Industrially developed probiotic mouthwashes have been frequently employed to support both the intestinal system and oral microbiota in recent years. Examining the effects of these new materials on the surface properties of restorative materials and dental tissues is important for their safe use [14]. In-vitro test models are recommended for testing these effects [15]. However, there is a lack of studies in the literature investigating the effect of mouthwashes containing probiotics on surface roughness and surface microhardness, two important parameters in the mechanical properties of restorative materials and dental tissues [16]. The present study investigated the effect of a new probiotic mouthwash on the surface roughness and surface microhardness of different restorative materials, and both permanent and deciduous teeth enamel and compared this with kefir, a widely used probiotic beverage.
Since restoration and tooth surfaces are not constantly exposed to probiotic mouthwashes, aging methods should be applied when examining the effect of these materials on surface properties in vitro [17]. Studies examining the effects of different solutions on restoration surfaces have shown that aging specimens by five years successfully reveals the effect of the solutions on the surface properties [13]. In the present study, in the light of the manufacturer's instructions for using probiotic solutions (twice a day for 2 min), the specimens were kept in probiotic solutions for 8 hours a day for 14 days to achieve a five-year aging effect.
Polishing and surface smoothness are important parameters in studies investigating material surface characteristics. Studies have shown that the smoothest surfaces in restorative materials are obtained in specimens kept in contact with a celluloid strip matrix [18, 19]. In this study, specimens prepared from restorative materials were covered with a celluloid strip matrix on the top and bottom surfaces and smoothed with a glass plate. Additional polishing processes were applied to the restorative material specimens, and the surfaces of the permanent and deciduous teeth specimens were cleaned using slurry pumice and brush and stored in sterile saline solution at room temperature.
Surface roughness is an important parameter that affects plaque involvement, discoloration, caries formation, the success of restorative materials and periodontal health [20]. While devices such as profilometers and electron microscopes are frequently used to measure surface roughness in the dentistry literature, AFM that analyzes high-resolution 3D images has also been used for this purpose in recent years. AFM offers advantages over profilometers in terms of higher-resolution nano-sized measurement and 3D image acquisition, and over electron microscopes because the tip structure is more suitable for restorative materials and dental tissue evaluation. It is also an easier method for specimen evaluation since coating and fixation are not required [21, 22]. In the light of all these advantages, an AFM device was used in the present study to evaluate the roughness of dental restorative materials and teeth surfaces.
The mean surface roughness values of the restorative material groups treated with probiotic mouthwash and kefir were higher than those of the control groups. Research has shown that the pH values of the solutions applied to restorative materials affect the material surface roughness, with low pH solutions increasing the surface roughness of restorative materials [23, 24]. The low pH value of the probiotics used in the present study may have increased the surface roughness by causing deterioration in the surface matrix structures and dissolution of the restorative material fillers. Ozan et al. [25] examined the surface properties of different restorative materials in their study and found that probiotics with low pH values increased the surface roughness values of RMGIC, compomer, and composite specimens. The mean surface roughness values of the restorative materials used in the present study were composite < compomer < RMGIC for all groups. Similarly to our results, Guler and Unal [7] observed that RMGIC restorative material exhibited higher surface roughness than composite resins, and found that this was due to the glass particles in the RMGIC. The differences between the mean surface roughness values of the composite, compomer, and RMGIC materials may be due to the glass particle content of compomer and RMGIC and the resin matrix structure of restorative materials in the present study.
In contrast to the restorative materials in this study, the mean surface roughness values decreased in the permanent and deciduous teeth groups treated with probiotic mouthwash and kefir. Probiotic-containing agents exhibit anti-plaque and antibacterial effects by adhering to dental enamel [26]. The high calcium content of these agents, their adhesion to tooth enamel, and their ability to penetrate micro-cracks on the enamel surface may have increased the surface smoothness. Although changes occurred in the mean surface roughness values of the restorative materials and teeth surfaces with probiotic treatment in this study, the differences were not statistically significant (p > 0.05). In the light of our results, the first null hypothesis that "probiotics do not affect the surface roughness of different restorative materials, or permanent and deciduous teeth" was partially validated.
Another important parameter to be considered in evaluating the surface properties of restorative materials is surface microhardness. This affects numerous factors such as tolerance to chewing forces, wear resistance, fracture and crack formation, preservation of matrix structure and plaque involvement [27, 28]. The Vickers hardness tester is frequently employed in the measurement of surface microhardness in dentistry because of its short tip structure, which provides ease of measurement and is able to determine the surface hardness of different materials [29]. In the light of these advantages, we also evaluated the surface microhardness of restorative materials and dental specimens using a Vickers hardness tester. No significant difference was found between the mean surface microhardness of probiotic mouthwash and kefir application to different restorative materials (p > 0.05). However, mean surface microhardness values increased in the compomer and RMGIC kefir groups. Altwaim et al.[15] also observed an increase in the mean surface microhardness value of RMGIC specimens exposed to probiotic mouthwash. Although those researchers suggested that the surface microhardness of restorative materials might be adversely affected by exposure to different mouthwashes due to water absorption, they also concluded that this effect might vary depending on the solution and material content. In the present study, the differences in the mean surface microhardness of the probiotic mouthwash and kefir groups may have stemmed from the restorative material contents, matrix structures, solution content and the adhesion of the solutions to the material surface.
In contrast to the restorative materials, the mean surface microhardness values of the permanent and deciduous teeth enamels exposed to probiotic mouthwash decreased significantly in this study (p < 0.05). The low pH value of the probiotic mouthwash employed may have caused softening of the tooth enamel with inorganic tissue dissolution. Devlin et al. [30] also observed that acidic solutions reduce the surface hardness of tooth enamel, and prevent the penetration of calcium and other ions to the tooth surface, thereby eliminating the buffering effect. In contrast to those findings, kefir with a low pH exhibited similar surface microhardness values to those of the control group. This may be because the calcium ions it contains penetrate the tooth surface and reduce the low pH effect of the solution. In the light of our results, the second null hypothesis that "probiotics do not affect the surface microhardness of different restorative materials, or permanent and deciduous teeth" was partially rejected.
Despite all these intriguing results, there are a number of limitations to this study. First, only the surface roughness and microhardness of the specimens were evaluated while examining the surface properties. Further studies might use more parameters such as energy-dispersive X-ray analysis to determine the effects of probiotics on the elemental composition of restorative materials or enamel. Second, limited restorative materials were used in this study. Further studies examining the effect of probiotics on a wider range of restorative materialsare now needed. Finally, this research is an in vitro study, and clinical studies are needed to evaluate the effects of probiotics on different restorative materials and tooth enamel.