Surface Micro-hardness of Methacrylate-based Composites polymerized with LED and QTH

Using equivalent radiant exposure with LED and QTH does not necessarily ensure comparable degree of polymerization of composite materials. The wavelength of the polymerization unit is an aspect equally important to consider.

Objective: To evaluate the effect of polymerization with quartz-tungsten-halogen (QTH) and light-emitting diodes (LED) on the surface microhardness of methacrylate-based resin composites.

Methods: Disc-shaped specimens (H=1.0 x D=5.0mm, shade A3, n=5) were made from Heliomolar, TetricEvoCeram (Ivoclar-Vivadent); Vit-l-escence (Ultradent); Supreme Plus (3M-ESPE); Premise (Kerr); Artiste (Pentron); Beautifil (Shofu) and Estelite Sigma Quick (Tokuyama Dental). The specimens were polymerized with either QTH (Elipar 2500, 3M-ESPE, 450mW/cm²) or LED (Bluephase, Ivoclar-Vivadent, 900mW/cm²) to produce equivalent radiant exposure of 20 J/cm². Surface microhardness was evaluated immediately after polymerization (T₀), at 24h (T_24h), 1 week (T_1wk) and 1 month (T_1mo) of dark storage in 100% humidity at 37°C. Surface microhardness was measured with a hardness tester (Leco M-400, LECO). A load of 300g was applied through a Vickers indenter with a dwell time of 15s. Five indentations were made on the specimens' surface, averaged and the data analyzed with a three-way ANOVA and Student-Newman-Keuls at a significance level of p<0.05.

Results: A significant effect of the composite, light, time and all their interactions was demonstrated (p<0.05). Supreme evidenced significantly higher hardness and Heliomolar significantly lower hardness relative to all other brands (p<0.001). QTH showed significantly higher hardness than LED (p=0.012). Microhardness was also significantly increased from T₀ to T_24h (p<0.001) and from T_24h to T_1wk (p=0.008). A significant reduction in microhardness was shown at T_1mo relative to T_1wk (p<0.001), but no significant differences were observed between T_{0 and} T_1mo (p=0.643).

Conclusion: Microhardness values were dependent on the composite brand and light curing unit.