Dentin Shear Bond Strength of 5th and 7th Generation Adhesives Co-Cured with Preheated Composite Resin
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Abstract
Co-curing adhesives with composite resins can simplify restorative procedures and reduce film thickness, but attenuation of light and polymerization shrinkage stress often lead to compromised bond strengths. While preheating composite material improves its flowability and degree of conversion, rapid thermal dissipation during clinical handling remains a significant limitation. This study aimed to investigate the synergistic effect of simultaneous preheating and co-curing techniques on dentin bond strength, utilizing both total-etch (5th-generation) and self-etch (7th-generation) adhesive systems.
Methods: Eighty human teeth specimens with exposed flat dentin surfaces were divided into two main experimental groups depending on the adhesive system used: 5th-generation total-etch (OptiBond Solo Plus; Kerr) and 7th-generation self-etch (Bond Force II; Tokuyama). Each group was further subdivided into two subgroups (n = 30) based on the curing protocol: conventional sequential polymerization versus simultaneous co-polymerization with a composite resin (Herculite XRV; Kerr) that was preheated (40–43 °C) directly on the tooth surface using a specialized metal heating device. Shear bond strength (SBS) was measured using an UltraTester testing machine at a crosshead speed of 1 mm/min. Data were recorded in pounds and converted to megapascals (MPa), followed by statistical analysis using Student’s t-test (p < 0.05).
Results: Co-polymerization of the preheated composite with the 5th-generation adhesive system significantly decreased the SBS to dentin by 1.3-fold compared to conventional sequential polymerization (p < 0.05). Conversely, co-polymerization of the preheated composite with the 7th-generation self-etch adhesive resulted in a significant 1.2-fold increase in SBS (p < 0.05). Under the co-polymerization protocol, no statistically significant differences were observed between the two adhesive types.
Conclusions: The impact of combining preheating and co-curing protocols is strictly dependent on the adhesive system generation. Direct in situ heating of a composite resin over a self-etch adhesive significantly enhances its bonding performance, likely due to enhanced chemical interaction between monomers and tooth hydroxyapatite, whereas it impairs the performance of total-etch systems.