Translucency and Masking Ability of Single-Shade Opaque Composite Resin Compared with Conventional Opaque Composite Resins According to Thickness

Article information

J Korean Acad Pediatr Dent. 2025;52(3):312-321

Publication date (electronic) : 2025 August 22

doi : https://doi.org/10.5933/JKAPD.2025.52.3.312

Junmo Jeong, Jongsoo Kim, Joonhaeng Lee, Jongbin Kim, Jisun Shin, Miran Han

Department of Pediatric Dentistry, College of Dentistry, Dankook University, Cheonan, Republic of Korea

Corresponding author: Miran Han Department of Pediatric Dentistry, College of Dentistry, Dankook University, 119 Dandae-ro, Dongnam-gu, Cheonan, 31116, Republic of Korea Tel: +82-41-550-1867 / Fax: +82-41-550-0118 / E-mail: miraneee@dankook.ac.kr

Received 2025 May 23; Revised 2025 July 12; Accepted 2025 July 14.

Abstract

Introduction

The restoration of the anterior teeth in pediatric and adolescent patients is often required because of caries, trauma, or discoloration. Compared with posterior restorations, anterior restorations demand greater esthetic considerations, requiring shade harmony with adjacent teeth as well as functional recovery.

Recently, a single-shade composite resin, Omnichroma (Tokuyama Dental, Tokyo, Japan), has become available. The material consists of homogenized spherical fillers measuring 260 nm, which allow ambient light to interact with the surrounding tooth structure and generate a shade that naturally blends with the adjacent dentition [1]. As a result, this single-shade composite resin may reduce clinical chair time by simplifying the shade-matching process [2]. However, for Class III or IV cavities or teeth with intrinsic staining, where sufficient background tooth structure is lacking, or the background shade is inappropriate, the esthetic outcome may be compromised [3].

These limitations can be addressed, for example, by using layering techniques with opaque-shade composite resins to mask undesirable background shades [4-6]. The Omnichroma Blocker (OB; Tokuyama Dental) is a highly opaque, single-shade composite resin designed for thinlayer applications prior to Omnichroma placement. It has a masking function similar to that of the conventional opaque-shade composite resins. Previous studies have reported that a minimum resin thickness of 1.0 – 2.0 mm is required to mask intraoral darkness simulated by a black background, and at least 0.5 – 1.5 mm is necessary to mask stained tooth structure represented by a C4-shade background [7]. However, few studies have specifically evaluated the masking ability of Omnichroma Blocker [3]. Therefore, this study aimed to investigate and compare the masking ability and translucency of Omnichroma Blocker with those of conventional opaqueshade composite resins of various thicknesses.

Materials and Methods

1. Materials

1) Opaque-shade Composite Resins

Three opaque-shade composite resins were used: Omnichroma Blocker, Filtek Z350 XT A2D (FZ; 3M ESPE, St. Paul, MN, USA), and Gradia Direct Anterior AO2 (GD; GC Corp., Tokyo, Japan). Details of the composite resins used in this study are listed in Table 1.

Table 1.

Composite resins used in this study

2) Background Plates

Three background plates were prepared: white tile, black tile, and a C4-shade porcelain plate. A white tile (L* : 95.78, a* : -0.23, b* : 2.73; Lucideon, Stoke-on-Trent, Staffordshire, UK) and a black tile (L* : 26.4, a* : -0.09, b* : -1.41; Lucideon, Stoke-on-Trent) were used for translucency measurements. The black tile served as a simulated intraoral dark background. A C4-shade porcelain plate (L* : 62.34, a* : 3.32, b* : 16.52; VITA VMK Master Dentine C4, VITA Zahnfabrik, Bad Säckingen, Germany) was used to simulate the discolored tooth structure.

3) Spectrophotometer

A spectrophotometer (CM-26d; Konica Minolta, Osaka, Japan) was used to measure the CIE L*a*b* values of the composite resin specimens against each background.

2. Methods

1) Specimen Preparation

Specimens were fabricated using circular Teflon molds with a diameter of 8.0 mm and thicknesses of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm for each opaque-shade composite resin. The composite resin was inserted into the mold, covered with Mylar strips on both surfaces, and pressed to form a flat surface. Polymerization was performed using a light-curing unit (B&LiteS; B&L Biotech, Ansan, Korea) according to the manufacturer’s instructions. Ten specimens with different thicknesses were prepared.

Additional specimens with a thickness of 4.0 mm were fabricated to evaluate the inherent color of each opaqueshade composite resin. A schematic illustration of the specimen preparation process is provided in Fig. 1, and the CIE L*a*b* values of the opaque-shade composite resins used in this study are presented in Table 2. According to a previous study [4], composite resins, even highly translucent enamel shades, exhibit inherent shades independent of the background influence when the thickness is 4.0 mm or greater.

Fig 1.

Schematic illustration for resin specimen preparation.

Table 2.

CIE L*a*b* values of 4.0-mm-thick specimens of the opaque-shade composite resins

2) Shade Measurement

The CIE L* , a* , and b* values of the resin specimens were measured against each background using a spectrophotometer. Calibration was performed before each measurement according to the manufacturer’s instructions. Measurements were repeated three times under the standard illuminant D65, and the average value was recorded for each specimen [5].

3) Translucency Measurement

Translucency was assessed by calculating the translucency parameter (TP) using the color differences of resin specimens measured on white and black tile backgrounds, using the following equation [4,8-10]:

TP=[(L*w−L*B)2 + (a*w−a*B)2 + (b*w−b*B)2]1/2

The subscripts W and B indicate the values measured against white and black tile backgrounds, respectively. A higher TP value indicates greater translucency.

4) Masking Ability Measurement

The masking ability against intraoral darkness was evaluated by calculating the color difference (ΔE^*_ab ) between each resin specimen placed on a black background and its intrinsic color. Similarly, the ability to mask discolored tooth structures was measured using the color difference between the resin specimens placed on the C4-shade porcelain and their intrinsic color. The following formula was used:

ΔE*ab=[(ΔL*)2+(Δa*)2+(Δb*)2]1/2

A lower ΔE^*_ab value indicates less influence of the background on the resin shade, reflecting better masking performance [6]. Based on ISO/TR 28642 and previous studies, the clinically acceptable color difference threshold was set at ΔE^*_ab ≤ 2.7 [11-13], and this criterion was used to determine the minimum resin thickness required for adequate masking.

Results

1. Translucency Parameter Measurement

The results of the TP measurements are presented in Table 3 and Fig. 2. At all thicknesses, the TP values in increasing order were FZ, OB, and GD, with significant differences observed among the three groups at each thickness (p < 0.0167). In all groups, the TP values decreased significantly as the specimen thickness increased (Table 6, p < 0.001).

Table 3.

Translucency parameters of opaque-shade composite resins at various thicknesses

Fig 2.

Translucency parameters of opaque-shade composite resins at various thicknesses.

TP: Translucency parameter; OB: Omnichroma blocker; FZ: Filtek Z350 XT A2D; GD: Gradia direct anterior AO2.

Table 6.

Correlation coefficients between specimen thickness and TP or ΔE*_ab values for each composite resin

2. Background Masking Ability

1) Black Background Masking Evaluation

The ΔE^*_ab values between the colors of the resin specimens on the black tile and their intrinsic colors are shown in Table 4 and Fig. 3. For black background masking, the ΔE^*_ab values in increasing order were OB, FZ, and GD across all thicknesses. Significant differences among the three groups were observed at thicknesses of 0.5, 1.0, 1.5, and 2.5 mm, and a significant difference between the OB and GD groups was found at 2.0 mm (p < 0.0167). All three groups exhibited ΔE^*_ab values below 2.7 at a thickness of 1.5 mm or greater, and ΔE^*_ab values decreased significantly with increasing thickness (Table 6, p < 0.001).

Table 4.

ΔE*_ab values between the colors of resin specimens on the black tile and their intrinsic colors

Fig 3.

ΔE^*_ab values between the colors of resin specimens on the black tile and their intrinsic colors. The horizontal line represents a clinically acceptable threshold.

OB: Omnichroma blocker; FZ: Filtek Z350 XT A2D; GD: Gradia direct anterior AO2.

2) C4-Shade Background Masking Evaluation

The ΔE^*_ab values between the colors of the resin specimens on the C4-shade porcelain plate and their intrinsic colors are presented in Table 5 and Fig. 4. For C4-shade background masking, all three groups showed ΔE^*_ab values below 2.7 at all tested thicknesses, with values significantly decreasing as thickness increased (Table 6, p < 0.001). Significant differences were observed between the OB and GD groups at 0.5 mm, between the FZ group and the other two groups at 1.5 mm, between the GD group and the others at 2.0 mm, and between the OB and FZ groups at 2.5 mm (p < 0.0167).

Table 5.

ΔE*_ab values between the colors of resin specimens on the C4-shade porcelain plate and their intrinsic colors

Fig 4.

ΔE^*_ab values between the colors of resin specimens on the C4-shade porcelain plate and their intrinsic colors. The horizontal line represents a clinically acceptable threshold.

OB: Omnichroma blocker; FZ: Filtek Z350 XT A2D; GD: Gradia direct anterior AO2.

Discussion

Esthetics is a primary consideration when restoring anterior teeth. Achieving shade harmony with adjacent teeth is a priority. The recently developed single-shade composite resin, which incorporates homogenized spherical fillers, enables natural color matching with the surrounding dentition [1]. Consequently, single-shade composite resin has the potential to shorten esthetic restoration procedures by simplifying the shade-matching process [2]. However, owing to its sensitivity to the underlying tooth color, inappropriate background shading may compromise shade integration [3].

In particular, extensive Class III or IV cavities and severely stained or discolored teeth pose challenges in achieving esthetic outcomes owing to a lack of surrounding tooth structure or the presence of undesirable background shades. To minimize the impact of a background shade, a thin layer of an opaque-shade composite resin should be applied prior to using single-shade composite resin. Variations in both the background shade and composite thickness can affect the final restoration outcome [14]. In this study, a black tile and a C4-shade porcelain plate were used as background materials. The black tile simulated intraoral darkness typically seen in Class III and IV cavities. In contrast, the C4-shade porcelain plate represented discolored teeth, as it is considered an extreme discoloration due to its lowest L* value among the VITA classical shade guides [15-17]. Previous studies have reported that the C4 shade closely resembles intrinsic tooth discoloration, such as that caused by trauma or medication-induced staining [18,19].

Kamishima et al. [4] reported that even highly translucent enamel shade resins exhibit inherent colors that are unaffected by background colors when applied at a thickness of 4 mm. They also showed that specimens with a thickness of 4 mm reflected their inherent shade without being influenced by the underlying background. Thus, 4.0-mm-thick specimens were fabricated in this study to evaluate the intrinsic color of each composite resin.

In the present study, ΔE^*_ab values decreased with increasing specimen thickness. These findings are consistent with the results from previous studies [3,6,20,21]. The clinical acceptability threshold for ΔE^*_ab varies in the literature (ΔE^*_ab ≤ 2.0 [22], ΔE^*_ab ≤ 2.7 [11,12], ΔE^*_ab ≤ 3.3 [23], and ΔE^*_ab ≤ 3.7 [24]). Although reported thresholds vary across studies, the present study adopted a ΔE^*_ab threshold of 2.7 in accordance with the ISO/TR 28642.

All groups were able to mask the black background at a minimum thickness of 1.5 mm (ΔE^*_ab ≤ 2.7). The OB group exhibited the lowest ΔE^*_ab values at all thicknesses, potentially due to its lower L* value and distinctive filler composition. A lower L* value was associated with a greater masking ability [6]. In contrast, the highest ΔE^*_ab values observed in the GD group at all thicknesses are likely associated with its higher TP values, as recorded in this study. Although slight variations in ΔE^*_ab were observed among the three materials at certain thicknesses, all groups masked the C4-shade porcelain background at all tested thicknesses (ΔE^*_ab ≤ 2.7). These results suggest that even a relatively thin layer of opaqueshade composite resin is sufficient to effectively mask discolored tooth structures. This finding is consistent with results from previous studies [3,7].

However, interpretations of these results depend on the selected ΔE^*_ab threshold [6], and these thresholds can be influenced by viewing conditions and inter-observer variability [10]. Moreover, measured values can differ significantly depending on the light source (e.g., natural vs. incandescent light) [25,26]. Another study that used the 25 VITA classical shade guides found that even tabs labeled with the same shade exhibited color differences ranging from 0.75 to 3.05 [27]. These findings highlight that experimental conditions and human perception can significantly affect color evaluation results. Therefore, when comparing findings across studies, it is important to consider potential variations arising from factors such as differences in measurement devices and experimental protocols. To address this issue, standardizing experimental conditions is essential, and adopting established guidelines such as those from the ISO standard may enhance the reliability and reproducibility of results and allow more meaningful comparisons across studies [28]. Specifically, ISO/TR 28642 outlines several recommendations for achieving such standardization. According to this guideline, color measurements are recommended to be performed under CIE standard illuminant D65 with a color rendering index (CRI) of 90 or higher. For observer parameters, the use of a 2° standard observer defined by the 1931 CIE system is recommended for the assessment of teeth and dental restorations. In terms of measurement geometry, the CIE recommends different types of optical geometries for reflection, including 45° /0° and diffuse/0° geometries, which are considered most suitable for visual color evaluation in dentistry. The surrounding area of the specimen background should be matte and neutral in color, preferably a light gray, to minimize environmental influence. In addition, the use of a spectrophotometer enables more flexible and comprehensive measurement compared to a colorimeter, making it preferable for instrumental color evaluation in dental research. Furthermore, when interpreting the measurement results, setting the perceptibility threshold at ΔE^*_ab = 1.2 and the acceptability threshold at ΔE^*_ab = 2.7 can provide a clinically applicable standard for addressing color matching issues in dentistry [13].

Translucency is the ability of a colored material to allow the appearance of an underlying background [29]. It is affected by various factors, including material thickness, filler type and content, and added pigments [8,30]. The translucency parameter (TP), which is derived from the color differences between black and white backgrounds, is commonly used to assess translucency [3]. Although TP is a commonly used index to represent the translucency of composite resins, it does not always accurately predict masking ability. In general, a positive correlation exists between TP and ΔE^*_ab, as more translucent materials tend to exhibit greater color differences [20]. However, a limitation of TP is that it reflects only the relative translucency of the material under standardized conditions and does not account for other clinically relevant factors, such as interactions with discolored backgrounds or the influence of underlying cement layers or stains. This represents a methodological disadvantage compared to ΔE^*_ab, which more directly assesses actual masking ability [28]. While Kamishima et al. [4] suggested a TP threshold of 2.0 to determine masking capacity against black backgrounds, another study suggested that ΔE^*_ab values, calculated between the black background and the material’s inherent shade, provide a more clinically relevant measure [6]. Therefore, both TP and ΔE^*_ab should be considered together to reliably assess the masking ability of composite resins.

In this study, the TP values decreased with increasing thickness, which is in agreement with current evidence [3,6,20,21]. Across all thicknesses, the TP values were the lowest in the FZ group, followed by the OB and GD groups, with significant intergroup differences. However, in the black background masking evaluation, the OB group exhibited significantly lower ΔE^*_ab values at 0.5, 1.0, 1.5, and 2.5 mm than the other groups. This indicates that TP is not the sole determinant of a material’s masking ability [6].

This study has several limitations that should be considered when interpreting the results. First, the masking ability of composite resin is influenced by a combination of multiple factors, including translucency, filler composition, opacifiers, and pigments. Therefore, it is difficult to clearly identify which specific factor had the strongest influence on the TP and ΔE^*_ab values [20]. Second, masking ability was evaluated only at selected thickness levels. Consequently, the exact minimum thickness at which the ΔE^*_ab value falls below the 2.7 threshold could not be determined. In particular, specimens thinner than 0.5 mm were not included, which restricts the evaluation of masking ability at clinically relevant thinner layers over C4-shade backgrounds. Third, in clinical practice, opaque-shade composite resins are typically layered beneath other composite resins to achieve optimal esthetics. However, this study evaluated the opaqueshade composite resins in isolation, which may not fully reflect their performance in multilayered restorations. Future studies should consider layered specimens to better simulate clinical application and visual outcomes. Finally, color measurements in this study were obtained using a spectrophotometer, which provides objective data but does not account for subjective human perception. Since esthetic outcomes are ultimately evaluated by clinicians and patients, additional studies incorporating visual assessments by observers could enhance the clinical relevance of the findings.

The findings of this study may serve as a clinical reference for the selection and application of opaque-shade composite resins for the restoration of stained or discolored teeth and extensive Class III and IV cavities. Clinically, restorations are performed in the intraoral environment; therefore, a more comprehensive approach is essential. Furthermore, when using a single-shade composite resin, the selective use of an opaque-shade composite resin, when needed, may further reduce the time and complexity of shade matching, making it particularly advantageous in pediatric dentistry, where shorter procedure times are preferred.

Conclusion

This study evaluated and compared the translucency and masking ability of three opaque-shade composite resins. In all groups, TP and ΔE^*_ab values decreased as specimen thickness increased, and TP values were lowest in the order of FZ, OB, and GD. The black background was masked at a thickness of 1.5 mm or greater for all composite resins, whereas the C4-shade porcelain background was masked at a thickness of 0.5 mm or greater.

In conclusion, the Omnichroma Blocker showed a masking ability comparable to that of conventional opaque-shade composite resins, Filtek Z350 XT A2D and Gradia Direct Anterior AO2. When using these resins for background shade masking, a minimum thickness of 1.5 mm is recommended for masking intraoral darkness, and at least 0.5 mm is recommended for masking stained or discolored tooth structures.

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