Pre-heating of high-viscosity bulk-fill resin composites: Effects on shrinkage force and monomer conversion

Objectives: To investigate the influence of pre-heating of high-viscosity bulk-fill composite materials on their degree of conversion and shrinkage force formation. Methods: Four bulk-fill composite materials (Tetric EvoCeram Bulk Fill-TECBF, x-tra fil-XF, QuixFil-QF, SonicFill-SF) and one conventional nano-hybrid resin composite (Tetric EvoCeram- TEC) were used. The test materials were either kept at room temperature or pre-heated to 68 degrees C by means of a commercial heating device, before being photoactivated with a LED curing unit for 20 s at 1170 mW/cm(2). Shrinkage forces (n = 5) of 1.5-mm-thick specimens were recorded in real-time for 15 min inside a temperature-controlled chamber at 25 degrees C (simulating intraoral temperature after rubber dam application) with a custom- made stress analyzer. Degree of conversion (n = 5) was determined at the bottom of equally thick (1.5 mm) specimens using Fourier transform infrared spectroscopy. Data were analyzed with Student's t-test, ANOVA and Tukey's HSD post-hoc test (alpha = 0.05). Results: Composite pre-heating significantly increased the degree of conversion of TECBF, but had no effect on monomer conversion of the other materials investigated. For each of the test materials, pre-heated composite generated significantly lower shrinkage forces than room- temperature composite. At both temperature levels, TECBF created the significantly highest shrinkage forces, and QF caused significantly higher shrinkage forces than both XF and TEC. Conclusions: Both the composite material and the pre-cure temperature affect shrinkage force formation. Pre-heating of bulk-fill and conventional restorative composites prior to photoactivation decreases polymerization- induced shrinkage forces without compromising the degree of conversion. Clinical significance: Composite pre-heating significantly reduces shrinkage force formation of high-viscosity bulk-fill and conventional resin composites, while maintaining or increasing the degree of monomer conversion, dependent upon the specific composite material used. (C) 2015 Published by Elsevier Ltd.