Introduction

In the aesthetic zone, direct composite resin restorations are considered a highly conservative and minimally invasive approach for the rehabilitation of missing, discolored, or structurally compromised tooth surfaces. Resin composites are multiphase materials composed of distinct substances that retain their individual identities while functioning as a unified restorative medium [1]. A wide variety of resin-based composites are commercially available, each differing in shade systems, optical properties, and handling characteristics. Achieving optimal shade matching remains a clinical challenge, particularly in anterior regions, due to individual variations in tooth color, translucency, and lighting conditions [2]. Traditional shade matching procedures require multiple composite shades and incremental layering techniques, increasing inventory requirements, treatment duration, and overall cost. Moreover, achieving the correct translucency, masking ability, and final color match can be unpredictable, as each layer has unique optical behavior [3, 4].

The optical behavior of resin composites depends on light absorption, scattering, and transmission within the material. Filler particle size, filler-matrix refractive index matching, material thickness, and surface texture are key factors in translucency and color perception. Long-term color stability may be affected by water sorption, staining, resin matrix degradation, and thermal stresses, which can alter the optical properties of restoration over time [5]. To overcome these limitations, single-shade composite resins were developed. In 2019, Tokuyama Dental introduced Omnichroma, the first universal shade matching composite resin, based on the principle of structural coloration. Unlike pigment-based color generation, these materials utilize light scattering and reflection through their internal microstructure to blend with surrounding tooth structures. This phenomenon, often termed the “chameleon effect” or “blending effect,” allows the material to mimic adjacent tooth shades without the need for multiple color options [6].

Vittra APS Unique is another universal shade composite that incorporates advanced polymerization system (APS) technology. This material combines optimized photo initiator systems with modified resin chemistry to improve light transmission, depth of cure, and optical integration with surrounding dental tissues. Its formulation is designed to balance translucency and opacity, enabling adaptive color matching while maintaining mechanical strength and polishing ability [7].

Although these materials offer simplified shade selection and reduced inventory, limited evidence exists regarding their actual clinical performance compared to traditional multi-shade composites, particularly in terms of shade matching and long-term color stability. Therefore, it is essential to assess whether these single-shade composites deliver the aesthetic outcomes indicated by their manufacturers. The following hypo­theses were proposed:

1. The color matching ability of single-shade composite resin is comparable to or better than that of conventional composite resin.

2. The tooth shade does not affect the color matching ability of single-shade composite resin.

3. The color stability of single-shade restorations is comparable to conventional materials and remains unaffected by thermocycling.

Objective

The present in vitro study aimed to evaluate and compare the color matching ability of two single-shade composite resins (Omnichroma and Vittra APS Unique) to one conventional multi-shade composite resin (Filtek Z350) in different tooth shades (A1, A2, and A3.5) using visual scoring and spectrophotometric analysis (ΔE). The study also sought to assess and contrast the color stability of these materials after undergoing thermocycling.

Material and methods

A total of 30 freshly extracted, intact human maxil­lary premolars indicated for orthodontic extraction were collected. For standardization, teeth were categorized into three experimental shade groups, such as A1, A2, and A3.5, based on their natural shade as determined by the VITA classical A1-D4^® shade guide (Vita Zahnfabrik), with 10 teeth assigned to each group (n = 10). Shade selection was performed by an experienced clinician under standardized natural daylight conditions, and was further confirmed by a spectrophotometer to ensure accuracy and reproducibility. Informed consent was obtained from all patients prior to extraction. The teeth were cleaned of debris, disinfected, and stored in distilled water at room temperature until further use. The three chosen shades represented a range of values and chroma typically observed in clinical practice.

Sample size

Sample size was calculated using nMaster 2.0 software, with an α of 0.05, power of 80%, and effect size of 1.5, yielding n = 10 per group.

Each group was subdivided based on restorative material used, as follow:

• Omnichroma (OM) (Tokuyama Dental, Tokyo, Japan);

• Vittra APS Unique (VITTRA) (FGM Dental Group, Joinville, SC, Brazil);

• Filtek Z350 XT (FT) (3M ESPE, St. Paul, MN, USA).

Specimen preparation

Standardized cavity preparations were made according to Black’s classification system. Class V cavities were prepared on the buccal and palatal surfaces, positioned 3 mm above the cementoenamel junction, using a high-speed round carbide bur under continuous water cooling. These cavities were standardized to a diameter of
3 mm and a depth of 1.5 mm. Additionally, a Class II slot preparation was made on the proximal surface of each premolar, standardized to dimensions of 3 mm × 3 mm × 1.5 mm. All cavity dimensions were verified using a periodontal probe. Each tooth received restorations at three distinct sites:

• buccal surface: restored with Omnichroma;

• palatal surface: restored with Vittra APS Unique;

• proximal (control): restored with Filtek Z350, shade-matched to VITA shades A1, A2, or A3.5.

Before composite placement, all cavities were treated using a standardized adhesive protocol. Enamel surfaces were etched with a 37% phosphoric acid gel for 15 seconds and dentin for 10 seconds, followed by thorough rinsing and gentle air drying. A universal adhesive (Scotchbond Universal, 3M ESPE, USA) was applied according to the manufacturer’s instructions, gently air-thinned and light-cured for 10 seconds using a LED curing unit. The same bonding protocol was uniformly applied to all samples to ensure consistency and elimi­nate variability in optical outcomes. Composite resin was then placed in a single layer, overlaid with a Mylar strip for surface smoothness, and light-cured for 20 seconds using a LED curing unit (Bluephase G2, Ivoclar Vivadent, Amherst, NY, USA) with an intensity of
1280-1300 mW/cm². All cavity preparations and restorations were completed by a single-trained operator, following consistent conditions and adhering strictly to the manufacturers’ instructions. All specimens were kept hydrated in distilled water at room temperature between each procedural step to prevent dehydration.

Color matching evaluation

Visual evaluation

Shade matching was visually assessed by three cali­brated dental professionals with normal color perception. Evaluations were conducted under natural daylight at a viewing distance of 25 cm, within a 25-second window per specimen. Observers were blinded to the restorative material, and assigned a visual score based on the ISO/ TR 28642:2016 scale [8], as follow: 1 = mismatch, 2 = poor/ hardly acceptable match, 3 = acceptable match, 4 = close match/ small difference, 5 = exact match. The average visual score (VS) for each restoration was calculated across all observers.

Instrumental evaluation

Color measurements were conducted using a spectrophotometer (Vita Easyshade V; VITA Zahnfabrik, Bad Säckingen, Germany) under standardized 6500K illumination. The probe was manually positioned perpendicular (at a 90° angle) and centered over each site; however, no customized jig was used, which may introduce minor variability in probe alignment. As the probe tip diameter (5 mm) exceeded the restoration diameter (3 mm), the probe was centrally aligned over the restoration, ensuring consistent positioning with partial overlap of the surrounding tooth structure at the restoration margins. Each site was measured three times, and the mean values of CIELAB coordinates (L*, a*, b*) were calculated for analysis. All measurements were performed by a single-calibrated operator to minimize variability. The overall color discrepancy (ΔE) between the restoration and adjacent natural tooth surface was determined by evaluating differences in three dimensions, including lightness (ΔL*), red-green axis (Δa*), and yellow-blue axis (Δb*), after which the following equation was applied:

∆E = √ [(∆L)² + (∆a)² + (∆b)²],

where ΔL* is the change in lightness, Δa* is the change along the red-green spectrum, and Δb* is the change along the yellow-blue spectrum.

Color stability evaluation (thermocycling)

To simulate intraoral aging, specimens underwent thermocycling for 1000 cycles between 5°C and 55°C [9]. Each cycle involved immersion for 30 seconds in each bath, with a 5-second transfer interval between baths. After thermocycling, color measurements were repeated using the same spectrophotometric procedure. ∆E values before and after thermocycling were compared to assess color stability.

Statistical analysis

Descriptive statistics (mean ± standard deviation) were computed. The normality of data distribution was verified using the Shapiro-Wilk test, and homogeneity of variances was checked using the Levene’s test. For intergroup comparisons of ΔE and visual scores among the three restorative materials within each tooth shade, one-way analysis of variance (ANOVA) was performed. When ANOVA indicated statistically significant diffe­rences, the Tukey’s honest significant difference (HSD) post hoc test was applied to determine pairwise differences between groups. For intragroup comparisons of color changes before and after thermocycling, paired t-tests were employed to assess the effect of thermocycling on each material. A p-value of < 0.05 was consi­dered statistically significant for all analyses.

Results

The visual scores (VS) for the three restorative materials are presented in Table 1 and Figure 1. Visual scores (VS) for all three restorative materials, including Omnichroma (OM), Vittra APS Unique (VITTRA), and Filtek Z350 (FT) fell within a favorable range of 4 to 5 across all groups. In the A1 group, Omnichroma (4.80 ± 0.42), Vittra APS Unique (4.60 ± 0.52), and Filtek Z350 (4.70 ± 0.48) demonstrated no significant differences (p = 0.647). Comparable results were observed in the A2 group, where Omnichroma (4.50 ± 0.53), Vittra APS Unique (4.30 ± 0.48), and Filtek Z350 (4.30 ± 0.48) again showed no statistical significance (p = 0.590). Similarly, in the A3.5 group, Omnichroma (4.50 ± 0.53), Vittra APS Unique (4.00 ± 0.67), and Filtek Z350 (4.20 ± 0.79) revealed no significant differences (p = 0.261). This indicated that the single-shade composites matched the aesthetic performance of the conventional multi-shade composite in visual evaluation.

Instrumental color difference (ΔE)

As shown in Figure 2 and Table 2, in the A1 group, ΔE values for Omnichroma (4.77 ± 0.82), Vittra APS Unique (5.90 ± 1.90), and Filtek Z350 (4.70 ± 0.99) were not significantly different (p = 0.092). However, significant differences were detected in the A2 group (p = 0.000), where Vittra APS Unique (7.01 ± 1.96) recorded substantially higher ΔE values than Omnichroma (3.55 ± 1.11) and Filtek Z350 (3.44 ± 0.40). A simi­lar trend was observed in the A3.5 group (p = 0.000), with Vittra APS Unique showing the highest ΔE value (9.00 ± 1.08), followed by Omnichroma (7.73 ± 1.15), while Filtek Z350 (5.60 ± 1.87) exhibited the lowest color difference. The Tukey’s HSD post hoc test confirmed that Vittra APS unique in the A3.5 subgroup presented the greatest deviation from the adjacent tooth structure, indicating the least effective shade matching among the three materials. Omnichroma performed somewhat better than Vittra APS unique, yet still did not match the color adaptation of Filtek Z350 in darker shades.

Color stability after thermocycling

The post thermocycling ΔE values are presented in Table 3 and Figure 3. Color changes following thermocycling were assessed by measuring post aging ∆E values. In the A1 group, Omnichroma (6.48 ± 1.39), Vittra APS Unique (5.82 ± 2.45), and Filtek Z350 (5.60 ± 1.71) showed no significant differences (p = 0.563). In contrast, statistically significant differences were found in the A2 group (p = 0.000), where Vittra APS Unique (7.04 ± 1.71) again displayed greater color change than Omnichroma (5.00 ± 1.80) and FT (4.00 ± 0.75). In the A3.5 group, Vittra APS Unique (8.89 ± 1.12) recorded the highest ΔE value after thermocycling, followed by Omnichroma (7.78 ± 1.12), while Filtek Z350 (6.06 ± 1.66) maintained the most stable color performance (p = 0.001). Among all groups, the highest ∆E after thermocycling was noted in Vittra APS Unique for the A3.5 shade, as shown in Figure 3 and Table 3, indicating the greatest color instability and least matching ability when compared to Omnichroma and Filtek Z350.

Statistical analysis indicated a significant difference in ∆E values among the three restorative materials in both the A2 and A3.5 shade groups (p < 0.05), highlighting the influence of both material type and initial tooth shade on the color stability of restorations.

Discussion

This study aimed to evaluate and compare the color matching performance of two single-shade composite resins, including Omnichroma (Tokuyama Dental) and Vittra APS Unique (FGM) against one multi-shade resin composite, Filtek Z350 XT (3M ESPE), across three VITA classical tooth shades, i.e., A1, A2, and A3.5.

Visual and instrumental evaluations were conducted to assess color matching, followed by thermocycling to evaluate color stability. The color difference (ΔE) was used as the primary measure of aesthetic blending ability.

The results indicated that Omnichroma outperformed Vittra APS Unique in terms of color matching, with Vittra APS exhibiting the highest ΔE values across all shades, suggesting poorer blending with the natural tooth structure. These findings are consisted with previous research that reported variation in the color matching performance among different single-shade composite systems. In particular, Yilmaz et al. [10] demonstrated that while certain single-shade materials achieve satisfactory shade adaptation, others show limitation particularly in darker shades. Our results align with this pattern, demonstrating that optimal performance of single-shade composites is material-dependent. Omnichroma’s superior performance can be attributed to its advanced structural color technology that utilizes uniform supra-nano spherical fillers to mimic the natural tooth color through light reflection and scattering. In contrast, Vittra APS’s filler and resin matrix technology appeared less effective in adapting to darker tooth shades [11].

The study by Alhamdan et al. [12] evaluated the single-shade composite Omnichroma, and reported that it exhibited inferior color matching when compared to conventional multi-shade composites. Furthermore, this study found that tooth shade has a significant impact on the color matching ability of single-shade composites. Single-shade composites exhibited better color matching with lighter tooth shades, such as A1, compared to darker shades, such as A3.5, supporting previous study by Iyer et al. [13], who noted a trend of better harmony between lighter shades and restorations. Moreover, a recent investigation by Mohammadipour et al. [14] revealed that the blending effect of singleshade composites is influenced by restoration thickness and underlying shade, showing superior color matching on lighter (A1) backgrounds and at thinner restorations (1.5 mm) than on darker (A3) backgrounds and greater thicknesses.

In the visual assessments, there was no statistically significant difference in color matching among the three composite materials across various tooth shades. Never­theless, Omnichroma consistently received higher scores, especially for the A1 shade (4.8 ± 0.42), indicating its enhanced aesthetic performance in lighter shades. This aligns with findings by Khayat et al. [15], who also noted Omnichroma’s effectiveness in matching lighter tooth shades. However, it is essential to acknowledge that visual evaluations are inherently subjective and can be influenced by variables, such as the observer’s expertise, lighting conditions, and individual visual perception [16]. Despite these potential biases, visual evaluation remains the most commonly used technique in clinical settings. Though not without its limitations, this method plays a significant role in influencing patient satisfaction and acceptance [17, 18].

Instrumental analysis with a spectrophotometer provided more accurate color matching data by measuring the CIELAB coordinates (L*, a*, b*) of both the restorations and the natural tooth structure [19, 20]. The spectrophotometric results demonstrated that the ΔE values for single-shade composites were generally higher than for multi-shade composites, indicating a more noticeable color mismatch. The highest ΔE was observed for the A3.5 shade restored with Vittra APS Unique, highlighting its suboptimal performance in darker shades. The techno­logy behind Omnichroma, which utilizes structural colo­ration, offers a “chameleon effect” that effectively mimic surrounding tooth color in lighter shades but show limitations in darker shades [21]. Owing to their translucency, single-shade composites reflect the color of surrounding cavity walls, indicating that cavity depth influences their blending effectiveness [22]. Several studies have demonstrated that at a cavity depth of approximately 2 mm, singleshade materials exhibit a markedly reduced capacity to mirror the underlying substrate, thereby failing to accurately reproduce the color of adjacent tooth structure. This diminished optical accuracy may account for our observation that single-shade composites displayed inferior color matching performance compared to multi-shade materials during instrumental analysis [23, 24].

In our study, the total color difference (ΔE) was evaluated using the CIELAB system and interpreted according to established perceptibility (PT) and acceptability (AT) thresholds. The 50% : 50% PT threshold for CIELAB was defined as ΔE = 1.2, indicating that 50% of observers can detect the difference while the other 50% cannot. A ΔE value below 2.7 was considered the AT limit for composite resin restorations; however, color differences greater than 2.7 may still be clinically acceptable in certain clinical conditions [25].

The study also assessed the color stability of the restorations after thermocycling, simulating aging to mimic mechanical degradation, such as wear, abrasion, and fatigue as well as chemical aging mechanisms, including hydrolytic breakdown, enzymatic activity, and acid-induced polymer degradation [26, 27]. Thermocycling for 1000 cycles was selected as it provides a reasonable and widely used short-term aging protocol in laboratory studies. The results revealed significant color changes in all composite materials, with Omnichroma exhibiting the least color change, followed by Filtek Z350. However, Vittra APS Unique showed the most significant ΔE increase after thermocycling, suggesting a decline in color stability, particularly in darker shades. This result is consistent with the findings of Tepe et al., who reported that aging adversely impacts the shade harmony of single-shade composites [28].

The superior color stability of Filtek Z350 can be attributed to its optimized filler technology and tailored pigmentation, which enhance its resilience to thermal aging and maintain a more consistent color match with the natural tooth. Iyer and Babani [13] also noted that multi-shade composites exhibit better stability under thermal stress compared to single-shade systems.

This study rejected all three hypotheses; it revealed that single-shade composites exhibited significantly lower color matching ability compared to conventional
multi-shade composites, with tooth shade exerting a notable effect on the materials’ matching performance. Moreover, thermocycling significantly affected the color stability of the restorations.

Nevertheless, this study was limited by its in vitro design and the use of only 1000 thermocycling cycles, which provides a restricted aging simulation. In addition, color measurements were obtained using a spectrophotometric probe with a larger diameter than the restoration size, which could have resulted in partial inclusion of adjacent tooth structure despite careful central positioning and averaging of repeated measurements. Future studies using customized positioning aids or smaller probe tips may further improve measurement precision. Additional cli­nical studies considering other aging factors, such as mechanical forces, pH variations, and staining agents should be conducted to better simulate real-life conditions.

Conclusions

Although single-shade composite resins offer practical advantages, such as ease of use and reduced inventory requirements, they may fall short in achieving ideal color matching, especially in cases involving darker tooth shades. Therefore, when aiming at optimal aesthetic outcomes, clinicians should take into account both the specific pro­perties of the composite material and the natural shade of the tooth. The findings emphasize the need for caution when using single-shade composites in patients with darker teeth or in those with high aesthetic expectations.

Disclosures

Author contributions: Conceptualization: T.M.; Metho­dology: T.M., R.V.; Investigation and data collection: S.K.B., R.V.; Formal analysis: S.K.B., R.V.; Data curation: S.K.B., R.V.; Writing of original draft: S.K.B., M.M.; Writing – review and editing: S.K.B., M.M.; Supervision: T.M., N.J.; Project administration: T.M. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board statement: Not applicable.

Informed consent statement: Not applicable.

Data availability statement: All the data generated or analyzed in this study are included in this manuscript. The data generated in this study may be requested from the corresponding author.

Acknowledgments: None.

Conflicts of interest: The authors declare no conflicts of interest.

AI use statement: Not applicable.

References