Micro-hardness and contraction stress evaluation of bulk-fill resins composites

Method: 50 specimens of bulk-fill resin composites were prepared with 5 different materials (N=10 each): SonicFil (Kerr), Tetric Bulk (Ivoclar), SDR (Dentsply), Extra Base (Voco), Filtek Bulk (3M ESPE). Composites were bulk-placed in semi-circular mould with 10mm diameter and 6mm depth. Light curing tip was placed in contact with the top surface of the sample and curing was performed for 40 s with a LED lamp (BluePhase 2). Vickers hardness was tested with a micro-indenter at top and bottom surfaces (4 measurements for each surface), and along the lateral side of the sample (6 measurements, one each mm) starting from the side in contact with the light curing tip. Contraction stress was tested with an Universal Machine. Data were statistically analysed with Two-Way ANOVA and Bonferroni test. Hardness progression along lateral side of each group was compared with Friedman test. Statistical significance was set at p=0.05.

Result: Means of Vickers hardness and contraction stress of the different groups are expressed in table 1. 

Conclusion: The null hypothesis (1) was partially accepted since only some bulk-fill materials showed a significant hardness decrease directly proportional to thickness. Null hypothesis (2) is accepted because there is no difference in contraction stress between the resins tested