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%)
  • 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

There's more to see -- the rest of this topic is available only to subscribers.