Thursday, May 7, 2009

What is a knee effusion with a baker's cyst?

excess of fluid in the knee joint space is called an effusion .

it could be a result of infection like with bacteria or inflammatory conditions like rheumatoid arthritis .

a bakers cyst is formed when this fluid extends into the calf.its significance is that if it ruptures it leads to diffuse swelling and pain and may mimick deep vein thrombosis(a much more serious condition)

Baker's cyst is an accumulation of joint fluid (synovial fluid) that forms behind the knee.

Alternative Names:

Popliteal cyst

Causes, incidence, and risk factors:

Baker's cyst is a fluid collection behind the knee. This cyst may be formed by the connection of a normal bursa (a normal lubricating fluid sac) with the knee joint. This type is more common in children.

The condition can also be caused by the herniation of the knee joint capsule out into the back of the knee, which is more common in adults. This type of Baker's cyst is commonly associated with a tear in the meniscal cartilage of the knee. In older adults, this condition is frequently associated with degenerative arthritis of the knee.

Nearly one half of Baker's cysts are found in children, where they appear as painless masses behind the knee that are more obvious when the knee is fully extended. A large cyst may cause some discomfort or stiffness but generally has no symptoms. Baker's cysts usually disappear spontaneously, but the time in which they do so is variable.

Background: Baker cyst, also termed popliteal cyst, is the most common mass in the popliteal fossa and results from fluid distension of the gastrocnemio-semimembranosus 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. The common underlying conditions were osteoarthritis and Charcot joint.

Pathophysiology: A Baker cyst is a synovial cyst located posterior to the medial femoral condyle between the tendons of the medial head of the gastrocnemius and semimembranosus muscles. This usually communicates with the joint via a slitlike opening at the posteromedial aspect of the knee capsule just superior to the joint line. A Baker cyst is lined by a true synovium, as it is an extension of the knee joint. Popliteal cysts range from 1-40 cm3 (median 3 cm3).

A popliteal 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 popliteal cyst but not in the reverse direction because of a valvelike communication, such as either a ball or Bunsen valve (Picture 1).

In the ball-valve mechanism, effusion is pumped from the knee joint into the Baker cyst, but fibrin serves as a one-way valve blocking return of effusion into the knee joint. In the Bunsen-valve mechanism, the enlarging Baker cyst exerts mass effect on the slitlike communication between the joint and cyst, trapping effusion. Trapped effusion is reabsorbed through the semipermeable membrane, leaving behind concentrations of fibrin. This explains the difficulty aspirating the thick, glutinous contents of these cysts. Rauschning and Lindgren studied 41 patients with popliteal cysts via arthrography, arthroscopy, or arthrotomy. Their study suggested that Baker cysts may form by 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. Scarring and irritation may form these folds. 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 disorder, which was demonstrated in 14 of 41 patients (34%) with popliteal cysts. Associated articular disorders included osteoarthritis, rheumatoid arthritis (RA), psoriatic arthritis, nonspecific synovitis, meniscal tears, and chondromalacia patellae.


In the US: Prevalence of Baker cyst depends on patient population and imaging modality, as shown in Tables 1 and 2.

Table 1. Prevalence of Baker Cyst Based on Diagnostic Modalities

Diagnostic Modalities Prevalence, %

MRI 5-18

Cadaveric dissections 30

Arthroscopy 37

Ultrasound 40-42

Arthrography 5-46

Table 2. Prevalence of Baker Cyst Based on Patient Populations

Patient Populations Prevalence, %

RA 5-58

Osteoarthritis 42

Internal derangements 5-18

Mortality/Morbidity: Refer to Special Concerns for a detailed discussion.

Race: No racial predilection exists.

Sex: No sex predilection exists.

Age: Popliteal cysts appear much less frequently in children than in adults.

Anatomy: Refer to Pathophysiology for a detailed discussion.

Clinical Details: Table 3 lists the most common symptoms in patients with popliteal cyst from a study of 38 patients by Bryan et al.

Table 3. Symptoms of Baker Cyst

Symptoms Frequency

Popliteal mass or swelling 29/38 76%

Aching 12/38 32%

Knee effusion 12/38 32%

Thrombophlebitis 5/38 13%

Clicking of the knee 4/38 11%

Buckling of the knee 4/38 11%

Locking of the knee 1/38 3%

A popliteal mass is the most common presenting complaint or symptom. A significant number of patients (13%) had symptoms simulating deep venous thrombosis (DVT), a syndrome termed pseudothrombophlebitis. Therefore, exclude DVT in patients with popliteal cyst and leg swelling.

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




Juvenile RA


Reiter syndrome


Systemic lupus erythematosus

Internal derangement (meniscal tears, anterior cruciate ligament [ACL] tears, osteochondral fractures)

Infection (septic arthritis, tuberculosis)

Chronic dialysis



Pigmented villonodular synovitis


Arthritis is the most common condition associated with Baker cyst. Of the arthritides, osteoarthritis is probably the most common cause of popliteal cyst. Although prevalence of Baker cyst in patients with inflammatory arthritis is higher than in patients with osteoarthritis, osteoarthritis is much more common than inflammatory arthritis. Fam et al demonstrated that 21 of 50 patients (42%) with osteoarthritis had popliteal cysts detected by ultrasound (US). Bilateral cysts were seen in 8 patients (16%). The occurrence of Baker cysts relates directly to the presence of knee effusion and severity of osteoarthritis.

In 99 consecutive patients with RA, Andonopoulos et al demonstrated Baker cysts on US in 47 patients (48%). Twenty patients (20%) had bilateral cysts. Of 198 patients' knees, 67 (34%) had popliteal cysts, yet only 29 cysts (43%) were diagnosed clinically.

Popliteal cysts appear much less frequently in children than in adults. The prevalence of popliteal cysts in asymptomatic children examined sonographically 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 associated ACL tear or meniscal tear. Four patients (8%) had osteochondritis dissecans (n=2), septic arthritis (n=1), and juvenile RA (n=1). In most children with popliteal cysts (82%), the cysts disappeared without intervention (84%) or caused no symptoms (16%). However, in children with arthritis, popliteal cysts are common.

In a study of 44 children with knee arthritis, US detected Baker cyst in 27 children (55%). Most of these children (80%) had juvenile RA. The remaining causes of arthritis in descending order of occurrence include spondyloarthritis, psoriatic arthritis, septic arthritis, and systemic lupus erythematosus. Cysts were followed prospectively with serial US 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 US in detecting popliteal cysts. In 24 patients with possible Baker cyst, arthrography of both knees was performed immediately after US. US detected 19 cysts in 48 knees (40%), while arthrography demonstrated 22 cysts (46%). The increased sensitivity of arthrography is probably the result of its ability to distend the bursa.

Guerra et al found a 30% incidence of popliteal bursa in cadaveric anatomic dissection of older patients. Johnson et al demonstrated a 37% incidence of popliteal bursa by diagnostic arthroscopy.

The incidence of popliteal cysts detected on knee MRI varies (5-18%), depending on the patient population. Initially, Fielding et al reported an association between Baker cyst and tear of the medial meniscus or complete tear of the ACL. Stone et al demonstrated that 84% of Baker cysts detected on MRI had meniscal tears. Most tears were observed in the posterior horn of the medial meniscus.

Subsequently, Miller et al confirmed a significant association of Baker cyst with effusion, meniscal tear, 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 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 et al reviewed the incidence of associated intra-articular disorders in a series of 1001 adult patients undergoing MRI of the knee. They found the most common associated lesions were meniscal tears, chondral lesions, and ACL tear.

Preferred Examination: Imaging evaluation of a popliteal cyst begins with conventional radiography to detect a soft tissue mass (Picture 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.

US is a very helpful imaging technique in the evaluation of a popliteal mass. US is an easy to use, rapid, relatively inexpensive examination to use in this setting. US determines whether the popliteal mass is a pure cystic structure or a complex cyst and/or solid mass (Picture 3, Picture 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 (Picture 5, Picture 6). US can concomitantly exclude a coexisting DVT created by subjacent mass effect. The weakness of US is related to the difficulty in establishing a true connection to the joint space proper, which is essential in discriminating a popliteal cyst from other potentially harmful conditions in the differential diagnosis (see discussion on MR evaluation below).

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

Previously, popliteal cysts were commonly detected by conventional arthrography or CT. 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 can delineate a low-to-intermediate attenuation mass, which normally measures from 20 to -10 Hounsfield units, in the posteromedial popliteal space. CT can easily delineate secondary findings such as intracystic osseous fragments, mass effect, wall thickening, and bony erosion.

In current radiologic practice, popliteal cyst often is detected on MRI evaluation of the knee performed for any indication. Sansone studied 1001 randomly selected patients submitted for a knee MRI examination and reported a frequency of 4.7%, but the frequency in the literature varies. The advantages of MRI are derived from the superior soft tissue contrast resolution and multiplanar capability, which help determine the extent of the popliteal cyst and its composition.

However, one of the most important strengths of MR imaging is the use of the axial plane to establish positive identification of a high signal intensity fluid-filled neck of the cyst that connects the cyst to the joint space (Picture 7). This allows us to discriminate between a benign popliteal cyst and uncommon but clinically important cystic tumors that are known to occur in the popliteal fossa, such as myxoid liposarcoma.

On US, 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 to distinguish cystic or necrotic lesions from solid cellular lesions. DIFFERENTIALS Section 3 of 10

Author Information Introduction Differentials Radiograph CT Scan MRI Ultrasound Intervention Pictures Bibliography

Deep Venous Thrombosis, Lower Extremity

Other Problems to be Considered:

Vascular masses

Popliteal artery aneurysm (most common in popliteal fossa)

Cystic adventitial degeneration of popliteal artery (Erdheim mucoid degeneration)

Nonvascular masses

Simple Baker cyst

Complicated Baker cyst - contains internal debris (Picture 12, Picture 17, Picture 18), septations (Picture 16), or MRI signal intensity atypical for simple cyst

Inflammatory arthritides

Septic arthritis

Postoperative changes (seroma, hematoma, abscess)

Hemorrhage within a cyst

Hemophilic arthropathy

Soft tissue tumor: Benign - peripheral nerve sheath tumors (neurolemmoma); Malignant - myxoid liposarcoma (adults), lipoblastoma (children, especially %26lt;5 y)

Meniscal cyst (occur more commonly laterally, but medial cysts have been identified)

Ganglion cyst

Traumatic tear of gastrocnemius muscle

RADIOGRAPH Section 4 of 10

Author Information Introduction Differentials Radiograph CT Scan MRI Ultrasound Intervention Pictures Bibliography

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

Quick Find

Author Information




CT Scan






Click for related images.

Related Articles

Deep Venous Thrombosis, Lower Extremity

Continuing Education

CME available for this topic. Click here to take this CME.

Patient Education

Click here for patient education.

CT SCAN Section 5 of 10

Author Information Introduction Differentials Radiograph CT Scan MRI Ultrasound Intervention Pictures Bibliography

Findings: On CT, 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 MR or US is more specific.

Degree of Confidence: CT is not as sensitive as MRI in detecting an internal derangement, which may be the cause of popliteal cyst. In addition, the definitive diagnosis of a Baker cyst may not be made without injection of air and/or iodinated contrast material into the knee joint. MRI Section 6 of 10

Author Information Introduction Differentials Radiograph CT Scan MRI Ultrasound Intervention Pictures Bibliography

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

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

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

Additionally, off-label usage of intraarticular gadolinium in MR arthrography, now common in establishing the presence of meniscal re-tear, is perhaps the most vivid way to display a popliteal cyst. MRI also can detect underlying internal derangements of the knee (Picture 14, Picture 15) , which may be etiologic in the formation of a popliteal cyst.

In a complex Baker cyst, calcified loose bodies can be detected, which appear as low signal intensity rounded foci within high signal intensity cystic fluid on fluid-sensitive images (Picture 17, Picture 18) . ULTRASOUND Section 7 of 10

Author Information Introduction Differentials Radiograph CT Scan MRI Ultrasound Intervention Pictures Bibliography

Findings: US is a very helpful imaging technique in the evaluation of a popliteal mass. US determines whether the popliteal mass is a cyst or solid mass. A simple Baker cyst appears as an anechoic mass with posterior acoustic enhancement that communicates with the knee joint. Findings on US relate to the criteria of a simple cyst, which include an anechoic mass, sharply defined posterior wall, and posterior acoustic enhancement. A complex popliteal cyst has internal echoes within the hypoechoic mass (Picture 3, Picture 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 within a gallbladder. An additional advantage of US is that it can exclude a coexisting DVT.

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

Author Information Introduction Differentials Radiograph CT Scan MRI Ultrasound Intervention Pictures Bibliography

Intervention: Treatment of popliteal cysts is conservative, including nonsteroidal anti-inflammatory agents, ice, and assisted weight bearing, in addition to 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 synoviorthosis can treat inflammatory arthritides and hemophilia. Prior to radioactive synoviorthosis, perform arthrography to exclude a leaking Baker cyst (Picture 19). A leaking Baker cyst would release radionuclide agent outside the knee joint, which is a contraindication to radioactive synoviorthosis.

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

Special Concerns:

Potential complications of popliteal cysts reported in the medical literature are as follows:

Pseudothrombophlebitis syndrome (rupture, dissection)


Pulmonary embolism




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 popliteal cyst may present as an enlarging mass in the calf.

Among patients with symptoms of DVT, the incidence of popliteal cysts is 3.1-4.1%, and the incidence of patients with Baker cyst and coexistent DVT is 0.2-11%. Therefore, consider a differential diagnosis of popliteal cyst in patients presenting with symptoms suggestive of DVT. Once DVT is excluded, evaluate the popliteal fossa carefully for a ruptured Baker cyst. Alternatively, if a popliteal 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 anatomic location, popliteal cysts can be a risk factor for DVT. Most commonly, Baker cyst is located between the tendons of the medial head of the gastrocnemius and the semimembranosus bursa. Occasionally, Baker cyst can be located 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 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 popliteal cyst can increase pressure within the deep posterior compartment of the leg, causing posterior compartment syndrome. Dissection of popliteal cysts can cause either anterior or posterior compartment syndrome, depending on the direction of dissection. A Baker cyst usually dissects through the muscles, primarily below the knee. Almost one third of dissections were into the thigh.

An infected Baker cyst is rare; review of the literature disclosed only 3 occurrences. The infected patients either had fever, an increased WBC count, or an elevated erythrocyte sedimentation rate.

An entity that may mimic infected Baker cyst is a popliteal cyst that contains gaslike lucencies. Only 2 patients have been reported with a gas-containing, noninfected popliteal cyst. Both patients had diabetes mellitus and RA. The authors did not suggest a mechanism for the appearance of the gaslike lucencies within the popliteal 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 popliteal cyst but not in the reverse direction because of a valvelike communication (either Bunsen or ball valve) (Picture 1). The effusion can be reabsorbed readily through the synovial membrane leaving behind concentrations of fibrin, which may appear as gaslike lucencies on radiographs. In our clinical experience, 1 patient was encountered with a noninfected Baker cyst containing gaslike lucencies (Picture 20, Picture 21, Picture 22). This patient also had diabetes mellitus and RA. Since gaslike lucencies in a popliteal cyst are rare, and an infected popliteal cyst is a serious condition, the former diagnosis must be one of exclusion, for which 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), popliteal artery aneurysm, vascular malformations, and loose calcified bodies trapped in a Baker cyst. Calcified bodies in Baker cyst 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 osteoarthritis, infection, neuropathic joints), or synovial (osteo)chondromatosis (ie, calcified or noncalcified bodies resulting from chondrometaplasia of synovial tissue) (Picture 8, Picture 9).

Distal migration of loose bodies supports the diagnosis of a dissecting popliteal 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 (Picture 10, Picture 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 popliteal cyst is located medially.

What is a knee effusion with a baker's cyst?
It means that you have terminal brain cancer.

Look it up on a medical website. Ask a doctor.
Reply:It is a tool for working on your car engine. It has a ratcheting splickle, with variable tuning and a large knob on the end (the "baker's cyst") that is used to adjust the spingle.
Reply:Effusion is the escape of fluid. A Baker's cyst is a cyst that is formed in the back of your knee which is filled with synovial fluid, due to arthritis. So, if you have a knee effusion with a Baker's cyst, you have a cyst which is leaking synovial fluid.

No comments:

Post a Comment