Effect of bonding resin strength on contraction gap formation

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Regarding with polymerizing contraction gap formation, mechanical properties of bonding resins immediately after light-curing seems to be important. The purpose of this study was to evaluate nano-hardness(H), elastic modulus(E) and microtensile strength(MTS) of bonding resins in three self-etching systems at 5minutes after light-curing the materials.The values of H and E at 5 minutes and 24 hours (as a control) after the 10 second irradiation for bonding resins were measured using a nano-indentation tester. MTSs of the materials at 5 minutes and 24 hours were also measured.Two-way ANOVA confirmed a significant increase in all the three mechanical properties at 24 hours compared with those at 5 minutes (p<0.05). As for the materials, the values of the mechanical properties were also varied among the materials.The variation of the mechanical properties observed in the different materials might be explained by differences in their monomer system, initiator system, stabilizer, filler and coupling agent. Within the same material, the difference between the mechanical properties at 5 minutes and at 24 hours might be due to a degree of conversion. It may be desirable that the tensile strengths of the bonding resins just before light-curing restoratives are large enough to compete with polymerizing contraction stresses. On the other hand, the elastic modulus of the bonding resins should not be so large considering the function of the adhesive layer as a stress breaker.As the mechanical properties of the bonding resins might be affected by mixture of primer, water on dentin and so on, further studies should focus on a relationship between bonding performance of the adhesive systems and the mechanical properties of adhesive layers at the resin-dentin interface.