Effects of Aging on Crack Formation Through Dentin and Composite

Objectives: We aimed to determine the influence of thermal cycling and flowable composite on microtensile bond strength (MTBS), crack formation and mechanical properties of the bonding interface using swept-source optical coherence tomography (SS-OCT) and nanoindentation.
Methods: MTBS test beams (0.9x0.9 mm²) were prepared from flat dentin of human premolars bonded with self-etch adhesive (Clearfil SE Bond, Kuraray Noritake Dental) and hybrid composite (Clearfil AP-X, Kuraray Noritake Dental) with or without lining using flowable composite (Estelite Flow Quick, Tokuyama Dental). The beams were then aged for 0 or 10,000 thermocycles (TC), resulting in 4 groups of specimens according to the placement technique and thermal cycling (n=10). MTBS test was performed at 1 mm/min crosshead speed. 2D images of specimens were recorded before thermal cycling, before and after bonding test using SS-OCT (Santec IVS-2000) to detect crack formed at interface. Nanoindentation Marten’s hardness and elastic modulus of eight successive layers across resin-dentin bonding area (100 μm axial spacing) were measured 100 mN load (ENT-1100a, Elionix). Data of bond strength, crack percentage and mechanical properties were statistically analyzed (α = 0.05).
Results: Two-way ANOVA showed that flowable lining significantly increased MTBS (p<0.05), while TC was not a significant factor (p>0.05). In thermocycled debonded beams, cracks were observed in both dentin and composite while in beams without aging process, cracks appeared only in dentin. TC significantly increased crack occurrence in composite while there was no significant difference in dentin crack. Thermal challenge significantly affected hardness of dentin and both resin composites (p<0.05).
Conclusions: Lining with low-viscosity composite improved the bond strength to dentin. Mechanical properties of dentin and resin composite significantly decreased after 10,000 thermocycles, which concurred with increased interface crack formation at both dentin and composite after MTBS test. OCT can visualize internal cracks after restoration debonding.