Color Stability and Surface Roughness of Single-Shade Composite Resin after Finishing and Polishing

Article information

J Korean Acad Pediatr Dent. 2024;51(3):197-207

Publication date (electronic) : 2024 August 26

doi : https://doi.org/10.5933/JKAPD.2024.51.3.197

Hyewon Shin¹, Haeni Kim¹, Minho Hong², Juhyun Lee^,1

¹Department of Pediatric Dentistry, Oral Science Research Center, College of Dentistry, Gangneung-Wonju National University, Gangneung, Republic of Korea

²Department of Dental Biomaterials and Research Institute of Oral Science, College of Dentistry, Gangneung-Wonju National University, Gangneung, Republic of Korea

Corresponding author: Juhyun Lee Department of Pediatric Dentistry, College of Dentistry, Gangneung-Wonju National University, 7 Jukheon-gil, Gangneung, 25457, Republic of Korea Tel: +82-33-640-2452 / Fax: +82-33-640-3113 / E-mail: ljh55@gwnu.ac.kr

Received 2024 April 25; Revised 2024 May 28; Accepted 2024 May 30.

Trans Abstract

Introduction

Composite resin is the most commonly used esthetic restorative material in the dental field [1,2]. With a growing emphasis on aesthetics from patients, clinicians endeavor to employ composite resin restorations with a range of shades to match them with the individual tooth shade of each patient [3]. However, purchasing multiple composite resins of different shades and spending extensive time on shade matching requires extra cost and chair time, respectively, which is disadvantageous for the clinician. Furthermore, the successful matching of restoration shade to tooth color can be contingent upon factors, such as the technical proficiency of the clinician or the environmental conditions prevailing at the time, which can hinder accuracy and consistency. In response to these challenges, single-shade composite resins have been introduced. Single-shade composite resins allow clinicians to reduce chair time owing to shortened shade-matching procedures, and to reduce the wastage of unused composite resin shades with lesser composite inventory [4]. These can be particularly advantageous for pediatric patients, who require expedited procedures with low chair time owing to a lack of cooperation [5].

Fillers of diverse shapes and sizes are used in conventional composite resins. Notably, the single-shade composite resins comprise uniformly sized supranano spherical fillers (260 nm spherical SiO2-ZrO2) that ensure consistent light reflection. Devoid of pigments, the single-shade composite resin relies on the specific microstructure formed by the filler to generate a “structural color” phenomenon that mirrors the adjacent tooth structure [6,7]. Structural color is the color generated by the reflection, scattering, and diffraction of light caused by the physical structure of the material, such as the specific shape or arrangement of particles [8]. Therefore, structural color is not an inherent color, but the visible color resulting from the light interference generated by the microstructure [8]. According to manufacturers, the red-to-yellow structural color of single-shade composite resin is generated by the microstructure of the filler itself, and it merges with the reflected light and shade of the surrounding tooth structures [9]. This feature facilitates esthetic restoration with a shade similar to that of the natural teeth of the patient without the need for multiple composite resins of varying shades.

Staining or discoloration of composite resin is one of the primary reasons for the replacement of restoration [10], therefore, color stability is a pivotal criterion in determining the clinical longevity of composite resin restorations. Over time, the colors of restorations change owing to extrinsic and intrinsic factors [11,12]. In particular, the roughness of the surfaces of composite resin restorations poses a challenge to color stability, with both wear and chemical degradation contributing to surface roughening [13]. This surface roughness adversely affects gloss and translucency, intensifying the discoloration of the composite resin [14]. Additionally, a rough surface can harbor more biofilm, and the production of organic acids softens the resin restorations [15,16], rendering the softened surface susceptible to staining [16-18]. To mitigate these issues associated with rough surfaces, a comprehensive finishing and polishing (F/P) process is imperative to reduce restoration surface roughness and, consequently, enhance the color stability of the composite resin [10,19-21].

Currently, little is known about the color stability and surface roughness of single-shade composite resins for primary teeth. This study aims to evaluate and compare the color stability and surface roughness of single-shade composite resin with those of other conventional multi-shade composite resins after F/P for primary molars.

The null hypothesis is that F/P will cause the single-shade resin to exhibit color stability and surface roughness that are not significantly different from those of multi-shade composite resins.

Materials and Methods

This study was conducted with the approval of the Institutional Review Board of Gangneung-Wonju National University Dental Hospital (GWNUDH-IRB2023-A016).

1. Color stability test

A total of 90 specimens were fabricated and divided into 3 groups based on the composite resin used (n = 30 per group): a single-shade composite resin (OM, OMNICHROMA, Tokuyama Dental, Tokyo, Japan) and two multi-shade composite resins (FT, Filtek^TM Z350XT, 3M ESPE, St. Paul, MN, USA; ES, ESTELITE^® SIGMA QUICK, Tokuyama Dental). Table 1 shows information on the 3 composites. Based on a previous study on the shade of primary teeth, both multi-shade composite resins were standardized to the A1 shade [22] (Table 1).

Table 1.

Information on the composite resins used in this study

The three composite resin groups were divided into 3 subgroups (n = 10 per subgroup): unpolished groups, Sof-Lex XT groups, and Sof-Lex Diamond groups.

1) Specimen preparation

Cavities that were 4.0 mm in width, 4.0 mm in height, and 1.5 mm in depth were prepared on the buccal surfaces of extracted primary second molars, using a #330 carbide bur. The cavities were then etched with 37% phosphoric acid gel (Vericom Co., Ltd., Chuncheon, Korea) for 15 seconds. The specimens were rinsed and dried, following which an 8^th-generation bonding agent (Scotchbond^TM Universal Adhesive, 3M ESPE) was applied and polymerized for 15 seconds. Subsequently, the cavities were filled with the prepared composite resins and pressed against a mylar strip and glass slab for flattening, followed by polymerization for 15 seconds. All specimens were then immersed in distilled water for 24 hours.

The unpolished subgroup was retained as prepared, while the Sof-Lex XT and Sof-Lex Diamond subgroups were subjected to F/P procedures. Briefly, a tungsten carbide finishing bur (Komet, Stuttgart, Germany) with a high-speed handpiece was used at 200,000 rpm for 30 seconds for the finishing process. The Sof-Lex XT subgroup was polished using Sof-Lex™ XT (3M ESPE) for 20 seconds in each step corresponding to three different abrasive grades: medium, fine, and superfine. In the Sof-Lex Diamond subgroup, polishing was conducted using a Sof-Lex™ Diamond (3M ESPE) with a low-speed handpiece in two steps. A coarse grit disk was used for 30 seconds, followed by a fine grit disk for an additional 30 seconds. All procedures were performed by a single operator to minimize variability.

2) Staining and color measurement

According to manufacturer instructions, the coffee solution was prepared by adding 0.9 g of coffee (Kanu, Dongsuh, Incheon, Korea) to 100 mL of water. Each specimen was then immersed in the coffee solution for 48 hours in a 37℃ environment. Before color measurement, each specimen was washed with distilled water for 5 minutes, and then dried with tissue paper.

Color shade measurements were conducted according to the Commission International de L’Eclainge (CIELab) color system (L*, a*, b* values) at baseline before immersion and after 48 hours using a colorimeter (Shade Eye-NCC, Shofu Co., Kyoto, Japan). The color difference (Δ E_ab) of each specimen was calculated using the following equation:.

∆Eab=∆L*2+∆a*2+∆b*21/2

Before every measurement, the colorimeter was calibrated corresponding to the manufacturer’s instructions. Color measurements of the top surfaces were performed three times for each specimen, and the average was computed by the same operator.

Results

1. Color stability test

Fig. 1 and Table 2 show the means and standard deviations of Δ E_ab. Among the control groups, FT demonstrated a significantly lower Δ E_ab than ES, but no significant differences were observed between the Δ E_ab values of OM and FT or OM and ES. No significant differences were observed between the 3 composites in the Sof-Lex XT and Sof-Lex Diamond subgroups. There were no significant differences among the 3 F/P methods in the OM and FT groups, but in the ES group, the control subgroup showed a significantly higher Δ E_ab than the Sof-Lex Diamond subgroup (Fig. 1, Table 2).

Fig 1.

Means and standard deviations of color change values (ΔE_ab) according to finishing and polishing methods.

Different uppercase letters indicate a significant difference between the three composite resins (p < 0.0167). Different lowercase letters indicate a significant difference between the three finishing and polishing methods (p < 0.0167).

Table 2.

Color change values (ΔE_ab) of specimens

2. Surface roughness test

The SEM images of the specimen surfaces, Fig. 2 show that the control subgroups exhibited the smoothest surfaces. AFM images show the surface roughness of specimens from each group in Fig. 3. The means and standard deviations of Ra are shown in Fig. 4 and Table 3. There were no significant differences between the 3 composites in all subgroups. The control subgroups showed significantly lower Ra values than the Sof-Lex XT subgroups of the 3 composites (Fig. 2 - 4, Table 3)

Fig 2.

Scanning electron microscope images of specimens from each group (original magnification, 2,500×). (A) Omnichroma - control subgroup, (B) Omnichroma - Sof-Lex XT subgroup, (C) Omnichroma - Sof-Lex Diamond subgroup, (D) Filtek Z350 XT - control subgroup, (E) Filtek Z350 XT - Sof-Lex XT subgroup, (F) Filtek Z350 XT - Sof-Lex Diamond subgroup, (G) Estelite sigma quick - control subgroup, (H) Estelite sigma quick - Sof-Lex XT subgroup, (I) Estelite sigma quick - Sof-Lex Diamond subgroup.

Fig 3.

Atomic force microscope images of surfaces of specimens from each group, sized 20 × 20 μm. (A) Omnichroma - control subgroup, (B) Omnichroma - Sof-Lex XT subgroup, (C) Omnichroma - Sof-Lex Diamond subgroup, (D) Filtek Z350 XT - control subgroup, (E) Filtek Z350 XT - Sof-Lex XT subgroup, (F) Filtek Z350 XT - Sof-Lex Diamond subgroup, (G) Estelite sigma quick - control subgroup, (H) Estelite sigma quick - Sof-Lex XT subgroup, (I) Estelite sigma quick - Sof-Lex Diamond subgroup.

Fig 4.

Means and standard deviations of surface roughness values (Ra) according to finishing and polishing methods.

Different uppercase letters indicate a significant difference between the three composite resins (p < 0.0167). Different lowercase letters indicate a significant difference between the three finishing and polishing methods (p < 0.0167).

Table 3.

Surface roughness (Ra) values of the specimens

Discussion

The single-shade composite resin devoid of pigmentation, relies on a uniform supra-nano filler structure to impart color [6,7]. The restoration color is manifested through reflection from the adjacent tooth structure, necessitating cavity filling before color determination. In this field, this study is the first to comparatively examine the color stability of single-shade composite resin with that of multi-shade composite resins for restorations in primary molars.

Discoloration of composite resin restorations is influenced by various internal and external factors [10,11]. Daily consumption of beverages, particularly wine, coffee, and tea, is identified as a major cause of discoloration [11]. Tannic acid is a common constituent of beverages such as coffee and tea, and is highly chromogenic [6]. Furthermore, coffee causes higher discoloration than tea and cola, with its low polarity allowing pigments to penetrate the matrix [23]. In our study, a coffee solution was used to induce color changes in specimens.

The higher the Δ E_ab value of composite resin is, the lower the color stability of the composite resin is. In our study, among the control groups, the Δ E_ab value of FT was significantly lower than that of ES, and there were no significant differences in Δ E_ab between OM and FT or OM and ES. Both triethylene glycol dimethacrylate (TEGDMA) and bisphenol A-glycidyl methacrylate (Bis-GMA) are hydrophilic, because they contain hydroxyl groups [24,25]. However, urethane dimethacrylate (UDMA) is hydrophobic because it does not contain hydroxyl groups [24,25]. According to a previous study [26], ethoxylated bisphenol A glycol dimethacrylate (Bis-EMA) has a lower water absorption capacity than Bis-GMA. Hydrophilic matrices allow excessive water absorption, which causes the resin matrix to expand and plasticize, resulting in the generation of microcracks that allow the penetration of a staining solution [27]. Therefore, ES, which comprises hydrophilic Bis-GMA and TEGDMA matrices, exhibits low color stability. Composite resins containing UDMA and/or Bis-EMA such as OM and FT are highly stain-resistant because of their low water absorption capacity and solubility.

For the Sof-Lex XT and Sof-Lex Diamond subgroups, there were no significant differences between the Δ E_ab values of the three composite resins, potentially owing to the removal of the matrix-rich layer at the surface [28,29]. Before the polishing process, the excess resin matrix at the surface results in more active and direct water absorption [28]. The F/P process removes the matrix-rich layer of the resin surface[29], thereby rendering a lower impact on the matrix, which could reduce the degree of discoloration.

In restoring primary molars, there were no significant differences between the color stability of OM and that of FT or ES regardless of the polishing methods. The results are in accordance with previous studies showing that the single-shade composite resin showed comparable color stability with other multi-shade composite resins [4,30]. Exploiting the effects of the monomers that exhibit lower water absorption, such as UDMA, and considering the structural color phenomenon through which minor changes in tooth color are effectively camouflaged, the single-shade composite resin demonstrated prominent color stability in primary teeth.

The mylar strip offered smoother surfaces than the other two F/P systems, and the results are consistent with previous reports [24,31]. The F/P methods may generate friction heat that causes microcracks on the surface of the restorations, consequently increasing the surface roughness [32]. However, a relatively unstable matrix-rich layer is formed when polymerized under the mylar strip, it was reported that this may produce a superficial layer that can reach the organic matrix, which could be more susceptible to staining and degradation [33].

In our study, there were no significant differences in surface roughness between the three composite resins after all the F/P methods. This aligns with the results of a prior study [34], which reported consistent surface roughness despite varying filler sizes. The critical threshold value of Ra is clinically recommended to be 0.2 μm [33,35,36]. When the surface roughness value surpasses the threshold, plaque accumulation, periodontal inflammation, and the risk of caries increase [33]. Meanwhile, the minimum surface roughness value that most patients can detect with the tip of their tongue is 0.3 μ m [37,38]. The adhesion of S. mutans, which is considered to be a major etiological factor of dental caries, decreased at surface roughness values below 0.15 μm [39]. In our study, because none of the surface roughness values exceeded 0.15 μm, they were considered clinically acceptable.

According to a previous study[40], the finishing process using the tungsten carbide bur exhibited a lower surface roughness than that with the Superfine grit diamond bur. The Sof-Lex Diamond system uses spirals of either aluminum oxide or diamond particles impregnated in thermoplastic elastomers, whereas the Sof-Lex XT system uses wheels of aluminum oxide. Sang et al. [41] reported that the Sof-Lex Diamond system results in a lower surface roughness for composite resins compared with the Sof-Lex XT system. In our study, the Sof-Lex Diamond subgroups showed a lower surface roughness than the Sof-Lex XT subgroups in all 3 composite resin groups, but there were no significant differences. This can be attributable to the difference in the application time for polishing.

There are some limitations of this study. First, the surfaces of restorations in clinical environments are uneven surfaces of concave and convex forms. The uneven surfaces might affect the polishing procedures and cause plaque accumulation [42]. Second, several factors affect the color stability of composite resins [43]. Future studies involving parameters such as different staining solutions and aging effects, are required to comprehensively understand the color stability of single-shade composite resins.

Consequently, we successfully proved the null hypothesis: there was no significant difference in color stability and surface roughness between the single-shade composite resin and the multi-shade composite resins that underwent F/P processing.

In a previous study [44], the single-shade composite resin showed the most remarkable color adaptability for primary molars, with a wide range of color-matching capabilities compared with the multi-shade composite resins. Single-shade resins are expected to be highly useful in clinical dentistry, particularly for pediatric treatment, owing to their excellent color stability, color adaptability, and fine surface roughness, as well as the minimal chair time required owing to the easy shade-matching procedure. Furthermore, to acquire additional information for the clinical use of single-shade composite resins, studies can be conducted comparing the mechanical characteristics of single-shade and multi-shade composite resins, such as their strength and wear resistance.

Conclusion

Single-shade composite resin demonstrated comparable color stability to that of multi-shade composite resins both before and after the F/P process in restoring primary molars. Among the three F/P methods, there were no significant differences in surface roughness between the three composite resins. The single-shade composite resin is expected to be useful in clinical pediatric dentistry with reduced chair time owing to the easy shadematching procedures, providing better esthetic and quality outcomes for the restoration of primary molars.

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