Assessment of the quality of resin-dentin bonded interfaces: an AFM nano-indentation, μTBS and confocal ultramorphology study

Objective: The aim of this study was to assess by using confocal microscopy (CLSM), AFM nano-indentation and microtensile bond strength test (μTBS) the quality of the resin-dentin interfaces created with selected bonding parameters.

Methods: Dentin conditioned with H(3)PO(4) or EDTA was bonded in ethanol- or water-wet condition using a HEMA-free or HEMA-containing adhesive. The resin-bonded teeth were stored in distilled water (24h) and sectioned as match-sticks (0.9 mm(2)) for μTBS. Further resin-bonded teeth were sectioned and analyzed using CLSM, and AFM nano-indentation. The AFM imaging and nano-indentation processes were undertaken using a Berkovich diamond indenter. The modulus of elasticity (Ei) and hardness (Hi) across the interface were evaluated with the specimens in a fully hydrated status. The AFM imaging was performed both in dry and wet conditions for evaluating the shrinkage of the hybrid layer on dehydration.

Results: The HEMA-containing adhesive applied onto H(3)PO(4)-etched ethanol or water-wet dentin created hybrid layers with the lowest biomechanical nano-properties (p<0.05); no significant differences in μTBS were found between the two wet-bonding techniques (p>0.05). However, the ethanol-wet bonding reduced the dye penetration into the adhesive layer created with the HEMA-containing adhesive. Hybrid layers with high biomechanical properties, low micropermeability and no shrinkage were only possible when using HEMA-free adhesive applied in ethanol wet-dentin. In particular, a significant increase in Ei and Hi was achieved at the hybrid layer and underneath the resin-dentin interface of ethanol-wet EDTA-treated dentin.

Significance: The use of HEMA-free adhesives applied onto ethanol-wet dentin may be considered as an alternative and suitable bonding strategy to achieve high quality resin-dentin interfaces.