Abstract

Objectives To evaluate the regional mechanical properties of resin composite under free and constrained conditions during polymerization. Methods Forty cavities (8 mm diameter and 5 mm depth) were fabricated in resin blocks. Half of the cavities were bonded and the other half left un-bonded. The cavities were bulk-filled with one of the following composites: flowable composite (Palfique Estelite LV, Unifil Lo Flo), and Universal composites (Clearfil AP-X, Palfique Estelite Σ), followed by photo-curing for 30 s. After 24 h storage, each specimen was sliced parallel to the long axis to harvest three slabs. The middle slab was serially sliced to harvest five sticks, which were trimmed to an hour-glass shape for measurement of regional ultimate tensile strength (UTS). The remaining semi-circular slabs were polished for microhardness measurement (KHN). Data were analyzed using three-way ANOVA followed by Tukey's HSD test and t-test ( α = .05). Results The KHNs of all the resin composites were not significantly different between the bonded and unbonded groups at each cavity depth ( p > 0.05). The UТS of the bonded group of flowable composites was significantly lower than those of the un-bonded group at the upper regions ( p < 0.05), while for universal composites, there were no significant differences in UTS between the bonded and un-bonded groups ( p > 0.05), although Clearfil AP-X had a trend toward lower UTS under the constrained condition at the upper regions. Significance The UTS of resin composite decreases due to polymerization shrinkage stress when polymerized under a constrained condition, however, these effects were dependent upon regions in the cavity and the resin materials.

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