Effect of elastic behavior of the hybrid layer on dentin bonding stability

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The effect of elastic properties of the hybrid zone on the dentin bonding efficacy was investigated in four commercialized adhesive systems.Evaluation of the elastic properties of the bonding area : Polished coronal dentin surface of human extracted molars were subjected to the study. Three brands of wet bonding system and a set-etching primer system were used in the study. The elastic properties was calculated from the difference. Between the total indentation depth after 10 gf loading for 30 seconds and permanent indentation depth after removal of the load measured by means of a universal microhardness testing machine. Indentation test was performed on the surfaces of polished dentin, etched/primed dentin, bonding resin applied and resin composite.Cavity adaptation in the dentinal cavity : On the flat coronal dentin, a cylindrical cavity 3.5 mm in diameter and 2 mm in depth, was prepared. After restored with the above restorative systems, the restoration was sectioned. Cavity adaptation was evaluated on the section along the cavity wall under a light microscope.Results and Discussion : Across the bonding area from the untreated dentin to resin composite, a drastic reduction of the elasticity was found at the bonding resin layer in the three wet bonding systems, while a self-etching primer system showed a gradient of the elastic properties. Concerning cavity adaptation. gap formation caused by the contraction stresses was seen at the line angle and/or cavity floor in the. wet bonding systems. A self-etching primer system resulted in a good sealing along the cavity wall. These results implied that. the hybrid layer might play a role of stress-releasing. The stress-releasing by the hybrid layer may be effective when the bonding area is graded in terms of elasticity and not related with the dimension of the hybridization.