Otimização dos parâmetros de fotoativação: efeitos do tempo e dose de energia em uma resina composta nanohíbrida

Abstract

The evolution of composite resins over the past decades represents one of the most significant advances in modern dentistry, offering superior aesthetic, functional, and biocompatibility benefits compared to traditional restorative materials. In this context, photoactivation has allowed for better control of clinical working time and improved the physicomechanical properties of these materials, with the light energy dose — directly dependent on the photoactivation time — being a critical factor for the formation of an adequate three-dimensional polymer network. Inadequate polymerization, whether due to insufficient or excessive energy dosage, can lead to failure and increased degradation. Therefore, optimizing polymerization requires a balance between the applied light energy and the material's characteristics. This study aimed to evaluate the influence of different energy doses (11,5 J/cm²; 23,0 J/cm²; 46,0 J/cm²; 69,0 J/cm²; and 138,0 J/cm²) by varying photoactivation times (10, 20, 40, 60, and 120 seconds), on the physicomechanical properties of a nanohybrid composite resin (Aura, shade DC2, SDI, Australia), including Knoop microhardness, elastic modulus, degree of conversion, diametral tensile strength, sorption and solubility, three-point flexural strength, and interlayer adhesion. Data were subjected to statistical analysis, including normality tests (Shapiro–Wilk) and homogeneity of variance (Levene). Two-way repeated measures ANOVA and Tukey’s HSD post-hoc test were used for microhardness, elastic modulus, degree of conversion, and flexural strength. Student’s t-test was applied for diametral tensile strength, and the Kruskal–Wallis test followed by Tukey was used for sorption, solubility, and interlayer adhesion. All analyses were conducted with a 95% confidence level (α=0.05). The results showed that shorter photoactivation times, such as 10 seconds, resulted in significantly lower mechanical properties (P<0.001), such as microhardness (top: 51.41 ± 5.57 KHN; bottom: 43.88 ± 3.97 KHN), elastic modulus (top: 9.93 ± 2.70 GPa; bottom: 8.76 ± 2.92 GPa), and flexural strength (54.85 ± 6.95 MPa). Additionally, a lower degree of conversion was observed at the bottom surface (23.16% at 10s; P<0.001), along with higher sorption (30.38 ± 4.55 µg/mm³; P<0.001) and solubility (13.15 ± 3.36 µg/mm³; P<0.001), and reduced interlayer adhesion (5.99 ± 0.17 MPa; P<0.001). Although the top surfaces reached an acceptable degree of conversion starting at 20s of activation (64.60%; p=0.159), the bottom regions required longer photoactivation times to achieve satisfactory performance. Diametral tensile strength was not significantly affected by activation time (P=0.442). It was observed that activation times between 40 and 60 seconds resulted in the optimization of most evaluated properties, while 120 seconds did not bring additional benefits and even led to a reduction in flexural strength (95.77 ± 4.91 MPa for 120s; P<0.001), possibly due to higher internal stresses. It is concluded that photoactivation time and energy dose are critical factors influencing the physicomechanical properties of composite resins. Photoactivation time and energy dose influenced the physico-mechanical properties of composite resins. Short exposure (10s) resulted in inferior performance, while optimized photoactivation occurred between 40 and 60 seconds. Excessive exposure (120s) provided no additional benefits and were, sometimes, detrimental. Thus, the delivered light energy dose defines the quality of polymerization, which is essential for the clinical success of resin-based restorations.