Polymerization of Bulk-fill Flowable and Packable Composites Under Different Compliances

Objectives: To explore the polymerization shrinkage stress (PS) evolution, curing kinetics, and exotherm development of bulk-fill flowable and packable composites under different compliances.
Methods: A bulk-fill flowable composite (Filtek Bulk-fill Flowable) and a conventional packable composite (Filtek Z250) from 3M ESPE were investigated. PS measurement was performed using a cantilever beam-based instrument, which was incorporated with an in situ near infrared spectrometer and a microprobe thermocouple for simultaneous measurement of degree of conversion (DC) and temperature, respectively. The measurements were carried out under various instrumental compliances (ranging from 0.33µm/N to 12.32 µm/N) that cover possible compliances of clinically prepared tooth cavities.
Results: The bulk-fill flowable material generated lower final PS value than the packable one under compliances less than 4 µm/N; while both composites generated statistically similar PS values under higher compliance (i.e. greater than 4 µm/N). The curing kinetics and DC as well as reaction exotherm of both materials were found independent of compliance. DC of the bulk-fill flowable material was slightly higher than that of packable one while the peak temperature rise of the bulk-fill flowable during polymerization was almost as twice higher as the packable. For the bulk-fill flowable, a characteristic decrease of PS was observed during the early stage of polymerization under all compliances studied. The time point of the maximum decrease coincided with the time point of peak temperature rise.
Conclusions: The result demonstrates that the bulk-fill flowable composite works better, from the perspective of PS, under lower compliances of constraint from tooth cavity walls. This validates the use of the bulk-fill flowable material as a base material adopted in clinical procedures.