Fixed Appliance Biomechanics and Treatment
Domain 3: Fixed Appliance Biomechanics and Treatment
The exam tests force systems, anchorage control, and mechanotherapy. You need to predict tooth movement from the moment-to-force ratio and to recognize when a planned movement is statically indeterminate.
Core rules
- The center of resistance of a single-rooted tooth is about one-third of the root length apical to the alveolar crest; for a multirooted tooth it is near the furcation. A force not passing through the center of resistance creates a moment.
- Moment-to-force ratio governs movement: below 5:1, uncontrolled tipping; 5-7:1, controlled tipping; 7-10:1, translation; above 10:1, root torque.
- Sliding mechanics on a continuous arch produce friction; the applied force must exceed friction before the tooth moves. Consider frictionless loop mechanics when friction is high.
- Bracket prescription is not a substitute for anchorage planning. MBT and Roth differ in tip and torque; pre-adjusted appliances still need wire bends when teeth are severely displaced.
- Light continuous forces of about 50-150 g for anterior retraction favor frontal resorption; heavy forces cause hyalinization and undermineralized resorption.
- Anchorage can be intraoral (transpalatal arch, lingual arch), extraoral (headgear), or skeletal (miniscrews). Miniscrews provide absolute anchorage but require primary stability and hygiene.
Worked example
A 14-year-old has had first premolars extracted and needs maximum retraction of the upper incisors. The canine is retracted on a 0.019 × 0.025 stainless steel archwire with a nickel-titanium closed coil spring delivering 150 g. If the force is applied at the bracket, the line of action is below the center of resistance, producing uncontrolled tipping and extrusion. The moment-to-force ratio is low. The fix is to add a power arm or retraction hook closer to the center of resistance, add torque in the archwire, and use a transpalatal arch or miniscrew for anchorage. The expected movement becomes controlled tipping or translation rather than uncontrolled tipping.
Traps
- Measuring force at the bracket and ignoring the moment arm.
- Assuming friction is negligible in sliding mechanics.
- Using heavy forces for faster movement; hyalinization slows it.
- Forgetting vertical control during retraction; incisors extrude and the bite opens.