Thermocycling effect on elastic modulus of dentin bonding interfaces

Objectives: Current hydrophilic dentin adhesives tend to absorb more water, which may result in decreased stiffness. The aim of this study was to determine the effects on the elastic modulus (EM) of the adhesive and the hybrid layer from thermocycling. Methods: Freshly extracted human molars were randomly divided into three groups (n=7/gp), cut to create flat dentin surfaces, bonded with light-cured composite using one of commercial adhesives (OptiBond FL [OP], Clearfil SE Bond [CL], and Xeno III [XE]). Each tooth was sectioned in halves, putting one into immediate bonding test and the other into thermocycling (×10,000), additionally sectioned from the midline to make a 2-mm thickness slab. One slab from immediate bonding and the other slab from thermocycling had two mirror-imaged surfaces and were utilized in comparison to minimize the histomorphological divergence between teeth. Nanoindentation test was performed to measure the EM of the adhesive and the hybrid layer, respectively, using atomic force microscope. Data was statistically analyzed by mixed-level ANOVA and ANCOVA and t-test (p<0.05). Results: After thermocycling, both in the adhesive and the hybrid layer EM showed a tendency towards decrease. In XE, EM decreased significantly in the adhesive layer (p=0.0246). Conclusion: Adhesives containing hydrophilic monomers was more prone to hydrolytic degradation induced by thermocycling, which was represented by reduced EM.