The knee rattles when the leg is straightened, commonly due to patellar maltracking, knee meniscus injury, cartilage injury and other causes, as follows: First, patellar maltracking: the patella will be tilted to the outside for congenital developmental reasons, and when the patient straightens the leg, the patella rubs against the outside of the femoral slide and produces a rattling sound, which in the long run will also lead to knee pain symptoms. Second, knee meniscal injury: If the anterior horn of the meniscus is torn, the patient will experience knee pain when the leg is straightened, accompanied by limited knee movement and increased pain when hyperextending. Third, knee cartilage damage: especially sports injuries and degenerative osteoarthritis, the patient’s joint surface will be significantly rough, which will cause excessive friction on the joint surface when the leg is straightened, resulting in joint pain and ringing. The above causes are common causes of knee rattling when the leg is straightened, and with active treatment, the rattling can be reduced or even disappear completely.