Nanoindentation Mapping and Bonding Strength of the Resin-dentin Interface

Objectives: To investigate the micromechanical properties within the resin-dentin interfacial regions based on nanoindentation technique and compare the bonding strength of different dentin positions in etch-and-rinse or self-etch mode.
Methods: Fourteen non-carious human third molars were sectioned horizontally to the occlusal surface to obtain superficial, middle and deep dentin slices, representing varying inter-tubular dentin/tubular ratios. Each dentin slice was divided into two parts for bonding with All Bond Universal in etch-and-rinse and self-etch mode. In situ nanoindentation was used to map out elastic modulus and nanohardness of resin-dentin bonding interface (N=6, one per group). Microshear bond strength (μSBS) was measured to evaluate the mechanical capabilities (N=60, ten per group). Field-emission scanning electron microscopy (FE-SEM) was utilized to examine the microstructure of dentin and fracture bonding surface.
Results: Nanoindentation mapping showed the continuous hierarchical structures of resin-dentin bonding interface, including the resin (R), adhesive (A), hybrid layer (HL), partially demineralized dentin (PDD) and normal dentin (D). Elastic modulus and nanohardness values of the HL and PDD were not significantly different regardless of the dentin location or adhesive mode (p > 0.05). The μSBS values of the superficial dentin in etch-and-rinse (15.68 ± 1.98 MPa) or self-etch mode (12.21 ± 2.58 MPa) were obviously higher than that of deep dentin (11.44 ± 3.34, 8.37 ± 2.99 MPa, respectively) (p < 0.05). The μSBS values of the superficial and middle dentin in etch-and-rinse mode were higher than in self-etch ones (p < 0.05).
Conclusions: Although similar microstructures were found at the resin-dentin bonding interfaces using nanoindentation mapping, different bonding strengths were obtained for the different regional dentin and adhesive modes. This indicates that mechanical interlocking between inter-tubular dentin and adhesives dominates the bonding strength while the interactions between the adhesive and collagen fibers within the resin tags contribute less.