Does reducing the acid-etching time increase the micro-tensile bond strength of a bulk fill composite to dentin using different universal adhesives?

Abstract

Objectives

The objective of this in vitro study was to evaluate whether a reduced phosphoric acid etching time would have an impact on the microtensile bond strength (µTBS) of different universal adhesives to dentin using a bulk-fill composite.

Methods

Forty-eight freshly extracted non-carious human molars were immersed in acrylic resin and cut to a height of 4 mm from the highest tubercle crest towards the cervical region. Dentin surfaces were subjected to 37% phosphoric acid pretreatment for 0, 3, or 15 s (s), rinsed with water for 20 s and gently dried, and wet-bonded. Two universal adhesives (Single Bond Universal, 3 M-ESPE; Adhese Universal, Ivoclar Vivadent) were used in this study. The Tofflemire matrix was added to each tooth, and a bulk-fill resin composite (Tetric EvoCeram Bulk-Fill, Ivoclar Vivadent) with a thickness of 5 mm was applied. The tooth composite blocks were cured using a light-emitting diode unit for either 20–40 s. Twelve groups were included (four teeth per group). All samples were immersed in distilled water for 24 h at 37°C. The samples were sectioned under water cooling to obtain multiple sticks approximately 1.0 × 1.0 × 8 mm³ in size using a non-trimming technique with water cooling. The µTBS test was performed (n = 20 sticks for each group). The data were analysed via a Welch analysis of variance (ANOVA) and the Games–Howell post hoc test (SPSS 23.0). The interactions between the three factors were analysed with a three-way ANOVA.

Results

Significant differences were observed among the experimental groups (p < 0.05). Both adhesives exhibited the highest bond-strength values in the groups for which acid was applied to the dentin for 3 s and the composite resin was light-cured for 40 s, resulting in statistically significantly higher values for these two groups compared with the other groups (p < 0.05).

Conclusion

Reducing the etching time to 3 s compared to the traditional etching time of 15 s increased the micro-tensile bond strength of the bulk-fill composite to dentin using universal adhesives.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-025-05602-5.

Introduction

Since Nakabayashi et al. [1] initially outlined the mechanism of the bonding interface, which is known as the hybrid layer, adhesive bonding agents, as well as bonding strategies, have been continuously developed. Universal adhesives, which are commonly called multimode adhesives, are among the most significant recent advancements in adhesive dentistry. Owing to benefits such as user-friendliness, low technique sensitivity, quick application, and adaptability, the demand for universal adhesives has increased among dentists [2]. In addition to these advantages, universal adhesives contain certain monomers, such as 10-MDP, which can chemically bond to apatite crystals at the bonding interface [3]. Furthermore, the importance of the amount of hydroxyapatite remaining at the adhesive interface for chemical bonding has been emphasised [4].

Owing to the complex biological structure of dentin, bonding to dentin is more difficult than adhesion to enamel, necessitating a more precise etch-and-rinse procedure [5]. Etching with phosphoric acid removes the smear layer and hydroxyapatite, which are crucial for the chemical interaction of acidic monomers with universal adhesives on the dentin surface of the prepared tooth [6]. Research has indicated that strong bonds are negatively affected by thick smear layers [7]. The traditional method involves using 30–40% H₃PO₄ for 15 s to completely remove the smear layer [8]. An alternative method for increasing the bond strength of universal adhesives to dentin involves reducing the etching time [8].

Another important recent innovation in adhesive dentistry has been the development of bulk-fill composites. These composites can be polymerised to a maximum depth of 4 mm [9]. In vitro investigations indicated that the degree of curing and micromechanical characteristics were consistent throughout a 4-mm-thick layer for an irradiation period of up to 20 s, confirming the manufacturer’s claims regarding the incremental thickness [10]. Nevertheless, there have been relatively few studies on the bond strength of bulk-fill composites to dental tissues especially when using different etching times with the universal adhesives; thus, further research on this subject is required.

The fundamental issue facing modern dental adhesives is achieving strong adhesion to various dental substrates (i.e. enamel, sound, carious, sclerotic dentin) [4]. However, because adhesion to tooth substrates is challenging due to different factors, research on contemporary multimode adhesives is being conducted to strengthen bonding to enamel and dentin [11]. Recently, researchers examined whether reducing the acid treatment time can increase the bond strength of universal adhesives [8]. Nevertheless, this topic needs to be extensively investigated owing to the lack of evidence. Therefore, the objective of the present in vitro study was to evaluate whether a reduced etching time increases the microtensile bond strength (µTBS) of different universal adhesives to dentin using bulk-fill composites with different curing times. To achieve this, dentin µTBS, failure mode, and scanning electron microscopy (SEM) analyses were performed. Three hypotheses were examined: (i) the bond strength of the bulk fill composite is unaffected by reducing the acid-etching time to dentin; (ii) the bond strength of the bulk fill composite to dentin is unaffected by different types of universal adhesives; (iii) increasing the curing time of the bulk fill composite resin increased the bond strength of bulk fill resin composite to dentin.

Materials and methods

Forty-eight non-carious intact human molars extracted within the past 6 months were collected for this study. A hand scaler was used to remove the remaining soft tissue from the tooth surface. The teeth were stored for 14 d in a 10% formalin solution for disinfection and then kept at room temperature in distilled water until they were utilised in the study.

A bulk-fill composite resin (Tetric Evo Ceram Bulk Fil, Ivoclar Vivadent, Schaan, Liechtenstein) and two universal adhesives (SU, Single Bond Universal, 3 M-ESPE, Neuss, Germany; AU, Adhese Universal, Ivoclar Vivadent, Schaan, Liechtenstein) were selected for this study. Descriptions of the materials used are presented in Table 1.

Table 1.

Materials used in this study

Brand name	Manufacturer	Remarks		Batch number
Single Bond Universal	3 M ESPE, Neuss, Germany	A one bottle, light-curing, multimod universal dental adhesive	10-MDP, dimethacrylate resins, 2-HEMA, Methacrylate-modified polyalkenoic acid copolymer, filler, ethanol, water, initiators, silane	80,801 A
Adhese Universal	Ivoclar Vivadent, Schaan, Liechtenstein	A light-curing, single component, multimod universal dental adhesive	10-MDP, 2-HEMA, BisGMA, MCAP, D3MA, highly dispersed silica, ethanol, water, photoinitiators	Y38234
Tetric Evo Ceram Bulk Fil	Ivoclar Vivadent, Schaan, Liechtenstein	Light-curing, radiopaque nano-hybrid bulk fill composite resin	Bis-GMA, Bis-EMA, UDMA, Barium glass filler, Ytterbium trifluoride, Mixed oxide Additive, Ivocerin (initiator), Stabilisers, Pigments	W12913
10-MDP:10-methacryloyloxydecyl dihydrogenphosphate, 2-HEMA: 2-hydroxyethyl methacrylate, BisGMA: Bisphenol-A glycidyl dimethacrylate, MCAP: Methacrylated carboxylic acid polymer, D3MA: Decandiol dimethacrylate, UDMA: Urethane dimethacrylate, Bis-EMA: Ethoxylated bisphenol A dimethacrylate

Each tooth was embedded in epoxy resin, leaving the crowns outside the resin. The occlusal surfaces were cut at the coronal third of the tooth using a water-cooled cutting device (IsoMet Low-Speed Saw; Buehler) with a diamond saw to create a flat dentin surface. These surfaces were then polished using a 400-grit silicon carbide abrasive paper.

The teeth were randomly allocated into 6 groups according to the surface treatments using a table of random numbers generated by “Research Randomized Program” (http://www.randomizer.org). Then, one of the following surface treatments was applied to the dentin surfaces in each group.

• i.SU only (SU in self-etch mode).
• ii.AU only (AU in self-etch mode).
• iii.Etching with 37% orthophosphoric acid for 3 s + SU.
• iv.Etching with 37% orthophosphoric acid for 3 s + AU.
• v.Etching with 37% orthophosphoric acid for 15 s + SU.
• vi.Etching with 37% orthophosphoric acid for 15 s + AU.

These main groups were divided into two subgroups according to polymerization time (20 s and 40 s), creating 12 groups.

Orthophosphoric acid (37%, Total Etch, Ivoclar Vivadent, Schaan, Liechtenstein) was applied for 0 (self-etching mode), 3, or 15 s. The acid-etched groups were washed with water for 20 s and then gently air-dried for 5 s to obtain slightly wet surfaces. After surface pretreatment, universal adhesives were directly applied and rubbed for 20 s for each adhesive according to the manufacturer’s instructions and light cured for 10 s with a light-emitting diode curing unit (1000 mW/cm², Woodpecker LED, Guilin, China). Following the surface treatments, a Tofflemire retainer was positioned on each tooth to leave a 4-mm empty space on the adhesive surface. Bulk-fill composite resins were applied using the bulk technique to fill the tooth space. A Mylar strip was used to close the composite resin surfaces, and polymerisation was completed using the same LED unit (Woodpecker LED) with a full-power mode. All samples were placed in distilled water and stored at 37 °C for 24 h.

The epoxy-resin blocks were securely attached to a water-cooled cutting device (Isomet Low-Speed Saw, Buehler). The blocks were cut into sections via water cooling, and a non-trimming technique was employed to produce numerous slender microbeam-shaped sticks, each having dimensions of approximately 1.0 × 1.0 × 8 mm³. Five sticks were obtained from the centre of each tooth composite resin block. Therefore, 20 dentin-composite resin sticks were used in each group. Samples were kept on a damp paper napkin for testing to prevent deterioration.

The study included 12 groups, with each group undergoing a µTBS test. Each sample was attached to a microtensile testing device (Micro Tensile Tester, BISCO, Schaumburg, IL, U.S.A.) using a cyanoacrylate adhesive (Pattex 2 K Professional Rapid Adhesive, Türk Henkel Kimya San. ve Tic. A.Ş., Istanbul, Turkey) at both ends. The samples were placed under tensile force at a crosshead speed of 1 mm/min until they fractured. The pressure at which breakage occurred was measured in newtons (N). Upon fracturing, the sample was carefully removed from the test apparatus using a spatula. Subsequently, the dimensions (a and b) of each fractured sample were measured to calculate the area of ​​the square on the stick’s attachment surface using a digital calliper, focusing on the area closest to the adhesion surface. These measurements were used to calculate the µTBS at the point of fracture, measured in megapascals (MPa). The formula MPa = N / (a × b) was used to convert N to MPa. One stick, which was not subjected to microtensile test, was separated and used in each group for SEM analysis to evaluate the interface between the resin composite and the dentin. The specimens were embedded in epoxy resin and polished with 600- and 1000-grit silicon carbide abrasive papers. They were then rinsed, and the polishing debris was ultrasonically removed for 5 min. After polishing, the samples were etched with 37% orthophosphoric acid for 15 s, washed, and treated with 0.1% NaOCl for 10 min. After all the samples were dried for 24 h at 37 °C and coated with gold, SEM (QUANTA 400 F Field Emission SEM, Thermo Fisher Scientific, Hillsboro, OR, USA) was performed. The mode of failure was determined using a stereomicroscope (Olympus SZ61, Tokyo, Japan).

We checked the normality of the data using the Shapiro–Wilk tests and assessed the homogeneity of variances using the Levene test. We conducted a three-way analysis of variance (ANOVA) to examine how the surface treatment, universal adhesive, and curing time individually and collectively affected the µTBS. The interactions between the three factors (acid etching time, adhesive type and composite curing time) were statistically significant therefore the combinations of these factors were considered. The data were analysed via a Welch ANOVA since the assumption of homogeneity of variances is failed. Moreover, for pairwise comparisons the Games–Howell test was conducted. All analysis were performed using SPSS Version 23 (SPSS Inc., Chicago, IL, USA).

Results

Table 2 presents the mean µTBS values along with the statistical differences. Significant differences were observed among the study groups (p = 0.001 < 0.05). Both adhesives exhibited the highest bond-strength values in the groups for which acid was applied to the dentin for 3 s and then light-curing was performed on the composite resin for 40 s; the values of these two groups were statistically higher than those of the other groups (p = 0.001 < 0.05), however statistically no difference was observed between the two groups showing these highest values (P = 1 > 0.05). The lowest µTBS was observed for the group where the composite was bonded with Adhese Universal Bond without pretreatment and cured for 20 s (AU-20-SE = 10.9 ± 3.1). There was no statistically significant difference between this group and one of the Single Bond Universal groups, where acid etching was applied for 15 s to dentin and the composite was polymerised for 20 s (SB-20-15 = 12.3 ± 1.1, p > 0.05). Failure modes were also given in Table 2.

Table 2.

Mean MPa values of the study groups

					MPa		Fracture types (Adhesive/Cohesive/Mixed)
Group	Adhesive	Composite Curing Time	Surface Treatment	n	Mean	SD
1 (SB-20-SE)	Single Bond Universal	20 s	-	20	19.7 d, e	2.4	18/0/2
2 (SB-20-3)			3 s AE	20	22.2 e	3.8	16/1/3
3 (SB-20-15)			15 s AE	20	12.3 a, b	1.1	17/0/3
4 (SB-40-SE)		40 s	-	20	19.2 c, d,e	5.3	16/0/4
5 (SB-40-3)			3 s AE	20	29 f	3	15/1/4
6 (SB-40-15)			15 s AE	20	16.8 c, d	2.7	18/1/1
7 (AU-20-SE)	Adhese Universal	20 s	-	20	10.9 a	3.1	15/0/5
8 (AU-20-3)			3 s AE	20	22 e	3.6	17/1/2
9 (AU-20-15)			15 s AE	20	15.7 b, c	2.1	18/0/2
10 (AU-40-SE)		40 s	-	20	20.2 d, e	3.5	17/0/3
11 (AU-40-3)			3 s AE	20	29.3 f	4.2	16/2/2
12 (AU-40-15)			15 s AE	20	19.4 d, e	4.8	17/1/2
Same superscript letters show statistically homogeneous subgroups (p > 0.05) SB: Single Bond Universal, SE: self-etch, AU: Adhese Universal, AE: asit-etching

Table 3 presents interactions between adhesive, composite resin curing time, and acid etching time—both individually and collectively providing the partial eta-squared (ηp2) values, according to the three-way ANOVA. The effect of the acid etching time on the µTBS values of the groups (ηp2 = 0.608, p < 0.05) was the strongest, followed by composite curing time (ηp2 = 0.368, p < 0.05). The adhesive type did not affect the µTBS (p > 0.05).

Table 3.

Interations between adhesive, composite resin, curing time, and acid etching time according to the three-way analysis of variance

Parameter	Eta-squared value	Significance (p values)
Adhesive a	0.002	0.49
Composite curing time	0.368	0.001
Acid etching time	0.608	0.001
Adhesive * Curing time	0.052	0.001
Adhesive * Etching time	0.149	0.001
Curing time * Etching time	0.036	0.015
Adhesive * Curing time * Etching time	0.108	0.001
The higher the eta-squared value, the stronger the interaction of the parameters on the bond strenght a Statistically no significant effect (p > 0.05)

Figure 1 shows SEM images of one sample from each group. In the groups without surface treatment (Fig. 1a and b), both adhesives did not penetrate the dentinal tubules and made a flat connection over the smear layer. In the groups for which acid etching was applied to the dentin surface for 3 s (Fig. 1c and d), partial penetration of the dentinal tubules by both adhesives was observed. However, the resin tags in the acid group applied for 3 s penetrated the dentinal tubules almost as well as those in the group acid-etched for 15 s. Deep penetration of both adhesives into the dentinal tubules was detected in the groups for which acid was applied to the dentin surface for 15 s (Fig. 1e and f).

Fig. 1.

SEM images of the samples. (a) SU without acid-etching; (b) AU without acid-etching: (c) SU with 3 s acid-etching, (d) AU with 3 s acid-etching; (e) SU with 15 s acid-etching; (f) AU with 15 s acid-etching SU; Single Bond Universal, AU; Adhese Universal

Discussion

In dental research, comparing the bond strengths of different adhesive systems is a critical area of study [12, 13]. Extensive research has been conducted on the bond strengths of different adhesive systems to sound dentin [14, 15]. These studies highlighted the importance of further investigating the performance of universal adhesives on sound dentin.

According to the results of this study, acid treatment of 3 s increased the bond strength compared to self-etch groups. However, an increase in the acid treatment time from 3 s to 15 s on sound dentin reduced the bond strength. Therefore, the first null hypothesis was rejected, which suggests that bond strength of the composite is unaffected by reducing the acid etching time on dentin. This result may be associated with the changes that occur on the dentin surface depending on the acid treatment time.

Since the adhesive interaction was not found significant, the second null hypothesis was accepted. The chemical composition of dental adhesives significantly affects their bonding capabilities. Akarsu and Karademir [16] reported that the polyalkenoic acid copolymers in the Single Bond Universal adhesive system differ from those in other adhesive systems because of the presence of carboxyl groups that can form ionic bonds with the calcium in hydroxyapatite. Because both adhesives used in this study contain a polyalkenoic acid copolymer, it can be inferred that Adhese Universal Bond and Single Bond Universal adhesives may share common chemical components, potentially leading to comparable bonding performance. Further research on the specific chemical compositions of these adhesives can provide deeper insights into their similarities or differences in bonding mechanisms.

In this study, increasing the curing time of the bulk-fill composite increased its bond strength. Therefore, the third null hypothesis is accepted. In agreement with our results, Ide et al. [17] reported that increasing the curing time of bulk-fill composites increased their bond strength. Furthermore, a longer curing time of bulk-fill composites leads to an increased depth of cure of the materials [18]. These finding supports the notion that longer curing times can promote deeper polymerisation within the bulk-fill composite, which may increase the bond strength to the dentin substrate.

Orthophosphoric acid etching is a critical step in tooth-surface modification. The duration of acid etching significantly affected the changes observed in the dentin substrates [19]. Collagen fibres were revealed and the micromechanical interlocking surface for adhesive bonding was preserved because the orthophosphoric acid demineralised the dentin surface and removed the smear layer [20]. The depth of demineralisation and collagen exposure can be influenced by the duration of acid etching, with longer etching times potentially leading to deeper demineralisation and increased collagen exposure [19]. According to Kharouf et al. [21], etching using 37% phosphoric acid gel for 15 s can remove a thick smear layer on the dentin surface. This duration permits the removal of the smear layer without compromising the integrity of the dentin surface. However, the dentin structure may deteriorate and demineralise when acid exposure exceeds 15 s [19]. Thus, in the present study, the maximum dentin etching time was 15 s.

The bonding mechanism of universal adhesives to dentin in the self-etch mode involves a complex interplay of chemical interactions that facilitate adhesion. When used in the self-etch mode, universal adhesives contain acidic monomers that dissolve the smear layer and penetrate the dentin substrate. Therefore, the hybrid layer served as an interface between the adhesive and dentin, facilitating micromechanical retention through resin tags and relying on chemical bonding via the interaction of functional monomers with hydroxyapatite and collagen [22]. Nevertheless, studies have been conducted to increase the bond strength of universal adhesives when they are used in the self-etch mode [23, 24]. These findings with SEM images support the notion that resin tag formation at the dentin–adhesive interface was sufficient with a reduced acid treatment time of 3 s. Nevertheless, there is limited research on this subject [8, 25]. Therefore, further research, including clinical studies, is needed to confirm our results.

The limitations of this study include the application of only two adhesives and one type of composite resin, as well as one reduced etching time of 3 s. In addition, a limited number of extracted teeth were used. These deficiencies should be addressed in future studies, which are needed to determine the extent to which the etching time can be reduced by eliminating the abovementioned limitations. These results should also be confirmed and extended by further clinical studies.

Conclusions

Within the limitations of this study, the following conclusions can be drawn:

1. Etching the dentin for 15 s provided higher bond strength than using the universal adhesive in self-etch mode without etching.

2. Reducing the etching time to 3 s increased the universal adhesive bond strength to dentin when compared to traditional etching time of 15 s.

3. The formation of resin tags at the dentin–adhesive interface with a reduced etching time of 3 s is comparable to that for the traditional dentin etching time of 15 s.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Author contributions

E.B: Idea, Supervision, Writting-Original draf preparation, Writing-Reviewing and Editing.O.T.M: Investigation, Laboratory Steps, Methodology.D.H.Y: Writting-Original draft preparation, Formal analysis, Validation, Visualization, Writing-Reviewing and Editing.

Funding

Not applicable.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Human Ethics and Consent to participate

This in vitro study was approved by the Ethics Committee of Hacettepe University (approval number: GO 22/652). The informed consent was obtained from all of the participants. This study was performed in accordance with the current version of the Declaration of Helsinki.