Impact of Increment Thickness, Preheating and Light Exposure Duration on Surface Hardness of Bulk-Fill Composite Cured in Covered Slot

Background: Experimental methods for evaluating composite filling materials and their use are essential for achieving reliable, predictable clinical results in tooth restoration. This study aimed to evaluate the impact of bottom increment thickness, preheating, and light exposure duration on microhardness and depth of cure of bulk-fill resin composite after polymerization in a covered slot.
Methods and Results: A total of 32 filling samples were made using Tetric® N-PowerFill 2 bulk-fill resin composite material (Ivoclar Vivadent AG). Composite filling samples were made in opaque plastic molds with isosceles trapezoidal slots. The slot was 1.5 mm wide. The lower and upper bases of the trapezoid were 3 and 6 mm, respectively. The height of the trapezoidal slot was 5 mm. Filling samples were divided into 8 groups of 4 each. All trapezoidal slots were filled with 2 successive layers of different thicknesses. In Groups 1, 3, 5, and 7, the height of the bottom horizontal layer was ~1mm, the top horizontal layer ~ 4 mm. In Groups 2, 4, 6, and 8, the height of the bottom horizontal layer was ~ 4mm, the top horizontal layer ~ 1 mm. Every layer was cured individually. In Groups 1, 2, 5, and 6, the light exposure for each layer was 20 sec, while in Groups 3, 4, 7, and 8, it was 40 sec. Filling samples in Groups 1, 2, 3, and 4 were made from a room-temperature composite (22-24°C). In Groups 5, 6, 7, and 8, the material was polymerized after heating in the slot up to 55-60°C. Photoactivation was performed with the Valo X LED lamp (Ultradent, USA) in standard mode. The surface microhardness of composite filling samples was assessed using the Vickers hardness tester after exposure to light and storage in a dark container for 24 hours. Measurements were performed using a “ПMT-3” tester with an indenter at a 50g load for a 15-second dwell time. Indentations were made in a linear order at levels of 0.5, 1.5, 2.5, 3.5, and 4.5 mm from the top surface.
Doubling the photoactivation time of the room-temperature composite from 20 seconds to 40 seconds increased the VHN of composite fillings at depths of 1.5, 2.5, 3.5, and 4.5 mm by 55%, but mostly insignificantly. The only difference of ~1.6 times was noted in VHN at a depth of 4.5 mm between the filling samples from Group 3 and Group 2 (p=0.0071), indicating the importance of both low increment thickness and prolonged irradiance.
The surface hardness of fillings at 0.5 mm in samples from Groups 1, 2, 3, and 4 was relatively high and did not show significant statistical differences among them. It was another confirmation of the crucial importance of close light source adjustment to the surface of light-cured material.
Photoactivation of a heated composite material in a covered slot had certain advantages over using a room-temperature composite, as demonstrated by the VHN of filling samples at different depths. For example, at all depths, the VHN of composite fillings in Group 8 was statistically greater than in Group 2, regardless of the thickness of the bottom increment. Moreover, the difference increased with depth, from 1.4 (P=0.0431) [at 0.5 mm] to 1.8 (P=0.0001) [at 4.5 mm]. However, it was noteworthy that prolonged irradiance of a 4 mm-thick layer of resin composite is beneficial and may offset the low polymerization kinetics of a room-temperature filling material.
Conclusion: Lowering the thickness of the bottom layer of bulk-fill composite, along with its preheating and prolonged photoactivation, cumulatively contributed to a significant increase in depth of cure and microhardness of filling samples made in a covered slot.