Baker's Cyst (Popliteal Cyst)

A Baker's cyst, or popliteal cyst, is a fluid-filled synovial cyst in the popliteal fossa: a distension of the gastrocnemius-semimembranosus bursa that communicates with the knee joint. William Morrant Baker described it in 1877 and took it for a primary disorder of the bursa. It is now recognised as almost always secondary to intra-articular knee pathology, which is why the knee, not the cyst, is what gets investigated and treated.

How common. Popliteal cysts are seen in approximately 5-32% of knees imaged, depending on the population and the imaging technique, with two age peaks at 4-7 years and 35-70 years (Handy, Semin Arthritis Rheum 2001, PMID 11590580). There is a slight female predominance. Cysts can be bilateral, especially in inflammatory arthritis.

Primary or secondary. In adults the cyst is secondary in 95%: something inside the joint is producing an effusion, and the cyst is its marker. In children Baker's cysts are rare, with a prevalence of about 6.3% on knee MRI (De Maeseneer 1999, PMID 10415188); they are usually primary, with no underlying knee pathology, and usually resolve spontaneously, so observation is appropriate and intervention is rarely needed.

What drives it. On MRI, joint effusion, meniscal tear and degenerative arthropathy are each independently associated with a Baker's cyst, and the probability of a cyst rises with the number of these features present (Miller, Radiology 1996, PMID 8816552). The recognised secondary causes:

• Meniscal tears, the most common, especially the posterior horn of the medial meniscus
• Osteoarthritis, through degenerative change and synovitis
• Rheumatoid arthritis and the other inflammatory arthropathies (psoriatic, reactive, gout), through chronic synovitis
• ACL tears, through chronic effusion, although Miller found no independent association with ACL injury
• Chondral injuries (chondral defects, osteochondritis dissecans), through synovial reaction
• Pigmented villonodular synovitis (PVNS)
• Post-infectious synovitis after septic arthritis

Because the cyst is a symptom rather than the disease, the pages that matter most are the ones covering what is driving it, most often a meniscal tear or knee osteoarthritis. Its dangerous mimic, and the reason a painful calf here is never assumed to be a burst cyst, is set out in venous thromboembolism; the tendinopathy that shares its anatomy in semimembranosus tendinopathy; and the solid lesion that can present as a popliteal swelling in tenosynovial giant cell tumour.

The popliteal fossa. A diamond-shaped space behind the knee, roofed by the deep fascia and floored by the posterior capsule, popliteus, and the posterior femur and tibia. Its boundaries:

• Superomedially, semimembranosus and semitendinosus
• Superolaterally, biceps femoris
• Inferomedially, the medial head of gastrocnemius
• Inferolaterally, the lateral head of gastrocnemius and plantaris

Its contents:

1. Popliteal artery, the deepest structure
2. Popliteal vein, superficial to the artery
3. Tibial nerve, the most superficial
4. Common peroneal nerve, laterally
5. Small saphenous vein, in the superficial fascia
6. Popliteal lymph nodes
7. Fat

The bursa. The gastrocnemius-semimembranosus bursa lies in the posteromedial part of the fossa, between the medial head of gastrocnemius anteriorly and the semimembranosus tendon posteriorly. A communication between the bursa and the knee joint may exist as a normal anatomical variant in some individuals, while in others the bursa arises primarily without a joint connection (Handy 2001, PMID 11590580).

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How the cyst forms. Intra-articular pathology produces a chronic effusion and intra-articular pressure rises. Fluid dissects through the posterior capsule at an area of weakness, and at the communication site, the posteromedial capsule, typically between the medial head of gastrocnemius and the joint capsule, a flap of tissue comes to act as a one-way valve. Intra-articular pressure during knee motion forces fluid into the bursa, the pressure in the bursa cannot overcome the valve to send it back, and the bursa progressively distends.

Neurovascular relationships. The tibial nerve lies lateral to the cyst, the popliteal vessels deep and lateral, and the common peroneal nerve lateral again, around the fibular head. Large cysts can compress any of these.

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A cyst is described by its cause, by what it is doing to the patient, and by its size and how far it has travelled. Each answers a different question about treatment.

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History. The usual story is a gradual onset of fullness or a mass behind the knee, an aching discomfort in the popliteal fossa, and a sensation of tightness or pressure that is worse with prolonged standing or activity. Ask about previous knee injury or arthritis. An acute onset of calf pain and swelling is the story of a ruptured cyst.

Differential diagnosis. The alternatives for a mass in the popliteal fossa, and how to tell them apart:

Popliteal artery entrapment, inflammatory (rheumatoid) synovitis, and a septic bursitis or abscess also belong on the list.

Inspection. Look for fullness in the popliteal fossa, better seen in extension, and compare with the other side; ecchymosis suggests rupture. The Foucher sign is pathognomonic: the cyst becomes more prominent and tense with the knee in extension, as fluid is pushed posteriorly, and less prominent in flexion, as gastrocnemius relaxes and the space increases. That behaviour distinguishes it from a solid tumour.

Palpation. A soft, fluctuant mass with smooth borders in the posteromedial fossa, non-tender unless ruptured or infected, and it may be compressible. Transillumination confirms a fluid-filled structure, and gentle compression may reduce the size temporarily. Warmth suggests inflammation or infection.

Range of motion and the knee. Movement is usually full, with perhaps mild flexion discomfort from a large cyst. The knee examination is essential, because it is the knee that will be treated:

• McMurray test and joint line tenderness (meniscal tear)
• Lachman and anterior drawer (ACL)
• Varus and valgus stress (collateral ligaments)
• Crepitus (arthritis)
• Effusion

Neurovascular examination. Test the tibial nerve (ankle plantar flexion, toe flexion, plantar sensation) and the common peroneal nerve (ankle dorsiflexion, toe extension, first web space sensation), feel the popliteal and pedal pulses, and examine the veins if DVT is suspected.

The ruptured cyst (pseudothrombophlebitis syndrome). Acute calf pain, swelling and calf tenderness, presenting identically to a DVT. The crescent sign, ecchymosis below the medial malleolus, is pathognomonic for a ruptured cyst. Homan's sign may be positive but is non-specific. DVT and a ruptured cyst can coexist, so a Doppler ultrasound is mandatory even when the crescent sign is present.

Ultrasound first. It confirms an anechoic, fluid-filled cyst between the medial head of gastrocnemius and semimembranosus, may show the neck communicating with the joint, and measures the size accurately. It is non-invasive, involves no radiation, is relatively inexpensive, and is dynamic, so the Foucher sign can be tested under the probe; it shows fluid tracking into the calf after rupture and guides aspiration if needed. Its limits are the joint: assessment of intra-articular pathology is limited, small meniscal tears may be missed, and it is operator dependent.

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MRI for the joint. MRI is the gold standard for comprehensive assessment, and its real value is not the cyst but the pathology behind it: it identifies the underlying knee pathology in 95% of cases. The cyst is a well-defined fluid-signal mass, low on T1 and high on T2, in the posteromedial fossa between gastrocnemius and semimembranosus, with a neck or stalk to the joint. The same study shows the meniscal tear (especially the posterior horn of the medial meniscus), cartilage defect, ligament tear or synovitis that will drive treatment. Request it to:

• Identify the underlying knee pathology (meniscal tears, ligament injuries)
• Characterise a complex or large cyst
• Look for complications (dissection, compression)
• Plan surgery
• Assess atypical features (solid components, septations)

It measures the cyst accurately in all planes, shows proximal or distal extension, multiloculation and septations, and maps the relationship to the neurovascular structures.

Plain radiographs have a limited role. They assess for osteoarthritis, exclude calcified masses and show bony abnormalities; the cyst itself appears at most as a posterior soft tissue shadow, and calcification is rare and suggests chronicity or another diagnosis. They cannot show the cyst well, its internal structure, or the soft tissue pathology behind it.

CT is rarely indicated. It shows the cyst as a low-density mass, is useful when there is concern about vascular pathology or bone involvement, and contrast shows the enhancement pattern.

Doppler ultrasound is essential when there is concern about DVT, which means the ruptured cyst presenting as pseudothrombophlebitis. It also assesses the popliteal artery to exclude an aneurysm and looks for venous compression.

Arthrography is historical: contrast injected into the knee joint filled the cyst and demonstrated the communication. It has been superseded by MRI and ultrasound.

Aspiration as a test. Clear, yellow synovial fluid confirms the diagnosis; send it for cell count, culture and crystal analysis if infection or inflammatory arthritis is a concern. As a treatment it gives temporary relief with a high recurrence rate and is not definitive, for the reasons and figures set out in Management.

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Algorithm

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The principle. The cyst is a manifestation of the knee, and simply treating the cyst is inadequate. Treatment is directed at the intra-articular condition, and the cyst often resolves once the effusion is controlled. Everything below is ordered around that: conservative measures and treatment of the joint first, aspiration to buy time, and surgery on the cyst reserved for specific indications.

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Rare complications. Compartment syndrome (very rare, from massive dissection), foot drop from mass effect on the peroneal nerve, and popliteal artery thrombosis (very rare).

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Most patients recover fully within 6-8 weeks of cyst excision, with low recurrence if the knee pathology has been addressed.

Follow-up. Review at 2 weeks for the wound, at 6 weeks for range of motion and clinical examination, at 3 months to confirm resolution without recurrence, and thereafter as needed if symptoms recur or concerns arise.

What the patient is told. Treatment of the underlying knee pathology continues after the cyst has gone, and any return of posterior knee swelling should be reported. Strengthening and weight management keep the knee working, and the arthritis or meniscal problem behind the cyst still needs addressing.

Left alone. Many cysts remain stable or resolve spontaneously, 30-40% increase in size over time, and a proportion rupture (the incidence is in Complications). Symptoms vary, and many patients tolerate the cyst well.

With conservative treatment. 80-90% improve symptomatically if the underlying knee pathology is addressed; the cyst may shrink or stay the same size, and recurrence is common if the pathology is not treated.

After surgery. 85-90% report good to excellent symptom relief, the complication rate is low in experienced hands, and recurrence is low when the knee pathology has been addressed. Return to full activity follows the timeline under Postoperative Care.

What predicts the outcome. The factors that matter:

• Underlying pathology: treatable causes (a meniscal tear) do better than degenerative disease (advanced osteoarthritis)
• Size: smaller cysts respond better to conservative treatment
• Duration: longstanding cysts are more likely to need surgery
• Age: younger patients may be more active and more symptomatic
• Treatment of the knee: the single most important factor

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Global epidemiology (PubMed-sourced):

Guidelines and practice variation:

There is no dedicated international society guideline (AAOS, NICE, BOA-BOAST, AO or EFORT) specific to Baker's (popliteal) cyst; management is guided by the evidence above and by guidelines for the underlying knee disorder (e.g. osteoarthritis, meniscal pathology, inflammatory arthritis). Likewise, popliteal cyst is a soft-tissue condition rather than an implant procedure, so it is not tracked by arthroplasty registries (NJR, AJRR, AOANJRR, SHAR, NZJR). Globally consistent principles are:

• Ultrasound first, MRI for the joint - sonography is the imaging method of choice; MRI characterises associated intra-articular pathology (Handy 2001, PMID 11590580; Miller 1996, PMID 8816552).
• Treat the cause, not the cyst - because adult cysts are usually secondary, recurrence is high after isolated excision (~63%; Rauschning & Lindgren 1979, PMID 525326) and far lower when the joint pathology and valve are corrected arthroscopically (~95% good/optimal; Sansone & De Ponti 1999, PMID 10355711).
• Image-guided aspiration/injection is a recognised, lower-morbidity option for symptomatic cysts (Smith et al 2015, PMID 26502415).
• Children differ - paediatric cysts are usually primary and self-limiting, so observation is the default (De Maeseneer 1999, PMID 10415188).

Practice variation: first contact and access to ultrasound/MRI differ by health system (primary-care vs direct specialist referral; publicly funded vs insurance-based imaging), but the diagnostic and treatment hierarchy above is broadly uniform across high-income settings.