Background: The most common dental materials are resin composite direct restorative materials and dentin adhesives, which are marketed with different techniques, application recommendations, and compositions, making it difficult to choose the most suitable material and method for different cases.
The present study aimed to investigate mechanical properties of teeth restored with universal and bulk-fill composites, by using different dentin adhesives and techniques, under in vitro conditions.
Material and methods: The study was carried out on freshly extracted premolar and molar teeth. After cavity preparation, the teeth were restored with conventional and bulk-fill resin composites, using different adhesive techniques. To assess the effect of the cavity preparation, the direct restoration, and the adhesive protocol on the tooth structure, a transillumination method and Vickers’ microhardness measurements were carried out.
Results: The universal composite showed an average hardness of 55.35 HV at the occlusal level of the restoration, while the bulk-fill composite showed an average of 79.93 HV at the same level. A statistically significant difference was found between the hardness values of the two composites (p = 0.02). The transillumination test revealed micro-fissures in the tooth structure in the first phase after cavity preparation and also after polymerization.
Conclusions: The bulk-fill composite showed higher hardness values than the universal composite. The tested dentin adhesives did not significantly affect the hardness of the dentin at the level of the adhesive interface. Fissures can appear in any phase of the direct restoration, after cavity preparation and polymerization.
