KEY FACTS
Terminology
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Synonym: Popliteal cyst
Imaging
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Fluid distention of semimembranosus-medial gastrocnemius bursa
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C-shaped cyst usually anechoic, thin walled, and well defined
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Isoechoic or hyperechoic material within cyst can be complex fluid, hemorrhage, &/or synovitis
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Cysts with inflammation &/or intracystic hemorrhage can be thick walled, hyperemic, or septated
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Intraarticular bodies are common and appear hyperechoic with shadowing
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Inferior extension occurs superficial to medial gastrocnemius and can extend as far as ankle
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Free fluid tracking adjacent to cyst indicates recent leakage
Clinical Issues
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Intraarticular knee pathology is very frequently associated with Baker cysts in adults
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Cyst rupture can cause edema &/or reactive cellulitis
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Chronic Baker cyst rupture can be confused with soft tissue mass
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Can aspirate &/or inject steroid percutaneously, if not too viscous, but high rate of recurrence
Scanning Tips
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Examine with patient prone, knees extended
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Follow medial belly of gastrocnemius proximally and locate interspace between medial gastrocnemius head and semimembranosus tendon
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Look for characteristic “talk-bubble” configuration of cyst on transverse imaging
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Scan inferiorly to find cyst margin and assess for rupture
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Routinely scan contralateral side; subclinical Baker cysts are common
located between the semimembranosus tendon 
and the medial belly of the gastrocnemius muscle 
.
with a “talk-bubble” configuration. The neck extends between the semimembranosus tendon 
and the medial belly of the gastrocnemius muscle 
.
within part of the Baker cyst 
. Osseous fragments within the knee joint can extend into a Baker cyst. Osseous fragments may also enlarge within Baker cysts.
tracking over the gastrocnemius muscle distal to the contained Baker cyst 
. Note the lack of a wall around the free fluid.
of a Baker cyst to the distal thigh. Note medial belly gastrocnemius muscle 
.
and the distended Baker cyst 
.
with a “talk-bubble” configuration. The neck extends between the semimembranosus tendon 
and the medial belly of the gastrocnemius 
.
and deep to 
the medial belly of the gastrocnemius muscle 
.
