Extraction space is closed by moving the neighboring teeth through.
Beginning
Middle
End
Teeth can be moved because of the periodontal ligaments on the roots (artificial implant has no periodontal ligament, thus is not movable).
Orthodontic force is transmitted to bone through periodontal ligaments.
When bone receives pressure, an inflammatory reaction will be triggered; followed by the activation of the bone-resorbing cells. Manifestation of inflammation is the sense of pain and discomfort.
The activated bone-resorbing cells then degrade the bone tissue. Inflammation will subside and pain gradually relieves.
The bone resistance diminishes from resorption, the teeth start to move.
When teeth move to the new position and orthodontic force declines, the new bone will start to form around the roots.
The similar cycle will repeat when next time a new orthodontic pressure is applied.