Polymerization stress evolution of a bulk-fill flowable composite under different compliances

Objective To use a compliance-variable instrument to simultaneously measure and compare the polymerization stress (PS) evolution, degree of conversion (DC), and exotherm of a bulk-fill flowable composite to a packable composite. Methods A bulk-fill flowable composite (Filtek Bulk-fill, FBF) and a conventional packable composite (Filtek Z250, Z250) purchased from 3 M ESPE were investigated. The composites were studied using a cantilever-beam based instrument equipped with an in situ near infrared (NIR) spectrometer and a microprobe thermocouple. The measurements were carried out under various instrumental compliances (ranging from 0.3327 μm/N to 12.3215 μm/N) that are comparable to the compliances of clinically prepared tooth cavities. Correlations between the PS and temperature change as well as the DC were interpreted. Results The maximum PS of both composites at 10 min after irradiation decreased with the increase in the compliance of the cantilever beam. The FBF composite generated a lower final stress than the Z250 sample under instrumental compliances less than ca. 4 μm/N; however, both materials generated statistically similar PS values at higher compliances. The reaction exotherm and the DC of both materials were found to be independent of compliance. The DC of the FBF sample was slightly higher than that of the packable Z250 composite while the peak exotherm of FBF was almost double that of the Z250 composite. For FBF, a characteristic drop in the PS was observed during the early stage of polymerization for all compliances studied which was not observed in the Z250 sample. This drop was shown to relate to the greater exotherm of the less-filled FBF sample relative to the Z250 composite. Significance While the composites with lower filler content (low viscosity) are generally considered to have lower PS than the conventional packable composites, a bulk-fill flowable composite was shown to produce lower PS under a lower compliance of constraint as would be experienced if the composite was used as the base material in clinical procedures.