This condition is caused by instability of the patellofemoral joint. The patellofemoral joint is usually affected by congenital patellofemoral dysplasia, or by an acquired injury to the quadriceps muscle or patellar tendon, which causes the patella to wear out, i.e. the patella is more severely worn on one side. In the process of squatting, the patellofemoral joint generates greater pressure and friction in this state, resulting in a ringing sound. After this happens, you should pay attention to keep your knee joint from getting cold, keep it warm, and use a warm water bag for warm compresses. Especially at night when the leg is straightened and cooled, it is most likely to produce a popping sound when the knee joint is flexed. In addition, you should reduce strenuous exercise, because the patellofemoral joint is worn out and the friction between the patella and the femur during strenuous exercise increases, making it easy to produce cartilage degeneration and osteophytes and calcification.