The knee joint is a contradiction in terms: it is stable because it can support the hip joint and the ankle joint to meet the body’s upright weight-bearing function; it is also flexible because it can adapt to the needs of the body’s technical movements in complex sports. According to Chinese medicine, the knee is the seat of the tendons, so the ligaments of the knee joint are important structures to maintain the coordination and balance of joint stability and flexibility. A rupture of the ligaments will seriously affect the function of the knee joint, causing an excessive increase in flexibility and destabilization! Traffic accidents, accidental falls and twisting impacts in sports and daily life can cause ligament rupture in the knee joint. According to the epidemiological statistics of the clinical cases of sports trauma all over the world, the ligament rupture of the knee joint is more often the rupture of the anterior cruciate ligament! Based on the general requirement of “early diagnosis, early treatment and early rehabilitation”, the authors believe that early and accurate diagnosis after accidental injury is the most important first step to seek medical treatment, and patients should have the ability to self-judge the relevant injury at the time of accidental injury in order to avoid aggravation of the injury! Therefore, based on the statistics of a large number of patients with acute ACL rupture of the knee and the relevant literature reports from home and abroad, the following “self-determination procedure” has been developed for patients with acute ACL rupture based on the patient’s complaints at the time of injury “1. Did the knee joint actually twist due to body instability or impact? Clinical experience suggests that patients with acute ACL ruptures have a clear history of injury! 2. Was the knee very painful within the knee joint during the acute injury? Clinical experience suggests that almost all patients with acute ACL ruptures complain of severe pain in the knee joint at the time of the first injury! 3. Do you hear or feel a “popping” sound in the knee joint during an acute injury? Clinical experience suggests that over 50% of patients complain of hearing or feeling a “popping” sound in the joint at the time of injury! Some may even describe the sensation of “bones being repositioned after being misaligned”! 4. After an acute injury, does the injured knee fall to the ground because it cannot hold its weight? Clinical experience suggests that almost all patients complain of “sitting or falling to the ground” while in severe pain, and that the injured knee cannot be flexed or extended, which is very painful! 5. Does the swelling of the injured knee occur soon after the acute injury? Clinical experience suggests that all patients complain of rapid swelling of the injured knee within three hours (possibly sooner, e.g. within a few minutes) of the injury, with an increase in the skin temperature of the knee! This indicates bleeding in the knee joint due to rupture of the anterior cruciate ligament, the so-called “joint hemorrhage”. 6. Did you go to the hospital for an x-ray after the acute injury? And was it confirmed that there were “no bone abnormalities”? Clinical experience suggests that most patients who go to the hospital for emergency care will have X-rays taken and will be confirmed to have “no bone abnormalities”. Note that X-rays do not show soft tissue structures such as ligaments of the knee, unless the acute ACL rupture is accompanied by an avulsion fracture of the anterior portion of the lateral tibial plateau (which is not very common!) In the case of a “Segond fracture”, this fracture can be detected on X-ray and indirectly proves that an ACL rupture has occurred that cannot be shown on X-ray! 7. How to decide if MRI is needed if the above mentioned items are present? Clinical experience suggests that if any of the above items are present, the patient is advised to alert the physician that an ACL rupture may have occurred. If signed by the patient or/and the delegated attorney, the physician will perform a puncture of the injured knee under strict sterile conditions (don’t worry, it is not too painful when done correctly and most patients are cooperative). If the blood is withdrawn, no fat droplets are present on the surface of the blood after a few minutes of resting, and the X-ray confirms that there is no intra-articular fracture, MRI is recommended to clarify the presence of an ACL rupture and possible combined injuries, such as “meniscal tears” or “bone contusions The MRI is recommended to identify ACL rupture and possible combined injuries such as “meniscal tear” or “bone contusion”, “medial and lateral collateral ligament injury” and “cartilage injury”. Therefore, a timely arthrocentesis can help guide the diagnosis of patients with the aforementioned high clinical suspicion of ACL rupture, with MRI being the final supporting evidence based on the above considerations! Of course, a reliable arthrocentesis can also extract a large amount of blood from the joint, allowing for intra-articular decompression and reducing inflammatory irritation for pain relief.