GPAtlas🧭 When to suspect
A Baker's cyst (popliteal cyst) is a fluid-filled distension of the gastrocnemius–semimembranosus bursa in the popliteal fossa. In adults it is almost always secondary: a one-way valve allows synovial fluid to track from a knee that is producing an effusion – most often osteoarthritis, a degenerate meniscal tear, or an inflammatory arthritis – into the bursa, which then swells. The cyst is best understood not as a disease in its own right but as a visible marker of intra-articular pathology.
Suspect it in an adult with a swelling, fullness or tightness behind the knee, often with aching that is worse on full flexion or extension and after walking. The swelling may fluctuate in size and is frequently found incidentally on knee imaging.
Two clinical priorities run through the whole topic: first, recognise the cyst is secondary and look for (and treat) the underlying joint problem; second, never miss the dangerous mimics – a ruptured cyst that is actually, or co-exists with, a DVT, and an enlarging popliteal mass that is a sarcoma or a popliteal aneurysm.
| Primary (uncommon) | Secondary (the usual adult cyst) |
|---|---|
• Mainly in children (typically aged 4–7) |
• Adults, typically aged 35–70 |
• No communication with the knee joint |
• One-way valvular communication with the joint |
• No underlying knee pathology |
• Driven by intra-articular disease – OA, meniscal tear, RA, gout |
• Often resolves spontaneously |
• Settles when the joint is treated; tends to recur if it is not |
Source: NICE NG226 · NICE NG158
🔒 Sign up free to read the full topic
You're viewing a free preview. Create a free account to unlock the rest.
Sign up free →Sample topics are open to everyone in the Free Sample Bundle.