ASSESSMENT BY NANO-INDENTATION OF THE HARDNESS AND ELASTICITY OF THE RESIN-DENTIN BONDING AREA

The hardness and Young's modulus of the successive layers across a res in-dentin bonding area were determined by nano-indentation for four co mmercially-available dentin adhesive systems, of which two were also a pplied with a different conditioning agent. With a computer-controlled nano-indentation technique, minute triangular indentations were made within a small area of a few micrometers' diameter at a load of a few milli-Newtons. The load and displacement of the indenter were continuo usly monitored during the loading-unloading sequence, so hardness and Young's modulus could be computed as a function of the indenter geomet ry and the applied load. The hardness of the resin-dentin interdiffusi on zone was significantly lower than that of unaltered dentin. A gradi ent of moduli of elasticity was observed from the rather stiff dentin over a more elastic resin-dentin interdiffusion zone and adhesive resi n layer to the restorative composite. That gradient was more substanti al in those systems that produced relatively thick adhesive resin laye rs or supplementally provided a filled low-viscosity resin as an inter mediate layer between the adhesive resin and the bulk restorative comp osite. Such an elastic bonding area might have a strain capacity suffi cient to relieve stresses between the shrinking composite restoration and the rigid dentin substrate, thereby improving the conservation of the dentin bond and, as a consequence, the marginal integrity and rete ntion of the restoration.