Objective To evaluate the effect of thermal cycling on surface microstructure of different light-curing composite resins. Methods A nanofilled composite (Z350) and 4 microhybrid composites (P60, Z250, Spectrum, and AP-X) were fabricated from lateral to center to form cubic specimens. The lateral surfaces were abrased and polished before water storage and 40 000 thermal cycles (5/55℃). The mean surface roughness (Ra) were measured and compared before and after thermal cycling, and the changes of microstructure were observed under scanning electron microscope (SEM). Results Significant decreases of Ra were observed in the composites, especially in Spectrum (from 0.164±0.024μm to 0.140±0.017μm, P<0.001) and Z250 (from 0.169 ± 0.035μm to 0.144 ± 0.033μm, P<0.001), whose Ra approximated that of P60 (0.121 ± 0.028μm) with smoothly polished surface. SEM revealed scratches and shallower pits on the surface of all the 5 resins, and fissures occurred on Z350 following the thermal cycling. Conclusions Water storage and thermal cycling may produce polishing effect on composite resins and cause fissures on nanofilled composite resins.

Objective To evaluate the effect of thermal cycling on surface microstructure of different light-curing composite resins. Methods A nanofilled composite (Z350) and 4 microhybrid composites (P60, Z250, Spectrum, and AP-X) were fabricated from lateral to center to form cubic specimens. The lateral surfaces were abrased and polished before water storage and 40 000 thermal cycles (5/55℃). The mean surface roughness (Ra) were measured and compared before and after thermal cycling, and the changes of microstructure were observed under scanning electron microscope (SEM). Results Significant decreases of Ra were observed in the composites, especially in Spectrum (from 0.164±0.024μm to 0.140±0.017μm, P<0.001) and Z250 (from 0.169 ± 0.035μm to 0.144 ± 0.033μm, P<0.001), whose Ra approximated that of P60 (0.121 ± 0.028μm) with smoothly polished surface. SEM revealed scratches and shallower pits on the surface of all the 5 resins, and fissures occurred on Z350 following the thermal cycling. Conclusions Water storage and thermal cycling may produce polishing effect on composite resins and cause fissures on nanofilled composite resins.