Orthodontics: Bone Resorption and Tooth Movement

Tooth Movement Biology and Biomechanics

• Tooth movement is a complex biological process involving alveolar bone remodeling, periodontal ligament reorganization, and tooth movement.
• Orthodontic tooth movement involves multiple cell types and chemical mediators coordinating their activity.
• Applying continuous force to a tooth surface results in compression and tension areas within the periodontal ligament.

Effects of Force on Periodontal Ligament

• The force applied to a tooth affects the periodontal ligament, leading to areas of compression and tension, influencing tooth movement.
• The magnitude of the force significantly affects the tissue response.
• Ideal orthodontic forces shouldn't exceed capillary pressure. Exceeding this pressure causes ischemia and tissue necrosis.

Optimal Force for Tooth Movement

• Optimal force for tooth movement depends on the root surface area of the tooth.
• Teeth with smaller roots require less force for movement compared to teeth with larger roots.

Different Types of Tooth Movement

• Tipping: This involves greater crown movement than root movement, resulting in a diagonal change in tooth position. Compressive forces act at the alveolar crest and root apex.
• Bodily Movement: Equal movement of crown and root in the same direction. Requires even compressive forces along one side.

Tooth Movement Rate

• Ideally, orthodontic forces are applied 24 hours a day to maximize efficient tooth movement.
• At least 6 hours of force delivery per day is clinically sufficient for minimal movement.
• Ideal tooth movement rate under optimal conditions is approximately 1mm per month, but individual variation exists.
• Adults experience slower tooth movement due to reduced cellularity and vascularity of the periodontal ligament and higher density alveolar bone.

Mechanisms Linking Force Application and Tooth Movement

• Pressure-tension theory: Pressure and tension changes within the periodontal ligament alter blood flow, triggering chemical messenger release and cellular responses for movement.
• Bioelectric theory: Bending of alveolar bone triggers electrical current generation, which in turn promotes desired cellular events for movement.

Biomechanics of Tooth Movement

• Forces: Forces (vectors) are described by magnitude and direction. Their components influence tooth movement.
• Moments: Moments of force (rotational effects) are calculated considering the force magnitude and perpendicular distance from the force application to the center of resistance.
• Couples: Moments from two equal and opposite forces that tend to cause rotation, commonly used in fixed appliance treatment.
• Torque: "Torque" refers to differential movement of tooth parts while restraining others - usually achieving root torque by applying force couples to the crown.

Types of Tooth Movement

• Tipping: The crown moves more than the root in a given direction, typically caused by moments from applied forces.
• Bodily Movement: Crown and root move together in the same direction, often accomplished by couples in fixed appliances.

Understanding Centre of Resistance

• Centre of resistance: The point in a tooth where resistance to tooth movement occurs. Single rooted teeth have a center halfway down the root. Multi rooted teeth have center of resistance in root furcation area. Maxilla's center of resistance commonly located in premolar area.

Factors influencing the rate of tooth movement

• Orthodontic forces: should be applied for 24 hours/day optimally for most efficient movement (at least 6 hours a day for minimal). -Capillary pressure: Excessive force creates tissue necrosis resulting in delayed or altered tooth movement.
• Time for tissue repair and remodelling: At least 4-6 weeks between successive appliance reactivations is needed for repair and remodelling of damaged tissues.

Cellular Responses to Orthodontic Forces

• Cellular responses within periodontal ligament are evident within hours of orthodontic force initiation.
• Chemical mediators like prostaglandin E, cytokines, and nitric oxide are detected within tissue after force application.
• Osteoclasts and osteoblasts actively remodel alveolar bone responding to force application.