium term in the presence of an associated cyst ¹¹. Should one conclude that rehabilitation is useless? No: the available data do not compare standardised strengthening programmes, and the best overall result is still obtained by combining rehabilitation and injection ²¹. The honest reading is a qualified one: physiotherapy keeps its full place on the underlying pathology, but an associated cyst often signals more advanced joint involvement, which must be taken into account in the goals and in the prognosis given to the patient.

It should be stressed that the literature does not, to date, provide a trial isolating the effect of a specific strengthening protocol (targeted on the quadriceps, for example) on the cyst itself: the reasoning rests on the validated management of the underlying osteoarthritis and meniscal lesion, on which the cyst depends ². Stating that uncertainty is better than masking it.

Surgery: rare, targeted on the lesion

Surgical excision of the cyst remains rare and is not the first-line answer. Here again the logic is causal: when a procedure is indicated it targets the intra-articular disorder, and postoperative rehabilitation depends largely on the condition associated with the cyst rather than on the cyst itself ². In children, surgery is reserved for a minority of resistant cases, the vast majority calling for simple monitoring ¹⁶¹⁵.

The red flag never to be missed

One last point is a matter of patient safety more than of rehabilitation. Rupture or dissection of a Baker's cyst can closely mimic a deep vein thrombosis (pseudothrombophlebitis): an acute, painful, swollen calf ¹⁹²⁰. In a series of patients with negative venography, 80 % in fact had a dissecting or ruptured cyst ¹⁹ ; and ruptured cysts are significantly more often discovered during the differential diagnosis with a thrombophlebitis ²⁵. Management of a DVT differs radically: faced with this picture, Doppler (duplex) ultrasound is the first-line investigation, because it reliably rules out DVT by showing normal venous compressibility and flow ²⁰. A suggestive but rare sign is the “crescent sign”: a dependent bruise below the medial malleolus, linked to the distal migration of synovial fluid along the fascial planes ²⁰. The physiotherapist's reflex is not to conclude, but to direct towards imaging.

An acute, swollen calf is never “just a cyst” until a phlebitis has been ruled out on Doppler ultrasound.

🗂️ What do concrete case reports teach us?

The case that follows is published and carries its PubMed identifier. It was selected because it brings together, in one patient, the two complications this chapter means to make you fear. They serve only to stage mechanisms solidly established by the literature. Every decision mentioned refers back to confirmed data: Baker's cyst as a secondary phenomenon to be treated through its joint cause, and cyst rupture as a red flag mimicking a deep vein thrombosis. Neither of these two accounts should be read as personalised management: they illuminate a line of reasoning, they replace neither clinical examination nor imaging.

Baker's cyst is almost never the disease: in adults, it is the visible sign of a knee in trouble.

A published case: an UNRUPTURED popliteal cyst compressing the tibial nerve and the vein

The case. Wang and colleagues report a symptomatic popliteal cyst combining, in the same patient, a compressive neuropathy and venous congestion, and that without the cyst being ruptured²⁶.

The authors' reminder. Popliteal cysts are a collection of synovial fluid in the popliteal fossa, forming in adults in association with trauma, degenerative change or inflammatory arthritis of the knee. Often asymptomatic, they can become problematic: by enlarging or by rupturing, they compress the neighbouring neurovascular structures, which produces oedema of the lower limb or a peripheral neuropathy²⁶.

What this changes in the session. Two received ideas fall with this case. The first is that a rupture would be needed for complications to occur: here the cyst is intact. The second is that a swollen calf points to thrombosis or to rupture, and nothing else: venous congestion from compression gives the same picture. A sensory or motor deficit in the tibial territory, with a popliteal swelling, should prompt referral rather than rehabilitation.

The treatment principle recalled earlier in this article does not change for all that: it is the knee that produces the cyst, and it is the knee that is treated. This case adds only that before treating the knee, one must check what the cyst is compressing.

Key points: the popliteal swelling

In adults the cyst is secondary: you treat the knee (meniscus, cartilage, osteoarthritis), not the “lump” ⁵².
A cyst associated with knee osteoarthritis may darken the prognosis at 6 months ¹¹.
In symptomatic forms, combining ultrasound-guided aspiration and injection with rehabilitation appears superior to either option alone: evidence still limited ²¹.

This case joins the general lesson: Baker's cyst in adults is a signal, not an isolated target. A signal of a damaged knee in case 2, a signal of a possible vascular trap in case 1. In both, the right reflex is not to treat the cyst, but to understand what it reveals.

🧭 How is this applied in practice?

The thread running through this whole section comes down to one idea: in adults, Baker's cyst is almost never the disease, it is its symptom. It arises from distension of the gastrocnemio-semimembranosus bursa, which communicates with the joint through a one-way valve mechanism, open in flexion and closed in extension ⁴²¹⁸. That fluid comes from the knee: if it builds up, there is an effusion, and therefore a joint reason to produce it. The physiotherapist's clinical reasoning must therefore constantly “work back” from the cyst to its cause.

You do not treat a Baker's cyst: you treat the knee that manufactures it.

A three-step reasoning algorithm

Faced with a posterior swelling of the knee or a painful knee in an adult, the approach can be structured as three successive questions.

Step 1: Rule out the emergency. The very first question is not “is this a cyst?” but “could this painful, swollen calf be a deep vein thrombosis (DVT)?”. A rupture or dissection of a cyst can closely mimic a DVT (pseudothrombophlebitis) and delay management ²⁰¹⁹²⁵. This is the central red flag, set out below.

Step 2: Confirm the cyst and look for its cause. Once the vascular emergency has been ruled out, the point is not to “see the cyst” but to understand why it is there. In adults it is secondary to intra-articular pathology in up to 94 % of cases ³⁶. The causes to hunt down are, in order of frequency, the meniscal lesion, above all the posterior horn of the medial meniscus, cartilage involvement and osteoarthritis, more rarely an anterior cruciate ligament lesion ^5417789.

94 %of Baker's cysts in adults are associated with an intra-articular disorder of the knee ⁶³

Step 3: Treat the cause, not the pouch. The practical consequence is direct: physiotherapy management targets the underlying osteoarthritis or meniscal lesion (load management, strengthening, control of the effusion), and not the cyst in isolation ⁵². Even after surgery, rehabilitation depends first on the associated pathology ².

The red flag never to be missed: the false phlebitis

This is the most important point in the whole section. A ruptured or dissecting cyst shows as acute pain and swelling of the calf that resemble a DVT feature for feature. Yet the two forms of management have nothing in common, and an unrecognised DVT is potentially serious. The distinction is therefore an absolute priority ¹⁹²⁰.

A few useful landmarks for the clinic:

The context orients : in a surgical series comparing 100 ruptured cysts with 100 intact ones, the ruptured cysts were significantly more often discovered in the context of the differential diagnosis with a thrombophlebitis (28 against 17; p = 0.04) ²⁵. Faced with an acute calf, cyst rupture is therefore legitimately among the hypotheses, but is never presumed.
A suggestive but rare sign : the “crescent sign”, a dependent bruise appearing below the medial malleolus, linked to the distal migration of synovial fluid along the fascial planes ²⁰. Its presence is an argument, its absence rules nothing out.
The golden rule : none of these clinical signs allows a DVT to be ruled out on its own. The first-line investigation is Doppler (duplex) ultrasound of the femoral vein and the popliteal fossa, precisely because it reliably rules out thrombosis by showing normal venous compressibility and flow ²⁰¹⁹.

Practical conduct: faced with an acute, painful, swollen calf, you never assert “it is only a cyst”, you direct towards imaging ¹⁹²⁵. It is the safety reflex that takes precedence over everything else.

Confirming and following the cyst: the place of ultrasound

When confirmation of the cyst is needed, ultrasound is the investigation of choice: in a meta-analysis (13 studies, 1011 subjects), its pooled sensitivity reached 0.97 and its specificity 1.00 against histopathology ²⁴. It has a double value: it sees the cyst, and it documents the effusion and the associated signs of osteoarthritis ¹³¹⁰. It is also what has shown how often the cyst is silent: in primary knee osteoarthritis, one study found an incidence of 21.1 %, of which close to 90 % were asymptomatic cysts ¹². In other words, finding a cyst does not mean it explains the patient's complaint.

Key messages to pass on to the patient

The way the cyst is explained governs adherence. Three messages, all supported:

“The cyst is a signal, not the disease.” In adults it reflects a knee problem (meniscus, cartilage, osteoarthritis, effusion) that needs managing ⁵¹⁷. That usefully shifts the patient's attention from the “lump behind the knee” to their knee.
“The size of the cyst is not the measure of your problem.” Many cysts are painless ¹². The aim is not to make the pouch disappear at all costs, but to improve pain and function.
“A calf that swells and becomes painful all at once calls for a prompt opinion.” The red flag is explained in simple terms, so that the patient seeks advice without delay rather than waiting ²⁰.

In children the message is different and reassuring: the cyst is most often primary, barely symptomatic and self-limiting. In a paediatric follow-up, 85 % of cysts decreased or disappeared, with no link to intra-articular morbidity ¹⁵¹⁴¹⁶. Simple monitoring is the rule.

Rehabilitation and associated procedures: what the evidence says

Rehabilitation addresses the cause: load management and strengthening in osteoarthritis or a meniscal lesion ². One point deserves honesty: the evidence of effectiveness for physiotherapy on its own on the cyst itself is limited. In a randomised trial in knee osteoarthritis patients with a cyst, physiotherapy alone did not maintain the fall in pain over time, whereas the groups receiving an ultrasound-guided corticosteroid injection (alone or combined) did: the best result on pain, function and cyst size being obtained by the combination of local procedure + rehabilitation ²¹. Another nuance worth knowing: when a cyst accompanies osteoarthritis, the improvement from conservative treatment tends to erode in the medium term (6 months) compared with knees without a cyst ¹¹.

Interventional procedures, aspiration with ultrasound-guided corticosteroid injection, are an option that reduces pain and cyst volume ²²²³, with a recurrence rate requiring a further aspiration of about 13 % ²³. They do not replace treatment of the cause: they add to it. Excision surgery, for its part, remains rare ².

When to refer?

The physiotherapist directs towards a medical opinion or further imaging in the following situations:

Urgently : an acute, painful, swollen calf raising the suspicion of a rupture, direction towards a Doppler ultrasound to rule out a DVT before any other hypothesis ²⁰¹⁹.
To identify the cause : a painful knee in an adult with a confirmed or suspected cyst, in order to look for the underlying meniscal or cartilage lesion or osteoarthritis (imaging, specialist opinion) ⁵¹⁷.
In case of failure or persisting trouble : a resistant symptomatic cyst, to discuss ultrasound-guided aspiration and injection alongside rehabilitation ²¹²².
Faced with an atypical sign : a rapidly growing, hard mass, or an unusual picture that falls outside the frame of an ordinary secondary cyst, caution then calls for an opinion.

Key points

In adults the cyst is secondary (up to 94 % of cases): treat the knee (meniscus, cartilage, osteoarthritis, effusion), not the pouch ³⁵.
Safety reflex : an acute, painful, swollen calf calls for a DVT to be ruled out by Doppler ultrasound before concluding that a cyst has ruptured ²⁰¹⁹.
Ultrasound confirms the cyst (Se 0.97 / Sp 1.00) and documents the effusion and the associated osteoarthritis ²⁴.
Size does not make the symptom : the majority of cysts on a background of osteoarthritis are silent ¹².
Rehabilitation directed at the cause ; the best result combines a local procedure (ultrasound-guided injection) and physiotherapy ²¹.
In children : most often primary and self-limiting, monitoring and reassurance ¹⁵¹⁴.

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❓ Frequently asked questions

What is a Baker's cyst (popliteal cyst)?

Baker's cyst, or popliteal cyst, is a distension of the gastrocnemio-semimembranosus bursa, situated at the back of the knee between the semimembranosus and the medial head of gastrocnemius ¹. This bursa is unusual because it communicates with the joint cavity of the knee through an opening in the capsule behind the medial femoral condyle ². That opening creates a one-way valve mechanism: the passage opens in flexion and closes in extension, letting synovial fluid build up in the posterior region of the knee with no return possible ⁴³. Its prevalence is about 5 % in adults and rises with age ⁴.

Is Baker's cyst a disease in itself?

No. In adults, the popliteal cyst is not a disease in itself but a phenomenon secondary to intra-articular pathology of the knee: its formation is associated with an intra-articular disorder in up to 94 % of cases ³⁶. Across 400 MRI scans, the probability of finding a cyst goes from 8 to 10 % in the presence of a single factor (effusion, meniscal lesion or degenerative arthropathy) to 38 % when all three are present ⁷. Management must therefore target the underlying knee pathology rather than the cyst itself ⁵.

Which knee lesions accompany a Baker's cyst?

Most often meniscal and cartilage lesions. In an arthroscopic series of 100 patients, those carrying a cyst had a far higher prevalence of medial meniscal lesions (70 % against 19 %) and of cartilage lesions (85 % against 28 %) than controls ⁵. Meniscal lesions predominate at the medial meniscus (82 % of cases) ahead of the lateral meniscus (38 %) ⁴. Across 1001 MRI scans, cysts were associated with one or more joint disorders in 94 % of cases, above all meniscal (83 %, often the posterior horn of the medial meniscus) and cartilaginous (43 %) ⁶. The physiotherapist's attention must therefore turn to the meniscus and the effusion as the causes to treat.

How is a Baker's cyst diagnosed?

Ultrasound is the investigation of choice. In a meta-analysis (13 studies, 1011 subjects), its pooled sensitivity was 0.97 and its specificity 1.00 against histopathological examination, with an area under the curve of 1.00 ²⁴. Ultrasound also detects cysts that are most often silent: in primary knee osteoarthritis, an ultrasound study found an incidence of 21.1 %, of which 89.28 % were asymptomatic cysts ¹². In a patient with knee pain, finding a cyst should prompt a search for underlying knee osteoarthritis or effusion ¹³.

Can a Baker's cyst be dangerous? Which red flag should be known?

Yes, through its rupture. A ruptured or dissecting Baker's cyst can closely mimic a deep vein thrombosis (pseudothrombophlebitis), with a painful, swollen calf, which leads to diagnostic uncertainty and delayed management ²⁰¹⁹. The distinction is essential because management differs from that of a phlebitis. Doppler (duplex) ultrasound is the first-line investigation: it reliably rules out deep vein thrombosis by showing normal venous compressibility and flow ²⁰. Faced with a painful, swollen calf, the physiotherapist must take this possibility on board and direct towards imaging rather than conclude straight away ²⁵.

How is a Baker's cyst treated?

By treating the cause, not the cyst in isolation. In adults, management targets the underlying intra-articular pathology (osteoarthritis, meniscal lesion) through load management and strengthening, excision surgery remaining rare ². For a symptomatic cyst on a background of knee osteoarthritis, the best improvement in pain, function and cyst size is obtained by combining physiotherapy with ultrasound-guided aspiration and corticosteroid injection, rather than either alone ²¹. Aspiration with corticosteroid injection under ultrasound guidance brings clinical improvement and a reduction in the volume of the cyst ²². In children, most cysts regress spontaneously or after treatment of the cause, which justifies a conservative and reassuring approach ¹⁴.

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