Effect of adhesion to cavity walls on the mechanical properties of resin composites

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 Tukeyʹs 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.