Orthodontics and Cellular Responses

Tooth Movement Biology and Biomechanics

• Tooth movement is a complex biological process involving alveolar bone remodeling, periodontal ligament reorganization, and coordinated cellular activity.
• Continuous force application to a tooth triggers these changes, leading to tooth movement.
• Orthodontic forces must not exceed capillary pressure in the periodontal ligament to avoid ischemia and necrosis.
• Tooth movement is influenced by root surface area; teeth with smaller roots require less force.
• Tipping involves greater compression and tension at the alveolar crest and root apex in opposite directions. Bodily movement occurs with uniform compressive loads on one side of the ligament.
• Force magnitude is crucial to tissue response.
• Intrusion requires the least force due to concentrated pressure at the root apex.
• Light forces result in direct bone resorption in compression areas and deposition in tension areas.
• Heavy forces lead to periodontal ligament necrosis, followed by delayed bone resorption beneath the necrotic area.
• Undermining resorption is a consequence of heavy forces.
• Bone resorption typically precedes deposition; hence, a widening periodontal space occurs during movement.

Mechanisms Linking Force to Tooth Movement

• Pressure-tension theory: Appears in periodontal ligament changes blood flow and releases chemical messengers initiating cellular reactions for movement.
• Bioelectric theory: Tooth movement is triggered by electrical currents generated by bone bending.
• Both mechanisms may contribute to orthodontic tooth movement.

Centre of Resistance

• Teeth, groups of teeth, and facial bones have a center of resistance.
• This is where resistance to movement is concentrated.
• Bodily tooth movement happens when force is applied directly to the center of resistance.
• For teeth, the center of resistance is not the same as the center of mass.
• It is defined by tooth shape and properties of alveolar bones and periodontal ligament.
• Center of resistance should be visualized in all planes of spaces.
• For single-rooted teeth, the center of resistance is approximately the halfway point down the root surface.
• For multi-rooted teeth, the center of resistance is located in the furcation area.
• Bone loss shifts the center of resistance apically.
• The center of resistance in the maxilla is near pre-molar roots; this is relevant for headgear application.

Forces, Moments, and Couples

• Orthodontic tooth movement depends on applied forces.
• A force is a vector with magnitude and direction.
• Forces have individual components (horizontal and vertical).
• Moments cause rotation—a pivotal concept in orthodontics.
• Moments equals the magnitude of the applied force multiplied by the perpendicular distance between appliance and center of resistance.
• A force applied away from the center of resistance results in rotation.
• Couples occur from pairs of equal but opposite forces acting at different points; used to create movement.
• Torque in orthodontics means differential movement of part of a tooth, while another part is restricted.
• Torque is often used to describe a moment or couple in tooth movement

Types of Tooth Movement

• Tipping: Crown moves more than the root in a particular direction; typically occurs during orthodontic therapy.
• Bodily movement: Crown and root move equally in the same direction (requiring a couple). This type of movement is achievable with fixed appliances.

Rate of Tooth Movement

• Ideally, forces should be applied 24 hours per day to speed up movement.
• Clinically, minimum movement occurs with a minimum of 6 hours per day of force application.
• In ideal circumstances, teeth move approximately 1 mm per month.
• Tooth initiation movement is slower in adults due to diminished periodontal ligament cellularity and vascularity, and greater alveolar bone density.