Popliteal Baker cyst
A popliteal (Baker) cyst is a synovial fluid-filled outpouching of the gastrocnemius-semimembranosus bursa driven by intra-articular knee pressure from osteoarthritis, inflammatory arthritis, or meniscal injury, and is managed by aspirating and injecting the knee joint rather than the cyst itself, since a ruptured cyst can masquerade as deep venous thrombosis.
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What this article covers
- Overview
A popliteal (Baker) cyst is a fluid-filled distention of the gastrocnemius-semimembranosus bursa, a normal anatomic bursa in the posteromedial knee that communicates with the knee joint capsule through a one-way valve-like opening.
- Epidemiology
Common in knee osteoarthritis: popliteal cysts are a recognized musculoskeletal complication seen frequently alongside knee osteoarthritis and other chronic knee joint disease.
- Risk factors
Knee osteoarthritis: the most common underlying joint disease driving chronic effusion and cyst formation.
- Pathophysiology
Pressure-driven outpouching: flexion of a joint with a chronic effusion markedly raises intra-articular pressure; because the gastrocnemius-semimembranosus bursa communicates with the joint space…
- Clinical features
Posterior knee fullness or a palpable, often fluctuant mass in the popliteal fossa, frequently more prominent with the knee extended and less prominent or non-palpable with the knee flexed (Foucher sign).
- Differential diagnosis
Deep venous thrombosis: acute unilateral calf swelling, warmth, and pain, the most important mimic of a ruptured popliteal cyst; distinguished by lower-extremity venous ultrasonography, and the crescent sign favors rupture over thrombosis.
- Diagnosis
A popliteal (Baker) cyst is confirmed by popliteal ultrasonography, which visualizes a well-defined, anechoic or complex fluid collection in the classic gastrocnemius-semimembranosus location and…
- Management
First, determine whether the cyst is symptomatic and whether it has ruptured, since asymptomatic unruptured cysts require no treatment.
- Complications
Cyst rupture with pseudothrombophlebitis: acute calf pain, swelling, and ecchymosis clinically indistinguishable from deep venous thrombosis without imaging.
- Prognosis
Asymptomatic cysts require no intervention and carry no independent morbidity beyond that of the underlying joint disease.
- Prevention
Effective long-term control of the underlying knee disease is the primary preventive strategy: treat inflammatory arthritis to a target of low disease activity or remission (DAS28 under 3.2), and for…
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