The standard edgewise appliance

The standard edgewise appliance originated from the work of Edward Angle, who experimented with a series of systems before developing the edgewise slot, on which most fixed appliances are now based (Angle, 1928). Initially placing the slot vertically, Angle found that by laying it horizontally within the bracket, greater control of the teeth could be obtained (Fig. 9.1): the interaction of a rectangular wire in a rectangular slot providing precise three-dimensional control of tooth position. The standard edgewise appliance became the fixed appliance of choice up until the late 1970s (Fig. 9.2), but it did suffer from several disadvantages. In particular, the passive bracket slot meant that final detailing of tooth position in rectangular wires was dependent upon many bends being placed within the archwire for each individual tooth (Fig. 9.3). This was time-consuming and required considerable skill on the part of the orthodontist. The presence of these bends also meant that space closure had to be carried out with closing loops, which were also complicated to bend (see Fig. 9.2). In addition, teeth are moved bodily along the archwire through alveolar bone using an edgewise appliance, which is demanding upon anchorage.

Figure 9.1 Edgewise slot.

Figure 9.2 Fully banded standard edgewise appliances with space-closing loops in the upper and lower archwires.

Figure 9.3 Typical finishing archwire incorporating individual tooth bends for an edgewise appliance.

Light wire appliances

In an effort to overcome the high anchorage demand associated with the standard edgewise appliance, an Australian orthodontist, P. Raymond Begg developed a fixed appliance system where tooth movement was based around the concept of differential force (Fig. 9.4) (Begg, 1956):

Figure 9.4 Begg treatment involves the concept of differential force.

The tooth crowns are first tipped into the desired position and then the roots are uprighted. This differential tooth movement requires less anchorage than bodily moving teeth.

It is much easier to tip a tooth than move it bodily and this requires less force, so the Begg technique was much lighter on anchorage and became very popular during the 1960s and 1970s (Fig. 9.5). Begg treatment mechanics are compartmentalized into three stages, each with specific objectives that have to be achieved before progressing (Cadman, 1975a, b) (Box 9.1). However, because it only uses round archwires, precise finishing is difficult and the use of auxiliaries in the final stages of treatment to upright teeth that often have been tipped through quite significant distances proved to be quite complex, difficult to control and also time-consuming. In an effort to address this, the Tip-Edge appliance was developed by Peter Kesling in the late 1980s (Kesling, 1988; Kesling et al, 1991). This appliance also tips the teeth during the initial phase of treatment, but allows later uprighting with more rigid three-dimensional control by closing the slot down around full-size rectangular archwires (Fig. 9.5).

Fig. 9.5 A fully banded Begg appliance (left) and bonded Tip-Edge® appliance (right).

Both these appliances use auxiliary springs (arrowed) to upright the teeth during the final stage of treatment.

The preadjusted edgewise appliance

After studying a large sample of untreated ideal occlusions, Lawrence Andrews published his six keys of occlusion (see Box 1.2) (Andrews, 1972) and introduced an edgewise bracket system that has revolutionized fixed appliance orthodontics (Andrews, 1979). The preadjusted edgewise or ‘straight-wire’ appliance that Andrews described is the most popular fixed appliance system in use today (Fig. 9.6). Unlike standard edgewise brackets, which are identical for each tooth and require bends within the archwire to generate individuality of tooth position, each tooth in the preadjusted edgewise system has a customized bracket. This built-in prescription was based around Andrews’ measurements from the untreated sample of ideal occlusions he studied and included a number of features (Fig. 9.7):

Figure 9.6 Preadjusted edgewise appliance.

Figure 9.7 Pre-adjusted Siamese edgewise brackets showing twin design and contoured base.

The bracket prescription will position the tooth in three dimensions, generating mesiodistal tip, torque and in/out positioning.

In this preadjusted system, the work in accurately positioning the teeth is done by the bracket prescription, significantly reducing the amount of wire bending required. A further advantage is that it also allows groups of teeth to be moved and spaces closed by sliding them in unison along a rigid archwire; because once tooth alignment has been achieved, the archwire sits passively in each bracket slot.

The original Andrews bracket prescription is still available, although there have been adaptations made as the appliance system has been developed clinically (Box 9.2). In particular, it was found that some of the torque prescriptions in the original Andrews appliance were not being fully expressed, most notably in the upper incisors due to the ‘slop’ or free space that inevitably exists between the wire and bracket slot. Therefore, many later prescriptions have increased torque values in the upper labial segment to compensate for this. Biological and anatomical variation, as well as mechanical deficiencies associated with the appliance, mean that one overall prescription does not fit all cases. A variety of modifications in bracket prescription and occasionally some wire bending are often required during the normal clinical use of a preadjusted appliance. These may be needed to overcome errors in bracket positioning, significant variations in tooth structure or position, and the presence of marked skeletal discrepancies (Creekmore & Kunik, 1993; Thickett et al, 2007).

Lingual appliances

One of the biggest problems associated with labial fixed appliance systems are the poor aesthetics. To overcome this, lingual appliance systems were introduced in the USA in the 1970s (Alexander et al, 1982). However, these proved to be clinically very difficult to use and with the introduction of aesthetic labial brackets, lingual orthodontics virtually disappeared from clinical practice in the USA. However, in Europe and Asia, several systems have been developed more recently and the technique is growing in popularity (Fig. 9.8) (Wiechmann, 2002). The principle advantages of lingual appliances are the improved aesthetics, lack of labial decalcification in cases with poor diet or plaque control and better bite opening due to a bite plane effect of the brackets. Disadvantages for the patient include problems with speech, soreness of the tongue and increased cost; whilst for the orthodontist, there is the inherent difficulty and time required for chairside adjustment.

Figure 9.8 Incognito® lingual fixed appliance.

Brackets

Orthodontic brackets are fixed to the tooth crown and mediate forces applied by the archwire and auxiliaries on the tooth. Brackets are either routinely cast or injection-molded from stainless steel; although to reduce the chance of allergic reaction, nickel-free brackets made from titanium or cobalt chromium are now available. Bonding techniques rely on a physical interaction being established between the bracket base and an etched enamel tooth surface. Bracket bases are therefore roughened or sandblasted to improve this bond (Fig. 9.9) and often curved in both the horizontal and vertical planes, which aids in bracket location and seating on the tooth crown (see Fig. 9.7).

Figure 9.9 Mesh design bracket base to improve bond strength.

Aesthetic edgewise brackets

A significant disadvantage of metallic orthodontic brackets is their poor aesthetics; edgewise bracket systems that are transparent, or more closely resemble natural tooth colour, have therefore been developed (Fig. 9.10). The early aesthetic brackets were made of acrylic and polycarbonate, which discolored quite rapidly and were prone to both permanent deformation and failure. To overcome these problems, plastic brackets were made in polyurethane or polycarbonate reinforced with ceramic or fiberglass fillers.

Figure 9.10 Ceramic preadjusted edgewise bracket.

Note the excessive composite ‘flash’ around a number of these brackets. This should be removed to prevent plaque accumulation in these areas.

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Ceramic brackets were introduced in the 1980s and provide higher strength, more resistance to wear and deformation, better colour stability and superior aesthetics. They are manufactured from aluminium oxide and are described as either mono- or polycrystalline, depending upon whether they are made from one or many crystals. Although the aesthetics are significantly improved, they are not without disadvantages compared to metal brackets (Russell, 2005). Ceramic brackets have low fracture toughness, which can lead to higher bracket breakage. There is also greater friction between the archwire and bracket slot, which can be reduced by the incorporation of a metal slot. Excessively high bond strengths, particularly in the earlier brackets, also increased the risk of enamel damage on bracket removal. One final problem is the fact that ceramics are harder than enamel. This can result in significant enamel wear if brackets are placed in a position of occlusal contact with the natural dentition, commonly the lower incisors in cases with an increased overbite. Despite these problems, adult patients often request ceramic brackets because of the improved aesthetics.

Light wire appliance brackets

The original light wire appliance was the Begg appliance, which utilized a simple bracket that was identical for each tooth. Begg brackets incorporate a narrow open-ended slot, into which a stiff round archwire is placed from the gingival aspect and held in position by the insertion of a small metallic auxiliary lock pin (Fig. 9.11). The loosely fitting round wire allows considerable scope for the teeth to tip under the influence of light intermaxillary elastic traction, which in combination with anchor bends placed in rigid archwires, allows rapid reduction of both overbite and overjet during initial treatment. Following this, a variety of auxiliary springs are required to upright and torque the teeth into the correct position. Unfortunately, this stage of treatment is quite complicated and the nature of the bracket slot means that precise control of tooth position is difficult. For these reasons, the Begg appliance has diminished in popularity during recent years.

Figure 9.11 Begg light wire bracket.

The Tip-Edge bracket has been modified from an edgewise design to facilitate the advantages of free tooth-tipping on round wires during early treatment, followed by accurate tooth positioning on rectangular wires during the later stages. This has been achieved by designing a narrow preadjusted edgewise bracket with wedges removed from each side of the archwire slot, which allow the bracket to tip up to 25° either mesially or distally. Lateral extensions or wings on the bracket provide good rotational control of tooth position and as the bracket tips, the dimensions of the slot increase. This allows the subsequent placement of rectangular wires; as the teeth are then uprighted with auxiliary side-winder springs, the slot dimension closes down and the prescribed tip and torque within the bracket is expressed (Fig. 9.12) (Parkhouse, 1998). More recently, the Tip-Edge PLUS bracket has been introduced, which eliminates the need for auxiliary springs by incorporating a tunnel deep to the main bracket slot (Fig. 9.13). A superelastic auxiliary archwire placed into this tunnel during the final stages of treatment provides the uprighting forces necessary for the bracket prescription to be expressed (Parkhouse, 2007).

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Figure 9.12 Tip-Edge® bracket.

As the tooth tips, the bracket slot dimension increases from 0.022 inches to 0.028. Uprighting on rectangular wires under the force of an auxiliary spring during the final stages of treatment closes down the slot and allows the bracket prescription to be expressed in three dimensions.

Figure 9.13 Tip-Edge PLUS® bracket with auxiliary tunnel (arrowed) deep to the main bracket slot.

A flexible nickel titanium archwire passes through the auxiliary tunnels and collectively does the work of individual side-winders. This allows simultaneous torque and tip delivery to be maintained as the teeth upright. During the final stage (three) of treatment, a passive rectangular archwire maintains arch shape and ensures correct torque delivery as a flexible superelastic archwire running through the auxiliary tunnels acts to provide the necessary uprighting tip. This tip brings the finishing surfaces of the main archwire slots into full contact with the rectangular main archwire and therefore, both tip and torque are corrected.

Reproduced with permission from Parkhouse (2008) Tip-Edge Orthodontics (St Louis: Mosby Elsevier); and with thanks to Richard Parkhouse. ®TP orthodontics, Inc. – La Porte, Ind. USA.

Self-ligating brackets

Friction between the bracket and the archwire can theoretically result in a loss of anchorage and slower tooth movement. In addition, individual bracket ligation with conventional edgewise appliances is time-consuming for both patient and orthodontist. In an attempt to reduce friction and appointment times, a range of brackets whose slot is closed by the use of a metal gate or clip are now available (Fig. 9.14). This technique is referred to as self-ligation, as the ligation system is built into the bracket. The concept is not new, having been originally pioneered in the 1930s. However, these bracket systems have undergone a renaissance over the past two decades, largely because of enhanced ingenuity and reliability. Self-ligating brackets provide a number of theoretical advantages over conventional appliances (Harradine, 2003):

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• Reduced friction;

• More robust ligation;

• More efficient tooth movement and sliding mechanics;

• Enhanced rotational control; and

• Reduced chairside adjustment time.

Figure 9.14 Damon MX® self-ligating bracket with the gate closed (left) and open (right).

Courtesy of David Birnie and Andy Price. ®Ormco corporation.

Archwires

The original archwires used in orthodontics were made of gold but metallurgical advances have meant that a wide range of metal alloys are now available. These different alloys all offer a variety of physical properties, which can be defined (Box 9.3). The ideal properties required of an orthodontic archwire will depend upon the stage of treatment and the type of tooth movements being carried out and no archwire material will offer all of these together (Kapila & Sachdeva, 1989). For this reason, a number of different archwires are required during a course of orthodontic treatment and these will vary in both the type of metal used and the dimensions. The particular sequence that an orthodontist chooses is often down to personal preference.

Tooth alignment during the initial stages of treatment requires the following properties:

For this reason, round nickel titanium, stainless steel multistrand or coaxial wires are generally used and these will progress in diameter from 0.012 to 0.018 inches, depending upon the degree of irregularity associated with the dentition. These are often followed by a short period with a rectangular nickel titanium archwire to begin torque expression.

Once initial alignment has been achieved, wires of increasing stiffness are used to complete the process of levelling the dentition, begin overbite reduction and allow sliding of teeth along the archwire. These are usually round stainless steel wires, which provide the necessary stiffness over a range of diameters from 0.016 to 0.020 inches.

During the later stages of treatment, the process of overbite reduction is completed and if necessary, space closure is carried out by moving blocks or individual teeth, either along the archwire with sliding mechanics under force provided by auxiliaries or with the archwire under force provided by closing loops. In both circumstances, the archwire will require the following properties:

For sliding mechanics, the frictional properties of the archwire will also have a significant influence on the efficiency of tooth movement, whilst for space closure with looped archwires, good formability will be important. Archwires used during the later stages of overbite reduction and for space closure tend to be made of rectangular stainless steel.

Archwire materials

The metal alloys currently used for the fabrication of orthodontic archwires include stainless steel, nickel titanium, cobalt chromium and beta titanium.

Stainless steel

Stainless steel is an alloy consisting of iron, chromium and nickel, which has a large modulus of elasticity. As such, stainless steel archwires are generally stiff and resist deformation, which makes them ideal as working archwires, providing support as teeth are moved along the wire. However, this stiffness makes them poor for initial alignment; it is difficult to tie a stainless steel archwire into a crowded dentition and if this is achieved, it is often accompanied by permanent deformation. Stainless steel wires used in orthodontics consist of iron (70%), chromium (17 to 20%), nickel (8 to 12%) and a maximum of 0.08% carbon. These are known as ‘18-8’ stainless steels, reflecting the amount of chromium and nickel in the alloy. Chromium makes the wire resistant to corrosion in the oral environment, whilst nickel increases ductility. Stainless steel can be softened by annealing, hardened by cold-working during manufacture and springback can be improved by increasing the length of wire between brackets, achieved by incorporating loops (Fig. 9.15). Alternatively, the flexibility of stainless steel archwires can be increased by winding several smaller wires together to form large multistrand or twistflex wires; or wrapping smaller wires around a larger central wire to produce a coaxial wire. This is because the force delivered by a wire is related to the radius of the individual wires that make up an archwire.

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Fig. 9.15 Looped steel archwire to align the lower incisors.

This is time-consuming and technically demanding, can result in difficulty with oral hygiene and can produce soft tissue trauma. This technique has been superseded by the advent of superelastic archwires.

Cobalt chromium

Commercially marketed as Elgiloy, cobalt chromium has a greater formability than stainless steel and similar stiffness, but with greater friction. It can be hardened by heat-treating in the laboratory and is used for the construction of auxiliaries, such as an intrusion arch or quadhelix.

Beta titanium

Introduced in the 1980s, beta titanium wires have good formability, with a stiffness of around one-third that of stainless steel; but they are also associated with higher friction. These archwires are used in the final stages of treatment, when finishing bends may be required to detail individual tooth position and achieve settling of the occlusion.

Nickel titanium

Nickel titanium or ‘NiTi’ archwires are characterized by high flexibility and the delivery of low force over long range. They also exhibit the phenomenon of shape memory; when deformed they will tend to return to their original shape; and superelasticity, the wire delivering the same force over a large range of deformation (Fig. 9.16). This behaviour occurs because nickel titanium undergoes a phase transition of crystalline structure on loading and unloading. These two phases are represented by the low stiffness martensitic crystal structure and the higher stiffness austenitic, with the superelasticity of NiTi archwires correlated to the coexistence of these two phases. It is also important to note that the deactivation plateau is lower than the activation, a phenomenon called hysteresis, which means that the force delivered by the wire to the tooth is lower than that needed to activate it.

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Figure 9.16 Plot of stress versus strain for a nickel titanium wire compared to stainless steel.

The plateau on the nickel titanium graph represents a phase shift in the crystalline structure of the metal on loading and unloading. The result is that the wire will deliver the same force over a large range of deformation, a property known as superelasticity. In addition, the deactivation plateau is lower than that for activation; less force is delivered on unloading than is required to activate the wire, a property known as hysteresis.

Nickel titanium was originally developed by the Naval Ordinance Laboratory in the USA in the early 1960s as nitinol and was introduced into orthodontics a decade later. The early wires had their crystalline structure stabilized or fixed in the martensitic form by cold working and exhibited greater flexibility but no shape memory. Later nickel titanium wires exhibited a phase shift in crystalline structure, induced by either mechanical stress such as ligation into a displaced tooth (pseudoelastic) or temperature change on placement into the oral cavity (thermoelastic). In combination, these properties give the wire its shape memory. The transitional temperature can be altered by changing the composition of the alloy, in particular by partially replacing the nickel with copper. In modern wires it is set at around body temperature. The wire outside the mouth is in the martensitic form and very flexible, allowing easy engagement into displaced teeth. As the wire reaches body temperature on placement in the mouth, the reverse occurs and it transforms into the austenitic form, returning to its original shape and imparting a gentle aligning force on the teeth. This force remains light because the pseudoelastic property of the wire ensures that when it is engaged in the bracket slot of a displaced tooth, the deflection of the wire (if it is sufficient enough) will generate a local martensitic transformation. As it unloads in these areas, the force will be lowered as the wire becomes superelastic, as exhibited by the plateau of the force–deflection graph (Fig. 9.16). Once the tooth is aligned, the reduction in wire deflection will restore the stiffer austenitic phase.

These wires are ideal for aligning teeth during the initial stages of treatment. However, the properties of low stiffness and poor formability make nickel titanium an unsuitable material for later stages of treatment where a stiffer archwire is required for space closure and overbite control.

Auxiliaries

A variety of auxiliary components are also required for the successful use of fixed appliances. These include both elastomeric and metallic products (Fig. 9.17).

Figure 9.17 Elastomeric and metallic auxiliaries used with fixed appliances.

Elastomerics include (A) ormolasts or modules; (B) elastics; (C) powerchain. Metallic auxiliaries include (D) long ligatures; (E) open and closed coil; (F) short ligatures; (G) Kobyashi ligatures.

Palatal and lingual arches

Palatal and lingual arches consist of a rigid stainless steel wire soldered onto molar bands. The orthodontist fits the relevant molar bands in the mouth but does not cement them in place. An impression is taken with the bands in situ and they are transferred with the impression, to be cast up on the working model by the technician who then constructs the relevant arch. The appliance is then cemented into place within the mouth.

• A transpalatal arch is constructed from a 0.9-mm stainless steel wire that traverses the hard palate and is attached to bands cemented onto the first or occasionally second molar teeth (Fig. 9.18). The arch fixes the maxillary intermolar distance, theoretically preventing these teeth from moving mesially into a smaller archform, and reinforcing their anchorage value. In reality, these arches are not particularly effective for anteroposterior anchorage, but can provide useful vertical anchorage; when mechanically erupting an impacted tooth or using an intrusion arch. A palatal arch will also prevent maxillary molars from flaring buccally when using high-pull headgear and can also be activated to de-rotate molars.

• Nance palatal arches extend towards the palatal vault and incorporate an acrylic button that lies on the palatal mucosa (Fig. 9.18). This also acts to fix the upper arch length and can be used for anchorage support. However, these auxiliaries can cause soft tissue irritation if the acrylic button embeds into the palatal mucosa, especially if left in situ during overjet reduction.

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• Lingual arches are used in the mandibular arch and are also constructed from 0.9-mm stainless steel wire (Fig. 9.19). The wire extends behind the cingulae of the lower incisors and extends to bands cemented onto the lower first molars. Lingual arches are generally used as space maintainers; either to prevent forward movement of the permanent molars following early loss of deciduous molars or to maintain the leeway space during the transition from mixed to permanent dentition.

Figure 9.18 Palatal arch (left) and palatal arch incorporating a Nance button (right).

Figure 9.19 Lingual arch.

Fixed expansion arches

Fixed expansion arches are also fabricated in the laboratory and require a cast model of the maxillary arch incorporating orthodontic bands for their construction in a manner similar to that of palatal and lingual arches.

• A quadhelix is made from either 0.9 to 1.0-mm stainless steel or 0.95-mm cobalt chromium wire and extends across the palate from bands cemented onto the first molars, but also incorporates helices to increase the range of action (Fig. 9.20). The quadhelix is activated by approximately one molar tooth width and produces 300–400 g of force, which gives mostly dental expansion (up to 4 mm), although in a preadolescent child it can produce some skeletal change.

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• A HYRAX (hygenic rapid expansion) or SUPERscrew telescopic expander, either attached to the first molar and first premolar teeth via cemented bands or cemented directly to the buccal dentition by acrylic capping (Fig. 9.21), can provide significant expansion of the maxillary dental arch. Each turn of the screw is approximately equal to 0.25-mm and for rapid maxillary expansion (RME) it is turned between 2 and 4 times per day until the maxillary arch is overexpanded. This generally equates to the maxillary palatal cusps occluding on the lingual inclines of the mandibular molar buccal cusps. Expansion is mostly skeletal before the age of 16 when there is patency of the midpalatal suture. Following expansion, the appliance is left in place for approximately three months as a retainer. Up to 10 kg of force can be achieved with these appliances and initially, up to 80% of the expansion achieved will be skeletal, more so anteriorly. However, over the longer term, a lot of this will relapse, with ultimately only around 50% being skeletal. RME can be effective in producing up to 10-mm of maxillary expansion.

Figure 9.20 Quadhelix.

Figure 9.21 Banded RME with HYRAX screw (left) and bonded RME with SUPERscrew (right).

Molar banding

Molars are generally banded because their restricted access can make moisture control and accurate positioning for bonding more difficult. In addition, the increased forces of mastication associated with the molar dentition can lead to increased bond failures in these regions. A number of specific indications exist for banding instead of bonding molar teeth:

To create space for the placement of bands, the contact points need to be opened by placing separators between the teeth for a few days (Fig. 9.22). The most common separators are small elastomeric rings; however, if a contact point is particularly tight, or a restoration is in the way, metal separators can also be used. The separators are removed immediately prior to band placement and the opened contact point facilitates easy band placement around the tooth. Orthodontic bands are seamless and come preformed in various sizes. The appropriate size is selected and the band fitted by finger pressure and then occlusal force as the patient is asked to bite down on a bite-stick, which seats the band in the correct position. Once the correct band has been selected and tried in, it is removed and then cemented in place. Glass ionomer cement is recommended, as it chemically bonds to enamel and releases fluoride, which helps prevent demineralization.

Figure 9.22 Separating elastics placed to create space to allow the placement of bands.

Bracket placement

Both standard edgewise and Begg brackets are simply placed at a specific distance from the occlusal edge of the tooth crown. However, this does not provide a consistent reference point, as the bracket position is determined primarily by the size of the crown. With contemporary preadjusted appliances, the aim is to place the bracket at the centre of the anatomical crown and parallel to the long axis of the tooth. This means that the bracket prescription will be expressed correctly in relation to mesiodistal tip, torque and in/out position, irrespective of the size and form of the tooth. Bracket positioning can be achieved in two ways:

Orthodontic brackets are generally bonded to enamel using mechanical locking created by acid-etching the enamel surface of the teeth. The steps involved in bonding are as follows (Fig. 9.23):

Figure 9.23 Direct bonding.

Moisture control is obtained, acid etch placed, the tooth surface washed and dried, and brackets positioned. Excess composite flash is removed and the composite either cures chemically or via the application of an external light source.

It is very important to clean up excess composite or ‘flash’ (see Fig. 9.10) as this can create problems in maintaining high levels of oral hygiene and result in demineralization around the bracket, a major risk of fixed appliance therapy.

Wire placement

In conventional fixed appliances the orthodontic archwires are held in place with ligatures that pass over the archwire and under the bracket tie-wings. These ligatures can be elastomeric or steel, the latter providing securer ligation and reduced friction, but taking longer to place. Self-ligating bracket systems use a variety of methods to mechanically lock the archwire into the bracket slot.

Removal of appliances

Bands are removed using special pliers, which exert an occlusally directed force from the gingival margin (Fig. 9.24). Because of the anatomy of molar teeth, the force is applied from the palatal aspect in the maxillary arch and buccally in the mandible.

Figure 9.24 Band removal.

In contrast to bands, the bond strength for bondable attachments must be higher to prevent repeated failures. The aim upon removal of a bonded fixed appliance is for the failure to occur between the bracket base and composite junction, and not at the junction between enamel and composite, to avoid the risk of enamel fracture. To achieve this, a shear force is applied across the bracket base with debonding pliers (Fig. 9.25). This distorts the bracket base and causes failure at the junction between bracket base and composite. Residual composite is removed from the tooth, ideally using a tungsten carbide fissure burr in a slow handpiece.

Figure 9.25 Bracket debonding and removal of composite with a tungsten carbide fissure burr.

Figure 9.26 Tooth alignment and levelling in the maxillary arch (note the use of a quadhelix to expand the arch).

Ceramic brackets initially had very high bond strengths, which resulted in a significant risk of enamel fracture on debonding. To overcome this, most contemporary systems have a failure point built into the bracket base, which means that when a shear force is applied, the bracket fractures down the midline, collapsing the tie-wings of the bracket that can then be easily removed.

Levelling and aligning

The first phase in all fixed appliance treatment is to align and level the dentition. Levelling means the correction of marginal ridge discrepancies as opposed to definitive overbite control. To achieve this, small-diameter flexible nickel titanium or multistranded steel arch wires are used (Figs 9.26 and 9.27).

Figure 9.27 Tooth alignment and levelling in the mandibular arch.

The aligning archwires are initially ligated into all the brackets, either fully or partially, unless a tooth is severely crowded and short of space. In this case, space will need to be created before the tooth can be aligned. This can be achieved by aligning the other teeth first and then placing a more rigid wire, such as stainless steel. A length of compressed coil spring placed on this archwire is then used to create space for the crowded tooth (Fig. 9.28). Once space has been created, the tooth can be aligned by placing a lighter, more flexible archwire as described earlier.

Figure 9.28 Space creation with push coil on round stainless steel archwire.

The alignment stage of treatment is completed when the working archwire is fully engaged in all the brackets. The working archwire when using a 0.022 × 0.028 inch bracket slot is usually a 0.019 × 0.025 inch stainless steel archwire. This provides sufficient rigidity for overbite reduction and control, and for space closure using sliding mechanics.

Overbite control

A number of techniques for facilitating overbite reduction are available and these vary between appliance systems. The Begg and Tip-Edge techniques are characterized by an initial stage of treatment concerned with tooth alignment that takes place simultaneously with both overbite and overjet reduction. This is achieved by using a combination of rigid steel archwires that bypass the premolar teeth and light intermaxillary elastic traction to tip the labial teeth. The steel wires have tip-back anchorage bends to create an intrusive force on the labial dentition, whilst the intermaxillary elastics have an extrusive effect on the molars. In combination, these mechanics are very effective in helping to reduce a deep overbite (Fig. 9.29).

Figure 9.29 Overbite and overjet reduction with a Tip-Edge® appliance.

Edgewise and preadjusted edgewise appliances begin the process of overbite reduction following initial tooth alignment and use continuous rigid archwires, often with a reverse curve of Spee in the lower archwire (Fig. 9.30 and see Fig. 11.15). The working archwire is usually rectangular and composed of stainless steel. Care needs to be taken to prevent lower incisor proclination with these archwires and lingual crown torque is usually placed in the labial segment of the wire to prevent this. Much like the Begg and Tip-Edge appliances, the use of class II intermaxillary elastics can also aid in bite opening, as the vertical component of force will produce molar extrusion.

Figure 9.30 Reverse curve of Spee in the lower archwire to reduce the overbite.

Edgewise appliances can also use utility arches and segmental mechanics to aid overbite reduction. Utility arches are auxiliary archwires that bypass the buccal segments and provide an intrusive force direct to the labial dentition. These arches are stabilized for anchorage posteriorly and act directly to intrude the incisors, being tied directly into the incisor bracket slots (Ricketts utility arch) or above an archwire that has been sectioned distally to the lateral incisors (Burstone intrusion arch). Utility arches are mechanically efficient, but do create a step in the archwire; they are also complex to fabricate and require triple tubes attached to the molar bands. However, they can be a useful adjunct in cases with deep bites (Fig. 9.31).

Figure 9.31 Burstone-type intrusion arch (left) and Ricketts utility arch (right) to reduce overbite.

Space closure and overjet correction

One of the great advantages of contemporary preadjusted edgewise systems is the use of sliding mechanics for space closure, which is impossible when using standard edgewise brackets because of the numerous bends that must be placed in the archwire to achieve tooth positioning. Therefore, for standard edgewise appliances, loops are bent into the wire (see Fig. 9.2), which pull the opposing segments together and close space. Whilst loop mechanics benefit significantly from reduced friction, the disadvantages include:

In preadjusted edgewise appliances, the first-, second- and third-order bends are incorporated within the bracket prescription; therefore following tooth alignment the archwire is both flat and straight. This facilitates space closure by simply sliding the teeth along the archwire (Fig. 9.32). The advantages of sliding mechanics are:

Figure 9.32 Space closure using sliding mechanics.

Force can be applied using simple elastics, elastomeric chain or coiled nickel titanium springs. However, a significant disadvantage of sliding mechanics is friction between the bracket and archwire as the tooth slides. Any applied force must overcome this before tooth movement can take place.

Anchorage support is often required during overjet correction. This can be provided by the use of class II or class III intermaxillary elastics (Fig. 9.33). These will have an extrusive effect on the molar dentition, which can result in a clockwise rotation of the occlusal plane and therefore should be avoided in cases with an increased maxillary–mandibular planes angle.

Figure 9.33 Class II and III intermaxillary traction provided by elastics.

Finishing

With earlier edgewise systems, considerable time had to be spent at the end of treatment placing artistic bends into the archwires to compensate for the standard bracket placed on each tooth. This was time-consuming and imprecise. With the introduction of preadjusted systems, the bracket prescription does a lot of the work in achieving the correct in/out position, angulation and torque for each tooth relative to its neighbours within the dental arch. Finishing has become less reliant upon wire bending and more of a reflection of correct bracket positioning. However, even with precise bracket positioning, a period of finishing to achieve an optimal aesthetic and occlusal result is often required. This usually involves removing the heavy rectangular steel working archwires, and replacing them with lighter wires that allow some occlusal settling. A good wire for this is beta titanium, which is not as stiff as stainless steel and has excellent formability. This is often supplemented with the use of intermaxillary settling elastics, which help to create maximum interdigitation by gently extruding the teeth (Fig. 9.34).

Figure 9.34 Settling elastics.