The ringing sound when the knee is straightened is considered to be related to osteoarthritis, meniscus injury, ligament injury and other causes. It can be treated by general treatment, physical therapy, medication, surgery and other methods according to the cause of the disease. 1. Osteoarthritis: patients will have degenerative damage of articular cartilage, when the knee is straightened, the damaged cartilage rubs against each other and rattles. Usually pay attention to rest, can apply non-steroidal anti-inflammatory drugs, such as celecoxib and other treatment. It can also be treated with heat therapy and microwave. If the condition is serious, after conservative treatment is invalid, artificial joint replacement is feasible. 2. Meniscus injury: it is mainly caused by trauma, chronic strain, etc. These factors will make meniscus degeneration and breakage, which will also cause knee straightening and rattling. Usually pay attention to rest, avoid excessive exercise, can also apply celecoxib and other treatment. If it is acute meniscus injury of third degree, it can be treated by surgery, such as minimally invasive meniscus suture surgery. 3. Ligament injury: After ligament injury, the joint will become loose and rattle when the knee is straightened. After the injury, we should pay attention to rest, you can apply cold compresses in the acute period and hot compresses in the recovery period. Mild ligament injuries can be treated with plaster immobilization. In case of ligament rupture, surgical repair and reconstruction is required. If there is a ringing sound when the knee is straightened, and accompanied by joint pain, activity limitation and other symptoms, you should go to the hospital for further diagnosis and treatment. The above drugs need to be applied under the guidance of a physician, avoid self-medication.