Tanta Dental Journal

ORIGINAL ARTICLE
Year
: 2018  |  Volume : 15  |  Issue : 2  |  Page : 91--98

Microshear bond strength of universal adhesives to dentin used in total-etch and self-etch modes


Ayad A Ahmed1, Mustafa M Hassan2, Ali I Abdalla2,  
1 Department of Operative Dentistry, Baghdad University, Baghdad, Iraq
2 Department of Operative Dentistry, Faculty of Dentistry, Tanta University, Tanta, Egypt

Correspondence Address:
Ayad A Ahmed
Baghdad University, Baghdad
Iraq

Abstract

Purpose To determine the dentin bonding ability of three new universal adhesive systems under different etching modes using microshear bond strength (μSBS). Materials and methods Futurabond U, Single Bond Universal, and Tetric N-Bond Universal were used in this study. Sixty noncarious human molars were assigned to three groups based on the type of the universal adhesives. Two etching modes (total-etch and self-etch) were employed for each adhesive group. The adhesives were applied on dentin surfaces according to the manufacturer's instructions then composite resin (Z350 XT, nanocomposite) was condensed through a polyethylene tube with a 1 mm internal diameter and 2 mm height attached firmly to dentin surfaces and light cured. All samples were thermocycled for 500 cycles from 5 to 55°C. The μSBS was measured by using universal testing machine at cross-head speed of 0.5 mm/min. The bonded surfaces were examined under a stereomicroscope at magnification ×50 to determine the mode of failure. Dentin slices were prepared for each group to evaluate the resin–dentin interface a resin composite with each adhesive were placed 2 mm thick to form composite core. The segments were sectioned longitudinally and prepared to be examined under scanning electron microscope. Results Among the universal adhesives, Futurabond U and Tetric N-Bond Universal in total-etch mode showed significantly higher μSBS values than in self-etch mode. Single Bond Universal did not show any significant difference in μSBS between the total-etch mode and self-etch mode. Conclusion Performance of universal adhesives was shown to be material-dependent. The results indicate that universal adhesives used on dentine performed better in total-etch mode than self-etch mode.



How to cite this article:
Ahmed AA, Hassan MM, Abdalla AI. Microshear bond strength of universal adhesives to dentin used in total-etch and self-etch modes.Tanta Dent J 2018;15:91-98


How to cite this URL:
Ahmed AA, Hassan MM, Abdalla AI. Microshear bond strength of universal adhesives to dentin used in total-etch and self-etch modes. Tanta Dent J [serial online] 2018 [cited 2018 Nov 18 ];15:91-98
Available from: http://www.tmj.eg.net/text.asp?2018/15/2/91/235136


Full Text

 Introduction



Resin composites have become one of the most popular materials used in teeth restorations because of their superior esthetics, durability, and minimal intervention required due to their adhesion to tooth structure [1]. However, resin composites relied heavily upon the development of adhesive technology to achieve a micromechanical bond to tooth structure, which have been rapidly advanced over the past years [2]. Currently, dental adhesives are generally classified into either 'etch-and-rinse' or 'self-etch' systems. Furthermore, the priming and bonding components can be separated or combined, resulting in three steps or two steps for etch-and-rinse systems, and two steps or one step for self-etch adhesives [3].

The etch-and-rinse strategy involves the prior application of phosphoric acid, which, at enamel, produces deep etch-pits in the hydroxyapatite-rich substrate and at dentin demineralizes up to a depth of a few micrometers to expose a hydroxyapatite-deprived collagen mesh [4],[5]. Self-etch materials do not require a separate acid step as demineralization and priming occur simultaneously [6]. The preliminary use of phosphoric acid increases the probability of clinical errors due to the need of rinsing and adequate management of dentin moisture [7]. Contrary to the etch-and-rinse approach, self-etch adhesives do not remove but incorporate the smear layer in the hybridized complex [8]. In recent years, self-etch adhesive system has become very popular due to the efficiency offered by the simplified bonding procedures. In addition, it is thought that the incidence of postoperative sensitivity appears to be lower relative to etch-and-rinse systems thanks to chemical bonding and reduced demineralization of dentin [9]. However, some laboratory studies have indicated that self-etch adhesive systems are not able to etch enamel as effectively as the phosphoric acids used in etch-and-rinse adhesive systems due to their lower acidity [10].

To achieve a durable bond to enamel, when using self-etch adhesive systems, selective etching with phosphoric acid prior to application of the self-etch adhesive has been recommended [11]. However, clinically, it may be difficult to precisely etch only the enamel region without affecting exposed dentin [12]. Therefore, inadvertent pre-etching of dentin could be a clinical risk, as resin monomers of self-etch adhesives may not be able to penetrate the entire depth of the deeply demineralized dentin, resulting in reduced dentin bonding quality [13]. Recently, a new type of single-step self-etch adhesive has been introduced. This type of self-etch adhesive is categorized as 'universal' or 'multi-mode' as they can be used either with the etch-and-rinse mode or the self-etch mode or as 'selective' etching mode (self-etch on dentin and etch-and-rinse on enamel) [14], which gives the dentist a more versatile adhesive system [15]. Universal adhesives are the latest innovation marketed by dental manufacturers for bonding of dental materials to tooth substrates. These multimodal adhesives can be used according to the clinician's preference [16].

Universal adhesive contains specific carboxylate and/or phosphate monomers that bond ionically to calcium in hydroxyapatite [17]. They differ from the current self-etch systems by the incorporation of monomers that can produce chemical adhesion to the dental substrates [18]. Among these monomers, methacryloyloxydecyl-dihydrogen phosphate (10-MDP) [19]. This functional monomer is considered one of the most effective monomers regarding chemical interaction and durability [20]. Its R-PO 3-4-bonds ionically to dentin, forming hydrolytically stable calcium salts on hydroxyapatite in the form of nanolayering 10-MDP-calcium salts [21]. It is believed that this incorporation may increase the durability of the bonds produced with simplified self-etch adhesives, which was shown to be limited for the current self-etch under in-vitro and in-vivo studies [22]. Clinically, the stability of this type of 10-MDP-mediated chemical bonding has resulted in an excellent 13-year retention rate for the two-step 10-MDP based Clearfil SE Bond [23].

There is still debate regarding the optimum application method of universal adhesives, whether it is better applied after acid etching or used as self-etch adhesives, especially on dentin. From a mechanical standpoint, some studies show additional phosphoric acid etching was beneficial for the dentin bond strength when using universal adhesives [24]. While in other studies, 24-h dentin bond strengths are similar for total-etch and self-etch approaches [18]. In a recent randomized controlled clinical trial, the 36-month clinical behavior of some universal adhesives did not depend on the bonding mode [25].

However, etching removes calcium from dentin, leaving a superficial network of collagen fibers surrounded by water. The removal of calcium from the interface might preclude any potential ionic bonding between calcium and the phosphate and/or carboxylate groups in the adhesive. In fact, the dentin sealing ability of universal adhesives worsens when they are used in total-etch mode on dentin. In contrast, when some universal adhesives were applied in self-etch mode on dentin, they resulted in the lowest immediate nanoleakage [26], as well after 1 year of water storage [27].

Because universal adhesives are marketed in fairly short time [28]. Relatively little information is available on their performance apart from those provided by the manufacturers, especially for the more recently introduced versions. As far as bonding to dentine is concerned, it is not known if equivalent bonding performance may be expected when these adhesives are used in either application mode, or whether this latest generation of adhesives has overcome some of the critical barriers associated with contemporary dentine bonding.

The purpose of this laboratory investigation was to determine the dentin bond quality of universal adhesives in different application modes using microshear bond strength (μSBS).

 Materials and Methods



Materials

Materials that have been used in this study illustrated in [Table 1].{Table 1}

Methods

Specimen preparation

Sixty freshly extracted caries free unrestored human third molars were selected for use in this study. Teeth were stored in distilled water containing 0.2% thymol antiseptic solution for 48 h at 37°C immediately after extraction. A written consent was taken from the patients after the study was approved by the Ethics Committee of Tanta University to ensure their agreement to use their teeth in the current study. The teeth were cleaned of debris using a rubber cup, pumice and a low-speed handpiece.

The teeth were mounted vertically in cold curing acrylic resin 2 mm below cement–enamel junction, using plastic circular molds. Superficial coronal dentin was exposed by horizontal trimming the occlusal surface of each tooth crown with a low-speed diamond disk (Edetal Golden S.A.W., Switzerland) under running water. After trimming, the resulting surfaces were flattened and finished using 600-grit silicon carbide papers (waterproof silicon carbide paper; Atlas, UK) to create a standardized smear layer. The prepared specimens were assigned to three groups of 20 each according to the tested adhesive system used. Each group was subdivided into two subgroups (10 specimens for each subgroup) according to application mode:

Group 1: Futurabond U (FBU) adhesive.

Group 2: Using Single Bond Universal (SBU) adhesive.

Group 3: Using Tetric N-Bond Universal (TNBU) adhesive.

Subgroup A: Total-etch mode.

Subgroup B: Self-etch mode.

Each adhesive system was applied according to the manufacturer's instructions over exposed dentin surface in each group as shown in [Table 1].

Polyethylene tubes with 1 mm internal diameter and 2 mm of height were firmly attached to the conditioned dentin surfaces and filled with resin composite and then cured for 40 s using a Bluephase C5 LED visible light-curing unit (Bluephase N; Ivoclar Vivadent) at a light intensity of 500 mW/cm 2 at zero distance. The specimens were stored in distilled water at room temperature for 24 h before being subjected to 500 thermocycling (5–55°C) with 20 s dwell time and 10 s transfer time (Mechtronik, Germany).

Microshear bond strength tests

All specimens were subjected to μSBS using universal testing machine (Instron Universal Testing Machine, Instron; UK) with a load cell of 5 kN at cross-head speed of 0.5 mm/min, until failure occurred and data were recorded using computer software. A 0.2 mm diameter stainless steel orthodontic wire was looped flush between the load cell projection and the resin cylinder contacting the lower half-circle of the cylinder and touching the tooth surface. Care was taken to keep the composite cylinder in line with the center of the load cell and to keep the wire loop parallel to the load cell movement direction and to the bonded surface to maintain a shear stress orientation at the bonding interface. The μSBS values (MPa) were calculated from the peak load at failure divided by the bonded surface area. After testing, the bonding site tooth surfaces and resin composite cylinders were observed under a stereomicroscope (Nikon MA 100; Nikon, Japan) at a magnification of ×50 to determine the bond failure mode. Based on the percentage of substrate area (adhesive–resin composite–dentin) observed on the debonded cylinders and tooth bonding sites, the types of bond failure were recorded as (i) adhesive failure, (ii) cohesive failure, (iii) mixed failure – partially adhesive and partially cohesive.

Scanning electron microscopy observations

Dentin slices were prepared for each adhesive (in both etching mode). After adhesive application according to manufacturer's instruction, a resin composite was placed 2 mm thick to form a composite core. The specimens were sectioned longitudinally into two halves. The fracture surface of each half was polished with an increased grit of silicon paper, under running water. The interface between composite and dentin was etched with 37% phosphoric acid for 30 s, rinsed with water spray, depolarized with 2% sodium hypochlorite for 60 s, and then rinsed with water to detect resin tags penetration within dentinal tubules and hybrid layer thickness. All specimens were gold sputtered and morphological examined under scanning electron microscope (JSm-5300 Scanning Microscopel; Jeol, Peabody, Massachusetts, USA) at constant magnification (×1000) to evaluate the resin–dentin interface.

Statistical analysis

A two-way analysis of variance (ANOVA) followed by Duncan's post-hoc test (P ≤ 0.05) was used for analysis of the μSBS data. Statistical analysis was performed using statistical package for the social sciences (SPSS) software version 20.0 for Windows (SPSS Inc., Chicago, Illinois, USA).

 Results



Microshear bond strength

The results of the μSBS for resin composite bonded to dentin by the universal adhesives in different etching modes are shown in [Table 2] and [Figure 1]. The two-way ANOVA revealed that the factor of adhesive system significantly influenced the μSBS values (P ≤ 0.001). Also, the factor of etching-mode (total-etch vs. self-etch) influenced the μSBS values (P ≤ 0.001). And the interaction between the factors was significant (P ≤ 0.001).{Table 2}{Figure 1}

The mean μSBS to dentin in total-etch mode [Table 2] ranged from 11.33 ± 1.3 to 20.71 ± 2.11 MPa, while the corresponding values for the specimens in self-etch mode ranged from 7.45 ± 2.66 to 14.58 ± 3.47 MPa. In total-etch mode TNBU showed a significantly higher μSBS value (P < 0.05) than all of the other. While in self-etch mode SBU showed the highest significantly μSBS value (P < 0.05). Among the universal adhesives, FBU and TNBU in total-etch mode showed significantly higher μSBS value (P < 0.05) than in self-etch mode. SBU did not show any significant difference in μSBS value (P > 0.05) between both etching mode.

Failure mode analysis of debonded specimens

The majority of the specimens (81.6%) showed adhesive/mixed failures. Cohesive failures were observed in 18.3% of the specimens in the present study as shown in [Table 3] and [Figure 2].{Table 3}{Figure 2}

Scanning electron microscopy observations

Scanning electron microscopy observations of the restorative–dentin interface are shown in [Figure 3], [Figure 4], [Figure 5]. The restorative–dentin interface for all the adhesives showed excellent adaptation to the dentin surface except for FBU regardless of the etching mode. In addition, for all the adhesives, longer resin tags were found in total-etch mode relative to that of self-etch mode.{Figure 3}{Figure 4}{Figure 5}

 Discussion



A new type of single-step self-etch adhesive that is categorized as 'universal' or 'multi-mode' has been recently introduced for patient care. Universal adhesives are 'universal' in two main ways. First, they can be used on a wide range of substrates [29]. They may be helpful for repair of resin composite restorations that involve different adherent substrates in the same region [30]. Second, they are recommended by dental manufacturers for use both with and without acid pretreatment of tooth surfaces. In order to overcome the lower bonding performance to enamel reported for self-etch adhesive systems [10] universal adhesives can be used with either etch-and-rinse or self-etch approaches [14]. However, this type of adhesive was only recently introduced to the market, and there is little information as to whether the different etching modes achieve equivalent bonding performance to dentin when it is subjected to repeated subcritical loading. Thus, the focus of this laboratory research investigation was to examine and compare bond effectivity of a resin composite using newer universal adhesives with different etching modes on a single substrate, dentin. Based on the results of this study, changes in dentin bond strength has recorded between the two-etching mode. In the current study, it was found that FBU and TNBU markedly improved their μSBS in the total-etching mode, while SBU showed almost no difference.

It is known that the smear layer constitutes a true physical barrier and makes it extremely difficult for the bonding and hybrid layer formation to be fully integrated with the dentine [31]. In this study, a thin artificial smear layer was created by means of grinding with 600-grit silicon carbide paper, which can be matched to clinical smear layer thicknesses produced by fine-grained diamond burs [32]. After preliminary etching with phosphoric acid, the smear layer and smear plugs is completely removed and superficial dentine is demineralized up to depths of few micrometers [33], depending on the concentration, pH, application duration, and viscosity [34]. After acid etching, a mineral-depleted collagen network is exposed, allowing resin infiltration through the nanometer spaces. The etching step ensures a deeper penetration of the adhesives into the dentine substrate, generating longer resin tags and better morphology [35], as well as thicker hybrid layers [13]. In this study when acid etching was applied prior to the adhesive (total-etch mode), all universal adhesives showed deeper penetration into dentine with formation of long resin tags when compared to their respective self-etch counterparts, this permitted a strong mechanical interlocking between the universal adhesives and dentin, which were associated to significantly improved μSBS values when compared to conventional application of the same adhesive in the self-etch mode [18].

Even though the application of the etching step prior to self-etch adhesives has shown to improve the hybrid layer thickness and resin tag formation, these interfaces showed significantly decreased bond strengths, with an increased number of adhesive failures [11]. The lower bond strength has been attributed to an incomplete infiltration of the demineralized collagen network by the bonding resin [36]. This shortcoming has been overcome in universal adhesives through the addition of low viscosity monomers like HEMA, that increase the affinity to the hydrophilic wet collagen network, as has been done earlier for one-step etch-and-rinse adhesives [14].

In case of FBU (pH: 2.3), bond strength of (11.33 ± 1.3 MPa) was reported in the total-etch mode. There was a significant reduction in bond strength in the self-etch mode (7.45 ± 2.66 MPa). FBU also had the inferior μSBS values as compared to the other tested universal adhesives in both etching modes, this may be due to high solvent content (25–50 wt%) [37], as compared to (10–15 and 15–25 wt%) in Single Bond U and TNBU respectively [38],[39]. Which leads to more residual solvent retained in the hybrid and adhesive layers [40], preventing the formation of a polymer with high reticulation [41]. consequently, a reduced degree of conversion [42] and making the adhesive interface more permeable after polymerization [43] and more prone to degradation over time [44], and low μSBS values [45] is produced, also in FBU the manufacturer describes its functional monomer only as a phosphate monomethacrylate. The absence chemical bonding agent in its formulation may be a reasonable speculation, since the refer to the presence of chemical bond agent not disclosed in the product information sheet.

This finding about FBU is in agreement with a recent study published by Chen and colleagues and Wagner et al. [14], who found that FBU showed the lowest significant microtensile bond strength as compared to the other tested universal adhesives, also FBU performed better in total-etch mode than self-etch mode. Wagner et al. [14] also found that there were significant increments in pretest failures for the FBU adhesive in self-etch mode after thermocycling, indicating a deleterious effect of the aging procedure on the bond interface of this adhesive. This could be related to an insufficient chemical interaction to dentine [14].

On contrary with Zhang et al. [46], who reported that FBU performed better in the self-etch mode than in total-etch mode after 24 h and 12 months aging. However, the difference in the results from other studies may resulted mainly from differences in their methodology [47]. Material type, bonding area, testing mode (shear, microshear, tensile or microtensile) and cross-head speed as factors that significantly influence bond strength, along with several others related to the substrate, specimen storage conditions, aging techniques and type of curing system [48].

SBU (pH: 2.7) is the only adhesive exhibited statistically similar μSBS value in both etching modes (14.83 ± 3.23 MPa) in total-etch mode and (14.58 ± 3.47 MPa) in self-etch mode. Also, it had the higher μSBS value among the tested adhesives in self-etch mode. This favorable performance of SBU in self-etch mode may have been resulted from its two-fold bonding mechanism. First, micromechanical interlocking created by process of a dentin–resin interaction zone with stable resin tags and mechanical retention between the dentin and resin composite.

Second, intense chemical interaction of the functional monomer, 10-MDP, present in SBU with residual hydroxyapatite (remaining around the exposed collagen fibrils within the hybrid layer) [11], SBU considered mild self-etch adhesive [49]. Mild self-etch adhesives partially demineralize dentine, leaving a substantial amount of hydroxyapatite crystals around the collagen fibrils [50]. Yoshida et al. [21] showed that an effective chemical interaction occurs between MDP and hydroxyapatite forming a stable nanolayer that could form a stronger phase at the adhesive interface, which increases the mechanical strength of the adhesive interface. In addition, stable MDP-Ca salt deposition along with nanolayering may explain the high bond stability [21], which has previously been proven both in laboratory and clinical research [51]. According to the adhesion-decalcification concept, the MDP-Ca salt complex is highly insoluble, and stable, and forms strong molecular bonds to hydroxyapatite-based substrates [52]. The first self-etch adhesive was to incorporate this component is Clearfil SE Bond. Studies with Clearfil SE Bond have demonstrated that MDP allows for a stable chemical bond to dentin over the course of time, both in vitro [11], and in vivo [53].

The other variable in the composition of the SBU which may account for the differences observed in this material is the presence of the polyalkenoic acid copolymer. SBU contains methacrylate modified polyalkenoic acid [Vitrebond Copolymer (VBCP)] first commercialized in 3M Vitrebond liner/base resin-modified glass ionomer cement. VBCP bonds chemically and spontaneously to hydroxyapatite in glass ionomer materials, and a recent study demonstrated that the presence of VBCP showed more bond strength than a VBCP free adhesive with the same composition [54]. VBCP in combination with MDP has shown contradictory results in the literature. Perdigao et al. [18] reported higher microtensile bond strength to dentine of SBU when compared to Clearfil SE Bond, which has only the MDP monomer, while Muñoz et al. [55] observed a lower microtensile bond strength of SBU when compared to the same adhesive. The VBCP may compete with the MDP monomer for Ca-bonding sites in HAp [21] and due to its high molecular weight, could even prevent monomer approximation during polymerization [55]. Wagner et al. [14] stated that there was not statistically significant difference in mean microtensile bond strength when SBU (self-etch mode) compared to All-Bond U (which only contains MDP). Yoshida et al. [21] hypothesized that VBCP may compete with the MDP present in SBU. However, Muñoz et al. [8] compared the longevity results of SBU (MDP-VBCP) with Adper Single Bond 2 (VBCP), two materials with similar compositions, the only difference being the presence of MDP in the former, it seems that the association MDP-VBCP enhanced the bonding ability, since SBU in both etching modes showed stable bonds even after 6 months of water storage [8].

Specimens bonded with TNBU, showed μSBS of 20.71 ± 2.11 MPa obtained in total-etch mode. There was significant reduction in its bond strength (10.88 ± 4.15 MPa) in self-etch mode. As SBU, TNBU is a 'mild' MDP-containing one-step self-etch adhesive (pH: 2.5) [56] that can chemically interact with calcium in hydroxyapatite [9]. However, TNBU does not have a polyalkenoic acid copolymer, such as VBCP, in its composition. The relevance of VBCP was recently studied using μTBS [57]. The lack of the additional chemical bonding provided by VBCP may be the reason why mean μSBS where lower for TNBU than for SBU in self-etch mode. Second reason, is that TNBU in its formulation, contains bisphenol-A diglycidyl methacrylate, which is a highly viscous monomer [58] which may result in limitation of the penetration of TNBU's resin monomers into the dentinal tubules and intertubular dentin, which consequently led to the low μSBS values in self-etch mode.

The result of this study agreed with Muñoz and colleagues and Tekçe and colleagues who reported that the immediate dentin bond strength increases when universal bonding systems are applied in the etch-and-rinse mode [21]. Therefore, using universal self-etch adhesives in total-etch mode may have positive impact on dentin bonding in the initial bonding performance. These findings are also in accordance with Sezinando et al. [59] who found that universal adhesives resulted in higher bond strength when applied in total-etch mode than in self-etch mode after 24 h and 6 months. On contrary, this result was disagreed with Jang and colleagues and Wagner et al. [14], who stated that similar bond strength values were observed for the universal adhesives regardless of application mode, which makes them reliable for working under different clinical conditions.

 Conclusion



From the results of this study, it was concluded that:

The influence of different etching modes on dentin bond quality of universal adhesives was dependent on the adhesive material, FBU and TNBU performed better μSBS to dentin in total-etch mode than self-etch mode and the differences were statistically significant, while SBU showed almost no differencePerformance of universal adhesives was shown to be material-dependent, in total-etch mode TNBU performed the best μSBS to dentin, while in self-etch-mode SBU had the highest bond strength mode and the differences were statistically significantAccording to ANOVA test, it was not found statistical significant difference between mixed, cohesive and adhesive mode of failure with μSBS.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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