Comparative Evaluation of Microhardness of Suprananospherical and Nanohybrid Composite after Various Bleaching Protocols: An In Vitro Study

Aim: To evaluate and compare the microhardness of suprananospherical composite and nanohybrid composite after bleaching with 15%; 35% hydrogen peroxide (HP); and 15% carbamide peroxide (CP).Materials and methods: A total of 64 disk-shaped composite specimens were prepared.Groups I and II constitute 32 composite disks made from suprananospherical composite and nanohybrid composite, respectively.Based on the bleaching agent used, each group was subdivided into four subgroups.Subgroups IA and IIA were subjected to bleaching with 15% HP for 15 mins for 10 days.Subgroups IB and IIB were subjected to bleaching with 35% HP for 45 minutes (three sequential 15-minute applications) once a week for three consecutive weeks.Subgroups IC and IIC were subjected to bleaching with 15% CP for 4 hours per day for 14 days.Subgroups ID and IID were kept as control groups where specimens were kept in distilled water for 14 days.After the bleaching procedure, composite disk specimens were subjected to the Vickers microhardness test.Results: Suprananospherical composite showed more reduction in microhardness values than nanohybrid composite.Suprananospherical composite bleached with 35% HP with a ΔH value of 1.248 ± 1.319 showed the highest decrease in microhardness.There was no statistically significant difference between the microhardness of suprananospherical composite and nanohybrid composite after bleaching.About 15% CP caused the least reduction in microhardness in nanocomposites compared to 15 and 35% HP.Conclusion: Suprananospherical and nanohybrid composites exhibited a reduction in microhardness after bleaching with no significant difference between the two composites.About 15% CP caused the least reduction in microhardness values of nanocomposites compared to 15 and 35% HP.Clinical significance: Direct composite resin materials due to their organic matrix are more prone to chemical alteration by bleaching.Reduced surface hardness may affect the durability of restoration.