Comparison of Marginal Ridge Fracture Resistance of Restored Primary Molar Teeth Using Flowable Bulk-Fill and Smart Monochromatic Composites: An In-Vitro Study

Background: The durability of marginal ridges has a significant effect on maintaining the structural integrity and functional performance of primary molar teeth. This study evaluated and compared marginal ridge fracture resistance of restored primary molars using two innovative restorative materials, namely, flowable bulk-fill and smart monochromatic composites. Methods: In this experimental study, 38 primary molar teeth were randomly divided into two binary groups. After the removal of caries and undermined enamel, conventional amalgam class II (≈ 3 mm×3 mm×4 mm) was prepared in one proximal surface. Flowable bulk fill universal composite (Palfique® Bulk Flow, Tokuyama, Japan) was applied in one group, and smart monochromatic universal composite (Omnichroma, Tokuyama, Japan) was placed and cured in another group. All samples were subjected to 5000 thermal cycles ranging from 5 °C to 55 °C. Next, the teeth were mounted with self-curing acrylic resin 1 mm above the CEJ. The fracture resistance of the teeth was measured using a universal testing machine with a maximum force of 1000 Newton. The intact marginal ridges of the opposite sides were also tested for resistance to failure as control groups. The force at which the marginal ridge was broken indicated the resistance to failure in Newton units. The data were analyzed using the Kolmogorov-Smirnov test. Results: The Kolmogorov-Smirnov test results revealed that fracture resistance measurements in both groups had a normal distribution (P>0.05). In addition, the comparison of the average resistance in the two composite groups showed that the resistance in the Omnichroma group was significantly higher than the flowable bulk-fill group (P<0.001). Conclusion: Based on the findings, smart monochromatic (Omnichroma) composites have higher marginal fracture resistance than flowable bulk-fill (Bulk-Flow U) composites in the restoration of posterior primary teeth.