It was initially said that orthodontists who are good at wire bending would achieve excellent clinical results. With the advent of pre programmed brackets, Dr. Ronald Roth noted that ‘at the heart of every excellent treatment result lies a well-placed appliance regardless of the type of appliance used’.
Introduction
Orthodontic treatment involves the movement of teeth initiated in response to force application on the tooth crown. This results in the resorption and deposition of bone in the alveolar bone surrounding the root. To achieve this biological stimulus leading to tooth movement, force is delivered through attachment(s) called bracket or a similar accessory attached to the tooth. A bracket on a tooth is analogous to a handle that can push, pull or twist the tooth. Literal meaning of a bracket is an object that is attached to a surface and used to support or hold up something.
During Angle’s era in 1907, the earliest orthodontic attachments on teeth were gold clamps. Orthodontists used full-mouth stainless steel pinched bands with soldered or welded bracket attachments after the discovery of stainless steel. These stainless steel bands were tightly adapted to the contours of the tooth and cemented with luting agents, the process known as banding of teeth. As technology advanced, preformed bands with laser-welded attachments were introduced to improve the fit on dental crowns, which significantly reduced attachment failures.
The traditional orthodontic treatment involves metal bands for holding the attachments, which has several drawbacks. One of the major disadvantages was that it gave a metallic look to the patient’s smile, which was aesthetically unappealing. Other issues included decalcification of enamel, white spot lesions (WSLs) and residual band spaces at the end of the treatment. Banding needed prolonged chairside time during appliance strap-up, causing gingival irritation and sometimes tongue and cheek lacerations.
However, the introduction of successful bonding orthodontic attachments improved clinical practice significantly. After extensive research, Newman first demonstrated bonding techniques in clinical practice. Although bonding has several advantages on banding, it is not an entirely safe procedure. Table 47.1 refers to the benefits and limitations of bonding in orthodontics.
TABLE 47.1
Advantages and disadvantages of bonding in orthodontics
| S.no. | Advantages of bonding | Disadvantages of bonding |
|---|---|---|
| 1. | Improved aesthetics |
|
| 2. | Improved patient comfort | Technique-sensitive procedure |
| 3. | Higher accuracy in the placement of brackets | The smaller size of brackets and better access to tooth cleaning surfaces do not guarantee better gingival health. |
| 4. | Reduced gingival irritation | Loss of enamel with etching |
| 5. | Oral hygiene maintenance becomes simpler than banded appliance: relative lower caries risk | Bracket bond failures do occur, and subsequent bonds are relatively weak. |
| 6. | An absence of bands allows interproximal reduction (IPR) or a proximal restoration during appliance in situ | Re-bonding is more time consuming. |
| 7. | Saves cost and time | Risk of accumulation of plaque if adhesive extends beyond bracket base. |
| 8. | Elimination of band spaces at debond | Enamel demineralisation and white spots around bonded attachments. |
| 9. | Bonding of attachment to porcelain or amalgam is possible |
|
| 10. | No increase in arch length by the presence of band material on proximal surfaces of teeth | Incomplete removal of the resin tags after debonding can result in discolouration of the tooth surface. Patients may sometimes complain of their teeth becoming yellow or darker. |
History of bonding on dental tissues
The bonding process in orthodontics refers to mechanically locking the adhesive material to the base of the orthodontic attachment and the irregularities present on the tooth enamel or dentine surface. The success of the bonding largely depends on appropriate tooth preparation that creates microscopic irregularities on enamel, proper attachment base design, and the strong adhesive bonding material sandwiched between the tooth and the bracket base.
Buonocore, in 1955, advocated the use of acid for the preparation of the enamel surface to make it more conducive for adhesion by creating a 10–50 μm porous outer surface. He suggested enamel conditioning using 85% phosphoric acid for 30 s. , This process, called acid etching, enhances the topography of the enamel, changing from a lower-reactive surface to a surface that is more susceptible to adhesion. The structure of enamel comprising prisms and interprism crystals is revealed by acid etching. The demineralisation is selective because of the morphologic disposition of the prisms. The difference in angulation of the prism crystals causes the acid to have a higher demineralisation potential at certain microregions. . This porous surface of the enamel leads to the creation of ‘resin tags’ of the bonding adhesive into the enamel, giving a mechanical interlocking. Direct bonding of resin adhesive to enamel surface was a significant breakthrough in dentistry and orthodontics. Landmark events related to bonding in dentistry and orthodontics are compiled in Table 47.2 .
TABLE 47.2
Landmark events in the bonding in dentistry and orthodontics
| Year | Author(s) | Event |
|---|---|---|
| 1955 | Buonocore | Introduced etching tooth surfaces with phosphoric acid—85% phosphoric acid for 30 s |
| 1962 | Bowen | Bowen’s Resin—bisphenol A glycidyl dimethacrylate |
| 1965 | Newman | Epoxy resin bonding orthodontic resins—diglycidyl ether of bisphenol A with a polyamide curing agent |
| 1968 | Smith | Bracket bonding with zinc polyacrylate cement |
| 1970 | Retief | Bonding with epoxy resin |
| 1972, 1976 | Silverman and Cohen | Pioneers of indirect bonding technique |
| 1974 | Dentsply/Caulk (Milford, DE) | First single-paste ultraviolet light curable bracket adhesive |
|
|
|
| 1977 | Zachrisson | First detailed published large sample data on the post-treatment evaluation of direct bonding |
| 1979 | Takao Fusayama | Total etch technique |
| 1979 | Maijer and Smith | Crystal growth technique |
| 1979 | Ormco (Orange, CA) | Technique to braze mesh to a metal foil pad in orthodontic brackets |
| 1982 | Nobuo Nakabayashi | Resin reinforced hybrid layer |
| 1985 | Suh | Bonding to flourosed enamel |
| 1986 | Orec (Beaverton, OR) | Colour change adhesive |
| 1992 | Kanca J et al. | Moist bonding technique |
|
|
10% maleic acid and polyacrylic acid as etchants |
| 1995 | Silverman E | Light cure GIC for orthodontic bonding without acid etching |
| 1996 | 3M Unitek | Metal bracket system with pre-application of light-cured adhesive to the bracket base |
| 1998 | Ortho Solo (Ormco), Assure (Reliance Orthodontic Products), MIP (3M Unitek) | New generation hydrophilic primers for bonding |
| 1999 | Sondhi A | Sondhi Rapid-Set Indirect Bonding Adhesive |
| 2003 | Pro Seal (Reliance Orthodontic Products) | Light-cured, fluoride-releasing filled sealants for non-porous enamel protection |
| 2004 | Tsuchiya et al. | Acid–base resistant zone (ABRZ) |
| 2009 | Basaran G et al. | Nanofiller SEP adhesive use compared with other SEPs |
| 2010 | Select Defense (ClassOne Orthodontics, Carlsbad, CA) | Enamel sealant containing selenium as an antimicrobial |
On an etched enamel surface, the bonded adhesive material can attain shear bond strength (SBS) in the 17–20 MPa range, which far exceeds the minimum requirements of an orthodontic attachment in clinical practice.
Acid etching patterns have been described and studied using a scanning electron microscope (SEM). The enamel dissolution mainly occurs around enamel rods in the interprismatic substance. Several authors have classified enamel etch patterns based on variations in the quality and quantity of etched enamel. The first classification was given by Poole and Johnson, followed by Silverstone et al., Brännström et al., and Galil and Wright. Further, it was proposed that the presence of aprismatic enamel may limit the development of the etch pattern and result in reduced resin penetration, leading to a weaker bond strength although there was no conclusive evidence of it. ,
Silverstone et al. suggested three basic patterns of enamel dissolution after acid etching : Type I, Type II and Type III. These patterns were later modified into six distinct types, from Type I to Type VI. This was the most comprehensive and widely used classification system to date ( Table 47.3 and Fig. 47.1 ).
TABLE 47.3
Microscopic patterns of enamel following acid treatment
| S.no. | Types | Description |
|---|---|---|
| 1. | Type I | Honeycomb appearance with enamel prism centres lost. Most favourably present in coronal buccal surfaces of teeth. |
| 2. | Type II | Cobblestone appearance with enamel prism edges lost. Most favourably present in coronal buccal surfaces of teeth. |
| 3. | Type III | Pitted enamel with a map-like appearance. Most favourably present in the middle part of buccal surfaces of teeth. |
| 4. | Type IV | Granulation of enamel with numerous holes, with no preferential dissolution of cores or peripheries. It is most commonly found in cervical areas and less in occlusal areas. |
| 5. | Type V | There are no prism outlines, a smooth and flat enamel surface, and a decrease in irregularities for resin penetration. It can be seen in teeth with fluoride treatment or fluorosis and locally in the occlusal regions of teeth. |
| 6. | Type VI | When phosphoric acid is applied for 90 s, a stark difference is observed between the ground (G) enamel and unground (UG) enamel surfaces. |
Microscopic patterns of enamel following acid treatment type I to type VI.
Source: Galil KA Wright GZ. Acid etching patterns on buccal surfaces of permanent teeth. Pediatr Dent. 1979;1:230–4.
Etching gels and/or solutions: Compositions and timings
Acid etching agents used in dentistry are available as acid etching solutions and gels. These are available in different viscosities and are applied for different time durations on the teeth. Of these, the most used solution for etching is orthophosphoric acid, which is used in various concentrations. A study evaluating the effectiveness of different etch solutions applied for varied durations (15, 30 or 60 s) for enamel etching of mandibular premolars found that 37% wt./wt. phosphoric acid was more effective than 2.5% wt./wt. nitric acid when applied for 30 s. Enamel etching with an orthophosphoric acid solution is known to produce a more effective and uniform etch compared to acid gel; possibly due to the increased number and more even distribution of resin tags with the etch solution. However, there were no significant differences in tensile bond strength between solution and gel, keeping other variables similar. ,
Different concentrations and durations of phosphoric acid etching have been tested. , It was seen that 50% phosphoric acid used for 60 s produced substrate monocalcium phosphate, a monohydrate that is easily removed after rinsing, whereas a concentration of 27% produces dicalcium dehydrate that is not easily rinsed off. Hence, based on the dissolution of calcium and depth of etching, a concentration of 30%–40% was found to be most suitable by Silverman et al. Current practice guidelines recommend 37% orthophosphoric acid for 20–30 s.
Etching procedure
The procedure for etching is as follows:
-
•
Etchants, whether in gel or solution form, are applied to the tooth surface with the help of disposable applicator tips/needles supplied by the manufacturer.
-
•
The etchant acid is gently rinsed off after 15–30 s with copious water spray. It is advisable to use a combination of water and air spray from the three-way syringe to facilitate the complete removal of the etchant from the tooth surface. Care should be taken to be gentle with the flow using the three-way syringe. Excessive rinsing may lead to oozing out of gingival crevicular fluid/blood, which may contaminate the operating field.
-
•
The tooth surface is dried with airflow free from moisture and oil to give a dull and frosty look, indicating successful etching.
-
•
When isolation of the tooth is compromised, or there is any salivary contamination during etching, the procedure needs to be repeated.
Acid gels are easy to control in terms of flow compared with acid solutions. They are restricted to the areas required for etching and, therefore, preferred for bonding surgically exposed teeth.
Etching of dentine
Although bonding an attachment to dentine is rarely required in orthodontic practice, understanding the difference is essential to any dental professional since the etching and bonding to dentine are entirely different from enamel. This is mainly due to the difference in the composition of inorganic hydroxyapatite, which is 95% in enamel and 45% in dentine. Also, there is a difference in the arrangement of hydroxyapatite, which has a more regular presence in enamel than dentine. Furthermore, a 0.5–4 μm smear layer on dentine acts as a diffusion barrier and complicates the dentine bonding. Although early animal studies indicated that acid etching caused moderate to severe pulpal reactions, there is a high probability that the pulpal irritation may have been due to the microleakage of bacteria and their products. Acid etching of dentine using contemporary dentine bonding systems does not show these reactions, thus highlighting the possibility of sealing the dentine effectively with these agents.
It is known that acid etching increases the permeability and wetness of dentine; hence, hydrophilic resins, which bond to both intratubular and peritubular dentine, are required for a successful bond of adhesive resins. Future trends are focussing on lowering both the concentration of acids and the time of dentinal etching. Milder acids, such as 10% phosphoric acid, 10% maleic acid and 2.5% nitric acid, have been effectively tested for the same. Thus, sealing the dentine properly with the newer bonding agents is the key to successful etching of dentine.
Alternatives to acid etching
Phosphoric acid was first introduced for enamel etching by Buonocore. Subsequently, many other alternatives for etching were studied, including acids and chelating agents like pyruvic acid, maleic acid, ethylenediaminetetraacetic acid (EDTA), nitric acid and polyacrylic acid. However, many shortcomings were recorded with these agents including ill-defined etch patterns, reduced bond strength and undesired significant enamel dissolution.
Some important alternatives to acid etching are as follows:
-
1.
Crystal growth
-
As described by Maijer and Smith, this bonding method involves a dense growth of tiny needle-shaped crystals on the enamel surface. These crystals are formed by the reaction of the sulphate component in the polyacrylic acid liquid with the calcium on the surface of the enamel. The crystals further grow in the so-called ‘spherulitic habit’. The build-up of crystals serves as a retentive mechanism whereby the resin adheres to the enamel surface by a micromechanical interlocking when the orthodontic attachment is bonded to the tooth. This phenomenon starkly contrasts the adhesion in conventional etching that solely relies on the penetration of resin tags into the enamel structure.
-
Composition of crystals: The crystals comprise calcium sulphate dihydrate after 30–60 s of enamel conditioning with 40% polyacrylic acid, which contains 3.8% sulphate ions.
-
Properties: The crystalline interface demonstrated tensile bond strength equivalent to a conventionally acid-etched surface. The main advantage is seen at debonding, where the fractures occur primarily in the crystal/resin interface when the brackets are pulled off the teeth.
-
An ultrasonic scaler and routine polishing procedures can easily remove the remnant adhesive. After debonding in this technique, the change in enamel colour is much less compared to the conventional phosphoric acid etching, as resin tags have no penetration into the enamel.
-
Advantages of crystal growth procedure :
-
i.
Faster and quicker debonding, which poses less damage to the enamel surface.
-
ii.
Nominal effect on the outer enamel surface containing fluoride.
-
iii.
The enamel surface is clean with the absence of resin tags after debonding.
-
iv.
Fluoride incorporation in the crystal interface provides anti-cariogenic properties.
-
v.
Acceptable debonding and clean-up procedures which is due to minimal iatrogenic damage of enamel surfaces or colour change.
-
i.
-
-
2.
Air abrasion
-
Enamel pre-treatment with air abrasion technique uses a high-speed stream of aluminium oxide (Al 2 O 3 ) particles of 50/90 μm size, propelled by air pressure onto the enamel surface, a process named sandblasting. The conditioning of the enamel surface in air-abrasion results in surface roughening of the enamel for bonding the orthodontic bracket directly without acid etching. The resulting bond strength with this method is only half that of acid etching. Research has shown that sandblasting the enamel, followed by etching with orthophosphoric acid, improves the bond strength more than etching with phosphoric acid alone. ,
-
-
3.
Lasers
-
For dental intra-oral applications, CO 2 and Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) lasers are used. The mechanism employed for enamel conditioning by lasers involves melting and recrystallisation of the surface enamel, which causes surface irregularities by creating micropores that allow resin penetration. Orthodontic bonding requires the use of hard tissue lasers like Erbium-doped Yttrium Aluminum Garnet laser (Er:YAG, wavelength of 2940 nm) and Erbium, Chromium-doped Yttrium Scandium Gallium Garnet laser (ErCr:YSGG, wavelength of 2780 nm). Type III etching pattern is usually obtained with correct duration, frequency and power settings. Ozer et al. recommended 1mm or a non-contact mode for laser etching. Using enough power for effective enamel conditioning by lasers is imperative.
-
-
4.
Maleic acid
-
Maleic acid in a concentration of 10% for 15s creates a surface morphology similar to orthophosphoric acid. It produces identical bond strength while causing significantly reduced demineralisation compared with phosphoric acid etching. Thus, it should be explored as an effective, more conservative alternative to orthophosphoric etching in future studies by confirming the validity of these claims.
-
Bonding agents
Dental composite resin materials have the advantages of improved strength, abrasion resistance, translucency, ease of application and ability to be polished; however, the major disadvantage is their poor bonding mechanism to the natural enamel and dentine surfaces. Bonding agents enable a strong bond between the composite resin and the tooth structure to withstand mechanical forces and stress. The prerequisites of an ideal bonding agent are mentioned in Table 47.4 .
TABLE 47.4
Requirements of an ideal bonding agents
| 1. | Biologically safe in the oral cavity. |
| 2. | Sufficient bond strength for orthodontic applications. A minimum bond strength of 6-8 MPa is required. |
| 3. | Not harmful to the hard and soft tissues of the oral cavity. |
| 4. | Optimum performance in the oral environment. |
| 5. | Easy removal without harming the dental tissues. |
The orthodontic bracket and/or auxiliary adheres to the tooth surface through various interfaces. The irregularities or mesh type design of the base of the brackets facilitates mechanical interlocking with composite resin material. Adhesion between composite resin material and bonding agent is attained by co-polymerising the adhesive resin with the resin matrix of composite material. On the tooth surface, there is micromechanical interlocking between the bonding agent and irregularities created by the etching procedure in the form of resin tags penetrating the etched enamel or dentine. Two different types of bonding agents, enamel bonding agents (EBA) and dentine bonding agents (DBA), have been developed to provide bonding with natural tooth surfaces. Different bonding agents use different adhesion mechanisms, such as mechanical (micromechanical) adhesion, adsorption adhesion, diffusion adhesion or electrostatic adhesion. The prerequisites of an ideal bonding agent are mentioned in Table 47.4 and the required properties of an ideal bonding adhesive are mentioned in Table 47.5 .
TABLE 47.5
Properties and clinical performance of orthodontic adhesive
| 1. | Adequate fluidity and wettability |
| 2. | Ease of manipulation with adequate working and setting time |
| 3. | Adequate shelf life |
| 4. | Quick attainment of optimum strength for early wire placement |
| 5. | Withstand the stresses of masticatory forces and thermal recycling in the oral cavity |
| 6. | Dimensionally stable with minimum shrinkage or expansion |
| 7. | Capable of enamel remineralisation |
| 8. | Antimicrobial properties or at least poor biohostility |
| 9. | Similar refractive index as of attachment like plastic and ceramic brackets |
| 10. | No discolouration or staining with time |
| 11. | Close shade matching with the enamel |
Evolution of bonding agents
Bonding, in general, has revolutionised adhesive dentistry. Material scientists, chemists and clinicians have strived to develop better handling and improve the mechanical properties of these materials.
-
First-generation bonding agents (FIGBA) : Buonocore et al. demonstrated glycerol phosphoric acid methacrylate containing resin and cyanoacrylates to bond to acid-etched dentine. The hydrophilic phosphate group from glycero-phosphate dimethacrylate (GPDM) resin increases the bonding of hydroxyapatite with the calcium ions. In the late 1960s, Buonocore claimed that the formation of resin tags caused the primary adhesion of resins to the acid-etched enamel. Subsequently, in 1967, he introduced a composite material, a combination of resin incorporating microscopic particles of glass or quartz (fillers) into the resin base. This marked the beginning of the dental composites. This material had compromised bond strength in water, so Bowen recommended N-phenylglycine glycidyl methacrylate (NPG-GMA), which is believed to form water-resistant bonds with dentinal calcium. This further led to the development of the next generations of DBA. ,
-
Second-generation bonding agents (SEGBA) : These systems were introduced in the late 1970s; majority of them are ‘halo phosphorous esters’ of unfilled acrylic resins such as bisphenol A-glycidyl methacrylate (Bis-GMA) or hydroxyethyl methacrylate (HEMA). These systems bonded to the dentine by an ionic bond to the calcium in the dentine by chlorophosphate groups in the resin. However, they had the disadvantage of high solubility in oral fluids, where dentinal moisture could result in debonding and microleakage. ,
-
Third-generation bonding agents (TGBA) : These systems used an additional dentine conditioning step and an intermediate primer in combination with a bonding agent. The conditioning agents are added to modify or remove the smear layer before the adhesive resin placement. Bowen used a dentine conditioner of 2.5% nitric acid or ferric oxalate followed by primers N-tolylglycine glycidyl methacrylate (NTG-GMA) and PMDM (pyromellitic dimethacrylate). Further modifications were made by replacing ferric oxalate with aluminium oxalate. ,
-
Fourth-generation bonding agents (FOGBA) : These are clinically successful bonding agents, which are based on three steps: first is acid etchant application, second is a primer application and third is the application of actual bonding agent or bonding resin. Etching removes the smear layer, opens the tubular dentine and decalcifies peritubular and intertubular dentine. The primer molecules HEMA, biphenyl dimethacrylate (BPDM) or 4-methacryloyloxyethyl trimellitate anhydride (4-META) have both hydrophilic and hydrophobic groups. Hydrophilic groups subsequently wet the dentine surface and raise its surface energy so that the resinous material can bind and form a resin reinforced layer or hybrid layer. This hybrid layer improves the bond strength of the FOGBA compared to the previous generations. ,
-
Fifth-generation bonding agents (F th GBA): These involve two-step procedures, including etching and resin polymerisation. It classically consists of a single bottle system, combining the primer and adhesive into one solution to be applied simultaneously after etching enamel and dentine. The etching in this method is done for 15 s, followed by rinsing and then gentle air-drying to keep the dentine moist. The prime and bond is then applied in one to five layers, gently air-dried and light cured. These bonding agents usually comprise aluminium borosilicate glass, fumed silica, ethanol, barium, Bis-GMA, HEMA, GPDM, sodium, hexafluorosilicate and camphorquinone (CQ).
-
Sixth-generation bonding agents (SIGBA) : These generation systems consist of self-etching primers (SEPs) and self-etching adhesives. SEP is an aqueous solution of 20% phenyl-P in 30% HEMA. Combining etching and priming steps reduces the working time and minimises collagen collapse risk. The self-etching adhesives are all-in-one adhesives containing acidic unreacted monomer that contacts the composite resin directly. The method involves etch and prime in one application without rinsing and then gentle air-drying. The bond is then applied in one layer and then gently air-dried and cured. They are available commercially in a single solution comprising etching, priming and bonding agents. Studies indicate that these bonding agents adhere well to dentine (41 MPa). However, the bond strengths of these systems to enamel are 25% weaker than fourth-generation systems.
-
Seventh-generation bonding agents (S th EGBA): It is a single-step procedure with ‘no mix’ or ‘all-in-one’ solution which combines etch, primer and bond without rinsing, simply air-drying and then light cure. Clinicians prefer using a one-step procedure over multistep etch–rinse–prime–adhesives as it reduces the number and complexity of clinical steps. However, it poses a challenge of decreased shelf life and bond strength due to the water permeability and incorporation of all the chemicals in a single vial. This generation of adhesives has proven to have the lowest initial and long-term bond strength, which is a significant limitation in clinical practice.
-
Eighth-generation bonding agents (EIGBA) : In 2010, Voco America introduced Voco Futurabond DC (dual cure), which contains nanosized fillers. , Nanofillers with an approximate particle size of 12 nm are incorporated into the adhesive systems. Nanoparticles facilitate improved penetration of resin monomers and increased hybrid layer thickness, which strengthens the mechanical properties of the bonding systems. EIGBA have several advantages, such as greater bond strength, stress absorption and shelf life. The acidic hydrophilic monomers in these new agents facilitate easy application on the moistened and etched enamel. The EIGBA include three functional monomers (4-META, methacryloyloxydecyl dihydrogen phosphate [MDP] and methacryloyloxydecyl dihydrogen thiophosphate [MDTP]) but exclude HEMA. These warrant enhanced stability and bond strength to the tooth surface and to all indirect substrates, including composite restorations and crowns or restorations made of precious and non-precious alloys.
Advanced bonding agents
-
Self-etching primers (SEPs) : These were introduced to eliminate separate enamel or dentine preconditioning and moist post-rinse control. , , The newer SEPs have one acidic primer solution, combining conditioning and priming agents. They have several advantages, including decreased enamel loss, chairside time and saliva contamination. Furthermore, eliminating rinsing in SEPs might reduce the technique sensitivity during bonding. These advantages have made SEPs more popular in orthodontics at present times. Furthermore, the SBS of brackets bonded using SEPs or conventional acid-etch technique is nearly similar. Even a minimal etch pattern in SEPs can provide adequate SBS, thus proving beneficial.
-
The pH of acidic functional monomers used in SEP exceeds phosphoric acid etchants. SEP is provided in an aqueous solution to aid in ionising functional monomers. They comprise of HEMA that increases the wettability of tooth surface, with additional bifunctional or multifunctional monomers to establish cross-linking of the matrix. Currently, SEP may be classified as follows:
-
•
One-step adhesive: A single solution comprising hydrophilic and hydrophobic acidic functional monomers, water and organic solvents. It is an all-in-one adhesive system combining etching, priming and bonding. It is also called ‘universal or multimode adhesive’.
-
•
Two-step adhesives: It comprises two steps: the first step is the application of a hydrophilic etching primer to etch and prime the tooth surface concurrently. The second step involves the application of a layer of hydrophobic bonding agent after the evaporation of the solvent to seal the tooth surface irregularities. Based on the acid dissociation constants (pKa values), they can be categorised for their etching aggressiveness as strong, intermediately strong, mild and ultra-mild. The strong SEPs show etching like phosphoric acid with deeper demineralisation and can be used for enamel bonding. Mild SEPs remove the smear layer and form a thin hybrid layer; hence, they can prove more effective for dentine bonding.
-
•
-
Thus, SEP has broad applicability, including silane for glass ceramics and primers for metal alloys and polycrystalline ceramics.
-
-
Moisture-insensitive primer (MIP) : The introduction of hydrophilic primers was a possible solution to the issue of decreased bond strength under moisture contamination. Transbond MIP (3M Unitek, Monrovia, CA, United States) is effective in unconventional areas like tooth impactions and bonding of partially erupted second or third molars where it is challenging to achieve isolation from moisture.
-
Hybrid bonding agents : Composite resins, though used widely as orthodontic bracket–bonding agents, hybrid bonding agents pose concerns of plaque accumulation and enamel demineralisation around the resin-bonded brackets. These drawbacks were eliminated by developing alternative adhesives, including composite resin–glass ionomer hybrid cement, having fluoride-releasing properties for decreasing enamel decalcification and WSLs around brackets. Their nomenclature visible light-cured glass ionomer cement (VLC-GIC) has, therefore, been modified to light-activated water-based cement. The resin-modified glass ionomer cement (RM-GIC) or resin-modified glass poly-alkenoates, are dual cured by employing both acid–base reaction and addition polymerisation (either light or chemically activated). RM-GIC offers two significant advantages: adhesion and fluoride release.
-
Compomers : Compomers are hybrids between ionomers and composites with intermediate properties but are anhydrous. Their primary bond is through physical contact with dry surface interfaces. They contain carboxyl-modified resin monomers, yet their packaging as single-component materials suggests a limited reactivity between the components of acid monomers and alkaline glass. Studies illustrating the setting reaction of compomers show that the setting after the light activation is very low despite the acid–base reaction. There is still no confirmation of carboxyl group chelation to enamel or dentine. Besides, the release of fluoride from compomers, which facilitates fluoride recharging and caries inhibition, is lower than GICs but greater than resins.
-
Nanocomposites : These are a new class of polymer nanocomposites containing 0.005–0.01 μm nanofillers. Nanocomposites are reported to have increased strength, smoother texture, less bacterial adhesion and less polymerisation shrinkage. Research shows that nanocomposites may be comparable to the traditional orthodontic adhesive for bonding brackets if there is a modification in its consistency to be more flowable to adhere to the bracket base. Nanocomposites were also tested with EIGBA to reduce microleakage and enhance bond strength.
-
Bonding agents and techniques have evolved over the years; however, an ideal bonding agent that could fulfil all the ideal requirements is yet to be discovered.
-
Orthodontic bonding adhesives
The bonding adhesives have evolved from a two-paste mix with the self-cure system to a no-mix light cure system.
The early days of bonding in orthodontics saw two-phase products, but these had several limitations in handling and application and were time-consuming and cumbersome. The material introduced many defects when mixing the two components, including surface porosity and air voids, which inhibit the polymerisation reaction due to oxygen in the atmospheric air. Thus, these materials slowly became obsolete.
Eventually, no-mix adhesives introduced the concept of inhomogeneous polymerisation, which intended to curtail the mixing-induced defects and decrease the steps involved in applying adhesive. This system establishes a catalyst gradient from the primed enamel surface towards the bracket surface by diffusion. This forms a disturbed cross-linked network, which decreases the resin strength but facilitates bond strength similarly to conventional systems.
In light-cured adhesives, various factors govern the extent of polymerisation for a given monomer system. It usually varies with the exposure time, the concentration of the photoinitiator, the intensity of curing light at the peak absorbance wavelength of the photoinitiator, the spectral distribution of the light source and the volume proportion of filler particles, which may bring about light refraction and scattering, thus reducing the intensity of light in the bulk material. The peak absorbance wavelength of the initiator is the measure of light intensity given by the manufacturer, which is usually 468 nm for most systems.
The last decade has extensively shifted from conventional two-phase systems to newer light-cured adhesives. Additionally, the advent of MIPs and water-activated adhesives has addressed the issues of bonding in the presence of moisture. Further, incorporating constituents that enhance the antimicrobial properties of adhesives is one of the significant developments in this field. Most contemporary adhesives are hybrid nanocomposites supplied in a dispensable dark tube for convenient handling.
Bonding techniques
One of the most critical steps in the process of clinical orthodontic practice is the bonding of orthodontic attachment to the teeth. Accurate and successful bonding greatly influences the treatment progress and attaining the final positions of the teeth in individual arches and occlusion.
The following aspects of bonding are most important:
-
1.
The technique of bonding to achieve adequate strength and maintain the health of the dental tissues.
-
2.
The precision in orthodontic attachment placement to achieve the desired tooth position.
The bracket position and placement vary with the type of orthodontic appliance, but the technique of bonding is usually classified as follows:
-
A.
Direct bonding technique
-
B.
Indirect bonding technique
-
A.
Direct bonding technique:
-
This refers to directly bonding brackets or attachments on the tooth surfaces. Direct bonding offers several advantages, including patient and operator convenience and reduced chairside time. Direct bonding is a frequently practised technique compared to indirect bonding, but the precision and accuracy of positioning brackets in this technique depend on operator’s experience and clinical expertise.
-
-
B.
Indirect bonding technique:
-
This technique was introduced by Silverman et al. in 1972.
-
Indirect bonding includes the following steps:
-
I.
Positioning of brackets on working casts with the aid of water-soluble or intermediary adhesives.
-
II.
This is followed by the fabrication of a transfer tray to transport the brackets to the patient’s dentition.
-
III.
The bonding is then done with orthodontic adhesives, which are either self-cured or cured with light emitting diode (LED) unit.
-
I.
-
The advantage of this technique over direct bonding is greater precision in bracket placement. Nevertheless, it is more technique-sensitive and time-consuming than direct bonding. The most used bonding agent in current times for indirect bonding is the Sondhi indirect bonding (IDB) agent.
-
Direct bonding technique
The direct bonding technique is the most common procedure in most orthodontic clinical practices ( Fig. 47.2 ).
Bonding is preferably carried out using a four-handed approach, with the orthodontist and the dental assistant (or dental resident, as seen above) working in unison.
The orthodontist carries out the main bonding procedure, while the assistant maintains an isolated oral environment for bonding and assists with instrumentation and the use of light emitting diode (LED) unit.
Armamentarium required for direct bonding:
-
•
Well-functioning dental chair unit with oil and moisture-free air, clean water and two three-way syringes.
-
•
The water used for irrigation spray through the three-way syringe should be free of any turbidity and should be sterile.
-
•
The compressed air should be free of moisture and oil.
-
•
An efficient noiseless suction with a transparent and disposable suction tip should be easy to bend for proper adaptation. Nola Dry Field System (Nola Specialties, Hilton Head, SC) works best for adequate isolation and contamination free working area.
-
•
The chair light should be shadowless with the capacity for an adjustable intensity.
-
•
Magnifying loupes with provision for LED light with amber filter for better visualisation.
The following instruments are arranged in a ‘tray setup’ labelled as a ‘bonding tray’:
-
•
Low-speed handpiece, polishing brushes and oil-free polishing paste.
-
•
Bracket height marker (Boone gauge).
-
•
Bracket positioner.
-
•
Bracket holding tweezers with preferably reverse tweezer action with different tips to facilitate holding of various sizes and shapes of attachments.
-
•
Regular tweezer with a lock/catch.
-
•
A hand scaler.
-
•
Nola/similar Dry Field System/self-retaining cheek retractors/tongue guard.
-
•
Adsorbent cotton rolls.
-
•
Dry angles absorb salivary flow from parotid and sub-mandibular ducts.
-
•
Disposable brushes/applicator tips.
-
•
Magnifying mouth mirror.
-
•
Sharp probe.
-
•
Williams graduated probe.
-
•
Bonding adhesive, primer and etching kit.
-
•
Efficient light-curing unit.
The instruments and armamentarium should be sterilised before clinical use ( Figs 47.3–47.4 ).
Armamentarium required for the bonding procedure.
Light-cure adhesive bonding agents are preferred over self-cure bonding agents.
Shows the instruments required for the bonding procedure.
Bonding tweezers and a bracket positioning gauge are essential bracket positioning devices.
Clinical steps in direct bonding
The steps involved in bonding procedures must be explained to the patient in detail. It is always a good practice to make the patient watch a video or observe a similar procedure being done on another patient to alleviate apprehensions and/or any fear.
The teeth to be bonded should be examined for the quality of enamel, aesthetic or metal restorations and any white spots, fractured cusps, attrition of teeth, cracks or other hard tissue defects. These findings should be documented appropriately in case records and recorded in intra-oral photographs.
It is necessary to decide the philosophy of bracket positioning as per the appliance prescription, the patient’s clinical requirements and the treatment planned by the orthodontist beforehand. The sequence of bonding the teeth should be pre-discussed with the dental assistant. The steps of the bonding are as follows:
-
1.
Cleaning and polishing: The patient is asked to brush thoroughly before starting the procedure. The tooth surface is polished with a rubber cup or bristle brush using low speed (<20,000 rpm) micromotor using low abrasive, high quality, oil-free pumice without glycerine. The polishing with pumice is believed to remove the organic pellicle layer and expose the aprismatic layer of enamel for resin penetration, resulting in a 50% increase in bond strength ( Fig. 47.5 A–B).
Figure 47.5 Steps in isolation and etching in the direct bonding.
(A) Cleaning with rubber cup using polishing paste without glycerine or only water slurry with pumice. (B) Cleaning with a bristle brush using bicarbonate based polishing paste with a particle size of 70 µm without glycerine or only water slurry with pumice. (C) Marking horizontal line suggestive of bracket slot height from incisor edge on each tooth. (D) Marking vertical line suggestive of bracket orientation along the long axis of each tooth. (E, F) Etchant is precisely applied in the area to be bonded with the orthodontic bracket, ensuring that the etchant occupies the optimal amount of enamel surface. (G) Water from three-way syringe is used to remove the etchant. The direction of the water spray should be from gingival to occlusal to prevent injury to and oozing from the gingival sulcus. (H) The etchant is gently removed with water from the three-way syringe under heavy duty suction with a broad nozzle to collect the etchant and water. (I) Gentle airflow is used to dry the enamel surface until a ‘frosty white appearance’ is observed uniformly. The direction of the airflow should also be from gingival to occlusal. (J) After the process of etching and drying, moisture control is the most crucial aspect until the bonding of the orthodontic brackets.
