Microtensile Bond Strength of Composite Restorations: Direct vs. Semi-Direct Technique Using the Same Adhesive System

The main purpose was to evaluate the in vitro adhesion strength of direct and semi-direct composite resin restorations in dentin, when the same adhesive system is applied, using microtensile testing (μTBS) and to observe the most recurrent types of failure in the different groups. For this study, 16 intact human mandibular molars without microscopic evidence of lesions were randomly divided into two test groups, according to the restoration strategy: direct restoration (DR) and semi-direct restoration (SR). For both restorative strategies, the same adhesive system (Clearfil SE Bond 2, Kuraray, Tokyo, Japan) was applied to the dentin surface using a two-step self-etching approach with no prior conditioning of the dentin, and the same composite resin (Ceram. x Sepctra ST HV, Dentsply Sirona, Charlotte, NC, USA) was used as a restorative material. The indirect restoration was cemented using resin cement (Variolink Esthetic LC, Ivoclar Vivadent, Schaan, Liechtenstein) within the interior side of the restoration. Each specimen was sliced into sections measuring approximately 1 mm2. The rods were then subjected to a microtensile bond strength test and the statistical analysis on the differences in μTBS between the groups were determined with the Mann–Whitney test. The surfaces were examined to determine the failure mode. The Chi-Square test was used to determine the association between the type of restoration and the failure mode. The DR group presented with a mean μTBS of 38.15 ± 10.75 MPa and a predominance of cohesive failures in the composite resin (69.5%). The SR group showed a mean μTBS of 25.45 ± 10.19 MPa and a predominance of adhesive failures (92.3%). There was not only a statistically significant difference in the adhesive strength of the DR and SR groups (p < 0.001), but also a statistically significant association between the type of restorative strategy and failure mode (p < 0.001). Even though Clearfil SE Bond 2 provided acceptable adhesion to the dentin, using the same two-step self-etch adhesive system, lower adhesive strength and more adhesive failures are expected in semi-direct restorations when compared to direct restorations.