Investigation of the physical and morphologicalchanges caused by the use of experimentalnanohydroxyapatite gel on the enamel surface afterthe application of different vital whitening agents

The aim of this study is to examine the physical and morphological changes caused by the use of experimental nanohydroxyapatite gel on the enamel surface after the application of two different vital whitening agents containing 40% hydrogen peroxide. In our study, 52 human maxillary incisors extracted for periodontal reasons without caries, cracks or hypoplastic defects were used. The teeth were separated from their roots and embedded in acrylic resin with the buccal surface of the crowns on top and polished for surface standardization. Chitosan added nanohydroxyapatite(n-HA) powder was synthesised. This powder was mixed with phosphate buffered saline solution to form a gel. Teeth were divided into four groups. Biowhiten In-office 40% n-HP (Group B), Opalescense Boost 40% PF (Group O), Biowhiten In office 40% +n-HA gel (Group Bn), Opalescense Boost 40% PF+ n-HA gel (Group On). Color, surface microhardness and surface roughness (Ra) measurements were done before, after and 1 week after bleaching from the tooth surfaces. SEM/EDS analysis and elemental mapping analysis of three samples from each group were performed. Ca/P ratios of enamel surfaces were evaluated. For surface microhardness and surface roughness results, analysis of variance test was used and Bonferroni test was used for pairwise comparisons. Welch ANOVA test was used to anayse the data of color change (∆E). Significance was determined at the p<0.05 level. There were no differences between the color changes, microhardness and roughness values of groups that was measured immediately, after bleaching and 1 week after bleaching (p>0.05). Visible color change was detected in all groups after bleaching treatment (p<0.05). There were no differences between the treatment groups in terms of color change (p>0.05). When the test groups were compared after bleaching, highest Ra value was observed in Group O (p<0.05). Group On showed statistically lower Ra value than Group O (p<0.05). Microhardness values were decreased immediately after bleaching in all groups (p<0.05). Group O showed the lowest microhardness value after bleaching (p<0.05). Group Bn showed statistically higher microhardness value than Group B (p<0.05). Group On showed a statistically higher microhardness value than Group O (p<0.05). As a result of SEM images, the roughest and most irregular surface was seen in Group O. Enamel surfaces were seen more regular, smooth and non-homogeneous particle deposition was determined in Group On, Group Bn, and Group B. As a result of EDS analysis, the lowest Ca/P value was observed in Group O. After n-HA application, an increase was observed in the Ca/P ratio of the enamel surface. In conclusion, Opalescense Boost PF 40% HP vital whitening agent caused the highest surface roughness and the lowest microhardness value on the tooth surface. In SEM EDX analysis, the lowest Ca/P ratio observed in this group supports microhardness and surface roughness data. It was observed that after applying Biowhiten In-Office 40% n-HP to the enamel surfaces, higher Ca/P was observed and a smoother surface was created.. The use of experimental n-HA gel with chitosan applied after the bleaching agent increased the Ca/P ratio on the enamel surface and it had a positive effect on the surface microhardness and roughness. It was concluded that the use of experimental n-HA gel gave positive results to the physical and morphological properties of the tooth surface without any negative effects on the whitening effectiveness. Keywords: Whitening, hydrogen peroxide, nanohydroxyapatite, microhardness, roughness, calcium.