From a medical point of view, the tooth is surrounded by bone tissue. When an external orthodontic force is applied to a particular tooth, the tooth transmits this external force to the bone tissue surrounding the tooth. The bone tissue reacts to the force by resorbing the bone tissue in the area of pressure to move the tooth forward, and the bone tissue in the area of tension to fill the gap after the tooth has moved, until the bone around the tooth returns to its original thickness. It can be seen that in the process of orthodontic treatment, whether the teeth are extracted or not, the teeth are as stable as they were at the end of the orthodontic treatment, except for a slight loosening of the teeth during the treatment.