Effect of Zirconia and Alumina Fillers on the Microstructure and Mechanical Strength of Dental Glass Ionomer Cements

2.1. Synthesis of the Modified Glass-ionomer

In this study, commercial resin-modified glass ionomer cement (GIC) (Vitremer, 3M, ESPE), named as VT , was used to synthesize a novel reinforced glass ionomer containing fillers based on yttria stabilized zirconia (Sigma- Aldrich) and aluminum oxide (Sigma- Aldrich), named as VTZ and VTA respectively from now on.

The commercial GIC (VT) was mixed in a ratio of 2.68 g/ml on a glass plate and injected into a polytetrafluoroethylene mold using a Mark II Snap-fit syringe (Centrix, USA) (Fig. 1). The matrix had a capacity to produce six cylindrical samples (6 x 4 mm) simultaneously. The samples were covered with an acetate tape during the light-curing for 40 s using a Coltolux 75 (Coltène/Whaledent, Altstätten, Switzerland) cure device (Fig. 1C). The samples were produced by a single operator according to the manufacturer's recommendations.

Reinforced GIC (VT) formulations were prepared by adding different amounts of zirconia (VTZ) and/or alumina (VTA) nano-fillers into the commercial GIC (VT) composition (Table 1). The cylindrical specimens of the reinforced GIC were prepared following the same method described for the commercial GIC (VT). A resin composite nano-hybrid (Great, VOCO; Germany), named GD from now on, was used in order to establish terms of comparison with the GIC (VT) tested . The cylindrical composite samples were prepared according to the manufacturer's instructions. A total of 210 cylindrical specimens were synthesized and divided into 9 groups (Table 1).

2.2. Mechanical Strength

At first, 90 specimens were evaluated by compressive tests (n = 10) before thermal cycling tests. Then, 5 groups (VT, VTA, VTZ, VTZA and GD) consisting of a total of 120 samples were assessed by compressive tests before and after thermal cycling. Compressive tests were performed at 25 °C by using a universal mechanical testing machine (Instron 8874, MA, USA) with a 25 kN load cell. The tests were monitored using the Trapezium software. Test were carried out at a speed of 1 mm/min. A number of 10 specimens of each condition were tested in order to obtain mean values ​of the compressive force.

2.3. Thermal Cycling

Thermal cycling is an experimental method of simulating the effect of temperature variations on restorative materials as found in the oral cavity [11]. For thermal cycling, the samples were immersed in two consecutive containers containing artificial saliva at 5 ± 3 °C and 60 ± 3 °C. The immersion time in each container was 15 s with a transfer time of 15 s between containers. A total of 40 specimens from 3 groups (VT, VTA, GD), making a total of 120 samples, were divided into 10 samples for each condition (n = 10) of thermal cycling: C0 - before thermal cycling; C1 - 3000 cycles; C2 - 6000 cycles; C3 - 10 000 cycles). Within the particle-reinforced GIC (VT), only VTA was selected for the thermal cycling test due to the best result obtained in the mechanical strength tests. The thermal cycling tests were carried out by immersing the samples in a Fusayama´s artificial saliva solution [23]. The composition of the Fusayama saliva is shown in Table 2. The pH of the artificial saliva solution was approximately 5.5.

2.4. Microscopic and Chemical Analyses

Two specimens of each group were prepared for field emission guns scanning electron microscopy (FEG-SEM). Specimens were wet-ground on SiC abrasive papers down to 1200 mesh. After griding, the specimens were cleaned in distilled water using an ultrasonic bath and then polished with a diamond (1 µm) paste. After preparation, the specimen´s surfaces were sputter coated with carbon to be analyzed by FEG-SEM (FEI NOVA 200; USA).

The microstructure of these materials was analyzed before and after thermal cycling at magnifications between 200x and 10000x under secondary (SE) and backscattered electron (BSE) under voltage of 5-15 kV. The chemical analysis was performed in specific areas of the organic matrix and inorganic nano-particles by energy dispersive spectroscopy (EDAX Pegasus X4M; USA).

2.5. Statistical Analyses

The results of the compressive tests of the different types of samples and after different thermal cycling conditions were analyzed by analysis of variance (ANOVA) using Origin software (8.5.1 OriginPro, OriginLab Corporation, MA, USA). Shapiro-Wilk Test assessed the normality of the population. The difference between the mean values for each group was considered statistically different when p < 0.05. The Tukey multiple comparisons test was used to compare the results with each other.

3.1. Microscopic and Chemical Analyses

FEG-SEM images recorded on glass-ionomer surfaces are shown in Figs. (2A-D). The distribution of inorganic fillers in the commercial GIC matrix is noticed in Fig. (2A), in which Zone “Z1” is identified for chemical analyses. Pores were noticed in the GIC (VT) microstructure by FEG-SEM analyses as shown in Fig. (2B). The results of chemical analysis by X-ray spectroscopy revealed the composition of the first zone “Z1” (Fig. 2A) having the highest intensity peaks corresponding to the following chemical elements: Si (15.8% wt); Al (14.5% wt); and O (22.8% wt). Furthermore, less intense peaks are found for Sr (34% wt) and F elements. Those results allow us to confirm this area as being part of the glass particles.

The distribution of the glass particles can be noticed on the GIC reinforced with alumina (VTA) (Fig. 2C) or zirconia (VTZ) (Fig. 2E). The results of chemical analysis by X-ray spectroscopy recorded on zone “Z1” shown in Fig. (2C) showed the presence of a higher proportion of elements Al (19.1%wt) and O (39.5% wt), which allows us to identify this particle as alumina (Al₂O₃) added to the commercial composition of GIC to synthesize a reinforced material on the VTA group. Chemical analysis by X-ray spectroscopy performed on zones “Z1” and “Z2” indicated in Fig. (2E) revealed a higher proportion of Zr (60.5% wt), O (16.8% wt) and Y (12.3% wt). That indicated fillers composed of ytria-stabilized zirconia in reinforced GIC on the VTZ group. The size of zirconia particles (agglomerates) was larger than 50 μm. The micrograph shown in Figs. (2D) and (2F) reveals the presence of pores and cracks in the microstructure of the reinforced GIC. That was similar to that noticed on the commercial composition (Fig. 2B).

On the other hand, the microstructure of resin composite is revealed in Fig. (2G). Chemical analysis by X-ray spectroscopy performed in the selected areas (Fig. 2H) revealed the presence of Si (29.89% to 42.79 %) and O (60-65%) on zone “Z1”. On zone “Z2” in Fig. (2H), the following elements were detected: Ba (18.51% to 25.40%), Si (20%), Al (17%) and O (35%). Those results suggested two different inorganic fillers based on colloidal silica and barium glass. Zone 3 was chosen in an attempt to acquire results for the matrix composition. No morphological changes were observed by scanning electron microscopy as a result of thermal cycling (Fig. 4). Also, a degradation of GIC or resin composite could not be detected by scanning electron microscopy performed after thermal cycling.

3.2. Compressive Strength Before and After Thermal Cycling

Results obtained by axial compressive tests recorded for commercial or reinforced GIC before thermal cycling are shown in Fig. (3). Statistical analysis showed a significant difference in the mean values ​of compressive strength between the commercial and reinforced GIC groups (p < 0.05) (Table 2). Results showed that alumina particles fillers (VTA1 and VTA2 groups) increase mechanical strength of the commercial glass-ionomer cement (VT). However, the addition of zirconia particles (agglomerates) was found to have a negative impact on the strength of the glass-ionomer material. A Tukey analysis (Fig. 3) showed a significant statistical difference (p < 0.05) between GIC reinforced with alumina particles (VTA1 and VTA2) and the two compositions of GIC with a higher percentage of zirconia particles (VTZ3 and VTZ4). The experimental formulation with simultaneous reinforcement of alumina and zirconia (VTAZ) exhibited similar mechanical strength to that of the GIC containing only alumina (VTA1 and VTA2). GIC containing alumina and zirconia (VTAZ) has also shown a more statistically significant compressive strength (p < 0.05) than the formulations with higher amounts of zirconia (VTZ3 and VTZ4) (Fig. 3 and Table 2). The highest mean values of compressive strength were recorded for resin composite (326.5 ± 45.3 MPa) when compared to resin-modified glass ionomer (174.2 ± 17 MPa) and conventional glass-ionomer (78.78 ± 13.3 MPa).

After thermal cycling, results of the maximum compressive strength for commercial and reinforced GIC are shown in Figs. (5) and (6). Mean values of compressive strength recorded on the specimens revealed that thermal cycling did not cause a significant deterioration of glass-ionomer or resin composite materials (Fig. 7). The compressive strength values ​recorded on the materials in different conditions of thermal cycling did not vary significantly (p > 0.05) when compared to those before thermal cycling (see Tables 3, 4, 5, and 6).