Straight Wire (Preadjusted Edgewise) Appliance
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Direct answer
Andrews built the bends into the bracket and called it the straight wire appliance. After deriving the six keys to normal occlusion from 120 non-orthodontic ideal casts (published 1972), he machined each tooth's average first-, second- and third-order values into a tooth-specific bracket, so a flat full-dimension wire theoretically expresses ideal in-out, tip and torque without hand-bending. Later prescriptions retuned his averages: Roth added overcorrection to fight relapse, and McLaughlin, Bennett and Trevisi (MBT, 1997–98) re-engineered values for use with lighter forces, lacebacks and bendbacks — most famously raising upper central incisor torque from Andrews' +7 to +17 degrees. The working reality is a 0.018 or 0.022 inch slot system climbed from light round nickel-titanium through rectangular transition wires to a 0.019 × 0.025 steel arch in the 0.022 slot, with bracket-positioning errors, not wire-bending skill, now the main source of finishing imperfection.
What you must remember
- Andrews' six keys (1972): molar relationship (upper mesiobuccal cusp in the lower groove), correct crown angulation (mesiodistal tip), correct crown inclination (labiolingual torque), absence of rotations, absence of spaces, and a flat occlusal plane with minimal curve of Spee.
- Three orders built in: first-order (in-out) via base thickness variations; second-order (tip) via bracket slot angulation — upper central 5 degrees in the original Andrews prescription; third-order (torque) via slot or base inclination — upper central +7 degrees originally.
- MBT prescription markers: upper central tip 4 degrees with torque +17 degrees; heavier incisor torque and reduced canine tip to offset the play of a 0.019 × 0.025 wire in a 0.022 slot.
- Roth's rationale: values overcorrected toward relapse — rotations, torque and Class II relationships deliberately overtreated so settling lands on the target.
- Slot geometry and torque play: a 0.019 × 0.025 steel archwire in a 0.022 slot leaves several degrees of slop before the wire engages the slot walls; prescriptions compensate by adding torque in advance.
- Bracket placement: heights charted from incisal edges with a gauge (upper central commonly 4.0 mm), the vertical axis aligned to the long axis of the clinical crown; errors here defeat the prescription, and repositioning the bracket — not adding bends — is the straight-wire answer.
- Related systems worth naming: Alexander's Vari-Simplex philosophy mixing slot sizes and force levels; lingual straight-wire equivalents derived later.
From bond-up to finish on one flat wire
Walk a first-premolar-extraction case through the appliance. Brackets bonded by height gauge, light 0.014 nickel-titanium engaged with lacebacks from molars to canines so the canines cannot flare during levelling. Alignment resolves in weeks because the tiny wire engages deep displacements at gram-level force. Transition through rectangular nickel-titanium brings torque expression; then the 0.019 × 0.025 steel working wire slides the segments together on nickel-titanium coil springs or active ties at 150–200 g. At finish, the orthopantomogram shows one lateral rotated slightly mesial-in — the bracket was bonded 10 degrees off the long axis at a rushed first visit. The straight-wire answer is to debond that single bracket, reposition it 2 mm gingivally or rotate the orientation, and run six more weeks, rather than to introduce an artistic bend that fights every other value in the arch. That philosophy — the appliance carries the information, the operator carries the placement — is what "preadjusted" genuinely means, and why the technique punishes sloppy bonding more than clumsy bending.
Where viva examiners probe
Attribution first: the six keys and the preadjusted appliance belong to Andrews; MBT is the 1990s revision by McLaughlin, Bennett and Trevisi; Roth is the overcorrection variant — mixing the three loses easy marks. The second probe is the claim that preadjusted means no bends; the honest answer is that tooth-size variation (a Bolton discrepancy), bracket-height error and play between wire and slot guarantee that finishing still demands first-order artistic bends, minor torque steps and repositioning. Third, candidates describe torque as built into the base only; both torque-in-base and torque-in-slot designs exist, and the 0.019 × 0.025 in a 0.022 slot trailing torque deadzone is the reason prescriptions front-load incisor torque values.
Frequently asked questions
What are Andrews' six keys to normal occlusion?
Molar relationship, crown angulation, crown inclination, absence of rotations, absence of spaces, and a flat occlusal plane — the template from which the original bracket prescription was derived.
What distinguishes the MBT prescription from Andrews' original?
MBT increases incisor torque (upper central +17 versus +7 degrees), adjusts tip values and is designed around lighter forces with lacebacks and bendbacks in a 0.022 slot.
Why does a 0.019 × 0.025 wire in a 0.022 slot underexpress torque?
The clearance between wire and slot walls leaves several degrees of play before engagement, so effective torque lags the nominal prescription unless values are over-engineered.
What is the first-order bend called, and what does it correct?
The in-out or artistic bend in the horizontal plane, correcting labiolingual positional deviations — though in preadjusted systems bracket repositioning is preferred.
How does the Roth prescription handle relapse?
By overcorrection — rotations, torque and molar relationships are deliberately overtreated so post-treatment settling finishes on the ideal rather than short of it.