The knee joint is the largest weight-bearing joint in the human body and contains bone, cartilage, surrounding muscles, tendons, ligaments, meniscus and other anatomical structures, with a complex structure and variable injury mechanisms. If the knee joint is injured in daily life or during sports, the following conditions should be seen immediately by a specialist orthopedic surgeon to avoid delaying treatment and causing future problems. (1) Severe pain in the knee joint after the injury. (2) The appearance of the injured knee is abnormal when compared to the other side. (3) Swelling of the knee joint within 30 minutes of the injury. (4) A ringing sound can be heard in the knee joint at the time of injury or after the injury. (5) The knee joint cannot be straightened and cannot bear weight after the injury. (6) The injured knee is unstable, often stuck or weak. (7) Signs of nerve or blood vessel injury, such as numbness or pins and needles, or pale, bruised skin, appear below the injured knee. (8) Previous injury to the knee joint and repeated injury again.