Invisalign fill the empty space after tooth extraction by the neighboring teeth moving, if need to inwardly retract the teeth, generally by the near middle neighboring teeth to fill most of the empty space, if need to lift the crowding of the teeth, generally by the near and far middle neighboring teeth to fill part of the empty space respectively, as follows. 1. The need for dental retraction: mainly for the correction of maxillary protrusion or antimandibular, through the extraction of orthodontic teeth, dental retraction, that is, the teeth before the empty space to move backward, to fill the empty space of the extracted teeth. 2. Need to lift the crowding: after the orthodontic teeth are removed, the remaining teeth need to be lined up and leveled, the process of empty space near and far in the teeth were filled with part of the empty space, in order to lift the original crowding of the teeth. Invisible orthodontic tooth extraction may also exist in other cases, the patient’s situation is different, the empty space is filled in different ways, it is recommended to go to the hospital for targeted treatment.