The purpose of this study was to evaluate the effect of the polymerization shrinkage and modulus of elasticity of composites on the cusp deflection of class V restoration in premolars. The sixteen extracted upper premolars were divided into 2 groups with similar size. The amounts of cuspal deflection were measured in Class V cavities restored with a flowable composite (Filtek flow) or a universal hybrid composite (Z-250). The bonded interfaces of the sectioned specimens were observed using a scanning electron microscopy (SEM). The polymerization shrinkage and modulus of elasticity of the composites were measured to find out the effect of physical properties of composite resins on the cuspal deflection. The results were as follows.

The amounts of cuspal deflection restored with Filtek flow or Z-250 were 2.18 ± 0.92 µm and 2.95 ± 1.13 µm, respectively. Filtek flow showed less cuspal deflection but there was no statistically significant difference (p > 0.05).

The purpose of this study was to determine the effects on the elastic moduli of the adhesive and the hybrid layer from thermocycling. Twenty one human molars were used to create flat dentin surfaces. Each specimen was bonded with a light-cured composite using one of three commercial adhesives (OptiBond FL [OP], Clearfil SE Bond [CL], and Xeno III [XE]). These were sectioned into two halves and subsequently cut to yield 2-mm thickness specimens; one specimen for immediate bonding test without thermocycling and the other subjected to 10,000 times of thermocycling. Nanoindentation test was performed to measure the modulus of elasticity of the adhesive and the hybrid layer, respectively, using an atomic force microscope. After thermocycling, XE showed a significant decrease of the modulus in the adhesive layer (p < 0.05). Adhesives containing hydrophilic monomers are prone to hydrolytic degradation. It may result in the reduced modulus of elasticity, which leads to the mechanically weakened bonding interface.