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.

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).

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 />

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