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Answer:
The speech bubble sign on axial knee MRI or transverse ultrasound is virtually diagnostic of a Baker cyst (popliteal cyst), a distended gastrocnemio-semimembranosus bursa in the posteromedial popliteal fossa. The cyst body is the round bubble; its narrow neck passing between the medial head of gastrocnemius and the semimembranosus tendon into the joint is the tail. Demonstrating this neck confirms communication with the joint and excludes mimics that lack it: parameniscal cyst (contiguous with a meniscal tear, not between these tendons), ganglion cyst (multilocular, no joint neck) and popliteal artery aneurysm (central, follows the artery, shows Doppler flow). Effusion, medial meniscal tear and degenerative arthropathy each independently raise the probability of an associated Baker cyst, so search for them once the sign is seen.
Why is it called so?
Pathophysiology
Alternative names: Talk bubble sign
Other associated named signs: None
References
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