Microleakage difference between total-etch and self-etch bonding in bulk fill packable composite restoration after carbonic acid immersion

Background: Bulk fill packable composite that can be applied to a depth of 4mm in cavities is widely used in posterior teeth restoration. Unfortunately, this composite is subject to potential microleakage which occurs due to erosion resulting from the consumption of carbonated drinks containing carbonic acid. Nevertheless, microleakage can be reduced by bonding applications the etch technique of which is divided into two forms; self-etch bonding and total-etch bonding. Purpose: This study aims to determine the difference in microleakage between total-etch and self-etch bonding in bulk fill packable composite following carbonic acid immersion. Methods: This study constitutes experimental laboratory research utilizing 28 incisors bovine teeth which were cleaned, immersed in 0.01% NaCl, and randomly divided into four groups. The cervical area of the teeth of all groups were prepared through the creation of cylindrical shapes 2mm in diameter and 3 mm deep. Groups I and III used total-etch bonding, while groups II and IV used self-etch bonding. Groups III and IV were control groups, whereas groups I and II were treatment groups (immersed in carbonic acid) for 24 hours. Thermocycling was carried out in all groups which were subsequently immersed in 1% methylene blue for 24 hours after which the teeth were cut in a buccolingual direction using a diamond disc wheel. Microleakage was subsequently evaluated by calculating the amount of methylene blue passing between the restoration wall and cavity using a Stereomicroscope and scanning electron microscope (SEM). The data was then analyzed using Kruskal Wallis and Mann Whitney U tests. Results: There was a significant difference between the control groups and treatment groups (p<0.05). The microleakage in Group 2 was higher than that of other groups. Conclusion: The microleakage of total-etch bonding was lower than self-etch bonding in bulk fill packable composite after carbonic acid immersion.


INTRODUCTION
Composite resin is considered a popular restorative material since it not only has high aesthetic value and strong physical mechanical properties, but also demonstrates long-term stability and can be employed in the treatment of almost all classifications of GV Black carious lesions. 1,2 A number of innovations in the development of composite materials have been introduced, one of which is to intended to simplify its restoration technique while maintaining its physical properties. Bulkfill composites are ones that can be applied up to a depth of 4mm and are consequently capable of reducing processing time. Another characteristic of bulk fill composites is that they are not only effective against polymerization shrinkage, durable, and not easily fractured, but also possess sound dimensional stability. 3 Bulk fill packable composite resins not only have high viscosity which enables them to adapt quickly to dentin, but also considerable mechanical strength rendering them suitable for use in posterior teeth. 4,5 Nevertheless, it is known that microleakage in bulk fill packable composites is greater than in flowable composites which can be prevented through the administration of dentin bonding. 5 Microleakage is caused by dimensional changes and unfavorable adaptation to the cavity wall. 6 A bonding material is one that increases the bond strength between the composite resin and tooth structure, enhances retention of restorations, and reduces microleakage along the surface between dentin and composite resin. 7 Bonding applications can be classified into one of two etching techniques, namely; total-etch and self-etch. 8 One required characteristic of a restoration is the ability to resist degradation which in the oral cavity is a complex phenomenon related to the disintegration and dissolution of the restorative material present there. 9 Degradation in composite resins can also take the form of material loss due to erosion. 10,11 Significantly, acidic drinks with a pH below the critical level for the demineralization of enamel (4.5-5.5) have the potential to cause erosion. 11 Carbonated drinks are one of the non-alcoholic varieties with a high acidity level. A major ingredient of carbonated drinks is carbonic acid which has a pH of 2.37 and a concentration of 56.693 mg/mL.12 Composite resins are said to be degraded due to acid exposure in artificial saliva with a low pH. 12 Unfortunately, it is not yet certain which bonding technique demonstrates the optimum attachment to dentine. A previous study posited that total-etch bonding has more minimal microleakage than the self-etch variety. 13 In contrast, another prior investigation stated that the selfetch bonding demonstrates less microleakage than the total-etch variety. 14 Meanwhile, another study revealed that no significant difference exists in microleakage resulting from the total-etch dentin bonding or its self-etch counterpart. 15 Hence, this study aims to determine the difference in microleakage between total-etch and self-etch bonding in bulk fill packable composite after carbonic acid immersion.

MATERIALS AND METHODS
This study constitutes an in vitro laboratory experimental research with post-only control group design which was granted ethical eligibility by the Ethics Commission of the Faculty of Dental Medicine, Universitas Airlangga (No. 368/HRECC.FODM/VI/2019). Certain stages were conducted in the course of this study. First, 28 samples of caries-and fracture-free bovine teeth (mandibular incisors) were removed intact from the jaw. 16 They were subsequently cleaned and soaked in solution before being randomly divided into four groups, each of which contained seven samples. All the samples were then prepared using low speed round and cylindrical diamond burs (NSK, USA) cylindrical in shape, 3 mm in diameter and 2 mm deep which were applied to the cervical area of the teeth. Cavity depth was examined by means of a straight probe. 17 In the fifth step, the members of Group I and Group III were treated with total-etch bonding technique (Ivoclar Vivadent, N-Etch® Schaan, Liechtensein, Germany) and separate bonding. Meanwhile, Group II and Group IV were treated with a self-etch bonding technique using universal bonding materials (Ivoclar Vivadent, Tetric® N-Bond Universal, Schaan, Liechtensein, Germany). All groups were restored using bulk fill packable composite (Ivoclar Vivadent, Tetric® N-Ceram Bulk Fill, Schaan, Liechtensein, Germany) and irradiated for 20 seconds by means of a curing unit (Woodpecker® Light Curing LED.C Wireless, USA). The samples were then prepared for thermocycling and carbonic acid immersion.
Thermocycling was carried out in all sample groups for 120 cycles at 5 o and 55 o C. The samples were covered apically with dental wax to prevent further penetration. The coronal part was then coated twice with nail polish in an area up to 1 mm around the restoration. Group I and Group II were subsequently immersed in carbonic acid with a pH of 2.33 or a concentration of 56.693 mg/mL for 24 hours while the Control Groups, Group III and Group IV were immersed in distilled water for 24 hours. All sample groups were dried with tissue paper before immersion in 1% methylene blue solution for 24 hours. Following immersion, a buccolingual direction was cut in the middle of the restoration using a diamond disc wheel. (Intensive® Diamond Dental Disc Wheels, Switzerland).
Microleakage in restoration was evaluated by examining the penetration of 1% methylene blue solution at the margins of the overlap in the occlusal and gingival walls. Each sample was then assessed by three observers by means of a stereomicroscope (Zeiss® Stremi DV4, Germany) at 15x magnification. The microleakage was then evaluated with a scoring method based on that proposed by Didron et al. (2013), 18 namely; 0 = no colour penetration; 1 = colour penetration up to half of the cavity wall; 2 = total colour penetration of the cavity wall; 3 = colour penetration up to half of the axial wall; and 4 = colour penetration more than half of the axial wall ( Figure 1).
More detailed observations of microleakage were conducted with a scanning electron microscope (SEM) (Zeiss® EVO MA 10, Germany) before the data was analyzed using a Kruskal Wallis Test and a Mann Whitney U Test with SPSS Software (IBM® SPSS Statistics 20, IBM, USA) with a p value of <0.05.

RESULTS
The contents of The microleakage in samples observed using a buccolingual 15x magnification stereomicroscopes that based on metylene blue as color penetration, the lowest microleakage score with a score of 1 occurred in Group III. Meanwhile, Group II recorded the highest microleakage score with a score of 4. Group I and Group IV produced almost identical microleakage scores with a score of 2. Furthermore, SEM observation results indicated that microleakage as a gap or cavity with an irregular shape and darker color than the surrounding tissue structure, (yellow arrows) was predominantly identified in Group II and Group IV. Contrastingly, no microleakage occurred in Group I and Group III (Figure 2).

DISCUSSION
The results of this study indicated a significant difference in microleakage between total-etch bonding and self-etch bonding after carbonic acid immersion. The bonding material employed was of a universal adhesive type which contains 10-30% more water than a conventional bonding variety. This caused greater ionization of acid monomers resulting in an accompanying increase in the depth of the demineralized dentin. Therefore, deeper resin penetration was possible in producing adequate adhesion. 19 The results of this study also revealed that Group I had fewer microleakage values than the other groups due to separate etching and rinsing application processes within the total-etch bonding. Therefore, the smear layer produced in the etching process would be eradicated during rinsing. The clean surface of the smear layer substrate is an indication of the exposure of the hydroxyl enamel group to hydroxyapatite crystals which renders the substrate hydrophilic with the result that wettability increases and results in deeper and stronger penetration of the bonding material monomer. 20 This process will then increase marginal adaptation culminating in the edge of the leakage that occurs in the total-etch bonding technique being smaller than that in its self-etch counterpart. 21 A previous study conducted by Tsujimoto et al. (2016), 22 similarly argued that the base surface of the enamel covered by the smear layer is hydrophobic resulting in a reduction in wettability.
Nevertheless, the acid etching application procedure can culminate in increased wettability in the enamel causing the formation of a small phi angle. As a result, bonding and composite resin material easily penetrates the entire cavity and good marginal adaptation is formed. 21 The increased wettability of the enamel will also alter the hydrophobic characteristics to hydrophilic ones through exposure to groups of cavities, thereby increasing chemical bonds on the substrate involved in the adhesive process. 23 Ultimately, the surface area and surface energy available for binding which are derived from the interaction between the composite resin and collagen dentin (hybrid layer) are greater. Therefore, the restoration is more resistant to thermocycling and erosion due to carbonic acid immersion. As with the results of this research, those of a previous study conducted by El Sayed et al. (2014) 7 argued that total-etch bonding experiences less extensive microleakage than self-etch bonding.
Furthermore, the results of this study found that Group II (using the self-etch bonding technique with carbonic acid immersion) was the group with the highest leakage. The self-etch bonding technique group did not go through the process of etching and rinsing and, consequently, the hydroxyapatite decalcification results (smear layer) were still present on the dentin surface. The smear layer chemically bonded with the functional monomer of acrylic phosphonic acid to form a smear plug (0.5 μm-5 μm) of total-etch bonding. These chemical bonds are stable but weak, leading to the formation of lower marginal adaptations. 24 37%, phosphoric acid, is the etching material most frequently used to produce consistent microporosity. The etching process with weak acid, in this case present in self-etch bonding material, will cause irregularly formed microporosity of insufficient depth, thereby causing inadequate micromechanical retention and susceptible edge leakage. 23 The influence of thermocycling and erosion caused by carbonic acid immersion causes not only self-etch bonding to demonstrate higher demineralization, but also induces the appearance of a gap allowing the low pH of the carbonic acid in carbonated drinks to soften Ca10 (PO4) 6 (OH) 2 hydroxyapatite crystals. Of the subsequently decomposing complexes formed, one is Ca +2 which will then be bound by carbonate ions to form CaCO 3 . 25 Demineralization occurs continuously during consumption of carbonated drinks which have a low pH of 2.37. This condition, in turn, provides an opportunity for carbonic acid to penetrate and react to form a gap between the cavity and tooth restoration, resulting in the development of enamel porosity at the cast edge. 22,24 Finally, the results of this study also confirmed no statistically significant difference in microleakage between Group III and Group IV (p>0.005). This is because both total-etch and self-etch techniques make equally effective marginal adaptations to the tooth structure surface. Similarly, a previous study conducted by Perdigão et al. (2003) 15 stated that both of these adhesive techniques produce microleakage that is not significantly different. Moreover, the application of universal bonding material in the two sample groups provided optimum adhesive strength to minimize microleakage (p<0.05). In conclusion, the level of microleakage in bulk fill packable composite restorations in total-etch bonding after carbonic acid immersion was lower than that in self-etch bonding.