Stability of Restorative Resin Composites Over a two-Year Period.

Objectives: To analyze the stability of five resin composite restorative materials of dissimilar composition over time through the analysis of flexural strength (FS).
Methods: Specimens (n=12) representing five resin composite categories (nanofilled - NF, microhybrid - MH, microfilled - MF, hybrid – HB and nanohybrid, NH) were produced by incremental placement (1000 mW/cm² blue light output) to the final dimensions of 2 x 2 x 25 mm (ISO 4049). All specimens were stored under dark conditions (ISO-9917) in physiologic 0.02M phosphate-buffered saline (PBS) at 37°C until the appointed flexural strength testing (FS - three-point bending) time: 24 hours, 1 month, 12 months, and 24 months. Data were analyzed using a two-way analysis of variance and Tukey HSD (α=0.05).
Results: There was a significant interaction effect between material and time. FS of NF and NH significantly decreased over time. HB and MF were stable over the two years period, but HB had significantly higher FS over time than MF.

There was a significant interaction effect between material and time (p=0.02). FS significantly decreased between 24h (181MPa) and 1 month (131MPa) for NF (p<0.05) and 24h (129MPa) and 24m (103MPa) for NH (p<0.05). HB (150MPa) and MF (96MPa) 24h values were stable over the two-year period, but HB had significantly higher FS over time than MF.
Conclusions: The addition of nanoparticles to the composition of composites affected their stability over time. Overall, a hybrid composite showed 50% higher flexural strength than a microfilled composite..