Objective: This study investigated simulated cuspal deflection and flexural properties of bulk-fill and conventional flowable resin composites. Methods and Materials: Five bulk-fill and six conventional flowable resin composites were evaluated. Aluminium blocks with a mesioocclusal- distal cavity were prepared and randomly divided into groups for each of the different measurement techniques and were further subdivided according to the type of flowable resin composite. The simulated cuspal deflection caused by the polymerization of resin composite within an aluminium block was measured using a highly accurate submicron digimatic micrometer or a confocal laser scanning microscope (CLSM). In addition, the flexural properties of tested resin composites were measured to investigate the relation between cuspal deflection and flexural properties, and the resin composites were observed using scanning electron microscopy. Results: Simulated cuspal deflection of some bulk-fill flowable resin composites was found to be significantly lower than or similar to those for conventional counterparts, regardless of the measurement method. There were statistically significant differences in flexural properties depending on the material, regardless of the type of flowable resin composite. Pearson correlation analysis did not reveal a statistically significant relationship between flexural properties and cuspal deflection. Conclusion: These results indicate that some bulk-fill flowable resin composites exhibit lower cuspal deflection with the bulk-filling technique than is shown by conventional flowable resin composites using the incremental filling technique. Simulated cuspal deflection can be measured using either a micrometer or CLSM, but this experiment failed to show a significant relationship between cuspal deflection and flexural properties of flowable resin composites.
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