Popliteal (Baker) Cyst
- A fluid-filled synovial sac arising in the popliteal fossa as a distention of (typically) the gastrocnemial-semimembranous bursa; not a true cyst
- Can be unilateral or bilateral
- Most frequent cystic mass around the knee
- Primary cysts are a distention of the bursa (arise independently without an intra-articular disorder).
- Secondary cysts occur if there is a communication between the bursa and knee joint, allowing articular fluid to fill the cyst.
- Associated with synovial inflammation
- Bimodal distribution
- Children ages 4 to 7 years
- Adults increasing with age
- Primary cysts usually seen in children <15 years
- Secondary cysts seen in adults
- Variable adult prevalence of 19–47% in symptomatic knees and 2–5% in asymptomatic knees
- In children: 6.3% in symptomatic knees; 2.4% in asymptomatic knees
Etiology and Pathophysiology
Associated intra-articular pathology includes
- Meniscal tears, mostly of the posterior horn
- Anterior cruciate ligament (ACL) insufficiency
- Degenerative articular cartilage lesions
- Rheumatoid arthritis (20%)
- Osteoarthritis (50%)
- Gout (14%)
- Other potential factors
- Infectious arthritis
- Villonodular synovitis
- Connective tissue diseases
- Extension or herniation of synovial membrane of the knee joint capsule or connection of normal bursa with the joint capsule
- May result from increased intra-articular pressure
- Commonly seen with knee effusions
- Direct trauma to the bursa is likely the primary cause in children because of no communication between the bursa and the joint.
- A valve-like mechanism allowing one-way passage of fluid from the joint to the bursal connection has been described.
- Osteoarthritis of knee (most common)
- Rheumatoid arthritis
- Meniscal degeneration or tear
- Advancing age
- Ligamentous trauma
- Ligamentous insufficiency
Commonly Associated Conditions
Any condition causing knee joint effusion
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