Sudden swelling of the knee may be caused by ligament rupture, trauma, knee osteoarthritis and so on. 1. Ligament rupture: Ligament rupture may be a sudden injury or stretching that affects the ligamentous tissues of the knee, causing edema, i.e. sudden swelling of the knee. 2. Trauma: Trauma may be a fracture caused by gravity impact, which causes soft tissue contusion and aseptic inflammation, which causes restriction of knee function and fluid buildup in the joint, resulting in sudden swelling of the knee. 3. Knee arthritis: knee arthritis may be due to joint osteophytes, meniscus degeneration, caused by a kind of aseptic knee arthritis, the knee in the inflammation of the stimulus, produce more joint fluid, thus causing sudden swelling of the knee. Sudden swelling of the knee may also be caused by septic arthritis, gout, etc. It is recommended to consult a doctor to confirm the cause of the disease and then follow the doctor’s instructions for treatment.