ABSTRACT
<p><b>OBJECTIVE</b>To evaluate the effect of thermal cycling on surface microstructure of different light-curing composite resins.</p><p><b>METHODS</b>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 degrees celsius;). 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).</p><p><b>RESULTS</b>Significant decreases of Ra were observed in the composites, especially in Spectrum (from 0.164±0.024 µm to 0.140±0.017 µm, P<0.001) and Z250 (from 0.169±0.035 µm to 0.144±0.033 µm, P<0.001), whose Ra approximated that of P60 (0.121±0.028 µm) 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.</p><p><b>CONCLUSION</b>Water storage and thermal cycling may produce polishing effect on composite resins and cause fissures on nanofilled composite resins.</p>