Long-term bond strength of restorations subjected to thermo-mechanical stresses over time

To evaluate the long-term effect of thermal and mechanical cycling on dentin bond strength to cervical margins of Class II restorations.

Sixty Class II slot cavities were prepared in bovine incisors. The cavities were restored with Single Bond and Z-250 composite according to manufacturer's instructions. The teeth were then divided into two groups (n = 30): specimens that would receive thermo-mechanical load cycling (2,000 cycles 5-55 degrees C/100,000 cycles of 50 N loading) (TM) and the control (C). Fifteen specimens from each group were tested at baseline and the remaining 30 specimens were stored in distilled and deionized water at 37 degrees C and tested after 1 year. For microtensile evaluation, the restorations were sectioned perpendicular to the cervical bonded interface into 0.7 +/- 0.2 mm thick slabs. The slabs were further trimmed at the interface to 1.4 < or = 0.2 mm with a fine diamond bur to produce a cross-sectional surface area of ca. 1 mm2. Specimens were then subjected to microtensile bond testing. The bond strength data were analyzed using two-way ANOVA and Fisher's PLSD test (P < 0.05). Fracture mode analysis was performed using SEM.

At baseline, bond strength of the TM group was significantly lower when compared to the C group (P = 0.012). However, after 1 year storage, a significant decrease in bond strength was observed for the C group compared to baseline. No significant differences were noted between the C and TM groups at 1 year. No interaction was observed between groups (C and TM) and storage time (P = 0.098). For the fracture mode evaluation, at baseline, mixed failure was predominant for the C group, and increased after TM. Decreased mixed failure was observed after 1-year storage, with a concomitant increase in interphase failure.