AUTHOR AND EDITOR INFORMATION

Section 1 of 10  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

INTRODUCTION

Section 2 of 10  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

Background

The most common mass in the popliteal fossa, Baker cyst, also termed popliteal cyst, results from fluid distention of the gastrocnemio-semimembranous bursa. 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.¹ The common underlying conditions were osteoarthritis and Charcot joint.

This chapter reviews Baker cyst anatomy, prevalence, complications, and treatment, as well as the cyst's association with certain medical conditions and the differential diagnoses of popliteal masses. Emphasis is placed on the radiologic significance of a Baker cyst and its appearance on different imaging modalities. (See also the eMedicine articles Cystic Lesions About the Knee and Knee Injury, Soft Tissue.)

Pathophysiology

A Baker cyst is a synovial cyst that is located posterior to the medial femoral condyle, between the tendons of the medial head of the gastrocnemius and semimembranous muscles. It usually communicates with the joint by way of a slitlike opening at the posteromedial aspect of the knee capsule just superior to the joint line. An extension of the knee joint, a Baker cyst is lined with a true synovium. Baker cysts range in size from 1-40 cm³ (median 3 cm³).

A Baker cyst may serve as a protective mechanism for the knee. Intrinsic intra-articular disorders cause joint effusion. The knee effusion is displaced into the Baker cyst, thus reducing potentially destructive pressure in the joint space. Jayson and Dixon postulated that joint effusion and fibrin are pumped from the knee joint into the Baker cyst but not in the reverse direction, because of a valvelike communication, such as a ball valve or a Bunsen valve (see Image 1).²

In the ball-valve mechanism, effusion is pumped from the knee joint into the Baker cyst, but fibrin acts as a 1-way valve that blocks the effusion's return to the knee joint. In the Bunsen-valve mechanism, the enlarging Baker cyst exerts mass effect on the slitlike opening between the joint and the cyst, trapping effusion. The trapped effusion is reabsorbed through the semipermeable membrane, leaving behind concentrations of fibrin. This explains the difficulty of aspirating the thick, glutinous contents of these cysts. Rauschning and Lindgren studied 41 patients with Baker cysts using arthrography, arthroscopy, or arthrotomy.³ Their study suggested that Baker cysts may form by the following 2 mechanisms:

• A primary, or idiopathic, cyst has a valvular connection with the joint cavity. Membranes, synovial bands, and folds were seen in all valvular cases in Rauschning and Lindgren's study. Scarring and irritation may create these folds in primary cysts. Alternatively, the synovial bands may be remnants of connective tissue interposed between the joint and bursal cavity. Idiopathic cysts usually are seen in young patients without symptoms. Cyst contents usually are viscous.
• A secondary, or symptomatic, cyst communicates freely with the knee joint and contains synovial fluid of normal viscosity. Secondary cysts reveal underlying articular disorders, which was demonstrated in 14 of 41 patients (34%) with Baker cysts in Rauschning and Lindgren's study. Associated articular disorders in the study included osteoarthritis, rheumatoid arthritis (RA), psoriatic arthritis, nonspecific synovitis, meniscal tears, and chondromalacia patellae. (See also the article Rheumatoid Arthritis: Diagnosis, on Medscape, and the eMedicine articles Rheumatoid Arthritis [in the Physical Medicine and Rehabilitation section], Rheumatoid Arthritis, [in the Rheumatology section], and Arthritis, Rheumatoid [in the Emergency Medicine section].)

Frequency

United States

The prevalence of Baker cysts depends on patient population and imaging modality, as shown in Tables 1 and 2.

Table 1. Prevalence of Baker Cysts Based on Diagnostic Modalities

Table 2. Prevalence of Baker Cysts Based on Patient Populations

Mortality/Morbidity

Refer to Special Concerns for a detailed discussion.

Race

No racial predilection exists.

Sex

No sex predilection exists.

Anatomy

Refer to Pathophysiology for a detailed discussion.

Clinical Details

Table 3 lists the most common symptoms in patients with a Baker cyst, as found in a study of 38 patients by Bryan and colleagues.⁴

Table 3.  Symptoms of Baker Cyst

In the study, a popliteal mass was the most common presenting complaint or symptom. A significant number of patients (13%) had pseudothrombophlebitis, a syndrome in which symptoms simulate those of deep venous thrombosis (DVT). Therefore, DVT should be excluded in patients with Baker cyst and leg swelling. (See also the article Venous Thromboembolism: Diagnosis, on Medscape.)

Medical conditions associated with Baker cysts, in descending order of frequency, are as follows:

• Arthritides
• • Osteoarthritis
  • RA
  • Juvenile RA
  • Gout
  • Reiter syndrome
  • Psoriasis
  • Systemic lupus erythematosus
• Internal derangement (meniscal tears, anterior cruciate ligament [ACL] tears, osteochondral fractures)
• Infection (septic arthritis, tuberculosis)
• Chronic dialysis
• Hemophilia
• Hypothyroidism
• Pigmented villonodular synovitis
• Sarcoidosis

Arthritis is the most common condition associated with Baker cysts, with osteoarthritis probably being the most frequent cause among the arthritides. (See also the eMedicine article Osteoarthritis in the Physical Medicine and Rehabilitation section.) Although the prevalence of Baker cysts in patients with inflammatory arthritis is higher than in patients with osteoarthritis, osteoarthritis is much more common than inflammatory arthritis. Using ultrasonography, Fam and colleagues found that 21 of 50 patients (42%) with osteoarthritis had Baker cysts.⁵ Bilateral cysts were seen in 8 patients (16%). The occurrence of Baker cysts relates directly to the presence of knee effusion and the severity of the osteoarthritis.

In 99 consecutive patients with RA, Andonopoulos and coauthors demonstrated Baker cysts on ultrasonograms of 47 patients (48%).⁶ Twenty of the 99 patients (20%) had bilateral cysts. Of 198 knees, 67 (34%) had Baker cysts, yet only 29 cysts (43%) were diagnosed clinically.

Baker cysts appear much less frequently in children than in adults. The prevalence of Baker cysts in asymptomatic children examined ultrasonographically was 2.4%. The prevalence of Baker cyst in children undergoing MRI examination of the knee was 6.3%. None of the children with Baker cyst demonstrated an associated ACL or meniscal tear. Four patients (8%) had osteochondritis dissecans (n = 2), septic arthritis (n = 1), or juvenile RA (n = 1). In most children with Baker cysts (82%), the cysts disappeared without intervention (84%) or caused no symptoms (16%). However, in children with arthritis, Baker cysts are common.

In a study of 44 children with knee arthritis, ultrasonography detected a Baker cyst in 27 children (61%), most of whom (80%) had juvenile RA.⁵⁰ The remaining causes of arthritis in the study included spondylarthritis, psoriatic arthritis, septic arthritis, and systemic lupus erythematosus. Cysts were followed prospectively with serial ultrasonography for 18-24 months. Cysts resolved with the resolution of suprapatellar effusion. Reports of Baker cyst associated with gout, Reiter syndrome, psoriasis, and systemic lupus erythematosus exist. The common underlying pathology for these medical conditions is synovial proliferation with effusion.

Arthrography is more sensitive than ultrasonography in detecting Baker cysts. In 24 patients with a possible Baker cyst, arthrography of both knees was performed immediately after ultrasonography. The latter detected a cyst in 19 of the 48 knees (40%), while arthrography demonstrated a cyst in 22 knees (46%). The increased sensitivity of arthrography is probably the result of its ability to distend the bursa.

Guerra and colleagues found a 30% incidence of popliteal bursa in cadaveric anatomic dissection of older patients.⁷ Using diagnostic arthroscopy, Johnson and coauthors demonstrated a 37% incidence of popliteal bursa.⁸ 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.⁹ Stone and colleagues demonstrated that 84% of Baker cysts detected on magnetic resonance images had meniscal tears.¹⁰ 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.¹¹ 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 is P = 0.19-0.21, and of all 3 variables is P = 0.38. However, no association was found between Baker cyst and ACL tear.

Sansone and colleagues reviewed the incidence of associated intra-articular disorders in a series of 1001 adult patients undergoing MRI of the knee.¹² They found that the most common associated lesions were meniscal tears, chondral lesions, and ACL tears.

Preferred Examination

Imaging evaluation of a Baker cyst begins with conventional radiography to detect a soft-tissue mass (see Image 2), internal calcifications, displacement of an atherosclerotic popliteal artery, and the unusual case of adjacent bony involvement from a large and/or long-standing cyst.

Ultrasonography is a very helpful imaging technique in the evaluation of a popliteal mass. The modality 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 Image 3, Image 4).

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 Image 5, Image 6). (See also the article Update on the Diagnosis and Management of Popliteal Aneurysm and Literature Review, on Medscape.)  Ultrasonography can concomitantly exclude a coexisting 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 in the differential diagnosis (see the discussion on magnetic resonance evaluation, below).

The communication with the joint by way of the gastrocnemio-semimembranous 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.

Previously, Baker cysts were commonly detected by conventional arthrography or by computed tomography (CT) scanning. Arthrography demonstrates the cyst only if the iodinated contrast material that is injected into the joint during arthrography communicates with the cyst under the pressure of the injection. 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.

In current radiologic practice, Baker cysts are often detected on MRI evaluations of the knee (performed for any indication).  Sansone and colleagues studied 1001 randomly selected patients who were submitted for a knee MRI examination; the authors reported the frequency of Baker cysts to be 4.7%, but the frequency in the literature varies.  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 to determine the extent and composition of the Baker cyst.

However, 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 Image 7).  This makes it possible to discriminate between a benign Baker cyst and one of the uncommon, but clinically important, types of cystic tumors, such myxoid liposarcoma, that can occur in the popliteal fossa.

On ultrasonography, myxoid liposarcomas appear as complex, hypoechoic masses that do not meet the criteria for a simple cyst. Myxoid liposarcoma can mimic a fluid-filled cyst on T2-weighted imaging. Contrast enhancement is helpful in distinguishing cystic or necrotic lesions from solid cellular lesions.
 

DIFFERENTIALS

Section 3 of 10  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

Other Problems to Be Considered

Vascular masses

• Popliteal artery aneurysm - Most common in the popliteal fossa
• Cystic adventitial degeneration of the popliteal artery - Erdheim mucoid degeneration

Nonvascular masses

• Simple Baker cyst
• Complicated Baker cyst - Characterized by internal debris (see Image 12, Image 17, Image 18), septations (see Image 16), or MRI signal intensity atypical for simple cyst
• • Inflammatory arthritides
  • Septic arthritis
  • Postoperative changes - Seroma, hematoma, or abscess
  • Hemorrhage within a cyst
  • Hemophilic arthropathy
• Soft-tissue tumor
• • Benign - Peripheral nerve sheath tumors (neurilemmoma)
  • Malignant - myxoid liposarcoma (adults), lipoblastoma (children, especially < 5 y)
• Meniscal cyst - Occurs more often laterally, but medial cysts have been identified
• Ganglion cyst
• Traumatic tear of the gastrocnemius muscle

RADIOGRAPH

Section 4 of 10  

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• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

Findings

Imaging evaluation of a popliteal mass begins with conventional radiography to detect a soft-tissue mass, calcifications, and bony involvement. A Baker cyst appears as a soft-tissue mass in the posteromedial knee joint (see Image 2). Occasionally, a Baker cyst is suggested by the presence of multiple, calcified, loose bodies in the cyst (see Image 8, Image 9). Rarely, a solitary loose body in a Baker cyst may mimic a fabella on a lateral radiograph of the knee (see Image 10). However, on frontal radiograph (see Image 11), the calcified body in the Baker cyst will be located behind the medial femoral condyle, whereas a fabella will be present behind the lateral femoral condyle.

CT SCAN

Section 5 of 10  

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• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

Findings

On a CT scan, a Baker cyst appears as a fluid-containing mass located behind the medial femoral condyle and between the tendons of the medial head of the gastrocnemius and semimembranosus muscles. A space-occupying lesion in the posteromedial knee suggests the diagnosis but is not always sufficient to exclude other etiologies, for which MRI or ultrasonography is more specific.

Degree of Confidence

CT scanning is not as sensitive as MRI in detecting an internal derangement, which may be the cause of a Baker cyst. In addition, the definitive diagnosis of a Baker cyst may not be made without the injection of air and/or iodinated contrast material into the knee joint.

MRI

Section 6 of 10  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

Findings

On MRI, a Baker cyst appears as a homogeneous, high–signal intensity, cystic mass behind the medial femoral condyle; a thin, fluid-filled neck interdigitates between the tendons of the medial head of the gastrocnemius and semimembranosus muscles (see Image 7).

An uncomplicated Baker cyst should demonstrate homogeneous high signal intensity on all fluid-sensitive pulse sequences, including on the second echo of a conventional or fast/turbo spin-echo sequence, on short tau inversion recovery (STIR), or on gradient echo (GRE)/fast field echo (FFE) sequences employing a low flip angle (10-30º). The axial plane (see Image 7) allows the cyst, neck, and joint space to be seen in contiguity, although the cyst may be demonstrated in all 3 planes.

In addition, intravenously administered gadolinium can detect synovial enhancement (see Image 12, Image 13) and pannus formation in RA in the cyst and in the joint space proper, prior to the radiographic detection of well-known signs of RA much later in the course of the disease (erosion, uniform joint space loss without marked osteophytosis, periarticular osteopenia, soft-tissue swelling).

Off-label usage of intra-articular gadolinium in magnetic resonance arthrography, now common in establishing the presence of a meniscal retear, is perhaps the most vivid way to display a Baker cyst. MRI also can detect underlying internal derangements of the knee (see Image 14, Image 15), which may be etiologic in the formation of a Baker cyst.

In a complex Baker cyst, calcified, loose bodies can be detected; these appear on fluid-sensitive images as low–signal intensity, rounded foci within high–signal intensity, cystic fluid (see Image 17, Image 18) .

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or magnetic resonance angiography (MRA) scans. As of late December 2006, the Food and Drug Administration (FDA) had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of theskin; yellow  spots  on  the whites of the eyes; joint stiffness with  trouble  moving  or  straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

ULTRASOUND

Section 7 of 10  

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• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

Findings

Ultrasonography is a very helpful imaging technique in the evaluation of a popliteal mass. The modality can be used to determine whether the popliteal mass is a cyst or a solid mass. A simple Baker cyst appears as an anechoic mass with posterior acoustic enhancement that communicates with the knee joint. Findings on an ultrasonogram relate to the criteria of a simple cyst, which include an anechoic mass, a sharply defined posterior wall, and posterior acoustic enhancement. A complex Baker cyst has internal echoes within the hypoechoic mass (see Image 3, Image 4). Calcified, loose bodies within a Baker cyst appear as mobile, intraluminal, echogenic foci with distal acoustic shadowing, an appearance similar to that of cholelithiasis in a gallbladder. An additional advantage of ultrasonography is that it can exclude a coexisting DVT.

Color Doppler ultrasonography can detect vascular flow within the mass to exclude a popliteal artery aneurysm. In cystic adventitial degeneration of the popliteal artery, ultrasonographic examination reveals multiple cystic structures surrounding a normal-sized artery (see Image 5, Image 6).

Degree of Confidence

This is the fastest, most cost-effective manner in which to diagnose a Baker cyst.

False Positives/Negatives

A cyst that is too large or complex may obscure visualization of the fluid-filled connection to the joint space proper, leading to a false-positive diagnosis.

INTERVENTION

Section 8 of 10  

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• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

The treatment of Baker cysts is conservative and includes the use of nonsteroidal anti-inflammatory agents, ice, and assisted weight bearing, in addition to the correction of underlying intra-articular disorders. Internal derangements of the knee can be treated with therapeutic arthroscopy. Total knee arthroplasty is reserved for severe osteoarthritis.

Radioactive synoviorthesis can be used to treat inflammatory arthritides and hemophilia. Prior to this treatment, arthrography should be performed to exclude a leaking Baker cyst (see Image 19). A leaking Baker cyst would release radionuclide agent outside the knee joint, which is a contraindication to radioactive synoviorthesis.

Radionuclide agents can be instilled during arthrography after the absence of leakage, rupture, or dissection has been documented. Colloidal chromic phosphorus-32 is a common radionuclide agent used in synoviorthesis. An orthopedic surgeon may perform cyst excision when a Baker cyst is unresponsive to all other therapies.

Special Concerns

• Potential complications of Baker cysts that have been reported in the medical literature are as follows:
• • Pseudothrombophlebitis syndrome (rupture, dissection)
  • DVT
  • Pulmonary embolism
  • Hemorrhage
  • Leaking
  • Infection
  • Posterior compartment syndrome
  • Trapped calcified bodies
• The most common complication of Baker cyst is the rupture or dissection of fluid into the adjacent proximal gastrocnemius muscle belly, which results in a pseudothrombophlebitis syndrome mimicking symptoms of DVT. The incidence of Baker cyst rupture is 3.4-10%. A ruptured Baker cyst may present as an enlarging mass in the calf.
• Among patients with symptoms of DVT, the incidence of Baker cysts is 3.1-4.1%; the incidence of patients with a Baker cyst who have coexistent DVT is 0.2-11%. Therefore, a differential diagnosis of Baker cyst should be considered in patients presenting with symptoms that are suggestive of DVT. Once DVT is excluded, the popliteal fossa should be carefully evaluated for a ruptured Baker cyst. Alternatively, if a Baker cyst is observed incidentally on a venous duplex examination, the authors suggest that the radiologist should then search carefully for a coexistent DVT.
• Because of their anatomic location, Baker cysts can be a risk factor for DVT. Most commonly, the Baker cyst is located between the tendons of the medial head of the gastrocnemius and the semimembranous bursa. Occasionally, the Baker cyst can be found between the heads of the gastrocnemius muscles. In a series of patients with Baker cyst simulating DVT, all patients demonstrated lateral deviation of the popliteal vein, and 30% of patients revealed compression of the popliteal vein. A report exists of a patient who developed a pulmonary embolism after receiving a diagnosis of Baker cyst and DVT.
• Posterior compartment syndrome usually is caused by trauma. Rarely, dissection or rupture of a Baker cyst can increase pressure within the deep posterior compartment of the leg, causing posterior compartment syndrome. Dissection of Baker cysts can also cause anterior compartment syndrome, depending on the direction of dissection. A Baker cyst usually dissects through the muscles, primarily below the knee. There is evidence that almost one third of dissections are into the thigh.
• An infected Baker cyst is rare; review of the literature disclosed only 3 occurrences. The infected patients had either fever, an increased white blood cell (WBC) count, or an elevated erythrocyte sedimentation rate.
• An entity that may mimic an infected Baker cyst is a Baker cyst that contains gaslike lucencies. Only 2 patients have been reported with a gas-containing, noninfected Baker cyst. Both patients had diabetes mellitus and RA. The authors did not suggest a mechanism for the appearance of the gaslike lucencies in the Baker cyst.
• Jayson and Dixon studied the valvular mechanisms in juxta-articular cysts and postulated that joint effusion and fibrin are pumped from the knee joint into the Baker cyst but—because of a valvelike communication (either a Bunsen or ball valve)—not in the reverse direction (see Image 1).² The effusion can be readily reabsorbed through the synovial membrane, leaving behind concentrations of fibrin, which on radiographs may appear as gaslike lucencies. In our clinical experience, 1 patient was encountered with a noninfected Baker cyst containing gaslike lucencies (see Image 20, Image 21, Image 22). This patient also had diabetes mellitus  and  RA. Because gaslike lucencies  in  a Baker  cyst  are  rare  and an infected Baker cyst is a serious condition, the former diagnosis must be one of exclusion; a CT scan with the appropriate window settings would allow discrimination between air and fibrin.
• Radiographic findings of calcified bodies posterior to the knee joint can be confusing. The differential diagnosis includes soft-tissue neoplasms with calcification (eg, extraskeletal soft-tissue sarcoma), a popliteal artery aneurysm, vascular malformations, and loose, calcified bodies trapped in a Baker cyst. Calcified bodies in Baker cysts are common, with an incidence of 6% in one series.
• Calcified bodies may derive from trauma (ie, loose bodies resulting from osteochondral fractures), arthropathy (ie, loose bodies associated with joint surface disintegration, such as that related to osteoarthritis, infection, and neuropathic joints), or synovial chondromatosis (that is, calcified or noncalcified bodies resulting from chondrometaplasia of synovial tissue) (see Image 8, Image 9).
• Distal migration of loose bodies supports the diagnosis of a dissecting Baker cyst. Rarely, a solitary, calcified, loose body in a Baker cyst may simulate a fabella on the lateral view of conventional radiography of the knee (see Image 10, Image 11). However, on the frontal view, a fabella is located in the lateral head of the gastrocnemius muscle, while a calcified, loose body in a Baker cyst is located medially.

MULTIMEDIA

Section 9 of 10  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

Media file 1:  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: semimembranous muscle)
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Media file 2:  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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Media file 3:  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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Media file 4:  Longitudinal ultrasonographic image of a Baker cyst in a patient who underwent recent knee arthroscopy.
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Media file 5:  Transverse ultrasonographic image of the popliteal fossa shows a complex, cystic mass (arrowhead).
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Media file 6:  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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Media file 7:  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 semimembranous muscles.
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Media file 8:  Anteroposterior radiograph of the knee shows calcifications (arrowhead) overlying the medial tibial plateau.
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Media file 9:  Lateral radiograph of the knee shows multiple calcified bodies (arrowhead) posterior to the knee, which is consistent with synovial osteochondromatosis.
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Media file 10:  Lateral radiograph of the knee shows a calcified body (arrowhead), which appears to be a large fabella, posterior to the knee.
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Media file 11:  Anteroposterior radiograph of the knee shows that a calcified body (arrowhead) is overlying the medial femoral condyle, making a fabella highly unlikely. The calcified, loose body is in a Baker cyst.
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Media file 12:  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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Media file 13:  Axial, T1-weighted magnetic resonance image of the knee after the intravenous administration of gadolinium shows synovial enhancement (arrowhead).
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Media file 14:  Sagittal, T2-weighted magnetic resonance image with fat saturation of the knee shows a large knee effusion and a Baker cyst.
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Media file 15:  Sagittal, T2-weighted magnetic resonance image with fat saturation of the knee, lateral to the view in Image 14, shows a complete rupture of the anterior cruciate ligament.
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Media file 16:  Sagittal, T2-weighted magnetic resonance image with fat saturation shows a complicated Baker cyst containing multiple internal septations.
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Media file 17:  Sagittal, T2-weighted magnetic resonance image shows a Baker cyst containing hypointense, loose bodies.
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Media file 18:  Axial, T2-weighted magnetic resonance image with fat saturation shows a Baker cyst containing hypointense, loose bodies.
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Media file 19:  Arthrography of the knee performed prior to radioactive synoviorthesis shows leakage of contrast (arrow) just superior to the sinus (arrowhead and "BB" marker).
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Media file 20:  Anteroposterior radiograph of the knee shows multiple tiny lucencies superior to the medial femoral condyle (arrowhead).
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Media file 21:  Lateral radiograph of the knee shows multiple tiny lucencies (arrowhead) posterior to the knee.
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Media file 22:  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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REFERENCES

Section 10 of 10

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Intervention
• Multimedia
• References

1. Baker WM. On the formation of synovial cysts in the leg in connection with disease of the knee-joint. 1877. Clin Orthop Relat Res. Feb 1994;(299):2-10. [Medline].
2. Jayson MI, Dixon AS. Valvular mechanisms in juxta-articular cysts. Ann Rheum Dis. Jul 1970;29(4):415-20. [Medline]. [Full Text].
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Article Last Updated: Nov 2, 2007