Considering the enhanced knowledge and higher demand of patients for smile esthetics, as well as the advances in dental science, tooth bleaching has become an increasingly popular dental procedure. Thus, several bleaching materials and techniques have been proposed for tooth bleaching. ¹ Evidence shows that 34% of adults are not satisfied with the natural color of their teeth, ² and most of them prefer shiny whiter teeth. As a result, many advances have been made in bleaching materials and techniques. To respond to this demand, the manufacturers introduced the bleach-shade composite resins, which are whiter than the whitest natural teeth.

The success of composite resins highly depends on their color stability over time. ³ , ⁴ However, despite great advances in this respect, poor color stability is still a drawback of many composite resins. ⁵

Esthetic failure and unacceptable color match are the main reasons for replacement of composite restorations. ² , ⁵ , ⁶ Furthermore, color change of composite resins due to their clinical service in the oral environment has been commonly reported. ⁷ Color change of composite resins depends on a number of factors such as the coloring agent, type of composite resin, and smoothness of the polished surface. The finishing and polishing procedures are performed to obtain optimal tooth anatomy and contour, and decrease the surface roughness and cracks, and are imperative for periodontal health, integrity of the margins, and reduction of wear. ⁸ Polishing of restoration surfaces minimizes plaque accumulation, gingival irritation, unesthetic appearance, surface discolorations, and risk of secondary caries. ⁹ A homogeneously smooth surface is imperative for optimal esthetics and long-term longevity and clinical service of composite restorations. ¹⁰ The optical properties and color stability of composite resins are influenced by the finishing and polishing procedures. ¹¹ A composite restoration should mimic the surface roughness and gloss of the adjacent natural teeth. ¹² Surface irregularities increase staining, and a rougher surface results in greater adsorption of pigments, leading to discoloration over time. ¹² , ¹³ , ¹⁴ A preliminary in vitro study indicated that a rougher composite surface was lighter and less chromatic than a smooth surface. ¹⁵ Thus, the composite surface should be comprehensively finished and polished to minimize color change. ⁵

Although using a Mylar strip would yield a highly smooth composite surface, additional finishing is often required. Clinically, dental clinicians use diamond and carbide burs and often diamond and aluminum oxide abrasive polishing systems for finishing and polishing of composite restorations in order to obtain a glossy smooth surface. ¹⁵ Some studies showed finishing and polishing systems containing diamond particles provides the least color difference on all composite groups. However, color differences of all composite resin groups were found to exceed the perceptibility threshold and acceptability threshold. ¹⁶ , ¹⁷

On the other hand, color change depends on the curing process as well. ¹⁸ , ¹⁹ Light-curing of composite paste changes its color. The in vitro color change (ΔE) of composite resins after curing, compared with baseline, is reportedly 3–12 units, and sometimes the L*, a*, and b* color parameters change in this process. ²⁰ In other words, the composites are darkened and become less chromatic by curing. Thus, it is recommended to use shade tabs of polymerized composites instead of unpolymerized composites for more precise shade matching. The degree of conversion of composite resins ranges from 60% to 75% in the clinical setting. Light-initiated polymerization increases the risk of incomplete conversion in the clinical operation. As a result, unreacted monomers remain. ²¹ Oxidation of unreacted double bonds is one cause of discoloration of composite resins. ³ , ¹⁴

The color stability of composite resins also depends on the type of curing unit and curing time. ¹⁸ , ¹⁹ Composite resins should be adequately polymerized to ensure their favorable optical and mechanical properties. Incompletely polymerized composite resins have higher susceptibility to water sorption and dissolution of unreacted monomers, which make them more susceptible to staining. ²² A previous study showed that composite specimens cured by a quartz tungsten halogen (QTH) curing unit had higher color stability than those cured by a plasma arc curing unit. Furthermore, they showed that 40 s of curing was ideal to achieve optimal color stability. ¹⁸ Another study revealed that curing time affected the color stability of composite resins, and specimens cured with plasma arc experienced much greater color change, irrespective of the curing time. ²³

Discoloration can be determined visually or by the use of advanced tools. ¹⁵ , ²⁴ Spectrophotometers and colorimeters are among the most commonly used tools for assessment of color change of restorative materials. Color change (ΔE) indicates the changes in L*, a*, and b* color parameters of specimens. ¹⁴

Since bleach-shade composite resins are relatively novel, there is a gap of information regarding their long-term color stability. The null hypothesis is:

The color parameters and polish ability of two composite resins would not change after curing with different settings of curing protocol

This in vitro experimental study was conducted on Gradia XBW and Filtek Z350 XT XBW Enamel Shade Table 1.{Table 1}

Specimen preparation and curing

A total of 64 disc-shaped specimens were fabricated from each composite with 8 mm diameter and 1 mm thickness using cylindrical plexiglass molds (each composite tube had at least 2 years until expiration). Two glass slides were placed at the bottom and on the top, and the mold containing composite was compressed between the two glass slides; a 5 kg load was applied for 30 s to obtain a uniform thickness of composite specimen and eliminate the voids. The following four modes were considered for curing of specimens in terms of intensity and duration of curing:

High-intensity curing (1000 mW/cm ²) for 20 s

The specimens were light-cured by a light-emitting diode (LED) curing unit (N Bluephase; Ivoclar Vivadent) according to the manufacturers' instructions. For all specimens the light guide was in contact with the glass slide and was in contact with the top side. The same was done for the bottom surface as well. The light intensity was controlled by a radiometer.

Composite discs with voids, fracture, or impurities were excluded and replaced. To allow complete polymerization, the specimens were stored in distilled water for 24 h.

Then, one point at the periphery of each disc was marked and perforated by a fissure bur.

Polishing procedures

Each group was then divided into two subgroups. Specimens in the first subgroup were polished with coarse, medium, fine, and extra-file Sof-Lex discs while the specimens in the second subgroup were polished with Jiffy polishing points Table 1.

Color measurement

Next, the primary color parameters of the specimens were measured (L0, a0, b0).

Vita EasyShade spectrophotometer (VITA Zahnfabrik, Rauter GmbH and Co., KG, Germany) was used to measure the primary color parameters of specimens according to the CIE L*a*b* color space. These values were recorded as baseline values. In this system, the L* parameter indicates lightness, and a* and b* indicate the chromatic coordinates (L* = 0 indicates darkness and L* = 100 indicates complete lightness; negative a* values indicate greenness and positive a* values indicate redness; negative b* values indicate blueness and positive b* values indicate yellowness). Color change in this system is indicated by ΔE. The color of specimens was measured against a white background, and the color parameters of each disc were measured three times. The device was calibrated after three color measurements using a calibration tile provided by the manufacturer.

Staining procedures

A thread was attached to each specimen using the previously created holes at the margins, and the specimens were immersed in tea solution for 3 h a day for a total of 24 days. For this purpose, each specimen was immersed in tea solution in a vertical position to minimize the deposition of stains on its surface. Furthermore, the specimens had no contact with the walls of the container. To prepare the tea solution, two tea bags (2 g; Yellow Label; Lipton, London) were immersed in 250 mL of boiling water for 3 min. The tea solution was prepared fresh daily. After completion of the staining period, the specimens were rinsed with distilled water for 1 min. It should be noted that only one side of the specimens was assessed in this study, and the other side was finished by a coarse disc after immersion to minimize color assessment errors. The color parameters of the specimens were then measured again and the L*, a*, and b* color parameters were recorded; ΔE was calculated and statistically analyzed.

Analysis and statistics

SPSS version 25 (IBM, SPSS Inc) was used for data analysis. Univariate four-way ANOVA was applied to assess the effects of composite type, curing time, curing intensity, and technique of polishing on ΔE of composite resins. Since the interaction effects were significant, subgroup analysis was performed by t-test. The level of significance for this test was set at 0.05.

According to t-test, curing time had a significant effect on ΔL, Δa, and Δb (P < 0.001) of Filtek Z350 XT XBW cured in high-intensity mode and polished with polishing points; however, it had no significant effect on ΔE (P = 0.222) Table 2.{Table 2}

Type of composite had a significant effect on ΔL (P < 0.001), Δa (P < 0.001), and ΔE (P = 0.027) and an insignificant effect on Δb (P = 0.229) of specimens cured in high-intensity mode for 20 s and polished by discs Table 2and Table 3.{Table 3}

Figure 1shows the ΔE of composite resins based on the curing intensity and time, and technique of polishing. Figure 1

ΔE of composite resins based on curing intensity and time, and technique of polishing.

This in vitro, experimental study assessed the effect of curing time and curing intensity of a LED curing unit as well as the effect of technique of polishing on color stability of two commonly used bleach-shade composite resins. The color change of composite resins was measured by EasyShade spectrophotometer using the CIEL*a*b* color space, which has advantages such as optimal repeatability, sensitivity, and objectivity. ²⁵ This technique has gained approval from the American Dental Association, ²⁶ and is suitable for assessment of even the slightest changes in color. According to some authors, ΔE values between 1 and 3 can be perceived by the naked eye, and values >3.3 are considered clinically unacceptable. ²⁷

In the present study, all specimens showed a color change visible to the naked eye (ΔE >3.3) after completion of the staining period, irrespective of their finishing technique. The use of tea as the staining solution in the present study was due to its high popularity among the Iranians. It has coloring agents that can cause significant color change in composite resins. All specimens were immersed in tea solution for 72 h (3 h a day for 24 days), corresponding to around 2 years of clinical exposure to coloring agents (24 h in vitro corresponds to 1 month in the clinical setting). This time period is suitable for assessment of long-term color stability of restorative materials. ²⁸ However, it should be noted that in vitro conditions of this study have considerable differences with the clinical setting in terms of mechanical washing, saliva, temperature, etc., The obtained results confirmed the first null hypothesis of the study regarding no effect of type of composite on its color stability since no significant difference was noted between the two composite resins regarding color stability (P > 0.05). The present study evaluated Z350 XT XBW nanofilled composite and Gradia XBW microhybrid composite.

Barakah and Taher ²⁹ reported that Z250 microhybrid composite showed a ΔE comparable to that of EvoCeram nanocomposite, which can be due to the replacement of TEGDMA with UDMA and bisEMA in this composite resin. They explained that the low stainability of the two composites was probably due to the low water sorption of hydrophobic resins. ²⁹ The similar ΔE values of Gradia GC microhybrid and Filtek Z350 XT nanocomposite in the present study may also be related to the presence of UDMA hydrophobic resin in their composition.

The present results were in contrast to those of Al Kheraif et al., ³⁰ who discussed that type of composite affects color stability. This difference in the results may be due to the fact that they used coffee solution as the coloring agent in their study. In line with our findings, Poggio et al. ³¹ found no significant difference in stainability of tested composite resins.

The present results rejected the second null hypothesis regarding no significant effect of finishing technique on color stability of composite resins. According to the results, the finishing/polishing technique has a significant effect on ΔE since it can affect the composite surface quality and influence the resistance of resin-based materials to staining. ¹² Beltrami et al. ²⁷ reported that finishing treatments significantly affected the color stability of esthetic restorative materials.

It has been confirmed that the smoothest surface is obtained by curing of restorative materials in direct contact with the Mylar strip. ³² Some abrasive tools such as flexible discs and finishing burs are used for recontouring of restorations and elimination of excess material. Evidence shows that polishing points with diamond particles create smoother surfaces than diamond finishing burs, tungsten carbide burs, or mounted stone polishing points. ⁹ , ³³ In the present study, Sof-Lex discs (Kerr) made from polyethylene, synthetic polymers, aluminum oxide, and epoxy resin glue and Ultradent Jiffy polishing points made from silicone rubber were used for polishing of specimens. Chung ³⁴ reported that aluminum oxide discs provided higher surface smoothness than the Enhance polishing system because they do not displace the composite fillers. ³⁴ Berastegui et al. ³⁵ reported that fillers of microfilled composites are very small; thus, their stiffness can easily decrease. Therefore, aluminum oxide discs are a better choice than tungsten carbide and diamond discs because they cause equal wear of filler and resin matrix due to their high flexibility. Herrgott et al. ³⁶ and Van Dijken and Ruyter ³⁷ discussed that aluminum oxide discs equally abrade the filler and resin matrix and therefore create a smoother surface. In the present study, specimens polished with the Sof-Lex discs experienced greater color change among almost all groups. Barbosa et al. compared eight finishing/polishing techniques and reported that complete set of Sof-Lex aluminum oxide discs used in the suggested order yielded the smoothest surface. The worst results were obtained by the use of diamond burs alone. ³⁸ Borges et al. ³⁹ reported that Sof-Lex aluminum oxide discs and Jiffy silicone polishing points yielded the smoothest surface compared with Enhance and KG polishing kits in the tested composites; however, they recommended silicone polishing points due to their easier clinical application. Dos Santos et al. ⁴⁰ compared the efficacy of Jiffy polishing points and Sof-Lex aluminum oxide discs and reported results similar to those of the present study. The Jiffy silicone polishing points caused lower roughness and resulted in lower color change compared with Sof-Lex discs.

The part of the third null hypothesis of the present study regarding no significant effect of curing time and curing intensity on color stability of composite resins was rejected. The results showed that curing time had no significant effect on color stability of composite resins while the curing intensity significantly affected it (P < 0.05).

With regard to the interaction effects of different variables, the results showed a lower color change in Filtek Z350 specimens cured with high-intensity mode and polished by polishing points (ΔE <19.24). However, comparison of high- and low-intensity modes showed that among the specimens polished with polishing points, those cured with low-intensity mode had higher color stability (ΔE <14.16). Comparison of polishing techniques revealed that Gradia XBW specimens that were cured with high-intensity mode and polished by polishing points experienced a lower color change (ΔE = 8.94). Comparison of high- and low-intensity modes also revealed that high-intensity curing mode caused a lower color change in specimens polished by polishing points while low-intensity curing mode caused a greater color change in specimens polished by discs (ΔE = 13.85). Thus, in total, all groups that were cured in high-intensity mode and polished with polishing points, except one, showed higher color stability than those cured in low-intensity mode and polished by polishing discs. This result confirmed the findings of Barakah and Taher, ²⁹ who reported superior results by polishing with polishing points. Teimourian et al. ⁴¹ evaluated the effects of curing time and intensity of a LED curing unit on color stability of methacrylate-based composite resins. Unlike the present study, they found that increasing the light intensity over the standard threshold (600 mW/cm ²) had no significant effect on color stability of composite specimens. However, changing the curing time significantly affected the color stability of methacrylate-based composite resins. Color change due to different curing times depends on the light intensity. In fact, they showed the significant interaction effect of light intensity and curing time on color stability of composite resins. Poorsattar Bejeh Mir and Poorsattar Bejeh Mir ⁴² assessed the effect of curing time on color change of composite resins and reported results different from our findings. They indicated that color change following longer curing time (20 s) was significantly greater than that following shorter curing time (10 s). This finding may be due to the immediate measurement of color change without aging. Pires-de-Souza et al. ⁴³ evaluated the color stability of three types of composite resins cured with QTH (500 mW/cm ²) and LED (320 mW/cm ²) curing units. They found no significant difference in the shades of the three composite resins, irrespective of the type of curing unit and light intensity. Teimourian et al. ⁴¹ revealed that doubling the light intensity did not cause a significant change in color stability of composite resins. Nonetheless, the generated heat in higher light intensities may compromise the vitality of the tooth. On the other hand, the standard curing time is 20 s in use of the majority of dental curing units. Assuming that light curing is performed in contact mode (light source is in contact with the restoration surface), this time would suffice for curing of a light-shade composite with 2 or 2.5 mm thickness. However, the tooth anatomy often prevents the close contact of curing unit with the restoration surface. Thus, 40 s of curing improves the degree of conversion at all depths, and is necessary for optimal curing of darker shades of composite resins. ⁴⁴ In total, it appears that the interaction of light intensity and duration of curing is the main factor affecting the color stability and stainability of composite resins (although prolonging the curing time may have no effect on the final result), as shown by Teimourian et al. ⁴¹ Therefore, polishing improves the staining resistance of composite resins. Nanocomposite resins did not exhibit better-staining resistance or surface roughness than microhybrid composite resin. ⁴⁵ Hence, it seems that more studies must be done about finishing procedures and curing time of different composite resins.

Filtek Z350 composite specimens polished by polishing points experienced lower color change than those polished with discs, compared with baseline. Gradia XBW specimens cured with high-intensity mode for 20 s and polished by polishing points experienced the lowest color change while maximum color change was noted in specimens cured with high-intensity mode for 20 s and polished with discs. In all groups, except one, polishing with polishing points and high-intensity curing resulted in lower color change than polishing with discs and low-intensity curing.

Financial support and sponsorship

Nil.

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.