Contemporary Orthodontic Appliances

Steps in Preparing the Aligners: Diagnostic records for CAT are not different from those for any other type of orthodontic treatment, but for Invisalign sequenced aligners, an intraoral optical scan (which also records the initial set of the patient’s bite) or PVS (polyvinyl siloxane) impressions and a bite registration (maximum intercuspation) are obtained. The scan or impressions and photographs are submitted to the company along with the doctor’s initial instructions. The production process begins when the intraoral scan or impressions are used to create an accurate three-dimensional (3-D) digital model of each dental arch (Figure 10-12). These records are transferred electronically to a digital treatment facility (presently in Costa Rica).

At the digital treatment facility, the teeth are digitally sectioned and cleaned up (obvious artifacts removed), the dental arches are related to each other, gingiva is added, movement is staged following the doctor’s instructions, and this preliminary plan is placed online for the doctor’s review as a “ClinCheck.” After the doctor is satisfied with the planned sequence of aligners, the set of digital models for a patient is transferred to a cast production facility, where a stereolithographic model for each step is fabricated (Figure 10-13). A clear plastic aligner is formed over each model, and the set of aligners is sent directly to the doctor.

Clinician’s Role in ClinCheck: With experience, doctors tend to be more specific in their initial prescription of what they want, but the sequence of steps and the amount of movement between steps is specified by algorithms built into the Treat software if this is not spelled out in detail in the prescription. In essence, when the ClinCheck is posted for the doctor to examine, the computer technician has sent a draft treatment plan for review (Figure 10-14). The software used by the computer technicians has default scenarios for different types of malocclusions and default rates of tooth movement. These defaults are satisfactory for simpler cases but not for the more complex ones.

For complex treatment, the doctor must customize the plan in terms of the amount and location of interproximal reduction of teeth (if any) that is to be done (Figure 10-15), the sequence of tooth movement steps, the rate of tooth movement with each subsequent aligner (often reducing the amount of movement at critical points), and the extent to which bonded shapes are to be used to increase the aligner’s grip on the teeth.

Considerations in Clinical Use of Clear Aligners: Although Invisalign is over a decade old, only a few studies of the outcomes of Invisalign treatment have been published in refereed professional journals.⁴ A recent prospective study used Invisalign’s software to evaluate the accuracy with which planned changes were accomplished, using the ratio of achieved to predicted change, and found that the highest accuracy (47%) was achieved during lingual constriction of the dental arches and the lowest (18%) for extrusion of maxillary incisors.⁴ Based on the existing studies and comments from experienced users, it seems clear now that Invisalign (and clear aligners more generally) do some things well and others not so well (Box 10-1). The limitations should be kept in mind when CAT is considered.

Several other considerations in the use of sequential aligners include the following:

The clinical use of clear aligners in adjunctive and comprehensive treatment is discussed in greater detail in Chapter 18.

E-Arch: In the late 1800s, a typical orthodontic appliance depended on some sort of rigid framework to which the teeth were tied so that they could be expanded to the arch form dictated by the appliance. Angle’s first appliance, the E-arch, was an improvement on this basic design (Figure 10-16). Bands were placed only on molar teeth, and a heavy labial archwire extended around the arch. The end of the wire was threaded, and a small nut placed on the threaded portion of the arch allowed the archwire to be advanced so that the arch perimeter increased. Individual teeth were simply ligated to this expansion arch. This appliance still could be found in the catalogs of some mail-order orthodontic laboratories as late as the 1980s, perhaps because of its simplicity, and despite the fact that it can deliver only heavy interrupted force.

Pin and Tube: The E-arch was capable only of tipping teeth to a new position. It was not able to precisely position any individual tooth. To overcome this difficulty, Angle began placing bands on other teeth and used a vertical tube on each tooth into which a soldered pin from a smaller archwire was placed. With this appliance, tooth movement was accomplished by repositioning the individual pins at each appointment.

An incredible degree of craftsmanship was involved in constructing and adjusting this pin and tube appliance, and although it was theoretically capable of great precision in tooth movement, it proved impractical in clinical use. It is said that only Angle himself and one of his students ever mastered the appliance. The relatively heavy base arch meant that spring qualities were poor, and the problem therefore was compounded because many small adjustments were needed.

Ribbon Arch: Angle’s next appliance modified the tube on each tooth to provide a vertically positioned rectangular slot behind the tube. A ribbon archwire of 10 × 20 gold wire was placed into the vertical slot and held with pins (Figure 10-17). The ribbon arch was an immediate success, primarily because the archwire, unlike any of its predecessors, was small enough to have good spring qualities and therefore was quite efficient in aligning malposed teeth. Although the ribbon arch could be twisted as it was inserted into its slot, the major weakness of the appliance was that it provided relatively poor control of root position. The resiliency of the ribbon archwire simply did not allow generation of the moments necessary to torque roots to a new position.

Edgewise: To overcome the deficiencies of the ribbon arch, Angle reoriented the slot from vertical to horizontal and inserted a rectangular wire rotated 90 degrees to the orientation it had with the ribbon arch, thus the name “edgewise” (Figure 10-18). The dimensions of the slot were altered to 22 × 28 mils, and a 22 × 28 precious metal wire was used. These dimensions, arrived at after extensive experimentation, did allow excellent control of crown and root position in all three planes of space.

After its introduction in 1928, this appliance became the mainstay of multibanded fixed appliance therapy, although the ribbon arch continued in common use for another decade.

Labiolingual, Twin Wire: Before Angle, placing attachments on individual teeth simply had not been done, and Angle’s concern about precisely positioning each tooth was not widely shared during his lifetime. In addition to a variety of removable appliances utilizing fingersprings for repositioning teeth, the major competing appliance systems of the first half of the twentieth century were the labiolingual appliance, which used bands on first molars and a combination of heavy lingual and labial archwires to which fingersprings were soldered to move individual teeth, and the twin-wire appliance. This appliance used bands on incisors as well as molars and featured twin 10 mil steel archwires for alignment of the incisor teeth. These delicate wires were protected by long tubes that extended forward from the molars to the vicinity of the canines. None of these appliances, however, were capable of more than tipping movements except with special and unusual modifications. They have disappeared from contemporary use.

Begg Appliance: Given Angle’s insistence on expansion of the arches rather than extraction to deal with crowding problems, it is ironic that the edgewise appliance finally provided the control of root position necessary for successful extraction treatment. The appliance was being used for this purpose within a few years of its introduction. Charles Tweed, one of Angle’s last students, was the leader in the United States in adapting the edgewise appliance for extraction treatment. In fact, little adaptation of the appliance was needed. Tweed moved the teeth bodily and used the subdivision approach for anchorage control, first sliding the canines distally along the archwire, then retracting the incisors (see Figure 9-33).

Raymond Begg had been taught the ribbon arch appliance at the Angle school before his return to Australia in the 1920s. Working independently in Adelaide, Begg also concluded that extraction of teeth was often necessary, and set out to adapt the ribbon arch appliance so that it could be used for better control of root position.

Begg’s adaptation took three forms: (1) he replaced the precious metal ribbon arch with high-strength 16 mil round stainless steel wire as this became available from an Australian company in the late 1930s; (2) he retained the original ribbon arch bracket, but turned it upside down so that the bracket slot pointed gingivally rather than occlusally; and (3) he added auxiliary springs to the appliance for control of root position. In the resulting Begg appliance (Figure 10-19),⁵ friction was minimized because the area of contact between the narrow ribbon arch bracket and the archwire was very small and the force of the wire against the bracket was also small. Binding was minimized because Begg’s strategy for anchorage control was tipping/uprighting (see Figure 8-21), and tipping minimizes the angle of contact between the wire and corner of the bracket.

Although the progress records with his approach looked vastly different, it is not surprising that Begg’s overall result in anchorage control was similar to Tweed’s, since both used two steps to compensate for resistance to sliding. The Begg appliance is still seen in contemporary use though it has declined in popularity and often appears now in a hybrid form, with brackets that allow the use of rectangular wires in finishing (Figure 10-20).⁶ It is a complete appliance in the sense that it allows good control of crown and root position in all three planes of space.

Automatic Rotational Control: In the original appliance, Angle soldered eyelets to the corners of the bands, so a separate ligature tie could be used as needed to correct rotations or control the tendency for a tooth to rotate as it was moved (see Figure 10-18). Now rotation control is achieved without the necessity for an additional ligature by using either twin brackets or single brackets with extension wings that contact the underside of the archwire (Lewis or Lang brackets) (Figure 10-21) to make it easier to obtain the necessary moment in the rotational plane of space.

Alteration in Bracket Slot Dimensions: The significance of reducing Angle’s original slot size from 22 to 18 mils and the implications of using the larger slot with undersize steel wires have been discussed in Chapter 9. In essence, there are now two modern edgewise appliances, because the 18 and 22 slot appliances are used rather differently. Chapters 14 to 16 focus on these differences.

Straight-Wire Prescriptions: Angle used the same bracket on all teeth, as did the other appliance systems. In the 1980s, Andrews developed bracket modifications for specific teeth to eliminate the many repetitive bends in archwires that were necessary to compensate for differences in tooth anatomy, and bonding made it much easier to have different brackets for each tooth. The result was the “straight-wire” appliance.⁷ This was the key step in improving the efficiency of the edgewise appliance.

In the original edgewise appliance, faciolingual bends in the archwires (first-order, or in-out, bends) were necessary to compensate for variations in the contour of labial surfaces of individual teeth. In the contemporary appliance, this compensation is built into the base of the bracket itself. This reduces the need for compensating bends but does not eliminate them because of individual variations in tooth thickness.

Angulation of brackets relative to the long axis of the tooth is necessary to achieve proper positioning of the roots of most teeth. Originally, this mesiodistal root positioning required angled bends in the archwire, called second-order, or tip, bends. Angulating the bracket or bracket slot decreases or removes the necessity for these bends.

Because the facial surface of individual teeth varies markedly in inclination to the true vertical, in the original edgewise appliance it was necessary to place a varying twist (referred to as third-order, or torque, bends) in segments of each rectangular archwire, in order to make the wire fit passively. Torque bends were required for every patient in every rectangular archwire, not just when roots needed to be moved facially or lingually, in order to avoid inadvertent movements of properly positioned teeth. The bracket slots in the contemporary edgewise appliance are inclined to compensate for the inclination of the facial surface, so that third-order bends are less necessary.

The angulation and torque values built into the bracket are often referred to as the appliance prescription. Obviously, any prescription based on a group average would precisely position only the average tooth and would not be correct for outliers in a normal population.

The edgewise appliance continues to evolve. Current commercially available edgewise appliances and their bracket prescriptions are reviewed in some detail at the end of this chapter. Before getting to them, let us examine banding versus bonding as the means of fixing the appliance in place.

1. Teeth that will receive heavy intermittent forces against the attachments. This is the primary indication for banding now. An excellent example is an upper first molar against which extraoral force will be placed via a headgear. The twisting and shearing forces often encountered when the facebow is placed or removed are better resisted by a steel band than by a bonded attachment.

2. Teeth that will need both labial and lingual attachments such as a molar with both headgear and lingual arch tubes. Isolated bonded lingual attachments that are not tied to some other part of the appliance can be swallowed or aspirated if something comes loose.

3. Teeth with short clinical crowns, so that bonded brackets are difficult to place correctly. If attached to a band, a tube or bracket can slightly displace the gingiva as it is carried into proper position. It is much more difficult to do this with bonded attachments. The decision to band rather than bond second premolars in adolescents is often based on the length of the clinical crown.

4. Teeth with extensive restorations. Although bonding to porcelain, gold, or amalgam is possible, bond strength tends to be low, and debonding from porcelain often will damage its appearance by removing the glaze. For that reason, if little intact enamel is available for bonding, banding may be easier and more efficient.

Separation: Tight interproximal contacts make it impossible to properly seat a band, which means that some device to separate the teeth usually must be used before banding. Although separators are available in many varieties, the principle is the same in each case: a device to force or wedge the teeth apart is left in place long enough for initial tooth movement to occur, so that the teeth are slightly separated by the appointment at which bands are to be fitted.

Two main methods of separation are used for posterior teeth: (1) separating springs (Figure 10-22), which exert a scissors action above and below the contact, typically opening enough space for banding in approximately 1 week; and (2) elastomeric separators (“doughnuts”), applied as shown in Figure 10-23, which surround the contact point and squeeze the teeth apart over a period of several days.

From the patient’s perspective, steel spring separators are easier to tolerate, both when they are being placed and removed, and as they separate the teeth. These separators tend to come loose and may fall out as they accomplish their purpose, which is their main disadvantage and the reason for leaving them in place only a few days, not for more than a week. Elastomeric separators are more difficult to insert but are usually retained well when they are around the contact and so may be left in position for somewhat longer periods. Because elastomeric separators are radiolucent, a serious problem can arise if one is lost into the interproximal space. It is wise to use a brightly colored elastomeric material to make a displaced separator more visible, and these separators should not be left in place for more than 2 weeks.

Fitting Bands: With the wide availability of preformed bands now, forming bands clinically is too inefficient. Almost all bands are supplied now with prewelded attachments. This saves clinical time and allows the use of templates to assure accurate placement of the attachment.

Fitting a preformed band involves stretching the stainless steel material over the tooth surface. This simultaneously contours and work-hardens the initially rather soft band material. It follows that heavy force is needed to seat a preformed band. This force should be supplied by the masticatory muscles of the patient, not by the arm strength of the dentist or dental assistant. Patients can bite harder and with much greater control, a fact best appreciated on the rare occasions when a patient is unable to bite bands to place and the orthodontist has to do it with hand pressure.

Preformed bands are designed to be fitted in a certain sequence, and it is important to follow the manufacturer’s instructions. A typical maxillary molar band is designed to be placed initially by hand pressure on the mesial and distal surfaces, bringing the band down close to the height of the marginal ridges. Then it is driven to place by pressure on the mesiobuccal and distolingual surfaces. The final seating is with heavy biting force on the distolingual corner. Lower molar bands are designed to be seated initially with hand pressure on the proximal surfaces and then with heavy biting force along the buccal but not the lingual margins. Maxillary premolar bands are usually seated with alternate pressure on the buccal and lingual surfaces, while mandibular premolar bands, like mandibular molars, are designed for heavy pressure on the buccal surface only.

Cementation: New cements specifically designed for orthodontic use have supplanted the zinc phosphate and early glass ionomer cements used in the twentieth century. These tend to be a composite of glass ionomer and resin materials and usually are light-cured. Their use has greatly reduced problems with leakage beneath bands that previously was a risk for decalcification of banded teeth.

All interior surfaces of an orthodontic band must be coated with cement before it is placed, so that there is no bare metal. As the band is carried to place, the occlusal surface should be covered so that cement is expressed from the gingival as well as the occlusal margins of the band (Figure 10-24).

Preparation of the Tooth Surface: Before bonding an orthodontic attachment, it is necessary to remove the enamel pellicle and to create irregularities in the enamel surface. This is accomplished by gently cleaning and drying the enamel surface (avoiding heavy pumicing), then treating it with an etching agent, usually 37% unbuffered phosphoric acid for 20 to 30 seconds. The effect is to remove a small amount of the softer interprismatic enamel and open up pores between the enamel prisms, so the adhesive can penetrate into the enamel surface (Figure 10-25). At present, etching and priming the tooth surface often are done in a single step, especially when rebonding after a bracket is loose or lost. The tooth surface must not be contaminated with saliva, which promotes immediate remineralization, but the new tooth preparation materials now minimize the need to have a perfectly dry tooth surface.

Surface of Attachments: The base of a metal bonded bracket or tube must be manufactured so that a mechanical interlock between the bonding material and the attachment surface can be achieved. Either chemical bonding or mechanical interlocking can be used with ceramic brackets. The strength of chemical bonds can become high enough to create problems in debonding, so mechanical retention now is preferred for ceramic as well as metal brackets.

Bonding Materials: A successful bonding material must meet a set of formidable criteria: it must be dimensionally stable; it must be quite fluid, so that it penetrates the enamel surface; it must have excellent inherent strength; and it must be easy to use clinically.

Until recently, light-activated filled acrylic (bis-GMA) resins were the preferred bonding materials, but new self-adhesive resin cements (which incorporate a self-etch primer or a self-adhesive component) are beginning to replace them because they require less preparation of the tooth surface. The new cements do not have as much bond strength with metal brackets as a widely used etch-and-rinse bonding material, but one of them already has the same strength with ceramic brackets.⁸

Modified glass-ionomer cements also can be used as orthodontic bonding agents. Their possible advantage is less decalcification around the brackets because of fluoride release (see further discussion below); their great disadvantage is significantly less strength and therefore a greater chance of loose brackets that require rebonding during treatment.

Direct Bonding: During direct bonding, bracket position is determined intraorally by the clinician during the bonding procedure. This technique can be used quite successfully as a routine clinical procedure. Even when most attachments are bonded indirectly (as described below), direct bonding is much more efficient whenever a single bracket must be repositioned. After preparation of the tooth surface, either a chemically activated composite resin with a very rapid setting time or a light-activated material can be used.

The major difficulty with direct bonding is that the dentist must be able to judge the proper position for the attachment and must carry it to place rapidly and accurately. There is less opportunity for precise measurements of bracket position or detailed adjustments than there would be at the laboratory bench. It is generally conceded that for this reason, direct bonding does not provide as accurate a placement of brackets as indirect bonding. On the other hand, direct bonding is easier, faster (especially if only a few teeth are to be bonded), and less expensive (because the laboratory fabrication steps are eliminated).

Steps in the direct bonding technique when using a light-activated resin for each bracket are illustrated in Figure 10-26. Light-cured resins now are used more frequently than chemically activated resins because the newer light-cured materials have more flexibility in working time and usually have higher bond strengths.

Indirect Bonding: Indirect bonding is done by accurately placing the brackets on dental casts in a laboratory, then using a template or tray to transfer the bracket positions to the patient. The advantage is more precise location of brackets than is possible with direct bonding because the teeth can be examined from all angles without the limitations of cheeks and saliva. Indirect setups can be done by the clinician in the office laboratory, but more frequently they now are done on stereolithographic casts made from impressions sent to a company (Cadent, E-Models, others) that also produces digital casts as part of diagnostic records. An alginate impression, poured relatively rapidly in the office lab, gives an accurate enough working cast for indirect bonding, but more stable impressions are needed for later digital scanning. Laboratory and clinical steps in indirect bonding are illustrated in Figure 10-27.

Obtaining optimal bond strength while minimizing flash around the bracket can be a challenge during indirect bonding, since the presence of a tray prevents removing uncured flash. One solution is to use “no-mix” chemically activated materials. The composite resin is placed on the tooth surface in unpolymerized form, while the polymerization catalyst is placed on the back of the brackets. When the tray carrying the brackets is placed against the tooth surface, the resin immediately beneath the bracket is activated and polymerizes, but excess resin around the margins of the brackets does not polymerize and can easily be scaled away when the bracket tray is removed. Some studies, however, have found increased bond failures with this technique because it relies on diffusion for proper polymerization. An alternative is to use a chemical cure resin that is mixed prior to application to the brackets and teeth, but care must be taken to minimize excess resin. Finally, a flowable light-cured material can be used with a transparent tray, but polymerizing the resin at each bracket through or around the tray takes more time than using a chemical cure. With any of these techniques, proper isolation is critical for obtaining adequate bond strength.

At present, routine use of indirect bonding is gaining popularity. Custom brackets that were manufactured for an individual patient require precise placement that can be achieved only by indirect bonding. More generally, the poorer the visibility, the more difficult direct bonding becomes and the greater the indication for an indirect approach. For this reason, indirect bonding is almost a necessity for lingual attachments. Bonding an isolated lingual hook or button is not difficult, but precisely positioning the attachments for a lingual appliance is, and even the placement of a fixed lingual retainer is done more easily with indirect technique and a transfer tray.

Debanding/Debonding: It is as important to remove a fixed appliance safely as to place it properly. Bands are largely retained by the elasticity of the band material as it fits around the tooth. This is augmented by the cement that seals between the band and the tooth, but a band retained only by cement was not fitted tightly enough. No orthodontic band cement bonds strongly to enamel (which is why band cements cannot be used to bond brackets). When the band is distorted by force to remove it, the cement breaks away from the band or the tooth, and there is almost no chance of damaging the enamel surface.

The greater strength of bonding adhesives becomes a potential problem in debonding. When a bonded bracket is removed, failure at one of three interfaces must occur: between the bonding material and the bracket, within the bonding material itself, or between the bonding material and the enamel surface. If a strong bond to the enamel has been achieved, which is the case with the modern materials, failure at the enamel surface on debonding is undesirable because the bonding material may tear the enamel surface as it pulls away from it. The interface between the bonding material and the bracket is the usual and preferred site of failure when brackets are removed. The safest way to remove metal brackets is to distort the bracket base, which induces failure between it and the bonding adhesive. This damages the bracket so that it cannot be reused. The major reason for not recycling and reusing brackets is the possibility of enamel damage when they are removed without distorting the base. If brackets can be removed without damage they can be cleaned, sterilized, and reused without risk to the patient in exactly the same way as other medical devices.

Ceramic brackets are a particular problem for debonding because their base cannot be distorted. They break before they bend. There are two ways to create adhesion between a ceramic bracket and the bonding adhesive: mechanical retention through undercuts on the bracket base, as is done with metal brackets, or chemical bonding between the adhesive and a treated bracket base. It is quite possible to create such a strong bond between the adhesive and a chemically treated bracket base that failure will not occur there, but then when the bracket is removed, there is a real chance of enamel surface damage. Reports of enamel damage on debonding began to appear soon after ceramic brackets were introduced and have been a problem ever since.

Modifications to ceramic brackets to enhance the chance of debonding at the right interface and electrothermal and laser techniques to weaken the adhesive during debonding are discussed in the section below on modern bracket materials.

Prevalence and Prevention: White spots, unsightly areas of decalcification around brackets, are a well-known problem that has become worse since bonded brackets have largely replaced bands and more of the tooth surface is exposed. Although bonded brackets do not directly damage the teeth, they make plaque removal more difficult. Maxillary incisors, particularly lateral incisors, are the greatest problem area. A recent study of 338 patients treated in a university orthodontic clinic reported that 36% had at least one lesion on a maxillary incisor tooth at the end of treatment despite preventive efforts. Risk factors were young age at the beginning of treatment, poor hygiene before the start of treatment, and citations for poor hygiene during treatment.⁹

The lesions can be carious or noncarious; carious lesions are rough and porous, noncarious are smooth and shiny. Some natural remineralization occurs, and the carious lesions have a better prognosis for this because the surface is porous. Nevertheless, the lesions are not likely to completely correct themselves. A recent 14-year follow-up showed that most of the white spot lesions noted at the end of treatment were still there.¹⁰

Fluoridated water and a fluoride-containing toothpaste are effective as caries-controlling measures in the general population and should be considered an essential part of a program to prevent white spot lesions. There is some evidence that a daily 0.05% neutral sodium fluoride rinse is effective in preventing white spots.¹¹ The major problem with the toothpaste and rinse, of course, is sporadic or no compliance. For caries-prone patients in general, a fluoride varnish application at 6-month intervals is recommended; for noncompliant orthodontic patients who are developing lesions, more frequent fluoride varnish applications may be helpful, though the evidence to support this is weak.¹² A short-term daily chlorhexidine rinse program (usually 14 days) could be a last resort approach to a noncompliant patient with persistent plaque accumulation, despite the staining of teeth that occurs.

A number of fluoride-releasing bonding agents have been offered commercially in the hope that they would control decalcification around brackets, but a 2010 review of the published data concluded that there is no good evidence that any are effective against white spots.¹² The problem is that the fluoride release is large initially, then dwindles to little or nothing long before a typical 18 to 24 month orthodontic treatment is completed. A recent report shows that in the laboratory, a reasonably sustained fluoride release from elastomeric modules can be obtained that is high enough to be clinically effective over a period of 25 days.¹³ Since elastomeric ligature ties are replaced at every appointment, if significant release can be maintained for appointment intervals of 6 to 8 weeks, this approach might be more successful, but no clinical data have yet been presented.

Treatment for White Spots: In a recent review, Guzman-Armstrong et al outlined a multistep approach to treatment that is the basis for the following discussion¹⁴:

The first step would appropriately be managed by the orthodontist. Beyond that point, involving the family dentist or a colleague in restorative dentistry would be wise.

Cast versus Metal-Injected-Molded Stainless Steel Brackets: The brackets and tubes for an edgewise appliance must be precisely manufactured so that the internal slot dimensions are accurate to at least 1 mil. Until the recent introduction of ceramic and titanium brackets, fixed appliances had been fabricated entirely from stainless steel for many years, and steel remains the standard material for appliance components.

There are two contemporary ways to produce steel edgewise brackets and tubes: by metal-injection molding (MIM) or by casting. Most of the brackets and tubes for contemporary appliances now are produced by MIM, but some are cast. The greatest precision of bracket slot size is achieved by milling the slot of a cast bracket, which corrects errors introduced by shrinkage of the casting as it cools.

Titanium as an Alternative to Stainless Steel: Nickel is a potentially allergenic material. Given the significant nickel content of stainless steel, it is fortunate for orthodontists that mucosal allergic reactions to nickel are much less prevalent than cutaneous reactions. Cutaneous sensitization to nickel often develops from skin contact with cheap jewelry, and 10% or more of the population now have some degree of sensitivity to nickel.¹⁶ Most patients who show skin reactions tolerate stainless steel orthodontic appliances quite satisfactorily, but a few do not, and there is concern that this number is increasing. Some European countries are now considering a ban on steel orthodontic appliances because of the risk of allergic responses.

The metal alternatives to steel are gold, long since abandoned because of performance and cost considerations, and titanium, which contains no nickel and is exceptionally biocompatible. Titanium archwires have been used since the 1980s, and the use of bonded titanium brackets and tubes has increased rapidly since the turn of the century. In addition to its hypoallergenic properties, titanium brackets and tubes seem to reduce the failure rate in bonding, perhaps because the material is more “wettable” and the bonding materials adhere better to the retention pad, perhaps because titanium is more resilient than steel and absorbs impacts better. For patients with nickel allergy, the choice would be between these brackets and nonmetallic ones.

Nonmetallic Appliance Materials: Recurring efforts have been made to make fixed appliances more esthetic by eliminating their metallic appearance. A major impetus to the development of bonding for orthodontic attachments was elimination of the unsightly metal band. Tooth-colored or clear brackets for anterior teeth (Figure 10-28) became practical when successful systems for direct bonding were developed. Although plastic brackets were introduced with considerable enthusiasm in the 1980s and have remained on the market ever since, they suffer from three largely unresolved problems: (1) staining and discoloration, particularly in patients who smoke or drink coffee; (2) poor dimensional stability, so that it is not possible to provide precise bracket slots or build in all the straight-wire features; and (3) friction between the plastic bracket and metal archwires that makes it very difficult to slide teeth to a new position. Using a metal slot in the plastic bracket helps the second and third problems, but even with this modification, plastic brackets are useful only when complex tooth movements are not required.

Ceramic brackets, which were first made available commercially in the late 1980s, largely overcome the esthetic limitations of plastic brackets in that they are quite durable and resist staining. In addition, they can be custom-molded for individual teeth and are dimensionally stable, so that the precise bracket angulations and slots of the straight-wire appliance can be incorporated. Several different types of ceramic brackets currently are available (Table 10-2).

Ceramic brackets were received enthusiastically and immediately achieved widespread use, but problems with fractures of brackets, friction within bracket slots, wear on teeth contacting a bracket, and enamel damage from bracket removal soon became apparent. Fractures of ceramic brackets occur in two ways: (1) loss of part of the brackets (e.g., tie wings) during archwire changes or eating and (2) cracking of the bracket when torque forces are applied. Ceramics are a form of glass, and like glass, ceramic brackets tend to be brittle. Because the fracture toughness of steel is much greater, ceramic brackets must be bulkier than stainless steel brackets, and the ceramic design is much closer to a wide single bracket than is usual in steel.

Most currently available ceramic brackets are produced from alumina, either as single-crystal or polycrystalline units. In theory, single-crystal brackets should offer greater strength, which is true until the bracket surface is scratched. At that point, the small surface crack tends to spread, and fracture resistance is reduced to or below the level of the polycrystalline materials. Scratches, of course, are likely to occur during the course of treatment.

Although ceramic brackets are better in this regard than plastics, resistance to sliding has proved to be greater with ceramic than with steel brackets. Because of the multiple crystals, polycrystalline alumina brackets have relatively rough surfaces (Figure 10-29). Even though monocrystalline alumina is as smooth as steel, these brackets do not allow good sliding, perhaps because of a chemical interaction between the wire and bracket material. For this reason, some ceramic brackets now have an integrated metal slot.

Many patients bite against a bracket or tube at some point in treatment. Contact against a steel or titanium bracket causes little or no wear of enamel, but ceramic brackets can abrade enamel quite rapidly. This risk is largely avoided if ceramic brackets are placed only on the upper anterior teeth, which is the location where improved esthetics is most important. Most patients who want the esthetic effect will accept ceramic brackets only where they are most visible and steel or titanium brackets elsewhere.

As noted above in the section on debonding, ceramic brackets also can be a problem when it comes time for bracket removal. Most manufacturers now offer a debonding pliers that is recommended for their ceramic bracket to take advantage of a unique feature engineered into the bracket to help in debonding. An alternative is to use a thermal or laser instrument to weaken the adhesive by heating it, to induce failure within the bonding agent itself. Thermal debonding of this type is quite effective in reducing the chance of enamel damage.¹⁷ Unfortunately, it introduces the chance of damaging the tooth pulp unless the heat application is controlled quite precisely, and for that reason has not been widely adopted.

It seems highly probable that composite plastic brackets will become the next advance in brackets in another few years. Composite plastics with better physical properties than any metal already exist and could be used for both brackets and archwires. It is just a matter of overcoming the engineering problems to produce brackets with better mechanical properties, and since the composite plastics can be almost any color, a better appearance is likely to be an additional benefit.

Compensations for First-Order Bends: For anterior teeth and premolars, varying the bracket thickness eliminates in-out bends in the anterior portions of each archwire, but an offset position of molar tubes is necessary to prevent molar rotation (Figure 10-31). For good occlusion, the buccal surface must sit at an angle to the line of occlusion, with the mesiobuccal cusp more prominent than the distobuccal cusp. For this reason, the tube or bracket specified for the upper molar should have at least a 10-degree offset, as should the tube for the upper second molar. The offset for the lower first molar should be 5 to 7 degrees, about half as much as for the upper molar. The offset for the lower second molar should be at least as large as for the first molar. Offsets in some typical commercially available appliances are shown in Tables 10-3 and 10-4 (the listed prescriptions are available in most instances from several different manufacturers).

Compensations for Second-Order Bends: In the original edgewise appliance, second-order bends, sometimes called artistic positioning bends, were an important part of the finishing phase of treatment. These bends were necessary because the long axis of each tooth is inclined relative to the plane of a continuous archwire (Figure 10-32). Contemporary edgewise brackets have a built-in tip for maxillary incisor teeth, which varies among the appliances that are now available (see Table 10-3). A distal tip of the upper first molar is also needed to obtain good interdigitation of the posterior teeth (Figure 10-33). If the upper molar is too vertically upright, even though a proper Class I relationship apparently exists, good interdigitation cannot be achieved. Tipping the molar distally brings its distal cusps into occlusion and creates the space needed for proper relationships of the premolars.

Compensation for Third-Order Bends: If the bracket for a rectangular archwire is placed flat against the labial or buccal surface of any tooth, the plane of the bracket slot will twist away from the horizontal, often to a considerable extent. With the original edgewise appliance, it was necessary to place a twist in each rectangular archwire to compensate for this. Failure to place third-order bends meant that in the anterior region, the teeth would become too upright, while posteriorly the buccal cusps of molars would be depressed and the lingual cusps elevated (Figure 10-34). Cutting the bracket slot into the bracket at an angle or forming the base so that the face of the bracket is at an angle (which are called placing torque in the bracket or torque in the base, respectively) allows a horizontally flat rectangular archwire to be placed into the bracket slots without incorporating twist bends.

The amount of torque recommended in the various appliance prescriptions varies more than any other feature of contemporary edgewise appliances (see Tables 10-3 and 10-4). Although a number of factors are important in establishing the appropriate torque, three are particularly germane to how much torque is used for any particular bracket: (1) the value that the developer of the appliance chose as the average normal inclination of the tooth surface (this varies considerably among individuals and therefore can be different in “normal” samples); (2) where on the labial surface (i.e., how far from the incisal edge) the bracket is intended to be placed (the inclination of the tooth surface varies depending on where the measurement is made, so that an appliance meant to be placed rather gingivally would require different torque values from one placed more incisally); and (3) the expected “play” in the bracket slot between the wire and the slot. As the tables demonstrate, the effective torque produced by undersized rectangular wires is far less than the bracket slot prescription might lead one to expect.

Self-Ligating Brackets: Placing wire ligatures around tie wings on brackets to hold archwires in the bracket slot is a time-consuming procedure. The elastomeric modules introduced in the 1970s largely replaced wire ligatures for two reasons: they are quicker and easier to place, and they can be used in chains to close small spaces within the arch or prevent spaces from opening.

It also is possible to use a cap or clip, attached over the bracket or built into the bracket itself, to hold wires in position. Three types of self-ligating mechanisms built into the bracket are available at present (with more probably on the way): a springy latching cap, springy retaining clips in the bracket walls, and rigid latching caps (Figure 10-35). A variety of claims have been made as advantages of self-ligating brackets, but as we have pointed out in Chapter 9, it is clear now that almost all of these are incorrect when clinical outcomes are reviewed. Reduction of friction, the most advertised claim, is true in laboratory testing but does not lead to less resistance to sliding a bracket along a wire or a wire through a bracket. A recent summary of claims versus evidence concluded that self-ligating brackets save a little time in ligation but do not produce a saving of treatment time or better results.¹⁸

This should not be taken to mean that there is anything wrong with the brackets. The problem is the advertising, not the product. As a group, the self-ligating brackets perform quite nicely, with no evidence that their latching mechanism makes any significant difference. It is important, however, that a self-ligating bracket is made so that when stabilization rather than tooth movement is needed or the latching mechanism has difficulty in holding a rectangular torquing wire in place, an archwire can be tied tightly in place with an external steel ligature.

Individually Customized Brackets: Because of the marked individual variations in the contour of the teeth, no appliance prescription can be optimal for all patients, and compensatory bends in finishing archwires often are necessary. Custom brackets for the facial surface of teeth offer the prospect of eliminating almost all archwire bending (i.e., they could provide the perfect straight wire appliance). The Insignia system now marketed by Ormco (Ormco/Sybron, Glendora, CA) makes just this claim.

Whether custom brackets are to be made for the facial or lingual surfaces, the technology is much the same. The first step is a 3-D scan with at least 50 micron resolution, now of dental casts on a laboratory bench but perhaps directly in the mouth in the future. This digital information is used to precisely cut each bracket using computer-aided design/computer-aided manufacturing (CAD/CAM) technology, so that the slot for each bracket has the appropriate thickness, inclination, and torque needed for ideal positioning of that tooth (Figure 10-36), and archwires with an arch form established for that patient are supplied. The result for custom brackets on the facial is “the ultimate straight-wire appliance,” with wire bending reduced to a minimum. Preliminary data indicate that treatment time is reduced in comparison to treatment with conventional prescription brackets, but that some adjustment of the final archwires still is required.

Individualized custom brackets must be attached to the teeth with precision equal to that used in making them, so an indirect bonding system with an accurate placement template is required. What happens when one of the custom brackets is lost and requires replacement and rebonding, or is loose and requires rebonding? Because the specifications for each bracket can be maintained in computer memory, it is possible to obtain a replacement bracket and bonding template within 2 to 3 weeks. Rebonding a loose bracket is done most efficiently by using the original bonding template, which should be kept with the patient’s records for this possible re-use.

At present, however, that is not the biggest problem. Even a set of modern CAD/CAM brackets formed on individual dental casts is still focused only on dental relationships and so, for example, the Class II patient who requires slightly more upright maxillary incisors and more proclined lower incisors would still receive brackets with “ideal” incisor inclinations. It remains important to introduce coordination with the patient’s individual skeletal and soft tissue pattern into this type of design. Attempts are being made now to integrate images of tooth–lip relationships into the data base for fabrication of the custom brackets.

Lingual Appliances: A major objection to fixed orthodontic appliances always has been their visible placement on the facial surface of the teeth. This is one reason for using removable appliances and is the major reason for the current popularity of clear aligners in treatment of adults. The introduction of bonding in the 1970s made it possible to place fixed attachments on the lingual surface of teeth to provide an invisible fixed appliance, and brackets designed for the lingual surface were first offered soon after bonding was introduced, but there were multiple problems in producing a bracket that intruded only minimally into tongue space and was at least reasonably easy to use. In the United States, most orthodontists who experimented with the lingual appliances available in the 1980s abandoned this approach as more trouble than it was worth, and lingual appliance treatment all but disappeared until quite recently.

Recent progress in Europe and Asia has made lingual orthodontics much more widely used there. One successful European approach that now is marketed in the United States (Incognito, 3M-Unitek) is to fabricate a custom precious metal pad that covers a large area on the lingual surface of each tooth and then attach low-profile brackets to the custom pads (Figure 10-37). These brackets, designed so the archwire can be inserted from the top, are the same for each tooth; wire bending is eliminated by using wire-bending robots to form the archwires.¹⁹ Computer-controlled wire-bending devices are particularly applicable to the fabrication of the lingual archwires but also can be used with labial appliances (see later).

A major test for any of the computer-assisted appliance systems is the accuracy with which the planned outcome (established during manufacture of the customized appliance) actually is achieved. A recent study, using a new method for analyzing the difference between the computer template and the final result, shows that Incognito outcomes are quite accurate representations of the template, except that second molars are not positioned as precisely as the other teeth (Figure 10-38).²⁰ Feedback of that type is needed for all of the computer-assisted approaches, both to improve the accuracy of the system and to allow better evaluation of the method.

Making Sensible Appliance Choices Based on Patient Preference: There are considerable differences in what patients indicate is the most attractive appliances, the one they would prefer to have.²¹ Most notably, this is related to patient age, but there are some minor gender differences. For 9 to 11 year olds, the preference is either for shaped brackets like Wildsmiles (Wildsmiles Braces, Omaha, NE) with or without colored elastomeric ties or for mini-twin brackets with colored elastomeric ties. Inconspicuous esthetic brackets are not a high priority for this group, so a durable appliance that provides excellent control and is modestly priced is appropriate. In the 12 to 14 age group, clear aligners and esthetic brackets are more highly prized, but mini-twin brackets with colored ties and the shaped brackets all are rated similarly. Clear aligners often are not practical with partially erupted teeth and continuing growth.

For the 15 to 17 year olds, clear aligners and esthetic brackets with a clear wire (see discussion later) are considered most attractive. For this group, clear aligners can make sense because the permanent teeth are fully erupted and rapid growth is completed. Adults prefer lingual appliances, clear aligners, and esthetic brackets, especially when combined with a clear wire. The spectrum changes from unique and colorful for young children to esthetic appliances with older adolescents and adults. With this combination of acceptable alternatives, it is possible to meet esthetic demands and accomplish the biomechanics for each individual case for almost every patient.

Selection of Arch Form for Individual Patients: As another contributor to increased efficiency, preformed archwires are an important part of the modern edgewise appliance, whether individualized custom brackets are used or not. When nickel-titanium (NiTi) and beta-titanium (beta-Ti; TMA) wires are needed, there is no choice but to use preformed archwires because these wires are almost impossible to shape to arch form without special tools. What arch form should be employed?

The concept that dental arch form varies among individuals is driven home to most dentists in full denture prosthodontics, where it is taught that the dimensions and shape of the dental arches are correlated with the dimensions and shape of the face. The same variations in arch form and dimensions of course exist in the natural dentition, and it is not the goal of orthodontic treatment to produce dental arches of a single ideal size and shape for everyone.

The basic principle of arch form in orthodontic treatment is that within reason, the patient’s original arch form should be preserved. Most thoughtful orthodontists have assumed that this would place the teeth in a position of maximum stability, and long-term retention studies support the view that posttreatment changes are greater when arch form is altered than when it is maintained (see Chapter 17).

As a more general guideline, if the maxillary and mandibular arch forms are incompatible at the beginning of treatment, the mandibular arch form should be used as a basic guide. In many patients with Class II malocclusion, the maxillary arch is narrow across the canines and premolars, and should be expanded to match the lower arch as overjet is reduced. Obviously, this guideline would not apply when mandibular arch form is distorted. That can happen in a number of ways, the most common being lingual displacement of the mandibular incisors by habits or heavy lip pressures, and unilateral drift of teeth in response to early loss of primary canines or molars. Although some judgment is required, the arch form desired at the end of orthodontic treatment should be determined at the beginning, and the patient’s occlusal relationships should be established with this in mind.

An excellent mathematical description of the natural dental arch form is provided by a catenary curve, which is the shape that a loop of chain would take if it were suspended from two hooks. The length of the chain and the width between the supports determine the precise shape of the curve. When the width across the first molars is used to establish the posterior attachments, a catenary curve fits the dental arch form of the premolar-canine-incisor segment of the arch very nicely for most individuals. For all patients, the fit is not as good if the catenary curve is extended posteriorly, because the dental arch normally curves slightly lingually in the second and third molar region (Figure 10-39). Most of the preformed archwires offered by contemporary manufacturers are based on a catenary curve, with average intermolar dimensions. Modifications to accommodate for a generally more tapering or more square morphology are appropriate, and the second molars must be “tucked in” slightly.

Another mathematical model of dental arch form, originally advocated by Brader and often called the Brader arch form, is based on a trifocal ellipse. The anterior segment of the trifocal ellipse closely approximates the anterior segment of a catenary curve, but the trifocal ellipse gradually constricts posteriorly in a way that the catenary curve does not (Figure 10-39, B). The Brader arch form therefore will more closely approximate the normal position of the second and third molars. It also differs from a catenary curve in producing somewhat greater width across the premolars.

Recently, several manufacturers have offered preformed archwires that appear to be variations of the Brader arch, with advertisements that suggest these wires are more compatible with expansion therapy than conventional arch forms. Expansion across the premolars often is thought to have esthetic advantages; whether the modified arch form to produce this has any effect on stability is unknown. More refined mathematical descriptions of typical human arch forms now are available,²² and it is likely that better mathematical models will improve the preformed archwires available in the near future.

It is important to keep in mind that neither the ligation method nor the prescription adjustments placed in straight-wire brackets have anything to do with arch form, which is still established by the shape of the archwires. Arch form is particularly important during the finishing stage of treatment, when heavy rectangular archwires are employed. Preformed archwires are best considered as “arch blanks” and sometimes are listed in the catalogs in that way. The name is appropriate, since this properly implies that a degree of individualization of their shape will be required to accommodate the needs of patients.

Wire-Bending Robots: Another approach to the goal of reducing the amount of clinical time spent bending archwires is to use a computer-controlled machine to shape the archwire as desired. If the effort to fabricate a complex archwire were eliminated, inexpensive “plain vanilla” brackets could be used instead of going to the trouble of producing custom brackets with elaborate prescriptions. A less complex bracket also could be smaller and have a lower profile.

In lingual orthodontics, the scanned casts needed for fabrication of custom bracket pads also provide the data needed to generate computer-fabricated archwires (Figure 10-40). For labial orthodontics, SureSmile (OraMetrix, Richardson, TX) uses data acquired via an intraoral scan to shape finishing archwires to the desired arch form and adjust it at each bracket to provide correct in-out, angulation, and torque bends.

In the SureSmile technique, it is recommended to obtain a first intraoral scan of the patient’s dentition so that the company can produce a digital setup of the finished result. The orthodontist can view and modify this as an aid in planning treatment. The first step in treatment is to bond non-customized brackets—the only requirement is that the characteristics of the brackets are known—and the teeth in both arches are aligned and leveled with light round wires as in typical treatment (see Chapter 14). At that point, a new intraoral scan is obtained and a second setup is produced in which the finishing details are incorporated. Once this is approved by the orthodontist, a robot forms superelastic rectangular archwires (using steel, beta-Ti, or NiTi as the archwire material as specified by the doctor) that are sent to the orthodontist, and these wires bring the teeth to their final positions (Figure 10-41).

In a study carried out at the University of Indiana, the first to provide good data for SureSmile outcomes in nonextraction treatment,²³ a group of 63 conventionally finished patients were compared with 69 SureSmile patients treated in the same office by the same clinician. The SureSmile group had a significantly shorter time in fixed appliances (mean of 23 versus 32 months) and a trend toward lower (better) scores on cast analysis that was not statistically significant. This must be interpreted with caution because the two groups had a somewhat different makeup: the SureSmile group had fewer complex malocclusions than the conventional group and a lower proportion of males (who have been shown in previous studies to generally finish with higher cast analysis scores). Although the SureSmile group had better scores for alignment/rotation correction and fewer interproximal spaces, the conventional group had better scores for both faciolingual (torque) and mesiodistal (tip) root angulation. The study concluded that the shorter treatment time with SureSmile was due at least in part to less severe malocclusions and less detailed finishing and that a randomized clinical trial would be needed to determine whether the use of computer-formed finishing wires really reduced treatment time for comparable outcomes. As we have noted previously, rectangular superelastic archwires provide lower moments within brackets than are ideal for both tip and torque, and although these were specified by the doctor, their use by SureSmile also may have contributed to the poorer root positioning.

Clear Polymer Archwires: Orthodontic archwires formed from clear polymers now are becoming available. They offer two potential advantages over stainless steel or titanium: better esthetics because the wire can be clear or the same color as the teeth, so that the wire becomes almost invisible when used with ceramic brackets (Figure 10-42) and physical properties that equal or exceed those of metal archwires.

Clear polymer archwires currently are being developed using two different approaches—a formable and a nonformable alternative. Burstone et al are pursuing the first approach using a polyphenylene polymer (Primospire, Solvay Advanced Polymers, Alpharetta, GA).²⁴ This material extruded as round and rectangular cross-section arches. It has properties similar to small dimension beta-Ti wires and formability similar to stainless steel wires.

The second approach is being advocated by SimpliClear Braces (Biomers, Singapore), who fabricate archwires from a polymer resin matrix reinforced with glass fibers. A specially modified methacrylate resin serves as the polymer matrix material. These wires are available in round and rectangular cross-sections, and can be paired with esthetic pretorqued and preangulated brackets of the practitioner’s choice. Although this clear wire is not formable clinically, auxiliaries like rotating wedges or bracket repositioning can be used to treat simple cases without custom wires.

For more complex cases, a series of preformed custom wires are made using either digital images from scans of dental casts or intraoral scans. This series is designed individually for every patient based on the treatment plan provided by the orthodontist prior to the onset of treatment. The wires are designed to sequentially and incrementally move teeth toward a predetermined position (Figure 10-43). In theory, they should provide better control than a sequence of clear aligners—conceptually, this is a fixed appliance version of Invisalign—but their clinical performance has not yet been thoroughly evaluated. This clear wire also can be incorporated into retainers as the labial bow to provide an esthetic solution that allows settling with good stability (see Figure 17-8, E).

Summary: At this point, it seems likely that most fixed orthodontic appliances of the not-too-distant future will be individualized using scans of the tooth surfaces and computer technology. It is still too soon to tell, however, whether the usual approach will be custom brackets that allow the use of preformed archwires with little or no manual wire bending, minimally compensated (and less expensive) brackets that are used in connection with a wire-bending robot, or a sequence of nonadjustable custom wires produced for that patient.

Stability: Short-term or primary stability is determined by mechanical retention of the screw in bone, which depends on bone properties, the engineering design of the screw, and placement technique. Long-term or secondary stability is defined by the biologic union of the screw to the surrounding bone. It is determined by the implant surface, bone characteristics, and bone turnover (especially in the context of cortical versus medullary bone) and is affected by the implant surface and the mechanical system used. To obtain good secondary stability it is important to limit micromovements that could lead to bone resorption and formation of a fibrous capsule. Over time, primary stability decreases while secondary stability increases; clinical stability is the sum of primary and secondary stability and is the major factor in clinical success (Figure 10-45).

Factors in stability/success that have been shown to be important are:

For a more detailed review of the factors that influence clinical stability and success rates, see Crismani et al.³⁵

Ease of Use: A second important aspect of any orthodontic TAD is its ease of use. This has two components: how easy or difficult it is to place the screw (Figure 10-46) and how easy it is to use the exposed screw head as an attachment for springs or wires.

Factors in ease of screw placement include:

A major factor affecting the ease of attaching springs or wires is whether direct or indirect anchorage is to be used. In direct anchorage the force is applied directly to a tooth or group of teeth from a bone screw. To do this, it is desirable to fit NiTi springs over the screw head without having to tie them, which means that the screw head should have an attachment area that will lock into place. With indirect anchorage, the bone screw anchors a tooth or group of teeth to which force is applied, and the goal is to prevent movement of these teeth. Even though some indirect anchorage can be gained by a ligature tie from the bone screw to the anchored tooth or teeth, indirect anchorage normally requires the screw head to have a rectangular slot or a hole where a wire can be locked or bonded in place (Figure 10-48).

In summary, the desirable characteristics of a bone screw TAD are listed in Box 10-2. Since no one device has all of them, this serves as a guide to the trade-offs in selecting the screw and suggests that efficient practice requires having more than one type of bone screw available.

• The number of screws with which the plate is attached. The success rate for plates of four designs placed by the same operator is shown in Figure 10-50. It appears that three screws provide more stability than two, but nothing additional is gained with four screws.

• The age of the patient. As Table 10-5 shows, the number of failures with miniplates in both North Carolina and Belgium was much greater in young patients who had not yet entered puberty and in the mandibular arch where miniplates were used only in young patients. This is a particularly pertinent observation with regard to use of Class III elastics to maxillary and mandibular miniplates (see Chapter 13): bone maturity does not reach the level for good retention of the screws before approximately age 11 (obviously, maturational rather than chronologic age).

• The amount of force that the miniplate can tolerate is significantly larger because the plate is held by multiple screws and usually is placed in an area with thicker cortical bone.

• If the miniplate has a locking mechanism (as it should), the direction of pull can be changed readily and the source of the force can be moved a considerable distance by extending wire hooks from the intraoral fixture (Figure 10-51; also see Figure 10-48). This also can be done with individual screws that have a slot for a wire extension, but putting a force on the extension introduces a moment that can overtighten or loosen the screw. With multiple screws holding the miniplate, this is not a problem.

• The miniplates can be placed well above the roots of the maxillary teeth, so that an interdental screw does not become a barrier to moving all the teeth mesially or distally. With individual interdental screws, relocation is necessary to accomplish distal movement of teeth anterior to the screw or mesial movement of teeth posterior to the screw.³⁹