Fixed, Removable & Functional Appliances MCQ

The appliance is how the force is delivered. Fixed appliances (brackets plus archwire) move teeth precisely in three planes. Removable appliances tip and retain. Functional appliances and headgear harness growth in the growing patient. Clear aligners stage tipping and bodily movements through a series of trays with known limits. Palatal expansion opens the midpalatal suture in growing patients. Matching the appliance to the goal, the patient, and the growth stage is the central decision of mechanics.

Brackets and the Straight-Wire Appliance

• Edgewise brackets have a rectangular slot that accepts a rectangular archwire; in the straight-wire appliance (Andrews), the bracket prescription is built into the slot angulation (tip), torque, and in-out (offset), so a straight final archwire produces the planned finish.
• Common prescriptions include Andrews, Roth (modified to allow finishing detail), and MBT; bracket prescription influences final tooth position more than the patient typically perceives.
• Self-ligating brackets replace the ligature with a built-in clip (passive or active); they reduce friction at the slot, which can speed alignment in some cases.
• Lingual orthodontics bonds custom brackets to the lingual surfaces of the teeth (hidden from view); it is a precision option with a steeper learning curve.

Archwire Sequence: NiTi, TMA, and Stainless Steel

• Initial alignment uses a thin, round NiTi (nickel-titanium) archwire because of superelasticity (force is nearly constant across a wide deflection range) and shape memory (the wire returns to a programmed shape).
• Working wires (the middle of treatment) are typically TMA (beta-titanium / titanium-molybdenum alloy) or stainless steel; they are stiffer than NiTi and allow precise three-dimensional control.
• Finishing wires are rectangular stainless steel that fill the bracket slot, allowing detailed torque and root paralleling.
• Auxiliary forces are delivered with elastomeric chains (light continuous force, but force decays in ~24-48 hours), nickel-titanium coil springs (more constant force over a wider range), and elastics (intra- or inter-maxillary) that depend on patient compliance.

Functional Appliances

• Functional appliances reposition the mandible (or maxilla) and harness growth in the growing patient. The Twin Block (two upper and lower removable blocks with inclined planes) and the Herbst (fixed telescopic appliance) are the most common Class II Div 1 functional appliances; both posture the mandible forward and recruit condylar growth.
• Other functional appliances include the activator and bionator (passive tissue-borne appliances) and the Fränkel (vestibular-shield appliance that allegedly alters muscle balance).
• Functional appliances work best at or just before the pubertal growth spurt; outside this window the skeletal effect is limited and the result is mostly dental.
• Headgear is a partner: cervical (low) pull adds posterior molar extrusion and distalization (suited to brachyfacial/Class II with deep bite tendencies that tolerate extrusion); high pull pulls maxillary molars superiorly and posteriorly (preferred in high-angle Class II patients to avoid extrusion); reverse-pull (facemask) protracts the maxilla (Class III in growing patients).

Clear Aligners

• Clear aligners (Invisalign and others) deliver tooth movement through a series of thermoformed thermoplastic trays, each tray staging a small movement of selected teeth.
• Composite ATTACHMENTS bonded to the tooth surface provide grip and create force vectors that the smooth aligner could not deliver (rotations, extrusions, complex bodily movements); IPR (interproximal reduction) creates space without extraction.
• Aligners work well for mild crowding (~1-5 mm), tipping, expansion, and IPR cases; they are limited for large rotations (especially canines), bodily movement of round-rooted teeth, large extrusions, large root torque, and complex Class II/III cases.
• Compliance is essential: aligners must be worn about 20-22 hours per day to track; refinement aligners are routinely planned to finish the case after the initial series.

Removable Appliances and Retainers

• Hawley appliances are acrylic-and-wire removable appliances used historically for tipping and now mainly for post-treatment retention; they cover the palate with finger springs/wires for selective adjustment.
• Other removable appliances include the inclined plane (interceptive use for anterior crossbite in young children) and the simple acrylic bite plate for deep bite cases.
• Retainers include Hawley (removable, palatal), Essix (clear thermoformed, full coverage), and fixed bonded lingual retainers (typically lower 3-3 wire bonded to the canines, or 4-4). Each balances compliance, occlusion, and hygiene access.
• Removable appliances tip and rotate teeth effectively but do not produce bodily translation; they are limited compared with fixed appliances for comprehensive treatment.

Palatal Expansion (RPE/Hyrax)

• Rapid maxillary expansion (RPE, often a Hyrax appliance) opens the midpalatal suture, producing SKELETAL maxillary expansion in growing patients; activation is typically 0.5-1 mm per day for 1-3 weeks until the desired expansion is achieved.
• Slow palatal expansion uses lighter forces over weeks to months and produces a more physiologic skeletal-plus-dental response with less tissue trauma.
• Successful skeletal expansion in growing patients is signaled by a diastema between the maxillary central incisors (the central incisors part as the suture opens); the diastema closes spontaneously after activation as the periodontal fibers recoil.
• The midpalatal suture progressively closes through mid-to-late adolescence; surgical-assisted rapid palatal expansion (SARPE) is needed in adults to obtain meaningful skeletal expansion.