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

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.

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 infrared (FTIR) spectroscopy following 0.1, 0.5, 1, 4, 24 and 48 h post-irradiation.

A total cuspal deflection was observed for EXL596 (2.5 +/- 0.9 microm) and H1 (6.0 +/- 1.8 microm) compared with Z100 (20.0 +/- 4.7 microm) and Filtek Z250 (16.5 +/- 3.3 microm) 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.

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.