Bending superelastic archwires — the missing constituent of orthodontic therapy

Dr. Suhail A. Khouri discusses the clinical benefits of wire bending

Introduction

When superelastic archwires entered orthodontic practice, their 100% spring-back elastic property created the lightest and most consistent force delivery range, which exceeds by far the elasticity increase obtainable by wire bending. They revolutionized teeth alignment without loop bending. But the impossibility of bending those brittle wires posed the challenge of finding a special tool to activate them.^1,2 This triggered the development of Bendistal pliers in 1995.³ These special pliers can insert permanent V-bends that activate those unbendable wires in all directions, leading to evolution of the new V-bend technique, which opened new potential in orthodontic therapies.^4-11

How elasticity of wires creates and delivers orthodontic forces

Orthodontic force is created from the energy stored by deflecting an elastic wire to a position just before its deformation point. This force will be delivered to teeth when the deflected wire returns to its original position. Bending archwires into loops, helices, and springs aims primarily at extending the elasticity range of the distant part of the bent wire that will be engaged in the bracket’s slots. The position of this distant part of already bent wire, before being engaged in brackets, becomes the targeted final position of the teeth. So, the distance from the target position and the bracket, the bent part of the wire to be tied to, is called “activation range.” It is the same range of extended spring-back range after appliance bending. The “deactivation range” is the same spring-back range. The pushing or pulling force starts acting on tooth/teeth, consistently moving the teeth all the way until the elastic range expires, and the teeth reach the target position.

Mechanical and clinical benefits of wire bending

The following factors determine levels of archwire elasticity, magnitudes, and continuity of force-delivery:

1. Metallurgic alloy structure of archwire exhibits its own specific level of elasticity.
2. Cross-sectional thickness. Wire that has a thinner cross section has a longer elasticity level and range than thicker wire, and vice versa.
3. Length of a wire. Longer wire has a higher level of elasticity and longer spring-back range than short ones, and vice versa.
4. Bending a wire extends its elasticity, creating lighter, more consistent force that moves teeth faster with less root
5. Bendability of a wire. Wire bending into appliances allows applying those forces in all directions.^9,10

The structural elasticity properties of superelastic archwires negates the need for the four benefits of wire bending; however, their bending remains urgently needed to apply their lightest and most consistent forces in all directions on teeth. Bendistal and Omni pliers were specifically designed just to satisfy the fifth objective and resolve this mostly important requisite.¹¹

Measuring archwire’s elasticity

The force magnitude required to deflect a wire a certain distance before it deforms is called “Load.” This maximum distance a wire can be deflected while maintaining full spring-back to its predefection position, is called “Deflection.” This is the distance along which orthodontic force is generated, delivered, and moves teeth with one adjustment. Relating the force magnitude over that distance makes the Load/Deflection ratio (L/D) ratio — the mathematical parameter that describes and measures the wire’s level of elasticity⁹ in gm, cm, or oz/inch. The lower value of this ratio indicates the wire’s higher elasticity that delivers lighter and more consistent force and moves teeth a longer distance with a single activation. A higher value of that ratio indicates that such wire has a lower level of elasticity, delivers stronger force magnitude, moves teeth a shorter distance, and requires frequent tying adjustments. Each wire’s metallurgic structure possesses its own level of elasticity (L/D rate) that distinguishes it from other kinds of wires.

Properties of superelastic wires

• Are brittle and unbendable (break upon acute sharp bend angle) and lose activation when bent in an obtuse angle.
• Have a 100% spring-back elasticity range that can move teeth the longest distance with just one tying adjustment.
• Have the lowest L/D rate value ever produced (delivers the lightest and most consistent force delivery).
• Maintain full activation range upon full wire-to-brackets engagement.^7,10
• Move teeth the longest distance with least number of adjustments in fastest treatment time.^12,13

Description of the Bendistal and Omni pliers (Figure 1)

Bendistal pliers’ jaws make a 45° angle to its long axis for easy intraoral manipulation and accessibility behind molar tubes. Each plier has two jaws — the occlusal jaw has a V-shaped trench, and the gingival jaw has a V-shaped gingival wedge that fits snugly in it to insert the lasting bends. A simple squeeze of those jaws will produce an optimal unbreakable activating bend. The two pairs of pliers insert V-bends in superelastic archwires in four quadrants of mouth, without annealing, breakage, or over twisting. One pair of pliers is designated to insert bends in the upper left and lower right mouth quadrants (UL-LR); and the other pair bends NiTi archwires in the upper right and lower left mouth quadrants (UR-LL). Engraved letters on both pairs of pliers help clinicians identify the correct pair that bends the archwire in the intended quadrants. Placing the pliers’ jaws around tied archwires determines the orientation of the inserted V-bend and its mechanical effect on the segments of teeth. Also, the location of the bend on the archwire determines the direction and planes of teeth segment movements as well. Vertically oriented intraoral V-bends can bend distal ends, tip back molars, and step-up/step-down, and intrude/extrude teeth. Transversely oriented V-bends activate archwires extraorally for step out/in, and arch expanding bends. The Omni Pliers is a new generation of Bendistal pliers that combines the exact jaw shapes and versatile functions of both pairs of Bendistal pliers into one pair of pliers.

Which bends can activate NiTi wires?

Because of the brittleness and superelasticity of NiTi wires, traditional pliers make bends that are shallow, quickly spring back, and lose their force-delivering ability. Inserting a permanent bend in those nonmalleable wires requires that the pliers’ bending jaws be V-shaped, not round. Bendistal and Omni pliers’ jaws design makes the bending of NiTi wires possible.

The crucial requirements for inserting the successful bends in superelastic archwires follow:

1. Inserting a permanent V-bend capable of delivering light and consistent force requires that the tip of the bend must be sharp.
2. The angle of the V-shaped bend must be not too acute to break the brittle wires or too obtuse to lose its force-delivering activation.
3. The bend must hold on after forcefully engaging bent wire in bracket slots.
4. V-bends must be inserted correctly at the first attempt. Attempting to repair a wrong bend will break the archwire, requiring replacement.

Inserting correct bends in straight NiTi wires (Figure 2)

Bendistal and Omni pliers’ jaws were optimally designed to insert successful unbreakable lasting V-bends, which do not dissipate upon bracket tying. Orienting these bends in the correct locations — the wire’s line and imaginary plane that moves teeth in the planned path — poses a challenge. This requires observing certain specific rules that instruct the correct orientation of the pliers’ jaw snout to lines and planes of archwires. These rules ensure inserting the correct bends that activate to apply their forces in the prescribed paths of teeth movements (Figure 2). Round and rectangular wires tend to slip during the bending process due to their elasticity and brittleness. This difficulty can be overcome by grabbing the wire in a correct orientation with a firm grip before squeezing plier jaws. If the wire slips after a loose grip, the bend will activate the wire in a wrong path of tooth movement.

Rules that ensure inserting correct V-bends in all planes follow:

1. The pliers’ jaws must grab the wire with a firm grip at a right angle (Figure 2C)
2. If the wire slips while bending due to lose grip, the bend will result in an obtuse or acute angle, activating the wires in a wrong direction and moving the teeth in unwanted oblique planes (Figures 2A and 2B).

Inserting correct vertical intruding bends in preformed NiTi archwires (Figures 3 and 4)

When inserting intrusion vertical bends in maxillary preformed NiTi archwire in the upper left quadrant, follow these rules:

1. Place the occlusal jaw of UL-LR Bendistal pliers under and in line with the imaginary archwire’s plane.
2. Pointing the tip of pliers’ jaws down or up to the archwire plane will deflect bent wire in a wrong oblique unwanted plane (Figures 3A and 3B).
3. Incorrect angle relation of the pliers’ snout to the archwire plane deflects wire in an incorrect intruding V-bend
   (Figures 3A and 3B).
4. Only observing the 90° angle rule to wire line, with jaws’ snout in line with archwire’s plane, produces correct intruding bend (Figure 4C).

Inserting transverse bends

To insert correct transverse V-bends that activate NiTi wires, the pliers’ snout must make a 90° angle to the archwire’s imaginary plane. At the same time, the snout should make a right angle with the archwire’s line sideways and anterio-posteriorly. Those two requirements ensure inserting bends in line with the archwire’s plane and wire’s line. Any deviation from that rule results in activating the bends in a different plane than the archwire’s. These bends are used to correct crossbites (expanding and constricting dental arches). Such bends include midline bends, canine constrictive or expanding bends, step-out, step-in bends, and mushroom bends (Figure 4A-4C).

Types of vertical intraoral V-bends

After clinicians master the preceding bending rules, extraorally, they can insert correct V-bends intraorally to activate tied NiTi archwires in the vertical direction. These bends include cinch-back bends without annealing; molar tip-back, anchorage, and space-regaining bends for second premolar eruption; and intrusion bends for anterior and posterior teeth segments (Figures 5A and 5B). When making intraoral V-bends, clinicians must observe all previously mentioned rules for correct lasting bends in a plane perpendicular to the archwire’s plane. Any oblique tilt of the pliers’ jaws before squeezing the pliers’ jaws will result in a wrong wire deflection that moves teeth in an unwanted oblique direction. For both mandibular and maxillary molar’s cinch-back, tip-back, and incisor segments intrusion, place the pliers’ occlusal jaw occlusal to the wires; and gingival jaw gingival to the wire. All intrusive bends produced must always have their sharp tip point occlusally. Reversing jaws, positions will result in forward molars tipping and incisor teeth extrusion, which may be used in closing open bites.

Discussion

This article introduces, for the first time, guidelines and instructions to help clinicians insert correct bends in those known-unbendable wires, and orient them in relation to archwire’s lines and imaginary planes to move teeth in the exact prescribed paths of movements.