Premature Polymerisation of Composite Resins Under Different Light-curing Conditions

Objectives: Premature polymerisation can affect the clinical performance of composite resins.  The aim of this study was to determine the amount of premature polymerisation and the differences in degree of conversion (DC) of selected composite resins under three different light-curing conditions. Methods: DC of four composite resins (Microhybrid: Filtek Z250; Nanohybrid:  Filtek Z350XT, Tetric N-Ceram and Grandio) was measured using Fourier Transform Infrared Spectroscopy (FTIR) after eight minutes of exposure to a fluorescent lamp, direct and indirect halogen operatory light and 20 seconds curing with a LED light curing unit. Change in viscosity was determined based on voids/clefts created using a viscosity test.  Two-way ANOVA, one-way ANOVA and Bonferroni post hoc test were used to analyse the data (α=0.05). Results: Light-curing conditions were found to have a statistically significant effect on DC of the composite resins and there was interaction between these factors (p=0.000). Under a fluorescent lamp, all composite resins exhibited some DC. With direct operatory light, Filtek Z250 and Z350XT had a lower DC compared to Tetric N-Ceram and Grandio. Filtek Z250 had a lower DC as compared to the nanohybrids under indirect operatory light. After an eight minute exposure to an direct operatory light, the composite resins exhibited DC in the range of 49%-65% of its maximal cure.The composite resins used demonstrated premature polymerization under different light-curing conditions at levels that may affect material handling and properties. Conclusions:   a) There are significant differences in the DC of the composite resins under the different light-curing conditions. b) DC of the different composite resins was material dependent.