Material-Dependent Microhardness Response to Preheating in Nanoparticulate Composite Resins Cured with High-Intensity Light.

Background/objectives: Composite resins are widely used in restorative dentistry due to their aesthetic properties and ease of handling. Preheating prior to light polymerization has been proposed to improve flowability, degree of conversion, and mechanical properties. This in vitro study aimed to evaluate the effect of preheating on the microhardness of three nanoparticulate composite resins-IPS Empress Direct (Ivoclar), Filtek Z350 XT (3M-ESPE), and Forma (Ultradent)-when cured with a high-power LED light.

Methods: Sixty disc-shaped samples (n = 20 per material) were fabricated and divided into preheated and non-preheated groups. After polishing and 24 h storage in distilled water at 37 °C, samples were subjected to Knoop microhardness testing under a 300 g load for 15 s. Statistical analysis was conducted using R software.

Results: Preheating produced a significant increase in surface microhardness for IPS Empress Direct (32.8%) and Filtek Z350 XT (5.8%) (p < 0.05 for both), whereas Forma showed no significant change.

Conclusions: Under the conditions of this in vitro study, preheating can enhance the mechanical performance of specific composite resins by increasing microhardness; however, the effect is material-dependent.