Experimental composite containing silicon dioxide-coated silver nanoparticles for orthodontic bonding: Antimicrobial activity and shear bond strength

ABSTRACT Objective: This study aimed to investigate the antimicrobial activity and shear bond strength (SBS) of orthodontic brackets to bovine enamel using experimental composites with different concentrations of silicon dioxide-coated silver nanoparticles (Ag@SiO2 NPs). Methods: Fifty bovine incisors were divided into five groups according to the composite (n = 10): G1 - Control Group (Transbond XT Resin), G2 - Experimental composite without Ag@SiO2 NPs; G3 - Experimental composite with 0.5% of Ag@SiO2 NPs; G4 - Experimental composite with 1% of Ag@SiO2 NPs; G5 - Experimental composite with 3% of Ag@SiO2 NPs. The SBS test was performed using a universal mechanical testing machine, and the adhesive remnant index (ARI) was analyzed by optical microscopy. For the antimicrobial activity evaluation, Streptococcus mutans (S. mutans) biofilm was formed for three days in hydroxyapatite discs. Posteriorly, S. mutans colony forming units (CFU) were evaluated. For SBS analysis, Analysis of Variance was used, followed by the Tukey test, at a 5% statistical significance level. The CFU data were analyzed by Kruskal-Wallis, followed by Dunn as a post-hoc test. The ARI results were analyzed descriptively. Results: There was no statistically significant difference in SBS values between the experimental and control groups (p>0.05). A 3% incorporation of Ag@SiO2 NPs statistically reduced the SBS values (p<0.05) compared to the 1% group. The addition of 3% of Ag@SiO2 NPs to the composites significantly reduced S. mutans biofilm formation, compared to group G2 (p<0.05). Conclusion: Composites incorporating 3% of Ag@SiO2 NPs presented similar SBS values compared to the control group, and showed significant antimicrobial activity.


INTRODUCTION
White spot lesions (WSLs) are a great concern for orthodontists.
The prevalence of WSLs, based on post-treatment evaluations, ranged from 25 to 97%. [1][2][3] This high frequency is related to the fact that orthodontic accessories increase the risk of biofilm retention (especially Streptococcus mutans, S. mutans), create numerous retention sites, and hamper adequate oral hygiene. [4][5][6] The incorporation of antimicrobial agents into dental materials has been pointed out as an effective strategy to reduce WSLs risk, since there is no need of patient compliance for effective biofilm control. 7 The ideal antimicrobial agent should present a broad-spectrum action, 8,9 long-term duration, 10,11 and no toxicity. 12 Furthermore, its incorporation should not negatively interfere with optical and mechanical properties. 13, 14 Silver particles present most of these desirable characteristics, such as a broad spectrum of action and low antimicrobial resistance. [15][16][17][18][19] Still, its addition causes significant material color changes, 7 compromising aesthetics, which is not acceptable, especially for patients with ceramic appliances. Therefore, recently, studies have proposed the synthesis of silicon dioxide-coated silver nanoparticles as an alternative to masking the material metallic coloration. 20 Aguiar RCO, Nunes LP, Batista ES, Viana MM, Rodrigues MC, Bueno-Silva B, Roscoe MG -Experimental composite containing silicon dioxide-coated silver nanoparticles for orthodontic bonding: Antimicrobial activity and shear bond strength 5 The antimicrobial efficacy of silver nanoparticles against the main caries pathogens, especially S. mutans, is well known. [21][22][23] Recently, studies have shown that silver nanoparticles (50 nm) addition to commercial composites reduced up to 40% of the S. mutans population. 22 When smaller particles (20 nm) were added, the results were even more expressive, providing a 94% reduction of S. mutans. 23 It is important to emphasize that, although some studies have shown that silver nanoparticles can have a cytotoxic potential and some side effects in the surrounding dental tissues, these harmful effects are depended on factors such as concentration and particle size used. 24,25 For the composites' synthesis, only a low concentration of silicon dioxide-coated silver nanoparticles is necessary, which does not present any cytotoxic potential. 20 Based on the fact that fixed orthodontic treatment carries a high risk of WSLs development, it seems promising to investigate the effect of incorporating different concentrations of silica-coated silver nanoparticles (Ag@SiO 2 NPs) on the mechanical property and antimicrobial activity of experimental composites.
The null study hypothesis was that the addition of Ag@SiO 2 NPs (0.5%, 1%, and 3%) would not reduce the S. mutans population around orthodontic brackets and would not affect its shear bond strength to the enamel.

MANIPULATION OF THE EXPERIMENTAL COMPOSITES
The organic matrix was prepared with BisGMA (2.2bis As a comparison standard, a commercial composite was also tested (Transbond XT, 3M ESPE, USA).

BRACKET-TOOTH SHEAR BOND STRENGTH (SBS)
Fifty freshly-extracted bovine incisors were randomly divided into five groups (n = 10). Before the adhesive procedure, prophylaxis was performed using rubber cups and pumice paste. Teeth surfaces were etched with 37% phosphoric acid  To assist the specimen's preparation for the SBS test (embedding and alignment inside the PVC pipe), a device with a system for fixing the tooth bracket into a rectangular orthodontic wire was used to guarantee perpendicularity of the specimen   followed by Dunn as a post-hoc test. The chi-square test was used to determine significant differences in ARI scores among the groups.

BRACKET-TOOTH SHEAR BOND STRENGTH (SBS)
The results of the SBS test are displayed in Table 1     The frequency distribution of the ARI and the chi-square comparison of the groups are displayed in Table 2. There was no significant difference between the groups (p > 0.05). The bond failure pattern was similar in all the evaluated composites. The most frequent failure pattern among the groups indicated less than half of the enamel bonding site covered with adhesive (ARI 1). Although there is no consensus on the mechanism of action of silver particles, authors believe that it occurs by superficial contact with microorganisms, 28 and that its antibacterial effect increases as smaller particles are used. 29 In this study, Ag NPs with 11 nm were used, and a small area of the material (bracket cementation line) was exposed to the oral environment. It is essential to highlight that, although the antimicrobial effect of It is also necessary to investigate its influence on mechanical properties, especially when used for bonding orthodontic brackets. 30 Ideally, the bracket should be maintained fixed to the tooth throughout the orthodontic treatment, and it should be removed without any damage to the enamel structure. 31 The SBS test represents the method of choice to evaluate the efficiency of orthodontic bonding systems. the Transbond XT group showed more frequently the score 0, with no significant enamel changes after debonding. should also be performed to monitor aging's impact, to verify the bonding stability and the antimicrobial activity during fully orthodontic treatment.

CONCLUSION
The addition of 3 wt% silicon dioxide-coated silver nanoparticles to dental adhesives reduced the S. mutans population around orthodontic brackets. Moreover, it did not affect its SBS to enamel, when compared to a commercial orthodontic composite resin.