Composite and bond strength tests: effects on adhesives/enamel interface

Objectives: To compare the micro-tensile (mTBS) and micro-shear (mSBS) enamel-resin bond strength of adhesive systems using composite resins, with different elastic modulus. Methods: The mesial, distal, buccal and lingual surfaces of 36 permanent molars were wet-grounded to obtain a flat surface and wet-polished in a 600-grit SiC paper (60s) to standardize the smear layer. The adhesive systems Adper Single Bond 2 (3M ESPE), Solobond Plus and Futurabond NR (VOCO) were applied on the enamel surfaces and light-cured (600mW/cm²) [Optilux 501, Kerr] according to the manufacturer's instructions. In order to obtain cylinder-shaped samples for mSBS, a tygon tube (0.75mm diameter; 0.4mm high) was filled either with Filtek Z250 or Filtek Flow Z350 (3M ESPE), placed on the enamel surfaces and light-cured. To obtain beam-shaped samples for TBS, 4mm high composite crowns were constructed with the composites on the enamel surface, light-cured, and sectioned to obtain 0.8mm² beams. Before testing, all samples were stored in water (24h/37ºC). The tests were conducted under tensile or shear loads at a cross-head speed of 0.5 mm/min. Average bond strength values were calculated in MPa and the data were subjected to a three-way repeated measures ANOVA (a= 0.05). Results: Only the main factors Composite and Test were significantly different (p=0.000, p=0.003, respectively). Higher bond strength values were observed for Z250 (31.9±8.6) composite than Filtek Flow (20.2±5.2) regardless the bond strength test employed. The values obtained under mTBS (mean 30.0±8.1) were significantly higher than those measured under mSBS (mean 22.1±5.9). Conclusion: The bond strength values of adhesive systems can be affected by the elastic modulus of the composite resin and by the bond strength test employed.