Tehran University of Medical Sciences Frontiers in Dentistry 2676-296X 16 6 2020 06 24 407 414 EN Ebrahim Yarmohammadi Department of Operative Dentistry, Hamedan University of Medical Sciences, Hamadan, Iran Shahin Kasraei Department of Operative Dentistry, Shahid Beheshti University of Medical Sciences, Tehran, Iran Yasaman Sadeghi Department of Operative Dentistry, Gilan University of Medical Sciences, Rasht, Iran 2018 09 11 2019 12 07 Objectives: It has been reported that bulk-fill composites simplify tooth restoration with no adverse effect on the success rate. This study sought to assess the cuspal deflection of premolars with mesio-occluso-distal (MOD) cavities restored with bulk-fill and conventional posterior composite resins. Materials and Methods: This in-vitro experimental study was conducted on 64 human maxillary premolars. MOD cavities were prepared on teeth and restored with Filtek P60 conventional composite and Filtek Bulk Fill flowable, X-tra fill, and X-tra base bulk-fill composites in four groups (n=16). Distance between the cusp tips was measured before, five minutes, 24 hours, 48 hours, and one week after restoration. The data were analyzed using repeated-measures analysis of variance (ANOVA) and Tukey’s test (α=0.05). Results: The mean±standard deviation (SD) of cuspal deflection at five minutes after the restoration was 13.5±5.3, 12.2±3.5, 11.3±4.4, and 10.4±3.7 µm for Filtek P60, Filtek Bulk Fill, X-tra fill, and X-tra base, respectively. ANOVA showed that bulk-fill composites did not cause a significant reduction in cuspal deflection compared to P60 (P>0.05). Cuspal deflection in all groups significantly decreased with time (P<0.05). Conclusion: Bulk-fill composites have no superiority over P60 in the reduction of cuspal deflection. The cuspal deflection was variable at different time points in all groups and decreased over time. https://fid.tums.ac.ir/index.php/fid/article/view/2804 https://fid.tums.ac.ir/index.php/fid/article/download/2804/1783