Objective: The aim of this study was to evaluate the void frequency (V F) and void volume (VV) in different flowable composites using swept-source optical coherence tomography (SS-OCT). Methods: Standard class I cavities were prepared and filled with six different flowable composites: Clearfil Majesty LV (MJ; Kuraray), MI Flow (MW; GC), MI Fil (ML; GC), Beautifil flow plus (BF; Shofu), Palfique Estelite low flow (EL; Tokuyama) and Surefil SDR flow (SF; Dentsply). The restorations were scanned under OCT. The OCT tomograms were analyzed and average VF and VV per restoration for each composite were calculated. Scanning electron microscope (SEM) was used to observe the structure of each composite. In addition, the flowability of the materials was evaluated measuring the displacement of each material placed up-right on a glass slide. VF and VV obtained by OCT were also compared to those calculated using micro-computed tomography (micro-CT). Results: Kruskal-Wallis ANOVA and Mann-Whitney U tests revealed significantly different VF and VV values (p < 0.05) among the composites. Voids ranging from 35 to 785 μm in diameter were detected in OCT tomograms. MJ showed highest VF and VV values followed by MW, but ML, BF, EL and SF showed no significant difference. Filler volume in composites showed a positive correlation with void formation, but flowability did not show a specific trend. Micro-CT evaluation validated the VF and VV calculation by OCT, with a significant correlation in void size (p < 0.001, r = 0.94). Conclusion: The results of this study indicate the reliability of SS-OCT for real-time void characterization of composite materials and restorations. Void formation in flowable composites is material dependent.
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