Baker Cyst

KEY FACTS

Imaging

• Fluid-filled sac with neck arising from interspace between medial gastrocnemius muscle & semimembranosus tendon

Cyst must have neck emerging from gastrocnemius muscle and semimembranosus tendon interspace

Gives “talk-bubble” configuration on transverse scan
• Cysts are typically well defined and thin walled
• As they communicate with knee joint, cyst contents usually mirror those in knee

Thick-walled cysts in inflammatory osteoarthritis, inflammatory arthropathy, or with intracystic hemorrhage

Thick-walled cysts often have hyperemic walls
• Cyst may be septated
• Contains anechoic synovial fluid; typically gelatinous in consistency
• Free-fluid tracking adjacent to cyst indicates recent leakage

Recent leakage is important to note as it can result in localized posteromedial knee pain
• Irregular and multiloculated cysts may result from repeated cyst leakage and collection
• Cysts usually extend distally superficial to medial belly gastrocnemius muscle
• Contralateral subclinical Baker cysts common
• US accurately confirms presence, extent, and content of Baker cyst

Examine with patient prone, knees extended

Follow medial belly of gastrocnemius proximally and locate interspace between medial gastrocnemius head and semimembranosus tendon

Routinely examine contralateral side

Can aspirate percutaneously if not too viscous, but high rate of recurrence

– ± steroid injection, especially if inflammatory component is present (inflammatory-type exudate, thick walled, hyperemia)

– Not possible to ascertain viscosity of cyst content from US appearance

TERMINOLOGY

Synonyms

• Popliteal cyst, popliteal synovial cyst

Baker cyst is preferred term since popliteal cyst can also refer to periarticular ganglion cyst or parameniscal cyst

Definitions

• Fluid distension of gastrocnemius-semimembranosus bursa

IMAGING

General Features

• Best diagnostic clue

Fluid-filled sac with neck arising from interspace between medial gastrocnemius muscle and semimembranosus tendon
• Location

Posterior to medial femoral condyle
• Size

Range: 1-30 cm across long axis or greater

– Larger cysts often seen with inflammatory arthropathy
• Morphology

Small unilocular sac to large multiloculated sac extending to mid 1/3 of calf

Radiographic Findings

• Radiography

Soft tissue swelling of posteromedial knee

– ± calcified debris/intraarticular loose bodies

– ± displacement of calcified popliteal artery

MR Findings

• T1WI

Hypointense, well-defined mass in expected location

Subacute hemorrhage in cyst seen as areas of hyperintensity
• T2WI

Hyperintense mass: Gastrocnemius-semimembranosus bursa

– ± fluid level, ± hypointense foci (hemosiderin), ± calcified bodies (hypointense), ± thick synovial wall

± surrounding free fluid

Ultrasonographic Findings

• Fluid-filled popliteal fossa mass

Anechoic with posterior enhancement

Must see neck arising from medial gastrocnemius-semimembranosus tendon interspace to make diagnosis of Baker cyst

– Characteristic “talk-bubble” configuration on transverse scan

Extension through posterior knee capsule not seen
• Cyst is typically well defined and thin walled
• As they communicate with knee joint, cyst contents usually mirror those in knee
• Thick-walled cysts suggest association with inflammatory osteoarthritis, inflammatory knee arthropathy, or intracystic hemorrhage

Thick-walled cysts often have hyperemic walls
• Cyst may be septated

Usually bilocular or trilocular
• Typically contains anechoic synovial fluid; gelatinous in consistency

Swirling fluid artifact is not feature

± fine echogenic speckles due to colloid aggregates

± small echogenic comet-tail artifacts (secondary to colloid aggregates)

± fibrinous-type exudate (particularly with inflammatory arthropathy)

± calcific particulate matter

± blood clots
• Iso- or hyperechoic fluid suggests recent intracystic bleeding or inflammatory exudate
• Free-fluid tracking adjacent to cyst indicates recent leakage

Irregular and multiloculated cysts result from repeated cyst leakage and recollection

Sharp angular margins (pointed or “stiletto tip”) to cyst contour suggest leakage

– Fluid leaked from Baker cyst will not have discernible wall

– Free-fluid tracts around posteromedial aspect of knee, into calf, and into distal thigh

Recent leakage is important to note as it can result in localized pain
• Cysts usually extend distally superficial to medial belly of gastrocnemius muscle

± extension distally deep to medial belly of gastrocnemius

± extension proximally deep to and around hamstring muscles, especially semimembranous muscle

± intramuscular extension (uncommon)

– Into medial belly gastrocnemius, vastus medialis

– Through muscular fascia weakened by trauma or perforating vessels
• Cannot usually ascertain viscosity of cyst content on US
• Contralateral subclinical Baker cysts are common

Routinely examine both popliteal fossae

Imaging Recommendations

• Best imaging tool

US

– Confirms presence, extent, content, and recent leakage
• Protocol advice

Examine with patient prone, knees extended

Follow medial belly of gastrocnemius proximally and locate interspace between medial gastrocnemius head and semimembranosus tendon

– Baker cyst arises from interspace between these 2 structures

– Check cyst margins closely looking for fluid leakage, especially most distal margin

Routinely examine contralateral side