Tehran University of Medical Sciences Frontiers in Dentistry 2676-296X 16 2 2019 04 30 136 143 EN Zahra Hasani Department of Pediatric Dentistry, School of Dentistry, Semnan University of Medical Sciences, Semnan, Iran Effat Khodadadi Dental Materials Research Center, Health Research Institute, Babol University of Medical Sciences, Babol, Iran Fariba Ezoji Dental Materials Research Center, Health Research Institute, Babol University of Medical Sciences, Babol, Iran Soraya Khafri Department of Biostatistics and Epidemiology, Faculty of Medicine, Babol University of Medical Sciences, Babol, Iran 2018 06 11 2018 09 03 Objectives: Microleakage is the most important factor responsible for the destruction of restoration margins. The aim of this study was to assess the effect of mechanical load cycling on microleakage of four types of glass ionomer cement (GIC) in comparison with a flowable composite resin. Materials and Methods: In this in-vitro experimental study, 100 Class V cavities were prepared on the buccal and lingual surfaces of 50 intact premolars. The prepared cavities were divided into five groups of (A) Z350, (B) Equia Forte, (C) encapsulated Fuji II LC, (D) hand-mixed Fuji II LC, and (E) Ketac Molar. All the samples were thermocycled (×2,000, 5-55°C), and half of the samples in each group were load cycled. All the teeth were then immersed in 0.5% basic fuchsine for 24 hours, sectioned, and observed under a stereomicroscope. Data were analyzed with Kruskal-Wallis and Mann-Whitney-U tests. Comparison between the incisal and gingival microleakage was made with Wilcoxon test. P<0.05 was considered statistically significant. Results: Load cycling and type of restorative material had a significant effect on microleakage. Gingival microleakage was significantly higher than occlusal microleakage with Equia Forte, encapsulated Fuji II LC, hand-mixed Fuji II LC, and Ketac Molar in the absence of loading, and with Z350 after loading. Conclusions: The sealing ability of Z350 under load cycling was better than that of Equia Forte, hand-mixed Fuji II LC, and Ketac Molar. The marginal integrity of encapsulated Fuji II LC was not significantly different than that of Z350. https://fid.tums.ac.ir/index.php/fid/article/view/2602 https://fid.tums.ac.ir/index.php/fid/article/download/2602/1749