Viscosity and Cavity Depth Effect on Bulk-Fill Composites Compressive Strength

Objectives: To compare the compressive strength of bulk-fill composites of high and low viscosities placed in different cavity depths.
Methods: Study protocol was reviewed and approved by the IRB. Thirty (30) human third molars slices were randomly distributed into six subgroups of 5 specimens each: 2 control subgroups using a high density packable composite (Filtek P60,3M ESPE), 2 subgroups using a low viscosity Bulk Fill composite (Filtek Bulk Fill, 3M ESPE) and 2 subgroups using a high viscosity Bulk Fill composite (Tetric N-Ceram Bulk Fill, Ivoclar, Vivadent). Posterior composites mentioned before were placed in class I tooth preparations of 2,5mm and 4mm depth each. Restorations were bonded to the teeth using their correspondent etching agents and adhesives in a total etch protocol. Composites were polymerized using a LED-curing unit. Specimens were storage in airtight plastic bags at relative humidity for 24 hours. Subsequently, compressive strength tests were made applying the force at center of the composite using the universal testing machine Instron® (crosshead speed of 0.5 mm/min). Statistical analysis was performed using Bonferroni, Shapiro Wilk, Bartlet, ANOVA and Tukey tests.
Results: No statistical significant differences were observed when compressive strengths (CS) of all groups were compared (p>0.05), except between Filtek P60/2.5mm and Tetric N-Ceram/4mm (p<0.05). However, CS of control groups (Packable composite) was higher than the strength observed at the Bulk Fill groups. CS of 2,5 mm depth subgroups for all composites was higher than the strength detected at 4mm depth restorations.
Conclusions: Compressive strength of high and low viscosity Bulk Fill composites is similar than the behavior reported by the high density composite (p>0.05). All three composites evaluated reported lower compressive strength values when occlusal preparations were deeper.