Effects of different antibacterial disinfectants on microleakage of bulk-fill composite bonded to different tooth structures

Composite materials are widely used in restorative dentistry due to their excellent esthetic properties [1, 2]. However, Polymerization shrinkage seems to be the most significant problem with the composite restorations, which may result in microleakage [3, 4], secondary caries, and subsequently may lead to bonding failure [5]. The presence of viable microorganisms that remain after cavity preparation aggravates the problems associated with microleakage, resulting in secondary or residual caries [6]. Incomplete caries removal, and weak adhesion between the restoration and tooth margins are among the main factors responsible for dental restoration failure, allowing the penetration of microorganisms and formation of secondary caries [7]. Studies demonstrated that after the preparation of cavity only a small section of teeth remain disinfected [8]. Therefore, the chances of recurrent dental caries is high which is considered the main cause for replacement of restorations [9]. In Class V restorations, up to 90% of recurrent caries was reported to be at the cervical margins, regardless of the type of restoration [9, 10]. Therefore, improper cleaning of bacterial remnants from the cavity walls during cavity preparation could contaminate the tooth margins and weaken the sealing ability of the restoration, hence leading to unwanted marginal adhesion failure.

The passage of oral fluids, molecules, bacteria, and ions at the interface of the cavity walls and cavity filling material is defined as, microleakage [11‐15]. For successful and durable cavity restoration, the deterrence of microleakage is imperative to avoid negative effects such as marginal colouring and fracture, secondary carries, corrosion, sensitivity, and inflammation of the pulp, [12]. These effects are due to the presence of bacteria, their nutrients or hydrogen ions, originating from plaque on the surface, leaking into the interfacial space [16, 17].

The mechanical caries removal techniques are not adequately enough to thoroughly eliminate caries [18]. Therefore, to prevent of secondary caries sequelae, the use of antibacterial disinfectants in conjunction with mechanical approaches is considered an effective method for caries debridement [19]. Chlorhexidine digluconate (CHX) is a powerful antimicrobial solution with high ability to suppress the growth of streptococci, and therefore, has the potential to prevent dental caries [20, 21]. When applied on dentin surface, CHX can prevent bacterial settlement over time due to the continuous release of positively charged molecules [22]. This antimicrobial effect is referred to as substantive antimicrobial activity (SAA) [23]. Barsani et al. found 2% CHX to be a potent disinfectant with effective SAA [23]. Moreover, since the formation of the dentin hybrid layer, during bonding procedure, may not be enough to stabilize the bond over time [24]. The substantive antimicrobial properties of CHX prevents collagen fibrils deterioration after immediate application, and therefore, may maintain a reliable dentin bond [25‐27]. Several studies have reported the effects of CHX on the bonding of resin composites to dentin, and these have been found to vary according to different factors, the most important of which are the concentration of CHX, the type of adhesive system, and the timing of CHX application [28‐30]. If etch-and rinse systems are used, CHX application has been recommended after acid etching, and before bonding step [31, 32], as the application of the CHX after acid etchant was reported to be highly effective in preserving the adhesive interface [31, 32]. However, studies that evaluated CHX during cavity preparation and its effect on the microleakage were mainly performed on conventional composites, and investigations of their effects and effectiveness on the fairly new bulk fill composites are limited.

Similarly, Salvadora persica (S. persica), Miswak, is a plant species that possesses significant antibacterial properties [33]. The documented natural antibacterial effects of this tree against cariogenic pathogens [34, 35] had made the World Health Organization (WHO) to encourage the use of disinfectants with miswak extracts in the fight against oral bacteria [29]. However, data on the effects of these disinfectants on tooth-colored materials, are scarce.

The aim of this study was to evaluate the impact of various disinfectants (2% CHX and Listerine Miswak antibacterial solutions) on the microleakage performance of a newly developed bulk fill nanocomposite bonded to different tooth structures by using single bond universal adhesive system. The null hypothesis was that no significant difference would be found in the microleakage performance of the bonded tooth structures pretreated with any one of the disinfectants. In addition, there would be no significant difference in the microleakage of pretreated (with disinfectants) and untreated tooth structures.

The tooth count recorded for each dye penetration score is presented in Table 2 for each of the three groups studied. Figure 2 shows microscopic images of dye penetration for various group samples.

Due to the significant antibacterial effects of disinfectants, increasing number of dental clinicians tend to apply them to cavity preparations before proceeding with dental restoration [39]. However, any unwanted interference of these materials with the success of the adhesion procedure of the restorative materials to tooth structures will cause their benefits to diminish. It has been reported in the literature that the techniques used to evaluate the sealing properties of composites by microleakage testing are not different [40]. In the current study, dye-penetration was used due to its well documented results [41]. Further, the microleakage of a recently developed bulk-fill composite bonded to Class V cavity preparations after pretreatment with 2% CHX and Listerine Miswak disinfectants was evaluated. A non-beveled preparation was used in an effort to standardize the preparations in the examined teeth, since it has been documented that beveling of enamel has no beneficial effect on either retention or marginal discoloration [42, 43].

The results of this study, revealed the superior sealing ability of the occlusal margins, against microleakage, compared with that in cervical margins, regardless of the use of disinfectant, when etch-and-rinse mode of the universal bond system was used. These results were in agreement with those of Yamauchi and co-workers, since they found that a separate etching step prior to the application of universal adhesive system resulted in better enamel marginal integrity [44]. This can be explained by the high percentage content of hydroxyapatite inorganic contents in enamel compared to dentin which contains higher amount of organic structures, and therefore, may interfere with the micromechanical adhesion procedure [45]. Several studies reported similar results on the effects of etch-and-rinse systems on both tooth structures [43, 46, 47]. In the current study, significantly increased microleakage, in cervical margins, compared to occlusal margins was found before and after pretreatment with both types of disinfectants. Therefore, the null hypothesis, that there would be no significant difference in microleakage between different tooth structures pretreated with different disinfectants, was rejected for all groups. The results revealed that, pretreatment of tooth structures with CHX or Listerine Miswak did not adversely affect the marginal seal of the restoration. These results were in agreement with those of multiple studies that evaluated CHX as a pretreatment disinfectant and found no significant effect on microleakage, regardless of the type of bonding system used [48‐50]. In both occlusal and cervical margins, CHX-pretreated teeth showed better marginal seal than those treated with Listerine Miswak; however, this difference was not statistically significant in both tooth structures. Therefore, the first null hypothesis, that there would be no significant difference in microleakage between disinfectants, cannot be rejected for both margin types. It is worth mentioning here that a statistically non-significant difference between the two disinfectants may not reflect their real effects in the case of a small sample size. In other words, increasing the power of the analysis by increasing the sample size may render this significant difference. The improvement in the cervical margins of CHX-pretreated specimens may be explained by the inherited properties of CHX to act as a matrix metalloproteinase inhibitor, which may prevent collagen destruction at the bonding interface [51]. Therefore, CHX application has been shown to maintain the dentin hybrid layer preservation [25, 26, 52]. In contrast, miswak extracts have been documented to disrupt the smear layer on dentin [34]. Recently, Khunkar et al. [53] compared the effects of miswak natural extracts with those of CHX on dentin collagen. The results revealed that, miswak extracts were found to have the ability to prevent collagen degradation at a lower effect than CHX. To the best of the authors knowledge, only one study (conducted by Salama et al.,) [36] has been found, where the effects of miswak extracts on microleakage were evaluated. The study reported no significant difference in microleakage of Class V conventional resin composites bonded to etched dentin margins of primary teeth pretreated with 1 mg/mL extracts of miswak or CHX. However, the CHX concentration they used was 0.2%. Listerine Miswak, used in the current study, is a chemical disinfectant with different components, other than miswak extracts. Therefore, its effect on microleakage, is attributed to the effect of the disinfectant rather than the miswak extract by itself. CHX has been recommended for use on etched dentin, with etch-and-rinse adhesives [54].

Universal adhesive system was used in the “etch-and-rinse” mode, which may explain the superior marginal sealing results in occlusal structure of the tooth, where all margins are in enamel. Conversely, cervical margins presented significantly high microleakage scores in all groups, especially in non-pretreated specimens. This could be attributed to the increased etching depth on the dentin margins that cannot be completely infiltrated with resin and yielded a weak bond which subsequently resulted in increased microleakage. However, when these margins were pretreated with either type of disinfectant, sealing abilities improved, which was significant in the CHX group only. Therefore, the second null hypothesis, that there would be no significant difference in microleakage between pretreated and non-pretreated groups, was rejected for the cervical margins of the CHX group.

Multiple studies reported the effectiveness of CHX in stabilizing the bond strength and improves the adhesion of composite restorations to dentin over time [26, 55, 56]. Carrilho et al. [26] reported that dentin pretreatment with CHX by etch-and-rinse systems was found to stabilize the bond after 14 months. Similarly, Ekambaram et al. reported that CHX maintained the bond strength of composite to tooth structures and decreased leakage after 1 year [55]. Komori et al. [50], studied the application of 2% CHX with etch-and-rinse adhesive systems and revealed that, CHX preserve the bond strength to sound dentin for up to 6 months. However, some studies reported no effect of CHX on long-term bond preservation [57, 58]. A study conducted by Angeloni et al. [58], showed no effect of CHX on the dentin bond with a self-adhesive system after 6 to 12 months. Similarly, Simões et al. [57] revealed that CHX application with an etch-and-rinse adhesive system had no effect on the sealing performance after 6 months. In a recently published meta-analysis, the sum report for the dentin bond strength after 6 months evaluations was found to be improved [59]. The results from the current study revealed that, the application of CHX to enamel did not improve the bond strength to composite restorations. These results were in agreement with previous studies by Kapdan et al. and Haidari et al.[60, 61], who used “etch-and-rinse” systems. Haidari et al., [61] used the same adhesive agent protocol used in the current study and found no effect of CHX on enamel microleakage. This may be explained by the high stability of bond to inorganic enamel before the application of disinfectant, as compared with that of dentin. It is worth mentioning that, majority of the studies evaluated the effects of CHX on bonding conventional composites to different tooth structures. Bulk-fill composites were not fully evaluated in that regard.

Although marginal integrity is not the only factor to determine the influence of certain clinical procedure or protocol during restorative treatment, microleakage is considered a common laboratory test to predict the longevity of dental restorations. However, in comparison with bacterial penetration, the characteristics of the molecular dyes used to detect microleakage may allow them to penetrate differently in spaces. Therefore, clinical correlations are necessary to validate these methods of assessment [62]. Clinical researches is believed to be the most appropriate method to evaluate the longevity of dental materials. However, clinical researches are difficult to standardize. In addition, they require continuous and long follow up to evaluate the performance of dental restorations during function [40].

In-vitro studies tend to test hypotheses in a controlled laboratory set-up. One of the measures applied in laboratory studies, to simulate clinical situations, is the artificial aging process which simulates the oral cavity environment.

Within the limitations of this study, it could be concluded that, Pretreatment of cavity preparations with 2% CHX or Listerine Miswak did not affect the microleakage between Filtek One bulk-fill composite material and different tooth structures bonded with the “etch-and-rinse” mode of the single bond universal adhesive system. On the contrary, pretreatment of the cervical margins with 2% CHX significantly improved the marginal seal.