Effect of filler size and filler loading on wear of experimental flowable resin composites

Abstract The relationship between wear resistance and filler size or filler loading was clarified for the universal resin composite; however, their relationship in flowable resin composites has not been clarified. Objectives The purpose of this study was to investigate the effect of filler size and filler loading on wear of experimental flowable resin composites by using a cyclic loading device. Material and Methods Nine experimental flowable resin composites consisting of three different sizes (70, 200 and 400 nm) and loading (50, 55 and 60 wt%) of filler were prepared. Bowl-shaped cavities were prepared on a flat surface of ceramic blocks using a No. 149 regular cut diamond point. The cavities were treated with a silane coupling agent and an all-in-one adhesive and then filled with each experimental flowable resin composite. The restored surfaces were finished and polished with a 1500-grit silicon carbide paper. The specimens were subjected to an in vitro two-body wear test using a cyclic loading device. The localized worn surfaces were evaluated at 10,000, 20,000, 30,000, and 40,000 cycles using a computer-controlled three-dimensional measuring microscope (n=5). The volumetric wear loss of the materials was calculated automatically by the equipment. Data were statistically analyzed with two-way ANOVA and post hoc Tukey test. Results Two-way ANOVA showed that the filler size significantly influenced wear volume (p<0.003), but the filler loading did not have a significant effect (p>0.05). A post hoc Tukey test detected significant differences in filler size between 70 nm and 400 nm, and 200 nm and 400 nm (p<0.007). Conclusion The experimental flowable resin composite containing a mean filler size of 400 nm exhibited significantly lower wear resistance in two-body wear compared with those containing mean filler sizes of 200 nm or 70 nm.


Introduction
First generation flowable resin composites were only used in low stress bearing areas because of inferior physical properties 1 . Seemann, et al. 15 (2011) reported that the physical properties of flowable resin composites were improved by increasing the filler particle concentration and modifying the filler size 15 .
In addition, flowable resin composites are easy to use to fill cavities using a direct-application-syringe.
Owing to such improvements in physical and handling properties, the application of flowable resins was expanded to posterior restorations. However, posterior restorations are considerably stressed by cyclic loading during mastication. Consequently, resin composite restorations are subjected to occlusal wear over time 12,14 . In vitro and in vivo studies have reported the wear resistance of universal resin composites, whose wear resistance was considerably improved by adding variously sized filler particles 3,6,[9][10][11]20,21 .
Although wear of universal hybrid resin composites is no longer considered a major clinical problem 4,11,13 , other research concluded that the wear resistance of flowable resin composites for posterior restorations was quite limited 2,14 . On the other hand, Sumino, et al. 18 (2013) reported that the localized wear and flexural properties of the flowable resin composites tested were equivalent to those of universal resin composites produced by the same manufacturers 18 .
Thus, the wear resistance of flowable resin composite is still a controversial area.
Our previous study 17 , which examined three-and two-body wear values of flowable resin composites for posterior restoration using a mechanical loading device, demonstrated that the wear resistance of the flowable resin composite containing nanofillers or spherical fillers was equivalent to that of a universal resin composite used as a control. The study also suggested that the size and shape of fillers in the flowable resin composite might influence both threeand two-body wear resistances. For the universal resin composite, the relationship between wear resistance and filler size or filler loading was clarified by many studies 2,5,8,9,11,18 . However, their relationship in flowable resin composites has not been clarified. The purpose of this study was to examine the effect of the size and loading of filler on the two-body wear resistance of experimental flowable resin composites using an in vitro wear simulator. The null hypothesis was that filler size and loading would not influence the two-body wear resistance of experimental flowable resin composites.

Material and methods
The materials used in this study are presented in

Two-body wear testing
The specimens were fixed to a stainless cup with an acrylic resin, and the cups were mounted on a cyclic loading device (Ito Electric Construction, Niigata, Japan). The resin restorations were subjected to a twobody wear test, in which a conical ceramic (aluminum nitride) stylus was used to apply a cyclic compressive load of 75 N to the surface of each restoration at a rate of 120 contacts/min.

Measurement of two-body wear of restorations
The localized worn surfaces of the restorations were scanned at 10,000, 20,000, 30,000, and 40,000 cycles with a computerized three-dimensional microscope (STM6DF, Olympus Corp., Tokyo, Japan). The volume of the worn area was obtained using a computer software package associated with the microscope.

Microscopic observation of the worn surfaces
The worn surfaces of a representative specimen in each group after 40,000 cycles were observed using a scanning electron microscope (SEM, S-800, Hitachi Corp., Tokyo, Japan) at ×40 and ×5,000 magnification.

Results
The mean wear volumes of each material at the respective wear cycles are presented in Table 1, and the correlation of the wear volumes between the two factors after 40,000 cycles is shown in Figure 4.
Although all of them tended to increase gradually with the number of wear cycles, the increase in groups 1, 2, and 3 was considerably larger compared with those of the other groups. After all wear cycles, two-way ANOVA showed that the filler size significantly influenced wear volume (p<0.003), but the filler loading did not significantly affect wear volume (p>0.05), and a significant interaction between these factors was not   (Table 1). However, there were no significant differences in wear volume among all the groups after 10,000 cycles (p>0.05) ( Table 1).    The adhesion between filler and polymerized resin matrix could also be related to the wear resistance of flowable resin composites. Tamura, et al. 19 (2013) reported that the filler loading is directly related to the occlusal wear of experimental resin composites containing four types of filler particles, including non-porous spherical silica, porous spherical silica, porous spherical zirconium silicate, and irregularshaped silica. However, the mechanical properties tested (flexural strength, elastic modulus, and Vickers hardness) showed no correlation with the occlusal wear. The study showed that the experimental resin composites containing porous spherical fillers exhibited significantly higher wear resistance compared with those containing non-porous spherical fillers. They speculated that the porous surface texture would produce a strong bond between the filler particles and resin matrix because of the mechanical retention of the resin matrix that penetrated into the tiny concavities.
From the results of the simulated occlusal wear test, they mentioned that the bonding between the filler particles and resin matrix dominantly influenced the occlusal wear, and indicated that the occlusal wear could not be directly influenced by the mechanical properties. The average filler particle size containing these resin composites was 1.7 to 2.5 μm, which was much larger compared with the experimental flowable resin composites used in this study. The protruded large filler particles might sustain greater friction with the stylus tip during occlusal wear testing.
From the results of our study, the null hypothesis that filler size and filler loading would not influence the two-body wear resistance of experimental flowable resin composites was partially rejected. Up to 40,000 wear cycles, the filler size significantly influenced wear volume, but the filler loading did not have a significant effect.

Conclusion
The experimental flowable resin composite containing a mean filler size of 400 nm exhibited significantly lower wear resistance in two-body wear compared with those containing mean filler sizes of 200 nm or 70 nm.