The ACL, also known as the anterior cruciate ligament, is the most important of the many ligaments in the knee joint for maintaining motion. The ACL is located in the center of the knee joint and is not palpable on the surface of the body, and its primary role is to maintain the stability of the knee joint. Specifically, the ACL limits excessive forward movement and rotation of the tibia, excessive extension of the knee joint, and excessive inversion and valgus, thus maintaining the stability of the knee joint and allowing the body to perform a variety of complex and difficult lower extremity movements. The ACL is susceptible to rupture when the body is rammed or moves at high velocities. There are three most common types of movements: the first is an inversion or valgus knee injury, such as a knee flexion valgus sprain after jumping up and landing during basketball shooting; the second is a knee hyperextension injury, such as when playing soccer and missing a kick, the leg is overstretched all of a sudden; in addition, a hyperextension injury can easily occur when the skis are inserted into the snow during a high-speed slide in skiing and the athlete trips and falls; the third is Knee direct impact injury, this is more rare, generally in rugby, athletes hit each other, thighs and knees are prone to this type of injury. These actions can easily cause an ACL rupture. When the ACL ruptures, patients often experience significant pain in the knee, along with a “clicking” sound. In addition, many patients also experience swelling in the knee. These patients are usually unable to continue playing sports, such as soccer or basketball, after the injury; if the patient is able to continue playing after a break, it is not usually an ACL rupture. It is important to note that the timing of the onset of knee swelling after an injury may indicate a difference in the site of injury. If the swelling is obvious immediately after the injury, it is more likely to be an intra-articular fracture or patellar dislocation, while if the swelling occurs 3-4 hours or more after the injury, it is likely to be a meniscal injury. So, what are the dangers to the patient when there is an injury or rupture of the ACL? The most important is to affect the stability of the knee joint, the following three common hazards. 1, can not stop sharply, affect the movement: after the ACL rupture, there will be pain, swelling, wait until after a period of rest, the pain and swelling will gradually disappear, the patient can walk normally, and even participate in sports. However, when the patient plays sports again, although he or she can run, he or she cannot make sharp stops and turns, such as crossing the ball with a ball, which requires high knee stability, which the patient often cannot do or dare not do. Thus, for athletes, it will affect their sports career; 2. Repeated sprains of the knee joint: after ACL rupture, if you force a sharp stop and turn movement, it will cause another sprain of the knee joint, many patients will repeatedly sprain the joint, and then seriously damage other structures in the knee joint (such as meniscus, cartilage, etc.); 3. 3. Secondary damage to structures in the knee: After an ACL rupture, even if you no longer participate in sports and only go about your normal work life, the loss of ligament protection can cause repeated minor misalignments in the joint, which can lead to secondary damage to the meniscus and cartilage in the knee over time. Studies have shown that secondary cartilage and meniscus damage is evident 3 months after the first ACL injury. In severe cases, after several years, even young patients’ joints even start to grow bone spurs, very similar to the joints of the elderly. Therefore, the consequences of a ruptured ACL, if not taken seriously, are very serious. So, what should a patient do when an ACL injury is suspected? On the one hand, it is necessary to find an experienced doctor for physical examination; on the other hand, MRI is recommended to determine whether the ligament is damaged. Some patients have visited the hospital when the ACL injury has just occurred, but only X-rays have been taken. However, X-rays mainly look at the condition of the bone, such as whether it is fractured, and cannot give a judgment on whether the ligament is damaged. As a result, many patients with early ACL injuries are thus missed, and by the time the disease transitions to the chronic stage and is examined again, some secondary damage has already occurred. Therefore, if ACL damage is suspected, MRI is recommended and a sports medicine specialist is consulted.