Acute sprain 1. Mild The pain is mild and not swollen when the knee is sprained. It improves with rest for 3-7 days. This is generally a mild injury to the joint capsule, and rest is sufficient. 2. Moderate There is pain when the knee is sprained, but it is still possible to move or walk, and the injury is judged by the swelling. One is swelling within a few hours after the injury, which is usually combined with knee ligament injury or joint capsule tear. The second is swelling found the day after the injury, which is usually a meniscal or cartilage injury. The third is swelling that does not occur after the injury, but occurs with activity after the pain has subsided. The fourth is slight swelling after the injury, which is usually a mild injury to the joint capsule. The first three cases must be seen as soon as possible, the last one can rest and move normally after the symptoms disappear. Zhen Dong, Department of Sports Medicine, Guizhou Orthopaedic Hospital 3. Severe The knee joint sprains with severe pain or immediate inability to move or a sense of dislocation, and swelling is evident within a few hours after the injury. These need to be seen as soon as possible, and some patients who can still walk after the injury, or even have a normal gait, should not delay the need for prompt medical attention. These injuries generally include large ligament tears such as medial patellofemoral ligament, anterior and posterior cruciate ligaments, meniscal tears or fractures of cartilage and bone, or even tendon ruptures. Chronic sprains that do not allow for fast running, sharp turns and stops, difficulty jumping on one leg, or inability to perform movements that were possible before the injury should be considered ligament ruptures, patellar dislocation injuries, or bony structural injuries and should be seen promptly. Painful popping in the knee joint, painful walking on level ground (especially definite pain in the medial or lateral space of the knee joint), and interlocking (sudden jamming of the knee joint that requires movement or help from others to unlock it) should be considered meniscal or cartilage injuries and should be seen promptly.