Mechanical characteristics and failure mechanisms of "nano-cluster" dental composites

Objectives: Reinforcement of a novel modern resin-based composite (RBC) (Filtek Supreme; FS) consists of clusters of sub-micron filler particles with porous interstices filled with silane. Controversy exists as to whether such novel “nano-filled” technology provides unique properties to existing microfilled or microhybrid RBCs. Consequently, the mechanical characteristics and failure mechanisms of novel nano-scale and conventional RBCs are assessed. Methods: Bi-axial flexure strength (BFS) and associated Weibull moduli (m) of disc-shaped specimens (12 x 2mm) of a conventional microhybrid (Filtek Z250; FZ), microfill (Heliomolar; HM) and novel “nano-cluster” RBC (FS) (n=20) were obtained for control groups under both wet and dry conditions. The BFS of further groups were tested under wet and dry conditions following pre-stress cyclic fatigue regimes of 100, 100 and 2000 cycles at 20, 50 and 100N. Results: FZ exhibited no significant difference in BFS or associated m following each of the cyclic fatigue regimes. A significant reduction (P=0.036) in BFS was observed for HM specimens pre-stressed for 100 cycles at 100N under wet (83 ± 8MPa) compared with dry (90 ± 8MPa) conditions, however, m was not significantly different (10.9 and 11.9, respectively). Although no decrease in BFS was reported for FS following 100 cycles at 50N, a significant decrease in m was identified compared with dry and wet control groups. For lower stress deformation of FS specimens (2000cycles/20N), increases in BFS and associated m were reported. Conclusions: The difference in defect distributions of FS compared with conventional microfill and microhybrid RBCs may result from modified failure mechanisms attributed to differences in the nature of the interface between composite phases.