Effects Of Carbamide Peroxide Bleaching On The Properties Of The Dental Restorative Materials

Since the introduction of nightguard vital bleaching by Haywood and Heymann, tooth bleaching has become increasingly popular in dentistry. As the least invasive option for improving tooth color, tooth bleaching is generally believed to be harmless to tooth hard tissues. In addition to the bleaching effects on tooth hard tissues, its effects on tooth-colored restorative materials have also received a lot of attention. However, most of the previous studies were performed in vitro and the influence of saliva has not often been considered in the experimental models. Further, some conflicting findings, regarding the effects of bleaching on the restorative materials, have been reported. Therefore, the effects of bleaching on the restorative materials need further investigation.Purpose:The aim of this PhD thesis was to evaluate the effects of bleaching agents (carbamide peroxide) on different properties of the dental restorative materials. This PhD thesis contained 5 studies, which investigated the bleaching effects on the surface microhardness, subsurface microhardness, color, surface structure, surface composition, and microleakage of dental restorative materials in situ and/or in vitro.1. The effects of bleaching on the flexural properties of dental restorative materialsMaterials and Methods:This in vitro study investigated the influence of carbamide peroxide on the flexural properties of the tooth-colored restorative materials at two environmental temperatures. Seven restorative materials were used, including four composite resins (Filtek Z350, Filtek Z250, Synergy Flow, and Filtek P60), a polyacid-modified composite (Dyract AP), a conventional glass-ionomer cement (Ketac Molar Easymix), and a ceramic (Vitablocs MarkⅡ). For each type of the material,80 bar-shaped specimens were fabricated and divided into 4 groups (n=20): bleaching group at 25℃, control group at 25℃, bleaching group at 37℃, and control group at 37℃. The specimens of the bleaching groups were treated with 10% carbamide peroxide gel for 8 hours/day, whilst the control specimens in the groups were stored in deionized water. After 14-day treatment, the flexural strength of the specimens was determined using a universal testing machine. All results were analyzed with analysis of variance (ANOVA) and Tukey's HSD post hoc tests with a significance level of 0.05. The data were also submitted to Weibull distribution. The Weibull modulus and characteristic strength were further calculated for each group.Results:The flexural properties of composite resins and ceramic tested were not affected by bleaching. However, the flexural strength and its Weibull distribution of polyacid-modified composite and glass-ionomer cement were more seriously affected by the bleaching agents than the composite resin and ceramic, especially at the higher environmental temperature. Both dentists and material scientists should be aware that, when evaluating the effects of bleaching agents on dental materials, the environmental temperature needs to be taken into account both in the design of a test protocol and the interpretation of the results.2. The effects of bleaching on the surface and subsurface microhardness of the dental restorative materialsMaterials and Methods:Seven restorative materials were tested in this in vitro study: four composite resins (Filtek Z350, Filtek Z250), a polyacid-modified composite (Dyract AP), a conventional glass-ionomer cement (Ketac Molar Easymix), and a ceramic (Vitablocs MarkⅡ). For each material 48 specimens were prepared and divided into four groups:bleaching group at 25℃, control group at 25℃, bleaching group at 37℃, and control group at 37℃. The samples from bleaching groups were treated with 10% carbarmide peroxide for 8 hours daily and for 14 days, while the control specimens were stored in deionized water. The surface and subsurface (0.1-1.0 mm) microhardness were determined by a Vickers microhardness tester to evaluate the bleaching effects. The data of surface microhardness were statistically analyzed using one-way analysis of variance (ANOVA) followed with Tukey HSD post-hoc tests. The data of subsurface microhardness were analyzed using repeated measures ANOVA. All the statistical analyses were perform using SPSS software with a significance level of 0.05.Results:All the materials showed a loss of surface microhardness except ceramic after bleaching at 37℃. However, only Dyract AP and Ketac Molar Easymix were found to have surface softening after bleaching at 25℃. For Dyract AP and Ketac Molar Easymix, bleaching affected microhardness at different subsurface layers. Further, bleaching at the higher temperature decreased the subsurface microhardness at deeper layers. In conclusion, the effects of bleaching on restorative materials were material dependent. Environmental temperature had an influence on bleaching effects on surface and subsurface microhardness of restorative materials. Accompanying with the temperature rose, bleaching showed greater softening effects on the surface and subsurface layers of dental materials.3. The effects of bleaching on the surface microhardness, structure, composition and color of dental restorative materials:an in situ studyMaterials and Methods:Four types of tooth-colored restorative materials, including a nano-hybrid composite resin (Filtek Z350), a packable composite resin (Filtek P60), a polyacid-modified composite (Dyract AP), and a conventional glass-ionomer cement (Ketac Molar Easymix), were used in this in situ study. The specimens of each material were divided into two groups (n=18):bleaching group and control group. Thirty-six Chinese volunteers were recruited and stone casts were constructed for their upper jaws. Two of the 4 types of restorative materials were randomly selected and mounted on the first or second molar of the cast for each subject. With respect to each material selected, two specimens from each group were put at a random side. Then custom-fabricated trays containing specimens were fabricated. The participants wore the tray in the daytime, and applied the bleaching gel for the side containing specimens of bleaching group during the night. Surface microhardness and color measurements were obtained on the surface of each sample on the following time periods:before bleaching,1 week,2 weeks,3 weeks and 4 weeks of bleaching, and 2 weeks after the end of bleaching. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to evaluate the changes in chemical composition and surface morphology of the specimens due to bleaching. The data of color and surface microhardness were statistically analyzed using one-way analysis of variance (ANOVA) and repeated measures ANOVA with a significance level of 0.05.Results:Fifteen percent carbamide peroxide bleaching did not produce any statistically significant effect on the surface microhardness of the composite resins. After bleaching, Ketac Molar Easymix showed a gradual increase in surface microhardness whilst a gradual reduction in surface microhardness was found on Dyract AP. For Ketac Molar Easymix, placing in the oral cavity produced statistically significant hardening effect. Significant color changes were found in all the materials after bleaching. The greatest color changes due to bleaching were found in Dyract AP. However, the color differences decreased after withdrawal of the bleaching. Surface dissolution and filler loss were detected in Dyract AP and Ketac Molar Easymix. Chemical composition alterations in Dyract AP and Ketac Molar Easymix were found after bleaching. However, no changes in surface structure and composition of composite resins after bleaching were detected by SEM. For all the control specimens, storing in situ had no effects on the surface color, structure, and composition of the restorative materials.4. The effects of bleaching on the staining susceptibility of dental restorative materialsMaterials and Methods:The tooth-colored restoratives used in this in vitro study were:a nano-hybrid resin composite (Filtek Z350), a packable resin composite (Filtek P60), a polyacid-modified composite (Dyract AP) and a conventional glass-ionomer cement (Ketac Molar Easymix). The specimens (10 mm in diameter,2 mm in thickness) of each material were divided into two groups (n=34):the bleaching group and the control group. This study included two treatment segments. In the first part (days 1-14), the specimens of the bleaching group were bleached with 15% carbamide peroxide gels at 37℃for 8 hours daily, while the specimens in the control group were stored in deionized water. Subsequently,4 specimens from each group were randomly selected for observation under an environmental scanning electron microscope. In the second part (days 15-42), the samples were stored in 5 different kinds of solutions (red wine, tea, cola, coffee, deionized water). Color measurements for each sample were taken using a spectrophotometer at 6 different time periods: before bleaching, after bleaching,1 day after immersing in solution,7 days after immersing in solution,14 days after immersing in solution,28 days after immersing in solution. The data were then analyzed using one-way analysis of variance (ANOVA) and repeated measures ANOVA with a significance level of 0.05.Results:After bleaching all the specimens showed statistically significant color changes compared with the control specimens. The bleaching agents seriously affected the surface morphology of Dyract AP and Ketac Molar Easymix. For the composite resins, there was not any change in surface morphology after bleaching. After immersing in the staining solutions (red wine, tea, cola, coffee), the bleached specimens exhibited greater staining susceptibility compared with the respective control specimens. The color changes due to immersing in the staining solutions were highlighted by a dramatic drop in L* values. Under the current experimental settings, the composite resins (Filtek Z350 and Filtek P60) exhibited the best color stability, while Dyract AP exhibited the least color stability. With regard to the staining ability of the solutions tested, red wine showed the greatest staining potential to the restorative materials tested.5. Protective effect of resin coating on the microleakage of ClassⅤrestorations following treatment with carbamide peroxide in vitroMaterials and methods:In this in vitro study,160 ClassⅤcavities were randomly restored with one of four different restorative materials (n=40):a polyacid-modified composite (Dyract AP), a conventional glass-ionomer cement (Ketac Molar Easymix), a resin modified glass-ionomer cement (FujiⅡLC) and a composite resin (Filtek Z350). For each kind of material,40 restorations were divided into 4 groups:bleached with resin coating (group BC); bleached without resin coating (group B); immersed in artificial saliva with resin coating (group SC); immersed in artificial saliva without resin coating (group S). In the groups B and BC, the specimens were bleached with 10% carbamide peroxide gel for 8 h daily, while the groups SC and S were stored in artificial saliva instead. The resin coating was performed on the restoration margins using a resin-based varnish (Seal & Protect). After 28-day treatment, all the samples were subjected to a dye penetration test using "multiple sectioning" technique. Further, an additional test was performed to investigate the color difference between coated and uncoated tooth surface after bleaching. The microleakage data were analyzed using the Kruskal-Wallis test followed by the Mann-Whitney U test and the color data were analyzed using the one-way analysis of variance with a significance level of 0.05.Results:There was a statistically significant increase in the cervical microleakage in group B specimens of FujiⅡLC and Ketac Molar Easymix compared with their respective control specimen (group S). However, these effects on microleakage were not found in the bleached specimens with resin coating (group BC). Importantly, there was no significant difference in the microleakage scores between the group BC and group SC, indicating the complete protection against bleaching by the resin coating. Further, the color differences were all under 1.8, indicating there was no visual detectable color difference between coated and uncoated tooth surface. In conclusion, the resin coating is an effective method to avoid the bleaching-induced microleakage of glass-ionomer cement. Dentist should consider apply resin-based varnish on the margin of glass-ionomer cement restoration before bleaching.ConclusionsUnder the condition of all the studies above, it can be concluded that carbamide peroxide is able to affect the properties of dental restorative materials. However, the effects of carbamide peroxide bleaching on the restorative materials were apparently material-dependent. Generally speaking, the composite resin has the best resistance to carbamide peroxide among the materials tested in this PhD thesis. The polyacid-modified composite resin and glass-ionomer cement can be easily affected by bleaching in many aspects. These findings were confirmed in both the in vitro and in situ experimental models.Based on the findings of this PhD thesis, we would like to offer some recommendations for dentists:1) Dentists should carefully examine the existed restorations before bleaching and renew insufficient filling prior to bleaching; 2) It is recommended that dentists should consider polish the restoration after bleaching to reestablish the restoration surface; 3) It is also recommended to apply resin-based varnish on the surfaces and margins of conventional glass-ionomer cement and resin-modified glass-ionomer cement before bleaching; 4) During bleaching, dentists should check the status of the restoration routinely. Importantly, we also have some recommendations for material scientists:researchers should consider the impact of environmental temperature on the results when analyzing the properties of dental materials after bleaching. The environmental temperature during experiment needs to be taken into account both in the study design and result interpretation.Although the mechanism regarding the bleaching effects on the restorative materials, the current literature offers some explanations:the alterations in color of the restorative materials have been attributed to oxidation of surface pigments and amine compounds, which have also been indicated as responsible for color instability of restorative materials over time. Differences in color change between different materials might be a result of different amount of resin and different degrees of conversion of the resin matrix to polymer. Also surface phenomena, such as increase in porosities, are discussed as a result of the deleterious impact of the oxidizing bleaching agents on the polymer-matrix of resin-based materials. Additionally, it was debated whether the negative influences of the oxidizing agents on the resin matrix led to water uptake of the restorative materials with complete or partial debonding of fillers causing reduced surface integrity and loss of hardness of the materials.In vivo or in situ studies can simulate the clinical situation maximally, but they are considered expensive, difficult, and time-consuming. Given the fact that the market of dental restorative materials is dramatically developing, it seems impossible to carry out all the tests in vivo or in situ. Based on our findings, results from the in vitro studies with all the variances controlled can give us enough information concerning the effects of bleaching on restorative materials.