Method: Two intensities, 1000mW/cm2 and 3200mW/cm2, of an LED light curing unit (Valo/Ultradent) were used to polymerize shades A1(L), and A3(D) of SonicFill (SF) and Filtek Supreme Ultra (FSU) and shades IVA(L) and IVB(D) of Tetric EvoCeram (TE). Stainless steel molds (5mm diameter) of two thicknesses, 2mm and 4mm, were filled with composite and cured for 10 seconds from the top. Surface microhardness was measured for top and bottom surfaces using a Tukon 2100B testing machine after 24 hours storage in the dark (n=10). Three readings of each surface were taken and bottom/top microhardness ratio was recorded. Acceptable depth of cure ratio (DOC) was at least 80%. Statistical analysis was done using SPSS and microhardness data were analyzed using multiple t-test and p<0.004 after Bonferroni correction.
Result: When comparing high (3200mW/cm2) and low (1000mW/cm2) light intensities at 2mm thickness, FS and SF had a DOC above 80% (B/T=0.82-0.96) for both; TE was slightly below 80% (0.78-0.79) at 1000mW/cm2 but 0.90-0.95 at 3200mW/cm2. At 4 mm thickness, all materials were at 0.65 or less at 1000mW/cm2 (0.13-0.65), but each material was significantly increased at 3200 mW/cm2 (0.30-0.80). Only SF in the light shade had a DOC of 80% at 4mm with 3200mW/cm2 and could be an acceptable bulk cure material. For 4mm samples, lighter shades had significantly greater DOC than darker shades but the range of values was low (0.13-0.72), except for SF at 3200mW/cm2 which reached 0.80.
Conclusion: Depth of cure was enhanced by decreasing incremental thickness, using lighter shades, and increasing intensity of LED light curing units. None of bulk-fill materials reached 80% depth of cure at 4mm except SonicFill shade A1.