Bond strength comparison of a fiber‐reinforced composite resin: Different dentin conditions and preparation techniques

This study aimed to compare the bond strength of a fiber‐reinforced composite resin with traditional and bulk‐fill composite resins under different dentin conditions and preparation techniques. Eighty molar teeth, excluding the mesio‐distal half of the occlusal dentin surfaces of each teeth, were isolated with acid‐resistant nail varnish and stored in a demineralisation solution (pH 4.5). After mechanical removal of the varnish, the teeth were buried in acrylic resin blocks. In every composite resin group, one‐half of the specimens were prepared with a diamond bur and another half with Er: YAG laser. Then, the specimens were divided into four groups of composite resins (Filtek Z250, G‐aenial Posterior, SonicFill 2, Ever X Posterior) (n = 10). Shear bond strengths were measured using a universal testing device, and failure types were determined with stereomicroscope images. SEM images were obtained at 1000× magnification. Data were analyzed using a three‐way analysis of variance (ANOVA), and Bonferroni correction was used for multiple comparisons (p = .05). Differences in the dentin surface affected the bond strength results (p < .05), whereas there was no significant difference between cavity preparation methods (p > .05). EverX Posterior showed the highest bond strength results. Within the limitations of this study, fiber‐reinforced composite resin exhibited successful bond strength results in addition to improved mechanical properties.

Dental caries is a microbiological, infectious, and multifactorial disease defined as a localized chemical dissolution of the tooth structure.
Dentin lesions occur as a result of the progression of enamel demineralisation.Translucent and subtransparent dentin caries layers are considered caries-affected dentin, which is demineralised but not contaminated by bacteria.Thus, in light of minimally invasive dentistry, it is not required to remove caries-affected dentin in the cavity preparation process (Ritter et al., 2019).
The endpoint of caries removal is still a subjective issue because of the difficulty of selective cavity preparation dentists often experience.Moreover, alternative methods for cavity preparation create residual dentin surfaces of various qualities, which may affect the adhesion process and long-term clinical success (Koyuturk et al., 2014).The FDA (Food and Drug Administration) approved Er: YAG (Erbium: Yttrium-aluminium-garnet) laser use for cavity preparation and dentin-enamel conditioning in 1997 (Dunn et al., 2005).Er: YAG lasers, which operate at a wavelength of 2940 nm, achieve selective ablation due to their affinity to water and hydroxyapatite, with absorption peaks of 3000 and 2800 nm, respectively (Fornaini, 2013).
The surface properties of the dentin surface after cavity preparation with an Er:YAG laser, including the absence of a smear layer or irregularities and its microretentive nature, are considered beneficial for adhesion (Guven & Aktoren, 2015).
Composite resin materials have a wide range of mechanical properties that may affect their bond strengths, and several studies have reported that an adhesive system's bonding performance is affected by composite resins (Goracci et al., 2011;Thomsen & Peutzfeldt, 2007).Therefore, advancements in recent filler technology have sought to improve mechanical properties without harming the bonding ability to tooth structures.
Glass fiber reinforcement of dental materials to improve their durability and mechanical properties has been studied for over two decades.In recent years, a new fiber-reinforced composite resin material, EverX Posterior (GC Corp., Japan) (Xenius Base was its predecessor), has been introduced to prevent common fracture failures in traditional composite restorations of posterior teeth (Fráter et al., 2014).EverX Posterior, a high-viscosity, fiber-reinforced bulk-fill composite, contains high AR (length-to-diameter aspect ratio) E-glass fibers (1-2 mm in length, 17 μm in diameter), and the monomers of its resin matrix form a semi-interpenetrating polymer network (semi-IPN) (Maas et al., 2017).This composition increases the material's fracture toughness (K IC ), leading to improved load-bearing capacity and higher flexural strength compared to conventional composite resins (Barreto et al., 2016).A capping layer consisting of a conventional composite resin should be employed in the proximal walls as well as in the occlusal layer when using EverX Posterior.This centripetal approach of layering is a good way of achieving a biomimetic restoration, with EverX Posterior (EverX) and conventional composite resin replacing the lost dentin and enamel, respectively.Another advantage of conventional composite resin capping is the aesthetic improvement of the restorations, as most of bulk-fill composite resins do not have different commercially available shades, including EverX Posterior.Previous studies have supported these advantages, claiming that fiber-reinforced composite build-up acts as a crack-prevention layer underneath the conventional composite resin (Garoushi et al., 2007).In the literature, some studies have evaluated the physical properties of this restorative material (Meenakumari et al., 2018) (Oja et al., 2021), but few have investigating its bond strength (Omran et al., 2019), and no study has examined laser preparation or different dentin conditions with regard to bond strength.
Another recent development in composite materials is bulk-fill composite resins, which allow the composite material to be placed in a single layer (min 4 mm) into the cavity (Leprince et al., 2014).Bulk-fill composite resins are a popular and rapidly improving category of restorative materials that have various advantages for clinicians.Two different types of bulk-fill composite resins, SonicFill 2 (SF 2) and EverX, were used in this study.SonicFill 2 (successor to SonicFill alongside SonicFill 3) (Kerr, USA), a high-low-viscosity bulk-fill composite resin that does not require a capping layer, has a unique application system, which includes sonic activation with a specialized handpiece that changes its viscosity, allowing the material to flow into the cavity.After this initial application, SonicFill 2 returns to a non-slumpy, easyto-manipulate state.Thus, it exhibits two different phases of viscosity, and there are clinical advantages of both of these states (Ozel et al., 2016).Manufacturers claim that this changeable high-lowviscosity quality provides a more comfortable replacement and better adaptation to the cavity.SonicFill 2 consists of an increased number of photoinitiators and specific optical properties, achieving a 5-mm lightcuring depth in a single increment (Ostapiuk et al., 2018).
The polymerization efficiency and physical properties of these materials, which overcome the disadvantages of the layering technique, are of particular interest in the dental materials field.
The aim of this study was to compare the bond strengths of a fiber-reinforced composite resin with those of conventional and bulkfill composites under different dentin properties and preparation method conditions.The null hypotheses are as follows.First, the bond strengths of the fiber-reinforced composite resin in sound dentin under different preparation methods will show similar results to conventional and bulk-fill composites.Second, the bond strengths of the fiber-reinforced composite resin in caries-affected dentin under different preparation methods will show similar results to conventional and bulk-fill composites.

| Study design
The sample preparation and study design steps are illustrated in T A B L E 1 The list of products used in the study.categorized as mixed when both adhesive and cohesive failures occurred.

| SEM analysis
Eighteen teeth were used for resin-dentin interface analysis of the specimens, and two slabs were obtained from each tooth.Resin-dentin slabs were etched using 10% phosphoric acid gel (5 s), rinsed with distilled water (15 s), air dried, and submerged first in an ethylenediamine tetraacetic acid (EDTA) solution (60 s) and then in a 5% sodium hypochlorite solution (10 min) to analyze the resin tags.Specimens were washed with distilled water for 5 min and sequentially dried using ascending grades of 60%, 70%, 80%, 90%, and 100% ethanol.Specimens were then transferred to a critical point dryer (Quorum SC7620, Quorum Technologies Ltd, East Sussex, UK) for desiccation, Au-Pd sputter coated, and then the dentin-adhesive interfaces were observed using an SEM (Jeol JSM-7001F, Tokyo, Japan) at 1000Â.
T A B L E 5 p-values of the comparisons between the composite groups according to different dentin and preparation conditions.F I G U R E 2 The distribution (%) of the different types of failure modes for the test groups.Four dentin slabs were obtained from two additional teeth for dentin surface imaging after bur and laser preparations, which were subjected to every specimen preparation step except for the adhesive system, composite resin applications, and thermal cycling.Dentin surface slabs also underwent the same preparation procedures for SEM imaging mentioned above.

| Statistical analysis
The normal distribution of the data was analyzed using the Shapiro-Wilk test.Data were analyzed using a three-way analysis of variance (ANOVA), and Bonferroni correction was used for multiple comparisons.

| RESULTS
According to the ANOVA analysis of the results, resin-based composite type and dentin conditions significantly affected the bond strength values (p = .000),while the preparation type did not (p = .305)(Table 3).
However, the bur-prepared groups' bond strength values were mostly higher than those of the laser-prepared groups (Table 4).
EverX demonstrated significant differences with all tested composite resins in only the caries-affected+bur condition (Table 4) (Z250: p = .000,GP: p = .014,SF 2: p = .032)(Table 5).Further, there were no statistical differences between any of the composite resins in the sound+laser condition (Table 4; p-values can be seen in Table 5).
Failure mode percentages were distributed as adhesive (52.5%), mixed (32.5%), and cohesive (15%) (Figure 2).EverX (caries-affected +laser) and Z250 (caries-affected+bur) had the highest percentages of adhesive failure (70%) (Figure 2).The purpose of this study was to analyze the bond strengths of a fiber-reinforced composite resin (EverX Posterior) with conventional and bulk-fill composites under different dentin properties and preparation method conditions.In sound dentin groups, EverX showed higher bond strength values, but no significant statistical difference was found between the other composite resins in laser-prepared groups ( p > .05)(Tables 4 and 5), whereas there was a statistically significant difference in the bur-prepared groups ( p < .05)(Tables 4 and   5).In light of these results, the first hypothesis that the bond strengths of the fiber-reinforced composite resin in sound dentin under different preparation methods would show similar results to conventional and bulk-fill composites was partially rejected.In laser-prepared caries-affected dentin groups, the EverX results were only significantly different from those of SF 2 ( p = .011)(Tables 4 and 5).In burprepared caries-affected dentin groups, the bond strength value of EverX was significantly higher than that of the other tested composite resins (p < .05)(Tables 4 and 5).In light of these findings, the second hypothesis that the bonding values of fiber-reinforced composite resin in caries-affected dentin would show similar results with conventional and bulk-fill composites under different preparation methods was also partially rejected.
The success of the bond strength and the durability of adhesion to the dental hard tissues are affected by the quality of the tooth structure, the effects of the cavity preparation on the prepared surfaces, the mechanism of the adhesive system, and the composition and other properties of the composite resin material.Hence, it has been reported that all these factors should be taken into consideration when evaluating the bonding performance of restorative materials (Cardoso et al., 2011).Self-etch adhesives reduce the number of clinical process steps, eliminating the etching and rinsing steps and decreasing the technical sensitivity of adhesive system application.
These adhesive systems continue to be developed to improve bonding to dentin, which is challenging due to dentin's complex structure (Pashley et al., 2011).SE Bond, which contains 10-MDP, widely considered to be a successful monomer in the adhesive dentistry, has demonstrated sufficient bonding values compared to other self-etch adhesives (Cardoso et al., 2011).In the study of Amsler et al., the shear bond strength of Z250 was investigated on sound and artificially eroded dentin surfaces using Clearfil SE Bond and Scotchbond Universal self-etch adhesive systems.The results in sound dentin were found to be better than the findings on eroded surfaces.Overall, the authors reported that Clearfil SE Bond performed better in eroded dentin, while Scotchbond Universal was more successful in sound dentin (Amsler et al., 2017).In all of the EverX groups, the highest bond strength value was observed in the bur-prepared sound dentin group (28.12 ± 2.66), whereas the laser-prepared caries-affected dentin group (19.96 ± 2.15) had the lowest result (Table 4).All of the sound dentin groups showed higher bond strength results than the caries-affected dentin groups, which is in accordance with the decrease in bond strength and durability of caries-affected dentin-adhesive interface generally reported in the literature, regardless of adhesive systems and bonding procedures (Pinna et al., 2015).This has been attributed to dentin tubules logged with acid-resistant mineral crystals, exposing collagen to form a thick layer with low surface hardness after adhesive application (Marquezan et al., 2009).In order to simulate clinical conditions in the study design as much as possible, both sound and cariesaffected dentin surfaces were included in the test groups of this study.Teeth with natural caries lesions can be used in bond strength studies on caries-affected dentin surfaces (Karaarslan et al., 2012), although artificial methods are also preferred by many researchers (Joves et al., 2013).The use of artificial methods has some advantages, such as providing standardized surfaces, avoiding the necessity of extracted teeth with a similar caries lesion depth, and obtaining both sound dentin and caries-affected dentin surfaces on the same tooth (Joves et al., 2013).Therefore, in this study, caries-affected dentin surfaces were achieved using a demineralisation solution followed by bur or Er: YAG laser preparations.
Dentin tubules are exposed without the formation of a smear layer after laser preparation on the dentin surface (Fornaini, 2013).Various handpieces are used during cavity preparation with a laser, none of which come into contact with the tooth surface, even in contact mode (very low distance).Therefore, the pressure formed during the preparation with the bur, and the surface cracks, formed by the pressure, are eliminated (Shirani et al., 2014), as confirmed by SEM images in this study (Figure 7).The effects of the laser on the dentin surface can also extend to the subsurface dentin layer, thereby forming a laser-modified layer of thermal denaturation.This layer is thought to be responsible for the reduction of the bond strength of adhesion systems to lasertreated dentin (Lopes et al., 2015).Conflicting results have been reported regarding the effect of laser preparation on bond strength.In the study of Shirani et al., shear bond strength results were found to be higher in the bur-prepared groups than in the laser-prepared groups.It was suggested that laser ablation might cause dentin collagen fibrils to collapse and reduce the interfibrillar area, complicating resin penetration into the interfibrillar areas (Shirani et al., 2014).It has also been reported that Er: YAG laser preparation increases the bond strength when combined with acid etching (Fornaini, 2013).In general, preparation with a bur or Er: YAG laser did not result in significant differences in bond strengths, except in the Z250 caries-affected dentin groups (1c and 1d) (Table 4), in which the laser-prepared group exhibited significantly higher bond strength (p = .003)(Table 4).In contrast to this study, Taneja et al. compared the microtensile bond strength of SF, the precursor of SF 2, with a traditional composite (Herculite Precis) at different thicknesses (4, 5, and 6 mm).The results for SF at 4 and 5 mm were higher than those for Herculite Precis (Taneja et al., 2016).Çolak et al. also compared the shear bond strength of SF with that of Tetric Evoceram, Herculite XRV Ultra, and Tetric Evoceram bulk-fill composite resins.Herculite XRV Ultra yielded the best results, and unlike the results in our study, no significant differences were found between bulk-fill composite resins (Colak et al., 2016).
Composite resin for posterior use, G-aenial Posterior, is a material with prepolymerised particles and inorganic fillers that contains fluoroaluminosilicate to increase its resistance to stress.A few studies have compared G-aenial Posterior and EverX Posterior in terms of their mechanical properties (Al et al., 2016;Oja et al., 2021).Baraba et al. compared the micro-tensile bond strength of EverX Posterior and G-aenial Posterior using self-etch adhesive (G-aenial Bond) and found that there was no significant difference between these groups (Baraba et al., 2021), which is similar to the results of the current study for the sound+bur condition with these composite resins (Table 5).
In the literature, in addition to being a reliable alternative for direct posterior restorations, it has also been argued that bulk-fill resin composite resins can achieve satisfactory bonding performance to dental hard tissues, which is one of the crucial aspects for the longevity of composite resin restorations (Akah et al., 2016;Colak et al., 2016;Mandava et al., 2017).Tsujimoto et al. examined the shear bond strength of EverX Posterior, comparing it with a microhybrid composite (Clearfil AP-X) and a nanohybrid composite (Filtek Supreme) using total-etch, two-step (Clearfil SE Bond), and one-step self-etch adhesive systems.They reported that EverX Posterior showed no significant differences from the composite resins, which is only similar to some of our findings (Tsujimoto et al., 2016).

Omran et al. investigated the bond strength performance of EverX
Posterior, two other bulk-fill composite resins (SDR and Tetric Evo-Ceram Bulk-Fill), and a conventional microhybrid composite resin (G-aenial Anterior) using a universal adhesive (Scotchbond Universal) in a one-step self-etch procedure.It was concluded that although there were no significant differences between the composite resins, EverX Posterior exhibited the highest bond strength values in 2 and 4 mm increment groups but not in the 6 mm increment group (Omran et al., 2017).These results partially align with those of the current study, in which the data suggest that with the same dentin

| CONCLUSION
According to the results of the research, and considering the conditions and limitations of an in vitro study, the following evaluations were made: • The bond strengths of sound dentin in all test groups were found to be higher than the caries-affected dentin values.
• Differences in preparation did not result in a significant difference in the bond strengths of the composite resins.Accordingly, it is possible to conclude that the use of an Er: YAG laser as an alternative to conventional cavity preparations generally does not lead to disadvantageous bond strength results.
• Among composite resins, in general, EverX yielded the best results, followed by GP, with no significant difference between Z250 and SF 2.

Figure 1 .
Figure 1.Eighty sound human third molars were used after they SEM images of the tested groups are shown in Figures 3, 4, 5, and 6, and SEM images of bur and laser preparation of sound dentin and caries-affected dentin surfaces are presented in Figure 7. F I G U R E 3 SEM images of Filtek Z250: Top left Group 1a, top right Group 1b; bottom left Group 1c, bottom right Group 1d.CR, composite resin; DT, dentinal tubules; RT, resin tags.

F
I G U R E 4 SEM images of G-aenial Posterior.Top left Group 2a, top right Group 2b; bottom left Group 2c, bottom right Group 2d.CR, composite resin; DT, dentinal tubules; RT, resin tags.

F
I G U R E 5 SEM images of SonicFill 2.Top left Group 3a, top right Group 3b; bottom left Group 3c, bottom right Group 3d.CR, composite resin; DT, dentinal tubules; RT, resin tags.
quality and preparation methods, EverX Posterior had the best test results.Moreover, based on the ANOVA analysis of the data, the composite resin type significantly affected bond strength values (p < .05)(Table 3).Bijelic-Donova et al. evaluated the shear bond strength values of EverX Posterior, a microhybrid (Z250), a F I G U R E 6 SEM images of EverX Posterior.Top left Group 4a, top right Group 4b; bottom left Group 4c, bottom right Group 4d.CR, composite resin; DT, dentinal tubules; RT, resin tags.nanofilled(SupremeXT), and a microhybrid silorane (Filtek Silorane) composite resin.Similar to the current study, they reported that a three-way ANOVA analysis showed that the composite material type had a significant effect on the shear bond strength results ( p < .001).They also observed that the bond strength values of EverX Posterior were comparable with those of the tested composite resins(Bijelic-Donova et al., 2015).SEM images of the dentin-adhesive interfaces (Figures3, 4, 5, and 6) were taken after the shear bond strength tests, illustrating the removal of hybrid layers and the resin tags from the dentin surface and tubules in some of the images.
List of the test groups.ANOVA table (three-way analysis of variance).
(GC Europe N.V.,Leuven, Belgium)Bis-GMA, PMMA, TEGDMA 53.6% vol, 74.2% wt E-glass short fibers, Ba-glass 16,01,291Primer: 10-MDP, HEMA, hydrophilic aliphatic dimethacrylate, camphorquinone, N, N-diethanol-p-toluidine, water Bond: 10-MDP, BisGMA, HEMA, N, N-diethanol-p-toluidine, silanated colloidal silica Primer was applied for 20 s on dried dentin.A mild air flow was applied for 20 s.Bond was applied for 10 s and gently air dried.Then light-cured for 10 s.lioglu Dental Inc., Ankara, Turkey) for 20 s.The tested composite resins and their properties can be seen in Table1.Overall, a total of 160 samples in 16 groups (Table2) were obtained and stored in distilled water for 24 h, at 37 C, in an incubator(INB 200, Memmert GmBH, Germany).Then the samples were artificially aged by thermal cycling (10,000 cycles, 5-55 C, 30 s of dwell time, 10 s of transfer time) (SD Mechatronik Thermocycler, SD Mechatronik GMBH, Westerham, Germany).2.3 | Bond strength test and failure mode analysisShear bond strength (SBS) was determined using a universal testing device (AGS-X Series, Schimadzu Europa GmbH, Germany), and the shear force was applied with a cross-head speed of 0.5 mm/min.SBS values were calculated using TrapeziumX 1.4.0 software (max debonding F(N)/adhesive area (mm 2 ), MPa).After the SBS tests, failure mode analyses were carried out using a stereomicroscope (Stemi 2000 C Carl Zeiss Microscopy, LLC, NY, USA).Failure modes were T A B L E 2 T A B L E 4 Mean bond strength values of the test groups.(x, y) within sound dentin/caries-affected dentin between bur and laser comparisons (p 2 values), (a, b) within bur/laser between sound dentin and caries-affected dentin comparisons (p 1 values); (X, Y) within related sound dentin groups between composite comparisons, (A, B) within related caries-affected dentin groups between composite comparisons.The same letters indicate no statistical difference (p > 0.05).