When teeth move positions during teeth straightening, we move the crown of the tooth and the tooth root. Usually, tooth roots are surrounded by strong, healthy bone that keeps them securely in place.
After the tooth roots move during orthodontics, it takes a while for the bone to remodel around the newly repositioned roots. There is a real risk that the roots could try to return to their original positions during the remodeling process. Wearing a retainer ensures that tooth roots cannot move, retaining their new positions in the jawbone.