Evaluation of the surface roughness and microhardness of three bulk fill nonflowable resin composites in comparison to one conventional resin compsite after polishing with different polishing systems

The aim of this in-vitro study was to evaluate the surface roughness and microhardness of three bulk fill nonflowable resin composites in comparison to one conventional resin composite after polishing with different polishing systems. A total of 96 specimens were prepared from all of the resin composite types used: SonicFill (Kerr), X-tra Fil (Voco), Tetric N-Ceram Bulk Fill (Ivoclar Vivadent) and Filtek Z250 (3M/ESPE). 24 specimens from each type, which were fabricated by inserting the resin composite material into a clear plastic split mold (10mm diameter × 4mm depth), for the bulk fill resin composites the mold was filled in one increment to the full depth; while, the conventional resin composite Filtek Z250 (3M/ESPE), it was inserted incrementally into the mold. The specimens were then covered with a transparent matrix strip and microscopic glass slide on the top, then the light curing procedure was made using LED light curing unit (Bluephase Style, Ivoclar Vivadent) for 20 seconds with light intensity of (>1000 mW/cm2 ). Immediately after the fabrication the specimens were stored in deionized water at 37°C for 24 hours. After the storage period, the specimens from each resin composite type were randomly divided into three groups (n= 8), one group was left intact to be used as control; whereas, the other two groups were wet grounded with 600 and 1200 grit silicon carbide papers respectively using a polishing machine, and each group was then assigned to one of the polishing systems used, Sof-Lex XT Discs (3M/ESPE) and PoGo (Dentsply/Caulk). The surface roughness was measured with a profilometer and the microhardness (VHN) was measured with a digital microhardness tester. The data were analyzed by: Two-way ANOVA test was used to determine the effect of resin composite material and polishing system on the surface roughness and microhardness, and the interaction between them; Two independent samples T test was used for comparison between the polishing systems; One-way ANOVA test was used to test the mean difference between more than two independent groups; Kruskall-Wallis test was used to test the median difference between more than two independent groups; when the overall significance was observed, pairwise post-hoc tests were used to compare between the multiple comparisons. P<0.05 was considered statistically significant. According to this in-vitro study, the conventional resin composite category Filtek Z250 showed significantly better results than the bulk fill resin composite categories for both of the surface roughness and microhardness, as lower surface roughness values and higher microhardness values were observed for the Filtek Z250. No significant differences were observed among the bulk fill resin composite categories in case of the surface roughness; however, there were significant differences among them in case of the microhardness. PoGo polishing system produced significantly less surface roughness on almost all of the resin composite investigated than Sof-Lex polishing system. But, they were not significantly different in terms of the microhardness. The surface roughness was both polishing system and resin composite material dependent; nonetheless, the microhardness was only resin composite material dependent. Even though, the bulk fill nonflowable resin composites used in this study showed less surface quality and microhardness after the polishing than the conventional one, they can be placed in increments of 4 mm thickness to restore the posterior teeth without affecting the clinical performance, esthetic, and longevity of such restorations, since all of the bulk fill resin composite categories evaluated in this in-vitro study showed acceptable surface roughness and microhardness. Considering the clinical steps during a cavity restoration procedure, bulk fill resin composites were introduced to save the clinical time, in this sense, PoGo One-step polishing system can add an advantage for the bulk fill concept. Keywords: bulk fill nonflowable resin composite; polishing; surface roughness; microhardness (VHN).