Limited reciprocity in curing efficiency of bulk-fill resin-composites

Objective: The aim of the study was to identify experimental limits of the general reciprocity hypothesis that the same photo-cure outcomes will result from applying essentially constant energy densities, despite reciprocal variations in the irradiance and irradiation time-period, for a representative set of bulk fill (BF) and non-BF resin composites. Methods: Six BF and two non-BF resin-composites were selected. The unset pastes were inserted into white acetal molds (5 mm id) with (n = 6) depths (1, 2, 3, 4, 5 and 6 mm). Three light curing units (LCUs) of increasing radiant emittance capability: 1200, 2000 and 3200 mW/cm ² were used. Composite specimen groups (n = 3, per depth, per LCU) were irradiated on the upper surface only. For each specific composite, the irradiation times for each LCU were reduced reciprocally, as the LCU irradiance increased, to deliver a constant energy density (J/cm ²) to that composite. However, the required energy density for a given composite differed in accordance with each composite manufacturer recommendations. After storing for 24 h at 37 °C, light transmission measurements were made through each specimen and re-expressed as Apparent Absorbance (A′). Vickers hardness (H _V) measurements (n = 10) were made on both top and bottom surfaces, for each specimen, and H _V versus “depth” profile plots created. From the top-surface data, a Depth-of-Cure parameter could be derived. Data were statistically evaluated for differences between top and bottom H _V values and for other predefined variables of interest. Results: Irradiation with the LCU of 1200 mW/cm ² generally gave the highest H _V/depth for most materials tested compared to the other curing lights with higher power output, regardless of top and bottom measurements (p < 0.001). However, this difference was material-dependent. With one BF composite, 1200 and 2000 mW/cm ² irradiance did not show a significant difference between top and bottom H _V. Composites with higher translucency showed reduced differences in top/bottom H _V than more opaque composites. Significance: Reciprocity was found to be limited with most materials examined, such that irradiance periods of 10 s, gave generally better H _V outcomes than by using LCUs of superior radiant emittance while reciprocally reducing irradiance time to maintain constant dose of energy density.