1http://dx.doi.org/10.20396/bjos.v18i0.8657247 Volume 18 2019 e191406 Original Article 1 Institute and Research Center São Leopoldo Mandic, Campinas, Brazil. 2 Department of Restorative Dentistry, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Brazil. Corresponding author: Vanessa Cavalli University of Campinas, Piracicaba Dental School Av. Limeira, 901, 13414-018 Piracicaba, SP, Brazil email: vcavalli@yahoo.com Received: September 13, 2018 Accepted: September 11, 2019 Do metal alloy primers increase the bond strength of orthodontic tubes? Keity Cristina Moreira de Oliveira1, Daylana Pacheco da Silva2, Cecília Pedroso Turssi1, Flávia Lucisano Botelho do Amaral1, Vanessa Cavalli2,* Aim: To evaluate the bond strength (BS) and failure mode of orthodontic tubes treated with different alloy primers at the interface among enamel, resin and orthodontic tubes. Methods: Orthodontic tubes were bonded to the enamel of 80 bovine incisors with the orthodontic resin (Transbond XT, 3M Unitek). Prior to bonding, the tubes were chemically treated with (n=20) Metal/Zirconia Primer (MZ, Ivoclar), Scothbond Universal (SB, 3M Espe); Orthoprimer (OP, Morelli) or left untreated (Control - C). Specimens were submitted to 5,000 thermal cycles (5 and 55o C) to age the bonded interface. A shear BS test and failure modes were conducted, and the results were analyzed using one-way analysis of variance and Fisher’s exact test, respectively. Results: No differences were observed among groups regardless of the type of alloy primer used (p = 0.254). However, no differences were observed among the failure modes of the groups tested (p=0.694). The adhesive failure mode between the resin and enamel was the most prevalent failure (45%) for groups OP and C, whereas cohesive failure in the orthodontic resin was the most prevalent failure (40%) for groups SB and MZ. Conclusion: Alloy primers were unable to increase the BS of the orthodontic tubes to enamel. Keywords: Shear strength. Dental bonding. Materials testing. Alloys. Orthodontics. 2 Oliveira et al. Introduction The use of tubes as an option for orthodontic treatment was responsible for a major advancement in orthodontics, and it was possible due to improvements in adhesive systems and orthodontic composite resins, which provide reliable and long-term enamel adhesion1. The orthodontic tubes offer advantages over traditional systems, such as orthodontic bands, as they facilitate hygiene, eliminate the need for spacers, and decrease the possibility of periodontal disease and secondary carious lesions2. To be effective, tubes must be able to withstand the forces that orthodontic mechan- ics generate, as well as masticatory forces. In addition, they should present a shear bond strength (BS) greater than 8 MPa, which is the minimum value necessary to maintain enamel adhesion under such circumstances3. However, under clinical con- ditions, enamel adhesive failures leading to the debonding of orthodontic tubes can occur4-6. In fact, debonding is the reason for many failures, causing 66% of total fail- ures in molars7. Adhesive failures are frequently associated with complications with the bonding technique, moisture contamination, occlusal contacts, the presence of aprismatic enamel, and changes in the enamel-etching pattern8. Although the tubes can be replaced or submitted to a new bonding procedure, these procedures involve additional chairside appointments, which compromises the treatment progress. The interface between the orthodontic tube and enamel is not the only site suscepti- ble to failure. Debonding can also occur between the tube and the orthodontic resin, and although a number of treatments are described to improve adhesion to enamel4-6, only a few are proposed to increase bonding on the metal surface. Mechanical treat- ments designed to increase roughness, such as aluminum oxide blasting, the use of a diamond drill9, and chemical treatment with the use of silane10, are amid the reported alternative treatments for the metal surface. Base metal alloys containing chrome in their composition (Ni-Cr, Ni-Cr-Be, Co-Cr) can oxidize when in contact with atmospheric air, forming a passivation layer11. Some adhesive monomers that can chemically bond to the oxide film on the metal sur- face have been incorporated into resin cements with the goal of improving bonding across the interface. Therefore, bonding between adhesive monomers and alloys is the result of a micromechanical interocking promoted by surface roughness, and a chemical interaction between the oxide film of the metal surface and the carboxylic of phosphate acidic monomer of the resin cements12. With the aim of overcoming the absence of adhesive monomers in the composition of resin cements, metal alloy primers were developed to be applied on the surfaces of alloys prior to cementa- tion13. Previous, studies have demonstrated alloy primers’ ability to improve bonding between polymer-based materials and metallic surfaces14,15. The first monomer to be used for adhesion to metal was 4-META (methacryloyloxyethy trimellitate anhydride), which was designed to eliminate the need for mechanical retention and increase adhesion16. Later, monomers containing either phosphoric or carboxylic acid groups, such as 11-methacryloxyunden, decarboxylic acid (MAC10), 10 methacryloxydecyl dihydrogen phosphate (10 MDP), vinylbenzyl-n-propylamino-tri- azine-dithiol (VBATDT), and 6-methacryloxyhexyl-2-thiouracil-5-caboxylate (MTU-6), 3 Oliveira et al. were synthesized14-16. The primers applied on the surface of the metal may contain MDP, VBTDT, MTU-6, MAC-10, or the combination of these monomers. The functional monomer of 10-MDP is able to react chemically with the chromium oxide of the cast- ing surface to promote adhesion14,16. MDP has two functional groups: One is a di-va- lent phosphoryl group that chemically bonds to the metal atoms of the metal surface, and the other is a methacryloyl group that copolymerizes with resin monomers either in the adhesive or in the resin cement composition14,16. Recently, “universal “or ”multi- mold” adhesives containing both silane and a functional monomer have been devel- oped and are indicated to bond to metal surfaces. However, these universal adhe- sives’ ability to bond to metal alloys is still being debated16. Since alloy primers are indicated for metal surfaces, the potential of increasing the BS of orthodontic tubes seems to be promising17. However, the literature is scarce in studies evaluating the use of primers in metal orthodontic devices. Therefore, the aim of this study was to evaluate the BS and failure mode at the interface among enamel, resin, and orthodontic tubes treated with various metal primers. The null hypothesis was that no differences would be found in the BS and failure mode among groups. MATERIALS AND METHODS Experimental design The experimental units consisted of 80 bovine incisors, and orthodontic tubes were bonded to the enamel buccal surfaces. Before bonding, the orthodontic tubes received (n=20) two alloy primers (Metal/Zirconia Primer – MZ and Orthoprimer - OP) and one multi-mode adhesive (Scothbond Universal – SB). The control group was left untreated (n = 20). The response variables were shear BS (in MPa) and the failure mode of the deboned area. The commercial name, manufacturer, and composition of the alloy primers are shown in Table 1. Table 1. Composition of the metal primers and orthodontic composite. Commercial name and manufacutrer Composition Metal/Zirconia primer (MZ, Ivoclar Vivadent, Schaan, Liechtenstein) Methacrylate of phosphonic acid and methacrylate cross-linked in organic solvent. Scotchbond Universal (SB, 3M/ESPE, St. Paul, MN, USA) BIS-GMA, HEMA, silane treated silica, water, ethanol, decamethylenedimethacrylate,10-decanediol phosphate methacrylate, acrylic copolymer and itaconic acid, camphorquinone, N, N-dimethylbenzocaine, 2-dimethylamonoethyl methacrylate, methyl ethyl ketone. Orthoprimer (OP, Morelli, Sorocaba, Brazil) Bis-GMA, TEG-DMA, HEMA, DMPT, camphorquinone, hydroxytoluenebutylated, dimethyl aminoethyl methacrylate. Transbond XT (3M-Unitek, Monrovia, USA) Primer: Bisphenol A diglycidyl ether dimethacrylate, TEG-DMA, triethylene glycol dimethacrylate, triphenylantimonium, 4- (dimethalamino) -benzethanol, d-1-camphorquinone, hydroquinone. Resin: Bis-GMA, bisphenol A bis (2-hydroxyethyl ether) dimethacrylate, quartz treated silane, silane treated silane, silanodimethacrylate, Diphenyliodonium hexafluorophosphate BIS-GMA- bisphenolglicedyl methacrylate; TEG-DMA-triethylene glycol dimethacrylate; HEMA-hydroxyethyl methacrylate; DMPT-dimethyl-p-toluidine. 4 Oliveira et al. Specimen preparation Eighty bovine incisors crowns with enamel free of defects or cracks were selected and were stored in a 0.1% thymol solution for 24 hours. After debridement and pumic- ing, the teeth were embedded in polystyrene resin with the buccal surfaces facing up and sonicated. Enamel bonding was performed according to the manufacturer’s instructions for the orthodontic composite (Transbond XT). Teeth were cleaned with water spray (15 s), air dried (15 s), and acid etched with 37% phosphoric acid gel for 30 s. The surface was rinsed with air-water spray for 20 s and air dried for 10 s. The primer of the Transbond XT system was applied on the enamel surface, then sprayed with a mild air spray for 5 s and light cured for 20 s. The orthodontic composite (Trans- bond XT) was applied on the inner surface of the tube and fixed on the enamel. Light curing was performed for 40 s (20 s in the mesial and 20 s in the distal sites) using a LED light-curing unit (Bluephase, Ivoclar, Liechtenstein, with irradiance of 1200 mW/cm2). Before the tube was bonded to the enamel surface, the base of the orthodontic tube was treated according to each experimental group: • MZ: The primer was applied on the base of the tube (180 s) and air dried (5 s). • OP: The primer was applied on the base of the tube (180 s) and air dried (5 s). • SB: The universal adhesive was applied on the base of the tube (20 s) and air-dried (5 s). • Control: No treatment was performed on the tube. Thermal cycling The samples were stored for 24 h in distilled water at 37 ºC, and for the purpose of aging the bonding interface, 5,000 thermal cycles were performed (MSCT-3, Marcelo Nucci ME, São Carlos, Brazil) at 5 and 55 °C (± 1°C) with a dwell time of one minute each. Shear bond strength test Forty-eight hours later, specimens were submitted to a shear test in the occlusal-cer- vical direction, with the blade placed at the enamel and resin/tube interface. The tests were performed in a universal testing machine (EMIC- DL 2000, Instron Brasil Scien- tific Equipment LTDA, São José dos Pinhais, Brazil) with a load cell of 1kN at a cross- head speed of 0.5 mm/min. The maximum force (N) up to failure was recorded. The shear BS (in MPa) was calculated from the force and the bonded area of the tube to the enamel surface. Failure mode The failure mode of the debonded interface was observed under a stereomicroscope at 40x magnification. Debonding was classified (Table 2) based on a previous report18. 5 Oliveira et al. Statistical analysis Shear BS data were submitted to exploratory analysis to verify normality and homosce- dasticity, and they were also submitted to parametric one-way analysis of variance (ANOVA). The fracture mode was analyzed using Fisher’s exact test. In all analyses, SAS software (SAS Institute Inc., Cary, NC, USA, Release 9.2, 2010) was used consid- ering the level of significance of 5%. RESULTS No significant difference was found among groups in terms of shear BS values (p = 0.254) as observed in Table 3. Additionally, no significant difference (p = 0.694) was observed in the failure mode distribution of the adhesive interface as a func- tion of the treatments. The most common failure mode was type 3 (adhesive failure between the orthodontic composite and enamel), in which no orthodontic composite remnant was found in the enamel (45% of the experimental units). The least prevalent failure was type 1 (only in the SB group), in which the base of the orthodontic tube did not exhibit a resin remnant and all of the resin remained on the enamel (1.2% of the experimental units) (Table 3). Table 3. Mean and standard deviation of shear bond strength and distribution of the failure mode according to treatments. Group Shear BS Failure mode Type 1 (Adhesive orthodontic composite/tube) Type 2 (Cohesive in orthodontic composite) Type 3 (Adhesive orthodontic composite/enamel) Type 4 (Cohesive in enamel) MZ 13.64 (6.24) a 0 (0%) 11 (55%) 5 (25%) 4 (20%) SB 12.26 (5.38) a 1 (1.25%) 11 (55%) 6 (30%) 2 (10%) OP 13.15 (4.18) a 0 (0%) 4 (20%) 13 (65%) 3 (15%) Control 10.40 (5.70) a 0 (0%) 6 (30%) 12 (60%) 2 (10%) Total 1 (1.25%) 32 (40%) 36 (45%) 11 (13.75%) MZ - Metal/Zirconia Primer; SB - Scotchbond Universal; OP – Orthoprimer; N- number of samples per group. Means followed by the same letter indicate no statistical differences. No differences were observed in failure modes among experimental groups, according to Fisher’s test (p = 0.1119). Table 2. Failure mode classification Standard Type of Failure 1 Adhesive failure between orthodontic composite and the base of the orthodontic tube (100% of the composite remains on enamel surface) 2 Cohesive failure in the orthodontic composite (50% of the composite remains at the base of the orthodontic tube and 50% bonded on enamel) 3 Adhesive failure between orthodontic composite and enamel (100% of the composite remains on the tube) 4 Cohesive enamel fracture 6 Oliveira et al. DISCUSSION The BS results showed no difference among the orthodontic tubes that were treated with various primers, even those containing acidic phosphate monomers (SB and MZ). Dias et al.19 observed that the application of an alloy primer (Kuraray) did not increase the BS of the resin cement to zirconia, and the primer did not prevent BS decrease after six months of water storage. The alloy primer contains two func- tional acidic monomers VBATDT (vinylbenzyl-n-propylamino-triazine-dithiol) and 10-MDP in an acetone-based solution, and it is indicated to cement metal fixed prosthodontics structures. Similarly, it was previously reported that an Orthoprimer application to polycarbon- ate-based brackets did not influence the BS results15, which is comparable to the find- ings of our study. The Orthoprimer agent does not contain acidic phosphate mono- mers except for methacrylate monomers and non-phosphoric hydrophilic monomers (TEGDMA e HEMA) (Table 1). Therefore, it is speculated that this primer is responsible for increasing wettability and improving the resin permeation of the composite in the irregularities of the base of the orthodontic tube. However, it does not chemically bond to the alloy surface as the function monomer does. Contrary to the present results, Cal Neto et al.20 observed that the application of an alloy primer with the acidic monomer 4-META (4-methacryloxyethyl trimellitic anhy- dride) increased the BS between the composite and the metallic brackets 48% com- pared with the control group, whereas in our research, alloy primers were able to increase the BS 17.9 to 31.2%. It should be noticed that these authors used a different primer (4-META) and that this agent could have been more effective in increasing BS compared with the primers selected in this research. The absence of thermal cycles to age the bonded interface, and the fact that the authors used human pre-molars instead of bovine incisors cannot be ruled out. In a previous study20, it was observed that treating the surface of zirconia with alloy primers increased the BS of the zirconia to the metallic bracket. That study used Z-Prime Plus (10-MDP and carboxylic acid) and the Zirconia Liner premium (silane with MDP)20. The authors credited the good performance of Z-Prime Plus to the pres- ence of MDP and the ability of the primer to co-polymerize with the resin monomers, as the functional group binds to the metallic oxide of the substrate via the phosphoric group21. It is important to notice that MZ, SB, and Z-Prime Plus exhibit different acidic phosphate monomers: the phosphonic acid methacrylate, 1,10-decanediol phosphate methacrylate, and 10-methacryloyloxydecyl dihydrogen phosphate, respectively. Thus, different monomer compositions and application modes may influence BS results22. Furthermore, the presence of organophosphate monomers in the universal adhesive SB could promote the instability of the silane component22. Therefore, it is possible that the presence of silane hampered the monomer performance and that the adhe- sive presented similar BS results compared with the other primers. Thermal cycling (5,000 cycles) was performed for all groups after bonding to age the interface. Previous studies observed that MDP-based primers were effective in preserving the bonding stability to zirconia even after thermal cycling22-24. On the other hand, Imai et al.25 observed that the BS between composite and alloy surfaces 7 Oliveira et al. decreased after 20,000 thermal cycles. The authors suggested that the lower BS could have been the result of water infiltration in microgaps between the composite and the alloy25. As no differences were observed in our study, it is possible that the number of cycles performed could have been insufficient for influencing the BS. No significant differences were found among the failure modes. In the current study, the shear BS test evaluated multiple interfaces—enamel, orthodontic resin, and the base of the orthodontic tube. Numerically, the most predominant failure was the adhe- sive type, between the enamel and orthodontic resin (45%). This may be an indication that the BS at the base of the orthodontic tube where the primer was applied was acceptable. In addition, it should be kept in mind that in shear bond tests, the knife of the apparatus slides down the bordering enamel, stressing the orthodontic resin more than the orthodontic tube. Therefore, as expected, the second more prevalent failure was cohesive failure in the orthodontic composite (40%). The least prevalent failure was the adhesive type between the orthodontic composite and the base of the ortho- dontic tube (1.25%), which might indicate that although the primers did not improve the shear BS, they did not compromise it. In addition, the fact that no differences were found among the BS and failure modes implies that the irregularities at the base of the orthodontic tubes were sufficient for promoting a reasonable BS regardless of the presence of primers. MZ, OP, and SB exhibited 15%, 20% and 10% respectively, of enamel cohesive defects. Although this type of failure indicates an acceptable performance of the primers applied to the metal, enamel fracture is undesirable when the tubes are removed at the end of orthodontic treatment. Therefore, because no differences were observed among groups, we believe that the decision to apply primers to metallic surfaces should be reexamined, as it could cause enamel fracture during debonding. Based on the above, the null hypothesis could be accepted, as the application of alloy primers to the base of the orthodontic tubes did not increase the BS to the enamel surface and did not influence the failure modes of the tubes bonded to the enamel surface. Moreover, based on the failure modes and the existence of enamel cohesive failures when primers were applied, we endorse that no need exists to apply alloy primers to orthodontic tubes. Considering that the application of alloy primers to orthodontic tubes can be dis- missed and that debonding occurs due to adhesives (enamel/orthodontic resin) at the orthodontic resin, future research should focus on developing an orthodontic resin that will not fail cohesively and that possesses optimal enamel adhesion that will resist shear forces without compromising enamel integrity when the tube is removed at the end of the orthodontic treatment. In conclusion, the application of alloy primers did not increase the BS of the ortho- dontic tube to the enamel. In addition, its application should be better evaluated, as enamel fracture could occur during debonding. CONFLICT OF INTEREST STATEMENT The authors do not have any financial interest in the companies whose materials are included in this article. 8 Oliveira et al. REFERENCES 1. Murray PG, Millett DT, Cronin M. Bonded molar tubes: a survey of their use by specialist orthodontists. 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