Biomechanics and Orthodontic Tooth Movement
What the exam tests
Force systems, centers of resistance, moment-to-force ratios, anchorage, friction, and the biology of PDL response.
Core rules
- The center of resistance of a single-rooted tooth lies roughly at 0.4 of root length from the alveolar crest; for the maxillary incisor it is near the apex region.
- Moment-to-force ratio controls movement type: uncontrolled tipping (≈0–2:1), controlled tipping (≈5:1), translation (≈10:1), and root torque (>10:1).
- Optimal force is light and continuous: roughly 50–100 cN for translation, less for intrusion (≈15–25 cN per incisor), more for multirooted teeth.
- Excessive pressure causes PDL hyalinization and undermining resorption, delaying movement; the rate is typically about 0.8–1.2 mm per month.
- Anchorage is classified as absolute, maximum, moderate, or minimum; TADs, palatal coverage, and segmental mechanics provide skeletal anchorage.
- Statics: sum of forces and moments must be zero; a couple is a pure moment; equivalent force systems can be relocated.
Worked example
A maxillary canine needs 150 g for translation. Required moment = force × bracket-to-center-of-resistance distance (≈10 mm) = 1500 g·mm, giving M/F ≈ 10:1. If only a simple point force is applied, the tooth tips; a power arm or T-loop generating the couple produces bodily movement.
Traps
- Applying a 10:1 ratio when tipping is intended, or vice versa.
- Believing heavier forces move teeth faster.
- Ignoring friction in sliding mechanics, which consumes up to 30–50% of applied force.
- Forgetting that intrusive forces concentrate at the apex and increase root resorption risk.