Hygroscopic expansion kinetics of dental resin-composites

To evaluate the extent and rate of hygroscopic expansion of resin composites at 37°C.

Eight resin composites were examined: 1 micro-hybrid (Bright Light(®)), 5 nano-hybrids (Experimental Vertise™; Nanoceram-Bright(®); Tetric EvoCeram(®); Grandio(®) SO; Ceram X™ duo) and 2 flowables (X-tra base; Venus(®) Diamond Flow). Five disks (15 mm×2 mm) of each material were prepared. The mean change in specimen diameter was recorded by a custom-built non-contact laser micrometer. Specimens were initially measured dry and then at fixed time intervals, over 150 days, after storage in distilled water at 37±1°C. Data were re-expressed in volumetric terms and analysed by repeated measures ANOVA, one-way ANOVA and Tukey's post hoc test (α=0.05).

The volumetric hygroscopic expansion ranged from 0.58 to 2.26 and can be considered in three bands. First, Experimental Vertise had the highest expansion (p<0.001). Venus Diamond Flow, Tetric EvoCeram and Ceram X duo were the second band. The third band, with still lower expansion, consisted of Bright light, Grandio So, Nanoceram-Bright and X-tra base, with no significant difference between them.

For the size (2mm thickness) and shape of specimen measured, equilibrium was attained in all cases by 60 days. Within this set of resin-composites the equilibrium expansion varied by almost 400% of the lowest material.