Objectives: To investigate the influence of thermo-cycling on bi-axial flexure strength (BFS) and reliability of strength data of a ‘nano'filled and microhybrid resin-based composite (RBC).

Method: Each group consisted of twenty disc-shaped specimens (12mm x 1mm); five groups each of the hybrid composite Filtek Z250 (FZ) and of the nanofilled composite Filtek Supreme in the Body (FS) and Translucent shades (FST) (3M ESPE, Seefeldt, Germany). One group was stored for 24h in a water-bath maintained at 37±1°C, the remaining four groups were thermo-cycled at 5-55°C with a dwell time of 6s at each temperature for 2500, 5000, 10000 and 15000 cycles. The mean BFS and associated Weibull moduli (m) were determined. The fracture surface, filler morphology and filler/matrix interface was examined utilising scanning electron microscopy (SEM).

Results: A significant decrease in BFS occurred in all systems as a consequence of thermo-cycling (P<0.05). The m of FZ following 2500 cycles increased compared with the control; 11.13(2.49), 8.71(1.95), respectively. Subsequent increase in cycles of FZ resulted in a significant decrease in m; 7.43(1.66), 6.16(1.38), 4.68(1.05), respectively. In contrast, the m of FS and FST was maintained following 2500 cycles; 6.15(1.38), 8.17(1.83) compared with the control; 5.98(1.34), 6.15(1.38) P>0.05. A significant reduction in m occurred following 5000 cycles of FS and FST; 3.96(0.89), 5.39(1.21), respectively. Interestingly, this level was maintained following 10000; 3.54(0.79), 5.99(1.34) and 15000 cycles; 4.61(0.93), 5.41(1.21). SEM highlighted the occurrence of voids/defects at the filler/matrix interface following the thermo-cycling regimes which corresponded with a decreased m.

Conclusion: The decreased BFS and reliability of strength data implies an increase in size and distribution of defects, whilst the maintenance of m for FS and FST following each thermo-cycling regime was attributed to a potential stabilising effect, induced by the nature of the interactions between resin matrix, nanoclusters and nanoparticles.

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