Clin Oral Invest DOI 10.1007/s00784-015-1496-2
ORIGINAL ARTICLE
TiF4 improves microtensile bond strength to dentin when using an adhesive system regardless of primer/bond application timing and method Juliana Bomfigli Tranquilin 1 & Enrico Coser Bridi 1 & Flávia Lucisano Botelho Amaral 1 & Fabiana Mantovani Gomes França 1 & Cecilia Pedroso Turssi 1 & Roberta Tarkany Basting 1
Received: 16 January 2015 / Accepted: 20 May 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Objectives This study evaluated the bond strength of a twostep conventional adhesive system to dentin pretreated with 2.5 % titanium tetrafluoride (TiF4) according to application timing (before or after acid conditioning) and primer/ adhesive application method (active or passive). Materials and methods Dentin surfaces were randomly treated with different adhesive procedures (n = 6): etching with phosphoric acid (PA) + primer/adhesive (Adper Single Bond 2/3M ESPE) actively applied; PA + primer/adhesive passively applied; TiF4 before PA + primer/adhesive actively applied; TiF4 before PA + primer/adhesive passively applied; TiF4 after
* Roberta Tarkany Basting [email protected] Juliana Bomfigli Tranquilin [email protected] Enrico Coser Bridi [email protected] Flávia Lucisano Botelho Amaral [email protected] Fabiana Mantovani Gomes França [email protected]
1
PA + primer/adhesive actively applied; and TiF4 after PA + primer/adhesive passively applied. A composite block was built onto the tooth, which was sectioned into sticks (adhesive area of approximately 1 mm2). Microtensile bond strength tests and the failure mode were determined. Results Two-way ANOVA revealed no significant interaction between TiF4 application timing and primer/adhesive application method (p = 0.184). The use of TiF4, before or after PA significantly increased bond strength values (p < 0.001). There was no significant difference in bond strength values when using TiF4 before or after PA. The primer/adhesive application method had no influence on bond strength, regardless of whether TiF4 was used (p = 0.906). Failure mode was predominantly adhesive. Conclusions The use of TiF4 promoted higher immediate bond strength to dentin. The conventional adhesive system may be applied either actively or passively, regardless of TiF4 application timing. Clinical significance Pretreatment with TiF4 increased bond strength in a conventional two-step adhesive system to dentin, regardless of the primer/adhesive application method and the timing of dentin pretreatment. Keywords Two-step conventional adhesive system . Hybrid layer . Microtensile bond strength . Titanium tetrafluoride
Cecilia Pedroso Turssi [email protected]
Introduction
Department of Restorative Dentistry, São Leopoldo Mandic Institute and Research Center, Rua José Rocha Junqueira, 13, Campinas 13045-755, São Paulo, Brazil
Titanium tetrafluoride (TiF4) is a fluoride compound available as a gel, varnish, or aqueous solution, which has been used topically on the enamel to prevent the rise of primary caries by
Clin Oral Invest
reducing demineralization in vitro [1] and inhibiting caries progression in situ and in vivo [2, 3]. In addition, it is capable of preventing erosion/abrasion lesions [4, 5], thus reducing dentinal hypersensitivity [6]. Sen and Büyükyilmaz [7] showed that a solid structure and modified smear layer are formed when TiF4 is applied at 4 %. Wiegand et al. [8] reported that pretreatment with TiF4, at 2.5 %, led to the formation of a globular lining surface that occluded the dentinal tubules. Since this surface lining is composed of organometallic complexes of titanium and dentinal organic matrix [9], it may provide mechanical protection for the dentin, enabled by the strong bond between the titanium and the oxygen atom from a phosphate group, thus forming a resistant vitreous layer [7, 10]. This bond occurs after the hydrolysis of TiF4, since titanium has a strong tendency to pair up with an oxygen atom; the titanium compounds formed therein are strongly bonded and not easily replaced [3, 11]. Titanium is a biologically acceptable element, because it is nontoxic in its pure metallic form [10]. Therefore, when applied to the dentin, particularly as a pretreatment following cavity preparation, a lower risk of secondary caries would be expected due to modifications in the smear layer, thus suggesting that the use of TiF4 to treat dentin prior to restorative procedures could be an important step [12], especially when using self-etching adhesive systems that can both demineralize and infiltrate the dentin to form a hybrid layer [13] containing titanium tetrafluoride. The use of TiF4 as a dentin pretreatment was initially proposed by Dündar et al. [14], who suggested the application of an aqueous TiF4 solution for 60 s on dentin previous to acid etching or acidic monomer application. Dentin pretreatment with TiF4 was also evaluated by Devabhaktuni and Manjunath [15], Bridi et al. [16], and Domingues et al. [17]. Bridi et al. [16] and Domingues et al. [17] did not report any influence of TiF4 as a pretreatment when using a self-etching adhesive system. In this respect, the use of self-etching adhesives of mild to moderate acidity may be suggested in order to dissolve the glaze layer formed following the application of TiF4. When using a conventional three-step adhesive system, Devabhaktuni and Manjunath [15] reported no significant effect on the bond strength of a composite resin to the dentin following pretreatment with TiF4 either before or after acid etching. Nonetheless, Basting et al. [18] have shown that the layer formed by pretreating the dentin with TiF4 was acid resistant and could not be removed, even after the use of citric acid for 30 min. According to Pupo et al. [19], applying the bonding agent actively results in greater hardness of the hybrid layer both superficially and in depth. In addition, mechanical pressure from the microbrush increases molecular agitation optimizing solvent evaporation and, consequently, improved monomer diffusion through the dentin, leading to an increased percentage of polymer formation, which is stable and resistant to hydrolysis, thus improving the mechanical properties of the
adhesive layer [20, 21]. Therefore, active application of the adhesive system containing water as a solvent would benefit bond strength [22–24], by allowing greater removal of water and further penetration of monomers through the dentinal collagen fibrils. The use of a TiF4 aqueous solution and active application of the adhesive system could promote greater removal of excess water from the pretreatment and improve diffusion of the monomer through the vitreous layer. The incorporation of TiF4 into the hybrid layer of conventional bonding systems could play an important role in restorative procedures, considering the possible inhibition of secondary carious lesions, as well as permitting higher longevity of the hybrid layer should TiF4 act as a dentinal biomodifier [14, 16, 18]. Due to ongoing controversy regarding TiF4 as a dentin pretreatment in combination with conventional adhesive systems as well as the lack of studies using conventional two-step bonding systems, the present study aimed to evaluate the bond strength of a conventional two-step adhesive system to dentin, which was pretreated with 2.5 % TiF4, in terms of application timing (before or after acid etching) and primer/ bond application method (active or passive).
Materials and methods Experimental design In order to obtain sticks for microtensile strength testing, the experimental units were formed of 36 blocks of dentin obtained from 36 extracted human third molars. Six blocks of dentin were allocated to each experimental group [16, 17]. The study factors were as follows: (A) TiF4 application timing on three levels, no treatment (no TiF4), before acid etching, and after acid etching; and (B) primer/bond application method on two levels, active or passive application. The response variable was microtensile bond strength, which was evaluated quantitatively and obtained in MPa. The response variable regarding failure mode was evaluated qualitatively by means of a scoring system. The micromorphology of the hybrid layer was also evaluated qualitatively. Teeth selection and preparation Following Ethics Committee approval (Sao Leopoldo Mandic School of Dentistry and Research Institute, registration 453.589), 36 recently extracted sound third molars were selected, cleaned using a periodontal curette, and stored in 0.1 % thymol aqueous solution. The teeth were inspected visually using a stereoscopic magnifier (EK3ST, CQA Comercial Química Americana Ltd., Americana, SP, Brazil) at ×40 magnification. Teeth presenting cracks of structural anomalies were excluded. The flat surfaces of the coronal aspect of the dentin were obtained by removing the root at 2 mm below the cementoenamel junction using a diamond disk mounted on a precision saw (Isomet 1000 Precision Diamond Saw, Buehler
Clin Oral Invest
Ltd., Lake Bluff, IL, USA). The occlusal enamel was subsequently removed, leaving a flat surface of exposed dentin at a right angle to the long axis of the tooth. The tooth fragments were finished on a water-cooled rotatory polisher (Politriz Aropol 2V, Arotec, Cotia, SP, Brazil), using aluminum oxide sandpaper (Imperial Wetordry, 3M, Sumaré, SP, Brazil), in a decreasing order of grain (400, 600, and 1000) in order to create smooth dentin surfaces. Following root removal, the exposed pulp chamber was filled with composite resin (Filtek Z350, 3M ESPE, Saint Paul, MN, USA) following by the bonding agent (Adper Single Bond, 3M ESPE, Saint Paul, MN, USA). The teeth were then divided into groups according to the combinations and levels based on the study factors (TiF4 application timing × primer/adhesive application method) (n = 6), as shown in Table 1. Dentin treatment and adhesive procedures For dentinal pretreatment, the TiF4 was obtained in the pro-analysis (P.A.) form and prepared at a concentration of 2.5 % in distilled deionized water (weight/volume and pH 1.2), according to Dündar et al. [14], Bridi et al. [16], and Domingues et al. [17]. The product was applied to the dentin of each group according to timing (before or after phosphoric acid etching) for 60 s, actively applied with a disposable brush, followed by brief drying for 5 s with blasts of air. For groups that only received the application of the adhesive systems (groups A and P), etching with phosphoric acid for 15 s was performed prior to the use of the adhesive system (Table 2). The two-step conventional adhesive system was applied passively, according to the manufacturer’s instructions, for the groups P, TiF4BEP, and TiF4AEP. For the remaining groups, A, TiF4BEA, and TiF4AEA, the primer/adhesive was applied actively, which consisted of mechanically smearing of the substance onto the surface using a disposable brush (Microbrush, Grafton, WI, USA), aiming to evaluate the Table 1
Study groups according to treatment
Groups/ abbreviations
Treatment
A
No treatment (no TiF4) + etching with phosphoric acid + active primer/adhesive application No treatment (no TiF4) + etching with phosphoric acid + passive primer/adhesive application TiF4 before etching with phosphoric acid + active primer/adhesive application TiF4 before etching with phosphoric acid + passive primer/adhesive application TiF4 after etching with phosphoric acid + active primer/ adhesive application TiF4 after etching with phosphoric acid + passive primer/adhesive application
P TiF4BEA TiF4BEP TiF4AEA TiF4AEP
dissolution of the vitreous layer that may have formed following TiF4 application. Following application of the adhesive system, a 5 × 5-mm composite resin block (Filtek Z350, 3M Espe, Saint Paul, MN, USA) was built and light-cured for 40 s in 2-mm thick increments. The block was eventually light-cured for 20 s on each side. Light-curing was carried out using a photoactivator halogen light equipment (Demetron Research Corporation, Danbury, CT, USA) set at a mean radiance of 495 mW/cm2 (422 to 569 mW/cm2). Specimen preparation for microtensile bond strength testing The teeth fragments were positioned on a metallographic saw (Isomet 1000 Precision Diamond Saw, Buehler Ltd., Lake Bluff, IL, USA), using acrylic plates for fixation. Buccal-lingual and mesial-distal sections were obtained of approximately 1 mm 2 measured using a digital caliper (Mitutoyo America Corporation, Aurora, IL, USA). The sticks were stored in Eppendorf tubes in distilled water at 37 °C for 24 h prior to microtensile bond strength testing. The latter was performed in a universal testing machine (DL2000, EMIC, São José dos Pinhais, PR, Brazil) at 0.5 mm/min and cell load of 50 N. The sticks were appropriately positioned using a specific device for microtensile bond strength testing aided by fast setting glue (Superbonder, Henkel Loctite Ltda, São Paulo, SP, Brazil). The bond strength values were obtained in MPa. The surface of the specimens submitted to the test was examined using a stereoscopic magnifier (EK3ST, Eikonal Equipamentos Ópticos e Analíticos, São Paulo, SP, Brazil) at 30 times magnification, in order to classify the specimens according to failure mode. The categories were adhesive (adhesion failure), cohesive failure in dentin (substrate failure), cohesive in resin (failure of the composite resin), or mixed (cohesion failure in resin). One or two sticks from each resin-tooth block were set aside for evaluation of the tooth/restoration interface by scanning electronic microscopy (SEM) (Jeol 5900LV, Jeol Ltd., Tokyo, Japan). The sticks were smoothed using watercooled sandpaper in decreasing grain (400, 600, and 1200) and then polished using diamond pastes of sequentially decreasing roughness (6, 3, 1, and 1/2 μm) on felt disks under mineral oil cooling. The samples were thoroughly rinsed, then demineralized for 30 s with 6 N HCl, rinsed again, deproteinized with 2.5 % NaOCl for 10 min [25], and serially dehydrated in 25, 50, 75, 95, and 100 % ethanol and chemically dried in HMDS for 10 min. The samples were mounted on aluminum stubs, sputter-coated with gold, and examined under SEM at an operating voltage of 10 Kv. Observations were carefully performed on the entire adhesive interface of the stick at ×500 magnification. Images of the most representative area of each specimen were then taken at ×1000 magnification. Differences in hybrid layer formation, shape,
Clin Oral Invest Table 2
Materials used, composition, protocol of use, and manufacturer
Materials
Composition/pH
Protocol of use
Manufacturer (city, state, country)
2.5 % titanium tetrafluoride
Titanium tetrafluoride P.A. + distilled deionized water/pH = 1.2 35 % phosphoric acid, silica/pH = 0.6
Apply the product for 60 s
Sigma-Aldrich (Saint Louis, MO, USA) 3M ESPE (3M ESPE, Saint Paul, MN, USA)
3M Scotchbond Etchant
Adper Single Bond 2
BisGMA, HEMA, dimethacrylates, ethanol, water, a novel photoinitiator system, and a functional copolymer of polyacrylic and polyalkenoic acids/pH = 4.3
length, and density of resin tags were assessed according to TiF4 application timing and primer/bond application method. Statistical analysis The data were digitalized and tabulated as Excel spreadsheets. Mean microtensile bond strength values were calculated for each tooth. Stabilization of variance and normality was obtained by transforming the data using the square root of the values. Descriptive statistics and two-way analysis of variance (ANOVA) were performed at a significance level of 5 % (SPSS Inc., Chicago, IL, USA). Failure mode was analyzed using descriptive statistics.
Results Two-way ANOVA on the transformed data revealed no significant interaction between the study factors (p = 0.184). Regardless of the application method, the use of TiF4, before or after acid etching, significantly increased bond strength (p < 0.001) (Table 3). No significant difference in bond strength values was observed when TiF4 was applied before Table 3 Mean values bond strength to dentin (MPa) and standard deviation (SD) according to the groups
Apply to the tooth for 15 s Rinse with water for 15 s Dry dentin with absorbent paper to maintain moisture Passive method (recommended by the manufacturer): Apply two consecutive layers of primer/adhesive passively for 15 s interposed by a 5-s blast of air Light-cure for 10 s Active method: Apply two consecutive layers of primer/adhesive actively (smearing), interposed by a 5-s blast of air Light-cure for 10 s
3M ESPE (3M ESPE, Saint Paul, MN, USA)
or after etching. Primer/adhesive application method did not influence bond strength, independently of TiF4 application (p = 0.906). The commonest type of failure observed was adhesive for all groups, followed by cohesive in resin. Dentin cohesion failure was infrequent, yet always present in the groups for which TiF4 was used to pretreat the dentin. No mixed failures were observed. Dentin cohesion failures were observed when primer/adhesive was applied passively (5 to 11 % of the failures). Figure 1 illustrates the failure modes across the groups. Micromorphological images (Fig. 2) revealed that the lack of TiF4 led to the formation of fewer resin tags when adhesive was applied either actively or passively. Although the tags were uniformly distributed throughout the hybrid layer, they presented lower diameter and depth when compared to the groups treated with TiF4. When TiF4 was applied (C to E), interface micromorphology was similar between the groups, regardless of application timing (before or after acid etching) and primer/adhesive application method (active or passive). A hybrid layer was formed in all types of treatment studied, showing a higher number of resin tags, which were also
Primer/adhesive application method TiF4 application timing
Active
Passive
No TiF4 TiF4 before etching TiF4 after etching Overall mean
10 (2.1) 16.1 (4.4) 16.3 (7.1) 14.1 (5.6) a
9.6 (1.9) 19.5 (7.3) 12.2 (1.2) 13.8 (6.0) a
Overall mean
9.8 (1.9) B 17.8 (6.0) A 14.2 (5.3) A –
Overall means followed by identical capital letters indicate no significant difference between the treatments. Overall means followed by identical lowercase letters indicate no significant difference between the application methods
Clin Oral Invest
Fig. 1 Failure modes distribution among groups
deeper and larger in diameter than in the groups for which TiF4 was not used.
Discussion The use of substances that act as biomodifiers or dentin pretreatment has been proposed as an approach to reduce degradation of the hybrid layer, stabilizing or reducing the incidence of secondary caries, as well as maintaining bond strength [26, 27]. Among these agents, TiF4 promotes the formation of a layer composed of titanium oxides or organometallic complexes upon contact with the organic material within the dentin, thus creating an acid-resistant stable lining and conferring higher resistance to tooth demineralization [7, 18]. Pretreating dentin with TiF4 could, therefore, be an important step prior to or concomitant to the application of bonding systems [12, 14, 16, 17], which would result in a biomodified hybrid layer that would be also acid resistant [18]. The results from the present study demonstrated that TiF4 application led to higher bond strength values to the dentin when using a conventional two-step adhesive system, regardless of application timing. This finding, contrary to what is reported for single-step [16] or two-step [16, 17] self-etching systems, shows that TiF4 may contribute to improve the mechanical properties of the hybrid layer, leading to a higher mechanical imbrication between dentinal collagen and the
monomers from the adhesive system, which can also be confirmed by observing the micromorphology of the adhesive interface. The formation of a vitreous layer when applying TiF4 to the dentin, which renders the surface nearly impermeable and resistant to acids, was observed by Sen and Büyükyilmaz [7] and Basting et al. [18]. Nonetheless, the use of self-etching adhesive systems has shown a capacity to infiltrate and penetrate through this layer [16, 17], possibly due to the established active application of the self-etching primer onto the dentin. The bond strength values and the micromorphology of the hybrid layer when using self-etching adhesive systems have shown similar results with or without TiF4 as a dentin pretreatment [16, 17]. The results from the present study, however, have shown that the bond strength values to dentin, when using TiF4 as a pretreatment, may be influenced by the bonding strategy used, considering that higher bond strength values were obtained with TiF4 pretreatment and a conventional two-step bonding system. Regardless of TiF4 application timing, a significant increase in dentin bond strength was observed, suggesting that TiF 4 may have contributed as an immediate dentinal biomodifier. Micromorphological evaluation revealed the formation of a vitreous layer obtained with the application of TiF4 onto the untreated smear layer (when applied before phosphoric acid etching) or onto the dentin that had had its smear layer (following acid etching) [18], where the primer/
Clin Oral Invest Fig. 2 Photomicrograph of the adhesive interface for the groups studied (×1000 magnification). a Active primer/adhesive application, showing few tags with low depth and low diameter. b Passive primer/adhesive application, showing similar tags to the active application group. c TiF4 application prior to phosphoric acid etching + active primer/adhesive application, showing numerous tags. d TiF4 application prior to phosphoric acid etching + passive primer/ adhesive application, showing the formation of tags with larger diameter and deeper into the substrate. e TiF4 application after phosphoric acid etching + active primer/adhesive application, showing a higher density of tags per area. f TiF4 application after phosphoric acid etching + passive primer/adhesive application, with tag formation similar to that from the passive application group
adhesive was able to penetrate through the vitreous layer, forming a hybrid layer containing resin tags with a deeper infiltration, larger diameters, and an increased density, thus explaining the higher bond strength values observed in comparison to the groups where no TiF4 was applied. These findings contradict those reported by Devabhaktuni and Manjunath [15], where no significant difference in bond strength when using a conventional three-step bonding system was observed for a composite material to the dentin following pretreatment with TiF4 after acid etching, possibly due to the strategy used, e.g., three-step instead of two-step, as per the present study, as well as the composition and pH of the adhesives used. It was expected that the TiF4 infiltrated in the hybrid layer could have an effect on increasing the mechanical properties of this layer. Bond strength increases as a consequence of filler addition in adhesive systems [28]. Likewise, particles in adhesives may increase the debonding energy, sometimes
leaving dentin at a disadvantage, because the dentinadhesive interface could be stronger than other substrates (resin or dentin), thereby increasing the susceptibility of failure propagation [28]. Accordingly, this mechanism may also explain the results observed for failure types, where dentin cohesion failure was always present in the groups for which TiF4 was used to pretreat the dentin. For this reason, the difference in fracture type patterns when TiF4 was applied could result in a crack trajectory moving away from the adhesive interface toward the adhesive dentin interface, as a result of increased resistance to fracture propagation. Regarding the application method for the bonding system, independently of the application of TiF4, active application showed similar bond strength values to passive application, which contradicts the studies by Hashimoto et al. [22], Jacobsen and Söderholm [23], and Dal Bianco et al. [24]. In general, all adhesive systems contain water or other solvents in their composition, which can be difficult to remove prior
Clin Oral Invest
to adhesive polymerization and, therefore, may compromise the quality of the polymer and the hybrid layer [23]. The resistance and durability of the bond strength of acetone or water-/ethanol-based bonding systems to dentin may be different due to the application method selected, considering that the pressure vapors for either solvent is relatively different (200 and 47.1 mmHg, respectively) [29]. In the case of the adhesive system used in this study, water and ethanol are present in its composition, which are more difficult to evaporate than acetone. Therefore, a maneuver to promote higher volatility for this kind of solvent is to increase application time [30], applying several layers of adhesives [22, 31], wait longer before light-curing the adhesive [32] or apply the adhesive under agitation [19, 24, 29], which would promote higher resistance of the polymer formed through the collagen fibrils and higher bond strength. In the present study, however, the dentin was kept moist prior to the application of primer/adhesive, both after etching and after applying TiF4, which favored adhesive permeability through the collagen fibrils for this type of solvent [24, 29] for both application methods, which could explain there being no difference in immediate bond strength between the application methods. This finding seems to corroborate the manufacturer’s instructions, which do not include application method for the primer/adhesive, despite prior knowledge that this type of simplified conventional bonding system contains monomers of high molecular weight that could show limited diffusion through moist demineralized dentin [22, 24]. Despite considering that active application may promote higher kinetics for the components and a greater diffusion of the monomers in depth, consequently allowing greater volatilization of the solvents, their application method did not influence the bond strength results nor did it have any impact on the micromorphology of the interface. In spite of the lack of difference in bond strength in terms of application method, Dal-Bianco et al. [24] suggest that active application intensifies removal of water from the demineralized dentinal matrix, replacing the space once occupied by minerals with resin monomers without allowing collapse of the collagen fibrils. The presence of residual water during monomeric diffusion or within the polymerized compound may induce hydrolytic degradation of the hybrid layer due to the formation of water pockets with hydrophilic properties, known as water-trees [33]. Active application is indicated for self-etching adhesive systems [19, 34, 35]. However, the need to apply the selfetching adhesive system actively refers to the increased need to modify and/or remove the smear layer to allow greater mechanical imbrication, as well as to improve the chemical interactions between the adhesive and the subjacent dentin, promoting higher bond strength values. Similarities in micromorphology were observed at the interface in both application methods, which may explain the
bond strength and failure mode findings, the latter predominantly adhesive for all groups. Microscopy has shown a similar pattern for resin tag formation in terms of depth and density between adhesive application methods. Despite the need of further in vitro and clinical studies to allow the recommendation of dentin pretreatment with TiF4 solution combined with a conventional two-step adhesive system, the use of TiF4 promoted higher immediate bond strength to dentin. The conventional adhesive system may be applied either actively or passively, regardless of TiF4 application timing.
Conclusions Pretreatment with TiF4 increased bond strength in a conventional two-step adhesive system to dentin, regardless of the primer/adhesive application method and the timing of dentin pretreatment. Acknowledgments The authors would like to thank CNPq foundation (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for providing grants (process number 125925/2013-1) and LME/LNNano/ CNPEM for the technical support during the electron microscopy work. Conflict of interest The authors declare that they have no competing interests.
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Jacobsen T, Söderholm KJ (1998) Effect of primer solvent, primer agitation, and dentin dryness on shear bond strength to dentin. Am J Dent 11:225–228 Dal-Bianco K, Pellizzaro A, Patzlaft R, de Oliveira Bauer JR, Loguercio AD, Reis A (2006) Effects of moisture degree and rubbing action on the immediate resin-dentin bond strength. Dent Mater 22:1150–1156 Radovic I, Vulicevic Z, Garcia-Godoy F (2006) Morphological evaluation of 2- and 1-step self-etching system interfaces with dentin. Oper Dent 31:710–718 Bedran-Russo AK, Pashley DH, Agee K, Drummond JL, Miescke KJ (2008) Changes in stiffness of demineralized dentin following application of collagen crosslinkers. J Biomed Mater Res B Appl Biomater 86:330–334 Bedran-Russo AK, Pauli GF, Chen SN, McAlpine J, Castellan CS, Phansalkar RS, Aguiar TR, Vidal CM, Napotilano JG, Nam JW, Leme AA (2014) Dentin biomodification: strategies, renewable resources and clinical applications. Dent Mater 30:62–76 Belli R, Kreppel S, Petschelt A, Hornberger H, Boccaccini AR, Lohbauer U (2014) Strengthening of dental adhesives via particle reinforcement. J Mech Behav Biomed Mater 37:100–108 Pashley EL, Zhang Y, Lockwood PE, Rueggeberg FA, Pashley DH (1998) Effects of HEMA on water evaporation from water–HEMA mixtures. Dent Mater 14:6–10 el-Din AK, Abd el-Mohsen MM (2002) Effect of changing application times on adhesive systems bond strengths. Am J Dent 15: 321-324. Mandava D, Ajitha P, Narayanan LL (2009) Comparative evaluation of tensile bond strengths of total-etch adhesives and self-etch adhesives with single and multiple consecutive applications: an in vitro study. J Conserv Dent 12:55–59 Cardoso PC, Loguercio AD, Vieira LCC, Baratieri LN, Reis A (2005) Effect of prolonged application times on resin–dentin bond strengths. J Adhes Dent 7:143–149 Tay FR, Pahsley DH, Yoshiyama M (2002) Two modes of nanoleakage expression in single-step adhesives. J Dent Res 81: 472–476 Loguercio AD, Stanislawczuk R, Mena-Serrano A, Reis A (2011) Effect of 3-year water storage on the performance of one-step self-etch adhesives applied actively on dentine. J Dent 39:578–587 Caneppele TM, Torres CR, Sassaki A, Valdetaro F, Fernandes RS, Prieto de Freitas C, Tay FR (2012) Effects of surface hydration state and application method on the bond strength of self-etching adhesives to cut enamel. J Adhes Dent 14:25–30
ORIGINAL ARTICLE
TiF4 improves microtensile bond strength to dentin when using an adhesive system regardless of primer/bond application timing and method Juliana Bomfigli Tranquilin 1 & Enrico Coser Bridi 1 & Flávia Lucisano Botelho Amaral 1 & Fabiana Mantovani Gomes França 1 & Cecilia Pedroso Turssi 1 & Roberta Tarkany Basting 1
Received: 16 January 2015 / Accepted: 20 May 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Objectives This study evaluated the bond strength of a twostep conventional adhesive system to dentin pretreated with 2.5 % titanium tetrafluoride (TiF4) according to application timing (before or after acid conditioning) and primer/ adhesive application method (active or passive). Materials and methods Dentin surfaces were randomly treated with different adhesive procedures (n = 6): etching with phosphoric acid (PA) + primer/adhesive (Adper Single Bond 2/3M ESPE) actively applied; PA + primer/adhesive passively applied; TiF4 before PA + primer/adhesive actively applied; TiF4 before PA + primer/adhesive passively applied; TiF4 after
* Roberta Tarkany Basting [email protected] Juliana Bomfigli Tranquilin [email protected] Enrico Coser Bridi [email protected] Flávia Lucisano Botelho Amaral [email protected] Fabiana Mantovani Gomes França [email protected]
1
PA + primer/adhesive actively applied; and TiF4 after PA + primer/adhesive passively applied. A composite block was built onto the tooth, which was sectioned into sticks (adhesive area of approximately 1 mm2). Microtensile bond strength tests and the failure mode were determined. Results Two-way ANOVA revealed no significant interaction between TiF4 application timing and primer/adhesive application method (p = 0.184). The use of TiF4, before or after PA significantly increased bond strength values (p < 0.001). There was no significant difference in bond strength values when using TiF4 before or after PA. The primer/adhesive application method had no influence on bond strength, regardless of whether TiF4 was used (p = 0.906). Failure mode was predominantly adhesive. Conclusions The use of TiF4 promoted higher immediate bond strength to dentin. The conventional adhesive system may be applied either actively or passively, regardless of TiF4 application timing. Clinical significance Pretreatment with TiF4 increased bond strength in a conventional two-step adhesive system to dentin, regardless of the primer/adhesive application method and the timing of dentin pretreatment. Keywords Two-step conventional adhesive system . Hybrid layer . Microtensile bond strength . Titanium tetrafluoride
Cecilia Pedroso Turssi [email protected]
Introduction
Department of Restorative Dentistry, São Leopoldo Mandic Institute and Research Center, Rua José Rocha Junqueira, 13, Campinas 13045-755, São Paulo, Brazil
Titanium tetrafluoride (TiF4) is a fluoride compound available as a gel, varnish, or aqueous solution, which has been used topically on the enamel to prevent the rise of primary caries by
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reducing demineralization in vitro [1] and inhibiting caries progression in situ and in vivo [2, 3]. In addition, it is capable of preventing erosion/abrasion lesions [4, 5], thus reducing dentinal hypersensitivity [6]. Sen and Büyükyilmaz [7] showed that a solid structure and modified smear layer are formed when TiF4 is applied at 4 %. Wiegand et al. [8] reported that pretreatment with TiF4, at 2.5 %, led to the formation of a globular lining surface that occluded the dentinal tubules. Since this surface lining is composed of organometallic complexes of titanium and dentinal organic matrix [9], it may provide mechanical protection for the dentin, enabled by the strong bond between the titanium and the oxygen atom from a phosphate group, thus forming a resistant vitreous layer [7, 10]. This bond occurs after the hydrolysis of TiF4, since titanium has a strong tendency to pair up with an oxygen atom; the titanium compounds formed therein are strongly bonded and not easily replaced [3, 11]. Titanium is a biologically acceptable element, because it is nontoxic in its pure metallic form [10]. Therefore, when applied to the dentin, particularly as a pretreatment following cavity preparation, a lower risk of secondary caries would be expected due to modifications in the smear layer, thus suggesting that the use of TiF4 to treat dentin prior to restorative procedures could be an important step [12], especially when using self-etching adhesive systems that can both demineralize and infiltrate the dentin to form a hybrid layer [13] containing titanium tetrafluoride. The use of TiF4 as a dentin pretreatment was initially proposed by Dündar et al. [14], who suggested the application of an aqueous TiF4 solution for 60 s on dentin previous to acid etching or acidic monomer application. Dentin pretreatment with TiF4 was also evaluated by Devabhaktuni and Manjunath [15], Bridi et al. [16], and Domingues et al. [17]. Bridi et al. [16] and Domingues et al. [17] did not report any influence of TiF4 as a pretreatment when using a self-etching adhesive system. In this respect, the use of self-etching adhesives of mild to moderate acidity may be suggested in order to dissolve the glaze layer formed following the application of TiF4. When using a conventional three-step adhesive system, Devabhaktuni and Manjunath [15] reported no significant effect on the bond strength of a composite resin to the dentin following pretreatment with TiF4 either before or after acid etching. Nonetheless, Basting et al. [18] have shown that the layer formed by pretreating the dentin with TiF4 was acid resistant and could not be removed, even after the use of citric acid for 30 min. According to Pupo et al. [19], applying the bonding agent actively results in greater hardness of the hybrid layer both superficially and in depth. In addition, mechanical pressure from the microbrush increases molecular agitation optimizing solvent evaporation and, consequently, improved monomer diffusion through the dentin, leading to an increased percentage of polymer formation, which is stable and resistant to hydrolysis, thus improving the mechanical properties of the
adhesive layer [20, 21]. Therefore, active application of the adhesive system containing water as a solvent would benefit bond strength [22–24], by allowing greater removal of water and further penetration of monomers through the dentinal collagen fibrils. The use of a TiF4 aqueous solution and active application of the adhesive system could promote greater removal of excess water from the pretreatment and improve diffusion of the monomer through the vitreous layer. The incorporation of TiF4 into the hybrid layer of conventional bonding systems could play an important role in restorative procedures, considering the possible inhibition of secondary carious lesions, as well as permitting higher longevity of the hybrid layer should TiF4 act as a dentinal biomodifier [14, 16, 18]. Due to ongoing controversy regarding TiF4 as a dentin pretreatment in combination with conventional adhesive systems as well as the lack of studies using conventional two-step bonding systems, the present study aimed to evaluate the bond strength of a conventional two-step adhesive system to dentin, which was pretreated with 2.5 % TiF4, in terms of application timing (before or after acid etching) and primer/ bond application method (active or passive).
Materials and methods Experimental design In order to obtain sticks for microtensile strength testing, the experimental units were formed of 36 blocks of dentin obtained from 36 extracted human third molars. Six blocks of dentin were allocated to each experimental group [16, 17]. The study factors were as follows: (A) TiF4 application timing on three levels, no treatment (no TiF4), before acid etching, and after acid etching; and (B) primer/bond application method on two levels, active or passive application. The response variable was microtensile bond strength, which was evaluated quantitatively and obtained in MPa. The response variable regarding failure mode was evaluated qualitatively by means of a scoring system. The micromorphology of the hybrid layer was also evaluated qualitatively. Teeth selection and preparation Following Ethics Committee approval (Sao Leopoldo Mandic School of Dentistry and Research Institute, registration 453.589), 36 recently extracted sound third molars were selected, cleaned using a periodontal curette, and stored in 0.1 % thymol aqueous solution. The teeth were inspected visually using a stereoscopic magnifier (EK3ST, CQA Comercial Química Americana Ltd., Americana, SP, Brazil) at ×40 magnification. Teeth presenting cracks of structural anomalies were excluded. The flat surfaces of the coronal aspect of the dentin were obtained by removing the root at 2 mm below the cementoenamel junction using a diamond disk mounted on a precision saw (Isomet 1000 Precision Diamond Saw, Buehler
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Ltd., Lake Bluff, IL, USA). The occlusal enamel was subsequently removed, leaving a flat surface of exposed dentin at a right angle to the long axis of the tooth. The tooth fragments were finished on a water-cooled rotatory polisher (Politriz Aropol 2V, Arotec, Cotia, SP, Brazil), using aluminum oxide sandpaper (Imperial Wetordry, 3M, Sumaré, SP, Brazil), in a decreasing order of grain (400, 600, and 1000) in order to create smooth dentin surfaces. Following root removal, the exposed pulp chamber was filled with composite resin (Filtek Z350, 3M ESPE, Saint Paul, MN, USA) following by the bonding agent (Adper Single Bond, 3M ESPE, Saint Paul, MN, USA). The teeth were then divided into groups according to the combinations and levels based on the study factors (TiF4 application timing × primer/adhesive application method) (n = 6), as shown in Table 1. Dentin treatment and adhesive procedures For dentinal pretreatment, the TiF4 was obtained in the pro-analysis (P.A.) form and prepared at a concentration of 2.5 % in distilled deionized water (weight/volume and pH 1.2), according to Dündar et al. [14], Bridi et al. [16], and Domingues et al. [17]. The product was applied to the dentin of each group according to timing (before or after phosphoric acid etching) for 60 s, actively applied with a disposable brush, followed by brief drying for 5 s with blasts of air. For groups that only received the application of the adhesive systems (groups A and P), etching with phosphoric acid for 15 s was performed prior to the use of the adhesive system (Table 2). The two-step conventional adhesive system was applied passively, according to the manufacturer’s instructions, for the groups P, TiF4BEP, and TiF4AEP. For the remaining groups, A, TiF4BEA, and TiF4AEA, the primer/adhesive was applied actively, which consisted of mechanically smearing of the substance onto the surface using a disposable brush (Microbrush, Grafton, WI, USA), aiming to evaluate the Table 1
Study groups according to treatment
Groups/ abbreviations
Treatment
A
No treatment (no TiF4) + etching with phosphoric acid + active primer/adhesive application No treatment (no TiF4) + etching with phosphoric acid + passive primer/adhesive application TiF4 before etching with phosphoric acid + active primer/adhesive application TiF4 before etching with phosphoric acid + passive primer/adhesive application TiF4 after etching with phosphoric acid + active primer/ adhesive application TiF4 after etching with phosphoric acid + passive primer/adhesive application
P TiF4BEA TiF4BEP TiF4AEA TiF4AEP
dissolution of the vitreous layer that may have formed following TiF4 application. Following application of the adhesive system, a 5 × 5-mm composite resin block (Filtek Z350, 3M Espe, Saint Paul, MN, USA) was built and light-cured for 40 s in 2-mm thick increments. The block was eventually light-cured for 20 s on each side. Light-curing was carried out using a photoactivator halogen light equipment (Demetron Research Corporation, Danbury, CT, USA) set at a mean radiance of 495 mW/cm2 (422 to 569 mW/cm2). Specimen preparation for microtensile bond strength testing The teeth fragments were positioned on a metallographic saw (Isomet 1000 Precision Diamond Saw, Buehler Ltd., Lake Bluff, IL, USA), using acrylic plates for fixation. Buccal-lingual and mesial-distal sections were obtained of approximately 1 mm 2 measured using a digital caliper (Mitutoyo America Corporation, Aurora, IL, USA). The sticks were stored in Eppendorf tubes in distilled water at 37 °C for 24 h prior to microtensile bond strength testing. The latter was performed in a universal testing machine (DL2000, EMIC, São José dos Pinhais, PR, Brazil) at 0.5 mm/min and cell load of 50 N. The sticks were appropriately positioned using a specific device for microtensile bond strength testing aided by fast setting glue (Superbonder, Henkel Loctite Ltda, São Paulo, SP, Brazil). The bond strength values were obtained in MPa. The surface of the specimens submitted to the test was examined using a stereoscopic magnifier (EK3ST, Eikonal Equipamentos Ópticos e Analíticos, São Paulo, SP, Brazil) at 30 times magnification, in order to classify the specimens according to failure mode. The categories were adhesive (adhesion failure), cohesive failure in dentin (substrate failure), cohesive in resin (failure of the composite resin), or mixed (cohesion failure in resin). One or two sticks from each resin-tooth block were set aside for evaluation of the tooth/restoration interface by scanning electronic microscopy (SEM) (Jeol 5900LV, Jeol Ltd., Tokyo, Japan). The sticks were smoothed using watercooled sandpaper in decreasing grain (400, 600, and 1200) and then polished using diamond pastes of sequentially decreasing roughness (6, 3, 1, and 1/2 μm) on felt disks under mineral oil cooling. The samples were thoroughly rinsed, then demineralized for 30 s with 6 N HCl, rinsed again, deproteinized with 2.5 % NaOCl for 10 min [25], and serially dehydrated in 25, 50, 75, 95, and 100 % ethanol and chemically dried in HMDS for 10 min. The samples were mounted on aluminum stubs, sputter-coated with gold, and examined under SEM at an operating voltage of 10 Kv. Observations were carefully performed on the entire adhesive interface of the stick at ×500 magnification. Images of the most representative area of each specimen were then taken at ×1000 magnification. Differences in hybrid layer formation, shape,
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Materials used, composition, protocol of use, and manufacturer
Materials
Composition/pH
Protocol of use
Manufacturer (city, state, country)
2.5 % titanium tetrafluoride
Titanium tetrafluoride P.A. + distilled deionized water/pH = 1.2 35 % phosphoric acid, silica/pH = 0.6
Apply the product for 60 s
Sigma-Aldrich (Saint Louis, MO, USA) 3M ESPE (3M ESPE, Saint Paul, MN, USA)
3M Scotchbond Etchant
Adper Single Bond 2
BisGMA, HEMA, dimethacrylates, ethanol, water, a novel photoinitiator system, and a functional copolymer of polyacrylic and polyalkenoic acids/pH = 4.3
length, and density of resin tags were assessed according to TiF4 application timing and primer/bond application method. Statistical analysis The data were digitalized and tabulated as Excel spreadsheets. Mean microtensile bond strength values were calculated for each tooth. Stabilization of variance and normality was obtained by transforming the data using the square root of the values. Descriptive statistics and two-way analysis of variance (ANOVA) were performed at a significance level of 5 % (SPSS Inc., Chicago, IL, USA). Failure mode was analyzed using descriptive statistics.
Results Two-way ANOVA on the transformed data revealed no significant interaction between the study factors (p = 0.184). Regardless of the application method, the use of TiF4, before or after acid etching, significantly increased bond strength (p < 0.001) (Table 3). No significant difference in bond strength values was observed when TiF4 was applied before Table 3 Mean values bond strength to dentin (MPa) and standard deviation (SD) according to the groups
Apply to the tooth for 15 s Rinse with water for 15 s Dry dentin with absorbent paper to maintain moisture Passive method (recommended by the manufacturer): Apply two consecutive layers of primer/adhesive passively for 15 s interposed by a 5-s blast of air Light-cure for 10 s Active method: Apply two consecutive layers of primer/adhesive actively (smearing), interposed by a 5-s blast of air Light-cure for 10 s
3M ESPE (3M ESPE, Saint Paul, MN, USA)
or after etching. Primer/adhesive application method did not influence bond strength, independently of TiF4 application (p = 0.906). The commonest type of failure observed was adhesive for all groups, followed by cohesive in resin. Dentin cohesion failure was infrequent, yet always present in the groups for which TiF4 was used to pretreat the dentin. No mixed failures were observed. Dentin cohesion failures were observed when primer/adhesive was applied passively (5 to 11 % of the failures). Figure 1 illustrates the failure modes across the groups. Micromorphological images (Fig. 2) revealed that the lack of TiF4 led to the formation of fewer resin tags when adhesive was applied either actively or passively. Although the tags were uniformly distributed throughout the hybrid layer, they presented lower diameter and depth when compared to the groups treated with TiF4. When TiF4 was applied (C to E), interface micromorphology was similar between the groups, regardless of application timing (before or after acid etching) and primer/adhesive application method (active or passive). A hybrid layer was formed in all types of treatment studied, showing a higher number of resin tags, which were also
Primer/adhesive application method TiF4 application timing
Active
Passive
No TiF4 TiF4 before etching TiF4 after etching Overall mean
10 (2.1) 16.1 (4.4) 16.3 (7.1) 14.1 (5.6) a
9.6 (1.9) 19.5 (7.3) 12.2 (1.2) 13.8 (6.0) a
Overall mean
9.8 (1.9) B 17.8 (6.0) A 14.2 (5.3) A –
Overall means followed by identical capital letters indicate no significant difference between the treatments. Overall means followed by identical lowercase letters indicate no significant difference between the application methods
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Fig. 1 Failure modes distribution among groups
deeper and larger in diameter than in the groups for which TiF4 was not used.
Discussion The use of substances that act as biomodifiers or dentin pretreatment has been proposed as an approach to reduce degradation of the hybrid layer, stabilizing or reducing the incidence of secondary caries, as well as maintaining bond strength [26, 27]. Among these agents, TiF4 promotes the formation of a layer composed of titanium oxides or organometallic complexes upon contact with the organic material within the dentin, thus creating an acid-resistant stable lining and conferring higher resistance to tooth demineralization [7, 18]. Pretreating dentin with TiF4 could, therefore, be an important step prior to or concomitant to the application of bonding systems [12, 14, 16, 17], which would result in a biomodified hybrid layer that would be also acid resistant [18]. The results from the present study demonstrated that TiF4 application led to higher bond strength values to the dentin when using a conventional two-step adhesive system, regardless of application timing. This finding, contrary to what is reported for single-step [16] or two-step [16, 17] self-etching systems, shows that TiF4 may contribute to improve the mechanical properties of the hybrid layer, leading to a higher mechanical imbrication between dentinal collagen and the
monomers from the adhesive system, which can also be confirmed by observing the micromorphology of the adhesive interface. The formation of a vitreous layer when applying TiF4 to the dentin, which renders the surface nearly impermeable and resistant to acids, was observed by Sen and Büyükyilmaz [7] and Basting et al. [18]. Nonetheless, the use of self-etching adhesive systems has shown a capacity to infiltrate and penetrate through this layer [16, 17], possibly due to the established active application of the self-etching primer onto the dentin. The bond strength values and the micromorphology of the hybrid layer when using self-etching adhesive systems have shown similar results with or without TiF4 as a dentin pretreatment [16, 17]. The results from the present study, however, have shown that the bond strength values to dentin, when using TiF4 as a pretreatment, may be influenced by the bonding strategy used, considering that higher bond strength values were obtained with TiF4 pretreatment and a conventional two-step bonding system. Regardless of TiF4 application timing, a significant increase in dentin bond strength was observed, suggesting that TiF 4 may have contributed as an immediate dentinal biomodifier. Micromorphological evaluation revealed the formation of a vitreous layer obtained with the application of TiF4 onto the untreated smear layer (when applied before phosphoric acid etching) or onto the dentin that had had its smear layer (following acid etching) [18], where the primer/
Clin Oral Invest Fig. 2 Photomicrograph of the adhesive interface for the groups studied (×1000 magnification). a Active primer/adhesive application, showing few tags with low depth and low diameter. b Passive primer/adhesive application, showing similar tags to the active application group. c TiF4 application prior to phosphoric acid etching + active primer/adhesive application, showing numerous tags. d TiF4 application prior to phosphoric acid etching + passive primer/ adhesive application, showing the formation of tags with larger diameter and deeper into the substrate. e TiF4 application after phosphoric acid etching + active primer/adhesive application, showing a higher density of tags per area. f TiF4 application after phosphoric acid etching + passive primer/adhesive application, with tag formation similar to that from the passive application group
adhesive was able to penetrate through the vitreous layer, forming a hybrid layer containing resin tags with a deeper infiltration, larger diameters, and an increased density, thus explaining the higher bond strength values observed in comparison to the groups where no TiF4 was applied. These findings contradict those reported by Devabhaktuni and Manjunath [15], where no significant difference in bond strength when using a conventional three-step bonding system was observed for a composite material to the dentin following pretreatment with TiF4 after acid etching, possibly due to the strategy used, e.g., three-step instead of two-step, as per the present study, as well as the composition and pH of the adhesives used. It was expected that the TiF4 infiltrated in the hybrid layer could have an effect on increasing the mechanical properties of this layer. Bond strength increases as a consequence of filler addition in adhesive systems [28]. Likewise, particles in adhesives may increase the debonding energy, sometimes
leaving dentin at a disadvantage, because the dentinadhesive interface could be stronger than other substrates (resin or dentin), thereby increasing the susceptibility of failure propagation [28]. Accordingly, this mechanism may also explain the results observed for failure types, where dentin cohesion failure was always present in the groups for which TiF4 was used to pretreat the dentin. For this reason, the difference in fracture type patterns when TiF4 was applied could result in a crack trajectory moving away from the adhesive interface toward the adhesive dentin interface, as a result of increased resistance to fracture propagation. Regarding the application method for the bonding system, independently of the application of TiF4, active application showed similar bond strength values to passive application, which contradicts the studies by Hashimoto et al. [22], Jacobsen and Söderholm [23], and Dal Bianco et al. [24]. In general, all adhesive systems contain water or other solvents in their composition, which can be difficult to remove prior
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to adhesive polymerization and, therefore, may compromise the quality of the polymer and the hybrid layer [23]. The resistance and durability of the bond strength of acetone or water-/ethanol-based bonding systems to dentin may be different due to the application method selected, considering that the pressure vapors for either solvent is relatively different (200 and 47.1 mmHg, respectively) [29]. In the case of the adhesive system used in this study, water and ethanol are present in its composition, which are more difficult to evaporate than acetone. Therefore, a maneuver to promote higher volatility for this kind of solvent is to increase application time [30], applying several layers of adhesives [22, 31], wait longer before light-curing the adhesive [32] or apply the adhesive under agitation [19, 24, 29], which would promote higher resistance of the polymer formed through the collagen fibrils and higher bond strength. In the present study, however, the dentin was kept moist prior to the application of primer/adhesive, both after etching and after applying TiF4, which favored adhesive permeability through the collagen fibrils for this type of solvent [24, 29] for both application methods, which could explain there being no difference in immediate bond strength between the application methods. This finding seems to corroborate the manufacturer’s instructions, which do not include application method for the primer/adhesive, despite prior knowledge that this type of simplified conventional bonding system contains monomers of high molecular weight that could show limited diffusion through moist demineralized dentin [22, 24]. Despite considering that active application may promote higher kinetics for the components and a greater diffusion of the monomers in depth, consequently allowing greater volatilization of the solvents, their application method did not influence the bond strength results nor did it have any impact on the micromorphology of the interface. In spite of the lack of difference in bond strength in terms of application method, Dal-Bianco et al. [24] suggest that active application intensifies removal of water from the demineralized dentinal matrix, replacing the space once occupied by minerals with resin monomers without allowing collapse of the collagen fibrils. The presence of residual water during monomeric diffusion or within the polymerized compound may induce hydrolytic degradation of the hybrid layer due to the formation of water pockets with hydrophilic properties, known as water-trees [33]. Active application is indicated for self-etching adhesive systems [19, 34, 35]. However, the need to apply the selfetching adhesive system actively refers to the increased need to modify and/or remove the smear layer to allow greater mechanical imbrication, as well as to improve the chemical interactions between the adhesive and the subjacent dentin, promoting higher bond strength values. Similarities in micromorphology were observed at the interface in both application methods, which may explain the
bond strength and failure mode findings, the latter predominantly adhesive for all groups. Microscopy has shown a similar pattern for resin tag formation in terms of depth and density between adhesive application methods. Despite the need of further in vitro and clinical studies to allow the recommendation of dentin pretreatment with TiF4 solution combined with a conventional two-step adhesive system, the use of TiF4 promoted higher immediate bond strength to dentin. The conventional adhesive system may be applied either actively or passively, regardless of TiF4 application timing.
Conclusions Pretreatment with TiF4 increased bond strength in a conventional two-step adhesive system to dentin, regardless of the primer/adhesive application method and the timing of dentin pretreatment. Acknowledgments The authors would like to thank CNPq foundation (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for providing grants (process number 125925/2013-1) and LME/LNNano/ CNPEM for the technical support during the electron microscopy work. Conflict of interest The authors declare that they have no competing interests.
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