Correlation between bond strength and nanomechanical properties of adhesive interface

The aims of this study were to evaluate the nanohardness and Young's modulus of the adhesive-dentin interface and to correlate them with the microtensile bond strength (μTBS) after storage in water for 24 h and 6 months.

Eighty human third molar teeth were bonded to composite resin with the Adper Scotchbond Multipurpose (SBMP), Adper Single Bond 2 (SB2), Clearfil SE Bond (CSE) or Clearfil S3 Bond (CS3) adhesive systems and stored in water for 24 h and 6 months. Three bonded teeth were selected for each group for nanoindentation methodology to obtain the nanohardness and modulus values (n = 3), while seven bonded teeth each group were used for μTBS (n = 7). Data were analyzed by two-way ANOVA and Tukey's test (α = 0.05). Spearman correlation between nanohardness and modulus and μTBS values was also calculated.

Nanohardness and Young's modulus values for the dentin and hybrid layer were not significantly different regardless of the adhesive system or water storage periods (p > 0.05). Nanohardness and Young's modulus values for the adhesive layer were significantly higher for SB2 than for SBMP, CSE, and CS3 systems in both water storage periods. The μTBS values for SBMP and CSE were significantly higher than for SB2 and CS3 in both storage periods. An inverse correlation between Young's modulus and μTBS was observed for the adhesive layer.

The water storage periods did not alter nanohardness, Young's modulus, and μTBS of adhesives. The adhesive layer of SB2 showed the highest nanohardness and Young's modulus, but the highest dentin bond strength was obtained with SBMP and CSE. Thus, a lower Young's modulus yielded high bond strength.

The inverse correlation between the Young's modulus of adhesive systems and dentin bond strength suggests adequate resistance of the adhesive to elastic deformation under stress, which are important properties to predict the success of the dental restoration.