Evaluation of the conversion degrees of current bulk fill composites using the microhardness method

Objective: The aim of this study is to evaluate the conversion degrees of current bulk fill composites in vitro by microhardness method. Materials and Methods: For this in vitro study, six bulk fill resin composites (SonicFill 2 (Kerr, USA), VisCalor (Voco GmbH, Germany), AdmiraFusion Extra (Voco GmbH, Germany), Filtek Bulkfill (3M Espe, USA), Fill-Up ( Coltene Holding AG, Switzerland) and GrandioSO Heavy Flow (Voco GmbH, Germany) were selected. Each of these six different bulk fill composites will be divided into 2 subgroups and subjected to the Vickers microhardness test (Micromet, Buehler, USA). A total of sixty samples, five in each group, were created.The samples were prepared from each bulk fill composite in cylindrical shape, five 2 mm deep and five 4 mm deep, using Teflon molds of 5 mm diameter at different depths. The lateral surfaces were marked after the Teflon was removed from the mold. The obtained samples were subjected to the Vickers microhardness test (Microtest, FM-310e, Turkey) The microhardness measurement of the samples was made after 24 hours to complete the polymerization. A load of 200 g was applied to the upper and lower surfaces of the samples with the device's indenter for 15 seconds at five different locations, thus creating five diagonal notches on each surface. The application of the load created a symmetrical indentation on each surface whose dimensions were determined under the microscope. After the notches were opened, the diagonal lengths of the diagonal shaped notches formed in the samples placed under the 50x magnifying lens of the microhardness device were determined with the help of the arms of the hardness device moving in the x-y-z plane, and the Vickers hardness (VH) value was automatically calculated by the device. The average of five measurements made from each surface was determined as the hardness value of that surface of the sample. One-way analysis of variance (One-way ANOVA), Tukey HSD multiple comparison test and independent t test were used for statistical analysis, and the level of significance was determined as p< 0.05. 10 Results: Considering the conversion degrees of the samples prepared with a thickness of 2 mm, the highest value was in the Sonic Fill 2 group; the lowest value was seen in the Fill-Up group. Fill-Up group is statistically significantly lower than all other groups. (p< 0,05) There is no statistically significant difference between the other groups. When the conversion degrees of the samples prepared with a thickness of 4 mm are examined, the GrandioSO Heavy Flow group is statistically significantly lower than the other groups. The highest degree of conversion was observed in the VisCalor group. There was no statistically significant difference between the AdmiraFusion Xtra, Filtek Bulk Fill, Sonic Fill 2 and VisCalor groups. Conclusion: The degree of conversion of the bulk fill composite resins used in the study decreased statistically significantly as the layer thickness increased. Upper surface microhardness values were found higher than lower surface microhardness values in all samples of bulk fill composite resins used in the study. In our study, it was found that the applied layer thickness affects the microhardness and degree of conversion of bulk fill resin composites.. Key words: Bulk fill resin composite, Vickers hardness value, Conversion degree.