Hardness, Young's Modulus and Ultrastructure of Bonded Primary Tooth Dentin

Objective: We evaluated the physical properties and ultrastructure of bonded sound and carious primary tooth dentin. Methods: Each of nine sound and carious primary molars were used. For sound teeth, flat occlusal dentin surfaces were prepared with a water-cooled high-speed diamond bur. For carious teeth, infected dentin was stained with a caries detector and removed with a water-cooled low-speed steel round bur and hand instruments. Prepared dentin was bonded with Bond Force (Tokuyama Dental Corp.) and restored with Clearfil Protect Liner F (Kuraray Medical Co.). The resin-dentin interface and dentin beneath the interface were measured with a nano-indentation tester and observed with a SEM and TEM. The data was statistically analyzed using one-way analysis of variance and post-hoc Fisher's PLSD at p <0.01. Results: For the sound teeth, the hardness (H) and Young's modulus (Y) at the interfacial dentin was significantly lower than those of the underlying dentin. For the carious teeth, there was no significant difference of the H and Y between the interfacial dentin and underlying dentin. For the interfacial dentin, the H of the sound teeth was significantly lower than that of the carious teeth, however, no significant difference of the Y between them. The H and Y of the underlying dentin of the carious teeth were significantly lower than those of the sound teeth. TEM showed no difference between sound and carious teeth. Nanoleakage and smear layer remnants in the interface and dentinal tubules were observed. Nanoleakage beneath the hybrid layer and streaks of silver deposits within the subsurface porous substrate were also observed. Conclusions: The Bond Force application decreased the hardness and Young's modulus of the interfacial dentin for sound teeth. Smear layer remnants was one reason of nanoleakage. Improvement of the bonding methods and adhesive composites may be required to obtain stable adhesion in primary sound and caries-affected dentin.