Baker cyst (popliteal cyst) is a synovial fluid-filled cystic lesion resulting from abnormal distension of the bursa between the semimembranosus tendon and medial gastrocnemius muscle in the posterior compartment of the knee. In adults, it is almost always associated with underlying intra-articular pathology (meniscal tear, osteoarthritis, chondral lesion, inflammatory arthritis) — joint fluid passes into the popliteal bursa through a one-way valve mechanism. Prevalence reaches 20-40% in the population over 50 years. On MRI, typical location (between semimembranosus-gastrocnemius) and characteristic T2 hyperintense fluid content are diagnostic. Complications include rupture (may mimic pseudomembranous phlebitis), internal hemorrhage, infection, and nerve/vessel compression. Ruptured Baker cyst can mimic DVT and must always be considered in differential diagnosis. Treatment is generally correction of the underlying pathology.
Age Range
30-70
Peak Age
50
Gender
Equal
Prevalence
Very Common
Baker cyst results from abnormal communication between the gastrocnemio-semimembranosus bursa (normal anatomic bursa) and the knee joint. In normal physiology, this bursa connects to the knee joint through a narrow channel (orifice). Intra-articular pathology (meniscal tear, osteoarthritis, synovitis) increases joint fluid production and with increased pressure, fluid passes through a one-way valve mechanism into the bursa — but cannot return because the orifice closes in knee extension. This mechanism leads to progressive cyst enlargement. The inner wall is lined with synovial membrane and contains synovial fluid. In chronic Baker cysts, the wall may thicken and content becomes proteinaceous — T2 signal decreases and T1 signal increases. In rupture, cyst fluid dissects along fascial planes between gastrocnemius and soleus muscles — 'falling sign' or 'flame-shaped' dissection is seen on MRI. This fluid collection causes pain, swelling, and ecchymosis in the calf clinically mimicking DVT. The bright T2 appearance of Baker cyst on MRI is directly related to its content being free water (synovial fluid) — free water protons produce high signal with long T2 relaxation time.
Pathognomonic diagnosis of Baker cyst is made with two components: (1) Location — cystic lesion between semimembranosus tendon and medial gastrocnemius muscle. (2) Neck sign — connection of cyst to knee joint through narrow orifice. The combination of these two findings differentiates Baker cyst from all other popliteal lesions (ganglion cyst, popliteal artery aneurysm, solid tumor).
On axial and sagittal T2 fat-sat images, a homogeneous T2 hyperintense cystic lesion is seen in the popliteal fossa between the semimembranosus tendon and medial gastrocnemius muscle. Cyst size ranges from a few mm to 10+ cm. Typically comma-shaped or speech-bubble-shaped extending beneath the medial gastrocnemius head. Content shows homogeneous fluid signal. The orifice (connection to knee joint) may be seen as a thin channel in the sagittal direction on axial images.
Report Sentence
A T2 hyperintense Baker cyst measuring approximately ___x___ cm is seen between the semimembranosus and medial gastrocnemius in the popliteal fossa.
In ruptured Baker cyst, the cyst wall is disrupted and fluid dissects along gastrocnemius-soleus fascial planes. On T2 fat-sat, flame-shaped or falling pattern high-signal fluid collection is seen in the calf. The cyst at its original location may be collapsed or decompressed. Edema and reactive changes in surrounding soft tissues accompany. This finding clinically mimics DVT and differential diagnosis should be made with US Doppler.
Report Sentence
The Baker cyst has ruptured with flame-shaped fluid dissection along the gastrocnemius-soleus fascial planes in the calf.
On US, an anechoic or hypoechoic cystic lesion is seen between semimembranosus and medial gastrocnemius in the popliteal fossa. Neck sign — demonstration of cyst connection to knee joint through a narrow neck — confirms Baker cyst diagnosis. Simple cyst shows anechoic content with posterior wall enhancement, while complex cyst may have internal echoes, septations, and thick wall. No internal vascularity on Doppler (differentiation from solid tumor).
Report Sentence
An anechoic cystic lesion measuring approximately ___x___ cm in the popliteal fossa with narrow neck connection to knee joint (neck sign) is seen, consistent with Baker cyst.
On T1-weighted images, hyperintense signal is seen within Baker cyst content different from simple fluid — this represents proteinaceous content or subacute hemorrhage (methemoglobin). In complex Baker cyst, internal septations, debris, and thickened wall may also be seen. In infected Baker cyst, pericystic edema and enhancement may be added. Contrast MRI may be needed to differentiate complex Baker cyst from synovial sarcoma or other popliteal tumors.
Report Sentence
T1 hyperintense signal within Baker cyst content is seen, suggesting complex Baker cyst with proteinaceous content or subacute hemorrhage.
On axial T2 images, the neck structure of Baker cyst between semimembranosus and medial gastrocnemius connecting to the knee joint is seen. This narrow channel (orifice) confirms the cyst's communication with the knee joint and differentiates Baker cyst from other popliteal cysts (ganglion cyst, meniscal cyst, etc.). The neck is a few mm in width and significantly thinner than the cyst body.
Report Sentence
Communication of Baker cyst to the knee joint space through a narrow orifice (neck sign) is seen.
Criteria
Homogeneous T2 hyperintense fluid content, thin wall, no septation
Distinct Features
Most common type, usually asymptomatic or mildly symptomatic, conservative treatment
Criteria
T1 hyperintense content, internal septation/debris, thickened wall
Distinct Features
Chronic cyst or internal hemorrhage, differential from solid tumor needed, contrast MRI
Criteria
Cyst wall disrupted, fluid collection dissecting into calf, falling/flame sign
Distinct Features
DVT mimic, acute calf pain and swelling, DVT exclusion with US Doppler needed
Distinguishing Feature
Medial meniscus tear is intra-articular meniscal pathology and one of the most common underlying causes of Baker cyst — both pathologies should be evaluated together
Distinguishing Feature
Chondral lesion is cartilage surface pathology and another common cause of Baker cyst; both frequently seen in osteoarthritis setting
Distinguishing Feature
Bone contusion is bone marrow pathology in completely different anatomic structure from Baker cyst; Baker cyst development after trauma is rare
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
6-monthThe fundamental approach in Baker cyst treatment is correction of the underlying intra-articular pathology — with meniscal tear repair, osteoarthritis treatment, or synovitis control, the cyst usually spontaneously regresses. US-guided aspiration + corticosteroid injection may be performed for symptomatic isolated cysts but recurrence rate is high (50-70%). Surgical excision is reserved as last resort and only considered for large cysts with compressive symptoms resistant to conservative treatment. In ruptured Baker cyst, treatment is symptomatic (analgesia, elevation, compression) and DVT exclusion. In complex Baker cyst, solid tumor exclusion (especially synovial sarcoma) with contrast MRI is important.
Baker cysts usually regress with treatment of the underlying joint pathology. Asymptomatic cysts can be followed. Symptomatic or large cysts may be treated with aspiration or surgical excision. Rupture can mimic DVT (pseudothrombophlebitis syndrome) and should be differentiated with US/Doppler.