Fixed appliances

16 May

17.1. PRINCIPLES OF FIXED APPLIANCES
Fixed appliances are attached to the teeth and are thus are capable of a greater range of tooth movements than is possible with a removable appliance. Not only does the attachment on the tooth surface (called a bracket) allow the tooth to be moved vertically or tilted, but also a force couple can be generated by the interaction between the bracket and an archwire running through the bracket (Fig. 17.1). Thus rotational and apical movements are also possible. The interplay between the archwire and the bracket slot determines the type and direction of movement achieved. A bewildering variety of different types of bracket are now manufactured, and the choice of archwire materials and configurations is extensive. Therefore, for clarity, we shall consider the edgewise type of bracket (Fig. 17.2) in this section; other bracket systems are described briefly in Section 17.6.
The edgewise bracket is rectangular in shape and is typically described by the width of the bracket slot, usually 0.018 or 0.022 inch. The depth of the slot is commonly between 0.025 and 0.032 inch. Modifying the shape of the bracket can affect tooth movement. For example, a narrow bracket (Fig. 17.3) results in a greater span of archwire between the brackets which increases the flexibility of the archwire. In contrast, a wider bracket reduces the interbracket archwire span, but is more efficient for de-rotation and mesiodistal control. Nowadays a wide variety of bracket designs are available. In most modern appliance systems each bracket is a different width corresponding to the type of tooth for which it is intended; for example, lower incisors have the narrowest brackets (see photographs of fixed appliances shown later in the chapter).
A round wire in a rectangular edgewise type of slot will give a degree of control of mesiodistal tilt, vertical height, and rotational position. The closer the fit of the archwire in the bracket, the greater is the control gained. However, with a round wire only tipping movements in a buccolingual direction are possible (Fig. 17.4). When a rectangular wire is used in a rectangular slot, a force couple can be generated by the interaction between the walls of the slot and the sides of the archwire and buccolingual apical movement produced (Fig. 17.5). However, some tipping movements will take place before the rectangular wires engage the sides of the bracket slot, with the degree of ‘slop’ depending on the differences between the dimensions of the archwire and the bracket slot (Fig. 17.6).
Thus fixed appliances can be used in conjunction with rectangular archwires to achieve tooth movement in all three spatial planes. In orthodontics these are described by the types of bend that are required in an archwire to produce each type of movement (Fig. 17.7):
  • First-order bends are made in the plane of the archwire to compensate for differing tooth widths.
  • Second-order bends are made in the vertical plane to achieve correct mesiodistal angulation or tilt of the tooth.

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  • Third-order bends are applicable to rectangular archwires only. They are made by twisting the plane of the wire so that when it is inserted into the rectangular bracket slot a buccolingual force is exerted on the tooth apex. This type of movement is also known as torque.
In the original edgewise appliance (see below) these bends were placed in the archwire during treatment so that the teeth were moved into their correct positions. Modern bracket systems have average values for tip (Fig. 17.8) and torque built into the bracket slot itself, and the bracket bases are of differing thicknesses to produce an average buccolingual crown position (known ingeniously as in–out). These ‘pre-adjusted’ systems have the advantage that the amount of wire bending required is reduced. However, they do not eliminate the need for archwire adjustments because average values do not always suffice. The disadvantage to these pre-adjusted systems is that a larger inventory of brackets is required as each individual tooth has different requirements in terms of tip, in–out, and torque. Pre-adjusted systems are discussed in more detail in Section 17.6.
Fixed appliances
Whilst it is possible to achieve a more sophisticated range of tooth movement with fixed appliances than with removable appliances, the opportunity for problems to arise is increased. Fixed appliances are also more demanding of anchorage, and therefore adequate training should be sought before embarking on treatment with fixed appliances.
17.2. INDICATIONS FOR THE USE OF FIXED APPLIANCES
  • Correction of mild to moderate skeletal discrepancies As fixed appliances can be used to achieve bodily movement it is possible, within limits, to compensate for skeletal discrepancies and treat a greater range of malocclusions.
  • Intrusion/extrusion of teeth Vertical movement of individual teeth, or tooth segments, requires some form of attachment on the tooth surface onto which the force can act.
  • Correction of rotations

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  • Overbite reduction by intrusion of incisors
  • Multiple tooth movements required in one arch
  • Active closure of extraction spaces, or spaces due to hypodontia Fixed appliances can be used to achieve bodily space closure and ensure a good contact point between the teeth.
Fixed appliances are not indicated as an alternative to poor cooperation with removable appliances. Indeed, if a successful result is to be achieved with the minimum of deleterious side-effects, treatment with fixed appliances should only be embarked upon in patients who are willing to:
  • maintain a high level of oral hygiene;
  • avoid hard or sticky foods and the consumption of sugar-containing foodstuffs between meals;
  • cooperate fully with wearing headgear or elastic traction, if required;
  • attend regularly to have the appliance adjusted.
17.3. COMPONENTS OF FIXED APPLIANCES
Tooth movement with fixed appliances is achieved by the interaction between the attachment or bracket on the tooth surface and the archwire which is tied into the bracket. Brackets can be carried on a band which is cemented to the tooth or attached directly to the tooth surface by means of an adhesive (known colloquially as bonds).
17.3.1. Bands
These are rings encircling the tooth to which buccal, and as required, lingual, attachments are soldered or welded (Fig. 17.9). Prior to the introduction of the acid-etch technique, bands were the only means of attaching a bracket to a tooth. With the development of modern bonding techniques, directly bonded attachments became popular. However, many operators still use bands for molar teeth because a band will remain in situ if cement failure occurs, whereas a debonded molar attachment (through which the end of the archwire passes) may traumatize a patient’s cheek. In addition, a molar band is more secure where headgear is to be used.
Bands can be used on teeth other than molars, most commonly following the failure of a bonded attachment or where de-rotation or correction of a crossbite dictate the need for both lingual and buccal attachments. However, this must be balanced against the poorer aesthetics of a band (Fig. 17.10).
Prior to placement of a band it may be necessary to separate the adjacent tooth contacts. The most widely used method involves placing a small elastic doughnut around the contact point (Fig. 17.11), which is left in situ for 2 to 7 days and removed prior to band placement. These separating elastics are inserted by being stretched, with either special pliers or floss (Fig. 17.12), and working one side through the contact point.
Band selection is aided by trying to guess the approximate size of the tooth from the patient’s study models. A snug fit is essential to help prevent the band from becoming loose during treatment. The edges of the band should be flush with the marginal ridges with the bracket in the midpoint of the clinical crown at 90° to the long axis of the tooth (or crown, depending upon the type of bracket). Most orthodontists use glass ionomer cement for band cementation.

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17.3.2. Bonds
Bonded attachments were introduced with the advent of the acid-etch technique and the modern composite (see Section 17.3.3). Adhesion to the base of metal brackets is gained by mechanical interlock (Fig. 17.13). More recently, ceramic brackets have been introduced (Fig. 17.14), but despite the obvious aesthetic advantages their use has been limited by a number of disadvantages which are currently the subject of considerable research. Ceramic brackets were originally marketed with a silane coupler designed to provide chemical adhesion between the bracket and the bonding composite. This was unfortunately so successful that enamel fracture occasionally occurred during debonding, because the bond between the bracket and the adhesive was so strong. Manufacturers have tried to overcome this problem in a variety of ways, for example using mechanical rather than chemical retention, with varying success. Ceramic brackets are brittle and are prone to fracture in clinical usage. Fracturing away of the wings of the bracket makes tying in the archwire difficult, and in addition the brackets tend to break up during removal of the appliance. The hardness of ceramic brackets can lead to wear of opposing teeth; therefore using ceramic brackets for lower incisors is inadvisable. The hard ceramic can also notch the archwire, which makes sliding the teeth along the wire difficult.
Edgewise brackets are subdivided according to the width of the bracket slot in inches. Two systems are widely used, 0.018 and 0.022. The depth of the slot varies between 0.025 and 0.032.
17.3.3. Orthodontic adhesives
The most popular cement for cementing bands is glass ionomer (Fig. 17.15), mainly because of its fluoride-releasing potential and affinity to stainless steel and enamel. Glass ionomers can also be used for retaining bonded attachments, but unfortunately the bracket failure rate with this material is greater than that with composite. Much current research work is directed towards hybrid compomer materials which it is hoped will combine the advantages of composites and glass ionomer adhesives.
Use of the acid-etch technique with a composite produces clinically acceptable bonded attachment failure rates of the order of 5–10 per cent for both self- and light-cured materials. Although conventional self-cured composites can be used for bonding, a modification has been manufactured specifically for orthodontics to circumvent the problem of air bubbles, which would obviously compromise bond retention. No-mix orthodontic composites (Fig. 17.16) comprise an activator, which is painted onto both the bracket base and the tooth surface (after etching). Following this, a small amount of the composite itself is applied to the bracket, which is then placed on the tooth surface under pressure. Squeezing the sandwich of composite and catalyst into a thin layer mixes the two components, and the material usually sets within a few minutes.
Whatever material is used, any excess should be cleared from the perimeter of the bracket before the final set to reduce plaque retention around the bonded attachment.
17.3.4. Auxiliaries
Very small elastic bands, often described as elastomeric modules (Fig. 17.17), or wire ligatures (Fig. 17.18) are used to secure the archwire into the archwire slot (Fig. 17.19). Elastic modules are quicker to place and are usually more comfortable for the patient, but wire ligatures are often preferred, particularly in the

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later stages of treatment, as they can be tightened to maximize contact between the wire and the bracket.
Intra-oral elastics for traction are commonly available in 2 oz, 3.5 oz and 4.5 oz strengths and a variety of sizes, ranging from 1/8 inch to 3/4 inch (Fig. 17.20). For most purposes they should be changed every day. Class II and Class III elastic traction is discussed in Section 15.6. Latex-free varieties are now available.
Palatal or lingual arches can be used to reinforce anchorage, to achieve expansion (the quadhelix appliance), or molar de-rotation. They can be made in the laboratory from an impression of the teeth (Fig. 17.21). Proprietary forms of most of the commonly used designs are also available, and these have the additional advantage that they are removable, thus facilitating adjustment (Fig. 17.22).
Springs are an integral part of the Begg technique (see Section 17.6.2).

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17.3.5 Archwires
Once an operator has chosen to use a particular type of bracket, the amount and type of force applied to an individual tooth can be controlled by varying the cross-sectional diameter and form of the archwire, and/or the material of its construction. In the initial stages of treatment a wire which is flexible with good resistance to permanent deformation is desirable, so that displaced teeth can be aligned without the application of excessive forces. In contrast, in the later stages of treatment rigid archwires are required to engage the archwire slot fully and to provide fine control over tooth position while resisting the unwanted effects of other forces, such as elastic traction.
The physical properties of an archwire material which are of interest to the orthodontist are as follows.
  • Springback This is the ability of a wire to return to its original shape after a force is applied. High values of springback mean that it is possible to tie in a displaced tooth without permanent distortion.
  • Stiffness The amount of force required to deflect or bend a wire. The greater the diameter of an archwire the greater the stiffness.
  • Formability This is the ease with which a wire can be bent to the desired shape, for example the placement of a coil in a spring, without fracture.
  • Resilience This is the stored energy available after deflection of an archwire without permanent deformation.
  • Biocompatibility
  • Joinability This is whether the material can be soldered or welded.
  • Frictional characteristics If tooth movement is to proceed quickly a wire with low surface friction is preferable.
The most popular wire is stainless steel (Fig. 17.23), because it is relatively inexpensive, easily formed and exhibits good stiffness. Because of these characteristics, stainless steel is particularly useful in the later stages of treatment. More flexible stainless steel wires have been developed which consist of three or more strands of fine stainless steel wire twisted or braided together. These are known as multistrand or twistflex wire (Fig. 17.24) and they are more flexible than a solid stainless steel wire of comparable diameter. However, whilst relatively inexpensive, multistrand wires can exert too high a force, and be distorted if tied into a markedly displaced tooth.
Alternatively, other alloys which have a greater resistance to deformation and greater flexibility can be used. Of these, nickel titanium (Fig. 17.25) is the most popular. Archwires made of nickel titanium are capable of applying a light force without deformation, even when deflected several millimetres, but this alloy is more expensive than stainless steel. By virtue of their flexibility, nickel titanium wires provide less control against the unwanted side-effects of auxiliary forces. Cobalt chromium has the advantage that it can be readily formed, and then the stiffness and rigidity of the archwire can be improved by heat-treatment. β-titanium, popularly known as TMA (tungsten molybdenum alloy), has properties midway between stainless steel and nickel titanium; it has been estimated that a β-titanium wire exerts approximately half the force of a stainless steel wire of comparable diameter. It is often employed in the later stages of treatment, being particularly useful when the operator wishes to torque individual teeth.
Archwires are described according to their dimensions. An archwire described as 0.016 inches (0.4 mm) is a round archwire, and an 0.016 × 0.022 inches (0.4 × 0.55 mm), is a rectangular archwire.
Archwires are available in straight lengths, as coils, or as preformed archwires (see Fig. 17.23). The latter variant is more costly to buy but saves chairside time. There are a wide variety of archform shapes; however, regardless of what design is chosen, some adjustment of the archwire to match the pretreatment archform of the patient will be required (see Section 17.4).
The force exerted by a particular archwire material is given by the formula
where d is the distance that the spring/wire is deflected, r is the radius of the wire, and l is the length of the wire.
Thus it can be appreciated that increasing the diameter of the archwire will significantly affect the force applied to the teeth, and increasing the length or span of wire between the brackets will inversely affect the applied force. As mentioned earlier, the distance between the brackets can be increased by reducing the width of the brackets, but the interbracket span can also be increased by the placement of loops in the archwire. Prior to the introduction of the newer more flexible alloys, multilooped stainless steel archwires were commonly used in the initial stages of treatment. Loops are still utilized in retraction archwires (see Section 17.5) and where a combination of a rigid archwire (to resist unwanted forces) with localized flexibility is required.
17.4. TREATMENT PLANNING FOR FIXED APPLIANCES
By virtue of their coverage of the palate, removable appliances inherently provide more anchorage than fixed appliances. It is important to remember that, with a fixed appliance, movement of one tooth or a segment of teeth in one direction will result in an equal but opposite force acting on the remaining teeth included in the appliance. In addition, apical movement will place a greater strain on anchorage. For these reasons it is necessary to pay particular attention to anchorage when planning treatment involving fixed appliances and, if necessary, this can be reinforced with headgear and/or a palatal or lingual arch (see Chapter 15).
The importance of keeping the teeth within the zone of soft tissue balance has been discussed in Chapter 7. Therefore care is required to ensure that the arch-form, particularly of the lower arch, present at the beginning of treatment is largely preserved. It is wise to check the dimensions of any archwire against a model of the lower arch, taken before the start of treatment (Fig. 17.26), bearing in mind that the upper arch will of necessity be slightly broader. Of course, there

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are exceptions, as discussed in Chapter 7. However, these should be foreseen at the time of treatment planning and, if necessary, the implications for retention of the final result discussed fully with the patient at that time.

17.5. PRACTICAL PROCEDURES
Accurate bracket placement is crucial to achieving success with fixed appliances. The ‘correct’ position of the bracket on the facial surface will depend upon the bracket system used. Some fixed appliance systems require the operator to position the bracket at different heights on each tooth to compensate for differing crown lengths. Others, notably the pre-adjusted systems, require the bracket to be placed in the middle of the tooth along the long axis of the clinical crown. Bracket placement is particularly important with these pre-adjusted systems, as the values for tip and torque are calculated for the midpoint of the facial surface of the tooth. Incorrect bracket positioning will lead to incorrect tooth position and ultimately affect the functional and aesthetic result; therefore errors in bracket placement should be corrected as early as possible in the treatment. Alternatively, adjustments can be made to each archwire to compensate, but over the course of a treatment this can be time-consuming.
As mentioned in Section 17.3.5, when a fixed appliance is first placed a flexible archwire is advisable to avoid applying excessive forces to displaced teeth, which can be painful for the patient and result in bond failure. Commonly, either a pre-formed nickel titanium archwire or a multistranded stainless steel archwire is used to achieve initial alignment. Alternatively, loops can be placed in a stainless steel archwire, as mentioned in Section 17.3.5, to increase the span of wire between brackets and thus increase flexibility. This approach is useful if a rigid archwire is desirable in other areas of the arch.
It is important to move on from these initial aligning archwires as soon as alignment is achieved, as by virtue of their flexibility they do not afford much control of tooth position. However, it is equally important to ensure that full bracket engagement has been achieved before proceeding to a more rigid archwire. In the edgewise or pre-adjusted appliance systems it is usual to progress through round archwires of increasing diameter to achieve progressively better intra-arch alignment. If tooth alignment alone is required, for example in a Class I malocclusion with rotations, a stiff round archwire which nearly fills the bracket slot will suffice. However, correction of inter-arch relationships and space closure is usually best carried out using rectangular wires for apical control. The exact archwire sequence will depend upon the dimensions of the archwire slot and operator preference.
Mesiodistal tooth movement can be achieved by one of the following:
  • Moving teeth with the archwire: this is achieved by incorporating loops into the archwire which, when activated, move a section of the archwire and the attached teeth as shown in Fig. 17.27.
  • Sliding teeth along the archwire (Fig. 17.28), usually under the influence of elastic force: this approach requires greater force to overcome friction between the bracket and the wire, and therefore places a greater strain on anchorage. This type of movement is known as ‘sliding mechanics’ and is more applicable to pre-adjusted appliances where a straight archwire is used. In the edgewise appliance the first-, second-, and third-order bends necessary in the archwire make sliding teeth along it difficult.
Adjustments to the appliance need to be made on a regular basis, usually every 6 weeks. Once space closure is complete and incisor position corrected, some operators will place a more flexible full-sized archwire, often in conjunction with vertical elastic traction, to help ‘sock-in’ the buccal occlusion.
The subject of retention is covered in more detail in the chapter on retention. However, in order to try and overcome the greater tendency for relapse of rotational or apical movements, some orthodontists overcorrect these aspects of a malocclusion.
17.6. FIXED APPLIANCE SYSTEMS
17.6.1. Pre-adjusted appliances
Because of their advantages these systems are now universally accepted. The need for first-, second-, and third-order bends in the archwire during treatment is considerably reduced because the brackets are manufactured with the slot cut in such a way that these movements are built in. Therefore plain preformed archwires can be used so that the teeth are moved progressively from the very start of treatment to their ideal position. Hence they are also known as the straight wire appliance.
As individual tooth positions are built into the bracket, it is necessary to produce a bracket for each tooth, but the time saved in wire bending and the superior results achieved more than compensate for the increased cost of purchasing a greater inventory of brackets. However, a pre-adjusted bracket system will not eliminate the need for wire bending as only average values are built into the appliance, and often additional individual bends need to be placed in the archwire.
Not surprisingly, there are many different opinions as to the correct position of each tooth, and many manufacturers keen to join a lucrative market. The result is an almost bewildering array of pre-adjusted systems, all with slightly differing

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degrees of torque and tip. Of these perhaps the best known are the Andrews’ prescription, developed by Andrews, the father of the straight wire appliance (see Table 17.2) and the Roth system.
17.6.2. Begg appliance
Named after its originator, the Begg appliance (Fig. 17.31) is based on the use of round wire which fits fairly loosely into a channel at the top of the bracket. Apical and rotational movement is achieved by means of auxiliary springs or by loops placed in the archwire. Begg used ‘differential force systems’ to accomplish tooth movement, claiming that the intra-oral forces were adjusted so that they were optimal for movement of the anterior segment teeth whilst ensuring that the posterior segment teeth acted as an anchorage unit. The Begg appliance was often used in conjunction with extractions to provide intra-oral anchorage, so that reliance was not placed on the patient wearing headgear. However, patient compliance with wearing elastics for the duration of treatment was required instead.
Apart from the problems experienced by patients cleaning around the auxiliary springs favoured in the Begg technique, the main drawback to this appliance is that it is difficult to position the teeth precisely at the end of treatment.
17.6.3 Tip Edge appliance
This appliance was designed with the aim of combining the advantages of both the straight wire and the Begg systems. Orthodontists disagree as to the extent to which the Tip Edge technique achieves this. The Tip Edge bracket (Fig. 17.32), allows tipping of the tooth in the initial stages of treatment when round archwires are employed, as in the Begg technique, but when full-sized rectangular archwires are used in the latter stages, the built-in pre-adjustments help to give a better degree of control of final tooth positioning.
17.7. DECALCIFICATION AND FIXED APPLIANCES
Placement of a fixed attachment upon a tooth surface leads to plaque accumulation. In addition, if a diet rich in sugar is consumed, this results in demineralization of the enamel surrounding the bracket and occasionally frank cavitation. The incidence of decalcification (Fig. 17.33) with fixed appliances has been variously reported as between 15 and 85 per cent. As any decalcification is undesirable, considerable interest has focused on ways of reducing this problem. The main approaches that have been used are as follows:
  • Fluoride mouth rinses for the duration of treatment. The problem with this approach is that the individuals most at risk of decalcification are those least likely to comply fully with a rinsing regime.
  • Local fluoride release from fluoride-containing cements and bonding adhesives. Variable results have been reported for those composites which have been marketed for their fluoride-releasing potential. Glass ionomer cements have been shown to be effective at reducing the incidence of decalcification around bands, whilst achieving equal or better retention results than conventional cements. Although glass ionomer cements appear effective at reducing decalcification around bonded attachments, this is at the expense of poorer retention rates (see Section 17.3.3).

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  • Dietary advice. This important aspect of preventive advice should not be forgotten. Patients are often advised to avoid chewy sweets during treatment, but the importance of avoiding sugared beverages and fizzy drinks, particularly between meals, should not be overlooked.
17.8. STARTING WITH FIXED APPLIANCES
It is extremely unwise to embark on treatment with fixed appliances without first gaining some expertise in their use. This is best achieved by a longitudinal course in the form of an apprenticeship with a skilled operator. It is mandatory that this is supplemented by a thorough appreciation of orthodontic diagnosis and treatment planning, so that the novice orthodontist realizes his or her limitations and is selective in the type of case tackled. The 2 or 3 day courses comprising a practical typodont with a small theoretical element are always heavily oversubscribed, but most serve to put off the general dental practioner (perhaps not unintentionally). They do not provide an adequate basis for launching into fixed appliances, unless a more experienced orthodontist is readily available for advice.
Some orthodontic supply companies offer the practioner a kit containing brackets, bands, and a few archwires in return for an impression and a fee. Of course, this is an expensive alternative and, in addition, bands selected from an impression are unlikely to be a good fit. Those interested in gaining further orthodontic skills are advised to gain adequate experience on a longitudinal basis and to buy an adequate stock of pliers, bands, and brackets to make fixed appliance treatment rewarding and successful for both the practitioner and patient.
PRINCIPAL SOURCES AND FURTHER READING
Howels, D. J. (1986). The straight-wire appliance. Dental Update, 13, 367–76.
•The background to, and use of, the first pre-adjusted system.
Williams, J. K., Isaacson, K. G., and Cook, P. A. (1995). Fixed orthodontic appliances. Principles and practices. Butterworth Heinemann, London.
An excellent book, which should be read by anyone using fixed appliances.
Kapila, S. and Sachdeva, R. (1989). Mechanical properties and and clinical applications of orthodontic wires. American Journal of Orthodontics and Dentofacial Orthopedics, 96, 100–9.
An excellent, and readable, account of archwire materials.
Kusy, R. P. (1997). A review of contemporary archwires: their properties and characteristics. Angle Orthodontist, 67, 197–207.
Millett, D. T. and Gordon, P. H. (1994). A 5-year clinical review of bond failure with a no-mix adhesive (Right-on). European Journal of Orthodontics, 16, 203–11.
This paper provides scientific justification for all the old wives’ tales about bond failure rates.
Rock, P. (1995). A practical introduction to fixed appliances: the straight wire appliance. Dental Update, 22, 18–21, 61–5.
O’Higgins, E. A. et al. (1999). The influence of maxillary incisor inclination on arch length. British Journal of Orthodontics, 26, 97–102.
A fascinating article — a ‘must read’ for those practitioners using fixed appliances.
Shaw, W. C. (ed.) (1993). Orthodontics and occlusal management. Wright, Bristol.
Chapter 15 on fixed appliances is well written and informative and is complemented by the chapter on common treatment procedures.
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