Occurrence of Polymerization Shrinkage Stress and Observation of Longitudinal Sectioned Cavities in the Initial Curing Stage of Bulk-fill Resin Composite

<p> Purpose: A bulk-fill resin composite was developed to reduce the chair time for direct composite restoration. However, there was concern that a large polymerization shrinkage stress was generated when filling a deep cavity with a bulk-fill resin composite. This study investigated the polymerization shrinkage stress during hardening and the mechanical properties of commercial bulk-fill resin composites.</p><p> Methods: Three bulk-fill resins, SDR (SDR), Bulk Base Hard (BBH), and Beautifil Bulk Flow (BBF), were used as experimental specimens. For these specimens, the behavior of shrinkage stress, bending strength, elastic modulus, polymerization shrinkage and polymerization rate were measured. In addition, in order to confirm the influence of shrinkage stress on the cavity and bulk-fill resin, metal cavities and bovine cavities were filled with bulk-fill resin and the longitudinal section of the cavities was observed with a microscope.</p><p> Results: The shrinkage stress of SDR and BBH showed a gradual rise, but BBF showed a sharp rise. The bending strength was small in the order of BBH, SDR and BBF, and a significant difference was observed in each specimen. The elastic modulus of SDR was significantly smaller than that of BBH and BBF. The polymerization shrinkage of BBH was significantly smaller than that of SDR and BBF. The polymerization rate was higher in the order of SDR, BBF, and BBH. In the observation of the longitudinal section of the metal cavities, SDR and BBH showed no contraction gap or cracks in the resin. BBF had no contraction gap, but cracks were observed in the resin. In the observation of the longitudinal section of the bovine tooth cavities, no contraction gap or cracks in the resin were observed in SDR, BBH and BBF.</p><p> Conclusion: The shrinkage stress generation behavior and mechanical properties of bulk-fill resins varied among the specimens. Cross-sectional observation of the metal cavities showed that only BBF had cracks in the resin due to the higher elastic modulus and polymerization shrinkage and lower bending strength than the other two specimens. However, no gaps or cracks were observed in the longitudinal section of the bovine cavities, suggesting that the effect of shrinkage stress on the cavities was low in the clinical use of bulk-fill composites.</p>