Dental bleaching is a simple and conservative procedure for the aesthetic restoration of vital and non-vital discolored teeth . Currently, many bleaching agents are commercially available with various components and concentrations, such as hydrogen and carbamide peroxide. The low pH of some bleaching agents, by-products of protein denaturation (e.g. urea), and prolonged exposure of tooth surface to the bleaching agents can cause tooth demineralization and sensitivity [2, 3].
Some remineralizing compounds, such as fluoride, calcium, bioactive glass, arginine/calcium carbonate, and nano-hydroxyapatite; have been investigated to reduce these adverse effects [4-6]. Recently, there has been interest in developing materials with bioactive potential. As the demand for environmentally friendly technology increases, synthesis of biomaterials from calcium-rich natural sources remains a viable and more economical option [7, 8].
Eggshells and Seashells are bioactive materials, a cost-effective, renewable, and rich source of calcium carbonate. Previous studies evaluated tooth enamel's successful remineralization using biowaste materials [7-9]. However; dentists are concerned about the post-treatment discoloration caused by such remineralizing agents, which could disappoint patients who prefer whiter teeth.
To objectively determine the difference between two perceived colors, a mathematical equation is used to calculate the colorimetric distance (ΔE). The colorimetric distance is defined as the value that represents the distance between the positions of two colors within the color space [10, 11].
The most frequently used color space to calculate the color of natural teeth and restorative materials has been the standard established by the Commission Internationale de l’Eclairage in 1976 (CIE L*a*b*) [10, 11]. Later, the Commission Internationale de l’Éclairage developed the CIE L*C*H* color space based on the previous standard. Since the new color space had a better corresponds better with the perception of the human eye, it has only recently been incorporated into the assessment of the natural color of teeth . CIE L*C*H* is based on a polar coordinate system, where L* (brightness), C* chroma (color intensity or saturation), and values of H* correspond to hues or shades (perceived color) [10, 11]
Until the publication of this paper, no studies used this color space to measure the color changes caused by such biowaste remineralizing materials after bleaching. Despite the great potential of eggshells and seashells and the high bioavailability of calcium in them, there is no consensus in the literature on the effectiveness of eggshells or seashells on the color changes of enamel after bleaching, so this in vitro study aimed to evaluate the color changes by these natural remineralizing agents on enamel after bleaching using the CIE L*C* H* color space.
The study tested two null hypotheses: 1) the use of eggshells and seashells after bleaching does not affect the color of the teeth, 2) there is a similar relationship between the L*, C*, and H* parameters and ΔE.
Tooth bleaching is a conservative and safe method of treating discolored teeth . Demineralization of dental structures is one of the important post-bleaching side effects caused by the low pH of the used materials. About two-thirds of patients who receive this treatment experience some degree of transient tooth sensitivity during and after bleaching [2, 15].The redox reactions of the bleaching material can lead to the dissolution of organic and mineral matrices to the extent that only carbon dioxide (CO2) and water remain. This compels dentists to prescribe remineralizing products .
Researches on different formulations of peroxide with calcium and fluoride showed that fluoride and/or calcium prevent microhardness reduction and accelerate the recovery of post-treatment microhardness to the pre-treatment state [1, 4]. Recently, researchers recommended natural biowaste products like seashell and eggshell nanoparticles; to increase the microhardness and remineralization of demineralized enamel [7-9].
Despite these benefits, using these medications after bleaching may lead to discoloration of the teeth, which may disappoint patients who desire whiter and more lustrous teeth. Therefore, this study aimed to evaluate the effect of seashell and eggshell nanoparticles on enamel color changes after bleaching.
To objectively determine the difference between the two perceived colors in this study, our work demonstrates for the first time; a mathematical equation in which the colorimetric distance (ΔE) obtained in the color space CIE L*C*H* in evaluating the tooth color changes after bleaching and mineralization. This color space is derived from another previously developed color space (CIE L*a*b*), which has been used to measure the color of teeth. However, CIE L*C*H* surpasses the previous versions since its color representation has a better correlation with the perception and interpretation of the human eye .
The results of this study showed a statistically significant total color alteration (ΔE) of the post-bleached treated samples, suggesting that the bleaching agent may have penetrated correctly into the enamel.
Regarding the effect of the treatments on the color change of bleached enamel, there was a statistically significant difference in the mean values of ΔE between different groups. All three remineralizing materials showed a significantly lower ΔE after remineralization compared to the post-bleaching.
The lowest mean value of ΔE recorded with eggshells, followed by 2% NaF, and the recorded ΔE was1.76 and 3.04, respectively. The human eyes can't perceive these changes as the lowest ΔE that can be differentiated by the human eyes are more than 3.3 as reported by previous study by Gawriołek et al. . The mean value of 3.3 (ΔE) is reported to be esthetically acceptable, and any difference beyond this limit is highly perceptible and clinically unacceptable [17, 18].
These findings can be explained by the fact that the remineralizing substances could be penetrated to a good depth through the bleached enamel making the teeth more similar in appearance to the natural teeth.
The current findings may be in the same line with a previous study by Tay et al. who reported that, 2% NaF didn't interfere with the bleaching effect of 35% H2O2 . Also, these results are consistent with the evidence provided by Armenio et al , who examined the effect of 1.23% fluoride gel applied for four minutes after the daily use of carbamide peroxide gel and reported that, NaF didn't disturb the whitening effect of the peroxide gel .
Chen et al. reported that, fluoridated bleaching agents and post-bleaching fluoride treatment did not interfere with the bleaching effect . Also, Maran et al. reported that, the incorporation of NaF in at-home bleaching gel containing 10% carbamide peroxide, does not affect the color change .
On the other hand, the current findings are in contradiction with a previous study by Malekipour et al, have recorded the highest ΔE with 0.05%NaF mouthwash use after bleaching compared to the other solutions (ΔE= 7.29) . Furthermore, a study by Kim et al. conflicts with the current findings . They reported that, NaF produces a visible discoloration after bleaching (ΔE=7.51), which contradicts our results (ΔE=3.04).
To the best of our knowledge, the present study is the first investigation on the discoloration potential of both eggshells and seashells; therefore, there is no reference for comparing of the results concerning these products. In addition, Kim et al. have reported that, nano-carbonate apatite prevented the re-staining after tooth bleaching in-vitro , this may be consistent with eggshells results in the current study, which recorded minimal color change after bleaching (ΔE=1.76).
However, the results of seashells challenge this report, we found that seashells can cause visible discoloration in bleached teeth (ΔE=7.21). The highest color changes were recorded after using seashells (ΔE=7.21), based on the above thresholds, this qualifies as clinically perceptible by human eyes and therefore, unacceptable color differences. This probably may be attributed to many reasons; first, the gray color of the seashell powder compared to the white color of eggshell powder. Second, the pores created after bleaching may not be filled for a good depth with seashell nanoparticles like eggshells. Therefore, the first null hypothesis reported that, there would be no difference between seashells and eggshells in color changes after bleaching was rejected.
In the current study, artificial saliva may cause perceptible unacceptable color changes in bleached teeth (ΔE=5.69). This fact probably may be explained by the pellicle formed by saliva on the teeth surface may increase its stain-ability.
Detailed analysis of each of the parameters that make up the color after treatment of the bleached enamel using eggshells; showed a significant increase in the brightness (∆L), and a non-significant decrease in the color intensity (chroma) (∆C) and hue (∆H), on the other hand, it showed a significant decrease in the total color difference (∆E) after remineralization with respect to the post-bleaching effect. This fact means that (∆E) is inversely proportional to the (∆L), however it is directly proportional to other color parameters; (∆H) and (∆C), thus the second null hypothesis, which stated a similar relationship between the L*, C*, and H* color parameters and ΔE is rejected,
These findings imply that eggshells positively affect bleached enamel and can restore teeth to their normal color. This may be related to the refractive index of eggshell crystalline structure calcite (1.63), which is closest to the refractive index of the normal tooth enamel(1.65) as reported by Sabuncu et al , hence the difference in the refractive index between enamel and nanoparticles is reduced, the enamel returns to its translucency and looks similar to the normal teeth.
Furthermore, seashells recorded the highest value for brightness (∆L) and color intensity (∆C) (chroma), implying that they could reversibly influence and mask the bleached color. This could be explained by three reasons; first; the fact that the micro-porosities of the bleached enamel, which is filled with seashell nanoparticles, may have a refractive index far away from that of enamel, thus the difference in the refractive indices between the porosities and enamel is increased
Second; related to the volume of minerals, seashell nanoparticles may not fill the full depth of the bleached enamel by restoring the surface rather than the body of the enamel, thus increasing the optical reflectivity as most of the detected signals come from lesions, not fully remineralized. Third, related to the crystalline structure of the calcium carbonate in seashells which is (aragonite), these are responsible for the iridescent colors of the shells, as reported by a previous study by Liu et al. .
LIST OF ABBREVIATIONS
|| = Color intensity (chroma)
|| = Hue
|| = Brightness
|| = Total color difference
Fatma Hussein is involved in all stages of the research processing, including research designing, analysis, and writing. Sahar El Marsafy acted as a research and academic supervisor to Fatma Hussein, who was involved in the designing, analysis and writing of the manuscript.
ETHICS APPROVAL AND CONSENT TO PARTI-CIPATE
Ethical approval of the local ethic committee of the Faculty of Dental Medicine, Al -Azhar University for Girls, following the international guiding principles was obtained(Code: RECOP-21-10).
HUMAN AND ANIMAL RIGHTS
No Animals were used in this research. All human research procedures were followed in accordance with the ethical standards of the committee responsible for human experimentation (institutional and national), and with the Helsinki Declaration of 1975, as revised in 2013.
CONSENT FOR PUBLICATION
Written informed consent was obtained from all the participants.
AVAILABILITY OF DATA AND MATERIALS
The data that support the findings of this study are available within the article.
CONFLICT OF INTEREST
The author declares no conflict of interest, financial or otherwise.
The authors declared that this study received no financial support or funding.