References

Bergstrom K, Halling A, Wilde B Orthodontic care from the patient's perspective: perceptions of 27 yr olds. Eur J Orthod. 1998; 20:319-329
Lim KF, Lew KKK, Toh SL Bending stiffness of two aesthetic orthodontic archwires: an in vitro comparative study. Clin Mater. 1994; 16:63-71
Husmann P, Bourauel C, Wessinger M, Jäger A The frictional behaviour of coated guiding archwires. J Orofac Orthop. 2002; 63:199-211
Neumann P, Bourauel C, Jäger A Corrosion and permanent fracture resistance of coated and conventional orthodontic wires. J Mater Sci Mater Med. 2002; 13:141-147
Dickson JA, Jones SP, Davis EH A comparison of the frictional characteristics of five initial alignment wires and stainless steel brackets at three bracket to wire angulations - an in vitro study. Br J Orthod. 1994; 21:15-22
Talass MF Orthodontic archwire.
Talass MF Optiflex archwire treatment of a skeletal class III open bite. J Clin Orthod. 1992; 4:245-252
Ireland AJ, Sherriff M, McDonald F Effect of bracket and wire composition on frictional forces. Eur J Orthod. 1991; 13:322-328
Watari F, Yamagata S, Imai I, Nakamura S The fabrication and properties of aesthetic FRP wires for use in orthodontics. J Mater Sci. 1998; 33:5661-5664
Rollins DA The potential application of fibre-reinforced composite as an orthodontic wire: biomechanical considerations. Am J Orthod Dentofacial Orthop. 1989; 98:560-561
Patel AP A study on the changes in mechanical properties and structure of fibre-reinforced composite wires as a result of thermoforming. Am J Orthod Dentofacial Orthop. 1991; 100:193-194
Jancar J, Dibenedetto AT, Hadziinikolau Y, Goldberg AJ, Dianselmo A Measurement of the elastic modulus of fibre-reinforced composite used as orthodontic wires. J Mater Sci Mater Med. 1994; 5:214-218
Kennedy KC, Kusy RP UV-cured pultrusion processing of glass-reinforced polymer composites. J Vinyl Addit Technol. 1995; 1:182-186
Kennedy KC, Kusy RP Pultruded fibre-reinforced plastic and related apparatus and method.
Imai T, Watari F, Yamagata S Mechanical properties and aesthetics of FRP orthodontic wire fabricated by hot drawing. Biomaterials. 1998; 19:2195-2200
Fallis DW, Kusy RP Variation in flexural properties of photo-pultruded composite archwires: analyses of round and rectangular profiles. J Mater Sci. 2000; 11:683-693
Huang ZM, Gopal R, Fujihara K Fabrication of a new composite orthodontic archwire and validation by a bridging micromechanics model. Biomaterials. 2003; 24:2941-2953
Zufall SW, Kennedy KC, Kusy RP Frictional characteristics of composite orthodontic archwires against stainless steel and ceramic brackets in the passive and active configuration. J Mater Sci Mater Med. 1998; 9:611-620
Newman GV Epoxy adhesives for orthodontic attachments: progress report. Am J Orthod. 1965; 51:901-912
Brandt S JCO interviews Dr Elliott Silverman, Dr Morton Cohen, and Dr A. J. Gwinnett on bonding. J Clin Orthod. 1979; 13:236-251
Birnie D Orthodontic material update: ceramic brackets. Br J Orthod. 1990; 17:71-75
Garcia-Godoy F, Martin S Shear strength of ceramic brackets bonded to etched or unetched enamel. J Clin Ped Dent. 1995; 19:181-185
Keith O, Kusy RP, Whitley JQ Zirconia brackets: an evaluation of morphology and coefficients of friction. Am J Orthod Dentofacial Orthop. 1994; 106:605-614
Summary of the AAO ceramic bracket survey. Bull Am Assoc Orthod. 1989; 7
In: West-brook JH, Conrad H US: ASM; 1973
Buzzitta VA, Hallgren SE, Powers JM Bond strength of orthodontic direct-bonding cement-bracket systems as studied in vitro. Am J Orthod. 1982; 81:87-92
Ghafari J Problems associated with ceramic brackets suggest limiting use to selected teeth. Angle Orthod. 1997; 62:145-152
Odegaard J, Segner D Shear bond strength of metal brackets compared with a new ceramic bracket. Am J Orthod Dentofacial Orthop. 1988; 94:201-206
Viazis AD, Cavanaugh G, Boris RR Bond strength of ceramic brackets under shear stress: an in vitro report. Am J Orthod Dentofacial Orthop. 1990; 98:214-221
Franklin S, Garcia-Godoy F Shear bond strengths and effects on enamel of two ceramic brackets. J Clin Orthod. 1993; 27:83-88
Mundstock KS, Sadowsky PL, Lacefield W, Bae S An in vitro evaluation of a metal reinforced orthodontic ceramic bracket. Am J Orthod Dentofacial Orthop. 1999; 116:635-641
Schwartz M Ceramic brackets. J Clin Orthod. 1988; 22:82-88
Reynolds IR A review of direct orthodontic bonding. Br J Orthod. 1975; 2:171-175
Newman GV Clinical treatment with bonded plastic attachments. Am J Orthod. 1971; 60:600-610
Bayne SC Dental biomaterials: where are we and where are we going?. J Dent Educ. 2005; 69:571-585
Powers JM, Sakaguchi RL, Craig RGMosby: Elsevier; 2006
Feldner JC, Sarkar NK, Sheridan JJ, Lancaster DM In vitro torque-deformation characteristics of orthodontic polycarbonate brackets. Am J Orthod Dentofacial Orthop. 1994; 106:265-272
Harzer W, Bourauel C, Gmyrek H Torque capacity of metal and polycarbonate brackets with and without a metal slot. Eur J Orthod. 2004; 26:435-441
Sadat-Khonsari R, Moshtaghy A, Schelgel V, Kahl-Nieke B, Moller M, Bauss O Torque deformation characteristics of plastic brackets: a comparative study. J Orofac Orthop. 2004; 65:26-33
Bazakidou E, Nanda RS, Duncanson MG, Sinha PK Evaluation of frictional resistance in esthetic brackets. Am J Orthod Dentofacial Orthop. 1997; 112:138-144
Kusy RP, Whitley JQ Degradation of plastic polyoxymethylene brackets and the subsequent release of toxic formaldehyde. Am J Orthod Dentofacial Orthop. 2005; 127:420-427
De Franco DJ, Spiller RE, von Fraunhofer JA Frictional resistance using teflon coated ligatures with various bracket-archwire combinations. Angle Orthod. 1995; 65:63-67
Edwards GD, Davis EH, Jones SP The Ex Vivo effect of ligation technique on the static frictional resistance of stainless steel brackets and archwires. Br J Orthod. 1995; 22:145-153

Aesthetic labial orthodontic appliances – an update

From Volume 4, Issue 3, July 2011 | Pages 70-77

Authors

Hemendranath V Shah

BDS, MFDS RCS(Eng), DDS, MOrth RCS(Ed), FDS(Orth) RCS(Eng)

FTTA in Orthodontics, Bristol Dental Hospital and Royal United Hospital Bath

Articles by Hemendranath V Shah

Stephen A Boyd

BDS, MFD RCSI

SpR in Orthodontics, Bristol Dental Hospital and Royal United Hospital Bath

Articles by Stephen A Boyd

Jonathan R Sandy

PhD, MSc, BDS, FDS MOrth, FMedSci

Professor in Orthodontics, Department of Child Dental Health, Bristol Dental Hospital, Bristol

Articles by Jonathan R Sandy

Abstract

This paper will give an update on recent developments in aesthetic archwires, brackets and accessories that are presently available to increase the options available for the provision of an aesthetic labial orthodontic fixed appliance.

Clinical Relevance: In a consumer driven market, the demands for aesthetic orthodontic appliances to correct malocclusions is increasing. Aesthetic labial fixed appliances have the advantage of precise three-dimensional control of tooth movement with the aesthetic requirements demanded by patients.

Article

The demand for orthodontic treatment among adult patients has steadily increased in recent years and, despite greater acceptance of conventional fixed appliances, the quest for a less visible alternative has driven the development of aesthetic orthodontic appliances. This has included developments in labial aesthetic brackets, archwires and accessories, along with lingual appliances and clear aligners such as Invisalign. Although a study carried out in Sweden found that, if treatment were needed, 67% of 27-year-olds would probably or definitely wear metal braces, the study highlighted the fact that nearly a third of those interviewed would not wear visible braces even when there is a clinical need.1 This clearly illustrates the potential market for an aesthetic alternative to conventional labially placed stainless steel appliances. The intention of this review is to update the reader on recent developments in labial aesthetic orthodontic appliances and to highlight the limitations of the currently available materials and products. Labial aesthetic appliances and materials can be classified under three main headings, which will be discussed in turn, namely:

  • Aesthetic archwires;
  • Aesthetic brackets;
  • Aesthetic orthodontic accessories.
  • Aesthetic archwires

    In an effort to produce aesthetic orthodontic archwires two broad approaches have been taken: firstly, the coating of metallic archwires on one or more surfaces and secondly, the development of non-metallic archwires.

    Coated metallic aesthetic archwires

    Stainless steel and super-elastic nickel titanium archwires, in both round and rectangular profiles, are available with aesthetic coatings made from either epoxy or TeflonTM.2 In addition to the improvement in aesthetics offered by these wires, in vitro studies have demonstrated that TeflonTM coated wires can reduce frictional losses to less than 10%, compared with non-coated wires3 and that the TeflonTM can also prevent the corrosion of the underlying metal archwire.4 Conversely, Dickson et al5 found that epoxy-coated steel wires produced significantly more friction in vitro, than uncoated archwires. This increase in friction seen with the epoxy coating is thought to be due to the material stripping off during the simulated tooth movement and causing binding within the system. However, in the oral environment, the frictional and corrosion characteristics may differ from those observed in the laboratory. In vitro studies have shown that the TeflonTM coating remains undamaged on nickel titanium archwires, but peels away from the surface of stainless steel wires when exposed to mechanical testing.4 Once again, this does not mean the same effects will be observed clinically. At the time of fitting, coated metallic archwires may provide superior aesthetics, but with use this advantage can soon be lost. The coating material can become discoloured and/or can wear off during clinical use due to abrasion. Table 1 lists examples of currently available coated metallic archwires.


    Coating Archwire Manufacturer/Distributor Base archwire material
    Epoxy Confidential TM ClassOne Orthodonticswww.classoneortho.com Regency® Nickel Titanium Stainless Steel
    Imagination TM Gestencowww.gestenco.com Super-elastic Nickel Titanium Stainless Steel
    Orthoform® Tooth-coloured Hawley Russellwww.hawleyrussell.com Super-elastic Nickel Titanium Stainless Steel
    Teflon Titanol® Cosmetic Forestadentwww.forestadent.com Super-elastic Nickel Titanium
    Aesthetic Micro-coated Archwires DB Orthodonticswww.dborthodontics.co.uk Super-elastic Nickel Titanium Heat-activated Nickel Titanium Stainless Steel
    Orthoform® White-coated Hawley Russellwww.hawleyrussell.com Super-elastic Nickel Titanium
    Tooth ToneTM Plastic Coating Tooth ToneTM Plastic-coated Archwire Ortho Technologywww.orthotechnology.com/TOC www.toc-uk.com Nickel Titanium Stainless Steel
    Rhodium Coating for Low Reflectivity Sentalloy® High Aesthetic Archwire GAC Internationalwww.gacintl.com/TOC www.toc-uk.com Super-elastic Nickel Titanium
    BioForce® High Aesthetic Archwire GAC International www.gacintl.com/TOC www.toc-uk.com BioForce Nickel Titanium

    An example of a commercially available coated archwire is Tooth ToneTM Plastic-coated Archwire manufactured by Ortho Technology (Tampa, Florida, USA) and distributed in the UK by TOC (Bristol, UK). This wire has a 0.002″ plastic coating covering the entire surface of the wire (Figure 1). In the labio-lingual or occluso-gingival dimension this will therefore increase the total thickness of the wire by 0.004″ and hence have effects on its mechanical properties. An example of a wire with a coating limited to the labial surface is the Aesthetic Micro-coated Archwire, supplied by DB Orthodontics (Silsden, West Yorkshire, UK) which has a 0.0005″ polytetrafluoroethylene (PTFE) tooth-coloured labial coating. The possible advantages of a single surface coating are more predictable mechanical properties of the archwire, including friction, whilst retaining an aesthetic appearance (Figure 2).

    Figure 1. 0.014″ Nickel titanium archwire with 0.002″ Tooth ToneTM coating around the entire surface of the wire.
    Figure 2. 0.019″ × 0.025″ Nickel titanium archwire with a 0.0005″ PTFE coating on the labial surface of the wire.

    An alternative to these two polymeric coatings is to apply a rhodium coating to the underlying metallic archwire. This produces a surface with low reflectivity, giving a matt white or frosted appearance. Examples of commercially available archwires with a rhodium coating are the Sentalloy® High Aesthetic and Bioforce® High Aesthetic, both of which are coated nickel titanium archwires by GAC International (Bohemia, New York, USA). The same coating is also available on the spring clips of the aesthetic self-ligating In-Ovation C bracket by the same manufacturer.

    Non-metallic aesthetic archwires

    The non-metallic aesthetic archwires can be subdivided into:

  • Coated non-metallic archwires;
  • Polymeric aesthetic archwires;
  • Fibre-reinforced composite archwires.
  • Coated non-metallic aesthetic archwires

    Optiflex® was an early non-metallic aesthetic archwire consisting of three layers, namely an innermost core of silicon dioxide, which provided the force for tooth movement, an intermediate layer of silicon resin, added to provide strength and to protect the core from moisture and an outermost layer made from nylon to improve stain resistance, to add strength and to prevent damage to the underlying core and resin components.6,7 An early in vitro study by Dickson et al5 encouragingly found that Optiflex® demonstrated the lowest static frictional resistance with stainless steel brackets, when compared to stainless steel, super-elastic nickel titanium, co-axial stainless steel and epoxy-coated stainless steel archwires. However, a separate, in vitro comparative study looking at three different archwires undertaken by Lim et al2 found that 0.017″ Optiflex® had a low stiffness, low resilience and a poor springback when compared to a 0.018″TeflonTM coated stainless steel or an 0.017″ uncoated stainless steel archwire. It was therefore concluded that the clinical efficacy was probably limited and, as a consequence, Optiflex® never achieved much in the way of commercial success as an aesthetic archwire.

    Single component polymeric aesthetic archwires

    Polyacetyl was used to create an early prototype polymeric aesthetic archwire. However, this material was found to lack the necessary stiffness and resilience for clinical use and never progressed beyond the laboratory testing phase.8

    Watari et al9 undertook an in vitro investigation, testing epoxy and poly(methyl methacrylate) polymeric aesthetic archwires. However, once again both materials lacked the necessary flexural rigidity for use as an orthodontic archwire.

    Fibre-reinforced composite aesthetic archwires

    The idea of a fibre-reinforced composite being of use as a possible orthodontic aesthetic archwire began in the late 1980s.10,11 Over the past two decades, several research groups have investigated the production of prototype fibre-reinforced composite aesthetic archwires. They have all explored different types of glass for the fibre component embedded within different resin matrices and have utilized various methods of manufacture.9,12,13,14,15,16,17,18

    A novel method of fibre-reinforced composite archwire production was described by Kennedy and Kusy13,14 in which photo-pultrusion was used to produce ultra-violet cured glass-reinforced polymer composites. Zufall et al18 investigated the frictional characteristics of a fibre-reinforced composite archwire made using this photo-pultrusion method. The prototype archwires were made from S2 glass® and a resin comprising 61% bisphenol-A diglycidyl methacrylate (Bis-GMA) and 39% triethylene glycol dimethacrylate (TEGDMA) by weight, and with a 0.020″ round profile. Tests for friction conducted in the dry state, using 0.022″ slot stainless steel, polycrystalline alumina and single crystal alumina brackets, demonstrated that composite archwires had coefficients of friction greater than stainless steel archwires, but less than nickel titanium and β-titanium archwire materials. The flexural properties of photo-pultruded composite archwires in both round (0.022″) and rectangular (0.021″ × 0.028″) cross-section were determined by three point bend testing.16 The investigators concluded that the elastic modulus of their prototype fibre-reinforced composite archwire lay midway between that of martensitic nickel titanium (33.4 GPa) and β-titanium (72.4 GPa). Additionally, the prototype archwire produced about 25% of the force of stainless steel wire per unit of activation. The flexural strength of the fibre-reinforced composite archwire was within the range of published values for β-titanium wires (1.3–1.5 GPa).

    Research by Watari et al9 investigated prototype fibre-reinforced polymeric (FRP) wires. These consisted of combinations of two polymers, epoxy and poly(methyl methacrylate), and two types of filler materials, namely long silane-coated alumina fibres and fibres made from calcium oxide, diphosphorus pentoxide, silicon dioxide and aluminium oxide (CPSA glass). Two methods of production were also investigated, namely mould polymerization and hot drawing, and using these techniques round 0.5 mm diameter wires were constructed. It was found that the FRP wires produced using the hot drawing method demonstrated the stiffness and strength needed for orthodontic applications. The epoxy/alumina wire contained small bubbles, making the wire more translucent than transparent. What is somewhat surprising is that this aesthetic wire was reported to have the desirable properties necessary for clinical orthodontics and yet it has never made it to the commercial market. This perhaps once again demonstrates how laboratory findings do not necessarily translate to the clinical environment.

    Using a different fibre and resin matrix combination, Huang et al17 created a composite archwire material using unidirectional E-glass with a fibre diameter of 9 mm and a 68:32% by weight epoxy and hardener matrix. Additionally, this study aimed to undertake only in vitro mechanical testing of the prototype archwire in order to determine the effects of the fibre reinforcement pattern upon the mechanical properties. The method of fabrication involved using tube shrinkage, rather than pultrusion, as it is claimed to prevent damage to the glass fibre component, which can arise during pultrusion. Another cited advantage is that tube shrinkage enables the fabrication of rectangular profile wires, which they claim is not possible with pultrusion. Placement of the glass fibres and epoxy resin in the tube is followed by vertical suspension and the application of heat via a soldering iron. This results in its contraction and the subsequent expulsion of any excess resin inferiorly. The resultant material is placed into an archwire-shaped mould and then heated in an oven at 100°C to complete the curing process. In this way, a 0.5 mm (0.020″) diameter prototype archwire was created with a 45% volume fraction of fibre. This archwire was designed to be used in the initial alignment phase of orthodontic treatment. The bending modulus was found to be 31.8 GPa, compared to 38.9 GPa for a 0.406 mm (0.016″) nickel titanium wire. The authors felt this prototype archwire was less flexible and more brittle than equivalent metal orthodontic archwires, which would pose a problem in aligning severely malposed teeth. It was suggested that ductility could be improved by the use of nanofibres, as these possess higher stiffness and strength, but are more ductile than micron diameter fibres. Further research by this group investigated the preliminary mechanical design of a composite archwire, by investigating changes in fibre volume fraction, whilst keeping the diameter of the wire constant, and vice versa. They believe that the desired mechanical properties of the archwire can be achieved at the design stage by changing the constituent materials, their ratios and the dimensions of the wire.

    At present, there is only one commercially available fibre-reinforced polymer composite aesthetic archwire. This is produced and manufactured by BioMers Pte Ltd (Singapore) (Table 2). It is made of a continuous fibreglass filament in a polymer resin composed of ethylene Bisphenol-A-Dimethacrylate, triethyleneglcol dimethacyrlate, glass frit and surrounded by a Di-p-Xylylene polymer coating. The constituent materials are inserted into a shrinkable die in the shape of an archform. The purported advantages of this technique is that it allows the uniform distribution of constituent materials throughout the wire, producing a wire with uniform cross-section and hence there is no need for secondary post processing to produce an archform. The wire is only available in round cross-section and for the levelling and alignment phase of treatment. At present a rectangular profile wire is not available for use during the intermediate and final stages of treatment, however, this is being developed.


    Archwire Manufacturer/Distributor Archwire material Available dimensions
    Transluscent Archwire/OPTISTM BioMers Products www.biomersbraces.com Fibre-reinforced polymer composite Available in round profile 0.014″, 0.016″ and 0.018″

    The manufacturers state that this wire is susceptible to fracture during treatment and should not be deflected beyond an angle greater than 60°. Whitening of the surface of the archwire is said to indicate crazing and is a sign that the wire could fracture in that location. Owing to the preformed nature of the wire, it cannot be reshaped and so artistic bends or stops cannot be placed. Any attempt to undertake this is likely to result in fracture of the wire. The manufacturers also suggest that the wire is supported with bumper tubing or metal coil over large spans, as masticatory function might also lead to fracture. The use of such supporting materials, particularly anteriorly, is likely to make the appliance more noticeable and less aesthetic. Interestingly, instructions on the use of the wire are supplied for both the clinician and the patient.

    Aesthetic brackets

    Orthodontic brackets have been constantly evolving in response to market pressures for more consistent production values, improved durability, less friction, better bonding and improved convenience. As time has progressed the demand for a more aesthetic alternative has emerged. To some extent this was aided by the introduction of direct bonding to enamel, that became common practice in the 1970s following work carried out by Newman.19 This enabled the orthodontist to bond to the enamel surface reliably without the need for a band around the tooth. Whilst this was an improvement in terms of aesthetics, there was still a significant amount of metal visible on the buccal surface of the teeth. The first truly aesthetic brackets appeared in 1963 and were injection moulded from polycarbonate, and bonded to the teeth using an unfilled polymethacrylate resin.20 Whilst these brackets initially showed promise in terms of improved aesthetics, there were significant failings in their clinical performance and they suffered from a high failure rate. Consequently, the development and introduction of aesthetic brackets stagnated somewhat until 1987 with the introduction of ceramic brackets.21

    Ceramic brackets

    Ceramic brackets can be classified into monocrystalline, polycrystalline and zirconia (Table 3).


    Material Bracket Manufacturer/ Distributor
    Ceramic Monocrystalline SPA DB Orthodonticswww.dborthodontics.co.uk
    Inspire ICE Ormcowww.ormco.com
    Radiance American Orthodonticswww.americanortho.com
    Pure Ortho Technologywww.orthotechnology.com
    Allure GAC Internationalwww.gacintl.com
    Purity Orthocarewww.orthocare.co.uk
    Polycrystalline Avex CXi Opal Orthodonticswww.opalorthodontics.com
    Clarity/Clarity SL 3M Unitekwww.solutions.3m.co.uk
    Integra Ortho Bytewww.ortho-byte.com
    Contour DB Orthodonticswww.dborthodontics.co.uk
    Aspire Forestadentwww.forestadent.com
    Virage American Orthodonticswww.americanortho.com
    Mystique MB GAC Internationalwww.gacintl.com
    Reflection Ortho Technologywww.orthotechnology.com
    Vision Hawley Russellwww.hawleyrussell.com
    InVu TP Orthowww.tportho.com
    Plastic Polyurethane Synthesis Ortho Bytewww.ortho-byte.com
    Avalon DB Orthodonticswww.dborthodontics.co.uk
    Polar Ortho carewww.orthocare.co.uk
    Polyoxymethylene Brilliant Forestadentwww.forestadent.com
    Polycarbonate Silkon Plus American Orthodonticswww.americanortho.com
    Elegance Dentarumwww.dentaurum.de

    Monocrystalline brackets are manufactured by heating aluminium oxide to temperatures in excess of 2100°C. The molten mass is cooled slowly, and the bracket is machined from the resulting crystal, creating a completely transparent, highly aesthetic bracket (Figures 3 and 4). Polycrystalline brackets are less complicated to produce and therefore more brands are available on the market (Figure 5). They are manufactured by blending aluminium oxide particles with a binder and the resultant mixture can be formed into a shape from which a bracket can be machined. The sintering process is conducted with temperatures above 1800°C in order to burn out the binder and fuse together the particles of the moulded mixture. The bracket can then be heat treated in order to remove surface imperfections. This process has been further refined by injection moulding of the aluminium oxide to create the final desired bracket shape, thereby reducing the amount of milling that is normally required to provide the final bracket shape. This reduces the likelihood of generating surface imperfections which can otherwise lead to in-service bracket failures. Even small surface imperfections have been shown to have a significant impact by reducing the load to fracture of ceramics.22 As the name suggests, polycrystalline brackets are composed of very many fused crystals rather than a single crystal, as is the case with monocrystalline brackets. As the individual crystal sizes increase so do the optical properties of the material. However, this is usually to the detriment of material strength. Therefore, while monocrystalline brackets are perhaps more aesthetic, they are less durable than polycrystalline brackets.

    Figure 3. Pure®, a monocrystalline bracket from Orthotechnology/TOC.
    Figure 4. SPATM Aesthetics, a monocrystalline bracket from DB Orthodontics.
    Figure 5. Clarity SLTM polycrystalline bracket from 3M Unitek.

    Zirconia brackets have the greatest toughness among all ceramics and the potential advantage as an orthodontic bracket would be fewer in-service failures, for example fractured tie-wings. In addition, the surface of this ceramic is relatively smooth and it was initially thought that this would reduce the friction encountered when in contact with metallic archwires. However, zirconia never became established as an orthodontic bracket material owing to the opaque appearance of the resulting bracket and the fact that it has no significant advantage over polycrystalline brackets in laboratory tests.23

    Disadvantages of ceramic brackets

    All ceramics are significantly harder than stainless steel, however, they are also harder than tooth tissue and this can result in problems with abrasion of enamel and dentine on occlusion. It can be a particular problem with upper incisor teeth if ceramic brackets have been placed on lower incisors where there is an increased and complete overbite, during space closure and during overjet reduction, as upper canines are brought into occlusion with lower ceramic brackets. This has previously been reported as the most common form of injury following ceramic bracket use.24

    As previously described, ceramic brackets have a low fracture toughness and, in particular, a low tensile strength. The toughness of ceramic brackets is commonly between 20–40 times less than stainless steel25 and so they have a tendency to fracture both during treatment, for example tie-wing fracture, and at debond.26 With improvements in bracket design and manufacturing control, the strength of ceramic brackets has improved, reducing the risk of mid treatment fracture, but this may present additional problems at debond.

    Ceramics cannot chemically bond to diacrylate-based adhesive resins owing to their inert composition and so, to overcome this problem, manufacturers introduced glasses to the bracket base which were coated with a silane coupling agent to act as a mediator between the bonding resin and bracket base.27 The resultant bond strengths were extremely high and were associated with an increased risk of enamel fracture at debond.28

    In an attempt to mitigate this risk, manufacturers have instead used slots and grooves on the bracket base to promote mechanical adhesion between the bracket base and bonding resin. This has been shown to produce more predictable bond failure either within the adhesive layer or at the adhesive/bracket base interface.29 Other methods used to reduce the risk of enamel fracture include the incorporation of a polymeric bonding base on the ceramic bracket, for example CeramaFlex and latterly MXi brackets by TP Orthodontics (La Porta, Indiana, USA). At debond the polymeric base separates from the ceramic, thereby reducing the risk of enamel failure as the polymeric base and bonding adhesive are easily removed using a debonding bur. Indeed, the force required to debond such brackets is reduced to a level which is similar to that produced between the adhesive and a conventional stainless steel bracket base.30 One of the problems with the original CeramaFlex brackets was that the bond strength was reduced so much that the brackets would debond during treatment. An alternative approach to reduce the risk of enamel fracture at debond is the introduction of a vertical notch in the centre of the bracket which acts as a stress concentrator, as may be seen with 3M Clarity® bracket (3M Unitek, Monrovia, California, USA). Preferential bond failure will arise at this stress raising notch, when appropriate forces are applied at debond, rather than at the enamel surface and so the risk of enamel failure is considerably reduced. Indeed, much of the adhesive remains on the enamel, further reducing the risk of enamel, damage.31

    A number of authors have examined the various options for removal of ceramic brackets and it was concluded by Schwartz32 that debonding pliers that result in tensile bond failure within the bracket/adhesive interface provided the safest and most effective method of reducing the risk of enamel fracture.

    Plastic brackets

    Initially marketed in the 1970s, early problems identified with plastic brackets included staining, lack of strength and stiffness, tie-wing fracture and permanent deformation. Significant water absorption also resulted in discoloration of the bracket.33 Fracture of these early plastic brackets was thought to be related to this increased water absorption.34

    Thermoplastic resins, such as polycarbonate used in the fabrication of plastic brackets, are particularly prone to deformation under load. This can either occur immediately, due to the inherent elasticity of the material, or it can happen over time and is known as creep. Such deformation is particularly relevant when torquing forces are applied. Full expression of torque relies on full engagement and interaction of the internal aspect of the bracket slot with the archwire. Any change in the internal dimensions of the bracket slot as a result of deformation, either immediate or delayed, therefore has a huge impact on the expression of torque.

    In order to try to reduce the degree of deformation and also to improve other properties, fillers are now added to plastic brackets. Generally, fillers improve stiffness, produce a material with higher elastic limits, improved fracture resistance and better wear characteristics.35 A range of filler particles are commonly used, including silicon dioxide, aluminium dioxide, barium, zirconium oxide, borosilicate and barium aluminium silicate glasses. Bonding of the filler to the resin matrix is achieved by coating the filler particles with a silane coupling agent, such as methacryloxypropyl trimethoxysilane. These bonds degrade when exposed to water, as might occur intra-orally, and as a result such particles might be lost from the bracket surface with time.36

    The clinical performance of reinforced brackets, when compared to unreinforced brackets, has been examined by a number of authors, including Feldner et al,37 who concluded that polycarbonate brackets that had been reinforced with glass filler particles had clinically more acceptable torque values than unreinforced brackets and showed markedly less deformation.

    Another method of decreasing the deformation of the bracket slot is the inclusion of a metal slot liner (Figure 6).

    Figure 6. ElationTM, a metal-reinforced plastic bracket from GAC International.

    Work carried out by Harzer et al38 compared a conventional stainless steel bracket with a plastic bracket with and without a reinforcing metal slot liner. They reported significantly higher torque losses with the plastic brackets when compared to the metal bracket and, although the metal liner reduced these effects, it did not act to reduce torque loss to the extent anticipated. An interesting study by Sadat-Khonsari et al39 investigated the effect torque had on permanent deformation of a range of plastic/composite brackets following simulated clinical ageing. The brackets were heat cycled and stored in water to simulate the ageing process. Despite the addition of glass fibre or ceramic filler particles, the deformation of the brackets under torquing forces was not significantly different from unfilled polycarbonate brackets. Instead, it was found that a metal liner was necessary to resist the forces generated during torquing. The results showed a significantly higher degree of deformation than previously found and highlighted the environmental effect of the structural integrity of these brackets. It must also be remembered that the majority of investigations into torque loss from plastic brackets involve immediate testing of the initial viscoelastic response to an applied force. The issue of creep over a prolonged period of time is of as much clinical importance as the initial deformation.

    As with ceramic brackets, the frictional characteristics of plastic brackets has been investigated. Surprisingly, it has been found that composite brackets demonstrate lower frictional forces compared to both the stainless steel bracket and ceramic brackets, regardless of the wire size used or the method of ligation. It was also found that the presence of a metal slot liner had no discernible effect on the friction generated.40

    The use of filler particles, metal liners and improved production techniques have steadily improved the performance of plastic bracket systems to the point that they are now a viable alternative to either stainless steel or ceramic brackets. Certainly, when compared to ceramic brackets, the reduced risk of enamel wear, reduced archwire friction (experimentally at least) and the reduced risk of enamel damage at debond suggest that there is considerable potential in the more modern plastic brackets. Issues regarding expression of torque and resistance to creep still require further work but, undoubtedly, with the development of better materials these problems can be reduced and hopefully eliminated.

    Safety considerations

    The quest for improved performance often leads to new materials being used and, in 1997, a new plastic bracket composed of polyoxymethylene (POM) was released. This bracket had significantly improved resistance to fracture compared to polycarbonate brackets and improved colour stability with reduced staining. One of the major problems with POM is that it can depolymerize under certain conditions, such as thermal, chemical or mechanical challenge, and produce formaldehyde. Formaldehyde has the potential to cause allergic skin reactions, nervous system depression, liver and kidney damage. Depolymerization of these brackets has been shown to occur under thermal challenge and, given the potential severity of side-effects, the authors felt that the use of POM is contra-indicated in orthodontic and paedodontic materials.41

    The superior aesthetics of both plastic and ceramic brackets are a significant advantage over conventional stainless steel brackets. However, the concerns over clinical performance still exist. Significant developments in product design have vastly improved the performance of aesthetic brackets; modifications to manufacturing processes, slot design and base design have in some way addressed the major flaws of the early brackets. Further developments are required to ensure that the standards of clinical performance demanded from conventional brackets are carried over to aesthetic brackets.

    Self-ligating aesthetic brackets

    In recent years a number of self-ligating ceramic brackets have been introduced. The advantage of such brackets is that there is no need to use elastomeric ‘O’ rings which can discolour with time. However, they still require the use of elastomeric chains for space closure which can discolour. Difficulties in manufacturing self-ligating brackets with aesthetic moving clip or slide mechanisms mean the choice at present is limited. The Oyster bracket (GAC, Bohemia, NY, USA) and the new Damon Clear brackets (Ormco, Glendora, CA, USA) are the only ones that can be considered as truly aesthetic brackets. The Oyster bracket is manufactured from reinforced polycarbonate, whilst the Damon Clear bracket consists of a polycrystalline ceramic bracket with a polycrystalline sliding clip mechanism. Other alternatives include In-Ovation C (GAC, Bohemia, NY, USA), which consists of a polycrystalline ceramic with a rhodium-plated metal clip which has a frosted white appearance. The Clarity SL bracket (3M Unitek, Monrovia, California, USA) is also a polycrystalline ceramic bracket which combines the Clarity bracket with the Smartclip ligation system, and so has minimal metal showing on the labial surface.

    Aesthetic orthodontic accessories

    Both epoxy and TeflonTM coated short (Figure 7 and 8) and long ligatures are currently available for clinical use and from a number of different suppliers (Table 4). Laboratory studies have demonstrated that TeflonTM coated stainless steel ligatures produce less friction than other methods of ligation.42,43 Like the coated archwires, the coating tends to come off, exposing the underlying metal ligature. Although coated ligatures offer enhanced aesthetics, they should be used cautiously with ceramic brackets. If such a ligature should scratch the surface of a tie-wing during placement or removal, it will increase the chance of fracture of the bracket during service.

    Figure 7. Preformed Shorty Ties coated with Tooth ToneTM coating.
    Figure 8. Coated Kobayashi Shorty Ties with Tooth Tone™ coating.

    Accessory Product Manufacturer/Distributor
    Tooth-coloured coated preformed long ligature ties Confidential TM ligature ties ClassOne Orthodonticswww.classoneortho.com
    Preformed ligature ties Ortho Technologywww.orthotechnology.com/TOC www.toc-uk.com
    Preformed ligature ties Forestadentwww.forestadent.com
    Pearl-tone preformed wire ligatures Precision Orthodonticswww.precisionorthodontics.com
    Pre-formed coated ligatures DB Orthodonticswww.dborthodontics.co.uk
    Pre-formed ligature wire 3M Unitekwww.solutions.3m.co.uk
    Tooth-coloured coated preformed short ligature ties Preformed Shorty Ties Coated Kobayashi Shorty Ties Ortho Technologywww.orthotechnology.com/TOC www.toc-uk.com
    Preformed Kobayashi Ligature Ties Forestadentwww.forestadent.com
    Pre-cut Short Coated Ligatures DB Orthodonticswww.dborthodontics.co.uk
    Tooth-coloured NiTi open coil spring NiTi Open Coil Spring Ortho Technologywww.orthotechnology.com/TOC www.toc-uk.com
    Tooth-coloured crimpable hooks Tooth Tone Aesthetic Coated Crimpable Ball Hooks Ortho Technologywww.orthotechnology.com/TOC www.toc-uk.com

    An alternative to the coated ligatures are transparent polyurethane elastomerics. However, these suffer from two disadvantages, namely; they tend to discolour rapidly in the oral cavity due to absorption of food stuffs, such as turmeric, making them visible and hence negating any aesthetic advantage. Secondly, when used with ceramic brackets, it is difficult to tie a figure of eight owing to the increased size of the bracket tie-wings compared to metallic brackets.

    In addition, coated Crimpable Ball Hooks (Figure 9) and NiTi Open Coil Spring (Figure 10) with Tooth ToneTM (Ortho Technology, Tampa, Florida, USA) are available to make the fixed appliance less conspicuous.

    Figure 9. Crimpable Ball Hook with Tooth ToneTM coating.
    Figure 10. NiTi Open Coil Spring with Tooth ToneTM coating.

    Conclusion

    In conclusion, it can be seen that developments in aesthetic archwires, brackets and accessories have led to the availability of many products which can produce a clinically reliable labial fixed appliance for use in patients who would not otherwise undergo orthodontic treatment with conventional metallic fixed appliances. However, there is still scope for further improvements.