Influence of Shrinkage and Viscosity of Flowable Composite Liners on Cervical Microleakage of Class II Restorations: A Micro-CT Analysis

This study determined the influence of shrinkage and viscosity of flowable composite liners on the cervical microleakage of Class II restorations using micro-CT. Seven composites of varying viscosities were selected and included five giomers (Shofu Beautifil II [BF], Flow Plus F00 and F03 [F00 and F03], Flow F02 and F10 [F02 and F10]) and 2 nano-filled composites (3M-ESPE Filtek Z350 [Z350] and Filtek Z350 Flowable [Z350F]). Polymerization shrinkage (n=7) was assessed with the Acuvol volumetric shrinkage analyzer while complex viscosity was determined with the advanced rheometric expansion system at 25 degrees C. Standardized Class II restorations incorporating 1-mm horizontal layers of different flowable liners and 3-mm oblique layers of BF or Z350 were subjected to a silver nitrate test for 24 hours and examined using micro-CT. Microleakage was determined at 0.1-mm intervals from the buccal to lingual surfaces providing 30 sites per specimen and scored accordingly. Statistical analysis was performed with the one-way ANOVA, Kruskal-Wallis test, and Spearman's rho correlation at a significance level of p<0.05. Mean volumetric shrinkage ranged from 5.33 +/- 0.17% to 2.35 +/- 0.02% for F02 to Z350, respectively. The flowable materials had significantly higher shrinkage than did their sculptable counterparts (BF and Z350). Complex viscosities ranged from 9.65 to 4.20 (Z350 and F10, respectively) at a frequency of 10 rad/s and from 8.16 to 3.28 (Z350 and F03, respectively) for 100 rad/s. Giomer restorations had significantly less leakage than did those restored with nano-filled composites. No microleakage was observed with restorations lined with F02 or F10. The use of flowable liners reduced cervical microleakage of Class II restorations. Interfacial integrity of Class II restorations was significantly correlated with liner viscosity, filler volume, and shrinkage.