Study on Polymerization Contraction Stress of Bulk-fill Resin Composites

Purpose: While incremental filling is recommended for deep cavities when a light-cured composite resin (CR) is used, because of issues such as polymerization contraction stress and irradiation depth, bulk-fill CRs, which can be filled in large cavities at once, have been developed and used in clinical application. This study examined the effects of the polymerization contraction stress of bulk-fill CRs in high C-factor cavities by conducting visual evaluation using polycrystallized glass blocks, shear bond strength tests on bovine dentin, and measurement of the residual volume after curing. Materials and methods: Bulk Base Hard (BH) and Beautifil Bulk Flow (BF) were used as bulk-fill CRs, and Gracefil Flow (GF) as a conventional CR. As bonding systems, Clearfil Mega Bond2 (MB) and Clearfil Universal Bond Quick ER (ER) were used. Bioram-M was used as the polycrystallized glass block. The groups filled with each CR after bonding treatment with MB were designated as the MBH Group, MBF Group, and MGF Group, and the groups filled with each CR after bonding treatment with ER as the EBH Group, EBF Group, and EGF Group. A cylindrical cavity of 4.5 mm diameter and 4.0 mm depth in Bioram-M was formed, the cavity was repaired, and the state of gap and crack generation was classified and scored. Furthermore, a flat dentin surface on a bovine front tooth was prepared, repaired, and then the tensile-shearing strength was measured immediately after bonding. A rubber ring jig with 4.5 mm diameter and 4.0 mm height was prepared and each CR was filled in a darkroom to prepare cylindrical samples. The specimen was soaked in acetone immediately after curing, and the unpolymerized parts of the CR were removed. The residual volume of each specimen was measured after removing the unpolymerized CR. Results: As a result of visual evaluation, a significant difference in the state of gap and crack generation was observed in the MBH and MBF Groups compared to the MGF Group, and in the EBH and EBF Groups compared to the EGF Group (p