Fixed and Removable Appliance Therapy
What the exam tests
Appliance design, bracket placement and prescription, mechanics for space closure, and the limits of removable and functional appliances.
Core rules
- Straight-wire appliances move the bends into the bracket; the prescription (Roth, MBT, Andrew's) encodes tip, torque, and in-out values.
- Bracket the tooth at the facial axis point; errors in height produce step and rotation.
- Slot size determines which wire fills the slot: a 0.019×0.025 wire in a 0.022 slot leaves play and less torque control than in a 0.018 slot.
- Space closure: sliding mechanics (friction, simpler) versus sectional/loop mechanics (frictionless, better anchorage control). The T-loop and vertical loop are common frictionless designs.
- Removable appliances excel at simple tipping, expansion, and retention; they cannot bodily move or torque teeth reliably, and depend on wear.
- Functional appliances (twin block, Herbst, activator) modify growth and posture; they need compliance (except Herbst) and a favourable growth stage.
Worked example
A growing 12-year-old with Class II division 1, 6 mm overjet, retrognathic mandible, and CVM CS3. Reasoning: a twin block during the growth spurt can reduce overjet by mandibular repositioning and dentoalveolar change, followed by fixed appliances for finishing. A Herbst is chosen if compliance is doubtful.
Traps
- Assuming brackets alone deliver the prescription with a round wire.
- Using a removable appliance for rotation or torque.
- Overlooking anchorage loss in sliding mechanics.
- Blaming appliance failure when the real problem is wear time.