Flexural strength and microhardness of anterior composites after accelerated aging

Background This study aimed to evaluate the flexural strength and microhardness of three different anterior composites after 10 000 thermocycles. Material and Methods The mechanical properties of a nano-fill composite (Filtek Ultimate Universal Restorative (FUR) (Enamel)), a nano-hybrid composite (Clearfil Majesty ES2 (ES2) (Enamel)), and a micro-hybrid composite (G Aenial Anterior (GAA)) were investigated in this study. For the microhardness test, 8-mm diameter and 2-mm thickness composite discs were used (n = 10), and for the flexural strength test, 25x2x2 mm bar-shaped specimens were prepared (n = 13). The specimens were tested at 24 h and after 10 000 thermocycles. Data were analyzed using two-way analysis of variance and the post-hoc Tukey test (p < .05). Correlations between hardness and flexural strength were calculated using Pearson’s correlation analysis. Results There was a significant difference in the microhardness values of the materials (p < .05). FUR exhibited significantly higher microhardness than ES2 and GAA. However, the flexural strength of three composites was statistically similar at 24 h (p > .05). Pearson correlation analysis revealed that there was a negative relationship between the mean hardness and flexural strength values (correlation coefficient = -0.367, p = .043). After 10 000 thermocycles, microhardness values of each material and flexural strength of ES2 and GAA decreased significantly according to 24 h. Conclusions The nano-fill composite FUR displayed significantly higher microhardness values. However, each resin composite was statistically similar for flexural strength values. Ten thousand thermocycles significantly affected microhardness and flexural strength. Key words:Flexural strength, microhardness, anterior composites.


Introduction
Resin composites are valued in restorative procedures of anterior teeth owing to their esthetic, physical, and mechanical properties (1)(2)(3). Development in fillers and polymers used in dental composites have allowed for a broad selection of materials that meet the requirements of each clinical situation (4). Anterior composite resin materials used in the restoration of anterior teeth, such as class III or class IV restorations located on incisal edges, must also be very fracture-resistant because high shearing forces cause stress to the restoration during chewing (5). Filler size, shape, and distribution directly determine all of these mechanical and esthetic properties of composites (6,7). Mechanical properties including microhardness and flexural strength are important properties for materials used in restorations where severe biting stress can cause defects, which result in inadequate protection against fracture (8). Clinical performance and shelf-life claims of new products are often tested using accelerated aging protocols to provide experimental data (9). Water is known to decrease the mechanical properties of the silane interface, cause filler debonding, and degrade resin in resin-based composites (10,11). Most polymer networks are generally considered as insoluble structures with high chemical and thermal stability (12). Nevertheless, these networks can absorb water and chemicals from their environment. Volumetric changes such as swelling, physical changes such as plasticization and softening, and chemical changes can be seen after aging protocols (12). Thus, the aim of the study was to compare the mechanical properties of three different anterior composites after 10 000 thermocycles. The following null hypotheses were tested: 1) there are no differences in the flexural strength and microhardness values between nano-fill, nano-hybrid, and micro-hybrid composites, 2) there is no relationship between mean hardness and flexural strength values, and 3) 10 000 thermocycles do not affect the flexural strength and microhardness values of anterior composites.

-Vickers Hardness Test
In total, 8-mm-diameter and 2-mm-thick 60 circular samples were prepared, 20 of which were FUR, 20 were ES2, and 20 were GAA. Uncured resin composite samples were condensed into a cylindrical stainless steel ring mold in one increment, and the mold was compressed between two glass microscope slides using finger pressure to remove excess resin and ensure a flat surface. The specimens were then polymerized for 40 s using an LED light (Elipar S10, 3M ESPE, St. Paul, USA) at 1200 mW/cm2. Afterwards, all samples were stored in distilled water at 37ºC for 24 h prior to testing. The top surfaces of the specimens were ground with 1200 grit silicon carbide (SiC) paper for 20 s under running water. Finishing and polishing was performed using Sof-Lex disks (3M ESPE, St. Paul, USA). The Vickers hardness numbers of the specimens were determined at 24 h and after 10 000 thermocycles using a Struers Duramin-5 microhardness tester (Struers Corp., Tokyo, Japan). Three indentations were pressed into the surface under a 300 g load with a 15 s dwell time. The average hardness value for each specimen was determined for 24-h measurements. Each sample was then submerged in thermocycling and identical measurements were taken after 10 000 thermocycles.

Restorative material
-Flexural strength test Preparation of fracture strength test specimens was determined in accordance with ISO 4049 (13) by conducting a 3-point bending test on bar-shaped specimens prepared in a 2×2×25-mm-stainless steel mold. In total, 78 circular samples were prepared: FUR (n=26), ES2 (n=26), and GAA (n=26). Half of the specimens from each composite were tested at 24 h and the remaining half were tested after 10 000 thermocycles. Each specimen was photo-polymerized for 20 seconds on both sides in 5 separate overlapping portions using a handheld light-polymerizing unit (Elipar S10, 3M ESPE, St. Paul, MN, USA). The edges of the specimens were manually finished with 1200-grit SIC-paper. The specimens were stored in distilled water at 37ºC for 24 h. The 3-point bend test was then conducted to half of 26 specimens from each composite using a universal material testing machine (LF Plus, LLOYD Instruments, Ametek, Inc., England) under a 0.5 mm/min cross-head speed, span length 20 mm, and a 2-mm-diameter indenter.
-Statistical analysis Statistical analysis was conducted using the Statistical Package for the Social Sciences v. 20.0 (SPSS, Chicago, IL, USA). Data distribution was checked for normality using the Kolmogorov-Smirnov test. Continuous variables are expressed as mean±standard deviation. Continuous variables for flexural strength and microhardness measurements were compared between the groups using two-way analysis of variance.

Results
The mean Vickers hardness (kg/mm 2 ), flexural strength (MPa) values, and standard deviations for each resin composite at 24 h and after 10 000 thermocycles are shown in table 2.
There was a significant difference in the microhardness values of the materials (p < .05

Discussion
This study was conducted to investigate the flexural strength and microhardness mechanical behaviors of a nano-fill, nano-hybrid, and micro-hybrid composite, and the influence of thermocycles on the flexural strength and microhardness of these three different anterior composites. It was demonstrated that FUR demonstrated significantly higher microhardness values than GAA and ES2, and ES2 was significantly higher than GAA. On the other hand, micro-hybrid composite GAA, nano-fill FUR, and nano-hybrid ES2 exhibited statistically similar flexural strength values. Therefore, the first null hypothesis that "there is no difference in the flexural strength and microhardness values between the nano-fill, nano-hybrid, and micro-hybrid composites" must be rejected. The second null hypothesis, "there is no relationship between Differences in small letters within rows represent statistically significant differences in mean values for measurement times (p<0.05).   the mean hardness and flexural strength values" must also be rejected, because there was a negative correlation between hardness and flexural strength for the anterior composites. Our third hypothesis, "Ten thousand thermocycles do not affect flexural strength and microhardness values of anterior composites" must be rejected because both parameters changed significantly after 10 000 thermocycles.
The highest microhardness values in the study were obtained with FUR. ES2 and GAA exhibited significantly lower microhardness values than FUR. Composites that include pre-polymerized fillers (ES2 and GAA) exhibited significantly lower microhardness values in the present study. Blackham et al. (14) reported that pre-polymerized fillers that contained composites (Gradia Direct Posterior, Premise) performed worse in strength tests than traditional hybrid composites (Z250, Esthet-X). According to Kim et al. (15) filler morphology and loading influenced mechanical properties of composites such as flexural strength and microhardness. The researchers reported that pre-polymerized filler particle-containing composites had significantly lower flexural strength compared with other composites. Pre-polymerized resin filler is primarily added into composites to reduce dimensional change during polymerization (16) and to reduce the amount of unpolymerized resin (17). However, use of pre-polymerized filler might result in an actual lower percentage of filler, which may result in poorer mechanical properties. The three different composites used in the present study that were all anterior composites and they incorporate with different combinations of silica, zirconium, barium glass, fluoroaluminosilicate, and pre-polymerized filler; barium glass for radiopacity, amorphous silica for better handling (15). FUR incorporates zirconium particles; higher microhardness values of FUR may be related with zirconia filler. Also, filler distribution or dimensions could affect hardness results (14,18).
In this study, nano-fill, nano-hybrid, and micro-hybrid composites were investigated. Beun et al. (19) compared the mechanical properties of nano-filled composites with universal hybrid and micro-filled composites. The authors revealed that micro-filled composites exhibited significantly lower mechanical properties than nano-filled and universal hybrid composites. In addition, nanofilled Grandio and universal hybrid Z 100 had significantly higher microhardness values than those of other composites. Similarly, we found that the micro-hybrid composite GAA showed significantly lower microhardness values than nano-fill FUR and nano-hybrid ES2.
One of the results of our study was consistent with a finding of Moraes et al. (20) who reported that nanohybrid resins generally demonstrated inferior properties compared with nano-filled composites, and the behavior of nano-hybrid resin composites was more closely related to that of micro-hybrid than nano-filled materials. Flexural strength is considered as the best measure of strength of dental materials and is defined as the maximum stress a material can resist before failure (23); considerable flexural stresses may occur during the complex process of mastication. Heintze et al. (24) reported that flexural strength was a good indicator for a material's durability under stress, and that it correlated well with clinical longevity. Beun et al. In accordance with studies in the literature, thermocycling significantly affected the microhardness of resin composites in the present study. The microhardness values of each material and flexural strength of ES2 and GAA decreased significantly after 10 000 thermocycles compared with 24-h water storage. The decreasing mechanical properties of materials after water storage results from the separation of polymer chains by water molecules (12). Water can cause the degradation of dental composites by weakening the silane interface and leaching filler particles, or softening the organic matrix due hydrolysis. Both effects result in a decrease of mechanical properties of composites. Different from our study, Bauer et al. (26)

Conclusions
FUR demonstrated significantly higher microhardness values than GAA and ES2; ES2 was significantly higher than GAA. On the other hand, the micro-hybrid com-posite GAA, nano-fill FUR, and nano-hybrid ES2 exhibited statistically similar flexural strength values. There was a negative relationship between the mean hardness and flexural strength values. The microhardness values of each material and flexural strength of ES2 and GAA decreased significantly after 10 000 thermocycles compared with 24-h water storage.