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Baker Cyst Imaging

Overview

The most common mass in the popliteal fossa, Baker cyst, also termed popliteal cyst, results from fluid distention of the gastrocnemio-semimembranosus bursa, which is located in the medial aspect of the popliteal fossa.
 The eponym honors the work of Dr William Morrant Baker. In 1877, Baker described 8 cases of periarticular cysts caused by synovial fluid that had escaped from the knee joint and formed a new sac outside the joint.
The common underlying conditions were osteoarthritis and Charcot joint.
(See the images below.)

Valvular mechanism of Baker cyst. Effusion and fib

Valvular mechanism of Baker cyst. Effusion and fibrin are pumped (large arrows) into the Baker cyst (long, thin arrows). In the Bunsen-valve mechanism, the enlarging Baker cyst exerts mass effect (feathered arrow) on the slitlike communication between the joint and the cyst, trapping effusion. In the ball-valve mechanism, fibrin serves as a 1-way valve that prevents the effusion’s return to the knee joint. Trapped effusion is reabsorbed through the semipermeable membrane (short, thin arrows), leaving behind concentrations of fibrin. (MFC: medial femoral condyle; MTP: medial tibial plateau; G: medial head of gastrocnemius muscle; SM: semimembranosus muscle)

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Anteroposterior radiograph of the knee shows unifo

Anteroposterior radiograph of the knee shows uniform joint-space loss in the medial and lateral knee compartments without osteophytosis. A Baker cyst is seen medially (arrowhead).

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Axial, T2-weighted magnetic resonance image of the

Axial, T2-weighted magnetic resonance image of the knee shows effusion, synovial proliferation (white arrowhead), and a Baker cyst that contains debris (black arrowhead).

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Contrast-enhanced, axial computed tomography (CT)

Contrast-enhanced, axial computed tomography (CT) scan of the knee shows multiple gaslike lucencies within a Baker cyst and synovial enhancement.

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Differential diagnosis  includes the following:

Ganglion cyst

Meniscal cyst

Myxoid liposarcoma

Popliteal artery aneurysm

Popliteal artery pseudoaneurysm

Synovial sarcoma

Osteochondrolipoma

Imaging

Plain radiographs are simple and readily available, but they provide limited information about the popliteal cyst. However, they should be obtained early in the evaluation, as they are useful for detecting other conditions commonly found in association with popliteal cysts, such as osteoarthritis, inflammatory arthritis, and loose bodies.
 

In the past, Baker cysts were commonly detected by conventional arthrography, but disadvantages include invasiveness and the use of ionizing radiation.​
 Ultrasound has largely replaced arthrography as the initial assessment for Baker cysts.  Ultrasonography is an easy-to-use, rapid, relatively inexpensive examination to employ in this setting. Ultrasonography determines whether the popliteal mass is a pure cystic structure or a complex cyst and/or solid mass (see the images below).
 It allows assessment of the size of the cyst; its relationship to adjacent muscles, tendons, and vessels; and the presence of intracystic loose bodies or septations. In addition, it can differentiate these cysts from popliteal aneurysms and ganglion cysts.
 The ability to detect Baker cysts is near 100%, but ultrasound lacks the specificity to differentiate Baker cysts from meniscal cysts or myxoid tumors. Another disadvantage is that it does not adequately visualize other conditions in the knee that are often associated with these cysts, such as meniscal tears.
  On ultrasonography, myxoid liposarcomas appear as complex, hypoechoic masses that do not meet the criteria for a simple cyst. Contrast enhancement is helpful in distinguishing cystic or necrotic lesions from solid cellular lesions.

Transverse ultrasonographic image of the knee in a

Transverse ultrasonographic image of the knee in a patient who had recent arthroscopy shows a complex, cystic mass (arrow) in the medial aspect of popliteal fossa. The mass communicates with the knee joint (arrowhead), which is consistent with a Baker cyst.

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Longitudinal ultrasonographic image of a Baker cys

Longitudinal ultrasonographic image of a Baker cyst in a patient who underwent recent knee arthroscopy.

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Chatzopoulos et al found that Baker cysts are a common ultrasonographic finding in knees with chronic osteoarthritic pain and also found that they are associated with synovial inflammation and its grade. Baker cysts were identified in 89 of 328 chronic osteoarthritic knees (27%), but only 1 cyst was found in 54 nonosteoarthritic knees (2%). Abnormal and intense tracer accumulations in early-phase bone scintigraphy were also significantly more frequent in osteoarthritic knees with Baker cysts than in those without Baker cysts.

Color Doppler imaging can confirm the absence of vascular flow within the mass to exclude a popliteal artery aneurysm or cystic adventitial degeneration of a popliteal artery (see the images below).
Ultrasonography can concomitantly exclude a coexisting deep venous thrombosis (DVT) created by subjacent mass effect. The weakness of ultrasonography is related to the difficulty in establishing a true connection to the joint space proper, which is essential for discriminating between a Baker cyst and other potentially harmful conditions.

Transverse ultrasonographic image of the popliteal

Transverse ultrasonographic image of the popliteal fossa shows a complex, cystic mass (arrowhead).

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Transverse color Doppler ultrasonographic image of

Transverse color Doppler ultrasonographic image of the popliteal fossa shows multiple cysts surrounding a normal-sized popliteal artery (A), which is consistent with cystic adventitial degeneration.

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The communication with the joint by way of the gastrocnemio-semimembranosus bursa is deep within the popliteal space, adjacent to the dense posterior femoral cortex. The ultrasonography probe is placed over the popliteal skin surface, and because this thin, necklike connection to the joint is anterior to the cyst, the mere presence of a large or complex Baker cyst may obscure the visualization of this connection.

CT scanning can delineate a low to intermediate attenuation mass, which normally measures from 20 to -10 Hounsfield units, in the posteromedial popliteal space. CT scanning can easily delineate secondary findings, such as intracystic osseous fragments, mass effect, wall thickening, and bony erosion.

Magnetic resonance imaging remains the gold standard for diagnosis of Baker’s cysts and differentiating them from other conditions. Its main disadvantage is the high cost; therefore, ultrasound should be considered as a screening modality if evaluation of the intra-articular structures is not necessary.  In current radiologic practice, Baker cysts are often detected on MRI evaluations of the knee (performed for any indication). The advantages of MRI are derived from the superior soft-tissue contrast resolution that it affords and from the modality’s multiplanar capability, which help determine the extent and composition of the Baker cyst. It allows assessment of the entire spectrum of related disorders, such as meniscal tears, chondral defects, loose bodies, synovitis, osteoarthritis, and ligament tears. Conditions such as meniscal cysts are more easily differentiated from Baker cysts with MRI than ultrasound.

One of the most important benefits of employing MRI is the ability to use the axial plane to establish positive identification of the high-signal intensity, fluid-filled neck of the cyst that connects the cyst to the joint space (see the image below). This makes it possible to discriminate between a benign Baker cyst and one of the uncommon, but clinically important, types of cystic tumors, such as myxoid liposarcoma, that can occur in the popliteal fossa.

Axial, T2-weighted magnetic resonance image with f

Axial, T2-weighted magnetic resonance image with fat saturation reveals a Baker cyst connected to the knee joint by way of a narrow neck between the tendons of the medial head of the gastrocnemius and semimembranosus muscles.

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Guerra and colleagues found a 30% incidence of popliteal bursa in cadaveric anatomic dissection of older patients.
Using diagnostic arthroscopy, Johnson and coauthors demonstrated a 37% incidence of popliteal bursa.
The incidence of Baker cysts detected through MRI of the knee varies (5-18%) according to the patient population. Initially, Fielding and colleagues reported an association between Baker cyst and tear of the medial meniscus or complete tear of the ACL.
Stone and colleagues demonstrated that 84% of Baker cysts detected on magnetic resonance images had meniscal tears. Most tears were observed in the posterior horn of the medial meniscus.

Subsequently, Miller and coauthors confirmed a significant association of Baker cyst with effusion, meniscal tears, and degenerative arthropathy.
The probability of a Baker cyst in the presence of any 1 variable (ie, association) is P=0.08-0.10; of any 2 variables, P=0.19-0.21; and of all 3 variables, P=0.38. However, no association was found between Baker cyst and ACL tear.

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