Also called a popliteal cyst. Its proposed cause, in some individuals, involves a communication between the semimembranosus/gastrocnemius bursa and the knee joint. Some have postulated a one-way valve effect in which synovial fluid moves from the knee to the bursa. Baker’s cysts can occur secondary to any process that produces synovial fluid (most commonly RA, osteoarthritis, or trauma). A ruptured cyst can occasionally dissect down the calf and be confused with deep venous thrombosis. It is diagnosed with ultrasound or arthrography.