GPAtlas🧭 When to suspect
A Baker's cyst (popliteal cyst) is a fluid-filled distension of the gastrocnemius–semimembranosus bursa in the popliteal fossa. The cyst is best understood not as a disease in its own right but as a visible marker of intra-articular pathology.
In adults it is almost always secondary: a one-way valve allows synovial fluid to track from a knee that is producing an effusion into the bursa, which then swells. The effusion is most often from:
• Osteoarthritis
• A degenerate meniscal tear
• An inflammatory arthritis
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
• Frequently found incidentally on knee imaging
Two clinical priorities run through the whole topic:
• Recognise the cyst is secondary and look for (and treat) the underlying joint problem
• 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) |
|---|
• Mainly in children (typically aged 4–7) |
• No communication with the knee joint |
• No underlying knee pathology |
• Often resolves spontaneously |
| Secondary (the usual adult cyst) |
|---|
• Adults, typically aged 35–70 |
• One-way valvular communication with the joint |
• Driven by intra-articular disease – Osteoarthritis (OA), meniscal tear, rheumatoid arthritis (RA), gout |
• Settles when the joint is treated; tends to recur if it is not |
Source: NICE NG226 · NICE NG158
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