Assessment of Debonded Interface After Thermocycling by Optical Coherence Tomography

Objectives: The purpose of this study was to determine the influence of thermal cycling and flowable composite on the microtensile bond strength (MTBS) and fracture mode using swept-source optical coherence tomography (SS-OCT) .
Methods: MTBS test beams (0.9x0.9mm²) were prepared from flat dentin of human premolars bonded with self-etch adhesive (Clearfil SE Bond, Kuraray) and universal composite (Clearfil AP-X, Kuraray Noritake Dental) with or without flowable lining (Estelite Flow Quick, Tokuyama). After storage (37^oC, 24h), half of specimens in each group were subjected to 10,000 thermocycles (n=10). MTBS test was performed at 1mm/min crosshead speed. 2D images of specimens were obtained after thermal cycling and bond test utilizing SS-OCT (Santec) to detect crack formed at interface. Representative specimens and fracture mode were visualized under confocal laser scanning microscope for confirmation of OCT findings.
Results: Two-way ANOVA showed that flowable lining significantly increased MTBS (p<0.05), regardless the usage of thermal cycling. Under SS-OCT, crack appeared in both dentin and composite part of thermocycled debonded beams, while there was only dentin crack of groups without thermal challenge. Fisher’s exact test revealed that thermal cycling significantly increased the crack frequency in groups using flowable lining (p<0.05). Placement technique did not affect the crack occurrence in tested groups (p>0.05).
Conclusions: Lining with low-viscosity composite improved the bond strength. Thermocycling induced crack in substrate and materials after MTBS. SS-OCT can visualize internal cracks after restoration debonding.