Introduction
Glass ionomer cement (GIC) is a self-adhesive direct restorative material that has been extensively employed in contemporary restorative dentistry [1]. GIC has favorable characteristics such as the ability to release fluoride over time, which leads to remineralization of dental hard tissues, biocompatibility, and chemical adhesion to tooth structures [2]. However, GIC has been reported to have relatively poor mechanical properties and a shorter working time than other restorative materials [3]. Numerous strategies have been devised to enhance the physical and mechanical properties of GIC. For example, Fuji IX GP (GC Corporation, Tokyo, Japan) was developed as a high-viscosity GIC with improved mechanical properties, demonstrating long-lasting durability as a posterior restoration material. Another major advancement in GIC technology is the development of glass-hybrid restorative materials, which incorporate highly reactive glass particles of various sizes into a standard filler. Equia Forte HT Fil (GC Corporation) is one of the glass-hybrid restorative materials, and the manufacturer claims that its enhanced reactivity leads to a significant improvement in its mechanical properties [4]. These materials have been reported to be appropriate for the durable restoration of posterior teeth [5].
A different strategy that has been proposed to overcome the vulnerability of GIC to moisture is the use of protective coatings. This aims to protect GIC from moisture contamination and improve its mechanical performance [6,7]. Protective coatings include various materials such as varnishes, bonding agents, and petroleum jelly [8,9]. Recently, a new type of resin coating material reinforced with nanofillers, known as the Equia Forte coat (GC Corporation), has become commercially available. This self-adhesive, nano-filled resin coating material contains uniformly distributed nanofillers, 50% methyl methacrylate, and 0.09% camphorquinone. The manufacturer claims that these nano-filled resin coatings, characterized by their low viscosity, create a resilient protective layer. This layer provides uniform sealing of the glass ionomer surface, potentially enhancing both its physical and aesthetic qualities. However, studies on the physical and mechanical effects of this new coating agent are limited.
This study aimed to assess the surface hardness and wear resistance of a novel glass-hybrid restorative material in comparison with those of high-viscosity GIC and to assess the impact of nano-filled resin coating application on these mechanical properties.
Materials and Methods
1. Specimen preparation
Based on power analysis using G Power software (Version 3.1.9.7, Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany) with an effect size of 0.71, a significance level of 0.05, and a power of 0.80, a total of 80 diskshaped specimens were fabricated [10]. The types, compositions, and manufacturers of the materials used in this study are specified in Table 1. Equia Forte HT Fil (GC Corporation) and Fuji IX GP (GC Corporation, Tokyo, Japan) were mixed for 10 s using a Rotomix (3M ESPE, Seefeld, Germany) according to the manufacturer’s instructions. After mixing, each material was injected into a cylindrical Teflon mold with dimensions of 7 mm across a depth of 3 mm. Transparent strips (Matrix-Strips, Orbis, Münster, Germany) were placed over the mold, followed by the placement of a transparent glass plate (HL-03242, Hanil Dental, Goyang, Korea) on top, and the setup was allowed to set for 5 min. To create standardized surfaces, each specimen was subjected to a standardized polishing process using silicon carbide paper with a grit size of 600 (CC-357, PACO Co., Ltd., Yeoju, Korea). After the preparation, the specimens were randomly divided into 4 groups for further testing. In the coated groups, an Equia Forte Coat (GC Corporation) was thinly applied to one surface of each specimen using a microbrush and light-cured for 20 s with a light-emitting diode (VALOTM, Ultradent Products Inc., South Jordan, UT, USA). All specimens were immersed in distilled water at 37°C for 24 h.
2. Vickers hardness test
Forty specimens were used, with 10 specimens allocated to each group. The microhardness of each specimen was evaluated using the Vickers method, and measurements were performed at 3 randomly selected locations using a Vickers microhardness tester (MMT-X7, Matsuzawa, Akita, Japan). At each point, a diamond-shaped indenter (Vickers pyramid) was pressed vertically with a 300 gf load for 15 s. The microhardness of each specimen was determined by calculating the mean value of three distinct measurements obtained at different surface locations.
3. Wear resistance
Forty specimens were used, with 10 assigned to each group. For assessment using a mastication simulator (TW-D811, Taewon Tech, Bucheon, Korea), the specimens were placed in a self-cured acrylic resin and subsequently affixed to the holder in the simulator chamber. A stainless-steel Ø5-mm indenter was used to repeatedly contact the center of each specimen, performing vertical and horizontal movements of 1.5 mm in each direction (Fig. 1). The testing conditions involved subjecting each specimen to 10,000 dynamic strokes at 2 Hz with a constant force of 50 N. This process was carried out in a thermal cycling environment ranging from 5 to 55°C, where the chamber was filled with water to submerge the specimens and then drained. Each temperature cycle lasted 36 seconds, alternating between both temperatures. The wear depth (μm) of the specimens was quantified by calculating the discrepancy in central thickness before and after wear, utilizing a digital point micrometer (342-261-30, Mitutoyo, Kawasaki, Japan). Measurements were performed twice for each specimen, and the average value was calculated. After measuring the wear depth, a Surftest (SJ-500, Mitutoyo) was used to acquire surface morphology data, allowing for a visual confirmation of the wear depth.
4. Statistical analysis
Data were analyzed using SPSS (version 29.0; IBM, Armonk, NY, USA). The normal distribution of the microhardness and wear measurement data were assessed using the Shapiro-Wilk test. To identify the differences in microhardness and wear depth among the four experimental groups, a one-way analysis of variance (ANOVA) was conducted. Subsequently, pairwise comparisons between groups were performed using Tukey’s honestly significant difference (HSD) test as a post-hoc analysis. For all statistical evaluations, the threshold for significance was set at α = 0.05.
Results
1. Vickers hardness test
The Vickers hardness numbers of each group are listed in Table 2. The Equia Forte HT Fil exhibited significantly greater hardness than Fuji IX GP (p < 0.05). However, the coating did not significantly influence the hardness of either GI group (p > 0.05).
2. Wear resistance
The wear depths for each group are presented in Table 3. In Fig. 2, each line represents the measured wear depth on the surfaces of selected specimens from each group. In the uncoated group, the Equia Forte HT Fil showed a significantly lower wear depth than the Fuji IX GP (p < 0.05). In the comparison before and after coating, both GI materials exhibited significantly lower wear depths following the application of the coating (p < 0.05). The Equia Forte HT Fil group with the coating exhibited the lowest wear depth among all tested groups. However, within the coated group, no significant difference in wear depth was observed between the 2 GI materials (p > 0.05).
Discussion
GICs are dental materials composed of fluoroaluminosilicate glass powder and polyacrylic acid formed through an acid-base reaction between these components. The setting process of GIC takes place in 2 stages. In the initial minutes of this process, the material exhibits sensitivity to moisture uptake as the polyacrylate matrix undergoes formation [11]. The second stage involves the continuation of the acid-base reaction over a 24-hour period and is susceptible to dehydration [12]. Consequently, the sensitivity of GIC to both hydration and dehydration during its prolonged setting time adversely affects its early mechanical properties and potentially compromises its overall performance [13]. Modifications of the structure and composition of these materials have been explored to enhance their resistance to moisture during setting [13]. The newly released Equia Forte HT Fil is characterized by the uniform dispersion of ultrafine, highly reactive glass nanoparticles and high-molecularweight polyacrylic acid throughout its structure. According to the manufacturer, this new GIC can form restorations with higher strength and hardness by using an improved glass-blend composition.
Hardness, defined as the resistance of a material to plastic deformation, is a crucial indicator for predicting the susceptibility of a material to wear and surface degradation [14]. Surface hardness can be measured by various methods, and in this study, we conducted the Vickers microindentation test, which is one of the most representative methods. This investigation revealed that Equia Forte HT Fil exhibited substantially greater hardness than Fuji IX GP, suggesting enhanced resistance to indentation in the former material. Similar to the results of this study, several previous studies have demonstrated that glass-hybrid restorative materials have higher hard-ness than conventional GI [15,16].
Surface coating increases the resistance to moisture contamination during the initial setting stage of GIC, consequently enhancing its physical properties [17]. The newly released Equia Forte coat is part of the Equia Forte system designed for the surface coating of GICs. According to the manufacturer, while this new coating protects against moisture contamination during the initial setting stage, similar to previous coatings, it features the incorporation of nanofillers to provide better wear resistance.
According to our research findings, the application of nano-filled coating agents resulted in enhanced microhardness for both materials compared to their uncoated counterparts. Regarding the comparison of the two GICs with the coating, EQUIA Forte HT Fil demonstrated a significantly higher hardness than Fuji IX GP. Similar to the results of this study, several previous studies have reported the enhancement of mechanical properties with Equia Forte Coat [15,18,19]. Handoko et al. [18] observed a significant increase in the surface hardness of Equia Forte Fil (not HT) after treatment with Equia Forte Coat, as determined by microhardness testing. Alqasabi et al. [19] reported that Equia forte coat increased the microhardness of Equia Forte HT Fil, and an improvement in hardness was observed due to the high filler content of Equia forte coat. These findings are consistent with our results. However, our study did not reveal any statistically significant differences between the coated and uncoated condition within each GIC.
Wear resistance is an important mechanical characteristic used to evaluate whether a restoration can resist external forces while maintaining its original form and function. Our investigation revealed that Equia Forte HT Fil exhibited a significantly lower wear depth than Fuji IX GP, suggesting superior wear resistance in the former material. Previous studies have demonstrated that Equia Forte HT Fil exhibits greater wear resistance than conventional GIs [20,21]. AlJamhan [20] demonstrated that Equia Forte Fill (not HT) exhibited superior wear resistance compared to Fuji IX GP, as evidenced by reduced volume loss during a toothbrush abrasion test. Brkanović et al. [21] demonstrated that Equia Forte HT Fil exhibited lower wear than Fuji IX GP under various pH conditions.
Our results revealed that for both materials, specimens treated with surface-coating agents exhibited significantly reduced wear depths compared to their untreated counterparts. During the GI setting process, the surface coating protects the material from moisture contamination, preventing the loss of carboxyl groups of polyacrylic acid and ions, thereby preventing the deterioration of mechanical properties [22]. The manufacturer of Equia Forte Coat reported that nanofillers were a key component of the product. Given the well-documented evidence from prior research demonstrating that nanofillers improve the mechanical characteristics of dental materials, it is reasonable to infer that these particles contribute to enhanced wear resistance [23]. Furthermore, prior research has demonstrated that applying resin coating agents to GIC enhances wear resistance through the infiltration of resin into porous areas, thereby improving surface quality [24].
This study has several limitations. As it was conducted in a laboratory setting over a short duration, this study was unable to replicate long-term masticatory forces. Additionally, it failed to accurately simulate clinical conditions such as temperature fluctuations and pH variations. This study did not evaluate surface roughness and was unable to compare with other coating agents. In addition, because the evaluation was performed after only 24 h of curing, it was not possible to determine the enduring impact of the surface coating over an extended period.
Conclusion
The glass-hybrid restorative material demonstrated significantly advanced performance in terms of both hardness and wear resistance compared with the highviscosity GIC. The nano-filled resin coating demonstrated no significant improvement in the hardness of either type of GIC; however, it substantially enhanced the wear resistance.
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