Background: Pressure changes can affect dental restorations especially in divers. The aim of the present study was to assess the fracture resistance and Micro-leakage of the MOD restorations using three types of composite resins undergoing pressure changes, simulating SCUBA-diving and aviation. Methods: For fracture resistance test 90 sound maxillary premolar teeth were randomly assigned to 3 groups of 30 MOD composite restorations using three types of composite resins (nanohybride 3M Z350, microhybride 3M Z250, packable 3M P60) were prepared. Each group was then divided to 3 subgroups of 10 for simulating SCUBA-diving (2 bar pressure cycle), Flight (0.5 bar pressure cycle) and control (atmosphere pressure). The teeth undergone pressure cycles for 1 month and then fracture resistance test was conducted on them using instron testing machine. For Micro-leakage test the same subgroups were made using 90 mandibular premolars (n=10) and using 2% Methylene blue dye for 24 hours Micro-leakage scores were recorded. Data were analyzed statistically. Result: In regard of the fracture resistance test packable composite resines had significantly higher scores than nanohybride and microhybride composite resins (p-value<0.05). Control group had significantly higher fracture resistance values in comparison to Dive groups whereas there was no significant difference between control and Flight group. (p-value=0.083). Considering Micro-leakage there was no significant difference between three restorative materials or between three pressure groups. Conclusion: Packable composite resins show grater fracture toughness than micro and nanohybride composite resins. Diving pressure cycles demonstrated to have adverse effect on fracture resistance amounts of all three types of composite resins.
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