Tooth movement is a relative concept of normal, teeth do have a physiological degree of movement, but the degree of movement is very small, about 0.02mm, if more than 0.02mm, the tooth is considered loose. There are three degrees of tooth loosening, degree I refers to the loosening within 1mm, and also refers to the loosening in the direction of the lip and cheek. Degree II refers to loosening within 2mm, and refers to loosening in both the labial-buccal and near-distal-medial directions. Ⅲ degree refers to loosening of more than 2mm, when the direction of movement, the tooth is loose in the labial-buccal side, near and distal-medial as well as vertical direction, and teeth with more than Ⅲ degree loosening need to be extracted.