Location of Baker's Cyst
A Baker's cyst is located in the popliteal fossa (the area behind the knee), specifically arising from the gastrocnemius-semimembranosus bursa situated along the medial side of the popliteal space. 1, 2
Anatomical Details
Primary Location:
- The cyst represents a fluid-filled distension of the bursa located beneath the medial head of the gastrocnemius muscle in the popliteal fossa 3
- It is positioned between the medial head of the gastrocnemius muscle and the semimembranosus tendon 1
- This bursa uniquely communicates with the knee joint space through an opening in the joint capsule posterior to the medial femoral condyle 2
Characteristic Imaging Appearance:
- On ultrasound, the cyst has a distinctive comma-shaped appearance when visualized in the posterior transverse scan between the medial head of gastrocnemius and semimembranosus tendon 1, 4
Patterns of Extension
While the cyst originates in the popliteal fossa, it can extend beyond this primary location:
Common Extension Patterns:
- The cyst typically extends along intermuscular planes around the knee joint 5
- Most commonly located in the inferomedial or superficial layers of the knee joint 5
- Can extend far into the thigh and calf muscles 1
Less Common Extensions:
- Lateral or proximal extension occurs less frequently 5
- Rarely, intramuscular extension can occur (such as into the vastus medialis muscle), though this is uncommon as the cyst typically respects intermuscular planes 5
Rupture Pattern:
- When ruptured, fluid leaks from the gastrocnemius-semimembranosus bursa and tracks inferiorly into the calf 6
- The fluid dissects along intermuscular planes between muscle groups, occasionally through muscle tissue itself 6