Description

• 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

Epidemiology

• 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%)
• Osteochondritis
• Gout (14%)
• Other potential factors
  • Infectious arthritis
  • Polyarthritis
  • Villonodular synovitis
  • Lymphoma
  • Sarcoidosis
  • 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.

Risk Factors

• 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