Tabriz University of Medical Sciences Journal of Dental Research, Dental Clinics, Dental Prospects 2008-210X 20 3 2026 09 30 Preheating Bulk-Fill Composite Resins: Effects on Microtensile Bond Strength to Dentin in High C-Factor Cavities 172 175 10.34172/joddd.44891 EN Ferdos Naserneysari https://orcid.org/0000-0003-3378-4801 Sana Dibazar https://orcid.org/0000-0001-9556-1422 Mahmoud Bahari Mehdi Esmaeili Journal Article 10.34172/joddd.44891 2026 06 08 2026 08 09 Introduction: Preheating composite resins has been suggested as a practical strategy to reduce material viscosity and improve adaptation to cavity walls. This approach may be particularly advantageous in deep Class I cavities with a high C-factor, where reliable dentin bonding remains challenging. Nevertheless, evidence on the influence of composite resin preheating on the dentin bond strength of bulk-fill composite resins under these conditions remains limited. Therefore, this study evaluated the effect of composite resin preheating on the microtensile dentin bond strength of a bulk-fill composite resin in such cavities. Methods: Thirty-six sound human mandibular third molars, extracted for orthodontic reasons, were included in this in vitro study. After debridement, the cusps were removed to obtain flat occlusal surfaces. Following acid etching and adhesive application, a 1-mm-thick composite resin layer was placed on the occlusal surface. Class I cavities were then prepared with the pulpal floor located in the mid-dentin region. The specimens were randomly allocated to two groups based on the composite resin preconditioning temperature. Composite resin syringes (x-tra fil) were stored in distilled water at either 23°C or 60°C for 30 minutes before placement. The cavities were subsequently restored, and the specimens were subjected to thermocycling prior to microtensile bond strength testing. Bond strength values (MPa) were compared using an independent-samples t-test. Results: The preheated composite group demonstrated significantly higher microtensile bond strength than the room-temperature group (P=0.002). Conclusion: Preheating the x-tra fil bulk-fill composite to 60°C significantly enhanced microtensile dentin bond strength in deep Class I cavities. Further studies are required to confirm the clinical relevance.