Orthodontic Treatment Principles: Forces, Extractions, and Appliance Mechanics

Response to Orthodontic Pressure

Light Pressure (<1 second to Hours)

• Less than 1 second: PDL fluid is incompressible; alveolar bone bends, and a piezoelectric signal is generated.
• 1-2 seconds: PDL fluid is expressed; the tooth moves within the PDL space.
• 3-5 seconds: Blood flow within the PDL is partially compressed on the pressure side and dilated on the tension side.
• Minutes: Blood flow is altered, and oxygen tension begins to change.
• Hours: Metabolic changes and chemical messengers affect cellular activity.
• Less than 4 hours: Cellular differentiation begins within the PDL.
• 2 days: Tooth movement occurs as osteoclasts and osteoblasts remodel bone.

Heavy Pressure (<1 second to 7-14 days)

• Less than 1 second: PDL fluid is incompressible; alveolar bone bends, and a piezoelectric signal is generated.
• 1-2 seconds: PDL fluid is expressed; the tooth moves within the PDL space.
• 3-5 seconds: Blood vessels within the PDL are occluded on the pressure side.
• Minutes: Blood flow is cut off in the compressed PDL, leading to sterile necrosis and a hyalinized, cell-free zone.
• Hours: Cell death occurs in the compressed area.
• 3-5 days: Cell differentiation occurs in adjacent marrow spaces, leading to undermining resorption.
• 7-14 days: Undermining resorption removes the lamina dura adjacent to the compressed PDL, leading to a period of stasis.

Orthodontic Extractions and Alternatives

Reasons for Tooth Extraction

1. Disproportion between teeth and arch size.
2. Malrelation between arches.
3. Unfavorable position of root apices.
4. Abnormal size of individual teeth.
5. Preservation of symmetry.
6. Extreme jaw relations.
7. Late treatment.

Modalities Other Than Extractions

• Interproximal reduction (stripping)
• Expansion
• Distalization

Serial Extraction in Orthodontics

Rules for Serial Extraction

1. Class I molar relationship bilaterally.
2. No skeletal problems.
3. Discrepancy of at least 5 mm.
4. Dental midlines coincide.

Advantages of Serial Extraction

• Simplifies mechanics.
• Decreases severity of malocclusion.
• Decreases treatment time.

Disadvantages of Serial Extraction

• Increased overbite.
• Lingual tipping of anterior teeth.
• Excessive space if done improperly.

Orthodontic Anchorage and Its Management

Factors Affecting Anchorage

1. Surface Area of Alveolar Bone.
2. Surface Area of the Root.
3. Distribution of Force Along the Root Relative to Root Axes.

Signs of Anchorage Loss

1. Mesial movement of molars.
2. Closure of extraction space.
3. Proclination of anterior teeth.
4. Spacing.
5. Increased overjet.
6. Change in molar relation.
7. Buccal crossbite.

Phases of Orthodontic Treatment

1. Preventive Phase: Maintaining the integrity of normal occlusion after early detection of potential causes of malocclusion.
2. Interceptive Phase: Malocclusion has started to develop; signs and symptoms of developing malocclusion are present, but full malocclusion has not yet occurred.
3. Habit Management: Addressing oral habits that contribute to malocclusion.
4. Corrective Phase: Malocclusion has occurred, and active steps are taken to correct it.
5. Retention Phase: The final phase to maintain orthodontic correction of dental and skeletal structures following the cessation of the active phase.

Advantages of Fixed Orthodontic Appliances

1. Higher biologic magnitude of force application.
2. Not dependent on patient's compliance.
3. Less bulky compared to some removable appliances.
4. Allows all types of tooth movement.