In clinical work, patients often ask the question: Why does a tooth move when it has already grown in? The exposed part of the tooth, called the crown, and the root, which is about two times the length of the crown, are rooted in the tooth bed (alveolar bone), and the tooth and the alveolar bone are not tightly bonded together. There are blood vessels and nerves inside the periodontium to provide local nutrition. Then why do the teeth move inside the bone of the toothbed after being subjected to external forces? This question is actually the basis of orthodontic treatment. The crown of the tooth was subjected to a tension F to the right, please note the change in the width of the periodontium on both sides. Then the periodontal membrane near the right side (A area) is under pressure, under the action of this pressure, the periodontal membrane is squeezed, in order to relieve this squeezed state, the cells in the periodontal membrane will differentiate osteoclasts, the function of osteoclasts is to be able to make the alveolar bone in the pressure area (black area) is absorbed away, as the alveolar bone is absorbed, the squeezed state of the pressure area is relieved. In the retraction zone (B zone), the periodontal membrane is pulled, producing a tendency of local space expansion, in order to alleviate this expansion tendency, the cells in the periodontal membrane in the retraction zone will differentiate into osteoblasts, the function of osteoblasts is to produce alveolar bone deposits on the alveolar bone surface (red area), with the continuous production of alveolar bone deposits, the local space expansion tendency in the retraction zone is alleviated. In this way, the environment around the tooth has returned to normal, and the tooth is stable again. Then we see from a macroscopic point of view that the tooth is moving in the direction of tension under the action of tension. It follows that as the tension continues, this activity of bone breaking and bone building continues, then the teeth move in the direction and position we have established, and eventually the effect of orthodontic treatment is achieved. Conversely, if there is no osteoclastic and osteogenic activity in the alveolar bone, then there is no way for the teeth to move at all. Through the above analysis, it is clear that the speed of tooth movement is not entirely determined by the size of the pulling force F. The fundamental controlling factor is the rate of bone growth, that is, the faster the rate of bone resorption in the black area/the faster the rate of bone deposition in the red area, the faster the tooth movement will be. Conversely, if the bone does not grow, then no amount of tension F can produce tooth movement. This explains why children’s teeth move faster than adults and the orthodontic treatment for children is shorter than that for adults: it is because the rate of bone growth plays a decisive role. Is it true that the greater the tension F, the faster the teeth will move? The answer is that within a certain range, this is true, but if the force F exceeds a certain range, then the effect is the opposite. If the tooth is subjected to a certain level of force F, then if the force is increased again, the tooth will move slower. Why? (To be continued)