The RI technique is a new technique used to prevent the development of caries, which can treat early caries without drilling or sacrificing healthy tooth structure around caries21).And now is demonstrated that restoring demineralization, dissolution and abrasion on enamel by resin infiltration is a promising approach for the aesthetic treatment of post-bleaching teeth. It confirms the results obtained in this study. The infiltrant resin used for restoration possesses very low viscosity, high surface tension, and low contact angle with the enamel, all of which are vital properties for penetration of the resin into the body of the enamel.
As is known to all, micropores could occur in the surfaces of enamel after tooth bleaching, which are visible by SEM. The relatively intact enamel surface is slightly damaged. There are many tiny pores that provide the channels for organic acid and minerals diffusing and moving, which can promote bacterial adhesion. After applied with RI, the color of the demineralization area can be similar to the normal teeth because the refractive index of Icon resin is 1.475, close to the refractive index of enamel (1.62). RI uses low-viscosity resin to penetrate, plug, and fill the micropores of the demineralization to form a barrier and avoid disintegration of the enamel22). The present experiment found that the Icon resin penetrated into the demineralization area through SEM observation. The resin penetration depth of etched teeth was deeper than that of the bleached ones without etching (Mann-Whitney test, P < 0.05). The result of deeper penetration may due to the acid. The acid etching technique can punch holes in the enamel surface and can completely remove the surface layer and fully expose the demineralization area23), which can make the resin penetrate into the micropores as deep as possible. While for the VHN value, though the penetration depth increased, the VHN value of the etched ones was less than the bleached ones without etching at the same time(Mann-Whitney test, P < 0.05). Moreover, we also found that Icon resin sometimes only penetrated to a certain depth, while some micropores still existed deep in the enamel. Based on our SEM result, etching with 15% HCl might cause excessive erosion. Though the resin penetration depth of the bleached teeth with etching were highest, it came at the cost of worse damage of micromorphology after etching. Tamer et al. created the artificial initial caries lesion with an average depth of 200 µm and it showed that the Icon penetration depth in the lesion was about 70%, 140 µm which was deep enough to prevent the further demineralization24). However, there was still some demineralization deep in the enamel due to the 15% HCl erosion. Whether deep demineralization can further affect the long-term function of bleached teeth is still unclear and needs further studies. Another reason we took into account was the saliva. The constant presence of saliva and fluoride may minimize changes in the enamel through the deposition of calcium fluoride crystals, mitigating the effects of demineralization, prolonging tooth health, and preventing changes that may damage the structure of bleached teeth25). Furthermore, extensive studies indicated that the bond strength of bleached teeth with a delay of resin bonding were higher than that of the bleached teeth bonded immediately, which had been attributed to the decomposition of oxygen free radicals accountable for the whitening effect in HP diffusing in enamel and adversely influencing the penetration of the bonding agent into the tooth surface and then inhibiting the polymerization of resin26,27).
Current study used artificial saliva for the experiment. The results showed that with a 5-day delay of RI by placing in artificial saliva for 5 days, the VHN values of the bleached teeth could regain closed to the normal one and better than the other groups(Mann-Whitney test, P༞0.05). Additionally, the penetration depths of the delayed ones were deeper than the bleached teeth which were resin infiltrated immediately༈Mann-Whitney test, P༜0.05༉. Based on the results, we inferred that several elements might account for the result. First, excessive erosion with 15% HCl leading the demineralization caused the VHN values decrease. Secondly, decomposition of oxygen free radicals inhibited the polymerization of resin which decreased the bonding strength. As for the delayed ones, it could be attributed to the partial loss of oxygen diffusion layer over time at the tooth and composite interface28) and the constant presence of saliva mitigating the effects of demineralization would account for it. Thus, it is suggested to delay the Icon resin infiltration treatment after tooth bleaching. However, many clinical doctors choose to perform the resin bonding after tooth bleaching immediately. Because both tooth bleaching and RI containing erosion, the immediate combination of these two techniques may cause excessive erosion, decreasing VHN value and polymerization. Barghi et al.19,20) indicated that ethanol could eliminate the oxygen free radicals. Thus it is suggested that if the patients required to perform the Icon resin infiltration after tooth bleaching immediately, 15% HCl etching was not needed.
In addition, all resins, including the Icon®-DMG Germany, have a disadvantage: aging. Furthermore, clinical long-term follow-up is lacking. Therefore, further studies of the combined application of tooth bleaching and Icon resin infiltration should be performed including long-term follow-ups to study the effects.