In vitro cuspal deflection and microleakage of maxillary premolars restored with novel low-shrink dental composites

Objectives: The development of novel low-shrink resin-based composites (RBCs) may offer a potential reduction in polymerisation shrinkage stresses generated at the tooth/restoration interface compared with current methacrylate RBCs. In vitro cuspal deflection and microleakage of meiso-occlusal-distal (MOD) cavities restored with experimental oxirane and silorane (EXL596 and H1) and methacrylate (Z100 and Filtek™ Z250) RBCs were assessed. Methods: Standardised pre-molar MOD cavities were prepared (n=10) and restored with each material. The flexure of buccal and palatal cusps was recorded 0.1 h following irradiation utilising a differential transformer deflection gauge. Each restored tooth was subjected to a thermocycling regime and microleakage of tooth sections were assessed following 24 h immersion in 0.2% fuschin dye. The degree of conversion (DC) of each RBC material was also assessed using Fourier transform infra-red (FTIR) spectroscopy following 0.1, 0.5, 1, 4, 24 and 48 h post-irradiation. Results: A total cuspal deflection was observed for EXL596 (2.5±0.9 μm) and H1 (6.0±1.8 μm) compared with Z100 (20.0±4.7 μm) and Filtek™ Z250 (16.5±3.3 μm) following 0.1 h. The cavities restored with EXL596 displayed significantly higher microleakage than any other RBC and H1 exhibited non-significant and significantly decreased microleakage compared with Z100 and Filtek™ Z250, respectively. The DC of EXL596 and H1 was significantly decreased compared with Z100 and Filtek™ Z250 following 0.1, 0.5 and 1 h. Significance: The ‘living’ polymerisation associated with the novel oxirane and silorane RBCs and the associated decrease in cuspal deflection may suggest a decrease in the magnitude of polymerisation shrinkage stress at the tooth/restoration interface. The decreased DC following 0.1 h of the oxirane compared with the methacrylate RBCs has provided information on the cure rate of cationic and free-radical polymerisation mechanisms, respectively. The inadequate marginal seal of cavities restored with EXL596 would preclude its use as a dental restorative. The reduction in cuspal deflection and decrease in microleakage of cavities restored with H1 compared with Filtek™ Z250 may be advantageous in terms of marginal integrity following placement. However, the non-significant difference in microleakage between the H1 and Z100 may only present modest decreases in the deleterious effects of shrinkage stress.