Methods: Four resin composite materials (SDR, Esthet X flow, Esthet X HD, Rebilda DC) were subjected to the following light-curing protocols, each employing a standardized energy density of 27 J/cm2: High-intensity continuous light (HIC; 30 s at 900 mW/cm2), low-intensity continuous light (LIC; 90 s at 300 mW/cm2), soft-start (SS; 15 s at 300 mW/cm2, 25 s at 900 mW/cm2), pulse-delay (PD; 3 s at 300 mW/cm2, 3-min delay, 29 s at 900 mW/cm2). Linear shrinkage (n=6) and shrinkage force (n=6) of 1.5-mm-thick specimens were measured for 15 min using custom-made devices. Knoop hardness (n=6) was determined at the bottom of equally thick (1.5 mm) specimens at the end of the observation period. Data were analyzed by ANOVA and Scheffé’s post-hoc test (p<0.05).
Results: No differences were observed among the curing protocols for both Knoop hardness and linear shrinkage, irrespective of the composite material. Esthet X HD caused the significantly lowest linear shrinkage, followed by SDR, Rebilda DC and Esthet X flow with all curing protocols. PD generated the significantly lowest shrinkage forces within SDR, Esthet X flow and Esthet X HD, and LIC produced lower shrinkage forces than HIC within Esthet X flow. HIC created the significantly highest shrinkage forces within Esthet X HD and Rebilda DC. SDR generated the significantly lowest shrinkage forces when HIC was used, while Esthet X HD caused the significantly lowest shrinkage forces for PD.
Conclusions: Both the composite material and the applied light-curing protocol control shrinkage force formation. Pulse-delay light-curing decreases shrinkage forces compared to high-intensity continuous light-curing without compromising the degree of hardening of restorative composites.