Microhardness vs Energy Density of Bulk-Fill Resin Composites: Possible Correlation?

Objectives: To evaluate the correlation between microhardness and energy density at the bottom level of six bulk-fill restorative resin composites, four bulk-fill base and one conventional resin composite.
Methods: Delrin discs of 6mm diameter and 2-4-5 and 6 mm depths were used to prepare samples. Aura (SDI), Filtek Bulk-Fill Posterior (3M/ESPE), SonicFill (Kerr), X-tra Fill (Voco), Tetric Evo Ceram Bulk-Fill (Ivoclar Vivadent), Admira Fusion X-tra (Voco) were the restorative bulk-fills used. Filtek Bulk-Fill Flowable (3M/ESPE), SDR (Dentsply), X-tra Base (Voco), Venus Bulk-Fill (Heraeus Kulzer) were the bulk-fill bases and Z100 (3M/ESPE) the control.
Each sample was placed on the bottom sensor of a visible light transmission spectrometer (Marc Resin Calibrator, BlueLight Analytics) and photo-polymerized for 20 s using a LED curing unit in standard curing mode. The tip was positioned 1 mm above the samples and energy densities at bottom levels were recorded. 24h later, Knoop hardnesses on bottom level were measured with a Knoop impression tester (Struers Duramin, Struers A/S) with 100 g load and 20s indentation time.
Paired t-test was used to compare two related means. Correlation analysis was used to determine the relation between the variables (p<0,05).
Results: For X-tra Fill, Tetric Evo Ceram Bulk-Fill, Filtek Bulk-Fill Flowable and Z100 groups energy density affected positively the microhardness but no correlation was observed for the remaining groups.
Conclusions: The microhardness of a material is not only depending on energy transfer. However, increasing the energy density may increase the monomer conversion thus the bottom hardness of some bulk-fill materials.