Fixed Appliance Therapy and Archwire Mechanics
Fixed Appliance Therapy and Archwire Mechanics tests bracket prescription, wire sequencing, space closure, and finishing. The exam expects you to connect mechanics to the force system rather than list appliances.
Core rules
- Bracket prescriptions differ in torque, angulation, and in-out values. Slot size affects wire fit and torque expression; a 0.022-inch slot accepts thicker working wires but expresses less torque with a given wire than a 0.018-inch slot.
- Wire sequence usually progresses from light nickel-titanium for alignment, to stiffer nickel-titanium or TMA for leveling, to stainless steel for working and finishing. Rectangular wires control torque; round wires do not.
- Space closure can be sliding mechanics with power chain or coil spring, or frictionless mechanics with loops. Sliding mechanics demand anchorage control and attention to friction.
- Archwire bends create force systems: a V-bend produces a couple and can control root movement; step bends produce vertical forces. Coordinate upper and lower arch forms before finishing.
Worked example
After extracting 14 and 24, a patient has 7 mm spaces and proclined incisors. Reasoning: use a 0.019 x 0.025 stainless steel archwire for torque and sliding mechanics. Apply a light continuous force with power chain or NiTi coil, but plan anchorage: maximum retraction of anterior teeth needs posterior anchorage, possibly miniscrews. Check canine and incisor torque; do not close spaces on a round wire.
Traps
- Trying to express torque with a round wire.
- Closing extraction spaces before leveling and aligning.
- Overloading posterior anchorage during en masse retraction.
- Finishing without checking marginal ridges, rotations, and arch coordination.