What is a Baker's Cyst?
A Baker's cyst is a fluid-filled sac that develops in the back of the knee, specifically representing a distention of the gastrocnemio-semimembranosus bursa that communicates with the knee joint space. 1, 2
Anatomical Definition
- The cyst forms from fluid accumulation in the bursa located between the gastrocnemius and semimembranosus muscles in the popliteal fossa 1, 2
- This bursa uniquely communicates with the knee joint through an opening in the joint capsule posterior to the medial femoral condyle 3
- The communication creates a valve-like mechanism that allows fluid to flow from the joint into the bursa but restricts return flow, contributing to cyst formation in adults 3
Clinical Presentation
Patients typically present with swelling and tenderness behind the knee that may worsen with exercise and can be present at rest. 2
Key symptoms include:
- Visible and palpable swelling in the popliteal fossa, especially with knee extension 2
- Tenderness in the popliteal fossa 2
- A feeling of tightness or fullness behind the knee 2
- Possible limitation in knee range of motion 2
- The cyst may extend far into thigh and calf muscles 2
Diagnostic Appearance
- Ultrasound reveals a characteristic comma-shaped extension between the medial head of gastrocnemius and semimembranosus tendon 1, 2
- The cyst appears as a fluctuant mass that may increase in size with knee extension 2
Associated Conditions
- Baker's cysts rarely occur in isolation and are most commonly found with intra-articular pathologies such as osteoarthritis, meniscus tears, and rheumatoid arthritis in adults 3
- In children, these cysts are more often incidental findings and less commonly associated with underlying joint pathology 3, 4
Important Complications
A ruptured Baker's cyst can mimic deep vein thrombosis clinically, presenting with sudden calf pain and swelling. 2, 5
- Rupture causes dissemination of fluid into the calf 1, 2
- This presents as pseudothrombophlebitis and requires differentiation from true DVT with appropriate imaging 1, 2
- Rarely, infection can occur 1
Critical Pitfall
Clinicians should not assume all popliteal masses are benign Baker's cysts and must obtain imaging to exclude popliteal artery aneurysm, especially in patients with a history of other arterial aneurysms. 2