Crowding is the most common form of malocclusion. Different methods have been used to resolve crowding, including extraction and nonextraction. Corticision is an important methods to accelerate tooth movement in crowded cases. Cortical bone incision without flap elevation enhances turnover rate of surrounding structures and decreases the time for leveling and alignment. In orthodontics, CBCT imaging has been applied to evaluate impacted teeth and root resorption. But the data are very rare regarding the application of CBCT in tooth movement. So CBCT will be used in this research to evaluate the amount of external apical root resorption (EARR) and presence of dehiscence in the lower anterior teeth in the crowded cases. Furthermore, the duration of alignment will be compared between corticision group and non-corticision group.