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- A fluid-filled synovial sac arising in the popliteal fossa
- Distention of the gastrocnemial-semimembranous bursa
- Can be unilateral or bilateral
- Most frequent cystic mass around the knee (1)
- Primary cysts are a distention of the bursa arising independently without an intra-articular disorder.
- Secondary cysts occur if a communication exists between the bursa and knee joint, allowing articular fluid to fill the cyst. Pathologic joint processes can also be transmitted in this manner.
- Associated with synovial inflammation
- Synonym(s): Popliteal cyst
- Bimodal distribution: Children ages 4–7, and adults increasing with age
- Primary cysts usually seen in children <15 years of age
- Secondary cysts seen in the adult population
- Varies by study
- Studies report a prevalence of 19–47% in symptomatic knees, 2–5% in asymptomatic knees.
- In children: 6.3% in symptomatic knees, 2.4% in asymptomatic knees
- Extension or herniation of synovial membrane of the knee joint capsule or connection of normal bursa with the joint capsule
- May be the result of increased intra-articular pressure
- Commonly seen with knee effusions
- Direct trauma to the bursa is likely the primary cause in children because there is no communication between the bursa and the joint in children.
- A valvelike mechanism allowing 1-way passage of fluid from the joint to the bursal connection has been described.
Associated intra-articular pathological findings include:
- Meniscal tears, posterior horn
- Anterior cruciate ligament (ACL) insufficiency
- Degenerative articular cartilage lesions
- Rheumatoid arthritis (20%)
- Osteoarthritis (50%)
- Gout (14%)
- Other potential factors: Infectious arthritis, polyarthritis, villonodular synovitis, lymphoma, sarcoidosis, and connective tissue diseases (3)
Commonly Associated Conditions
Any condition causing knee joint effusion