Effect of different light-curing modes and curing times on degree of conversion and microhardness of three different bulk-fill composites: an in vitro study.

Objectives: This in vitro study aims to compare the effect of different light curing modes and curing times on the degree of conversion (DC) and microhardness (MH) of three different bulk fill composites at two different depths.

Materials And Methods: A total of 224 cylindrical specimens (4 mm height, 6 mm diameter) were prepared using four composites: SDR Plus, Ever X Posterior (EXP), Beautifil Bulk Restorative (BBR), and Tetric N Ceram (TNC). Each material group (n = 56) was subdivided into four curing subgroups based on light-curing mode (Low Power or High Power) and exposure time (20-30 s). Following curing, all samples were stored at 37 °C for 24 h. From each subgroup, seven specimens were allocated for DC testing using Raman spectroscopy and seven for MH testing using the Vickers microhardness test.

Results: All the materials achieved acceptable (> 80%) bottom-to-top Vickers hardness ratio in high power (HP) mode for both curing times. However, in the low power (LP) mode, EXP could not achieve this ratio at 20 s and BBR failed to achieve this ratio in both curing times. BBR could not achieve an acceptable DC (> 55%) in both curing modes and both curing times. EXP did not demonstrate acceptable DC in LP mode at 20 s curing time, while SDR showed acceptable DC in both curing modes and both curing times.

Conclusion: Compliance with manufacturers' instructions yielded acceptable outcomes in most cases. Each bulk fill material has different requirements pertaining to curing intensity and time to achieve desirable microhardness and degree of conversion. Clinicians should be aware of the curing protocol for the specific bulk fill materials.

Clinical Relevance: BBR consistently exhibited low degrees of conversion, even when the applied energy density exceeded the manufacturer's recommended levels. Further research is warranted to establish optimized curing protocols for giomer-based composites and to investigate the long-term impact of the degree of conversion on the mechanical properties and clinical durability of these restorations.