INTRODUCTION

Shade matching of resin composite restorations is of crucial concern in esthetic dentistry. However, there seems to be several difficulties in shade matching for resin composite restorations. Regarding tooth color, Russel and others revealed that teeth become brighter after drying.1 Therefore, for filling procedures, all shade matching procedures should be performed before dehydration. When using the Vita Lumin shade guide tab system, it was noted that color differences exist between some composites and the corresponding Vita shades.2–4 In addition, as each Vita Lumin shade guide tab actually represents a gradation of different shades from the cervical to the incisal aspect,4 the color varies along the length of the shade tab. Thus, the Vita Lumin shade guide system seemed to be inadequate as a “standard” for precise shade selection of tooth-colored restorative materials. As for individual manufacturers' shade guides, Kim and others reported that the majority of the shade guides do not accurately depict the true shades of resin composites, as the shade guides are often made of acrylic resin.5 Hence, the authors suggested making custom shade guides from the material itself. The custom shade guide seems to be one of the best solutions for accurate shade selection, but it involves time and material in making the shade guide, which may be inconvenient for clinicians.

An alternative way to accurately shade match is to compare the color of the tooth to be restored with the shade of the paste of the material itself.5–6 This direct shade matching procedure is as follows: a small amount of resin composite is placed on a tooth to be restored and is polymerized, then shades of the resin and tooth are compared.6 This procedure seems to be especially beneficial in clinical cases in which the background color has a direct effect on the shade of the restorative resin composite used, as the material contacts the background directly. For example, when restoring shallow, discolored cervical lesions or fractures in the incisal part of anterior teeth, the clinician can estimate the effect of the background color by employing direct shade matching.

In direct shade matching, light curing is necessary, as materials often show perceptible color changes during polymerization.6–9 One of the problems in direct shade matching is the time wasted with each material placed.5 However, if the optical properties, such as color and translucency of the resin composites do not change during light curing, direct shade matching can be performed without light curing, resulting in less time spent. Therefore, stability in color and translucency during light curing is an important property in esthetic restorative materials.

This study evaluated color and translucency changes caused by light curing newer restorative resin composite materials. The null hypothesis tested was that the color and translucency of resin composite materials do not change during light curing.

METHODS AND MATERIALS

Tooth-colored Materials

The materials (Table 1) selected for this study were 2 newer resin composites, Solare (GC, Tokyo, Japan) and Filtek Supreme (3M, St Paul, MN, USA) and a resin composite that has been in clinical use for some time, Charisma (Heraeus-Kulzer, Irvine, CA, USA). The A2 and OA2 shades of Charisma, A2 and AO2 shades of Solare and the A2B and A2D shades of Filtek Supreme were used. As the names of the same opaque shades vary in different products, for simplicity, the OA2, AO2 and A2D shades were given the generic name “opaque A2” in this study. Similarly, the A2B of Filtek was described as “A2.”

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Calculation of the Translucency Parameter

The translucency of the materials 2-mm thick before and after light curing was calculated using the translucency parameter (TP) formula10–12:

where the subscript “W” refers to the CIELAB values for each 2-mm thick specimen on a white backing and the subscript “B” refers to the values for specimens on a black backing. TP is the color difference between a uniform thickness of the material on black and white backings and corresponds directly to common visual assessments of translucency.10–12

This calculation of TP was performed for the values obtained before and after light curing. To detect any statistical changes in TP during light curing, as primary factors, 2-way ANOVA and Tukey's post-hoc test were carried out regarding materials/shades and before or after light curing. When this analysis revealed interaction of any of the primary factors, 1-way ANOVA and Tukey's post-hoc tests were employed to detect statistically significant differences between groups (p<0.05).

Calculation and Evaluation of Color Differences (ΔE*) During Light Curing

The color change of each specimen (ΔE*) during light curing was calculated using the equation:

, where L*_after, a*_after, and b*_after were CIELAB values of each specimen evaluated on the backing of the material itself after light curing and L*_before, a*_before, and b*_before were those values of each specimen evaluated on the backing of the material itself before light curing.

The averages of ΔE* during light curing were statistically analyzed using 2-way ANOVA, considering the products and shades were primary parameters. When this analysis revealed interaction of any of the primary factors, 1-way ANOVA and Tukey's post-hoc tests were employed to detect statistically significant differences between groups (p<0.05).

The L*, a* and b* changes during light curing were also evaluated statistically.

RESULTS

The values for TP before and after light curing are indicated in Table 2. Statistical analysis was performed using 1-way ANOVA, and Tukey's post-hoc tests, as an interaction between the 2 primary factors, was detected in the 2-way ANOVA.

Table 2 Translucency Parameters Before and After Light Curing

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Regarding TP changes during light curing, the A2 and opaque A2 shades of Charisma showed a statistically significant increase, although no difference was observed in the other products. As for the TP value after light curing, the opaque A2 shade of Charisma revealed a higher TP compared to the opaque A2 of the other 2 materials, although no significant difference was observed in the A2 shade among the 3 products.

Table 3 summarizes the results in color differences (ΔE*) during light curing. The ΔE* of A2 shade was a significantly greater value compared with the opaque A2 shade. As for the products, Charisma showed significantly greater color changes during light curing than Solare and Filtek Supreme.

Table 3 Color Change (ΔE*) During Light Curing

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Changes in L*, a* and b* values are indicated in Table 4. A 2-way ANOVA was attempted for each parameter; however, interactions were detected in all parameters. In addition, the hypothesis of homogeneity of variance was rejected in the Levene test. Hence, 1-way ANOVA and the Games-Howell tests were performed for each parameter.

Table 4 L*, a*, b* Before and After Light Curing

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Comparing the L* values before and after light curing, no significant differences were detected except for the opaque A2 shade of Filtek Supreme. No significant differences were observed in the a* values except for the A2 shade of Solare. However, regarding the parameter of b*, statistically significant decreases were indicated in all products and shades.

DISCUSSION

The inherent translucency of resin composites may contribute to shade matching with a tooth by allowing the shade of the adjacent and underlying tooth structure to shine through. Clinicians have commonly observed this “chameleon” effect of resin composites.13

However, in situations where there is no tooth structure to provide a backing for the restoration, such as in a large Class III or IV cavities, translucent materials may provide relatively poor color matches. More specifically, a grayish shade is often seen in comparison with the surrounding tooth structure, as relatively translucent materials are probably affected by the darkness of the oral cavity. In such situations, opaque-shade resin composites have been utilized. 1214–15

Charisma was selected in this study because, in a previous study, this product showed a relatively small change in TP and color during light curing,10 while the remainder of the products are newer resin composites. In this study, the A2 and opaque A2 shades of Charisma showed a statistically significant increase in TP. Hence, the null hypothesis on translucency was rejected for this product. This increase in TP may result in a more grayish shade after light curing when used in large Class III and IV cavities. Therefore, direct shade matching for a large Class III or IV cavity using uncured paste may be inadequate in this product. Furthermore, the TP of the opaque A2 shade in this product after light curing showed a statistically greater value than that of the other 2 products. The greater TP value might be a disadvantage against the dark background of the oral cavity.

As for the TP and color change of Charisma, the TP values before and after light curing were smaller, and the color change during light curing was greater in this study compared with that of another study.10 The reason for the differences may be due to differences in methodology, such as in specimen preparation and the colorimeter employed.

It was previously reported that color differences of less than 3.3 CIELAB units were clinically acceptable to match tooth structure.16 This implies that the opaque A2 shades of Solare and Filtek Supreme may help to mask a dark background color more effectively. However, as too much opacity of resin composite may be less desirable against relatively translucent natural teeth, there is a case for the layering technique, as recommended by the manufacturer of Filtek Supreme.

There are numerous reports about factors that contribute to the opacity of resin composites. Inokoshi and others17 stated that, the greater the difference between the refractive indices of inorganic particles and the matrix phase of resin composites, the greater the opacity of the materials, due to multiple reflection and refraction at the matrix particle interfaces. Campbell, Johnson and O'Brien18 stated that, in experimental PMMA resin composites, the efficiency of light scattering for a quartz filler decreased as the size of the filler increased. Kawaguchi, Fukushima and Miyazaki15 mentioned that certain types of hybrid resin composites could show smaller transmission coefficients because of the wide range of particle size. Johnston and Reisbick10 insisted that the color and translucency of esthetic restorative materials is determined not only by more macroscopic phenomena, such as matrix and filler composition as well as filler content, but also by relatively minor pigment additions and potentially by all other chemical components of these materials. In this study, the relatively small translucency changes in the newer materials, Solare and Filtek Supreme, may be due to minimal change in differences between the refractive index of the glass filler and that of the resin matrix during light curing.

Regarding the TP change observed in Charisma in this study, it is likely that light curing brought about a change in the optical characteristics of the resin matrix. Hence, the TP increase observed could be caused by a corresponding decrease in the refractive index difference between the inorganic particles and the matrix phase of the resin composite.

As for the color change (ΔE*) values during light curing, a significantly smaller ΔE* was observed in Solare and Filtek Supreme compared to Charisma. However, all the ΔE* values were above 3.3, which was considered a clinically unacceptable color difference as mentioned above.16 Therefore, direct shade matching using uncured resin composites seemed inadequate for these products. This study has highlighted the fact that, in order to get a precise shade match, direct shade matching of these materials should be performed by using the cured material.

In terms of the L*, a* and b* change during light curing, no statistical change was indicated, except for the opaque A2 of Filtek Supreme in L* change and the A2 of Solare in a* change, though all the products/shades showed a significant decrease in b* value. The same phenomenon, in terms of a decrease in b* values, was also reported by Seghi and others.7 These authors explained that the reason for the color change was due to a decrease in absorption of blue light by photo initiators, such as camphoroquinone, after light curing.7 Although Johnston and others10 attached importance to the L* value as an indicator of color change, in this study, the b* values appeared to reflect color changes, which agrees with Seghi and others.7

The technique employed in this study generated considerable data to evaluate color and translucency change during light curing. One of the advantages of the technique is that the CIELAB parameters on the 3 backings before and after light curing can be obtained from the same specimen in order to make the data “paired data.” As a result, in this study, statistical analysis of TP and color change during light curing became possible. In addition, the authors could evaluate inherent colors of the specimens by means of the estimation on the backing of the material itself. As resin composites are more or less translucent, evaluation on the backing of the material itself may minimize the influence of the backing color in the evaluation of inherent color of the materials.

CONCLUSIONS

This study showed that, depending on the irradiation; newer restorative resin composite products tend to show less change in TP and color during light curing. However, changes in color during light curing may still be within the range of unacceptable color change. Therefore, for precise shade matching, direct shade matching of these materials should be performed using cured paste or a shade guide made with the resin composite itself.