Violent hyperextension or hyperextension of the knee joint can cause ACL injury. If external force is applied to the femur from front to back when the knee is flexed, or if external force strikes the upper tibia from back to front, it can cause ACL rupture. What are the symptoms of cruciate ligament injury of the knee? Anterior dislocation of the knee is often caused by hyperextension, which is bound to injure the ACL. If it is caused by over-extension, medial ligament rupture may occur at the same time, and ACL injury combined with medial meniscus injury is also common. When the knee is flexed, external forces strike the upper tibia from front to back, causing excessive posterior displacement of the tibia, which can lead to posterior cruciate ligament injury and even posterior dislocation of the knee. This disease is more common in adolescents, more males than females, and is most common in athletes. When injured, the sound of ligament rupture can sometimes be heard, and soon there is swelling, pressure pain and fluid (blood) in the knee joint due to severe pain, and spasm of the knee muscles. There are obvious clinical symptoms such as curled up ligament rupture can be felt. The anterior cruciate ligament starts from the front of the tibial intercondylar ridge and ends backward, upward, and outward at the inner surface of the femoral epicondyle; the posterior cruciate ligament starts from the back of the tibial intercondylar ridge and ends forward, upward, and inward at the outside of the femoral inner ankle. The anterior cruciate ligament prevents the tibia from moving forward and the posterior cruciate ligament prevents the tibia from moving backward. If one of these ligaments is sprained, ruptured or relaxed, the stability and physiological function of the knee joint can be affected.