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A Baker’s cyst, which was named after Dr William Baker, the surgeon who first described it, is commonly located in the posteromedial aspect of the popliteal fossa. Technically, it is a nonmalignant, fluid-filled swelling formed by distention of the semimembranosus-gastrocnemius bursa (Figs. 1A, B).1,2 A Baker’s cyst can be classified as a “primary” cyst if the distended semimembranosus-gastrocnemius bursa arises independently without communication to the knee joint or a “secondary” cyst if there is an open communication between the bursa and the knee joint cavity (Figs. 2A, B). Whereas most of Baker’s cysts are secondary cysts and associated with degenerative knee joint diseases, primary cysts are less common and occur primarily in children.3
A variety of differential diagnoses (e.g., popliteal aneurysm, meniscal cyst, thrombophlebitis) may also present as palpable lumps in the popliteal region and can be easily mistaken for Baker’s cyst on physical examination.2–4 Although magnetic resonance imaging is often considered as the gold standard for imaging soft tissue pathologies, it is more expensive and usually requires advanced scheduling.5 On the other hand, ultrasonography is a noninvasive, readily available, accurate, reliable, and cost-effective imaging tool, which has been gaining popularity in diagnosing soft tissue pathologies in the knee region.
As seen in Figures 1C and D, bulging of the popliteal fossa can be caused by a popliteal aneurysm that mimics a Baker’s cyst.3,5 Ultrasonography can be used to assess the lesion’s relationship to the adjacent structures and to ascertain its vascular characteristics before proceeding with procedures such as needle aspiration or corticosteroid injection.
Ultrasound Examination of the Baker’s Cyst
The patient undergoing ultrasound examination to rule out Baker’s cyst should lie in a prone position, with the knees extended and both feet hanging over the edge of the examination table. To begin, the transducer is placed in the popliteal aspect of the knee and viewed transversely (Fig. 3A). In a short axis view, a Baker’s cyst appears as a well-defined, anechoic, or hypoechoic cystic lesion with posterior acoustic enhancement, representing enlargement of the semimembranosus-gastrocnemius bursa.6 It is typically crescentic in shape, composed of a body (the larger and superficial component), a base (the smaller and deep component), and a neck connecting the body and the base (Fig. 3B). According to Ward et al.,2 a definite diagnosis of a Baker’s cyst is established by the identification of a fluid-filled neck between the tendon of semimembranosus and medial head of gastrocnemius. It is important not to confuse between the neck and the communication stalk of the cyst, as the latter did not necessarily appear in primary Baker’s cysts.
In a long axis view (Fig. 3C), a Baker’s cyst generally has a rounded appearance at its proximal and distal ends (Fig. 3D). A sharp end usually indicates rupture of the Baker’s cyst, whereas anechoic or hypoechoic fluid beyond the margin of the cyst represents leakage of fluid from the ruptured cyst.2 Because a Baker’s cyst typically communicates with the knee joint cavity, the content within the cyst is synovial fluid, which typically appears as either an anechoic or a hypoechoic image on ultrasound. Complicated Baker’s cysts have contents of varying echogenicity that may include debris, synovial hypertrophy, hemorrhage, calcification, or loose bodies.6
Ultrasound-Guided Aspiration of the Baker’s Cyst
For symptomatic relief, a Baker’s cyst can be aspirated, with or without concomitant corticosteroid injection into the cyst.3,7 Palpation-guided aspiration carries a relatively high risk of puncturing to the neurovascular bundle of the popliteal space, whereas ultrasound-guided aspiration has the advantage of accurate needle placement, followed by appropriate drainage. An 18- or 20-gauge needle is preferred because the content of the cyst tends to be very viscous. Needles with a larger tip also prevent obstruction of the needle pore by debris. To begin the procedure, the affected knee is extended to make the cyst more prominent. The transducer is applied on the popliteal region to identify the Baker’s cyst, which is first revealed in a short axis view (Figs. 3A, B). Then, the transducer is turned 90 degrees into a long axis view, and the needle is introduced into the cyst along the long axis plane of the cyst (Figs. 4A, B). The fluid within the cyst can be aspirated until the cyst is completely collapsed, while being monitored on ultrasound (Fig. 4C).7 Please refer to the attached video for a step-by-step demonstration of the aspiration procedure under ultrasound guidance.
On musculoskeletal ultrasound, the diagnosis of a Baker’s cyst can be established by identification of a popliteal cystic lesion, with a fluid-containing neck between the tendon of semimembranosus and medial head of gastrocnemius. A color Doppler view should be obtained to rule out any vascular lesion. Under real-time ultrasound guidance, aspiration of the Baker’s cyst can be safely and accurately performed.
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