Thermal Expansion of Bulk-fill Composite Resins

Objectives: To measure and compare the coefficient of thermal expansion (CTE) of several bulk-fill composites(listed in the Table).
Methods: Rectangular composite specimens (5mm*5mm*50mm=thickness*width*length, n=3) were fabricated from each composite for CTE measurement. Specimens in stainless-steel molds covered with Mylar sheets were cured on both sides using Demi Plus (Kerr) spot by spot across the entire specimen following manufacturer’s instruction, and then conditioned in water at 37^oC for 12h. CTE measurements use a fixture composed of a strain gauge (Mahr Federal µmaxµm XL) and ~0.5m-length quartz tube with an outer jacket and an internal quartz rod. The initial and final displacement were recorded respectively after specimen was stabilized in water bath at 0^oC for 1h then after temperature ramp up to 40^oC at ~1^oC/min. The measurement was repeated 3 times for each specimen. Statistical analysis was performed using One-way ANOVA and Tukey’s method for pair-wise comparison to determine significant differences among groups (p<0.05).
Results: ANOVA analysis revealed that low-viscosity (flowable) bulk-fill composites have statistically (p<0.05) higher CTE than those of high-viscosity bulk-fill composites. The higher CTE for low-viscosity bulk-fill composites can be attributed to their relatively higher resin content. There are no statistically significant (p>0.05) differences in CTE among the high-viscosity bulk-fill composites.
Conclusions: The CTE of SonicFill 3 is much lower than those of flowable Bulk-fill composites, but comparable to those of high-viscosity bulk-fill composites. The lower CTE of SonicFill 3 is expected to generate lower level of thermal stress at the margins or bonding interface of the restoration when subjected to large swings of oral temperature, thereby leading to reduced chance for microleakage.