Baker Cyst — SCE Rheumatology MCQ

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EasyMusculoskeletal ImagingBaker CystSCE Rheumatology

A 45-year-old woman with rheumatoid arthritis has active right knee synovitis and an effusion. Ultrasound shows a popliteal cyst communicating with the knee joint. She subsequently develops acute right calf pain and swelling. Duplex ultrasound shows fluid tracking from the cyst into the calf, with fully compressible deep veins and no intraluminal thrombus. What is the most likely diagnosis?

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Correct answer: E — Ruptured Baker cyst (popliteal cyst)

The diagnosis is a ruptured Baker cyst. Rheumatoid knee synovitis and effusion can distend the gastrocnemius–semimembranosus bursa, producing a popliteal cyst that communicates with the knee joint. Rupture allows synovial fluid to track into the calf, causing acute pain and swelling—the pseudothrombophlebitis presentation. Deep vein thrombosis is the key clinical differential, but fully compressible veins without intraluminal thrombus exclude it on this scan. A gastrocnemius tear would usually follow trauma or forceful contraction and would not arise directly from a communicating popliteal cyst. Compartment syndrome causes disproportionate pain and tense compartments, while cellulitis typically produces superficial erythema, warmth and systemic or infective features. Suspected rupture still requires urgent assessment because DVT cannot be distinguished reliably from symptoms alone.

Reference: NHS. Baker's cyst, symptoms, causes and assessment. Updated 26 June 2025. https://www.nhs.uk/conditions/bakers-cyst/