Influence of Curing Protocol on Bond Strength of Self-etching Adhesives

Objectives: To evaluate the effect of different curing protocols of dual-cure one-step self-etching adhesives on shear bond strength (SBS) of chemical-cure resin composite to dentin.
Methods: A total of 120 anterior bovine teeth were divided into 12 groups (n=10) according to the two experimental factors investigated, namely adhesive type and curing protocol. The adhesives used were: Single Bond Universal (SBU, 3M ESPE, Germany), Futurabond M+, Futurabond U (FBM+ and FBU, VOCO, Germany) and Gradia Core SE (GrSE, GC, Belgium). The curing protocols were: light curing at 1mm, 10mm distance, or left to cure chemically. Adhesives were applied on exposed labial dentin surfaces following manufacturers' instructions, over which chemical-cure resin composite cylinders (2mmx4mm, Rebilda SC, VOCO) were built-up. The specimens were submitted to SBS test (0.5mm/min). Fracture analysis was evaluated using a digital microscope x40. Representative de-bonded specimens were inspected under SEM. ANOVA and Tukey's tests (p=0.05) were applied.
Results: The experimental factors and their interactions (p=0.000) had significant effects on SBS (MPa). At 1mm, SBU (9.4±2.5) = FBM+ (8.2±2.8) = FBU (8.2±2.8); and FBM+ (8.2±2.8) = FBU (8.2±2.8) = GrSE (5.6±1.3). At 10mm, FBM+ (8.1±2.6) = SBU (9.9±2.1) > GrSE (6.5±2.0) > FBU (3.9±0.6); and FBM+ (8.1±2.6) = GrSE (6.5±2.0). In chemical-cure protocol, SBU (2.6±0.6) = FBU (2.7±0.8) = GrSE (3.4±1.0) > FBM+ (1.1±0.5). Within each adhesive, SBU, FBM+ and GrSE didn't show any significant differences at 1mm and 10mm distance, while chemical-cure protocol yielded the least significant SBS. FBU had no significant difference at 10mm and chemical-cure mode, which were significantly lower than 1mm distance. Failure modes observed were 82.5% adhesive, 15% mixed and 2.5% cohesive in dentin. SEM revealed that most of specimens fractured at the bottom of the hybrid layer.
Conclusions: Increasing light curing distance had no influence on SBS except for FBU. Chemical-cure protocol failed to produce acceptable bond strengths.