Glass ionomer cements (GICs) are widely used in dentistry due to their biocompatibility, chemical adhesion to dental tissues, and anticariogenic potential (1-3). Their relatively weak mechanical properties, however, have prompted numerous research efforts focused on improving their overall hardiness and clinical performance as long-term fillings in posterior teeth (4-7).
For example, water balance has been shown to play an important role in achieving optimal physical properties in glass ionomer materials. In the initial setting phase, which includes the neutralization reaction between metal cations released from the glass and polymeric acid, glass ionomer materials are sensitive to water and the cement sets within 3-6 minutes (2, 8). Dissolution of metal cations during the setting process can be avoided by protecting the cement from additional water. Once the cement has set, the maturation process occurs during the next 24 hours and up to a year afterward (2). During maturation, care must be taken to avoid the cement becoming dehydrated, which leads to surface cracking and a chalky appearance (9).
For this reason, there are several surface protection coatings commonly used in clinical practice that are applied in order to prevent the early loss of ions participating in the setting reaction, and also to prevent water loss later on (10). Mechanical properties of the glass ionomer—in particular, the surface hardness—were shown to improve after a coating was applied (11). The coat is responsible for the glaze effect that enhances the aesthetics of the set material; it also provides effective protection during the water-sensitive initial setting phase and has been shown to increase the glass ionomer’s compressive strength after fatigue strength and reduce abrasive wear of the filling (12, 13).
Equia Coat is a hydrophilic, low-viscosity nano-filled coating agent that consists of 50% methyl methacrylate and 0.09% camphorquinone. It is a component of a restorative system based on GIC technology that also consists of Equia Fil. In 2015, Equia Forte (GC, Tokyo, Japan) was released as a new restorative material based on glass hybrid technology where a glass filler matrix combines fillers of different sizes. It consists of microlaminated Equia Forte Fil with a nano-filled coat (Equia Forte Coat, GC, Tokyo, Japan) (14). Similarly as in the Equia Coat, in the Equia Forte Coat, nanofiller particles are dispersed in the liquid. Additionally, the Equia Forte Coat contains a multifunctional monomer that, as the manufacturer claims, improves surface hardness and wears resistance.
The standard techniques for assessing the strength of dental materials include determining compressive strength (CS) (15, 16). To determine compressive strength, the mechanical properties of tested materials are analyzed over a longer period of time and under simulated conditions where cyclic loading replicates masticatory loading and thermocycling simulates thermal oscillations in the oral cavity (17). The purpose of this in vitro study was to assess CS values for the two new restorative materials, with and without coating after cyclic loading and thermocycling, and to determine whether the nano-filled coating influences CS values after the initial setting and before moisture contamination. The hypothesis of the study was that the compressive strength of the tested materials would be higher after cyclic loading and thermocycling if they had been treated initially with a nano-filled coating.
Materials and methods
This in vitro study was approved by the Ethics Committee of the School of Dental Medicine, University of Zagreb, approval number 05-PA-15-12/2017.
The two restorative materials used in this study were Equia Fil GC (Tokyo, Japan) and Equia Forte Fil GC (Tokyo, Japan). The samples were divided into uncoated groups and groups coated with either Equia Coat or Equia Forte Coat (GC, Tokyo, Japan).
Cylindrical aluminum molds (6 mm diameter x 3 mm height) were used to prepare the samples. The materials were prepared according to the manufacturer’s instructions and packed into the molds at room temperature. The top surface of each specimen was covered with a celluloid strip and a glass slide, and the specimen was allowed to set in a moist environment in an incubator for 10 min. The specimens were then removed from the molds by applying pressure at one side of the samples. Equia Coat was applied on every second Equia Fil sample, and Equia Forte Coat was applied on every second Equia Forte Fil sample; both types of the coated samples were light-cured from each side for 20 s using a Bluephase C8® Light Unit (Vivadent, Schaan, Liechtenstein). There were four experimental groups, each containing seven samples: (1) Equia Fil coated, (2) Equia Fil uncoated, (3) Equia Forte Fil coated, and (4) Equia Forte Fil uncoated. After coating, the samples were stored in a moist environment at 37°C for 24 h. The compositions of the samples used in the study are shown in Table 1.
Compressive strength measurements and SEM evaluation of the samples
The specimens were subjected to thermocycling (10,000 cycles of 5°C and 55°C, 100 s per cycle, and a 5 s interval to remove water from the chambers). After thermocycling, wear simulation was performed using a chewing simulator (MOD, Esetron Smart Robotechnologies, Ankara, Turkey). A mass of 5 kg, comparable to 49 N of chewing force, was exerted (18). According to the previous studies, 240,000–250,000 loading cycles in a chewing simulator are equivalent to approximately one year of chewing (19, 20). The wear test included 240,000 loading cycles to clinically simulate the one-year chewing condition. After cycle loading, the specimens were stored in distilled water at room temperature for one week prior to compressive strength measurements.
The compressive strength measurements for the tested materials were performed at room temperature (23±1°C) according to ISO 9917-1:2007. The compressive strength was determined by loading at a crosshead speed of 1 mm/min until specimen failure. A universal mechanical testing machine was used (Instron, Lloyd, UK). The fracture load was recorded for each sample, and compressive strength was calculated using the equation CS=4F/(π D2), wherein CS stands for compressive strength, F is the maximum applied load in Newtons (N), and D is the diameter of the specimen in mm (app. 4 mm).
SEM evaluation of the samples
The specimens were analysed using SEM after thermocycling. They were placed into an electrically conductive polymer mass; grinded at 300 rpm using water cooling and sandpaper (P320, P500, P1000, P2400, P4000); and polished at 150 rpm with 30 N force applied using diamond pastes (3 µm and 1 µm) and lubricant. Subsequently, they were gold sputter-coated and observed under SEM (JSM-6400 SEM, JEOL, Tokyo, Japan) at 20, 200, and 1,000 times magnification.
Statistical analysis was performed using the SAS statistical package. The Shapiro–Wilk test was used to analyze the normality of distribution. The Kruskal–Wallis test and factorial ANOVA were used to compare the differences between the sample groups at the level of significance p=0.05.
The distribution of compressive strength measurements for all groups of samples was normal (the Shapiro–Wilk test). The Kruskal–Wallis test and factorial ANOVA analysis showed that there were no statistically significant differences between Equia Fil and Equia Forte Fil irrespective of the coating (Table 2), but a trend of increasing compressive strength in the coated samples was observed (Figure 1).
The SEM evaluation showed that when the samples were not coated prior to thermocycling, Equia Fil and Equia Forte Fil specimens were abraded with surface microcracks present (Figure 2).
The results of this study showed that the compressive strength of Equia Fil and Equia Forte Fil after cyclic loading and thermocycling was higher when the samples were coated after initial setting with Equia Coat and Equia Forte Coat, respectively, but the differences were not statistically significant. These results are in agreement with some previous studies that also reported that applying a low-viscosity coating agent after initial setting enhanced the mechanical properties of the material (11). A recent report claims that there has been improvement in mechanical properties of the fast-setting GICs with simplified application procedures, i.e., in the absence of a protective varnish (21). However, this finding cannot be interpreted as inconsistent with the results of this research since nano-filled coating agents were used instead of varnish.
Furthermore, in this study, Equia Forte Fil exhibited higher compressive strength than Equia Fil. This can be explained by the fact that the glass-hybrid concept of the Equia Forte restorative system, where the filler particles of approximately 25 µm are combined with highly reactive smaller particles (4 µm), contributes to the increase in the material’s strength (22). Furthermore, the nano-multifunctional monomer within the Equia Forte Coat improves the physical properties of the overall restorative system. This monomer used in Equia Forte Coat is functionalized with more functional groups to promote crosslinking, flexibility or adhesion.
Although we did not evaluate clinical performance, we can assume that the improved physical properties should contribute to enhanced clinical performance (14). We can therefore correlate our findings with the results of some clinical studies that reported that the coating did not significantly improve clinical performance of the GIC materials in terms of reduced occlusal wear and increased survival rates (23, 24).
The observed increase of CS in the coated samples, although not statistically significant, could be explained by the reduction of undesirable early water exchange (25). The application of a coating on the newly placed GIC was, therefore, recommended to prevent the water escape before it became strongly bound by hydration of the cations released from the glass or siloxane groups on the surface of glass particles (26, 27). In the present study the specimens were stored in distilled water before the compressive strength measurements were taken, but it was not expected that the storage in water would influence the water sorption in the uncoated groups since the escape of loosely bound water was shown to be critical at this stage, as mentioned earlier (27).
After the acid-base reaction, clinically described as the initial setting, the maturation process takes place within the material until complete setting occurs (28). The water absorbed in the maturation phase occupies coordination sites around metal cations or hydration regions around the polyanion chain within the set cement (9). This enables shrinkage compensation, and the proportion of loosely bound water decreases relative to the proportion of tightly bound water as the cement matures (9, 29). The maturation results in altered mechanical properties for GICs, and compressive strength increases asymptotically to a stable value higher than the one found at 24 hours (28, 30).
The increase in compressive strength during the initial period of a few weeks after placement was established for conventional glass ionomers decades ago, and it seems to occur faster in modern, highly viscous glass ionomers, where the increase in CS is significant during the first day and rises further until one week after placement (28, 31, 32). In this study, the compressive strength of the tested GIC materials was not determined after the initial setting or after 24 h, hence we cannot draw any conclusions about how the CS values changed over time within one group of samples. However, when comparing compressive strength values exhibited by the materials used in this study with the compressive strength values of other fast-setting glass ionomers, we can observe that the values obtained after aging simulations are similar to those obtained after 24 hours in a previous study (33). This might imply that thermocycling does not reduce compressive strength in GIC materials, as already reported (34).
Our results further suggest that coating Equia Fil and Equia Forte Fil rendered their surface free of microcracks, possibly due to more favorable water balances during the first 24 hours that enabled complete maturation. The SEM pictures taken after thermocycling show smooth and microcrack-free surfaces of glass ionomer samples coated with Equia Coat (Figure 2). This also shows that the nano-filled coat does not readily wear off. In this research, we did not focus on occlusal wear or volumetric loss of the loaded material based on GIC technology, but we can assert that the coat visualized on SEM images after thermocycling is consistent with the previously reported reduced occlusal wear of a GIC-based material protected by a surface resinous coating (12).
The clinically important mechanical property of compressive strength after thermocycling and cyclic loading was not found to be significantly improved by coating glass ionomer based restorative materials with nano-filled resinous coats, although a trend toward increase was recorded in the coated samples.