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ScienceDirect Tanta Dental Journal 12 (2015) 99e110 www.elsevier.com/locate/tdj

The effect of pre-curing waiting time of different bonding resins on micro-tensile bond strength to dentin W. El-Malky a,b,*, K.M. Abdelaziz b b

a Operative Dentistry Department, Suez Canal University, Egypt Department of Restorative Dental Sciences, College of Dentistry, King Khalid University, Abha, Saudi Arabia

Received 27 January 2015; revised 23 February 2015; accepted 28 February 2015 Available online 23 April 2015

Abstract Objective: To evaluate the effect of 30 s waiting time of different bonding resins on their micro-tensile bond strength (mTBS) to dentin. Method: The occlusal enamel of freshly extracted human third molars was removed to expose the middle dentin. The specimens were randomly assigned into four groups namely, Group I, two-steps-total-etch bonding system (Adper Single Bond 2/Filtek Supreme XT, ASB2/FSXT), Group II, two-steps-self-etch bonding system (Silorane System Adhesive/Filtek Silorane, SSA/FS), Group III, two-steps-self-etch bonding system (Adper SE Plus/Filtek Supreme XT, ASEP/FSXT) and Group IV, one-step-self-etch bonding system (Adper Easy One/Filtek Supreme XT, AEO/FSXT). Each group was further divided into two sub-groups according to the application time of the bonding resin (following manufacturer recommendation and 30s more than manufacturer recommendation). The corresponding resin composite (shade A3) was applied in 4 increments of 0.5e1 mm thickness each to build resin composite stump of 3e4 mm height. Each increment was light polymerized for 20 s using LED at intensity 1000 mW/cm2. The specimens were stored in deionized water at 37
C for 24 hrs. The specimens were cut into bars according to the non-trimming technique (n ¼ 20/group) with surface area of 0.81 mm2. The mTBS of the bars was tested using a micro-tensile device on a Universal testing machine at a crosshead speed of 1 mm/min. Result: Two-way ANOVA revealed a statistical significant difference in the two main effects, the types of bonding systems (P ¼ 0.0024) and the application times of bonding resins (P ¼ 0.0005). There was no statistical significant interaction between the two main effects (P ¼ 0.9257). The methacrylate-based two-steps-total-etch bonding system and two-steps-self-etch bonding system showed significant higher values than that of the one-step-self-etch bonding system. Conclusion: i. Thirty-seconds pre-curing waiting time for bonding resins has a role in achieving higher mTBS to dentin. ii. Onestep-self-etching bonding system ranked as the lowest bonding system. © 2015, Hosting by Elsevier B.V. on behalf of the Faculty of Dentistry, Tanta University. Keywords: mTBS; Dentin; Bonding systems; Pre-curing waiting time; Dwell

* Corresponding author. Suez Canal University, Faculty of Dentistry, Operative Dentistry Department, Ismailia, Egypt. Tel.: þ20 1001119804. E-mail address: [email protected] (W. El-Malky). Peer review under the responsibility of the Faculty of Dentistry, Tanta University.

1. Introduction and review of literature Since the innovative work of Buonocore, the concept of bonding has been continuously developing over the years till now. Bonding to enamel has been

http://dx.doi.org/10.1016/j.tdj.2015.02.002 1687-8574/© 2015, Hosting by Elsevier B.V. on behalf of the Faculty of Dentistry, Tanta University.

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demonstrated to be simply attainable and can be durable [1]. Bonding to dentin is far more complicated due to the complex challenging nature of dentin. Since the nineties, two main approaches for bonding systems has been endlessly evolving namely; total-etch (etch and rinse) and self-etch (etch and dry) [2,3]. The concept of total-etch approach is to apply an acid to simultaneously etch enamel, remove the smear layer, demineralize mainly the superficial inter-tubular dentin at different depths, and to open the collagen network. After rinsing those etching products, a primer is applied, which is a double function highly hydrophilic monomer carried on a solvent vehicle. The primer penetrates and infiltrates the demineralized collagen network to intermingle with its fibers increasing its low surface energy. This will be followed by the application of a hydrophobic photopolymerizable co-monomer to link the resin impregnated collagen network (hybrid layer) with the subsequently applied resin composite. The aforementioned approach was marketed as total-etch-three-steps bonding systems [3,4]. Alternatively, manufacturers started to launch modifications to total-etch-three-steps bonding systems such as the total-etch-two-steps (primer and bonding resin together) and the self-etch bonding systems. Such modifications were heavily marketed based on a concept that three-steps-total-etch bonding systems are more complicated, time-consuming and techniquesensitive. Self-etching systems were introduced to mainly overcome the sensitivity to humidity of the etch-and-rinse technique as well as to simplify the clinical procedures of adhesive application and to reduce its clinical time [5e9]. The concept of self-etch approach is to instantaneously condition and prime the dental substrate by the action of their hydrophilic acidic monomers. Therefore, these self-etching primers do not require a separate etching step and do not require rinsing after conditioning. Such promising features have led to the gradual growing popularity of self-etch bonding systems in the dental profession. Currently, self-etch bonding systems are available as two-steps systems or one-step systems. Two-steps-self-etch system is provided as a self-etching primer in one bottle and bonding resin (usually solventfree) in another bottle. One-step-self-etch system is provided either as two-component system (two separate solutions to be mixed together) or one single-component system (one single solution that do not require mixing). The single-component one-step-self-etch system is considered as the only true one-bottle or all-in-one bonding system [10,11].

The key factor in the success or failure of a resin composite restoration depends on many factors, but mainly on the performance of the bonding system and the performance of the resin composite. The bonding system is supposed to efficiently infiltrate and impregnate the entire demineralized and opened collagen network to overcome its humidity and to form a high cross-linking polymer inside the collagen network [3,12]. The consequential resinedentin hybrid layer would have an increased resistance to degradation [13]. Resin composite restorative materials have been extensively used in dental practice, but yet they still have inherent shortcomings. One of the main shortcomings of the resin composite is polymerization shrinkage. Therefore, manufacturers have developed many formulas to overcome such inherent deficiencies of resin composites as; the innovative low-shrinking monomers (silorane-based resin composite) [14] and the use of monomers with increased molecular weight [15,16]. Also, manufacturers optimized filler morphology, utilized a range of filler particle sizes and smaller average particle size to reduce polymerization shrinkage and to improve material's mechanical and esthetic properties [17]. To have a strong and durable bond with dentin, researchers also proposed many application techniques to overcome the effect of resincomposite-polymerization-shrinkage-stresses on the bond interface with dentin as; different photoactivation protocols [16] and incremental technique [18]. Manufacturers of bonding systems and resin composites are always competing with each other claiming that their products are less technique sensitive, have shorter time of application, less complicated and have better performance and durability. Even the same manufacturer has different bonding systems that follow both main bonding approaches as well as different types of resin composites. Manufacturers trials to suite all the preferences, demands and believes of different natures of practitioners sometimes confuses them. Adding to the confusion, in the last few years, studies evolved over the failure of bonding resin to efficiently cover the collagen fibers of demineralized dentin. Partially covered collagen fibers will trigger the action of collagen matrix metalloproteinase (MMPs) especially in the presence of residual water in the hybrid layer leading to declined bond strength to dentin upon aging of restorations [19e21]. Resinedentin interfaces formed under current manufacturer recommended application times seems insufficient to produce a stable polymer within the hybrid layer. Meanwhile, certain modifications in the manufacturer recommended

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application techniques of bonding systems produced higher bond strength between resin composites and dentin. Such higher bond strength signifies the appropriate penetration of the bonding resin to the demineralized dentin resulting in better quality of dentineresin hybrid layer [13]. Indeed, every effort should be exerted to ensure efficient infiltration of bonding resin within demineralized dentin to effectively cover its collagen fibers. Researchers have suggested many modifications for the recommended application techniques to overcome the complex nature of dentin to maximize the infiltration and to ease the sensitivity of the application such as; increased application times of primers/bonding resin [22], multiple adhesive coating [23] delayed polymerization [13,24,25] adhesive rubbing [26] and longer light exposure times of bonding resins [27]. Reviewing the dental literature revealed few data related to the effect of extending the application time of bonding resin for different categories of bonding systems on their mTBS to dentin. Consequently, it was believed that an investigation to elucidate such effect might be a contribution to the dental literature. The aim of the present study was to evaluate (mTBS) to dentin of four different bonding systems after 30 s precuring dwell waiting time of only their bonding resins more than what was recommended by their manufacturer. Bonding systems tested in the present study characterize the current approaches of bonding systems as well as the new developments in resin composites. The null hypothesis was that 30 s pre-curing dwell waiting time of bonding resins would not affect their micro-tensile bond strength to dentin. 2. Materials and method 2.1. Dentin surface preparation

Eight caries-free freshly extracted human third molars were used in the present study. The occlusal enamel was cut perpendicular to the long axis of the tooth and pulpal to the estimated level of the dentinoenamel junction to reach coronal dentin using a precision saw machine,1 Figs. 1,2. In case of any remaining enamel on the occlusal surface, another thin slice was done to remove it using thin diamond disc on the same saw machine. The occlusal dentin surface was manually ground using Silicon carbide papers of 600 grits under running water for 30 s to form a standardized smear layer [28]. 1

ISOMET 1000, Buehler, Lake Bluff, IL, 60044-1699, USA.

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Fig. 1. First cut to remove the occlusal enamel to reach dentin.

Fig. 2. Coronal dentin surface.

2.2. Bonding procedures

The specimens were divided into four groups according to the types of bonding systems, (Table 1). Group I, two-steps-total-etch bonding system (Adper Single Bond 2/Filtek Supreme XT, ASB2/FSXT), Group II, two-steps-self-etch bonding system (Silorane System Adhesive/Filtek Silorane, SSA/FS), Group III, twosteps-self-etch bonding system (Adper SE Plus/Filtek Supreme XT, ASEP/FSXT) and Group IV, one-step-selfetch bonding system (Adper Easy One/Filtek Supreme XT, AEO/FSXT). Each group was further divided into two subgroups according to the application times of bonding resins; manufacturer recommendation (application time of bonding resin according to manufacturer instructions) and 30s waiting time (increasing the manufacturer application waiting time by additional 30 s before curing). All other manufacturer instructions regarding bonding procedures were followed, (Table 1). It should be noted that bonding resin was left undisturbed during those extra 30 s waiting time before evaporation of solvent and light curing. The corresponding resin composite, (shade A3) Table 1, was applied in 4 increments of 0.5e1 mm thickness to build resin composite stump of 3e4 mm height. Each increment was light polymerized for 20 s using LED2 at intensity 1000 mW/cm2. The specimens were

2

Elipar Free Light 2, 3M/ESPE, St. Paul, MN, USA.

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Table 1 Type, Manufacturer, Classification, Composition, Patch no., Lot no., and technique of application as recommended by the manufacturer. Type/manufacturer

Classification

Composition

Patch no./Lot no.

Technique of application by manufacturer

Adper Single Bond 2 3M ESPE, St. Paul, MN, 55144-1000, USA

Total-etch Two-steps Methacrylate-based

BisGMA (bisphenol a diglycidyl ether dimethacrylate), HEMA (2hydroxyethyl methacrylate), dimethacrylates, ethanol, water, a novel photoinitiator system and a methacrylate functional copolymer of polyacrylic and polyitaconic acids.

51202/7 KE

Silorane system adhesive 3M ESPE, St. Paul, MN, 55144-1000, USA

Self-etch Two-steps Silorane-based

4713P & 4713B/8AT & 8AK

Adper SE Plus 3M ESPE, St. Paul, MN, 55144-1000, USA

Self-etch Two-steps Methacrylate-based

Self-etch primer: phosphoric acidmethacryloxy-hexylesters mixture, 1,6-hexanediol dimethacrylate, copolymer of acrylic and itaconic acid, phosphine oxide, (dimethylamino) ethyl methacrylate, Bis-GMA and HEMA, water and ethanol, camphorquinone, silane treated silica filler with a primary particle size of about 7 nm with filler loading 8e12 wt%. Bonding resin: Substituted dimethacrylate, TEGDMA, Phosphoric acid methacryloxyhexylesters, 1,6hexanediol dimethacrylate, camphorquinone, silane-treated silica fillers with filler loading 5e10 wt%. Liquid A Water, HEMA, polyethylene epolypropylene Glycol (Surfactant), Rose Bengal dye (Pink colorant) Liquid B UDMA (di-urethane dimethacrylate),TEGDMA (triethylene glycol dimethacrylate), TMPTMA (hydrophobic trimethylopropane trimethacrylate), HEMA phosphates, MHP (6methacryloxyexacryloxyphosphate, Bonded zirconia nanofiller, Initiator system based on camphorquinone.

Apply Scotchbond™ Etchant to dentin, Wait 15 s and Rinse for 10 s. Blot excess water using a cotton pellet, the dentin surface should appear shiny without pooling of water. Immediately after blotting, apply 2-3 consecutive coats of adhesive for 15s with gentle agitation using a fully saturated micro brush. Gently air thin for 5s to evaporate solvent. Lightcure for 10s using Elipar Free Light2. Self-etch primer was applied with a micro brush on dentin surface and rubbed for 15s; gently air dried and cured for 10s using Elipar Free Light2. The bottle of the bonding resin was well agitated first, and then the bonding resin was applied with a micro brush, gently air thinned for 10s and cured for 10s using Elipar Free Light2.

83030/7AC & 7AE

Liquid A is applied using a micro brush, to cover all dentin surface, followed by application of Liquid B. The pink color of liquid A disappears quickly. This color change indicates the activation of the acidic monomers in Liquid B and the beginning of the etching process. Agitation of Liquid B on the surface for 20s to ensure a proper etch, then air-dried for 10 s to remove water. Liquid B (adhesive) is re-applied to provide a hydrophobic overcoat then air thinned for 10s and light cured for 10s using Elipar Free Light 2. (continued on next page)

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Table 1 (continued ) Type/manufacturer

Classification

Composition

Patch no./Lot no.

Technique of application by manufacturer

Adper Easy One 3M ESPE, St. Paul, MN, 55144-1000, USA

Self-etch One-step (All-in-one) Methacrylate-based

85010/298027

The liquid was applied and agitated on the dentin surface for 20s using micro brush. Air-dried for 5 s, then lightcured for 10s using Elipar Free Light 2.

Filtek supreme XT 3M ESPE, St. Paul, MN, 55144-1000, USA

Methacrylate-based Nano-hybrid

3911A3B/6 GE

e

Filtek Silorane 3M ESPE, St. Paul, MN, 55144-1000, USA

Silorane-based Micro-hybrid

HEMA, Bis-GMA, Methacrylated phosphoric esters, 1,6 hexanediol dimethacrylate, Methacrylate functionalized polyalkenoic acid, Finely dispersed bonded silica filler with 7 nm primary particle size, Ethanol,Water, Initiators based on camphorquinone, Stabilizers bisphenol A glycol dimethacrylate; bisphenol A ethoxylated, methacrylate, urethane dimethacrylate, Triethylene glycol dimethacrylate. Fillers: Agglomerated 20 nm nano silica filler, zirconia/silica nanocluster 5e20 nm; cluster particle size 0.6e1.4 mm/78.5 wt.% 3,4-Epoxycyclohexylethylcyclopolymethylsiloxane, Bis-3,4Epoxycyclohexylethylphenylmethylsilane. Fillers: Silanized quartz, yttriumfluoride; 0.1e2.0 mm/76 wt.%

4712A3/N148837

e

stored immediately in deionized water for 24 h at 37
C before the sectioning procedures [28e30]. 2.3. Micro-tensile bond strength test procedures

The specimens were sectioned according to the nontrimming technique [31] for mTBS test using the same precision saw machine used to remove the occlusal enamel. The technique depends on sectioning the specimens in the buccal-lingual direction into serial slabs followed by slicing those slabs in the mesialdistal direction to produce multiple bars out of each tooth [28,29,31], Figs 3,4. After exclusion of the slabs containing enamel, 20 bars (n ¼ 20) of 0.81 mm2 were used in the mTBS testing for each subgroup. All bars didn't show any premature de-bonding during the sectioning procedures, and then the cross-sections of the bars were measured using digital micrometer.3 The bars were fixed with cyanoacrylate adhesive4 to a micro-tensile testing device5 [28,29]. The specimens were tested under tension using a universal testing machine at a crosshead speed of 1 mm/min.

2.4. Scanning electron microscope (SEM) analysis

Some bars from each group where kept for SEM analysis for another study. Few will be presented in the current study to show the difference, if any, between the manufacturer recommendation and the 30 s precure waiting time groups. 2.5. Statistical analysis

Bond strength data obtained for all the sub-groups were analyzed using StatView statistical analysis software version 5 (SAS, USA). Two-way ANOVA test at a ¼ 0.05 was used to test the two main effects. Posthoc testing was performed using Bonferroni/Dunn

3

Mitutoyo, 700-113, Model SC-6, China. Zap-It, Dental adventures of America, Corona, CA, 91720, USA. 5 Geraldeli's jig, Minnesota Dental Research Center for Biomaterials and Biomechanics, School of Dentistry, University of Minnesota. 4

Fig. 3. Cutting from buccal to lingual direction after removal of the slabs containing enamel.

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Fig. 4. Cutting in mesial to distal direction after removal of the slabs containing enamel.

test at a ¼ 0.05 to determine the source of difference between the sub-groups. 3. Results The mean mTBS values and standard deviations for the eight sub-groups are presented in both (Table 2) and (Fig 5). Two-way ANOVA was used to test the two main effects namely, the types of bonding systems (resin composite-bonding system combination) and the pre-curing dwell waiting time of bonding resins. The first main effect (types of bonding systems) had four levels namely, the ASB2/FSXT, SSA/FS, ASEP/FSXT and AEO/FSXT. The second main effect (pre-curing dwell waiting time of bonding resins) had two levels namely, manufacturer recommendation (pre-curing dwell waiting time of bonding resin according to manufacturer instructions) and 30 s pre-curing dwell waiting time (additional 30 s more than the manufacturer recommendation of pre-curing dwell waiting time). Two-way ANOVA revealed a statistical significant difference in the two main effects, the types of bonding systems (P ¼ 0.0024) and application times of bonding resins (P ¼ 0.0005). There was no statistical significant interaction between the two main effects (P ¼ 0.9257). Post-hoc Bonferroni/Dunn test at a ¼ 0.05 revealed that mTBS mean values of the ASB2/FSXT group was statistically higher than that of the AEO/FSXT group

Fig. 5. Representative SEM image of ASB2/FSXT group according to manufacturer recommendation. It shows the thickness of hybrid layer (H and double head arrow), resin tags (T) and incomplete lateral canal infiltration (single head arrows).

(P ¼ 0.0003) Meanwhile, there was no statistical significant difference between the mTBS mean values of the ASB2/FSXT group and that of the SSA/FS group (P ¼ 0.05) or ASEP/FSXT group (P ¼ 0.3528). There was no significant difference between the mTBS mean values of SSA/FS group and that of ASEP/FSXT group (P ¼ 0.2862) or AEO/FSXT group (P ¼ 0.0939). mTBS mean values of the ASEP/FSXT group was statistically higher than that of the AEO/FSXT group (P ¼ 0.0066). The mTBS mean values of the 30 s waiting time subgroups were statistically higher than that of the manufacturer recommended time (P ¼ 0.0005). The null hypothesis was rejected as significant differences were found between subgroups. 4. Discussion Although high immediate resinedentin bond strength values can be achieved with two-steps-totaletch bonding systems, they still suffer substantial deterioration after in vitro aging [3,32]. The polymer

Table 2 Mean mTBS and standard deviations of all groups. mTBS mean (MPa) ± SD Groups Adper Single Bond 2/Filtek Supreme XT, ASB2/FSXT Silorane System Adhesive/Filtek Silorane, SSA/FS Adper SE Plus/Filtek Supreme XT, ASEP/FSXT Adper Easy One/Filtek Supreme XT, AEO/FSXT Subgroups with the same letters are not significantly different (p > 0.05).

Manufacturer recommendation 43.608 38.639 41.635 33.158

± ± ± ±

a

13.636 10.621 ac 11.532a 9.323c

30 s Waiting time 49.490 44.446 46.802 41.495

± ± ± ±

12.124d 8.607 de 11.433d 11.399e

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inside the hybrid layer is susceptible to hydrolytic degradation due to its hydrophilic nature, which allows water sorption and increase of permeability [33,34]. Furthermore, it was shown that, a significant number of unprotected collagen fibrils could be found at the hybrid layer due to incomplete resin monomer infiltration [3,32]. These collagen fibrils become a target for the matrix metalloproteinases (MMPs) [19e21] and cysteine cathepsins [35], which weakens the bond of the restorative material to dentin. Recent studies confirmed and emphasized the role of MMPs and cysteine cathepsins on the dentin bonding degradation upon using both self-etch and total-etch bonding systems [36e39]. Consequently, it seems that immediate curing of bonding resin without a waiting period to infiltrate resin monomers into etched dentin created by total-etch bonding systems has been shown to be only partially effective. Likewise, Self-etch bonding systems claimed to penetrate along all demineralized dentin also, seems not totally true [38e40]. Extending the application time of the bonding resin (dwell time) means a longer waiting time after bonding resin application with no further disturbance before light curing. That concept of dwell time was first stated by Kanca in 1998 [41]. It was assumed that such waiting time would facilitate the removal of unbound water and residual solvents due to their evaporation [22,41]. This technique modification also, would help in the reduction of nano-pores and enable the formation of a highly cross-linked polymer. Accordingly, the resultant

Fig. 6. Representative SEM image of ASB2/FSXT after 30 s precuring waiting time group. It shows double the thickness of hybrid layer (H) and double head arrow, resin tags (T), deep complete lateral canal infiltration (single head arrows) and deep sidewall hybridization of dentinal tubules (stars).

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resinedentin hybrid layer would have better quality, bond strength and resistance to degradation [13]. There are very few publications related to the effect of bonding resin dwell time on bond strength. Therefore, the purpose of present in vitro investigation was to evaluate the mTBS to dentin of four different bonding systems after 30 s pre-curing dwell waiting time for their bonding resins more than what was recommended by their manufacturer before curing. The authors of the present study are fully aware and convinced that a randomized clinical study is the goldstandard method for the real-life differentiation between different types of combinations between bonding systems and resin composites. However, randomized clinical studies are complicated, time consuming, have a prohibitive cost and are difficult to get funding support and usually manufacturers launch newer products during the course of such clinical studies. Although laboratory-testing methods have their limitations and cannot simulate the clinical conditions, they could be useful in answering screening questions [42,43]. Studies which utilized the mTBS testing method showed that the specimens had better stress distribution during loading and less critical flaws resulting in fewer cohesive failures in dentin than those found with the conventional shear or micro-shear testing methods. Therefore, the results of mTBS testing method are more representative to the true interfacial bond strength [42,44] and more discriminative than the conventional and micro-shear methods [43,44]. Selection of the tested resin-composite-bondingsystem combinations was based on representing the recent nano-filled-methacrylate-based resin composites (claimed to be of low shrinkage) and the innovative low-shrinkage recently introduced silorane-based resin composite. Moreover, to represent the currently available bonding approaches, namely, methacrylate-based bonding systems (two-steps-total-etch, two-steps-selfetch, one-step-self-etch {all-in-one}) and siloranebased two-steps-self-etch bonding system. It is worth mentioning here that there was a clear manufacturer recommendation to use silorane-based resin composite only with its own silorane-based bonding system due to their unique chemistry. All products were selected from the same manufacturer and were exposed to the same curing conditions for one type only of methacrylate-based resin composite in a trial to investigate the solitary effect of increasing the time of application of these different tested bonding resins on the mTBS between the resin composite and dentin. The statistical analysis of the present study revealed

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statistically significant differences among the tested subgroups; therefore, the formulated null hypothesis had to be rejected. 4.1. Effect of increasing the pre-curing waiting time of the bonding resin on mTBS The results of the present study disclosed that the mTBS of 30s waiting time subgroups in all the tested bonding systems were statistically higher than that of the subgroups which followed the manufacture recommended pre-curing waiting time. This finding was in agreement with a study [22] that investigated the shear bond strength to dentin after increasing the precure waiting time for the bonding resin of a twosteps-total-etch bonding system (Single Bond) by 10, 20, 30, 40 s. They found that, 30 s and 40 s increase of the pre-cure waiting time resulted in significantly higher shear bond strength. Furthermore, another study [13] investigated the mTBS of two total-etch-two-steps bonding systems (Single Bond and One Step) after precure waiting time of 40, 90, 150 and 300 s for their bonding resin, one-day after bonding and after three years of storage. They concluded that increasing the application time of bonding resins of two-steps-totaletch bonding systems produced more stable resinedentin interfaces due to the formation of a more stable polymer within the hybrid layer. Although, 40s waiting time didn't result in significant higher mTBS values, but 300 s waiting time did with the Single Bond. It is worth mentioning that they air-dried the etched and rinsed dentin for 30 s then they re-wetted it again with water. Also, they used Z250 resin composite and photo-cured every increment for 30s using halogen light of 600 mW/cm2. Such differences in their methodology than the methodology of the present study, might explain why the 40s increase in waiting time did not show significant effect. The 300 s pre-cure waiting time sounded clinically as an unrealistic modification. Moreover, it should be declared here that there was but meager data in the literature to compare such kind of technique modification. Manufacturer instructions of the total-etch-twosteps bonding system used in the present study recommended the application of 2e3 consecutive layers of the bonding resin, before air-drying it. Moreover, Manufacturer instructions for the methacrylate-basedself-etch bonding systems used in the present study recommended rubbing the bonding resin for 20 s before air drying and curing. This multiple application and rubbing of bonding resins could be considered a form of increasing the time of application [23,26]. The

manufacturer knows that time is an important factor in achieving better bond strength, but it seems that competition in the market forced them to squeeze the application time to be able to advertise that their products have less clinical chair-side time. Analysis for SEM of SB2/FSXT groups was done to represent and to explain the results of the effect of the 30 s dwell waiting time on the quality of resin infiltration. It was clear that 30 s dwell waiting time produced thicker and denser hybrid layer than what had been produced by following the manufacturer instructions. Moreover, resin tags were also thicker and showed several lateral canals infiltration. It was interesting to visualize multiple sidewall hybridization of dentinal tubules (Figs. 5,6). The better resin infiltration and hybridization might explain why the mTBS of 30 s dwell waiting time subgroups in all the tested bonding systems were statistically higher than that of the subgroups which followed the manufacture recommended pre-curing waiting time. 4.2. Total-etch-two-steps bonding system The results of the present study demonstrated that the total-etch-two-steps bonding system group was ranked with the highest mTBS mean values after following the manufacturer recommended application time and also after 30 s dwell time. Moist bonding is recommended for total-etch-based-systems to avoid the collapse of the collagen network that was held by hydroxyapatite crystals as; water maintains the network in an extended state, which facilitates resin penetration. The hydrophilic monomers (primer/ bonding resin) in Adper Single Bond 2 used in the present study are dissolved in ethanol solvent with low concentrations of water in the same bottle. The vapor pressure of ethanol is 40 mmHg while; it is 17 mmHg for water. When primer/bonding resin is applied to the demineralized collagen network, it diffuses into the spaces occupied by water, and the water diffuses down its concentration gradient into the primer/bonding resin. The rate of diffusion depends on the concentration of the water in the bonding resin; the type of solvent and the amount of water within the demineralized dentin surface and it is time dependent [45]. The organic-water-miscible ethanol stiffens the collagen matrix as they chemically dehydrate it, thereby stabilizing the dimensions of the inter-fibrillar spaces and facilitating the resin infiltration [46]. The quality of resin infiltration into the exposed collagen affects the quality of the dentin-resin bond [13,23]. It was speculated that ethanol-saturated dentin allows bonding

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resins to flow into the catalytic sites of MMPs, then after polymerization, those enzymes could be inactivated [47].

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such possibilities which might explain that the mTBS of 30 s waiting time subgroup of Adper Easy One was statistically higher than that of following the recommended manufacturer application time.

4.3. Self-etch-one-step bonding system 4.4. Self-etch-two steps bonding system The results of the present study revealed that Adper Easy One/Filtek Supreme XT group was ranked the lowest mTBS values either following the manufacturer recommended time or after 30 s waiting time before curing. Also, mTBS mean values of the Adper Easy One/Filtek Supreme XT group were statistically lower than that of the Adper Single Bond 2/Filtek Supreme XT group. This finding was in agreement with another study, which investigated the tensile bond strength of Adper Single Bond 2, XP Bond total-etch, and two allin-one adhesives, Adper Easy One and Xeno V. They found that Adper Easy One showed significantly lower tensile bond strength than Adper Single Bond 2 on moist dentin [48]. One-step-self-etch bonding system is a complex mixture of acidic hydrophilic and hydrophobic monomers, which has low-viscosity and high hydrophilicity. Those systems have been shown to contain a higher concentration of acidic components (methacrylated phosphoric acid esters), water, and organic solvents than total-etch bonding systems to be able to simultaneously etch and infiltrate the dentin surface [10,11]. However, mixing all these heterogeneous components together in one bottle has initiated shortfalls to their shelf-life [10,11]. Moreover, the onestep-self-etch bonding systems are more hydrophilic in nature, so they act as permeable membranes, absorbing significant amounts of water when polymerized [43]. One-step-self-etch bonding system used in the present study is ultra-mild with pH > 2.5, which makes them hydrolytically unstable as a result of the methacrylatebased components [10,11]. Studies that investigated the hybrid layer of onestep-self-etch bonding systems showed nanometersized spaces within the hybrid layer as those systems suffer monomer-solvent phase separation, which forms water droplets in the adhesive after solvent evaporation and polymerization [10,11]. Those spaces allow fluid movements and result in plasticization of the resin matrix and removal of unconverted monomers, leading to a reduction in the bond strength [49]. It was hypothesized that mild-self-etch bonding systems have an advantage of keeping collagen encapsulated and protected by hydroxyapatite so; they might provide the potentials for chemical interaction with the hydroxyapatite [50]. It is assumed that giving more waiting time for one-step-self-etch adhesives might increase

The results of the present study unveiled that the mTBS mean values of the Adper SE Plus/Filtek Supreme XT group was statistically higher than that of the Adper Easy One/Filtek Supreme XT group. These findings are in agreement with another study [51], which compared the mTBS of the same self-etch-onestep and self-etch-two-steps bonding systems that are used in the present study, with mild two-step-self-etch adhesive Clearfil SE using Z100 resin composite. They found that self-etch-one-step (Adper Easy Bond) was statistically lower than that of self-etch-two-steps (Adper SE Plus) and Clearfil SE. But, self-etch-twosteps system (Adper SE Plus) was not statistically different from Clearfil SE which is considered the gold-standard among self-etch bonding systems in the in vitro studies [3] and in vivo studies [52]. The actual bonding performance of self-etch bonding system depends on their pH and the actual functional monomer in their resin formulation [11]. Ultra-mild self-etch bonding systems (pH > 2.5) can infiltrate dentin to few hundreds of nanometers (nanointeraction), while strong self-etch bonding systems (pH  1), as Adper SE Plus used in the present study, can infiltrate dentin for several micrometers deep [51]. Hence, the strong self-etch adhesives can form resin tags in dentin, while ultra-mild self-etch bonding systems, hardly might demineralize the smear plugs and subsequently resin-infiltrate it. Moreover, the second application of the bonding resin recommended by the manufacturer for the two-steps-self-etch bonding system (Adper SE Plus) used in the present study provides a separate hydrophobic resin layer as final step before curing, which can prevent the absorption of water after polymerization, which would enhance the bond strength and could resist the hydrolytic degradation [43]. 4.5. Silorane bonding system The results of the present study revealed that Silorane System Adhesive/Filtek Silorane subgroups recorded lower mTBS values than that of Adper Single Bond 2/Filtek Supreme XT and that of Adper SE Plus/ Filtek Supreme XT, but these differences were not significant. Moreover, the mTBS values of Silorane

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System Adhesive/Filtek Silorane subgroups were higher than that of Adper Easy One/Filtek Supreme XT, but those differences were not significant. Silorane bonding system is an ultra-mild self-etch-two-step, with pH about 2.7. Silorane-based resin exhibits the ability to reduce polymerization stresses without a proportional reduction in mechanical properties [53]. Studies showed that silorane-based resin composite has 0.94 vol.% polymerization shrinkage. The reduction of the polymerization shrinkage stresses could allow the survival of the bond by decreasing the stresses at the tooth restoration interface [14,18]. In the present study, the authors believe that the mTBS testing method is for screening and ranking of bonding systems rather than an absolute fact in predicting the clinical performance of bonding systems or resin composites. This believe is emphasized by a study done by Burgess et al., in 2013 [54] who compared the clinical performance of Filtek Supreme Plus resin composite restorations bonded with Adper Single Bond Plus, Adper SE and Adper Easy Bond and found no statistically significant difference in clinical performance between any of the three adhesives after a period of two years. In vitro mTBS test, as any other in vitro test, has its limitations and can be over estimating the bond strength [42,43], but still, it can rank bonding systems after a modification in the application technique to see whether such modification has an effect or not [55]. At that time, when it is worth it, another elaborate sophisticated testing method and a clinical study can confirm such effect. It is worth mentioning here that there is lack of recent literature investigating the effect of extension of application time of different bonding resins on their performance or their bond strength to dentin. It could be due to the fact that, manufacturers are rushing towards formulas that claim that the faster the better. The authors of the present study chose to start with mTBS test to see if such idea of extending the application time than that recommended by the manufacturer will make a difference or not. Since the authors found a statistical difference, as some other studies [22,13], the authors recommend further sophisticated investigations on such effect in the future especially after storage for different periods of time, which eventually could be supported by clinical studies. 5. Conclusion Within the limits of the present in vitro study, the following could be concluded:

i. Thirty-seconds pre-curing waiting time for bonding resins has a role in achieving higher mTBS to dentin. ii. One-step-self-etching bonding system ranked as the lowest bonding system.

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