Evaluation of Color Assimilation and Translucency of Monoshade Resin Composites: An In Vitro Study

INTRODUCTION

Color of a restorative material is influenced by the surrounding environment as a result of visual perception and subjective interpretation.¹ Visual perception is the critical factor in the overall acceptance of patients.² Instrumental methods such as spectrophotometer, colorimeter, and digital camera provide the details of color, its magnitude, and direction of their differences in an accurate manner.^3,4

Discrepancy in shade selection or shade guide results in unacceptable shade match of resin composite restorations.⁵ Restorations mimic the adjacent tooth structure when viewed together than in isolation by a phenomenon that is referred to many relative terms.⁶ They are color assimilation (CA), blending effect, color induction, and Bezold-Brücke effect.^1,6–8 Resin composites exhibit marked color-matching ability due to CA and translucency.⁴ Translucency can be measured using Commission Internationale de l’éclairage L*a*b* (CIELAB) units with minimal subjectivity.⁹

Recently, a monoshade resin composite (MC) has been introduced (Omnichroma, Tokuyama Dental Corporation, Tokyo, Japan) for direct anterior and posterior restorations.¹⁰ It reduces the patient’s chair side time and composite inventory in the dental clinic due to its versatile shade-matching properties. Sanchez et al. observed better color matching of MC than other branded enamel and cloud shade composites when compared with the posterior acrylic denture tooth.¹¹ A question exists as to whether the color matching of MC restorations would be acceptable when compared with conventional shaded resin composites (SCs) in general and especially anterior teeth.

Therefore, the aim of this research was to determine the difference in color and translucency using disks followed by CA of MC in comparison with different SCs, using anterior and posterior teeth-like composite models. The null hypothesis tested was that there is unacceptable color-matching ability of MC with the commonly used different SC. The alternate hypothesis tested was that the MC has acceptable color-matching ability with the different SC.

MATERIALS AND METHODS

This study was conducted at the College of Dentistry, Jazan University, Saudi Arabia. Five VITA shades (B1, A1, B2, A2, and A3) of supra-nano filled SC (Estelite Sigma Quick, Tokuyama Dental Corporation)¹² and supra-nano filled MC (Omnichroma) were used to prepare disk and teeth-like models (Table 1). The sample calculation was performed by using the G*Power version 3.1.3 for Windows software (Microsoft, Redmond, WA, USA). Based on the effect size (3.7) from the previous study,¹³ the calculated sample size was 12 by applying the alpha level of 0.05 and power of 95%. Although human teeth were not involved in this study, approval from the institutional review board was obtained as part of the protocol (CODJU-19121). Flowchart 1 represents the overall experimental plan in a comprehensive manner.

RESULTS

The ΔL*, Δa*, Δb*, and ΔE* values of MC vs SC disks are represented in Table 2. The mean ΔE* value was lowest for B1 (6.4 ± 2.2) followed by A1, B2, A2, and highest for A3 (p = 0.0005; one-way ANOVA). Intergroup comparison revealed no difference when comparing A1 (6.6 ± 2.2) with B1 (6.4 ± 2.2) and B2 (9.3 ± 2.0) (p %3E; 0.05; Tukeys post hoc). The TP value varied from 15.3 to 6.6 (Table 3) with highest value for MC (15.3 ± 4.3), suggesting its high translucency (p < 0.01; Tukeys post hoc).

Mean and standard deviation (SD) for VS and CA of MC vs SC disks and models are given in Table 4. Mean ± SD values of VS for disks unfolded a small difference of 4.0 ± 0.4 and 4.2 ± 0.2 for B1 and A1 shades, respectively; hardly acceptable difference of 2.1 ± 0.3 for B2; totally unacceptable difference of 1.3 ± 0.1 and 1.2 ± 0 for A2 and A3, respectively. Mean VS of models revealed a small difference in the range of 4–5 for all shades except for an acceptable difference of 3.6 ± 0.1 and 3.9 ± 0.1 in A2 (anterior) and B1 (posterior), respectively.

The CA values for both anterior and posterior models were very low in the range of 0.1–0.3 for shades B1 and A1, respectively. Meanwhile, CA ranged from 2.1 to 3.4 for the shades B2, A2, and A3 for the twain models. The CA of both the models increased with increase in ΔE* having a strong positive correlation (r = 0.8). The TP showed a weak negative correlation with decreasing values for increase in CA of anterior (r = –0.2) and posterior models (r = –0.3). The CA of anterior models showed a very strong positive correlation (r = 1) with that of posterior models.

Overall mean ± SD values of interobserver agreement were 64% ± 21%, 61% ± 13%, and 58% ± 11% for the disks, anterior models, and posterior models, respectively; whereas overall intraobserver agreement was 84% ± 6%, 76% ± 10%, and 80% ± 11% for the disks, anterior models, and posterior models, respectively.

DISCUSSION

The results of the present study revealed that MC was showing significant color-matching ability (ΔE*, TP, and CA) among all the shades of SC with large F value (Tables 2 to 4). Hence, the null hypothesis was rejected and the alternate hypothesis was accepted for the investigation.

All the eight observers selected for this study were tested for normal color vision using the Ishihara test to rule out deuteranomaly, protanomaly, tritanomaly, and achromatopsia.⁶ Value is one of the three dimensions of color that is determined by the tendency of teeth to reflect more or less light from the illuminating source.¹⁸ The types of SC selected for the present experiment are the commonly used shades arranged in the order of increasing darkness based on value.¹⁹ The diameter of disk was decided as 10 mm to closely approximate the tooth size and enable convenient measurement using a spectrophotometer.⁴ In addition to ΔE* and TP, the size of the restoration influences the CA (often referred as the blending effect in literature) of a material.⁷ Therefore, standardized dimensions of class I and class IV cavities were prepared for all models.

Measuring the tooth color using the instrumental method includes measurement of spectral reflectance either directly using spectrophotometer or indirectly using spectroradiometer. Most of the commercially available spectrophotometer is able to measure the chromacity coordinates and CIELAB values in a most accurate manner.¹⁸ The spectrophotometer used in this study captures a digital image of selected area using LED lights and analyzes the spectral data in a computer by the CIELAB method. The edge loss error on curved tooth surfaces was prevented by the noncontact mode of this device.²⁰

Overall, as we proceed from lighter to darker shades, the ΔE* was gradually increasing due to variation in the optical light reflectance properties of different SC materials.¹⁸ On the other hand, TP values were showing a decline toward darkness with MC being the topmost on the list, which is attributed to uniform structurally colored filler particle size owing to high transparency (Table 1).^7,10

The viewing distance in visual assessment was decided as 25 cm to facilitate minimum viewing angle of 2° similar to the previous studies.^3,4 Although the categorical-judgment method has been recommended instead of the mean category method in order to avoid the linearity in the VS, the results of the later method have been proven to be simple, valid, and reliable.⁷ Hence, the CA was calculated as difference in mean VS of the teeth-like models and disks using the mean category method.

The two common phenomena that decide the color-matching ability of restorations are simultaneous color contrast and CA. The former occurs due to color shift of resin composite to the complimentary color of the surrounding material thereby increasing the perceived color difference in large surface area. The latter occurs due to reduction in perceived color difference when viewed together in small surface area near the margins of the restoration.^6,7 Color shifting of restoration comprises of CA and translucency. It occurs either way at the tooth-restoration junction depending upon the straight-line and diffuse light transmission properties of the resin composite used.²⁰

Various factors influence the light transmission characteristics of tooth structure and restorative materials. They are particle size, distribution, surface texture, gloss, shade, color, and translucency.^7,21 Polishing and size of the restoration have direct influence on the CA of a material.²² Actually, CA represents the optical phenomenon of interaction between materials to minimize the color difference when viewed together. Therefore, higher the values of CA, higher will be the visual perception of color matching in reality.¹³ Consequently, it enables the clinician to compensate for the color mismatch or unavailability of a particular shade of resin composite material while performing the shade selection.

In the current study, CA values are inversely proportional to TP (r = −0.2 and −0.3) and directly proportional to ΔE* (r = 0.8). It is in disagreement with the reports of Paravina et al.^4,7 but consistent with that of Abdelraouf et al.¹³ This could be attributed to the difference in the type of material tested and methodology used for the investigation. The former evaluated microhybrid composites of different brands using the categorical-judgment method whereas the later evaluated the universal shade hybrid and shaded nanohybrid composites of same manufacturer using the mean category method. Omnichroma had been proven to have relatively appreciable color-matching ability.^3,11 It is a nonpigmented structurally colored material with uniform supra-nanospherical filler particles produced by the sol-gel method. The size and shape of filler particles enables them to reflect red–yellow wavelength of the incident light. The reflected light of MC combines with that of surrounding tooth structure to give a perfect match.^10,11 Accordingly, Omnichroma was able to adjust the CA according to the surrounding SC as deemed necessary, especially for darker shades where the color matching is of pivotal importance.

Well-trained eight standard observers performed the visual assessment. Therefore, a strong positive correlation (r = 0.8) was observed between CA (visual perception) and ΔE* (instrumental evaluation), which is consistent with the previous studies.^4,7 Interestingly, a very strong positive correlation was observed between the CA of anterior and posterior models (r = 1). This could be attributed to the standard observation of disks and models against the gray background in contrast to intraoral situation. On the contrary, intraoral anterior restorations require a thin lingual wall of radiopaque blocking agent for high esthetics.¹⁰ The interobserver differences were lower than that of intraobserver due to increase in number of observers.^4,7,13 Intraobserver difference was ranging from 76 to 84%, which is closely approximating (81–88%) with that of previous studies.^4,7,13 Nevertheless, both the inter- and intraobserver agreement was remarkably higher than that of Barna et al.²³

The limitations are the use of teeth-like resin composite models in our study in order to mitigate the effect of variables associated with natural teeth such as color variations. However, the natural teeth might behave in a different manner that needs further exploration. Moreover, the color compatibility of resin composites for a given shade among different manufacturers is a concern eventually demanding evaluation of MC with SC of different manufacturers.

CONCLUSION

Under the constraints of this investigation, MCs displayed high translucency and convincing perceptual CA with all the tested shades of both anterior and posterior models, particularly the darker shades. Further investigations are required to evaluate the monoshade composite as anterior restorations in the patients in order to appreciate its color-matching ability amidst interferences offered by various intraoral structures.

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