What is a Baker's Cyst?
A Baker's cyst is a fluid-filled swelling that develops in the back of the knee, specifically in the bursa located between the medial head of the gastrocnemius muscle and the semimembranosus tendon, which typically communicates with the knee joint space. 1, 2
Anatomical Definition
- Baker's cysts represent fluid accumulations in the gastrocnemius or semimembranosus bursa that frequently communicate with the knee joint through an opening in the joint capsule posterior to the medial femoral condyle 3
- The cyst appears as a characteristic comma-shaped extension on ultrasound imaging when viewed in the posterior transverse scan between the medial gastrocnemius head and semimembranosus tendon 1, 2
- This bursa is unique because it communicates with the knee joint, unlike other periarticular bursae, creating a valve-like mechanism that contributes to cyst formation when joint effusion is present 3
Clinical Presentation
Typical symptoms include:
- Swelling and tenderness behind the knee that may worsen with exercise and can be present at rest 2
- A feeling of tightness or fullness in the popliteal fossa 2
- Pain that worsens with knee movement or exercise, unlike vascular claudication which resolves quickly with rest 2
- Visible and palpable fluctuant mass in the popliteal fossa, especially with the knee extended 2
- Possible limitation in knee range of motion 2
Underlying Causes
The etiology differs significantly between adults and children:
- In adults, Baker's cysts are almost always associated with intra-articular pathology, including meniscal tears, osteoarthritis, or rheumatoid arthritis 4, 3
- In children, these cysts are usually incidental findings on physical examination or imaging and generally lack clinical significance 4, 5
Important Complications to Recognize
Cyst rupture is the most clinically significant complication:
- A ruptured Baker's cyst can cause sudden calf pain and swelling that clinically mimics deep vein thrombosis (DVT) 2, 6
- Fluid disseminates into the calf muscles, creating a pseudothrombophlebitis presentation 2, 3
- Appropriate imaging is essential to differentiate ruptured cysts from DVT 1, 2
Critical Diagnostic Pitfall
Do not assume all popliteal masses are benign Baker's cysts—imaging is mandatory to exclude popliteal artery aneurysm, especially in patients with a history of other arterial aneurysms. 2 Additional red flags suggesting malignancy rather than a benign Baker's cyst include: symptoms disproportionate to cyst size, absence of joint damage that would explain the cyst, unusual topography, bone erosion, cyst size greater than 5 cm, and tissue invasion of the joint capsule 4