HSS Ultrasound of the Month Case 96

Clinical History Two patients who were referred to the ultrasound department for ultrasoundguided aspiration of a cyst in the popliteal fossa of the knee

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Diagnosis: Popliteal (Baker’s) Cyst and Ganglion Cyst of The Popliteal Fossa of the Knee Discussion Palpable mass or fullness in the back of the knee is a common clinical scenario for which the first line imaging modality is ultrasound.

Ultrasound is a fast, accurate, and cost-effective modality for identifying the source of a mass in the back of the knee.

Cysts, including Baker’s cysts and ganglion cysts, are commonly found, but less common sources of “masses”, including thrombosed veins, popliteal artery aneurysms, hematomas, and tenosynovitis, can be accurately diagnosed on ultrasound.

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Discussion Baker’s cysts are an extension of the synovial compartment of the knee and are commonly seen in knees with chronic inflammation such as osteoarthritis.

By definition, they communicate with the joint via a thin neck located between the semimembranosus and the medial head of the gastrocnemius.

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Discussion The long term benefit of ultrasound-guided aspiration +/- cortisone injection for Baker’s cysts is controversial, although it is very safely performed and can be an effective treatment for symptomatic Baker’s cysts in the short term.

Other cysts in the popliteal fossa that are not located in the characteristic location for Baker’s cysts are ganglion cysts and do not communicate with the knee joint.

Ultrasound-guided aspiration +/- cortisone injection is a safe and effective treatment for ganglion cysts of the popliteal fossa, similar to ganglion cysts elsewhere in the body.

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