Aesthetic labial orthodontic appliances – an update

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 Teflon^TM.² In addition to the improvement in aesthetics offered by these wires, in vitro studies have demonstrated that Teflon^TM coated wires can reduce frictional losses to less than 10%, compared with non-coated wires³ and that the Teflon^TM 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 Teflon^TM 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.

Table 1. Examples of coated aesthetic metal archwires currently available.

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 Tone^TM 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).

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.

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).

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.

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

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).

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.

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 Teflon^TM 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 Teflon^TM 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.

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 Tone^TM (Ortho Technology, Tampa, Florida, USA) are available to make the fixed appliance less conspicuous.

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.