[Effects of light-curing modes on the polymerization shrinkage and surface hardness of composite resins]

To investigate the effects of light-curing modes on the polymerization shrinkage and surface hardness of resins and to explore the related clinical relevance.

Resins with filler content of 76%(mass fraction) were light-cured by high intensity, low intensity and soft start curing modes for 10 s and 20 s, respectively. Specimens for detecting volumetric shrinkage and surface hardness were prepared. Volumetric shrinkage was measured with Acuvol (n=7) and surface hardness were tested with an indenter (n=5).

The volumetric shrinkage of composites cured by high intensity, low intensity and soft-start curing mode was: 2.95% ± 0.08%/3.06% ± 0.03% (10 s/20 s), 2.98% ± 0.12%/3.05% ± 0.13% (10 s/20 s), and 3.03% ± 0.05%/3.11% ± 0.07% (10 s/20 s), respectively. No significant difference existed among polymerization shrinkage of composites cured by the three light-curing modes (P>0.05). The hardness of composites cured by high intensity, low intensity and soft-start curing mode was: (36.82 ± 4.45) MPa/(47.58 ± 3.16) MPa (10 s/20 s), (32.30 ± 1.33) MPa/(41.60 ± 1.83) MPa (10 s/20 s), and (34.56 ± 1.38) MPa/(44.62 ± 2.13) MPa (10 s/20 s), respectively. There existed significant difference among hardness of composites cured by the three light-curing modes (P<0.05). Polymerization shrinkage was correlated with energy density (r=-0.363,P=0.018). Surface hardness was also correlated with energy density (r=-0.890,P<0.001).

It would be better to use high intensity curing mode to improve the physical properties of restorations. In order to keep the physical properties of composites, it is necessary to prolong the curing time using soft-start/low intensity curing modes to increase the energy density.