الفهرس 
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Abstract
Objective: To evaluate the marginal adaptation of thermoviscous bulk-fill, preheated, and conventional nanohybrid composite resins in class II cavities. Materials and Methods: Thirty extracted human sound molars were selected in this in vitro study. Simple class II cavities were prepared using carbide bur. The cervical margin was established 1.5 mm above the cemento-enamel junction. The overall dimensions and depth of cavities were standardized as follows: 4 mm length occlusocervically, 4 mm width buccolingually, and 2 mm depth axially. The cavity depth and length were judged with a permanent mark on the bur and then verified using a periodontal probe. The teeth were randomly divided into three groups (n=10 each). Futurabond DC (one-step self-etch adhesive) was applied in all groups. group 1: Cavities were restored with VisCalor bulk composite, group 2: Cavities were restored with Grandio composite, group 3: Cavities were restored with preheated Grandio composite. The specimens of each group were thermocycled in a thermocycling apparatus by alternating immersion in water bath at 5°C and 55°C with a dwell time of 2 min for 600 thermal cycles. Impressions of the teeth were made and then poured with epoxy resin and replicas were examined under SEM to examine marginal gaps. All data were collected, tabulated and statistically analyzed. Result: The highest percentage of marginal gap length was recorded for group 2, recording 22.93%, while there was no significant difference between group 1 and group 3, recording 13.44% and 13.83%, respectively. One-way ANOVA test was used to compare the tested groups at a level of significance (P ≤ 0.05). Conclusion: The marginal gaps decreased when VisCalor bulk and Grandio composite resins were applied after preheating so preheating composite resins considerably improves marginal adaptation. Keywords: resin composite; VisCalor bulk; preheating; marginal gap.