Popliteal Cyst (Baker's Cyst)
A Baker's Cyst is a fluid-filled swelling in the popliteal fossa. In adults, it is almost always secondary to intra-articular pathology (like a "safety valve" relieving joint pressure).
1. Anatomy
It is a herniation of the synovial membrane through the posterior capsule, located between the Medial Head of Gastrocnemius and the Semimembranosus tendon.
2. Causes
- Children: Usually primary/idiopathic. Self-limiting.
- Adults: Secondary to Meniscal Tears (usually medial) or Osteoarthritis/RA. A one-way valve mechanism pumps fluid into the cyst but not out.
3. Clinical Features
- Swelling behind the knee.
- Foucher's Sign: The cyst feels tense/hard on extension and soft on flexion (because the fascia relaxes).
- Rupture (Pseudothrombophlebitis): Sudden severe calf pain mimicking DVT.
4. Management
- Treat the Cause: Treating the arthritis or meniscus tear usually resolves the cyst.
- Aspiration/Injection: High recurrence rate.
- Surgery: Excision is rarely needed and has high recurrence if the underlying pathology isn't fixed.
25 Practice MCQs
Q1. Baker's Cyst is located between which two muscles?
Answer: A). Classic anatomical landmark.
Q2. In adults, Baker's cyst is most commonly secondary to:
Answer: A). It is a sign of intra-articular pathology.
Q3. Foucher's Sign describes:
Answer: A). Due to fascial tension. Distinguishes from aneurysm (which remains pulsatile).
Q4. Ruptured Baker's Cyst mimics:
Answer: A). "Pseudothrombophlebitis".
Q5. Best imaging modality to confirm diagnosis and underlying cause is:
Answer: B). Shows the cyst, the valve, and the meniscus tear.
Q6. Primary treatment for a symptomatic Baker's cyst in an adult involves:
Answer: A). Treating the cyst alone leads to recurrence.
Q7. Why does fluid accumulate in the cyst?
Answer: A). The gastrocnemius-semimembranosus bursa connects to the joint.
Q8. "Crescent Sign" (Ecchymosis around malleolus) indicates:
Answer: A). Synovial fluid tracks down and causes bruising at the ankle.
Q9. In children, Baker's cysts are usually:
Answer: A). Rarely communicate with the joint.
Q10. A pulsatile mass in the popliteal fossa suggests:
Answer: A). Important differential diagnosis. Do not aspirate!
Q11. Does a Baker's cyst contain pus?
Answer: B). Clear/yellow viscous fluid.
Q12. Can a Baker's cyst cause nerve compression?
Answer: A). If very large.
Q13. Ultrasound is useful to:
Answer: A). Doppler helps rule out vascular issues.
Q14. Aspiration often leads to:
Answer: A). Because the valve remains open.
Q15. Most common associated meniscal tear:
Answer: A). Location matches the cyst opening.
Q16. Rheumatoid Arthritis patients develop cysts because:
Answer: A). High intra-articular pressure pushes fluid out.
Q17. "Safety Valve" theory suggests:
Answer: A). Reduces pressure inside the main joint capsule.
Q18. Differential diagnosis of a lump behind the knee includes:
Answer: A). Must rule out dangerous causes.
Q19. Steroid injection into the cyst is:
Answer: A). Often combined with aspiration.
Q20. Dissection of a cyst can be difficult due to:
Answer: A). Popliteal artery/vein and Tibial nerve are nearby.
Q21. Calcification inside the cyst suggests:
Answer: A). "Osteochondral bodies".
Q22. Is physiotherapy useful?
Answer: A). Treating the cause treats the cyst.
Q23. Homan's sign checks for:
Answer: A). Positive in both DVT and ruptured cyst.
Q24. Do Baker's cysts occur in healthy knees?
Answer: B). Always look for the cause.
Q25. MRI T2 weighted images show the cyst as:
Answer: A). Water is bright on T2.
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