(1) Release crowding and free up space for alignment. (2) To improve the protruding facial shape by inwardly retracting the anterior teeth. (3) Harmonize the upper and lower arches. As for the need of tooth extraction, how many teeth to be extracted and which teeth to be extracted, it is up to the doctor to decide based on clinical examination, X-ray examination, model analysis, etc. In order to ensure the coordination of upper and lower teeth and symmetry between left and right, it is more common to extract four bicuspids (one each for upper, lower, left and right). Removal of four bicuspids for orthodontic treatment will not affect the function and health of the teeth.