Introduction
In direct tooth restoration using composite resins, shade-matching is crucial for aesthetics. Natural teeth vary in color due to their position and individual differences [1]. Therefore, clinicians must choose the appropriate composite shades that match the restored and adjacent teeth. Traditional layering techniques require dental practitioners to use multiple shades of material to replicate the various teeth [2]. However, in response to the demands of everyday dental practice, there is a growing preference for ideal restorative materials that simplify the process by offering easier shade selection and more efficient application, ultimately reducing chairside time [2,3].
Introducing single-shade composite resins represents a paradigm shift in this approach. This innovative material was designed to adapt to the surrounding tooth structure, eliminate the need for shade selection, and simplify the restorative process [4,5]. Single-shade composite resins use a structural color phenomenon based on precisely sized spherical fillers (approximately 260 nm in diameter) designed to generate a red-to-yellow structural color as light passes through the material [6,7]. Instead of the inherent color of traditional pigments, the visible color is based on light interacting with the nanostructure of the material. This color range corresponds to the primary color components of natural teeth, theoretically allowing the material to adapt to various tooth shades without requiring added pigments or dyes [6].
The blending effect is a visual phenomenon in which the color disparity between natural teeth and restorative materials appears less noticeable when they are directly adjacent to each other [8]. This effect makes the restoration appear to match the surrounding tooth color more closely than if viewed separately. Various factors, including the optical properties of the restorative material, the characteristics of the surrounding tooth structure, and restoration dimensions, influence the blending effect [9]. Of these, the translucency of restorative materials plays a crucial role. It is characterized by the proportion of light that traverses an object after absorption and scattering [8,10]. Translucency enables the restoration to pick up the color of the surrounding tooth, potentially leading to more natural-looking results [1].
Color adjustment potential (CAP) was introduced to quantify the ability of a material to blend with the surrounding tooth structure. CAP is typically measured by comparing the color difference between the restoration material and the tooth in isolated and adjacent configurations [11]. A higher CAP value indicates a better ability of the material to match the surrounding tooth color when placed in a cavity.
Single-shade composite resins offer promising advantages, but various clinical factors, such as size and thickness of the restoration, degree of the color difference between the tooth and restoration, amount of remaining tooth structure, and surrounding tissues, may influence their performance [8,12]. As the thickness of a restorative material increases, its optical properties and ability to blend with the surrounding tooth structure may change [2,8]. This is particularly relevant in the treatment of dental caries, where cavity depth varies depending on the extent of decay. In deeper cavities, the increased thickness of the restorative material can affect its color properties and ability to match the surrounding tooth structure, potentially impacting the aesthetic outcome of the treatment.
Despite the growing interest in single-shade composite resins, previous research in this field has several limitations. Most studies have focused on evaluating shade matching ability at a single depth, failing to account for the variety of cavity depths encountered in clinical practice [6,7]. This knowledge gap makes it challenging to predict the performance of single-shade composites across different clinical scenarios. Furthermore, while some studies have examined either CAP or translucency individually, comprehensive research investigating both properties simultaneously and their potential interrelationship is lacking [6,7,13]. To the best of our knowledge, little is known about the effect of cavity depth on CAP and translucency, specifically in primary teeth. The purpose of this study was to evaluate the influence of cavity depth on the CAP and translucency of single-shade composite resins compared with multi-shade composite resins in primary molar denture teeth. By examining these properties across different cavity depths, we aimed to provide insights into optimal clinical single-shade composite resin applications and contribute to the ongoing development of aesthetic restorative materials.
Materials and Methods
1. Color adjustment potential test
In this study, we used a single-shade composite resin (OM, Omnichroma, Tokuyama Dental, Tokyo, Japan) and two multi-shade composite resins (FT, Filtek™ Z350 XT, 3M ESPE, St. Paul, MN, USA; ES, Estelite® Sigma Quick, Tokuyama Dental, Tokyo, Japan) (Table 1). Drawing from a study on primary tooth shade, we selected the A1 shade as the shade for both multi-shade composite resins [14]. We also used primary maxillary second molar denture teeth from Resin Natural Teeth™ (Nissin Dental Products Inc., Kyoto, Japan) in color group No.1 of type A (25BZ0023, Lot: 060238). The prepared denture teeth were classified according to the study process (Fig. 1, Table 2).
1) Specimen preparation
The evaluation included two types of specimens: single (Fig. 2) and dual (Fig. 3). For single specimens, a total of 30 primary second molar denture teeth replicas were created using the experimental composite resins with molds made from silicon (Exafine Putty Type, GC Co., Tokyo, Japan) to duplicate the teeth (n = 10 per group). Curing was performed using an LED light (VALO™, Ultradent, South Jordan, UT, USA), with an output power of 1,400 mW. The specimens were polished using Sof-Lex™ XT (3M ESPE, St. Paul, MN, USA) for 30 seconds in each step, corresponding to four different abrasive grades with coarse to superfine. After this procedure, all specimens were submerged in distilled water for 24 hours.
For dual specimens, a total of 60 specimens were fabricated and divided into three groups (n = 20 per group) depending on the composite resin used: OM, FT, and ES. Based on the cavity depth, we divided each of the three main groups into two subgroups (n = 10 per subgroup): 1.5 mm subgroups and 3 mm subgroups. Standardized cavities (4.0 × 4.0 × 1.5 mm for the 1.5 mm subgroups and 4.0 × 4.0 × 3.0 mm for the 3 mm subgroups) were prepared centered on the cervical end of the buccal groove on each primary maxillary second molar denture tooth. The cavities were created using a #330 carbide bur in a high-speed handpiece. The size of cavities was standardized using a digital caliper (Navimro, Seoul, Korea) to ensure uniform preparation. The preparation process began by etching the cavities with a 37% phosphoric acid gel (Vericom Co., Ltd., Chuncheon, Korea) for 15 seconds. After thorough rinsing and drying, an 8th-generation bonding agent (Scotchbond™ Universal Adhesive, 3M ESPE, St. Paul, MN, USA) was applied and light-cured for 15 seconds. The cavities were filled with the experimental composite resins. A mylar strip and glass slab were pressed against the filled cavities before light curing for 15 seconds. This step was designed to create a flat surface, ensuring consistent cavity depths and uniform restorative material thickness across all specimens. The specimens were polished using Sof-Lex™ XT for 30 seconds in each step, corresponding to four different abrasive grades from coarse to superfine. Following this procedure, all specimens were submerged in distilled water for 24 hours.
2) Color value measurement
A colorimeter (Shade Eye-NCC, Shofu Co., Kyoto, Japan) was used to measure the Commission Internationale d’ Eclairage (CIEL*a*b*) color parameters (L*, a*, b* values) with D65 lighting, corresponding to average daylight.
The color difference (ΔEab*) for each specimen was determined using this formula:
ΔEab* = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2.
ΔL* represents changes in lightness, while Δa* and Δb* indicate shifts in red-green and yellow-blue chromaticity, respectively. We calculated the CAP of each experimental composite resin using the formula below:
CAP = 1 - (ΔEDual / ΔESingle).
ΔESingle was computed as the color difference between the single specimen and the untreated primary second molar denture teeth. ΔEDual represented the color difference between the center of restoration in the dual specimens and the denture teeth prior to creating the restoration.
The colorimeter was calibrated using the manufacturer’s instructions. For each specimen, the color value measurements were repeated three times, and the same operator calculated the average values.
2. Translucency test
A total of 60 specimens were fabricated and divided into 3 groups (n = 20 per group) depending on the composite resin used: OM, FT, and ES. Based on the specimen thickness, we divided each of the three main groups into two subgroups (n = 10 per subgroup): 1.5 mm subgroups and 3 mm subgroups. Table 2 shows the classifications of specimens in the study.
1) Specimen preparation
Each specimen was created using the experimental composite resins. The specimens measured 8.0 mm in diameter and had thicknesses of 1.5 mm and 3.0 mm. The process involved filling a Teflon mold with each composite resin, followed by compression using a Mylar strip beneath a glass slab to ensure a smooth surface and remove any excess material. Curing was performed using an LED light with an output power of 1,400 mW, following the manufacturer’s instruction. After curing, all specimens were submerged in distilled water and left at 37℃ for 24 hours.
2) Color value measurement
A colorimeter measured the CIEL*a*b* color parameters at the center of the top surface of each specimen with D65 lighting, corresponding to average daylight. These measurements were taken against black (L* = 24.9, a* = 0.6, and b* = -0.8) and white (L* = 93.0, a* = 1.0, and b* = -3.2) backgrounds. The colorimeter was calibrated before each measurement according to the manufacturer’s guidelines. The color value measurements were repeated three times for each specimen, and the same operator calculated the average values.
The translucency parameter (TP) was calculated using the following formula:
TP = [(L*B ‒ L*W)2 + (a*B ‒ a*W)2 + (b*B ‒ b*W)2]1/2.
‘W’ and ‘B’ represent the CIEL*a*b* values for each specimen when placed on a white background and a black background, respectively. This calculation quantified the difference in color coordinates between the two background conditions to assess the translucency of the material.
3. Statistical analysis
Statistical analysis was performed with IBM SPSS version 28.0 (SPSS Inc., Chicago, IL, USA). We used the Shapiro-Wilk test to assess data normality. Statistical significance was determined using the Kruskal-Wallis test. For more detailed comparisons between specific pairs, we applied the Bonferroni post-hoc analysis. We set the significance threshold at e 0.0167 for comparison between the composite resin groups. In addition, the Mann-Whitney U test was applied with a significance threshold of p < 0.05 for comparison between two thickness or depth subgroups.
Results
1. Color adjustment potential test
The means of ΔEab* of single and dual specimens were demonstrated in Table 3. The highest ΔESingle was shown in OM, meanwhile, OM exhibited the lowest ΔEDual in both specimens of 1.5 and 3.0 mm depth. Fig. 4 and Table 4 show the means and standard deviations of CAP. OM demonstrated the highest ΔESingle compared with the other two composite resins. Between the other two multi-shade composite resins, ES showed a significantly higher ΔESingle than FT. In the 1.5 mm and 3.0 mm subgroups at dual specimens, OM demonstrated the smallest ΔEDual among the three experimental composite resins. OM showed a significantly higher CAP than FT and ES (p < 0.0167 for all comparisons), but the CAP values of FT and ES did not differ significantly (p = 0.114 in 1.5 mm subgroups; p = 0.017 in 3.0 mm subgroups), regardless of the cavity depth. In all three experimental composite resin groups, the 1.5 mm depth dual specimens showed significantly higher CAP values than the 3.0 mm depth dual specimens (p < 0.05 for all comparisons).
2. Translucency test
Fig. 5 and Table 5 show the means and standard deviations of TP. For both specimen thicknesses, OM showed a significantly higher TP than the other two multi-shade composite resins (p < 0.0167 for all comparisons). No significant difference was observed between the TP for FT and ES (p = 0.222 in 1.5 mm subgroups; p = 0.037 in 3.0 mm subgroups). The 1.5 mm thickness subgroups showed significantly higher TP values than the 3.0 mm thickness subgroups for all three composite resin groups (p < 0.05 for all comparisons).
Discussion
In this study, we aimed to evaluate the influence of cavity depth on the CAP and translucency of a single-shade composite resin compared with two multi-shade composite resins. CAP quantifies a material’s ability to blend with the surrounding tooth structure [11], while translucency describes the extent to which light can pass through a material[1], both being crucial for the natural appearance of restorations. Our results demonstrated that the single-shade composite resin had superior CAP and translucency across different cavity depths and specimen thicknesses, offering insights into these materials’ performance in clinical scenarios.
The higher CAP values observed for OM in the 1.5 and 3.0 mm cavity depths suggest a greater ability to match the surrounding tooth color compared to the multi-shade composite resins. Our finding aligns with previous studies, which showed that the single-shade composite resins have higher CAP than other multi-shade composite resins [11,15]. Upon polymerization, the precisely engineered monomer dimensions and configuration result in a structural color production, eliminating the need for supplementary colorants [2]. Interestingly, while OM showed the highest ΔEab* values in single specimens, it demonstrated the smallest ΔEab* in dual specimens for both cavity depths, suggesting that the single-shade composite resin has a greater capacity to blend with the surrounding tooth structure when placed in a cavity, despite having a more noticeable color difference when viewed in isolation.
Clinically acceptable color differences are crucial for evaluating the performance of dental restorative materials. According to previous studies, a ΔEab* value of 3.7 or less is considered excellent and clinically imperceptible [11]. In our study, OM showed a ΔEab* of 3.6 in the 1.5 mm depth cavities, which is below the excellent threshold, suggesting that OM can achieve a clinically imperceptible color match in shallow cavities. In contrast, both FT and ES showed higher ΔEab* values even in shallow cavities, indicating a noticeable color difference.
The translucency results further supported the superior optical properties of the single-shade composite resins. The higher TP values observed for OM in both 1.5 and 3.0 mm thicknesses may contribute to its enhanced blending effect and CAP. According to a previous study, the higher the translucency of the composite resin restoration, the higher the CAP [6]. A higher translucency allows more of the underlying tooth color to influence the final appearance of the restoration, potentially explaining the improved color matching capabilities of the single-shade composite [10,16,17]. In composite resins, translucency is influenced significantly by the interaction between the resin matrix and inorganic fillers [18]. Similar refractive indices between these components result in minimal light scattering, leading to high translucency [18,19]. Conversely, disparate refractive indices cause increased light refraction and reflection at the matrix-filler boundaries, thereby reducing translucency [18,20]. During the polymerization process, OM refractive index changes from 1.47 to 1.52, increasing transparency after curing [6]. The manufacturer attributes the high translucency of OM to a carefully engineered combination of filler characteristics, specifically uniform spherical particles measuring 260 nm, and a precise filler fraction of 79% by weight [21]. This formulation was designed to achieve a close match between the refractive indices of the polymerized resin matrix and the filler [21]. In addition, the effectiveness of the structural color of composite resins depends on critical factors, such as the filler size and refractive index [15]. Manufacturers employ a bottom-up colloidal assembly approach known as the ‘sol-gel method’ to achieve the precise filler characteristics required for structural color [15,22]. This technique allows for the creation of uniformly sized spherical particles with a defined refractive index [15,22].
Consistent with previous studies on the blending effect, we observed that CAP and TP values decreased with increasing cavity depth from 1.5 mm to 3.0 mm for all tested composite resins [1,8]. This decrease in both CAP and translucency as cavity depth increases can be attributed to several interconnected factors. As the thickness of the composite resin increases, the path length for light transmission becomes longer, resulting in more opportunities for light to interact with the material’s components. This leads to increased scattering and absorption, consequently reducing both the blending effect and overall translucency as less light reaches the underlying tooth structure and returns to the surface [19]. In thicker layers, light encounters more filler particles, increasing the likelihood of scattering events. This cumulative effect of multiple scattering interactions contributes to reduced light transmission and increased opacity [23]. Additionally, the difference in refractive indices between the resin matrix and filler particles becomes more pronounced in thicker layers. This mismatch causes light to bend and scatter at each resin-filler interface, further reducing translucency and the material’s ability to blend with the underlying tooth structure as the number of these interfaces increases with depth [24]. Furthermore, as the restoration thickness increases, multiple layers of composite resin are often required due to curing depth limitations. Each layer interface can introduce additional light scattering and refractive index changes, further compromising the overall optical properties of the restoration [25]. These layer interfaces can act as optical discontinuities, potentially reducing both CAP and translucency of the final restoration [25].
Understanding these mechanisms is crucial for clinicians, as it underscores the importance of considering depth-dependent optical properties when selecting materials and application techniques, especially in deeper cavities. This finding highlights the challenge of achieving optimal aesthetics in deeper restorations and suggests that clinicians may need to consider additional techniques or approaches, even when using a single-shade composite. To address these challenges, several strategies can be employed. Opaque blockers can enhance the seamless integration between the restorative material and the surrounding dental structure [21]. Using opaque blockers can help clinicians overcome some of the limitations observed in deeper cavities, potentially improving the overall esthetic outcome of single-shade composite resin restorations [21]. Additionally, the use of bulk-fill type composite resins can offer advantages in deeper cavities. By allowing placement in a single, thicker layer, bulk-fill composites can reduce the number of composite-composite joins [25]. This reduction in layer interfaces that leads to an increase in diffusion light transmission can lead to improved overall translucency and light transmission, potentially enhancing CAP and resulting in better color adaptation and matching with the surrounding tooth structure [25].
The selection of 1.5 mm and 3.0 mm cavity depths in this study serves multiple clinical and practical purposes. A 1.5 mm depth exemplifies a conservative approach, particularly suitable for early-stage caries. In contrast, the 3.0 mm depth represents more extensive lesions. Such deeper cavities are commonly encountered by practitioners when addressing advanced decay or when replacing substantial existing restorations. Furthermore, the choice of these two depths allows for an evaluation of different application methods. Given that many composite resins are typically applied in layers no thicker than 2 mm to ensure proper curing, the 1.5 mm and 3.0 mm depths permit a comparison between applications with fewer layers and those with more layers [26]. This comparison is crucial for understanding how placement methodology might influence the restoration’s overall performance and aesthetic outcomes.
Various factors in the oral cavity, such as tooth shape, position, existing dental work, the dark background, and surrounding tissues, can affect how well the color of the restoration matches the natural teeth [11]. While the appearance of anterior tooth restorations is often considered crucial, this study faced challenges in evaluating single-shade composite resin in primary incisors due to their small size and thin enamel and dentin layers. To address these limitations and ensure standardization, artificial denture teeth were used instead of natural teeth. This decision was made after careful consideration. Natural teeth posed challenges such as potential pulp exposure in deep cavities and insufficient depth in shallow cavities for meaningful comparisons. Although artificial teeth may not perfectly replicate the optical properties of natural teeth, they offered significant advantages in terms of standardization. These standardized specimens provided a consistent size, shape, and baseline shade across all samples, which is challenging to achieve with natural teeth due to their inherent variability [2]. This standardization was crucial for isolating the effects of the restorative materials and allowing for a more controlled comparison of the CAP and translucency of the tested composites.
However, it’s important to acknowledge that using artificial teeth was a limitation of this study. Natural teeth possess unique optical properties, including varying degrees of translucency, fluorescence, and opalescence, which can significantly influence the final appearance of restorations [1]. These factors, along with the complex environment of the oral cavity, are not fully replicated in artificial teeth. Considering these limitations, future research should focus on two main areas. Studies on primary incisors should be designed to overcome the challenges related to tooth size and cavity depth. Additionally, in vivo studies should be conducted to provide a more clinically relevant assessment of single-shade composite resins. These studies would evaluate the materials’ behavior in real clinical scenarios, taking into account the complex optical properties, structural variations, and environmental factors present in the oral cavity. Such studies would significantly enhance our understanding of the clinical efficacy and limitations of single-shade composite resins, potentially leading to improved materials and application techniques for better aesthetic outcomes in dental restorations.
While this study provides crucial insights into the inherent CAP and translucency of single-shade composite resins across different cavity depths, the inclusion of blockers, particularly in deep cavities, could offer additional clinically relevant information. Future studies should incorporate various types and colors of blockers to simulate more diverse clinical scenarios, especially for deep cavities. Such research could improve our understanding of how different blockers interact with single-shade composite resins and affect shade matching and overall aesthetic outcomes.
Conclusion
Within the limitations of this in vitro study, the single-shade composite resin exhibited superior shade-matching ability with structural color phenomenon and high translucency. However, the CAP of the composite resins decreased with increasing cavity depth, which can cause problems in deeper cavities. Clinicians should be aware of the limitations of deeper cavities and consider appropriate techniques or additional products in such cases.
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