Journal of Investigative and Clinical Dentistry (2014), 5, 1–9
ORIGINAL ARTICLE Silane-Based Primers and Bond Strength
Effect of experimental silane-based primers with various contents of 2-hydroxyethyl methacrylate on the bond strength of orthodontic adhesives Ammar A. Mustafa1, Jukka P. Matinlinna2, Aziz A. Razak3 & Akbar S. Hussin4 1 2 3 4
Faculty Faculty Faculty Faculty
of of of of
Dentistry, Dentistry, Dentistry, Dentistry,
Department of Dental Materials Science, International Islamic University Malaysia, Kuantan, Pahang, Malaysia Department of Dental Materials Science, The University of Hong Kong, Hong Kong, China Biomaterials and Technology Unit, University of Malaya, Kuala Lumpur, Malaysia Department of Orthodontics, International Islamic University Malaysia, Kuantan, Malaysia
Keywords 2-hydroxyethyl methacrylate, adhesion promotion, orthodontic adhesives, silane coupling agents, silane-based primers. Correspondence Assistant Professor, Dr Ammar. A. Mustafa, Faculty of Dentistry, Department of Dental Biomaterials, International Islamic University Malaysia, Kuantan Campus, Bandar Indera Mahkota, Jalan Sultan Ahmad Shah, 25200 Kuantan, Pahang, Malaysia. Tel: +60199202975 Emails: [email protected]; [email protected] Received 26 March 2013; accepted 22 October 2013. doi: 10.1111/jicd.12083
Abstract Aim: To evaluate in vitro the effect of different concentrations of 2-hydroxyethyl methacrylate (HEMA) in experimental silane-based primers on shear bond strength of orthodontic adhesives. Methods: Different volume percentages of HEMA were tested in four experimental silane-based primer solutions (additions of HEMA: 0, 5.0 vol%, 25.0 vol% and 50.0 vol%). An experimental silane blend (primer) of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) was prepared and used. The experimental primers together with the control group were applied onto acid-etched premolars for attachment of orthodontic brackets. After artificial aging by thermocycling the shear-bond strength was measured. The fractured surfaces of all specimens were examined under scanning electron microscopy (SEM) to evaluate the failure mode on the enamel surface. Results: The experimental primers showed the highest shear-bond strength of 21.15 MPa (SD 2.70 MPa) and with 25 vol% showed a highly significant increase (P < 0.05) in bond strength. The SEM images showed full penetration of adhesive agents when using silane-based primers. In addition, the SEM images suggested that the predominant failure type was not necessarily the same as for the failure propagation. Conclusions: This preliminary study suggested that nonacidic silane-based primers with HEMA addition might be an alternative to for use as adhesion promoting primers.
Introduction Bond strength and durability are important factors in keeping the integrity of orthodontic adhesive systems. The success of orthodontic brackets in long-term service is highly dependent on the ability of orthodontic adhesive systems to overcome failure to a variety of forces that are exerted on the bracket–adhesive interface and on the adhesive–enamel interface. At the same time, bonding orthodontic brackets with conventional adhesive resins ª 2014 Wiley Publishing Asia Pty Ltd
involves a series of technique-sensitive steps that require a completely dry operative field during the bonding procedure.1 However, in many instances, even with extraprecautious measures, a wet environment is unavoidable prior to bonding the brackets. In order to accomplish this challenge in orthodontic treatments, there is still a need to improve the bonding effectiveness by increasing the bond strength with a system that performs effectively in wet conditions and, at the same time, without endangering the enamel surfaces 1
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during the debonding procedure. Orthodontic adhesives should allow long-term service without any debonding failure. In addition, orthodontic adhesives and cements should allow easy removal of brackets at the end of treatment without damaging the enamel surface.2–4 Given this, bonding of resin materials to enamel substrate is more favorable and reliable than that to dentin because of the hydrophobic nature and the high inorganic content of enamel.5,6 Enamel is known as being a relatively hydrophobic substrate with 4% water, and has only 1–2 wt% of organic constituents. However, demineralized enamel by acid etching with orthophosphoric acid creates a high free-energy surface that allows hydrophobic resin adhesives to wet the surface and to form a strong and durable bond.7 Whereas in dentin–adhesive bonding, the adhesive resin penetrates the collagen network, resulting in a mechanical interlocking with dentin to form a “hybrid layer” or “resin-infiltrated layer”.8–11 Moreover, the use of primer solutions has proven to significantly increase the bond strength of adhesives to both metal and tooth substrate by keeping the tubuli (and other pores) opened.12,13 However, some studies have shown that there are still concerns about long-term debonding failures of orthodontic brackets because of failures at the interface area. This could be attributed to water sorption, impaired penetration of the adhesive, and degradation of the adhesive resin layer.14 In adhesive dentistry, the acid etching technique for bonding adhesive resins to the enamel surface is commonly used to attach orthodontic brackets. This takes place by facilitating the penetration of the resin into enamel micropores, providing the mechanism of micromechanical attachment between the resin and the tooth tissues.15–17 Bond strength can also be affected by the small surface area of the bracket base as well as other factors, such as bracket-base design, treatment of bracket base, and the conditioning treatment at the interface areas.18–21 Many methods have been suggested to increase the bond strength of the orthodontic bracket and hence to achieve long-term service without premature failure, that is, debonding.22–26 The introduction of cross-linking agents as primers may produce significantly higher shear bond strength between some metals and tooth substrates to resincomposite cement.27 Functional silane coupling agents promote significant adhesion between dissimilar materials in various applications,28,29 and in dentistry they have found numerous applications in resin composite luting cements, resin composite restorative filling materials, and as pretreatment to silica-coated30,31 substrates.32 Functional silane monomers usually need to be activated by hydrolysis before they can react with the substrate, through –OH groups on the substrate surface, to form 2
durable bonds.28,32 In the past years, different formulas of primer solutions (some of these primers are based on active hydrophilic constituent such as 2-hydroxyethyl methacrylate [HEMA]) and moisture-active adhesives based on cyanoacrylate have also been introduced to overcome the shortcomings of bonding in a wet field.33–35 The use of a functional silane and a cross-linking silane, a so-called novel silane system,36 may provide enhanced and more durable resin-composite bonding onto Ti and zirconia.37–39 All the above-mentioned materials exhibit excellent biocompatibility in dental applications.39 2-hydroxyethyl methacrylate is a low molecular weight (Mw = 130.14 g/mol) monomer that possesses multiple applications in dentistry, such as in aqueous primer solutions as well as in adhesive resins. It has the ability to act as a co-solvent and to increase the miscibility of hydrophobic and hydrophilic components into the solution by minimizing phase separation.40–42 The aim of this study was to assess: (a) the effect of three different concentrations of HEMA (0,5.0, 25.0, and 50.0 vol%) on bond strength, when mixed with a silane-based experimental primer consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) and bis-1,2-(triethoxysilyl) ethane (BTSE). (b) the shear bond strength of metallic orthodontic brackets bonded with the four different formulas of experimental primers after aging in thermocycled conditions. (c) the failure mode of tested specimens by scanning electron microscopy (SEM) to investigate the surface morphology at the adhesive–enamel interface and the effect of the test primer solutions on the bonding adhesion to enamel. The hypothesis was that the use of different proportions 2-hydroxyethyl methacrylate in the experimental primer based on bis-1,2-(triethoxysilyl) ethane and 3-isocyanatopropyltrimethoxysilane (ICMS) could produce different shear-bond-strength values. In addition, such a formulation would significantly increase the shear bond strength of orthodontic adhesives to enamel and hence will improve the long-term durability of orthodontic brackets in place. The null hypothesis stated that the incorporation of HEMA into the formula of silane-based primers has no significant effect on shear bond strength of orthodontic adhesives.
Materials and methods Specimen preparation Human premolars (N = 125) were used in the current study and they were extracted for orthodontic purposes. ª 2014 Wiley Publishing Asia Pty Ltd
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The selection criteria was based on choosing unrestored, caries-free teeth with no observed damage on their surfaces, in addition to no history of orthodontic treatment or chemical treatment for bleaching. The teeth were stored in distilled water containing 0.2% (wt/vol) thymol to inhibit bacterial growth.42,43 The teeth were then embedded into acrylic moulds to form specimens complying with the test procedures. The specimens (N = 125) were randomly divided into four test groups coded G1–G4 (n = 25) and into one control group (n = 25).
(d) Group 4 (n = 25): the teeth surfaces were applied (for 5 sec) with a primer solution consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) in 45.0% ethanol + 5.0% water + 50.0 vol% 2-hydroxyethyl methacrylate (HEMA). (e) Control group (n = 25): no application of primer solutions. Chemical formulas of the constituents of the test primer solutions are given in Figure 1.
Activation of test primer solutions
Bracket-bonding application
Four experimental silane-based primers were prepared in ethanol:distilled water (95:5), with additions of HEMA (deionized milli-Q water). The pH was adjusted to 4.5 with 1 M acetic acid. The solutions were allowed to stabilize for 24 h at room temperature before use. In this solution, the four sets of silane primers, consisting of 1.0 vol % 3-Isocyanatopropyltrimethoxysilane (ICMS) and 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE), were prepared in 50.0 mL polyethylene bottles and allowed to hydrolase (activate) for 1 h.42
Stainless steel premolar orthodontic brackets with a mesh base were used (Gemini, UniTek 3M, Monrovia, CA, USA). The average surface area of the bracket bases was measured to be 13.0 mm2. The surface area was measured by using a digital caliper (Absolute, Miyutoyo Corp., Kawasaki, Japan). The brackets were attached using OneStep Ortho-Adhesive (Alpha Dent, Austin, TX, USA). In testing, each bracket was exposed to a compressive force using a weight of 300 g for 10 sec, using Force Gram Indicator with round blunt feeler 48 mm (a maximum force 500 g; Correx Co., Bern, Switzerland). The entire excess bonding adhesive was removed using a sharp instrument.44 After storing the specimens in distilled water for 7 days,45 all the groups were subjected to artificial aging by thermocycling, which comprised 500 cycles in a water bath between 5°C and 55°C according to (ISO) TR 11450 standards (1994).46
Bonding procedure For all groups, 0.3 mL water was introduced as a contaminant to the vestibular area of the tooth1 by means of a 0.3 mL insulin syringe (UltiCareTM, St Paul, MN, USA). Enamel surfaces of all the test groups were etched with a 37.5% phosphoric acid gel (Gel etchant 3; SDS Kerr, Orange, CA, USA) for 15 sec, then rinsed with water spray for 10 sec, and then gently air dried with oil-free air for 10 sec. The experimental primer solutions, labeled as OIWA5–OIWA8, were applied to the bonding test groups G1–G4 respectively as follows: (a) Group 1 (n = 25): the teeth surfaces were applied (for 5 sec) with a primer solution consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) in 95.0% ethanol + 5.0% water. (b) Group 2 (n = 25): the teeth surfaces were applied (for 5 sec) with a primer solution consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) in 90.0% ethanol + 5.0% water + 5.0 vol% 2-hydroxyethyl methacrylate (HEMA). (c) Group 3 (n = 25): the teeth surfaces were applied (for 5 sec) with a primer solution consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) in 70.0% ethanol + 5.0% water + 25.0 vol% 2-hydroxyethyl methacrylate (HEMA). ª 2014 Wiley Publishing Asia Pty Ltd
Figure 1. The molecular structures of the two silane compounds used, 3-isocyanatopropyltrimethoxysilane and 1,2,-bis-(triethoxysilyl) ethane (ICMS and BTSE), and the molecular structure of 2-hydroxyethyl methacrylate monomer (HEMA).
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Debonding test Shear bond strength was investigated by debonding the specimens using a precision universal tester (Table top AGS-X, Shimadzu, Japan) at a crosshead speed of 1.0 mm/min. The first failure by the machine represented by the first crack formation was enumerated as the record for shear bond strength, which was calculated in MPa. Debonding was continued until complete separation in order to further study the interfaces as well as the mode of cohesion failure.
ler, Dusseldorf, Germany) and examined under the SEM to study the penetration of the adhesive into the enamel prisms. Numerical data were analyzed using the one-way analysis of variance (ANOVA) and Tukey’s tests with statistical significance set at 0.05. Post-hoc tests were conducted on the analyzed data by using the Dunnett “t” test to compare the test groups against the control group. All data were analyzed using the SPSS 16 (SPSS, Chicago, IL, USA) software. Results
Scanning electron microscopy analysis After testing, all the specimens were examined under a SEM (EVO 50, Zeiss, Jena, Germany) to investigate the surface morphology at the adhesive–enamel interface, and also the remnants of adhesive resin on both enamel surfaces and base of metal brackets. Then, certain specimens with full remnants of adhesive resin on the enamel surface were sectioned longitudinally (IsoMet 1000 Bueh-
Means for shear bond strength and the standard deviations of all groups are given in Tables 1 and 2. The data obtained showed that the highest value for shear bond strength was obtained by group 3 (OIWA7) followed by groups 4 (OIWA8), 2 (OIWA6), and 1 (OIWA5) respectively. Groups 3, with 25% HEMA, and 4, with 50% HEMA, displayed higher shear bond strength means than the other groups (P < 0.05), even though no statistically significant
Table 1. Mean shear bond strength values with standard deviations for all test groups (control + 4 experimental primers) 95% confidence interval for mean Group Experimental Experimental Experimental Experimental Control – no Total
HEMA-free primer 0.5 vol% HEMA-based primer 25.0 vol% HEMA-based primer 50.0 vol% HEMA-based primer application of primer
N
Mean
Standard deviation
Lower bound
Upper bound
Minimum
Maximum
25 25 25 25 25 125
11.7800 14.3215 16.9630 14.3565 8.3045 13.1451
4.77105 5.84396 3.00445 2.46184 4.60958 5.15047
9.5471 11.5864 15.5569 13.2043 6.1472 12.1231
14.0129 17.0566 18.3691 15.5087 10.4618 14.1671
1.50 2.18 11.34 10.14 0.00 0.00
16.75 20.83 22.98 18.50 17.34 22.98
Table 2. Shear bond strength (MPa) results using Dunnett T3 test for multiple comparisons 95% confidence interval
(I) Bond material
(J) Bond material
Experimental HEMA-free primer
Experimental 0.5 vol% HEMA-based primer Experimental 25.0 vol% HEMA-based primer Experimental 50.0 vol% HEMA-based primer Control – no application of primer Experimental 25.0 vol% HEMA-based primer Experimental 50.0 vol% HEMA-based primer Control = No Application of primer Experimental 50.0 vol% HEMA-based primer Control – no application of primer Control – no application of primer
Experimental 0.5 vol% HEMA-based primer Experimental 25.0 vol% HEMA-based primer Experimental 50.0 vol% HEMA-based primer
Mean difference I–J 2.54150 5.18300* 2.57650 3.47550 2.64150 0.03500 6.01700* 2.60650* 8.65850* 6.05200*
Standard error
Significance
1.68693 1.26075 1.20049 1.48343 1.46933 1.41797 1.66433 0.86855 1.23034 1.16852
0.752 0.003 0.016 0.209 0.544 1.000 0.009 0.046 0.000 0.000
Lower bound 7.5466 8.9545 6.1980 0.9162 7.0747 4.3493 1.0750 0.0297 4.9828 2.5322
Upper bound 2.4636 1.4115 1.0450 7.8672 1.7917 4.2793 10.9590 5.1833 12.3342 9.5718
*Mean difference is significant at the 0.05 level.
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differences were found between groups 3 and 4 (P > 0.05). Moreover, no statistically significant increase of the shear bond strength (P > 0.05) was found with group 1, which is based on HEMA-free primer solutions. All the enamel surfaces showed obvious roughness when examined under the SEM. Selected specimens were further fractured manually to study the enamel prisms. The SEM images of enamel surfaces of test groups 2–4 showed that enamel prisms were totally saturated with
Silane-based primers and bond strength
bonding material after the debonding process was performed on the surfaces (Figures 2 and 3). Failure mode assessment showed that the origin of cracks was not necessarily the same as for the failure propagation (Table 3). The predominant mode of failure was found at the adhesive part of the first crack on debonding (47.2%). Similarly, the predominant mode of failure was at the same site after complete debonding (44%) (Figures 4 and 5). Discussion
Figure 2. An scanning electron microscopy image showing an area of longitudinally sectioned enamel after the debonding test. The specimen was treated with 25 vol% of HEMA-based silane primer. The section shows enamel structure saturated with the bonding agent.
In the process of bonding orthodontic brackets to enamel surfaces, the conventional prebonding procedure combines the use of acid etch and a primer solution and then the adhesive resin for bonding.47–49 Primers are used effectively to increase the bond strength of the bracket adhesives to tooth substrates.43,50–52 They can enhance the shear bond strength even in nonetchable materials.32 In the current study, four formulas of experimental silane-based primers with different proportions of the HEMA solution were investigated for their effect on the shear bond strength of orthodontic bracket adhesives to enamel surface. The first group of the test series was HEMA-free primer, whereas the other groups were based on HEMA with different volume concentrations. In a fundamental study about HEMA and adhesion promotion, Munksgaard and Asmussen53 concluded that the bond strength was highly dependent on the HEMA concentration. Interestingly, incorporation of hydrophilic materials into enamel bonding can facilitate the process of adhesive resin infiltration into the already etched enamel surface at the prism level. This mechanism of bonding between enamel and resin adhesive is based on reducing interfacial porosity and hence bonding defects.54 When using acetone-based adhesives, wetting the enamel surface by immediate priming is a mandatory step to guarantee that
Figure 3. Scanning electron microscopy images showing enamel prisms fully saturated with bonding agents. Similar results were obtained with both 25 and 50 vol% of HEMA-based primers.
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Table 3. Failure mode assessment and classification of the tested specimens
Debonding failure First crack formation site on debonding
Site of failure propagation after complete debonding
Primer group 1 2 3 4 Control Total 1 2 3 4 Control Total
n
Debonding at enamel-adhesive interface (%)
Debonding within the adhesive resin (%)
Debonding at bracket-adhesive interface (%)
25 25 25 25 25 125 25 25 25 25 25 125
64 28 12 12 84 40 56 40 24 24 68 24.4
28 48 80 72 8 47.2 36 44 64 60 16 44
8 24 8 16 8 12.8 8 16 12 16 16 13.6
Figure 4. An scanning electron microscopy image showing adhesive remnants on the bracket base after debonding. The results were obtained after treating the specimen with 5vol% HEMA-based silane primer.
Figure 5. An scanning electron microscopy image showing full remnants of adhesive on the bracket base after debonding. The results were obtained after treating the specimens with 25 and 50vol% HEMA-based silane primers.
all enamel irregularities are filled by the primer solutions and the adhesives.48,55 Generally, it has been well demonstrated that the use of HEMA can improve the bond strength to dentin, enhancing wetting of the dentin subsurface,47 and preventing phase separation between the hydrophobic components and water.56 Water sorption in one-step self-etch adhesives is dependent upon HEMA concentration. Therefore, the incorpora-
tion of HEMA into the formula of primers can reduce the inadequacies that occur when HEMA is incorporated into the formula of the adhesive resins, and this can reduce the undesirable effect of water sorption by the adhesive resin.57–59 In the current study, however, no water sorption measurements were made but left for other future studies. Orthodontic adhesives should display shear bond strength more than 6–8 MPa, so as to accomplish suitable
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clinical performance.60 Thus, all the tested primer formulas have shown greater bond strength values, which are adequate for bonding metal orthodontic brackets. Significant enhancement in bond strength was achieved with group 3 (21.1 2.7 MPa) by adding 25.0 vol% of HEMA in the formulation of the experimental primer, with no significant difference from group 4 with 50.0 vol % of HEMA (20.3 2.6). This could be attributed to the fact that the higher contents of HEMA could increase the degradation of dental adhesive resins61 and at the same time could induce better wetting and infiltration properties of the adhesive. Adversely, when HEMA is added in higher concentrations, bond strength may be impaired due to increased hydrophilicity, decreased removal of water, and reduced co-polymerization.34,56 The formulation of group 2 with 5.0 vol% HEMA showed an increase in the recorded shear bond strength values (20.9 2.7 MPa) but with lower numerical records than groups and 4. The test primer, a HEMA-free group 1 showed no significant increase in the shear bond strength. Shear-bond-strength values less than that exhibited by using HEMA-based primers indicated that incorporation of HEMA into the primer solution has significantly increased the shear bond strength of orthodontic adhesives to enamel surfaces. Immediate micromechanical retention to already etched enamel surfaces is better achieved when the bonding agent invades deeply into the enamel prisms without porosities. In addition to the desirable effect of HEMA, the hydrophilic characters of the test primer solutions also could be attributed to the presence of the ethanol solvent and water, on which they provide a definite tolerance to wet conditions.1 The use of ethanol as a solvent to HEMA has a certain effect in producing high bond strength when compared to other solvents.62,63 With the exception of a few specimens, no predictable enamel fracture was observed during the study. Fracture of enamel is an actual possibility during References 1 Vicente A, Toledano M, Bravo LA et al. Effect of water contamination on the shear bond strength of five orthodontic adhesives. Med Oral Patol Oral Cir Bucal 2010; 15: 820–6. 2 Klocke A, Tadic A, Kahl-Nieke B et al. An optimized synthetic substrate for orthodontic bond strength testing. Dent Mater 2003; 19: 773–8. 3 Rock WP, Abdullah SB. Shear bond strengths produced by composite and compomer light cured orthodontic adhesives. J Dent 1997; 25: 243–9.
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the debonding process of the brackets, especially if the tooth is nonvital.64,65 The 2-hydroxyethyl methacrylate additions used in this study seemed to improve the penetration of the adhesive into the enamel, hence increasing the shear bond strength of the orthodontic brackets to enamel. At the same time, the shear bond strength reached values higher than 25 MPa without endangering the enamel surface by causing perceptible fracture throughout the study. The use of all test formulae produced clinically suitable shear bond strength even after storage and thermocycling between 5 and 55°C for 500 cycles. This study has indicated that nonacidic monomers based on the two silane monomers (ICSM and BTSE) and varying additions of HEMA to replace ethanol, might be used clinically as an adhesion promoting comonomer for orthodontic adhesives that lack etching and priming constituents. Conclusion It can be concluded that by increasing the volume percentage of HEMA additions, together with incorporating varying percentages of ethanol solvent, would at least partially resolve the problem of bonding to enamel. This effect might be reflected confidently on the durability and integrity of the fixed orthodontic appliances. The clinical use of the experimental primer solutions on etched enamel for direct bonding of orthodontic brackets can be strongly encouraged. Acknowledgments The research team would like to thank Research Management Center, IIUM for funding this study. The authors would like also to thank Dr Lim Chin Choon MBBS (UM) for helping in the statistical analyses for this study.
4 Sunna S, Rock WP. An ex vivo investigation into the bond strength of orthodontic brackets and adhesive systems. Brit J Orthod 1999; 26: 47–50. 5 Pashley DH. Dentin: a dynamic substrate-a review. Scanning Microsc 1989; 3: 161–76. 6 Soderholm K-JM. Correlation of in vivo and in vitro performance of adhesive restorative materials: a report of the ASC MD156 Task Group on test methods for the adhesion of restorative materials. Dent Mater 1991; 7: 74–83.
7 Summitt JB, Robbins JW, Hilton TJ, Schwartz RS. Fundamentals of operative dentistry: a contemporary approach. Hanover Park, IL: Quintessence Publishing Co., 2006. 8 Perdigao J, Swift EJ, Denehy GE Jr et al. In vitro bond strengths and SEM evaluation of dentin bonding systems to different dentin substrates. J Dent Res 1994; 73: 44–55. 9 Nakabayashi N, Kojima K, Masuhara E. The promotion of adhesion by the infiltration of monomers into tooth substrates. J Biomed Mater Res 1982; 16: 265–73. 7
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10 Erickson RL. Mechanism and clinical implications of bond formation for two dentin bonding agents. Am J Dent 1989; 2: 117–23. 11 Van-Meerbeek B, Inokoshi S, Braem M et al. Morphological aspects of the resin-dentin interdiffusion zone with different dentin adhesive systems. J Dent Res 1992; 71: 1530–40. 12 Dias de Souza GM, Thompson VP, Braga RR. Effect of metal primers on microtensile bond strength between zirconia and resin cements. J Prosthet Dent 2011; 105: 296–303. 13 Bulbul M, Kesim B. The effect of primers on shear bond strength of acrylic resins to different types of metals. J Prosthet Dent 2010; 103: 303–8. 14 Shinoda Y, Masatoshi N, Keiichi H et al. Effect of smear layer characteristics on dentin bonding durability of HEMA-free and HEMA-containing one-step self-etch adhesives. Dent Mater J 2011; 30: 501–10. 15 Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surface. J Dent Res 1955; 34: 849–53. 16 Watanabe T, Tsubota K, Takamizawa T et al. Effect of prior acid etching on bonding durability of single-step adhesives. Oper Dent 2008; 33: 426–33. 17 Jou GE, Leung RL, White SN et al. Bonding ceramic brackets with lightcured glass ionomer Cements. J Clin Orthod 1995; 3: 184–7. 18 Maijer R, Smith DC. Variables influencing the bond strength of metal orthodontic bracket bases. Am J Orthod 1981; 79: 20–34. 19 Regan D, van Noort R, O’Keefe C. The effects of recycling on the tensile bond strength of new and clinically used stainless steel orthodontic brackets: an in vitro study. Brit J Orthod 1990; 17: 137–45. 20 Chadwick RG. Thermocycling: the effects upon compressive strength and abrasion resistance of three composite resins. J Oral Rehabil 1994; 21: 533–43. 21 Willems G, Carels CE, Verbeke G. In vitro peel/shear bond strength eval-
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24
25
26
27
28
29
30
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uation of orthodontic bracket base design. J Dent 1997; 25: 271–8. McAlarney ME, Brenn P. A modified direct technique versus conventional direct placement of brackets: in vitro bond strength comparison. Am J Orthod 1993; 104: 575–83. Lopez JI. Retentive shear strengths of various bonding attachment bases. Am J Orthod 1980; 77: 669–78. Vicente A, Bravo LA, Romero M et al. Effects of 3 adhesion promoters on the shear bond strength of orthodontic brackets: an in-vitro study. Amer J Orthod Dentofacial Orthop 2006; 129: 390–5. Yi GK, Dunn WJ, Taloumis LJ. Shear bond strength comparison between direct and indirect bonded orthodontic brackets. Amer J Orthod Dentofacial Orthop 2003; 124: 577–81. Pulido LG, Powers JM. Bond strength of orthodontic direct-bonding cement–plastic bracket systems in vitro. Am J Orthod 1983; 83: 124–30. Lung CYK, Matinlinna JP. Resin bonding to silicatized zirconia with two isocyanatosilanes and a crosslinking silane. Part I: experimental. Silicon 2010; 2: 153–61. Lung CYK, Matinlinna JP. Silanes for adhesion promotion and surface modification. In: Moriguchi K, Utagawa S, ed. Silanes: chemistry, applications and performance, 1st edn. Hauppauge, NY: Nova Science Publishers Incorporated, 2012: 87–109. Mustafa AA, Matinlinna JP, Choi AH et al. Fracture strength and fractographic analysis of zirconia copings treated with four experimental silane primers. J Adhes Sci Technol 2013; 27: 68–80. Heikkinen TT, Lassila LVJ, Matinlinna JP et al. Dental zirconia adhesion with silicon compounds using some experimental and conventional surface conditioning methods. Silicon 2009; 1: 199–201. Lung CYK, Matinlinna JP. Aspects of silane coupling agents and surface conditioning in dentistry: an overview. Dent Mater 2012; 28: 467–77.
32 Matinlinna JP, Lassila LVJ, Kangasniemi I et al. Isocyanato- and methacryloxysilanes promote bis-GMA adhesion to titanium. J Dent Res 2005; 84: 360–4. 33 Vicente A, Bravo LA, Romero M et al. Shear bond strength of orthodontic brackets bonded with selfetching primers. Am J Dent 2005; 18: 256–60. 34 Van Landuyta KL, Snauwaert J, De Munck J et al. Systematic review of the chemical composition of contemporary dental adhesives. Biomaterials 2007; 28: 3757–85. 35 Felizardo KR, Lemos LVFM, DeCarvalho RV et al. Bond strength of HEMA-containing versus HEMA-free self-etch adhesive systems to dentin. Braz Dent J 2011; 22: 468–72. 36 Matinlinna JP, Lassila LVJ, Vallittu PK. The effect of three silane coupling agents and their blends with a crosslinker silane on bonding a bis-GMA resin to silicatized titanium (a novel silane system). J Dent 2006; 34: 740–6. € 37 Matinlinna JP, Ozcan M, Lassila LVJ et al. Effect of the cross-linker silane concentration in a novel silane system on bonding resin-composite cement. Acta Odontol Scand 2008; 66: 250–5. 38 Matinlinna JP, Tsoi JKH, de Vries J et al. Characterization of novel silane systems on titanium implant surfaces. Clin Oral Implants Res 2013; 24: 688–97. 39 Matinlinna JP, Choi AH, Tsoi JKH. Bonding promotion of resin-composite to silica-coated zirconia implant surface using a novel silane system. Clin Oral Implants Res 2013; 4: 290–6. 40 Mallineni SKS, Nuvvula S, Matinlinna JP et al. Biocompatibility of various dental materials of contemporary dentistry: a narrative insight. J Invest Clin Dent 2013; 4: 9–19. 41 Nakabayashi N, Saimi Y. Bonding to intact dentin. J Dent Res 1996; 75: 1706–15. 42 Scougall-Vilchis RJ, Ohashi S, Yamamotoc K. Effects of 6 self-etching primers on shear bond strength of
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orthodontic brackets. Amer J Orthod Dentofacial Orthop 2009; 135: 424–5. Bishara SE, Soliman M, Laffoon J et al. Effect of antimicrobial monomer-containing adhesive on shear bond strength of orthodontic brackets. Angle Orthod 2005; 75: 397–9. Bishara SE, VonWald L, Laffoon JF et al. Effect of using a new cyanoacrylate adhesive on the shear bond strength of orthodontic brackets. Angle Orthod 2001; 71: 466–9. Feilzer AJ, De Gee AJ, Davidson CL. Relaxation of polymerization contraction shear stresses by hygroscopic expansion. J Dent Res 1990; 69: 36–9. The International Organization for Standardization (ISO). TR 11450 Standard. Geneva, Switzerland: Dental Materials–Guidance on Testing on Adhesion to Tooth Structure, 1994. Van Meerbeek B, De Munck J, Yoshida Y et al. Buonocore memorial lecture. Adhesion to enamel and dentin: current status and future challenges. Oper Dent 2003; 28: 215–35. Chigira H, Koike T, Hasegawa T et al. Effect of the self-etching dentin primers on the bonding efficacy of dentine adhesive. Dent Mater J 1989; 8: 86–92. Bishara SE, VonWald L, Laffoon JF et al. Effect of using a self-etch primer/ adhesive on the shear bond strength of orthodontic brackets. Amer J Orthod Dentofacial Orthop 2001; 119: 621–4.
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50 Nakabayashi N. Dentinal bonding mechanisms. Quintessence Int 1991; 22: 73–4. 51 Shinya M, Shinya A, Lassila LV et al. Treated enamel surface patterns associated with five orthodontic adhesive systems – surface morphology and shear bond strength. Dent Mater J 2008; 27: 1–6. 52 Powers JM, Sakaguchi R. Craig’s restorative dental materials, 12th edn. St. Louis: Mosby Elsevier, 2006. 53 Munksgaard EC, Asmussen E. Materials science bond strength between dentin and restorative resins mediated by mixtures of hema and glutaraldehyde. J Dent Res 1984; 63: 1087–9. 54 Brantley WA, Eliades T. Orthodontic materials: scientific and clinical aspects. Stuttgart: Thieme Medical, 2001: 203. 55 Tay FR, Gwinnett AJ, Pang KM et al. Resin permeation into acid-conditioned, moist, and dry dentin: a paradigm using water-free adhesive primers. J Dent Res 1996; 75: 1034–44. 56 Van Landuyt KL, De Munck J, Snauwaert J et al. Monomer-solvent phase separation in one-step self-etching adhesives. J Dent Res 2005; 84: 183–8. 57 Shinoda Y, Nakajima M, Hosaka K et al. Effect of smear layer characteristics on dentin bonding durability of HEMA-free and HEMA-containing one-step self-etch adhesives. Dent Mater J 2011; 30: 501–10.
58 Nishitani Y, Yoshiyama M, Hosaka K et al. Use of Hoy’s solubility parameters to predict water sorption/solubility of experimental primers and adhesives. Eur J Oral Sci 2007; 115: 81–6. 59 Hosaka K, Nakajima M, Takahashi M et al. Relationship between mechanical properties of one-step self-etch adhesives and water sorption. Dent Mater 2010; 26: 360–7. 60 Reynolds IR, von Fraunhofer JA. Direct bonding of orthodontic attachments to teeth: the relation of adhesive bond strength to gauze meshsize. Brit J Orthod 1975; 3: 91–5. 61 Collares FM, Ogliari FA, Zanchi CH et al. Influence of 2-hydroxyethyl methacrylate concentration on polymer network of adhesive resin. J Adhes Dent 2011; 13: 125–9. 62 Carvalho RM, Mendoncßa JS, Santiag SL et al. Effects of HEMA/solvent combinations on bond strength to dentin. J Dent Res 2003; 82: 597–601. 63 Garcia FCP, Otsuki M, Pashley DH et al. Effects of solvents on the early stage stiffening rate of demineralized dentin matrix. J Dent 2005; 33: 371–7. 64 Joseph VP, Rossouw E. The shear bond strengths of stainless steel and ceramic brackets used with chemically and light-activated composite resins. Amer J Orthod Dentofacial Orthop 1990; 97: 121–5. 65 Bishara SE, Fehr DE. Ceramic brackets: something old, something new; a review. Semin Orthod 1997; 3: 178–88.
9
ORIGINAL ARTICLE Silane-Based Primers and Bond Strength
Effect of experimental silane-based primers with various contents of 2-hydroxyethyl methacrylate on the bond strength of orthodontic adhesives Ammar A. Mustafa1, Jukka P. Matinlinna2, Aziz A. Razak3 & Akbar S. Hussin4 1 2 3 4
Faculty Faculty Faculty Faculty
of of of of
Dentistry, Dentistry, Dentistry, Dentistry,
Department of Dental Materials Science, International Islamic University Malaysia, Kuantan, Pahang, Malaysia Department of Dental Materials Science, The University of Hong Kong, Hong Kong, China Biomaterials and Technology Unit, University of Malaya, Kuala Lumpur, Malaysia Department of Orthodontics, International Islamic University Malaysia, Kuantan, Malaysia
Keywords 2-hydroxyethyl methacrylate, adhesion promotion, orthodontic adhesives, silane coupling agents, silane-based primers. Correspondence Assistant Professor, Dr Ammar. A. Mustafa, Faculty of Dentistry, Department of Dental Biomaterials, International Islamic University Malaysia, Kuantan Campus, Bandar Indera Mahkota, Jalan Sultan Ahmad Shah, 25200 Kuantan, Pahang, Malaysia. Tel: +60199202975 Emails: [email protected]; [email protected] Received 26 March 2013; accepted 22 October 2013. doi: 10.1111/jicd.12083
Abstract Aim: To evaluate in vitro the effect of different concentrations of 2-hydroxyethyl methacrylate (HEMA) in experimental silane-based primers on shear bond strength of orthodontic adhesives. Methods: Different volume percentages of HEMA were tested in four experimental silane-based primer solutions (additions of HEMA: 0, 5.0 vol%, 25.0 vol% and 50.0 vol%). An experimental silane blend (primer) of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) was prepared and used. The experimental primers together with the control group were applied onto acid-etched premolars for attachment of orthodontic brackets. After artificial aging by thermocycling the shear-bond strength was measured. The fractured surfaces of all specimens were examined under scanning electron microscopy (SEM) to evaluate the failure mode on the enamel surface. Results: The experimental primers showed the highest shear-bond strength of 21.15 MPa (SD 2.70 MPa) and with 25 vol% showed a highly significant increase (P < 0.05) in bond strength. The SEM images showed full penetration of adhesive agents when using silane-based primers. In addition, the SEM images suggested that the predominant failure type was not necessarily the same as for the failure propagation. Conclusions: This preliminary study suggested that nonacidic silane-based primers with HEMA addition might be an alternative to for use as adhesion promoting primers.
Introduction Bond strength and durability are important factors in keeping the integrity of orthodontic adhesive systems. The success of orthodontic brackets in long-term service is highly dependent on the ability of orthodontic adhesive systems to overcome failure to a variety of forces that are exerted on the bracket–adhesive interface and on the adhesive–enamel interface. At the same time, bonding orthodontic brackets with conventional adhesive resins ª 2014 Wiley Publishing Asia Pty Ltd
involves a series of technique-sensitive steps that require a completely dry operative field during the bonding procedure.1 However, in many instances, even with extraprecautious measures, a wet environment is unavoidable prior to bonding the brackets. In order to accomplish this challenge in orthodontic treatments, there is still a need to improve the bonding effectiveness by increasing the bond strength with a system that performs effectively in wet conditions and, at the same time, without endangering the enamel surfaces 1
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during the debonding procedure. Orthodontic adhesives should allow long-term service without any debonding failure. In addition, orthodontic adhesives and cements should allow easy removal of brackets at the end of treatment without damaging the enamel surface.2–4 Given this, bonding of resin materials to enamel substrate is more favorable and reliable than that to dentin because of the hydrophobic nature and the high inorganic content of enamel.5,6 Enamel is known as being a relatively hydrophobic substrate with 4% water, and has only 1–2 wt% of organic constituents. However, demineralized enamel by acid etching with orthophosphoric acid creates a high free-energy surface that allows hydrophobic resin adhesives to wet the surface and to form a strong and durable bond.7 Whereas in dentin–adhesive bonding, the adhesive resin penetrates the collagen network, resulting in a mechanical interlocking with dentin to form a “hybrid layer” or “resin-infiltrated layer”.8–11 Moreover, the use of primer solutions has proven to significantly increase the bond strength of adhesives to both metal and tooth substrate by keeping the tubuli (and other pores) opened.12,13 However, some studies have shown that there are still concerns about long-term debonding failures of orthodontic brackets because of failures at the interface area. This could be attributed to water sorption, impaired penetration of the adhesive, and degradation of the adhesive resin layer.14 In adhesive dentistry, the acid etching technique for bonding adhesive resins to the enamel surface is commonly used to attach orthodontic brackets. This takes place by facilitating the penetration of the resin into enamel micropores, providing the mechanism of micromechanical attachment between the resin and the tooth tissues.15–17 Bond strength can also be affected by the small surface area of the bracket base as well as other factors, such as bracket-base design, treatment of bracket base, and the conditioning treatment at the interface areas.18–21 Many methods have been suggested to increase the bond strength of the orthodontic bracket and hence to achieve long-term service without premature failure, that is, debonding.22–26 The introduction of cross-linking agents as primers may produce significantly higher shear bond strength between some metals and tooth substrates to resincomposite cement.27 Functional silane coupling agents promote significant adhesion between dissimilar materials in various applications,28,29 and in dentistry they have found numerous applications in resin composite luting cements, resin composite restorative filling materials, and as pretreatment to silica-coated30,31 substrates.32 Functional silane monomers usually need to be activated by hydrolysis before they can react with the substrate, through –OH groups on the substrate surface, to form 2
durable bonds.28,32 In the past years, different formulas of primer solutions (some of these primers are based on active hydrophilic constituent such as 2-hydroxyethyl methacrylate [HEMA]) and moisture-active adhesives based on cyanoacrylate have also been introduced to overcome the shortcomings of bonding in a wet field.33–35 The use of a functional silane and a cross-linking silane, a so-called novel silane system,36 may provide enhanced and more durable resin-composite bonding onto Ti and zirconia.37–39 All the above-mentioned materials exhibit excellent biocompatibility in dental applications.39 2-hydroxyethyl methacrylate is a low molecular weight (Mw = 130.14 g/mol) monomer that possesses multiple applications in dentistry, such as in aqueous primer solutions as well as in adhesive resins. It has the ability to act as a co-solvent and to increase the miscibility of hydrophobic and hydrophilic components into the solution by minimizing phase separation.40–42 The aim of this study was to assess: (a) the effect of three different concentrations of HEMA (0,5.0, 25.0, and 50.0 vol%) on bond strength, when mixed with a silane-based experimental primer consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) and bis-1,2-(triethoxysilyl) ethane (BTSE). (b) the shear bond strength of metallic orthodontic brackets bonded with the four different formulas of experimental primers after aging in thermocycled conditions. (c) the failure mode of tested specimens by scanning electron microscopy (SEM) to investigate the surface morphology at the adhesive–enamel interface and the effect of the test primer solutions on the bonding adhesion to enamel. The hypothesis was that the use of different proportions 2-hydroxyethyl methacrylate in the experimental primer based on bis-1,2-(triethoxysilyl) ethane and 3-isocyanatopropyltrimethoxysilane (ICMS) could produce different shear-bond-strength values. In addition, such a formulation would significantly increase the shear bond strength of orthodontic adhesives to enamel and hence will improve the long-term durability of orthodontic brackets in place. The null hypothesis stated that the incorporation of HEMA into the formula of silane-based primers has no significant effect on shear bond strength of orthodontic adhesives.
Materials and methods Specimen preparation Human premolars (N = 125) were used in the current study and they were extracted for orthodontic purposes. ª 2014 Wiley Publishing Asia Pty Ltd
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The selection criteria was based on choosing unrestored, caries-free teeth with no observed damage on their surfaces, in addition to no history of orthodontic treatment or chemical treatment for bleaching. The teeth were stored in distilled water containing 0.2% (wt/vol) thymol to inhibit bacterial growth.42,43 The teeth were then embedded into acrylic moulds to form specimens complying with the test procedures. The specimens (N = 125) were randomly divided into four test groups coded G1–G4 (n = 25) and into one control group (n = 25).
(d) Group 4 (n = 25): the teeth surfaces were applied (for 5 sec) with a primer solution consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) in 45.0% ethanol + 5.0% water + 50.0 vol% 2-hydroxyethyl methacrylate (HEMA). (e) Control group (n = 25): no application of primer solutions. Chemical formulas of the constituents of the test primer solutions are given in Figure 1.
Activation of test primer solutions
Bracket-bonding application
Four experimental silane-based primers were prepared in ethanol:distilled water (95:5), with additions of HEMA (deionized milli-Q water). The pH was adjusted to 4.5 with 1 M acetic acid. The solutions were allowed to stabilize for 24 h at room temperature before use. In this solution, the four sets of silane primers, consisting of 1.0 vol % 3-Isocyanatopropyltrimethoxysilane (ICMS) and 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE), were prepared in 50.0 mL polyethylene bottles and allowed to hydrolase (activate) for 1 h.42
Stainless steel premolar orthodontic brackets with a mesh base were used (Gemini, UniTek 3M, Monrovia, CA, USA). The average surface area of the bracket bases was measured to be 13.0 mm2. The surface area was measured by using a digital caliper (Absolute, Miyutoyo Corp., Kawasaki, Japan). The brackets were attached using OneStep Ortho-Adhesive (Alpha Dent, Austin, TX, USA). In testing, each bracket was exposed to a compressive force using a weight of 300 g for 10 sec, using Force Gram Indicator with round blunt feeler 48 mm (a maximum force 500 g; Correx Co., Bern, Switzerland). The entire excess bonding adhesive was removed using a sharp instrument.44 After storing the specimens in distilled water for 7 days,45 all the groups were subjected to artificial aging by thermocycling, which comprised 500 cycles in a water bath between 5°C and 55°C according to (ISO) TR 11450 standards (1994).46
Bonding procedure For all groups, 0.3 mL water was introduced as a contaminant to the vestibular area of the tooth1 by means of a 0.3 mL insulin syringe (UltiCareTM, St Paul, MN, USA). Enamel surfaces of all the test groups were etched with a 37.5% phosphoric acid gel (Gel etchant 3; SDS Kerr, Orange, CA, USA) for 15 sec, then rinsed with water spray for 10 sec, and then gently air dried with oil-free air for 10 sec. The experimental primer solutions, labeled as OIWA5–OIWA8, were applied to the bonding test groups G1–G4 respectively as follows: (a) Group 1 (n = 25): the teeth surfaces were applied (for 5 sec) with a primer solution consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) in 95.0% ethanol + 5.0% water. (b) Group 2 (n = 25): the teeth surfaces were applied (for 5 sec) with a primer solution consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) in 90.0% ethanol + 5.0% water + 5.0 vol% 2-hydroxyethyl methacrylate (HEMA). (c) Group 3 (n = 25): the teeth surfaces were applied (for 5 sec) with a primer solution consisting of 1.0 vol% 3-isocyanatopropyltrimethoxysilane (ICMS) + 0.5% bis-1,2-(triethoxysilyl) ethane (BTSE) in 70.0% ethanol + 5.0% water + 25.0 vol% 2-hydroxyethyl methacrylate (HEMA). ª 2014 Wiley Publishing Asia Pty Ltd
Figure 1. The molecular structures of the two silane compounds used, 3-isocyanatopropyltrimethoxysilane and 1,2,-bis-(triethoxysilyl) ethane (ICMS and BTSE), and the molecular structure of 2-hydroxyethyl methacrylate monomer (HEMA).
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Debonding test Shear bond strength was investigated by debonding the specimens using a precision universal tester (Table top AGS-X, Shimadzu, Japan) at a crosshead speed of 1.0 mm/min. The first failure by the machine represented by the first crack formation was enumerated as the record for shear bond strength, which was calculated in MPa. Debonding was continued until complete separation in order to further study the interfaces as well as the mode of cohesion failure.
ler, Dusseldorf, Germany) and examined under the SEM to study the penetration of the adhesive into the enamel prisms. Numerical data were analyzed using the one-way analysis of variance (ANOVA) and Tukey’s tests with statistical significance set at 0.05. Post-hoc tests were conducted on the analyzed data by using the Dunnett “t” test to compare the test groups against the control group. All data were analyzed using the SPSS 16 (SPSS, Chicago, IL, USA) software. Results
Scanning electron microscopy analysis After testing, all the specimens were examined under a SEM (EVO 50, Zeiss, Jena, Germany) to investigate the surface morphology at the adhesive–enamel interface, and also the remnants of adhesive resin on both enamel surfaces and base of metal brackets. Then, certain specimens with full remnants of adhesive resin on the enamel surface were sectioned longitudinally (IsoMet 1000 Bueh-
Means for shear bond strength and the standard deviations of all groups are given in Tables 1 and 2. The data obtained showed that the highest value for shear bond strength was obtained by group 3 (OIWA7) followed by groups 4 (OIWA8), 2 (OIWA6), and 1 (OIWA5) respectively. Groups 3, with 25% HEMA, and 4, with 50% HEMA, displayed higher shear bond strength means than the other groups (P < 0.05), even though no statistically significant
Table 1. Mean shear bond strength values with standard deviations for all test groups (control + 4 experimental primers) 95% confidence interval for mean Group Experimental Experimental Experimental Experimental Control – no Total
HEMA-free primer 0.5 vol% HEMA-based primer 25.0 vol% HEMA-based primer 50.0 vol% HEMA-based primer application of primer
N
Mean
Standard deviation
Lower bound
Upper bound
Minimum
Maximum
25 25 25 25 25 125
11.7800 14.3215 16.9630 14.3565 8.3045 13.1451
4.77105 5.84396 3.00445 2.46184 4.60958 5.15047
9.5471 11.5864 15.5569 13.2043 6.1472 12.1231
14.0129 17.0566 18.3691 15.5087 10.4618 14.1671
1.50 2.18 11.34 10.14 0.00 0.00
16.75 20.83 22.98 18.50 17.34 22.98
Table 2. Shear bond strength (MPa) results using Dunnett T3 test for multiple comparisons 95% confidence interval
(I) Bond material
(J) Bond material
Experimental HEMA-free primer
Experimental 0.5 vol% HEMA-based primer Experimental 25.0 vol% HEMA-based primer Experimental 50.0 vol% HEMA-based primer Control – no application of primer Experimental 25.0 vol% HEMA-based primer Experimental 50.0 vol% HEMA-based primer Control = No Application of primer Experimental 50.0 vol% HEMA-based primer Control – no application of primer Control – no application of primer
Experimental 0.5 vol% HEMA-based primer Experimental 25.0 vol% HEMA-based primer Experimental 50.0 vol% HEMA-based primer
Mean difference I–J 2.54150 5.18300* 2.57650 3.47550 2.64150 0.03500 6.01700* 2.60650* 8.65850* 6.05200*
Standard error
Significance
1.68693 1.26075 1.20049 1.48343 1.46933 1.41797 1.66433 0.86855 1.23034 1.16852
0.752 0.003 0.016 0.209 0.544 1.000 0.009 0.046 0.000 0.000
Lower bound 7.5466 8.9545 6.1980 0.9162 7.0747 4.3493 1.0750 0.0297 4.9828 2.5322
Upper bound 2.4636 1.4115 1.0450 7.8672 1.7917 4.2793 10.9590 5.1833 12.3342 9.5718
*Mean difference is significant at the 0.05 level.
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differences were found between groups 3 and 4 (P > 0.05). Moreover, no statistically significant increase of the shear bond strength (P > 0.05) was found with group 1, which is based on HEMA-free primer solutions. All the enamel surfaces showed obvious roughness when examined under the SEM. Selected specimens were further fractured manually to study the enamel prisms. The SEM images of enamel surfaces of test groups 2–4 showed that enamel prisms were totally saturated with
Silane-based primers and bond strength
bonding material after the debonding process was performed on the surfaces (Figures 2 and 3). Failure mode assessment showed that the origin of cracks was not necessarily the same as for the failure propagation (Table 3). The predominant mode of failure was found at the adhesive part of the first crack on debonding (47.2%). Similarly, the predominant mode of failure was at the same site after complete debonding (44%) (Figures 4 and 5). Discussion
Figure 2. An scanning electron microscopy image showing an area of longitudinally sectioned enamel after the debonding test. The specimen was treated with 25 vol% of HEMA-based silane primer. The section shows enamel structure saturated with the bonding agent.
In the process of bonding orthodontic brackets to enamel surfaces, the conventional prebonding procedure combines the use of acid etch and a primer solution and then the adhesive resin for bonding.47–49 Primers are used effectively to increase the bond strength of the bracket adhesives to tooth substrates.43,50–52 They can enhance the shear bond strength even in nonetchable materials.32 In the current study, four formulas of experimental silane-based primers with different proportions of the HEMA solution were investigated for their effect on the shear bond strength of orthodontic bracket adhesives to enamel surface. The first group of the test series was HEMA-free primer, whereas the other groups were based on HEMA with different volume concentrations. In a fundamental study about HEMA and adhesion promotion, Munksgaard and Asmussen53 concluded that the bond strength was highly dependent on the HEMA concentration. Interestingly, incorporation of hydrophilic materials into enamel bonding can facilitate the process of adhesive resin infiltration into the already etched enamel surface at the prism level. This mechanism of bonding between enamel and resin adhesive is based on reducing interfacial porosity and hence bonding defects.54 When using acetone-based adhesives, wetting the enamel surface by immediate priming is a mandatory step to guarantee that
Figure 3. Scanning electron microscopy images showing enamel prisms fully saturated with bonding agents. Similar results were obtained with both 25 and 50 vol% of HEMA-based primers.
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Table 3. Failure mode assessment and classification of the tested specimens
Debonding failure First crack formation site on debonding
Site of failure propagation after complete debonding
Primer group 1 2 3 4 Control Total 1 2 3 4 Control Total
n
Debonding at enamel-adhesive interface (%)
Debonding within the adhesive resin (%)
Debonding at bracket-adhesive interface (%)
25 25 25 25 25 125 25 25 25 25 25 125
64 28 12 12 84 40 56 40 24 24 68 24.4
28 48 80 72 8 47.2 36 44 64 60 16 44
8 24 8 16 8 12.8 8 16 12 16 16 13.6
Figure 4. An scanning electron microscopy image showing adhesive remnants on the bracket base after debonding. The results were obtained after treating the specimen with 5vol% HEMA-based silane primer.
Figure 5. An scanning electron microscopy image showing full remnants of adhesive on the bracket base after debonding. The results were obtained after treating the specimens with 25 and 50vol% HEMA-based silane primers.
all enamel irregularities are filled by the primer solutions and the adhesives.48,55 Generally, it has been well demonstrated that the use of HEMA can improve the bond strength to dentin, enhancing wetting of the dentin subsurface,47 and preventing phase separation between the hydrophobic components and water.56 Water sorption in one-step self-etch adhesives is dependent upon HEMA concentration. Therefore, the incorpora-
tion of HEMA into the formula of primers can reduce the inadequacies that occur when HEMA is incorporated into the formula of the adhesive resins, and this can reduce the undesirable effect of water sorption by the adhesive resin.57–59 In the current study, however, no water sorption measurements were made but left for other future studies. Orthodontic adhesives should display shear bond strength more than 6–8 MPa, so as to accomplish suitable
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clinical performance.60 Thus, all the tested primer formulas have shown greater bond strength values, which are adequate for bonding metal orthodontic brackets. Significant enhancement in bond strength was achieved with group 3 (21.1 2.7 MPa) by adding 25.0 vol% of HEMA in the formulation of the experimental primer, with no significant difference from group 4 with 50.0 vol % of HEMA (20.3 2.6). This could be attributed to the fact that the higher contents of HEMA could increase the degradation of dental adhesive resins61 and at the same time could induce better wetting and infiltration properties of the adhesive. Adversely, when HEMA is added in higher concentrations, bond strength may be impaired due to increased hydrophilicity, decreased removal of water, and reduced co-polymerization.34,56 The formulation of group 2 with 5.0 vol% HEMA showed an increase in the recorded shear bond strength values (20.9 2.7 MPa) but with lower numerical records than groups and 4. The test primer, a HEMA-free group 1 showed no significant increase in the shear bond strength. Shear-bond-strength values less than that exhibited by using HEMA-based primers indicated that incorporation of HEMA into the primer solution has significantly increased the shear bond strength of orthodontic adhesives to enamel surfaces. Immediate micromechanical retention to already etched enamel surfaces is better achieved when the bonding agent invades deeply into the enamel prisms without porosities. In addition to the desirable effect of HEMA, the hydrophilic characters of the test primer solutions also could be attributed to the presence of the ethanol solvent and water, on which they provide a definite tolerance to wet conditions.1 The use of ethanol as a solvent to HEMA has a certain effect in producing high bond strength when compared to other solvents.62,63 With the exception of a few specimens, no predictable enamel fracture was observed during the study. Fracture of enamel is an actual possibility during References 1 Vicente A, Toledano M, Bravo LA et al. Effect of water contamination on the shear bond strength of five orthodontic adhesives. Med Oral Patol Oral Cir Bucal 2010; 15: 820–6. 2 Klocke A, Tadic A, Kahl-Nieke B et al. An optimized synthetic substrate for orthodontic bond strength testing. Dent Mater 2003; 19: 773–8. 3 Rock WP, Abdullah SB. Shear bond strengths produced by composite and compomer light cured orthodontic adhesives. J Dent 1997; 25: 243–9.
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the debonding process of the brackets, especially if the tooth is nonvital.64,65 The 2-hydroxyethyl methacrylate additions used in this study seemed to improve the penetration of the adhesive into the enamel, hence increasing the shear bond strength of the orthodontic brackets to enamel. At the same time, the shear bond strength reached values higher than 25 MPa without endangering the enamel surface by causing perceptible fracture throughout the study. The use of all test formulae produced clinically suitable shear bond strength even after storage and thermocycling between 5 and 55°C for 500 cycles. This study has indicated that nonacidic monomers based on the two silane monomers (ICSM and BTSE) and varying additions of HEMA to replace ethanol, might be used clinically as an adhesion promoting comonomer for orthodontic adhesives that lack etching and priming constituents. Conclusion It can be concluded that by increasing the volume percentage of HEMA additions, together with incorporating varying percentages of ethanol solvent, would at least partially resolve the problem of bonding to enamel. This effect might be reflected confidently on the durability and integrity of the fixed orthodontic appliances. The clinical use of the experimental primer solutions on etched enamel for direct bonding of orthodontic brackets can be strongly encouraged. Acknowledgments The research team would like to thank Research Management Center, IIUM for funding this study. The authors would like also to thank Dr Lim Chin Choon MBBS (UM) for helping in the statistical analyses for this study.
4 Sunna S, Rock WP. An ex vivo investigation into the bond strength of orthodontic brackets and adhesive systems. Brit J Orthod 1999; 26: 47–50. 5 Pashley DH. Dentin: a dynamic substrate-a review. Scanning Microsc 1989; 3: 161–76. 6 Soderholm K-JM. Correlation of in vivo and in vitro performance of adhesive restorative materials: a report of the ASC MD156 Task Group on test methods for the adhesion of restorative materials. Dent Mater 1991; 7: 74–83.
7 Summitt JB, Robbins JW, Hilton TJ, Schwartz RS. Fundamentals of operative dentistry: a contemporary approach. Hanover Park, IL: Quintessence Publishing Co., 2006. 8 Perdigao J, Swift EJ, Denehy GE Jr et al. In vitro bond strengths and SEM evaluation of dentin bonding systems to different dentin substrates. J Dent Res 1994; 73: 44–55. 9 Nakabayashi N, Kojima K, Masuhara E. The promotion of adhesion by the infiltration of monomers into tooth substrates. J Biomed Mater Res 1982; 16: 265–73. 7
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10 Erickson RL. Mechanism and clinical implications of bond formation for two dentin bonding agents. Am J Dent 1989; 2: 117–23. 11 Van-Meerbeek B, Inokoshi S, Braem M et al. Morphological aspects of the resin-dentin interdiffusion zone with different dentin adhesive systems. J Dent Res 1992; 71: 1530–40. 12 Dias de Souza GM, Thompson VP, Braga RR. Effect of metal primers on microtensile bond strength between zirconia and resin cements. J Prosthet Dent 2011; 105: 296–303. 13 Bulbul M, Kesim B. The effect of primers on shear bond strength of acrylic resins to different types of metals. J Prosthet Dent 2010; 103: 303–8. 14 Shinoda Y, Masatoshi N, Keiichi H et al. Effect of smear layer characteristics on dentin bonding durability of HEMA-free and HEMA-containing one-step self-etch adhesives. Dent Mater J 2011; 30: 501–10. 15 Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surface. J Dent Res 1955; 34: 849–53. 16 Watanabe T, Tsubota K, Takamizawa T et al. Effect of prior acid etching on bonding durability of single-step adhesives. Oper Dent 2008; 33: 426–33. 17 Jou GE, Leung RL, White SN et al. Bonding ceramic brackets with lightcured glass ionomer Cements. J Clin Orthod 1995; 3: 184–7. 18 Maijer R, Smith DC. Variables influencing the bond strength of metal orthodontic bracket bases. Am J Orthod 1981; 79: 20–34. 19 Regan D, van Noort R, O’Keefe C. The effects of recycling on the tensile bond strength of new and clinically used stainless steel orthodontic brackets: an in vitro study. Brit J Orthod 1990; 17: 137–45. 20 Chadwick RG. Thermocycling: the effects upon compressive strength and abrasion resistance of three composite resins. J Oral Rehabil 1994; 21: 533–43. 21 Willems G, Carels CE, Verbeke G. In vitro peel/shear bond strength eval-
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uation of orthodontic bracket base design. J Dent 1997; 25: 271–8. McAlarney ME, Brenn P. A modified direct technique versus conventional direct placement of brackets: in vitro bond strength comparison. Am J Orthod 1993; 104: 575–83. Lopez JI. Retentive shear strengths of various bonding attachment bases. Am J Orthod 1980; 77: 669–78. Vicente A, Bravo LA, Romero M et al. Effects of 3 adhesion promoters on the shear bond strength of orthodontic brackets: an in-vitro study. Amer J Orthod Dentofacial Orthop 2006; 129: 390–5. Yi GK, Dunn WJ, Taloumis LJ. Shear bond strength comparison between direct and indirect bonded orthodontic brackets. Amer J Orthod Dentofacial Orthop 2003; 124: 577–81. Pulido LG, Powers JM. Bond strength of orthodontic direct-bonding cement–plastic bracket systems in vitro. Am J Orthod 1983; 83: 124–30. Lung CYK, Matinlinna JP. Resin bonding to silicatized zirconia with two isocyanatosilanes and a crosslinking silane. Part I: experimental. Silicon 2010; 2: 153–61. Lung CYK, Matinlinna JP. Silanes for adhesion promotion and surface modification. In: Moriguchi K, Utagawa S, ed. Silanes: chemistry, applications and performance, 1st edn. Hauppauge, NY: Nova Science Publishers Incorporated, 2012: 87–109. Mustafa AA, Matinlinna JP, Choi AH et al. Fracture strength and fractographic analysis of zirconia copings treated with four experimental silane primers. J Adhes Sci Technol 2013; 27: 68–80. Heikkinen TT, Lassila LVJ, Matinlinna JP et al. Dental zirconia adhesion with silicon compounds using some experimental and conventional surface conditioning methods. Silicon 2009; 1: 199–201. Lung CYK, Matinlinna JP. Aspects of silane coupling agents and surface conditioning in dentistry: an overview. Dent Mater 2012; 28: 467–77.
32 Matinlinna JP, Lassila LVJ, Kangasniemi I et al. Isocyanato- and methacryloxysilanes promote bis-GMA adhesion to titanium. J Dent Res 2005; 84: 360–4. 33 Vicente A, Bravo LA, Romero M et al. Shear bond strength of orthodontic brackets bonded with selfetching primers. Am J Dent 2005; 18: 256–60. 34 Van Landuyta KL, Snauwaert J, De Munck J et al. Systematic review of the chemical composition of contemporary dental adhesives. Biomaterials 2007; 28: 3757–85. 35 Felizardo KR, Lemos LVFM, DeCarvalho RV et al. Bond strength of HEMA-containing versus HEMA-free self-etch adhesive systems to dentin. Braz Dent J 2011; 22: 468–72. 36 Matinlinna JP, Lassila LVJ, Vallittu PK. The effect of three silane coupling agents and their blends with a crosslinker silane on bonding a bis-GMA resin to silicatized titanium (a novel silane system). J Dent 2006; 34: 740–6. € 37 Matinlinna JP, Ozcan M, Lassila LVJ et al. Effect of the cross-linker silane concentration in a novel silane system on bonding resin-composite cement. Acta Odontol Scand 2008; 66: 250–5. 38 Matinlinna JP, Tsoi JKH, de Vries J et al. Characterization of novel silane systems on titanium implant surfaces. Clin Oral Implants Res 2013; 24: 688–97. 39 Matinlinna JP, Choi AH, Tsoi JKH. Bonding promotion of resin-composite to silica-coated zirconia implant surface using a novel silane system. Clin Oral Implants Res 2013; 4: 290–6. 40 Mallineni SKS, Nuvvula S, Matinlinna JP et al. Biocompatibility of various dental materials of contemporary dentistry: a narrative insight. J Invest Clin Dent 2013; 4: 9–19. 41 Nakabayashi N, Saimi Y. Bonding to intact dentin. J Dent Res 1996; 75: 1706–15. 42 Scougall-Vilchis RJ, Ohashi S, Yamamotoc K. Effects of 6 self-etching primers on shear bond strength of
ª 2014 Wiley Publishing Asia Pty Ltd
A.A. Mustafa et al.
43
44
45
46
47
48
49
orthodontic brackets. Amer J Orthod Dentofacial Orthop 2009; 135: 424–5. Bishara SE, Soliman M, Laffoon J et al. Effect of antimicrobial monomer-containing adhesive on shear bond strength of orthodontic brackets. Angle Orthod 2005; 75: 397–9. Bishara SE, VonWald L, Laffoon JF et al. Effect of using a new cyanoacrylate adhesive on the shear bond strength of orthodontic brackets. Angle Orthod 2001; 71: 466–9. Feilzer AJ, De Gee AJ, Davidson CL. Relaxation of polymerization contraction shear stresses by hygroscopic expansion. J Dent Res 1990; 69: 36–9. The International Organization for Standardization (ISO). TR 11450 Standard. Geneva, Switzerland: Dental Materials–Guidance on Testing on Adhesion to Tooth Structure, 1994. Van Meerbeek B, De Munck J, Yoshida Y et al. Buonocore memorial lecture. Adhesion to enamel and dentin: current status and future challenges. Oper Dent 2003; 28: 215–35. Chigira H, Koike T, Hasegawa T et al. Effect of the self-etching dentin primers on the bonding efficacy of dentine adhesive. Dent Mater J 1989; 8: 86–92. Bishara SE, VonWald L, Laffoon JF et al. Effect of using a self-etch primer/ adhesive on the shear bond strength of orthodontic brackets. Amer J Orthod Dentofacial Orthop 2001; 119: 621–4.
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Silane-based primers and bond strength
50 Nakabayashi N. Dentinal bonding mechanisms. Quintessence Int 1991; 22: 73–4. 51 Shinya M, Shinya A, Lassila LV et al. Treated enamel surface patterns associated with five orthodontic adhesive systems – surface morphology and shear bond strength. Dent Mater J 2008; 27: 1–6. 52 Powers JM, Sakaguchi R. Craig’s restorative dental materials, 12th edn. St. Louis: Mosby Elsevier, 2006. 53 Munksgaard EC, Asmussen E. Materials science bond strength between dentin and restorative resins mediated by mixtures of hema and glutaraldehyde. J Dent Res 1984; 63: 1087–9. 54 Brantley WA, Eliades T. Orthodontic materials: scientific and clinical aspects. Stuttgart: Thieme Medical, 2001: 203. 55 Tay FR, Gwinnett AJ, Pang KM et al. Resin permeation into acid-conditioned, moist, and dry dentin: a paradigm using water-free adhesive primers. J Dent Res 1996; 75: 1034–44. 56 Van Landuyt KL, De Munck J, Snauwaert J et al. Monomer-solvent phase separation in one-step self-etching adhesives. J Dent Res 2005; 84: 183–8. 57 Shinoda Y, Nakajima M, Hosaka K et al. Effect of smear layer characteristics on dentin bonding durability of HEMA-free and HEMA-containing one-step self-etch adhesives. Dent Mater J 2011; 30: 501–10.
58 Nishitani Y, Yoshiyama M, Hosaka K et al. Use of Hoy’s solubility parameters to predict water sorption/solubility of experimental primers and adhesives. Eur J Oral Sci 2007; 115: 81–6. 59 Hosaka K, Nakajima M, Takahashi M et al. Relationship between mechanical properties of one-step self-etch adhesives and water sorption. Dent Mater 2010; 26: 360–7. 60 Reynolds IR, von Fraunhofer JA. Direct bonding of orthodontic attachments to teeth: the relation of adhesive bond strength to gauze meshsize. Brit J Orthod 1975; 3: 91–5. 61 Collares FM, Ogliari FA, Zanchi CH et al. Influence of 2-hydroxyethyl methacrylate concentration on polymer network of adhesive resin. J Adhes Dent 2011; 13: 125–9. 62 Carvalho RM, Mendoncßa JS, Santiag SL et al. Effects of HEMA/solvent combinations on bond strength to dentin. J Dent Res 2003; 82: 597–601. 63 Garcia FCP, Otsuki M, Pashley DH et al. Effects of solvents on the early stage stiffening rate of demineralized dentin matrix. J Dent 2005; 33: 371–7. 64 Joseph VP, Rossouw E. The shear bond strengths of stainless steel and ceramic brackets used with chemically and light-activated composite resins. Amer J Orthod Dentofacial Orthop 1990; 97: 121–5. 65 Bishara SE, Fehr DE. Ceramic brackets: something old, something new; a review. Semin Orthod 1997; 3: 178–88.
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