Orthodontic Therapy and Tooth Movement

Tooth movement occurs due to bone remodeling around the tooth, primarily mediated by the periodontal ligament (PDL)
The PDL occupies a 0.5 mm wide space around the root, comprising parallel collagenous fibers, cellular elements, tissue fluids, and vascular and neural elements

The PDL has a network of parallel collagenous fibers inserting into cementum on one side and the lamina dura on the other
Supporting fibers run at an angle, attaching farther apically on the tooth than on the adjacent alveolar bone
The ligament contains cellular elements, including mesenchymal cells, vascular and neural elements, and tissue fluids
The collagen of the ligament is constantly remodeled and renewed during normal function

Biological response to orthodontic therapy includes the response of the PDL and growing areas distant from the dentition
Forces applied to teeth can affect bone apposition and resorption at sites distant from the teeth, particularly the sutures of the maxilla and bony surfaces on both sides of the temporomandibular joint

During masticatory function, teeth and periodontal structures are subjected to intermittent heavy forces
Tooth contacts last for 1 second or less, with forces ranging from 1 or 2 kg to 50 kg depending on the object being chewed
Heavy loads on a tooth cause quick displacement within the PDL, transmitted to the alveolar bone, which bends in response
Bone bending in response to normal function generates piezoelectric currents, stimulating skeletal regeneration and repair
Piezoelectricity is the production of a flow of current when a force is applied to a crystalline structure, such as bone or collagen

Pain is normally felt after 3 to 5 seconds of heavy force application
Prolonged force, even of low magnitude, produces a different physiologic response—remodeling of the adjacent bone
Orthodontic tooth movement is made possible by the application of prolonged forces