The causes of knee swelling and pain are infection, osteoarthritis and synovitis. The details are as follows: 1. Infection: when the knee joint is infected, septic arthritis will occur, and due to the inflammatory reaction and purulent exudation, joint distension and pain will occur, and it will be accompanied by symptoms such as chills and high fever. 2. Osteoarthritis: Osteoarthritis is characterized by degenerative disease of articular cartilage and secondary osteophytes, which will involve the articular cartilage and the whole joints, and joint swelling and pain will also occur. 3. Synovitis: Synovitis is caused by degeneration of joints, trauma, infection, etc. Synovitis secretes a large amount of synovial fluid, which can also cause joint swelling and pain. There are many other causes of knee swelling and pain, such as trauma, gouty arthritis and so on, the specific cause of the disease can go to the hospital, the physician based on the medical history, symptoms and related tests to determine, and actively cure.