Tehran University of Medical Sciences Frontiers in Dentistry 2676-296X 18 0 2021 04 03 3385 3385 Elmira Jafari Navimipour Department of Operative Dentistry, Dental Faculty, Tabriz University of Medical Sciences, Tabriz, Iran Fatemeh Pournaghi Azar Department of Operative Dentistry, Dental Faculty, Tabriz University of Medical Sciences, Tabriz, Iran Sajjad Keshipour Department of Nanotechnology, Nanoresearch Center, Urmia University, Urmia, Iran Saba Nadervand Department of Operative Dentistry, Dental Faculty, Tabriz University of Medical Sciences, Tabriz, Iran 2019 12 21 2020 09 29 Objectives: This study aimed to evaluate the thermal stability and monomer elution of bulk fill composite resins cured at different irradiation distances. Materials and Methods: Forty cylindrical-shaped (3×4mm) specimens were fabricated from two composite resins (X-tra fil, X-tra base) and cured from 0 and 7mm distances. In 9 specimens, the degree of conversion was determined by the release of monomers. For this purpose, after curing of composites, they were immersed in 5 mL of 99.9% methanol and stored at 37°C for 24h. The eluted monomer was then analyzed by gas chromatography (GC). Also, thermal stability of one sample from each group was assessed by thermogravimetric analysis (TGA) at a rate of 10°C/min. Data were analyzed using two-way ANOVA and Tukey’s post-hoc test (P<0.05). Results: X-tra fil had significantly higher degree of conversion than X-tra base (P=0.001). Specimens cured at 7mm distance had significantly lower degree of conversion compared with those cured at 0 mm distance (P=0.001). The interaction effect of composite type and distance of light curing unit from the surface of samples was statistically significant (P=0.001). Regarding the TGA results, the lowest and the highest percentages of weight loss were detected in X-tra fil cured at 0 mm distance and X-tra base cured at 7mm distance, respectively. Conclusion: X-tra fil composite cured at 0mm distance had the highest degree of conversion and thermal stability, and X-tra base composite cured at 7mm distance had the lowest values. https://fid.tums.ac.ir/index.php/fid/article/view/3385 https://fid.tums.ac.ir/index.php/fid/article/download/3385/1845