A young patient with a sprained knee from landing while playing basketball had no particular discomfort after 2 weeks of swelling and pain in the knee. He was still able to play, but the joint felt unstable when he exerted force. He went to the hospital and was diagnosed with an ACL injury, and the doctor recommended ACL reconstruction surgery. The patient wondered why he needed surgery now that he was not particularly uncomfortable. In fact, the doctor’s recommendation for surgery was made through a comprehensive evaluation. After an ACL injury, the treatment plan needs to take into account the patient’s age, the degree of ligament damage, the degree of joint instability and the patient’s sports needs. The goal of ACL reconstruction is to restore knee stability and kinematics to avoid further injury or limitation of motion due to joint instability and premature joint degeneration. ACL injuries produce joint swelling, pain, and limitation of motion in the acute phase. If there is no persistent interlocking of the meniscus after the initial injury, the swelling and pain will generally subside significantly after 2-3 weeks, and mobility will gradually return. If the initial ACL injury causes instability in the knee joint, the patient may re-train during sports or may not be able to perform certain sports or movements due to fear of instability or re-train. In the case of the ACL itself, the initial injury often results in a complete rupture of the ligament, so there is no second injury to the ligament, which results in new articular cartilage and meniscus damage. If the initial injury to the ACL causes a partial injury, there is a possibility of a complete rupture with a second injury. The initial ACL injury or a second sprain due to joint instability can be combined with a meniscal injury, which can also cause localized pain, transient or persistent joint interlocking. Injury to the ACL can be accompanied by damage to the articular cartilage, and subsequent joint instability can exacerbate the cartilage damage, resulting in joint pain, stiffness, and limited motion. In terms of joint stability, after a complete rupture of the ACL, the joint tends to be reasonably stable in the early stages. However, over time, objective instability tends to increase and the most severe state occurs. This may be the cause of further laxity of the lateral ligaments of the knee as well as diminished stability of the meniscus, especially in patients with previous re-injury. Patients with partial ACL injuries tend to have better stability in the early stages. Patients who do not enjoy sports generally have little change in stability later on, while those who enjoy sports may have further increased laxity. First, because partial injuries do not heal on their own, and second, the remaining ligament fibers often cannot withstand the full load of the original motion and may become completely ruptured. Surgery is recommended for patients with re-injury or multiple injuries. Re-injury indicates that the stability of the knee joint does not meet the functional requirements of the knee joint or lower extremity. Surgery is also recommended for patients who have had premature osteoarthritis attacks to avoid premature progression of osteoarthritis. Anterior cruciate ligament injuries are often combined with meniscus injuries. Surgery is required when meniscal injuries cause symptoms that require early relief (e.g., persistent interlocking, etc.) or when various types of meniscal injuries require early repair. In these cases, although the primary goal of surgery is to treat the meniscus injury, ligament reconstruction or strengthening surgery may also be performed at the same time. In this case, ACL reconstruction is an incidental procedure. If the ACL injury is combined with a cartilage injury that requires surgical treatment, ACL reconstruction may be performed incidentally. If the patient has a single injury or a primary injury that is not combined with a meniscal or articular cartilage injury that requires surgery, an MRI is required to determine the integrity of the ligament. If the injury is partial, it can be treated conservatively with regular follow-up to see how the stability of the joint has changed and how the integrity of the ligament has changed. If it is a complete rupture, the objective stability of the joint will need to be understood. If the stability is fair, the auxiliary stabilizing structures of the knee are generally functioning well and can be treated conservatively, but regular follow-up is needed to understand changes in the stability of the knee. If there is significant instability, it is important to understand the patient’s exercise requirements. Surgery is recommended for any patient with athletic requirements, regardless of age, and the goal of surgical treatment is to prevent re-injury. Patients who do not desire to play sports will need to have further treatment options determined by age. If the patient is elderly, surgical treatment is not required. In these patients, daily activities are generally unaffected after the initial injury subsides and re-injury is rare; in the long term, the effects of accelerated joint degeneration due to objective joint instability may not be felt. If the same condition occurs in young and middle-aged adults, surgery is usually recommended. The purpose of surgery in this case is not to relieve the current symptoms, but to prevent premature degeneration of the joint in the long term. This is because even though the patient will not sprain again, there is still an abnormal kinematic state of the joint during daily walking (minor dislocation-reset), which can still exacerbate the degeneration of the joint. Patients whose original ligament is partially damaged, it is important to follow up regularly for changes in the integrity of the ligament. In case of conversion to a complete rupture, further treatment needs to be determined according to the appropriate treatment procedures. In patients with complete ligament rupture and good knee stability, it is also important to follow up regularly to see how the stability of the knee joint has changed. This is because the trend is for the joint to become less stable, not more stable. In the event of significant instability, further treatment needs to be decided according to the appropriate procedures.