Many factors affect human smile design such as form, position, and color of teeth. In a study,¹ tooth related factors were found to be more important than orthodontic problems. This shows that the appearances of teeth are more significant than the position of teeth. Bleaching is the most conservative tooth whitening method. Vital bleaching techniques include in-office technique, referred to as power bleaching, and at home bleaching. These techniques may be used separately or combined.²

Oxygenating agents such as hydrogen peroxide (HP) are used in tooth bleaching to diffuse into enamel and dentin.³ Some studies have demonstrated that the sodium fluoride in bleaching agents improves enamel remineralization through increasing the inorganic crystals containing fluoride. Using different desensitizing agents by the patients either combined with whitening agent or separately before or after bleaching may affect bonding of resin to tooth structure.⁴

High concentration of HP used in office bleaching materials is accelerated by light or heat.⁵ Light sources such as light-emitting diode (LED) light-curing units, lasers, and ultraviolet (UV) light (ZOOM! Advanced) accelerate bleaching process with different mechanisms. In a study, Wetter et al. showed that diode laser causes less tooth and gingival sensitivity than HP alone does.⁶ Today, LED is very popular among dentists because it is available in almost all dental offices, but its affection on microshear bond strength (MSBS) of composite resin to dentin and enamel is not clear yet. ZOOM! Advanced power system (Discus Dental, USA) has a different effect on HP other than lasers. This system accelerates tooth bleaching with UV waves, and in this way, it improves bleaching agents' effectiveness and consequently decreases the chairside time.⁷

There may be composite resin restorations in the teeth subject to bleaching. Little is known about the effect of in-office bleaching on bond strength of composite resin restorations to tooth structure. In a study,⁸ the effect of bleaching on the bond strength of restoration to tooth was evaluated and found that it depends on the type of adhesive system and restoration material. However, another studies showed that bleaching cannot damage bond interface.²,⁵

One of the most reliable methods to determine tooth color is spectrophotometery. This method shows color of teeth in CIELAB system. The three last letters in the CIELAB refer to the three opponent process dimensions: A* for red-green contrast; b* for yellow-blue contrast; and L* for luminosity dimension or whiteness, and it is proportional to the power of the light reflected from the object's surface.⁹

Hence, the aim of this study is in vitro evaluation of the effect of different light sources on the MSBS of a composite resin to enamel and dentin and also on teeth color in office bleaching. The null hypothesis is whether new methods of bleaching could be more reliable to use on composite restorations.

Specimen preparation

In this in vitro study, approved by the Human Research Ethics Board of Babol University of Medical Sciences, Babol, Iran, 50 extracted noncarious human third molars were collected and stored in 0.2% thymol solution at 4°C for no longer than 3 months. Teeth were cleaned and randomly divided into two groups of E and D equally. Class V cavities were prepared with the dimensions of 4 mm × 4 mm × 1 mm (length × width × depth) in Group E to have enamel axial walls and 4 mm × 4 mm × 2 mm in Group D to have dentinal axial walls. Cavities were prepared 2 mm cervical to the buccal cusp tip.

Bonding procedure

Cavities were etched by applying 35% phosphoric acid (3M ESPE, St. Paul, MN, USA) Table 1 for 30 s on enamel and 15 s on dentin and then were rinsed. Enamel surfaces were gently air-dried, and dentinal surfaces were dried with cotton pellet. Cavities were then treated with Single Bond 2 (3M ESPE, St. Paul, MN, USA) Table 1 adhesive system, gently air-dried, and cured with UltraLume 2 (Ultradent Products, Inc., South Jordan) LED-curing unit (400 mw/cm²) for 20 s. Composite resin (Filtek Z250, A2) (3M ESPE, St. Paul, MN, USA) Table 1 was placed incrementally with 1 mm in thickness for any increment and cured for 40 s. Finishing and polishing were done with Sof-Lex finishing and polishing disc system (3M ESPE, St. Paul, MN, USA) from coarse to superfine in 30 s. Teeth were stored in 24°C incubator in water until perfuming the bleaching procedure.{Table 1}

Bleaching procedure

The teeth in Groups E and D were divided equally into five subgroups of E1 and D1 through E5 and D5, respectively. Subgroups E1 and D1, as the control subgroups, received no bleaching treatment.

Subgroups E2 and D2 were bleached with 25% HP (chairside whitening gel, Discus Dental, USA) Figure 1a. Double syringe and self-mixing tip delivery devices, designed for this system, were used to mix the two components in the right proportion and preparing an activated 25% HP. The devices were washed and refilled with fresh bleaching materials every 15 min for 1 h. In all other groups in addition to applying bleaching material in the same way as subgroups 2, the teeth were irradiated with UV-emitted unit (Zoom! Advanced power, Discus Dental, USA) Figure 1b for 60 min, in Subgroups E3 and D3. And with UltraLume2 LED curing unit Figure 1d for 5 * 30 s with 15 min intervals (400 mw/cm2) in E4 and D4 subgroups Also in E5 and D5 subgroups, irradiation with diode laser (Dr. Smile, Italy, 815 nm) Figure 1c for 5 *10s with 15 min intervals. In all the subgroups, the manufacturer's instructions were followed.Figure 1

(a) Twenty-five percent hydrogen peroxide, (b) Ultraviolet-emitted unit (Zoom! Advanced power), and (c) diode laser (Dr. Smile). (d) Light-emitting diode-curing device (UltraLume 2).

Table 2 displays the means and standard deviations of MSBSs of composite resin to enamel and dentin. One-way ANOVA for enamel MSBSs did not show significant difference between the groups. The same analysis showed a significant difference in MSBSs of composite resin to dentin (P < 0.00). In these groups, MSBS values decreased, but there were no significant differences between them. Table 3 shows the modes of failure in enamel and dentin subgroups. Adhesive fracture in all of the groups was the most frequent failure mode.{Table 2}{Table 3}

Table 4 shows the mean and standard deviation values of ΔE. Kruskal–Wallis test showed that ΔE significantly increases with diode laser, UV, and LED accelerators (P < 0.034). Laser was the most effective instrument to change ΔE, followed by UV and LED, respectively.{Table 4}

In this study, Tukey's test showed that MSBS of composite resin to enamel did not decrease significantly after bleaching but decreased significantly to dentin. Other studies also showed similar results in dentin with different concentrations of carbamide peroxide and HP.⁸,¹⁰ About 25% HP forms different amounts of oxygen radicals depending on temperature, light, PH, co-catalysts, and other conditions. Hydroxyl free radical (HO₂⁻) that is formed from H₂O₂ is extremely reactive and has great oxidative power. It may break up large stain macromolecules into smaller molecules and by diffusion expel them to the surface.¹¹ It is also thought to be attached to stain molecules in the inorganic structure as well as matrix proteins.¹² The free radicals eventually combine to form oxygen molecules and water. Some aspect of this chemical process may accelerate the hydrolytic degradation of composite resins.¹³ Hence, bleaching agents might have some effects on bond degradation of composite resin if it could penetrate to tooth structure. Some studies show that bond strength would not decrease after bleaching.²,⁵ On the other hand, many studies suggest that some composite resins may be more susceptible to alteration, so various adhesive systems and composite resins show varying results.¹⁴,¹⁵ Cavalli et al., 2005, evaluated the effect of bleaching on the composite resin restorations bonded with self- or total-etch adhesive systems. The results showed that bond strength of composite resin to enamel would decrease if it was bonded with self-etch CLEARFIL SE bond. However, total-etch adhesive system (Single Bond 2) had stronger initial bond and bleaching would not decrease it so much. Furthermore, bleaching agents at higher concentrations can cause more changes than lower ones.¹⁴ Far and Ruse, 2003, found that fracture strength of composite resin to dentin would decrease after bleaching with carbamide peroxide at higher concentrations than 15% and re-bonding of restorations was necessary after bleaching procedures.¹⁶ Stocks et al., 1992, stated that HP has negative effects on shear bond strength of resin to enamel.¹⁵ Ulukapi et al., 2003, and Turkun et al., 2004, suggested that nonvital bleaching with 10% carbamide peroxide can increase microleakage score.¹⁷,¹⁸ Moosavi et al., 2009, also found postoperative bleaching with carbamide peroxide increased microleakage in dentinal margins.¹⁰ Klukowska et al., 2008, explored the effects of different concentrations of HP and carbamide peroxide agents on the enamel margin microleakage of composite restorations.¹⁹ In their study as well as in White's research,²⁰ bleaching agents could not increase the microleakage scores of Filtek Z250 bonded with scotch bond. This controversy is maybe because of different study design and using older generations of adhesive systems.

In this study, the lowest MSBS to dentin after bleaching was belonged to UV group. It can be related to special treatment plan of this device. The latest version of Zoom! Advanced power uses White Speed LED bleaching lamp (LED) in the range of 350–400 nm (violet light). An important aspect of the new White Speed unit is that it emits some UV light too. After application of HP, Zoom! Advanced power (UV wave) irradiates for 60 min, but with LED and laser, the duration of irradiation is 30 and 15 s, respectively. The negative effects of UV irradiation on the bond interface between composite resin and tooth structure may be time sensitive. On the other hand, we should know that if bleaching agent could penetrate into tooth structure, it may affect bond strength.

The most frequent mode of fracture in enamel and dentin groups was adhesive fracture, but it was less frequent in enamel groups Table 3, and it is consistent with the lowered bond strength, due to bleaching, in enamel groups compared to dentin groups Table 2. More frequent cohesive failure or mixed fracture represents the higher strength of bond compared to the strength of substrates.

Different light sources can accelerate bleaching process. In this study, diode laser was the most effective light in tooth color change followed by UV and LED, respectively. Several other investigations also showed this result. In a study that compared the effect of plasma arc and diode laser on tooth whitening and tooth sensitivity, the authors mentioned that while both devices improved the effectiveness of bleaching agents, diode laser lowered tooth sensitivity more significantly that was more acceptable for the patients.²¹ Luk et al. 2004 studied the effect of different laser devices on bleaching.²² They found that light sources accelerated only the special bleaching agent's reaction and also pulp temperature must be monitored during the process. CO2 laser increases pulp temperature and can damage it, but diode laser increases temperature under the deleterious limit (5.5°C).

Zoom! Advanced power system also had positive effects on whitening the teeth. Gallagher et al., 2002, and Toko et al., 2005, in their studies found that this system is better than halogen and xenon lamps and Opalescence Xtra boost kit for teeth bleaching.²³,²⁴

Then, under limitations of this study, the followings may be concluded:

All light sources in this study could accelerate bleaching process and cause brighter teeth and using them is suggested

Acknowledgment

We would like to thank Babol Dental Material Research Center and Research Council of Babol University of Medical Sciences.

Financial support and sponsorship

Research Council of Babol University of Medical Sciences.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.