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Bonding with glass ionomer cements and resin-modified glass ionomer cements
Bonding with glass ionomer cements and resin-modified glass ionomer cements
16
C. Rahiotis, S. Schricker
16.1
Introduction
Orthodontic brackets are cemented either to labial or to lingual tooth surfaces and act as a medium for the delivery of forces applied by the archwire and auxiliaries to the teeth. The factors that are the main contributors for the successful transfer of orthodontic forces to a tooth include the following: the preparation of the enamel surface for bonding; the type of adhesive cement used; and the shape, material, and surface finish of the bracket.1e3 Among these factors the adhesive cements have a key role in this procedure. The ideal cement used for orthodontic bracket bonding should exhibit enough retention to resist displacement during normal oral function and transmit the required orthodontic forces to the tooth. Furthermore, it should be easily removed once the treatment is complete, without causing any damage to the tooth surface and, ideally, without leaving residues that need to be removed by drilling or air abrasion.4 Commonly used adhesives for bonding brackets to teeth have been the glass ionomer cements (GICs) and resin-modified glass ionomer cements (RMGICs). Each adhesive displays a different adhesion mechanism. This chapter presents the chemistry of the GICs and RMGICs, discusses their manipulation, bonding, and debonding in orthodontics, and concludes with comments on the causes of bond failures.
16.2
Conventional glass ionomer cements
The conventional glass ionomer materials form a chemical bond through calcium ions between the enamel surface and the bracket.5e7 During the last two decades the beneficial properties of GICs and the composite resin adhesives (discussed in Chapters 9 and 10) have been combined into the new category of RMGICs, where the acidbased curing reaction of the GIC has been combined with the light-activated polymerization of 2-hydroxyethyl methacrylate and a methacrylate-functionalized polyacid.8 Fig. 16.1 shows the structure of a methacrylated polyacid and the reaction with 2-hydroxyethyl methacrylate in parallel with the acidebase reaction. GICs were first described by Wilson and Kent, and their physical properties were a combination of those of silicate and polycarboxylate cements.9 Their long setting time, Orthodontic Applications of Biomaterials. http://dx.doi.org/10.1016/B978-0-08-100383-1.00016-3 Copyright © 2017 Elsevier Ltd. All rights reserved.
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Figure 16.1 The dual-cure reaction of a resin-modified glass ionomer cement.
poor durability, high water absorption, and solubility subsequently led to the development of ion-leachable glass with better physical and clinical properties.10 Currently, the GICs are a group of materials based on the acid/base reaction between polyalkenoic acid and a basic ion-leachable silicate glass. They bond to dental hard tissues because of ionic interactions between the calcium ions of the hydroxyapatite of the enamel and carboxylic groups of the cement.11 The polyalkenoic acid is typically a copolymer of acrylic acid with a variety of comonomers, notably itaconic acid. Fig. 16.2 shows the chemical structure of polyacrylic acid along with several comonomers. The reaction between the polyacid and the acidic base will release calcium and aluminum ions that will bind to the carboxylic groups of the polyacid. These ions CH2COOH
CH2∙CH
CH2∙C CH2∙CH COOH COOH
COOH Poly(acrylic acid)
Poly(acrylic acid-co-itaconic acid) COOH CH2∙C CH2∙CH COOH CHCH2COOH COOH
COOH CH CH CH2∙CH COOH COOH Poly(acrylic acid-co-maleic acid)
Poly(acrylic acid-co-3-butene 1,2,3 tricarboxylic acid)
Figure 16.2 Structures of common polyacids.
will serve as cross-linkers to stabilize the polyacid gel network. Fig. 16.3 shows polyacid chains, depicted as wavy lines, cross-linked by ions. The basic glass is now bound in the polyacid gel matrix. The GICs possess a combination of properties that are potentially useful in clinical orthodontics. First, they adhere to different substrates such as enamel and metal.12 For bonding to enamel, GICs have the advantage of achieving a chemical bond without
Bonding with glass ionomer cements and resin-modified glass ionomer cements
COOCa2+
COOCa2+ COO-
COO-
255
COOAl+++
-OOC
COO-
Figure 16.3 Salt bridges that cross-link the polyacids.
etching, reducing the clinical steps while protecting the enamel surface from dissolution.13 The polyalkenoic acids slightly pit the enamel to form a thin hybrid layer, and its removal leads to less enamel damage compared to that caused by the removal of composite resins.14 Another advantage of GICs is their ability to release fluoride for long periods of at least 12 months.15 They also have the ability to absorb fluoride from different sources such as fluoride toothpastes, recharging their fluoride reservoir.16 The deposition of fluoride in the cement around the orthodontic bonding area leads to a decrease of the populations of Streptococcus mutans and Lactobacillus bacteria,5,17 which consequently leads to less enamel demineralization and less white spot formation.18,19 To increase more the antibacterial spectrum of GICs, studies suggest the incorporation of either chlorhexidine (CHD)20 or ethanolic extracts of propolis (EEP),21 which seem to increase the antibacterial effect of GICs for a long period of time. The addition of CHD does not significantly influence the shear bond strength, whereas EEP seems to slightly enhance the mechanical properties of GICs. Another way to increase the antibacterial ability of GICs is the incorporation of zinc oxide, which serves as an activator of enzymes that can be toxic to microbes at concentrations as low as 0.5 ppm.22 While its antimicrobial effect lasts at least for 1 month the mean bond strength of a GIC mixed with zinc oxide falls in the lower range of shear bond strength recommended for GICs, decreasing even more as the concentration of ZnO increases.23 With regard to saliva contamination and the bonding field, conventional GICs are less demanding to apply in comparison to composite resins, since they are not hydrophobic. Accordingly, the speed and ease of bracket placement are increased. Besides their advantages, GICs appear to have disadvantages as well, such as their slow curing reaction and the weak, yet clinically acceptable, bond strength. The latter is initially low and reaches its maximum value 24 h after bonding. Nevertheless, the relatively weak bond strength is a feature that makes GICs easy to remove when orthodontic treatment is completed.
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Enamel preparation
There are no clear guidelines about the best method of enamel treatment prior to bonding with conventional GICs, yet there are several contradictory suggestions. Some authors suggest that pretreatment of the enamel with pumice slurry and acids, such as polyacrylic acid or tannic acid, for 5 s, improves bond strength to about 60% of the value achieved using an acid-etching enamel/composite resin technique.24 Others suggest that no further pretreatment than drying the tooth with a cotton swab is required. With regard to pretreatment of the enamel surface with phosphoric acid, the outcome is disputed. Some investigators found that it produced a significantly poorer bond,25 while another investigator26 found an increase in mean bond strength, although it was not significantly greater than that of unetched enamel. Recently, the use of 5.25% sodium hypochlorite as a deproteinizing agent has been suggested, since it is claimed to remove the organic elements both from the enamel surface and the biofilm acquired by the GICs, thereby increasing the bond strength.27 Enamel deproteinization with NaOCl increases the bond strength of metal brackets bonded with either conventional or RMGICs, yet not significantly.28 Apart from all of the above, sandblasting the enamel surface for 3 s seems to increase, significantly, the in vitro shear bond strength and the mean survival time of metal brackets having a mesh base that are bonded with a GIC.29
16.2.2
Effect of powder/liquid ratio
The powder/liquid ratio utilized for preparation of a GIC is critical for successful bonding. However, it seems to vary among different studies. Some investigators followed manufacturer instructions,30,31 whereas other groups used a slightly thicker mix32 or encapsulated glass ionomers.33 The encapsulated GICs have higher powder/liquid ratios than hand-mixed cements, which seem to be clinically acceptable. The manufacturer instructions for mixing GICs depend upon their use as filling materials, bases, or liners in restorative dentistry. There are no guidelines about material proportioning for orthodontic bonding. An ideal powder/liquid ratio for orthodontic use would provide flow properties, while optimizing bond strength.34
16.3
Resin-modified glass ionomer cements
To overcome the disadvantages of both composite resins, such as white spots formation, dry bonding field, and enamel destruction, and GICs, such as low bond strength, a new category of bonding materials was created, the RMGICs. These new materials incorporate a resin matrix (hydrophilic monomers, such as hydroxyethylmethacrylate (HEMA) and polymerization initiators) into GICs. Light-activated RMGICs have the advantages of GICs (fluoride release, even if this seems to be a little inferior to that of GICs, chemical bonding to both enamel and metal, and adhesion in moist fields) and the mechanical and physical properties of composite resins.35 Clinically, RMGICs eliminate the demand for working in a dry field, along with the need for etching
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and priming enamel surfaces, and they enable bracket repairs to be made quickly and easily. According to a number of studies, RMGICs appear to achieve lower bond strengths in comparison to composite resins, yet higher bond strengths in comparison to conventional GICs.36e39 To date, in vitro studies have shown that resin-reinforced GICs retain brackets significantly better than conventional GICs.40,41 The fact that RMGICs combine the ability of bonding to moist enamel surfaces with the ability to release fluoride makes them ideal as lingual retainer adhesives. Conventional GICs sustain their maximum strength after 24 h. The resin addition to the cement formulation has facilitated light curing, allowing a snap set and rapid development of strength.42 RMGICs are usually encapsulated and dual cured. Once the liquid and powder are mixed, both the acidebase reaction and the light-initiated free-radical polymerization of resin occur. The resin phase polymerizes quickly, whereas development of the glass ionomer phase proceeds slowly via an acidebase reaction over a period of time, with the material achieving its full bond strength 24 h after its application. Light curing activates the free-radical polymerization of HEMA and two other monomers, thus forming a poly-HEMA matrix and causing an immediate setting of the material.
16.3.1 Enamel pretreatmentdprimers Before the introduction of RMGICs, most previous studies followed the recommendations from manufacturers and used 10% polyacrylic acid for surface treatment before bonding. Polyacrylic acid contains carboxylic acid functional groups potentially capable of bonding to the tooth surface.43 On the other hand, with conventional composite resin, 37% phosphoric acid is popular for surface treatment before bonding. Phosphoric acid creates microporosities on the enamel surface, into which resin tags extend after curing, forming a mechanical bond to the enamel surface. It is observed that teeth conditioned with 10% polyacrylic acid provide bond strength significantly lower than teeth conditioned with 37% phosphoric acid before bonding with RMGICs.44 Increasing the polyacrylic acid concentration to 20% results in an eightfold increase in bond strength, though still significantly lower than that achieved with phosphoric acid.45 Self-etching primers have also been applied to the enamel surface prior to bonding with RMGICs, resulting in even greater bond strength values, in comparison to both polyacrylic acid and phosphoric acid.46 Etching the enamel surface with 37% phosphoric acid produces higher bond strength, similar to that of composite resins, when compared to nonetched surfaces,47 while etching with polyacrylic acid seems to provide a similar bond strength to that of nonetched surfaces. When RMGICs are used as adhesives, the bond strength seems to increase even more when the enamel surface is contaminated either with saliva or plasma. Moisture is required for optimal adherence of both GICs and RMGICs, which contain hydrophilic monomers. Sandblasting could also be applied in the case of bonding with RMGICs.
16.3.2 Cytotoxicity Despite the improvement in mechanical properties, the cytotoxic effects of RMGICs are of more potential concern in comparison to conventional GICs, since they
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incorporate monomers such as HEMA along with initiators. As with composite resins, incomplete polymerization results in inadequate conversion of monomers into polymers, and these residual monomers can cause a significant cytotoxic effect when liberated into the oral environment. There is a direct relationship between the degree of conversion and RMGIC cytotoxicity, which, after the initial polymerization, decreases over time.48
16.4 16.4.1
Bonding to different substrates than enamel Introduction and perspective
This section presents recommendations and protocols for the bonding of brackets to different substrates other than enamel that are encountered for orthodontic patients. Generally, the quality of bonding is assessed by some laboratory test that evaluates bond strength from the measurement of debonding force. While such tests can provide useful comparative information, the limitation of their efficacy is discussed at length in Chapter 11.
16.4.2
Amalgam
In adult orthodontic patients, and occasionally in adolescents as well, amalgam restorations exist on the buccal surfaces of posterior teeth. In such cases, successful bonding of orthodontic attachments to amalgam surfaces is challenging. This clinical problem led to the investigation of several procedures to improve the bond strength in such cases. These procedures include surface treatment and the use of intermediate resins and adhesives that chemically bond to metals. Surface treatment procedures include roughening the amalgam surface with a diamond bur,49 sandblasting,50,51 galliumetin (GaeSn) liquid application,52 and chemical corrosion.53 Sandblasting is the most common method used for surface preparation, since it creates scratch-like irregularities that increase bond strength. In addition to mechanical retention, bonding to metal has the advantage of chemical adhesion. Therefore GICs and RMGICs chemically bonded to amalgam are recommended.53 The mean bond strength values of stainless steel orthodontic brackets bonded to amalgam surfaces are significantly lower than those of brackets bonded to etched enamel, yet clinically acceptable.54 Bond failures occur at the amalgam-adhesive interface, regardless of the adhesive system, and without any damage to the amalgam restoration.
16.4.3
Ceramics
Apart from amalgam restorations, many adult patients seeking orthodontic therapy have metal-ceramic and all-ceramic restorations. When bonding to dental porcelain or other dental ceramics, adequate bond strength is desired, with easy removal to avoid damage of the restored teeth. Several techniques have been used to bond brackets to porcelain surfaces, and they differ in the surface preparation and bonding agents
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applied. The adhesives used to bond brackets to ceramics (composite resins, RMGICs) seem to provide similar bond strength values.56 Surface preparations include the application of different acids (orthophosphoric or hydrofluoric), treatment by different laser techniques, roughening by diamond burs, sandblasting, and silanization. The use of hydrofluoric acid (HF) greatly increases the bond strength. This is due to the ability of the acid to react with the silica phase, creating micromechanical retention through the formation of microchannels. Over time, the glassy matrix partially dissolves, and the formation of such retentive channels increases. A longer etching time increases the bond strength, as it allows the acid to react with the ceramic matrix further. However, considering the harmful effects of etching with HF, mechanical roughening with sandblasting or diamond burs is recommended. In any case, the bond strength of brackets bonded to porcelain is further improved by the application of silane, which has the ability to form chemical bonds with inorganic and organic surfaces.57,58 The conventional techniques of HF etching and silanization, sandblasting and silanization, orthophosphoric acid etching and silanization, and HF etching alone show higher shear bond strength values than laser etching in combination with silane application, whereas orthophosphoric acid etching alone and sandblasting alone show lower bond strength values than laser application alone.59,60 The neodymium-doped yttrium aluminium garnet laser seems to be an acceptable substitute for HF etching; however, the erbium-doped yttrium aluminium garnet laser is not an acceptable option.61 The best reported protocol for bonding to porcelain is acid etching with 9.6% HF, rinsing for 30 s, air drying, and silanization.62 However, there are differences between various ceramic surfaces and brands, such as dissimilar particle sizes and microstructural phases, leading to higher or lower bond strengths. Higher shear bond strength values are presented by Empress II (lithium disilicate; Ivoclar) and Finesse (leucite-containing porcelain; Dentsply); the metal-ceramic surface, and In-Ceram (alumina matrix; Vident) have comparable shear bond strengths; and IPS Empress (leucite-containing porcelain; Ivoclar) shows the weakest bond strength among these ceramics.63e66
16.4.4 Bleached teeth Various bleaching agents and methods exist to whiten discolored teeth at the dental office or at home. The results of studies about the effect of bleaching on the bond strength of orthodontic brackets are ambiguous. One study reported no adverse effect on the bond strength of orthodontic brackets.67 Another study reported considerable reduction of bond strength values subsequent to bleaching.68 The reduced bond strength may be attributed to several sources. There can be alterations in the microstructure of bleached enamel surfaces after acid etching, including reduced microhardness and calcium loss, and overetching can cause the loss of enamel prisms.69 Another potential mechanism is the release of oxygen radicals on the enamel surface by residual components of bleaching agents.70 To avoid bonding failure on bleached teeth, several methods have been proposed. These include (1) avoidance of the bleaching process until the orthodontic treatment is completed, (2) delay of bracket bonding up to 4 weeks after bleaching, and
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(3) pumicing the bleached teeth and the application of antioxidant agents, such as 10% sodium ascorbate or 10% a-tocopherol, prior to bracket bonding to neutralize the effect of released oxygen radicals from residual components.70
16.5
Debonding
When a bonded bracket is removed, failure can occur at one of the three interfaces: between the adhesive and enamel surface, within the bonding material itself, or between the cement and the bracket. The interface between the adhesive cement and the bracket is the usual site of failure,71 and the remaining adhesive must be removed. The use of adhesive-removal pliers may cause pain, while physical changes to the enamel can occur as well, ranging from surface roughening to microscopic fractures. A wide variety of instruments and procedures have been introduced as a result of the search for an efficient and safe method of adhesive removal after debonding. These include (1) manual removal with the use of a scaler or a band-removing plier, (2) various shapes of tungsten carbide burs with high- or low-speed handpieces, (3) specially designed burs that are less aggressive to the enamel, (4) Soflex discs (3M ESPE), (5) special composite resin finishing systems with zirconia paste or slurry pumice, and (6) ultrasonic applications; methods such as CO2 laser application or powder abrasive systems are quite promising.72 The color of the enamel can also be affected both by debonding and the subsequent cleaning procedures.73 Changes in the color of the enamel may also result from the discoloration of the residual resin that has irreversibly penetrated the surface, despite the cleaning procedures. The average depth of penetration ranges between 8 and 15 mm, with maximum tag lengths ranging up to 50 mm. Removing all these residues would result in a considerable loss of sound tooth structure. Most desirable would be the availability of a bonding agent having both a minimal discoloration potential and a simple protocol for removal of its residue. When the brackets are bonded with the etchand-rinse or self-etching systems, removing the adhesives with Stainbuster (Abrasive Technology) burs is recommended, whereas for the RMGICs, tungsten carbide burs may provide less enamel discoloration over long time periods. The combination of etch-and-rinse system and tungsten carbide burs is not recommended for clinical use, since this seems to cause the greatest color change.74 In general the weaker chemical bonding between GICs/RMGICs and enamel facilitates the cleanup process of removing the adhesive remaining on the enamel surface after debonding, in comparison to the cleanup process when brackets are bonded with composite resin.
16.6
Bond failures
Bonded brackets should ideally remain attached to the tooth surface throughout the whole treatment, and the bond strength of the adhesive material should be sufficient to resist tensile, shear, torque, and peel functional stresses.75 Bond failure, however,
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is encountered frequently during treatment and may be influenced by the microstructure of the bracket base, the etching time, the etching system, the bonding agent, and the bonding technique used. In addition, factors related to the operator, such as moisture control during bonding procedure, choice of bonding material, choice of brackets, or instructions given to the patients, and patient factors, such as sex, age, malocclusion, and dental hygiene, are likely to influence the failure rate of any bonding system. Most fixed-appliance orthodontic treatments last about 18 months. The more limited the bond failure is during this period of time the better for the clinical result. The acceptable levels of bond failure for in vivo use are 4e10%.76 According to the literature, composite resins have an average failure rate of about 6% (study results range between 4.7% and 8.3%).76 These failure rates are comparable to those of RMGICs, when applied in combination with prior etching. Their average failure rate is 7% (with studies reporting failure rates from 5% to 8.9%). Higher failure rates are expected when the enamel is dried prior to bonding with RMGICs.77 Both composite resins and RMGICs have significantly lower bond failure rates than GICs, which seem to have a failure rate ranging from 12% to 50%.76 The disadvantage of extra bracket failures appears to outweigh any potential advantages when considering GICs for bonding of orthodontic brackets. Even though bonds can fail on any tooth at any time, some generalizations have been made: (1) most failures occur at the bonding visit or some time before the first postbonding visit, (2) incisors and canines have fewer failures than premolars, (3) maxillary canine bonds are more successful than mandibular canine bonds, and (4) bonds on anterior teeth separate more at the bracketeresin interface, whereas bonds on posterior teeth are more likely to demonstrate an enameleresin failure.78 In summary, in any case bond failure is of particular concern clinically, and its cause should be ascertained and addressed accordingly whenever possible.
References 1. Barry GR. A clinical investigation of the effects of pumice prophylaxis on band and bond failure. Br J Orthod 1995;22:245e8. 2. Bearn DR, Aird JC, McCabe JF. Ex vivo bond strength of adhesive precoated metallic and ceramic brackets. Br J Orthod 1995;22:233e6. 3. Bin Abdullah MS, Rock WP. The effect of etch time and debond interval upon the shear bond strength of metallic orthodontic brackets. Br J Orthod 1996;23:121e4. 4. Banerjee A, Paolinelis G, Socker M, Watson TF, McDonald F. An in-vitro investigation of the effectiveness of bioactive glass air-abrasion in the selective removal of orthodontic resin adhesive. Eur J Oral Sci 2008;116:488e92. 5. Hallgren A, Oliveby A, Twetman S. Fluoride concentration in plaque adjacent to orthodontic bracket appliances retained with glass ionomer cement. Caries Res 1993;27:51e4. 6. Millett DT, McCabe JF, Bennett TG, Carter NE, Gordon PH. The effect of sandblasting on the retention of first molar orthodontic bands cemented with glass ionomer cement. Br J Orthod 1995;22:161e9. 7. Millett DT, McCabe JF. Orthodontic bonding with glass ionomer cement e a review. Eur J Orthod 1996;18:385e9.
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8. Silverman E, Cohen M, Demke RS, Silverman M, Linwood NJ. A new light-cured glass ionomer cement that bond brackets to teeth without etching in the presence of saliva. Am J Orthod Dentofac Orthop 1995;108:231e6. 9. Wilson AD, Kent BE. A new translucent cement for dentistry. Br Dent J 1972;132:133e5. 10. Walls AW. Glass polyalkenoate (glass-ionomer) cements: a review. J Dent 1986;14: 231e46. 11. Voss A, Hickel R, M€olkner S. In vivo bonding of orthodontic brackets with glass ionomer cement. Angle Orthod 1993;63:149e53. 12. Hotz P, McLean JW, Sced I, Wilson AD. The bonding of glass ionomer cements to metal and tooth substrates. Br Dent J 1977;142:41e7. 13. van Dijken JW. The effect of cavity pretreatment procedures on dentin bonding: a four-year clinical evaluation. J Prosthet Dent 1990;64:148e52. 14. Charles C. Bonding orthodontic brackets with glass-ionomer cement. Biomaterials 1998; 19:589e91. 15. Maijer R, Smith DC. A comparison between zinc phosphate and glass ionomer cement in orthodontics. Am J Orthod Dentofac Orthop 1988;93:273e9. 16. Coups-Smith KS, Rossouw PE, Titley KC. Glass ionomer cements as luting agents for orthodontic brackets. Angle Orthod 2003;73:436e44. 17. Hallgren A, Oliveby A, Twetman S. L(þ)-lactic acid production in plaque from orthodontic appliances retained with glass ionomer cement. Br J Orthod 1994;21:23e6. 18. Sadowsky PL, Retief DH, Bradley Jr EL. Enamel fluoride uptake from orthodontic cements and its effect on demineralization. Am J Orthod 1981;79:523e34. 19. Marcusson A, Norevall LI, Persson M. White spot reduction when using glass ionomer cement for bonding in orthodontics: a longitudinal and comparative study. Eur J Orthod 1997;19:233e42. 20. Farret MM, de Lima EM, Mota EG, Oshima HM, Barth V, de Oliveira SD. Can we add chlorhexidine into glass ionomer cements for band cementation? Angle Orthod 2011;81: 496e502. 21. Hatunoglu E, Ozt€urk F, Bilenler T, Aksakallı S, Sims¸ek N. Antibacterial and mechanical properties of propolis added to glass ionomer cement. Angle Orthod 2014;84:368e73. 22. Abu Alhaija ES, Al-Wahadni AM. Evaluation of shear bond strength with different enamel pre-treatments. Eur J Orthod 2004;26:179e84. 23. Lee BS, Hsieh TT, Lee YL, Lan WH, Hsu YJ, Wen PH, et al. Bond strengths of orthodontic bracket after acid-etched, Er:YAG laser-irradiated and combined treatment on enamel surface. Angle Orthod 2003;73:565e70. 24. Fischer-Brandies H, Tr€agner-Born J. Glass ionomer cements used as bonding materials in orthodontics. Orthod Fr 1989;60(Pt 2):827e34. 25. Cook PA, Youngson CC. An in vitro study of the bond strength of a glass ionomer cement in the direct bonding of orthodontic brackets. Br J Orthod 1988;15:247e53. 26. Wiltshire WA. Shear bond strengths of a glass ionomer for direct bonding in orthodontics. Am J Orthod Dentofac Orthop 1994;106:127e30. 27. Espinosa R, Valencia R, Uribe M, Ceja I, Saadia M. Enamel deproteinization and its effect on acid etching: an in vitro study. J Clin Pediatr Dent 2008;33:13e9. 28. Pereira TB, Jansen WC, Pithon MM, Souki BQ, Tanaka OM, Oliveira DD. Effects of enamel deproteinization on bracket bonding with conventional and resin-modified glass ionomer cements. Eur J Orthod 2013;35:442e6. 29. Millett D, McCabe JF, Gordon PH. The role of sandblasting on the retention of metallic brackets applied with glass ionomer cement. Br J Orthod 1993;20:117e22.
Bonding with glass ionomer cements and resin-modified glass ionomer cements
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30. Norevall LI, Sj€ogren G, Persson M. Tensile and shear strength of orthodontic bracket bonding with glass ionomer cement and acrylic resin. An In vitro Compcomparison. Swed Dent J 1990;14:275e84. 31. Oen JO, Gjerdet NR, Wisth PJ. Glass ionomer cements used as bonding materials for metal orthodontic brackets. An in vitro study. Eur J Orthod 1991;13:187e91. 32. Rezk-Lega F, Ogaard B. Tensile bond force of glass ionomer cements in direct bonding of orthodontic brackets: an in vitro comparative study. Am J Orthod Dentofac Orthop 1991; 100:357e61. 33. Evans R, Oliver R. Orthodontic bonding using glass ionomer cement: an in vitro study. Eur J Orthod 1991;13:493e500. 34. Millett DT, McCabe JF. Orthodontic bonding with glass ionomer cementea review. Eur J Orthod 1996;18:385e99. 35. Godoy-Bezerra J, Vieira S, Oliveira JH, Lara F. Shear bond strength of resin-modified glass ionomer cement with saliva present and different enamel pretreatments. Angle Orthod 2006; 76:470e4. 36. Jobalia SB, Valente RM, de Rijk WG, BeGole EA, Evans CA. Bond strength of visible light-cured glass ionomer orthodontic cement. Am J Orthod Dentofac Orthop 1997;112: 205e8. 37. Komori A, Ishikawa H. Evaluation of a resin-reinforced glass ionomer cement for use as an orthodontic bonding agent. Angle Orthod 1997;67:189e95. 38. Owens Jr SE, Miller BH. A comparison of shear bond strengths of three visible light-cured orthodontic adhesives. Angle Orthod 2000;70:352e6. 39. Sfondrini MF, Cacciafesta V, Pistorio A, Sfondrini G. Effects of conventional and highintensity light-curing on enamel shear bond strength of composite resin and resinmodified glass-ionomer. Am J Orthod Dentofac Orthop 2001;119:30e5. 40. Joseph VP, Harris AMP, Grobler SR. Bond strength of orthodontic brackets using a new glass ionomer [Abstract]. J Dent Res 1994;73(Special Issue):197. 41. Supak LA, Burgess JO. Shear bond strength of orthodontic brackets bonded with four cements [Abstract]. J Dent Res 1994;73(Special Issue):413. 42. Mount GJ. Buonocore Memorial Lecture. Glass-ionomer cements: past, present and future. Oper Dent 1994;19:82e90. 43. Powis DR, Follerås T, Merson SA, Wilson AD. Improved adhesion of a glass ionomer cement to dentin and enamel. J Dent Res 1982;61:1416e22. 44. Bishara SE, Vonwald L, Laffoon JF, Jakobsen JR. Effect of altering the type of enamel conditioner on the shear bond strength of a resin-reinforced glass ionomer adhesive. Am J Orthod Dentofac Orthop 2000;118:288e94. 45. Bishara SE, VonWald L, Laffoon JF, Jakobsen JR. Effect of changing enamel conditioner concentration on the shear bond strength of a resin-modified glass ionomer adhesive. Am J Orthod Dentofac Orthop 2000;118:311e6. 46. Cacciafesta V, Sfondrini MF, Baluga L, Scribante A, Klersy C. Use of a self-etching primer in combination with a resin-modified glass ionomer: effect of water and saliva contamination on shear bond strength. Am J Orthod Dentofac Orthop 2003;124:420e6. 47. Kirovski I, Madzarova S. Tensile bond strength of a light-cured glass ionomer cement when used for bracket bonding under different conditions: an in vitro study. Eur J Orthod 2000; 22:719e23. 48. dos Santos RL, Pithon MM, Martins FO, Romanos MT, Ruellas AC. Evaluation of cytotoxicity and degree of conversion of glass ionomer cements reinforced with resin. Eur J Orthod 2012;34:362e6.
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49. Zachrisson BU, B€uy€ukyilmaz T, Zachrisson YO. Improving orthodontic bonding to silver amalgam. Angle Orthod 1995;65:35e42. 50. B€ uy€ukyilmaz T, Zachrisson BU. Improved orthodontic bonding to silver amalgam. Part 2. Lathe-cut, admixed, and spherical amalgams with different intermediate resins. Angle Orthod 1998;68:337e44. 51. Skilton JW1, Tyas MJ, Woods MG. Effects of surface treatment on orthodontic bonding to amalgam. Aust Orthod J 2006;22:59e66. 52. Gross MW, Foley TF, Mamandras AH. Direct bonding to Adlloy-treated amalgam. Am J Orthod Dentofac Orthop 1997;112:252e8. 53. Sperber RL, Watson PA, Rossouw PE, Sectakof PA. Adhesion of bonded orthodontic attachments to dental amalgam: in vitro study. Am J Orthod Dentofac Orthop 1999;116: 506e13. 54. Zachrisson BU, Buyukyilimaz T. Recent advances in bonding to gold, amalgam and porcelain. J Clin Orthod 1993;27:661e75. 55. Germec D, Cakan U, Ozdemir FI, Arun T, Cakan M. Shear bond strength of brackets bonded to amalgam with different intermediate resins and adhesives. Eur J Orthod 2009;31: 207e12. 56. Rambhia S, Heshmati R, Dhuru V, Iacopino A. Shear bond strength of orthodontic brackets bonded to provisional crown materials utilizing two different adhesives. Angle Orthod 2009;79:784e9. 57. Eslamian L, Ghassemi A, Amini F, Jafari A, Afrand M. Should silane coupling agents be used when bonding brackets to composite restorations? An in vitro study. Eur J Orthod 2009;31:266e70. 58. Costa AR, Correr AB, Puppin-Rontani RM, Vedovello SA, Valdrighi HC, CorrerSobrinho L. Vedovello Filho M. Effect of bonding material, etching time and silane on the bond strength of metallic orthodontic brackets to ceramic. Braz Dent J 2012;23:223e7. 59. An KM, Sohn DS. The effect of using laser for ceramic bracket bonding of porcelain surfaces. Korean J Orthod 2008;38:275e82. 60. Akova T, Yoldas O, Toroglu MS, Uysal H. Porcelain surface treatment by laser for bracketporcelain bonding. Am J Orthod Dentofac Orthop 2005;128:630e7. 61. Poosti M, Jahanbin A, Mahdavi P, Mehrnoush S. Porcelain conditioning with Nd:YAG and Er:YAG laser for bracket bonding in orthodontics. Lasers Med Sci 2012;27:321e4. 62. Grewal Bach GK, Torrealba Y, Lagravere MO. Orthodontic bonding to porcelain: a systematic review. Angle Orthod 2014;84:555e60. 63. Abu Alhaija ES, Al-Wahadni AM. Shear bond strength of orthodontic brackets bonded to different ceramic surfaces. Eur J Orthod 2007;29:386e9. 64. Karan S, B€uy€ukyilmaz T, Toroglu MS. Orthodontic bonding to several ceramic surfaces: are there acceptable alternatives to conventional methods? Am J Orthod Dentofac Orthop 2007;132. 144.e7e14. 65. Saraç YS¸, K€ul€unk T, Elekdag-T€urk S, Saraç D, T€urk T. Effects of surface-conditioning methods on shear bond strength of brackets bonded to different all-ceramic materials. Eur J Orthod 2011;33:667e72. 66. T€urk T, Saraç D, Saraç YS, Elekdag-T€urk S. Effects of surface conditioning on bond strength of metal brackets to all-ceramic surfaces. Eur J Orthod 2006;28:450e6. 67. Bishara SE, Oonsombat C, Soliman MM, Ajlouni R, Laffoon JF. The effect of tooth bleaching on the shear bond strength of orthodontic brackets. Am J Orthod Dentofac Orthop 2005;128:755e60. 68. Lai SC, Tay FR, Cheung GS, Mak YF, Carvalho RM, Wei SH, et al. Reversal of compromised bonding in bleached enamel. J Dent Res 2002;81:477e81.
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69. T€urkkahraman H, Adanir N, G€ung€or AY. Bleaching and desensitizer application effects on shear bond strengths of orthodontic brackets. Angle Orthod 2007;77:489e93. 70. Bulut H, Kaya AD, Turkun M. Tensile bond strength of brackets after antioxidant treatment on bleached teeth. Eur J Orthod 2005;27:466e71. 71. Proffit WR. The third stage of comprehensive treatment: finishing. In: Proffit WR, Fields HW, Sarver DM, editors. Contemporary orthodontics. 5th ed. St. Louis: Mosby/ Elsevier; 2013. p. 582e605. 72. Eminkahyagil N, Arman A, Cetins¸ahin A, Karabulut E. Effect of resin-removal methods on enamel and shear bond strength of rebonded brackets. Angle Orthod 2006;76:314e21. 73. Eliades T, Kakaboura A, Eliades G, Bradley TG. Comparison of enamel colour changes associated with orthodontic bonding using two different adhesives. Eur J Orthod 2001;23: 85e90. € € Effects of different orthodontic adhesives and resin 74. Boncuk Y, Cehreli ZC, Polat-Ozsoy O. removal techniques on enamel color alteration. Angle Orthod 2014;84:634e41. 75. Hanks CT, Strawn SE, Wataha JC, Craig RG. Cytotoxic effects of resin components on cultured mammalian fibroblasts. J Dent Res 1991;70:1450e5. 76. Hitmi L, Muller C, Mujajic M, Attal JP. An 18-month clinical study of bond failures with resin-modified glass ionomer cement in orthodontic practice. Am J Orthod Dentofac Orthop 2001;120:406e15. 77. Cacciafesta V, Bosch C, Melsen B. Clinical comparison between a resin-reinforced selfcured glass ionomer cement and a composite resin for direct bonding of orthodontic brackets. Part 2: bonding on dry enamel and on enamel soaked with saliva. Clin Orthod Res 1999;2:186e93. 78. Egan FR, Alexander SA, Cartwright GE. Bond strength of rebonded orthodontic brackets. Am J Orthod Dentofac Orthop 1996;109:64e70.
16
C. Rahiotis, S. Schricker
16.1
Introduction
Orthodontic brackets are cemented either to labial or to lingual tooth surfaces and act as a medium for the delivery of forces applied by the archwire and auxiliaries to the teeth. The factors that are the main contributors for the successful transfer of orthodontic forces to a tooth include the following: the preparation of the enamel surface for bonding; the type of adhesive cement used; and the shape, material, and surface finish of the bracket.1e3 Among these factors the adhesive cements have a key role in this procedure. The ideal cement used for orthodontic bracket bonding should exhibit enough retention to resist displacement during normal oral function and transmit the required orthodontic forces to the tooth. Furthermore, it should be easily removed once the treatment is complete, without causing any damage to the tooth surface and, ideally, without leaving residues that need to be removed by drilling or air abrasion.4 Commonly used adhesives for bonding brackets to teeth have been the glass ionomer cements (GICs) and resin-modified glass ionomer cements (RMGICs). Each adhesive displays a different adhesion mechanism. This chapter presents the chemistry of the GICs and RMGICs, discusses their manipulation, bonding, and debonding in orthodontics, and concludes with comments on the causes of bond failures.
16.2
Conventional glass ionomer cements
The conventional glass ionomer materials form a chemical bond through calcium ions between the enamel surface and the bracket.5e7 During the last two decades the beneficial properties of GICs and the composite resin adhesives (discussed in Chapters 9 and 10) have been combined into the new category of RMGICs, where the acidbased curing reaction of the GIC has been combined with the light-activated polymerization of 2-hydroxyethyl methacrylate and a methacrylate-functionalized polyacid.8 Fig. 16.1 shows the structure of a methacrylated polyacid and the reaction with 2-hydroxyethyl methacrylate in parallel with the acidebase reaction. GICs were first described by Wilson and Kent, and their physical properties were a combination of those of silicate and polycarboxylate cements.9 Their long setting time, Orthodontic Applications of Biomaterials. http://dx.doi.org/10.1016/B978-0-08-100383-1.00016-3 Copyright © 2017 Elsevier Ltd. All rights reserved.
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Figure 16.1 The dual-cure reaction of a resin-modified glass ionomer cement.
poor durability, high water absorption, and solubility subsequently led to the development of ion-leachable glass with better physical and clinical properties.10 Currently, the GICs are a group of materials based on the acid/base reaction between polyalkenoic acid and a basic ion-leachable silicate glass. They bond to dental hard tissues because of ionic interactions between the calcium ions of the hydroxyapatite of the enamel and carboxylic groups of the cement.11 The polyalkenoic acid is typically a copolymer of acrylic acid with a variety of comonomers, notably itaconic acid. Fig. 16.2 shows the chemical structure of polyacrylic acid along with several comonomers. The reaction between the polyacid and the acidic base will release calcium and aluminum ions that will bind to the carboxylic groups of the polyacid. These ions CH2COOH
CH2∙CH
CH2∙C CH2∙CH COOH COOH
COOH Poly(acrylic acid)
Poly(acrylic acid-co-itaconic acid) COOH CH2∙C CH2∙CH COOH CHCH2COOH COOH
COOH CH CH CH2∙CH COOH COOH Poly(acrylic acid-co-maleic acid)
Poly(acrylic acid-co-3-butene 1,2,3 tricarboxylic acid)
Figure 16.2 Structures of common polyacids.
will serve as cross-linkers to stabilize the polyacid gel network. Fig. 16.3 shows polyacid chains, depicted as wavy lines, cross-linked by ions. The basic glass is now bound in the polyacid gel matrix. The GICs possess a combination of properties that are potentially useful in clinical orthodontics. First, they adhere to different substrates such as enamel and metal.12 For bonding to enamel, GICs have the advantage of achieving a chemical bond without
Bonding with glass ionomer cements and resin-modified glass ionomer cements
COOCa2+
COOCa2+ COO-
COO-
255
COOAl+++
-OOC
COO-
Figure 16.3 Salt bridges that cross-link the polyacids.
etching, reducing the clinical steps while protecting the enamel surface from dissolution.13 The polyalkenoic acids slightly pit the enamel to form a thin hybrid layer, and its removal leads to less enamel damage compared to that caused by the removal of composite resins.14 Another advantage of GICs is their ability to release fluoride for long periods of at least 12 months.15 They also have the ability to absorb fluoride from different sources such as fluoride toothpastes, recharging their fluoride reservoir.16 The deposition of fluoride in the cement around the orthodontic bonding area leads to a decrease of the populations of Streptococcus mutans and Lactobacillus bacteria,5,17 which consequently leads to less enamel demineralization and less white spot formation.18,19 To increase more the antibacterial spectrum of GICs, studies suggest the incorporation of either chlorhexidine (CHD)20 or ethanolic extracts of propolis (EEP),21 which seem to increase the antibacterial effect of GICs for a long period of time. The addition of CHD does not significantly influence the shear bond strength, whereas EEP seems to slightly enhance the mechanical properties of GICs. Another way to increase the antibacterial ability of GICs is the incorporation of zinc oxide, which serves as an activator of enzymes that can be toxic to microbes at concentrations as low as 0.5 ppm.22 While its antimicrobial effect lasts at least for 1 month the mean bond strength of a GIC mixed with zinc oxide falls in the lower range of shear bond strength recommended for GICs, decreasing even more as the concentration of ZnO increases.23 With regard to saliva contamination and the bonding field, conventional GICs are less demanding to apply in comparison to composite resins, since they are not hydrophobic. Accordingly, the speed and ease of bracket placement are increased. Besides their advantages, GICs appear to have disadvantages as well, such as their slow curing reaction and the weak, yet clinically acceptable, bond strength. The latter is initially low and reaches its maximum value 24 h after bonding. Nevertheless, the relatively weak bond strength is a feature that makes GICs easy to remove when orthodontic treatment is completed.
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Enamel preparation
There are no clear guidelines about the best method of enamel treatment prior to bonding with conventional GICs, yet there are several contradictory suggestions. Some authors suggest that pretreatment of the enamel with pumice slurry and acids, such as polyacrylic acid or tannic acid, for 5 s, improves bond strength to about 60% of the value achieved using an acid-etching enamel/composite resin technique.24 Others suggest that no further pretreatment than drying the tooth with a cotton swab is required. With regard to pretreatment of the enamel surface with phosphoric acid, the outcome is disputed. Some investigators found that it produced a significantly poorer bond,25 while another investigator26 found an increase in mean bond strength, although it was not significantly greater than that of unetched enamel. Recently, the use of 5.25% sodium hypochlorite as a deproteinizing agent has been suggested, since it is claimed to remove the organic elements both from the enamel surface and the biofilm acquired by the GICs, thereby increasing the bond strength.27 Enamel deproteinization with NaOCl increases the bond strength of metal brackets bonded with either conventional or RMGICs, yet not significantly.28 Apart from all of the above, sandblasting the enamel surface for 3 s seems to increase, significantly, the in vitro shear bond strength and the mean survival time of metal brackets having a mesh base that are bonded with a GIC.29
16.2.2
Effect of powder/liquid ratio
The powder/liquid ratio utilized for preparation of a GIC is critical for successful bonding. However, it seems to vary among different studies. Some investigators followed manufacturer instructions,30,31 whereas other groups used a slightly thicker mix32 or encapsulated glass ionomers.33 The encapsulated GICs have higher powder/liquid ratios than hand-mixed cements, which seem to be clinically acceptable. The manufacturer instructions for mixing GICs depend upon their use as filling materials, bases, or liners in restorative dentistry. There are no guidelines about material proportioning for orthodontic bonding. An ideal powder/liquid ratio for orthodontic use would provide flow properties, while optimizing bond strength.34
16.3
Resin-modified glass ionomer cements
To overcome the disadvantages of both composite resins, such as white spots formation, dry bonding field, and enamel destruction, and GICs, such as low bond strength, a new category of bonding materials was created, the RMGICs. These new materials incorporate a resin matrix (hydrophilic monomers, such as hydroxyethylmethacrylate (HEMA) and polymerization initiators) into GICs. Light-activated RMGICs have the advantages of GICs (fluoride release, even if this seems to be a little inferior to that of GICs, chemical bonding to both enamel and metal, and adhesion in moist fields) and the mechanical and physical properties of composite resins.35 Clinically, RMGICs eliminate the demand for working in a dry field, along with the need for etching
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and priming enamel surfaces, and they enable bracket repairs to be made quickly and easily. According to a number of studies, RMGICs appear to achieve lower bond strengths in comparison to composite resins, yet higher bond strengths in comparison to conventional GICs.36e39 To date, in vitro studies have shown that resin-reinforced GICs retain brackets significantly better than conventional GICs.40,41 The fact that RMGICs combine the ability of bonding to moist enamel surfaces with the ability to release fluoride makes them ideal as lingual retainer adhesives. Conventional GICs sustain their maximum strength after 24 h. The resin addition to the cement formulation has facilitated light curing, allowing a snap set and rapid development of strength.42 RMGICs are usually encapsulated and dual cured. Once the liquid and powder are mixed, both the acidebase reaction and the light-initiated free-radical polymerization of resin occur. The resin phase polymerizes quickly, whereas development of the glass ionomer phase proceeds slowly via an acidebase reaction over a period of time, with the material achieving its full bond strength 24 h after its application. Light curing activates the free-radical polymerization of HEMA and two other monomers, thus forming a poly-HEMA matrix and causing an immediate setting of the material.
16.3.1 Enamel pretreatmentdprimers Before the introduction of RMGICs, most previous studies followed the recommendations from manufacturers and used 10% polyacrylic acid for surface treatment before bonding. Polyacrylic acid contains carboxylic acid functional groups potentially capable of bonding to the tooth surface.43 On the other hand, with conventional composite resin, 37% phosphoric acid is popular for surface treatment before bonding. Phosphoric acid creates microporosities on the enamel surface, into which resin tags extend after curing, forming a mechanical bond to the enamel surface. It is observed that teeth conditioned with 10% polyacrylic acid provide bond strength significantly lower than teeth conditioned with 37% phosphoric acid before bonding with RMGICs.44 Increasing the polyacrylic acid concentration to 20% results in an eightfold increase in bond strength, though still significantly lower than that achieved with phosphoric acid.45 Self-etching primers have also been applied to the enamel surface prior to bonding with RMGICs, resulting in even greater bond strength values, in comparison to both polyacrylic acid and phosphoric acid.46 Etching the enamel surface with 37% phosphoric acid produces higher bond strength, similar to that of composite resins, when compared to nonetched surfaces,47 while etching with polyacrylic acid seems to provide a similar bond strength to that of nonetched surfaces. When RMGICs are used as adhesives, the bond strength seems to increase even more when the enamel surface is contaminated either with saliva or plasma. Moisture is required for optimal adherence of both GICs and RMGICs, which contain hydrophilic monomers. Sandblasting could also be applied in the case of bonding with RMGICs.
16.3.2 Cytotoxicity Despite the improvement in mechanical properties, the cytotoxic effects of RMGICs are of more potential concern in comparison to conventional GICs, since they
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incorporate monomers such as HEMA along with initiators. As with composite resins, incomplete polymerization results in inadequate conversion of monomers into polymers, and these residual monomers can cause a significant cytotoxic effect when liberated into the oral environment. There is a direct relationship between the degree of conversion and RMGIC cytotoxicity, which, after the initial polymerization, decreases over time.48
16.4 16.4.1
Bonding to different substrates than enamel Introduction and perspective
This section presents recommendations and protocols for the bonding of brackets to different substrates other than enamel that are encountered for orthodontic patients. Generally, the quality of bonding is assessed by some laboratory test that evaluates bond strength from the measurement of debonding force. While such tests can provide useful comparative information, the limitation of their efficacy is discussed at length in Chapter 11.
16.4.2
Amalgam
In adult orthodontic patients, and occasionally in adolescents as well, amalgam restorations exist on the buccal surfaces of posterior teeth. In such cases, successful bonding of orthodontic attachments to amalgam surfaces is challenging. This clinical problem led to the investigation of several procedures to improve the bond strength in such cases. These procedures include surface treatment and the use of intermediate resins and adhesives that chemically bond to metals. Surface treatment procedures include roughening the amalgam surface with a diamond bur,49 sandblasting,50,51 galliumetin (GaeSn) liquid application,52 and chemical corrosion.53 Sandblasting is the most common method used for surface preparation, since it creates scratch-like irregularities that increase bond strength. In addition to mechanical retention, bonding to metal has the advantage of chemical adhesion. Therefore GICs and RMGICs chemically bonded to amalgam are recommended.53 The mean bond strength values of stainless steel orthodontic brackets bonded to amalgam surfaces are significantly lower than those of brackets bonded to etched enamel, yet clinically acceptable.54 Bond failures occur at the amalgam-adhesive interface, regardless of the adhesive system, and without any damage to the amalgam restoration.
16.4.3
Ceramics
Apart from amalgam restorations, many adult patients seeking orthodontic therapy have metal-ceramic and all-ceramic restorations. When bonding to dental porcelain or other dental ceramics, adequate bond strength is desired, with easy removal to avoid damage of the restored teeth. Several techniques have been used to bond brackets to porcelain surfaces, and they differ in the surface preparation and bonding agents
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applied. The adhesives used to bond brackets to ceramics (composite resins, RMGICs) seem to provide similar bond strength values.56 Surface preparations include the application of different acids (orthophosphoric or hydrofluoric), treatment by different laser techniques, roughening by diamond burs, sandblasting, and silanization. The use of hydrofluoric acid (HF) greatly increases the bond strength. This is due to the ability of the acid to react with the silica phase, creating micromechanical retention through the formation of microchannels. Over time, the glassy matrix partially dissolves, and the formation of such retentive channels increases. A longer etching time increases the bond strength, as it allows the acid to react with the ceramic matrix further. However, considering the harmful effects of etching with HF, mechanical roughening with sandblasting or diamond burs is recommended. In any case, the bond strength of brackets bonded to porcelain is further improved by the application of silane, which has the ability to form chemical bonds with inorganic and organic surfaces.57,58 The conventional techniques of HF etching and silanization, sandblasting and silanization, orthophosphoric acid etching and silanization, and HF etching alone show higher shear bond strength values than laser etching in combination with silane application, whereas orthophosphoric acid etching alone and sandblasting alone show lower bond strength values than laser application alone.59,60 The neodymium-doped yttrium aluminium garnet laser seems to be an acceptable substitute for HF etching; however, the erbium-doped yttrium aluminium garnet laser is not an acceptable option.61 The best reported protocol for bonding to porcelain is acid etching with 9.6% HF, rinsing for 30 s, air drying, and silanization.62 However, there are differences between various ceramic surfaces and brands, such as dissimilar particle sizes and microstructural phases, leading to higher or lower bond strengths. Higher shear bond strength values are presented by Empress II (lithium disilicate; Ivoclar) and Finesse (leucite-containing porcelain; Dentsply); the metal-ceramic surface, and In-Ceram (alumina matrix; Vident) have comparable shear bond strengths; and IPS Empress (leucite-containing porcelain; Ivoclar) shows the weakest bond strength among these ceramics.63e66
16.4.4 Bleached teeth Various bleaching agents and methods exist to whiten discolored teeth at the dental office or at home. The results of studies about the effect of bleaching on the bond strength of orthodontic brackets are ambiguous. One study reported no adverse effect on the bond strength of orthodontic brackets.67 Another study reported considerable reduction of bond strength values subsequent to bleaching.68 The reduced bond strength may be attributed to several sources. There can be alterations in the microstructure of bleached enamel surfaces after acid etching, including reduced microhardness and calcium loss, and overetching can cause the loss of enamel prisms.69 Another potential mechanism is the release of oxygen radicals on the enamel surface by residual components of bleaching agents.70 To avoid bonding failure on bleached teeth, several methods have been proposed. These include (1) avoidance of the bleaching process until the orthodontic treatment is completed, (2) delay of bracket bonding up to 4 weeks after bleaching, and
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(3) pumicing the bleached teeth and the application of antioxidant agents, such as 10% sodium ascorbate or 10% a-tocopherol, prior to bracket bonding to neutralize the effect of released oxygen radicals from residual components.70
16.5
Debonding
When a bonded bracket is removed, failure can occur at one of the three interfaces: between the adhesive and enamel surface, within the bonding material itself, or between the cement and the bracket. The interface between the adhesive cement and the bracket is the usual site of failure,71 and the remaining adhesive must be removed. The use of adhesive-removal pliers may cause pain, while physical changes to the enamel can occur as well, ranging from surface roughening to microscopic fractures. A wide variety of instruments and procedures have been introduced as a result of the search for an efficient and safe method of adhesive removal after debonding. These include (1) manual removal with the use of a scaler or a band-removing plier, (2) various shapes of tungsten carbide burs with high- or low-speed handpieces, (3) specially designed burs that are less aggressive to the enamel, (4) Soflex discs (3M ESPE), (5) special composite resin finishing systems with zirconia paste or slurry pumice, and (6) ultrasonic applications; methods such as CO2 laser application or powder abrasive systems are quite promising.72 The color of the enamel can also be affected both by debonding and the subsequent cleaning procedures.73 Changes in the color of the enamel may also result from the discoloration of the residual resin that has irreversibly penetrated the surface, despite the cleaning procedures. The average depth of penetration ranges between 8 and 15 mm, with maximum tag lengths ranging up to 50 mm. Removing all these residues would result in a considerable loss of sound tooth structure. Most desirable would be the availability of a bonding agent having both a minimal discoloration potential and a simple protocol for removal of its residue. When the brackets are bonded with the etchand-rinse or self-etching systems, removing the adhesives with Stainbuster (Abrasive Technology) burs is recommended, whereas for the RMGICs, tungsten carbide burs may provide less enamel discoloration over long time periods. The combination of etch-and-rinse system and tungsten carbide burs is not recommended for clinical use, since this seems to cause the greatest color change.74 In general the weaker chemical bonding between GICs/RMGICs and enamel facilitates the cleanup process of removing the adhesive remaining on the enamel surface after debonding, in comparison to the cleanup process when brackets are bonded with composite resin.
16.6
Bond failures
Bonded brackets should ideally remain attached to the tooth surface throughout the whole treatment, and the bond strength of the adhesive material should be sufficient to resist tensile, shear, torque, and peel functional stresses.75 Bond failure, however,
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is encountered frequently during treatment and may be influenced by the microstructure of the bracket base, the etching time, the etching system, the bonding agent, and the bonding technique used. In addition, factors related to the operator, such as moisture control during bonding procedure, choice of bonding material, choice of brackets, or instructions given to the patients, and patient factors, such as sex, age, malocclusion, and dental hygiene, are likely to influence the failure rate of any bonding system. Most fixed-appliance orthodontic treatments last about 18 months. The more limited the bond failure is during this period of time the better for the clinical result. The acceptable levels of bond failure for in vivo use are 4e10%.76 According to the literature, composite resins have an average failure rate of about 6% (study results range between 4.7% and 8.3%).76 These failure rates are comparable to those of RMGICs, when applied in combination with prior etching. Their average failure rate is 7% (with studies reporting failure rates from 5% to 8.9%). Higher failure rates are expected when the enamel is dried prior to bonding with RMGICs.77 Both composite resins and RMGICs have significantly lower bond failure rates than GICs, which seem to have a failure rate ranging from 12% to 50%.76 The disadvantage of extra bracket failures appears to outweigh any potential advantages when considering GICs for bonding of orthodontic brackets. Even though bonds can fail on any tooth at any time, some generalizations have been made: (1) most failures occur at the bonding visit or some time before the first postbonding visit, (2) incisors and canines have fewer failures than premolars, (3) maxillary canine bonds are more successful than mandibular canine bonds, and (4) bonds on anterior teeth separate more at the bracketeresin interface, whereas bonds on posterior teeth are more likely to demonstrate an enameleresin failure.78 In summary, in any case bond failure is of particular concern clinically, and its cause should be ascertained and addressed accordingly whenever possible.
References 1. Barry GR. A clinical investigation of the effects of pumice prophylaxis on band and bond failure. Br J Orthod 1995;22:245e8. 2. Bearn DR, Aird JC, McCabe JF. Ex vivo bond strength of adhesive precoated metallic and ceramic brackets. Br J Orthod 1995;22:233e6. 3. Bin Abdullah MS, Rock WP. The effect of etch time and debond interval upon the shear bond strength of metallic orthodontic brackets. Br J Orthod 1996;23:121e4. 4. Banerjee A, Paolinelis G, Socker M, Watson TF, McDonald F. An in-vitro investigation of the effectiveness of bioactive glass air-abrasion in the selective removal of orthodontic resin adhesive. Eur J Oral Sci 2008;116:488e92. 5. Hallgren A, Oliveby A, Twetman S. Fluoride concentration in plaque adjacent to orthodontic bracket appliances retained with glass ionomer cement. Caries Res 1993;27:51e4. 6. Millett DT, McCabe JF, Bennett TG, Carter NE, Gordon PH. The effect of sandblasting on the retention of first molar orthodontic bands cemented with glass ionomer cement. Br J Orthod 1995;22:161e9. 7. Millett DT, McCabe JF. Orthodontic bonding with glass ionomer cement e a review. Eur J Orthod 1996;18:385e9.
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Orthodontic Applications of Biomaterials
8. Silverman E, Cohen M, Demke RS, Silverman M, Linwood NJ. A new light-cured glass ionomer cement that bond brackets to teeth without etching in the presence of saliva. Am J Orthod Dentofac Orthop 1995;108:231e6. 9. Wilson AD, Kent BE. A new translucent cement for dentistry. Br Dent J 1972;132:133e5. 10. Walls AW. Glass polyalkenoate (glass-ionomer) cements: a review. J Dent 1986;14: 231e46. 11. Voss A, Hickel R, M€olkner S. In vivo bonding of orthodontic brackets with glass ionomer cement. Angle Orthod 1993;63:149e53. 12. Hotz P, McLean JW, Sced I, Wilson AD. The bonding of glass ionomer cements to metal and tooth substrates. Br Dent J 1977;142:41e7. 13. van Dijken JW. The effect of cavity pretreatment procedures on dentin bonding: a four-year clinical evaluation. J Prosthet Dent 1990;64:148e52. 14. Charles C. Bonding orthodontic brackets with glass-ionomer cement. Biomaterials 1998; 19:589e91. 15. Maijer R, Smith DC. A comparison between zinc phosphate and glass ionomer cement in orthodontics. Am J Orthod Dentofac Orthop 1988;93:273e9. 16. Coups-Smith KS, Rossouw PE, Titley KC. Glass ionomer cements as luting agents for orthodontic brackets. Angle Orthod 2003;73:436e44. 17. Hallgren A, Oliveby A, Twetman S. L(þ)-lactic acid production in plaque from orthodontic appliances retained with glass ionomer cement. Br J Orthod 1994;21:23e6. 18. Sadowsky PL, Retief DH, Bradley Jr EL. Enamel fluoride uptake from orthodontic cements and its effect on demineralization. Am J Orthod 1981;79:523e34. 19. Marcusson A, Norevall LI, Persson M. White spot reduction when using glass ionomer cement for bonding in orthodontics: a longitudinal and comparative study. Eur J Orthod 1997;19:233e42. 20. Farret MM, de Lima EM, Mota EG, Oshima HM, Barth V, de Oliveira SD. Can we add chlorhexidine into glass ionomer cements for band cementation? Angle Orthod 2011;81: 496e502. 21. Hatunoglu E, Ozt€urk F, Bilenler T, Aksakallı S, Sims¸ek N. Antibacterial and mechanical properties of propolis added to glass ionomer cement. Angle Orthod 2014;84:368e73. 22. Abu Alhaija ES, Al-Wahadni AM. Evaluation of shear bond strength with different enamel pre-treatments. Eur J Orthod 2004;26:179e84. 23. Lee BS, Hsieh TT, Lee YL, Lan WH, Hsu YJ, Wen PH, et al. Bond strengths of orthodontic bracket after acid-etched, Er:YAG laser-irradiated and combined treatment on enamel surface. Angle Orthod 2003;73:565e70. 24. Fischer-Brandies H, Tr€agner-Born J. Glass ionomer cements used as bonding materials in orthodontics. Orthod Fr 1989;60(Pt 2):827e34. 25. Cook PA, Youngson CC. An in vitro study of the bond strength of a glass ionomer cement in the direct bonding of orthodontic brackets. Br J Orthod 1988;15:247e53. 26. Wiltshire WA. Shear bond strengths of a glass ionomer for direct bonding in orthodontics. Am J Orthod Dentofac Orthop 1994;106:127e30. 27. Espinosa R, Valencia R, Uribe M, Ceja I, Saadia M. Enamel deproteinization and its effect on acid etching: an in vitro study. J Clin Pediatr Dent 2008;33:13e9. 28. Pereira TB, Jansen WC, Pithon MM, Souki BQ, Tanaka OM, Oliveira DD. Effects of enamel deproteinization on bracket bonding with conventional and resin-modified glass ionomer cements. Eur J Orthod 2013;35:442e6. 29. Millett D, McCabe JF, Gordon PH. The role of sandblasting on the retention of metallic brackets applied with glass ionomer cement. Br J Orthod 1993;20:117e22.
Bonding with glass ionomer cements and resin-modified glass ionomer cements
263
30. Norevall LI, Sj€ogren G, Persson M. Tensile and shear strength of orthodontic bracket bonding with glass ionomer cement and acrylic resin. An In vitro Compcomparison. Swed Dent J 1990;14:275e84. 31. Oen JO, Gjerdet NR, Wisth PJ. Glass ionomer cements used as bonding materials for metal orthodontic brackets. An in vitro study. Eur J Orthod 1991;13:187e91. 32. Rezk-Lega F, Ogaard B. Tensile bond force of glass ionomer cements in direct bonding of orthodontic brackets: an in vitro comparative study. Am J Orthod Dentofac Orthop 1991; 100:357e61. 33. Evans R, Oliver R. Orthodontic bonding using glass ionomer cement: an in vitro study. Eur J Orthod 1991;13:493e500. 34. Millett DT, McCabe JF. Orthodontic bonding with glass ionomer cementea review. Eur J Orthod 1996;18:385e99. 35. Godoy-Bezerra J, Vieira S, Oliveira JH, Lara F. Shear bond strength of resin-modified glass ionomer cement with saliva present and different enamel pretreatments. Angle Orthod 2006; 76:470e4. 36. Jobalia SB, Valente RM, de Rijk WG, BeGole EA, Evans CA. Bond strength of visible light-cured glass ionomer orthodontic cement. Am J Orthod Dentofac Orthop 1997;112: 205e8. 37. Komori A, Ishikawa H. Evaluation of a resin-reinforced glass ionomer cement for use as an orthodontic bonding agent. Angle Orthod 1997;67:189e95. 38. Owens Jr SE, Miller BH. A comparison of shear bond strengths of three visible light-cured orthodontic adhesives. Angle Orthod 2000;70:352e6. 39. Sfondrini MF, Cacciafesta V, Pistorio A, Sfondrini G. Effects of conventional and highintensity light-curing on enamel shear bond strength of composite resin and resinmodified glass-ionomer. Am J Orthod Dentofac Orthop 2001;119:30e5. 40. Joseph VP, Harris AMP, Grobler SR. Bond strength of orthodontic brackets using a new glass ionomer [Abstract]. J Dent Res 1994;73(Special Issue):197. 41. Supak LA, Burgess JO. Shear bond strength of orthodontic brackets bonded with four cements [Abstract]. J Dent Res 1994;73(Special Issue):413. 42. Mount GJ. Buonocore Memorial Lecture. Glass-ionomer cements: past, present and future. Oper Dent 1994;19:82e90. 43. Powis DR, Follerås T, Merson SA, Wilson AD. Improved adhesion of a glass ionomer cement to dentin and enamel. J Dent Res 1982;61:1416e22. 44. Bishara SE, Vonwald L, Laffoon JF, Jakobsen JR. Effect of altering the type of enamel conditioner on the shear bond strength of a resin-reinforced glass ionomer adhesive. Am J Orthod Dentofac Orthop 2000;118:288e94. 45. Bishara SE, VonWald L, Laffoon JF, Jakobsen JR. Effect of changing enamel conditioner concentration on the shear bond strength of a resin-modified glass ionomer adhesive. Am J Orthod Dentofac Orthop 2000;118:311e6. 46. Cacciafesta V, Sfondrini MF, Baluga L, Scribante A, Klersy C. Use of a self-etching primer in combination with a resin-modified glass ionomer: effect of water and saliva contamination on shear bond strength. Am J Orthod Dentofac Orthop 2003;124:420e6. 47. Kirovski I, Madzarova S. Tensile bond strength of a light-cured glass ionomer cement when used for bracket bonding under different conditions: an in vitro study. Eur J Orthod 2000; 22:719e23. 48. dos Santos RL, Pithon MM, Martins FO, Romanos MT, Ruellas AC. Evaluation of cytotoxicity and degree of conversion of glass ionomer cements reinforced with resin. Eur J Orthod 2012;34:362e6.
264
Orthodontic Applications of Biomaterials
49. Zachrisson BU, B€uy€ukyilmaz T, Zachrisson YO. Improving orthodontic bonding to silver amalgam. Angle Orthod 1995;65:35e42. 50. B€ uy€ukyilmaz T, Zachrisson BU. Improved orthodontic bonding to silver amalgam. Part 2. Lathe-cut, admixed, and spherical amalgams with different intermediate resins. Angle Orthod 1998;68:337e44. 51. Skilton JW1, Tyas MJ, Woods MG. Effects of surface treatment on orthodontic bonding to amalgam. Aust Orthod J 2006;22:59e66. 52. Gross MW, Foley TF, Mamandras AH. Direct bonding to Adlloy-treated amalgam. Am J Orthod Dentofac Orthop 1997;112:252e8. 53. Sperber RL, Watson PA, Rossouw PE, Sectakof PA. Adhesion of bonded orthodontic attachments to dental amalgam: in vitro study. Am J Orthod Dentofac Orthop 1999;116: 506e13. 54. Zachrisson BU, Buyukyilimaz T. Recent advances in bonding to gold, amalgam and porcelain. J Clin Orthod 1993;27:661e75. 55. Germec D, Cakan U, Ozdemir FI, Arun T, Cakan M. Shear bond strength of brackets bonded to amalgam with different intermediate resins and adhesives. Eur J Orthod 2009;31: 207e12. 56. Rambhia S, Heshmati R, Dhuru V, Iacopino A. Shear bond strength of orthodontic brackets bonded to provisional crown materials utilizing two different adhesives. Angle Orthod 2009;79:784e9. 57. Eslamian L, Ghassemi A, Amini F, Jafari A, Afrand M. Should silane coupling agents be used when bonding brackets to composite restorations? An in vitro study. Eur J Orthod 2009;31:266e70. 58. Costa AR, Correr AB, Puppin-Rontani RM, Vedovello SA, Valdrighi HC, CorrerSobrinho L. Vedovello Filho M. Effect of bonding material, etching time and silane on the bond strength of metallic orthodontic brackets to ceramic. Braz Dent J 2012;23:223e7. 59. An KM, Sohn DS. The effect of using laser for ceramic bracket bonding of porcelain surfaces. Korean J Orthod 2008;38:275e82. 60. Akova T, Yoldas O, Toroglu MS, Uysal H. Porcelain surface treatment by laser for bracketporcelain bonding. Am J Orthod Dentofac Orthop 2005;128:630e7. 61. Poosti M, Jahanbin A, Mahdavi P, Mehrnoush S. Porcelain conditioning with Nd:YAG and Er:YAG laser for bracket bonding in orthodontics. Lasers Med Sci 2012;27:321e4. 62. Grewal Bach GK, Torrealba Y, Lagravere MO. Orthodontic bonding to porcelain: a systematic review. Angle Orthod 2014;84:555e60. 63. Abu Alhaija ES, Al-Wahadni AM. Shear bond strength of orthodontic brackets bonded to different ceramic surfaces. Eur J Orthod 2007;29:386e9. 64. Karan S, B€uy€ukyilmaz T, Toroglu MS. Orthodontic bonding to several ceramic surfaces: are there acceptable alternatives to conventional methods? Am J Orthod Dentofac Orthop 2007;132. 144.e7e14. 65. Saraç YS¸, K€ul€unk T, Elekdag-T€urk S, Saraç D, T€urk T. Effects of surface-conditioning methods on shear bond strength of brackets bonded to different all-ceramic materials. Eur J Orthod 2011;33:667e72. 66. T€urk T, Saraç D, Saraç YS, Elekdag-T€urk S. Effects of surface conditioning on bond strength of metal brackets to all-ceramic surfaces. Eur J Orthod 2006;28:450e6. 67. Bishara SE, Oonsombat C, Soliman MM, Ajlouni R, Laffoon JF. The effect of tooth bleaching on the shear bond strength of orthodontic brackets. Am J Orthod Dentofac Orthop 2005;128:755e60. 68. Lai SC, Tay FR, Cheung GS, Mak YF, Carvalho RM, Wei SH, et al. Reversal of compromised bonding in bleached enamel. J Dent Res 2002;81:477e81.
Bonding with glass ionomer cements and resin-modified glass ionomer cements
265
69. T€urkkahraman H, Adanir N, G€ung€or AY. Bleaching and desensitizer application effects on shear bond strengths of orthodontic brackets. Angle Orthod 2007;77:489e93. 70. Bulut H, Kaya AD, Turkun M. Tensile bond strength of brackets after antioxidant treatment on bleached teeth. Eur J Orthod 2005;27:466e71. 71. Proffit WR. The third stage of comprehensive treatment: finishing. In: Proffit WR, Fields HW, Sarver DM, editors. Contemporary orthodontics. 5th ed. St. Louis: Mosby/ Elsevier; 2013. p. 582e605. 72. Eminkahyagil N, Arman A, Cetins¸ahin A, Karabulut E. Effect of resin-removal methods on enamel and shear bond strength of rebonded brackets. Angle Orthod 2006;76:314e21. 73. Eliades T, Kakaboura A, Eliades G, Bradley TG. Comparison of enamel colour changes associated with orthodontic bonding using two different adhesives. Eur J Orthod 2001;23: 85e90. € € Effects of different orthodontic adhesives and resin 74. Boncuk Y, Cehreli ZC, Polat-Ozsoy O. removal techniques on enamel color alteration. Angle Orthod 2014;84:634e41. 75. Hanks CT, Strawn SE, Wataha JC, Craig RG. Cytotoxic effects of resin components on cultured mammalian fibroblasts. J Dent Res 1991;70:1450e5. 76. Hitmi L, Muller C, Mujajic M, Attal JP. An 18-month clinical study of bond failures with resin-modified glass ionomer cement in orthodontic practice. Am J Orthod Dentofac Orthop 2001;120:406e15. 77. Cacciafesta V, Bosch C, Melsen B. Clinical comparison between a resin-reinforced selfcured glass ionomer cement and a composite resin for direct bonding of orthodontic brackets. Part 2: bonding on dry enamel and on enamel soaked with saliva. Clin Orthod Res 1999;2:186e93. 78. Egan FR, Alexander SA, Cartwright GE. Bond strength of rebonded orthodontic brackets. Am J Orthod Dentofac Orthop 1996;109:64e70.