Effects of irradiance, wavelength, and thermal emission of different light curing units on the Knoop and Vickers hardness of a composite resin

Abstract

The aim of this study was to evaluate the effects of irradiance, light emission wavelength, and heating of different light curing units on the Knoop and Vickers hardness of a hybrid composite resin. The specimens were irradiated during 40 s with ten different light curing units, LEDs, and halogen lights. The spectral emission of each light curing unit was assessed by a spectrometer, the irradiance was measured by two commercial radiometers, and the heating measured with a thermocouple. After 48 h of storage in a dark recipient under a 100% humidity condition, the Knoop and Vickers hardness tests were carried out. The hardness results were analyzed by ANOVA, and Tukey HSD test (p < 0.05). The results showed that the surface hardness of the composite resin depends not only on the irradiance, but strongly on the emission wavelength and heating of the light curing units. It was observed, a linear correlation between the conversion degree and radiant exposure. In addition, it is suggested that the well known base to top surface hardness ratio convention of 80-90% is not appropriate to evaluate curing efficiency of composites, since the top surface is not always sufficiently polymerized.

Wavelength of Experimental LEDS: Hardness, Elastic Modulus, Degree of Conversion and Temperature Rise of a Microhybrid Composite

“…The lowest hardness and elastic modulus values were found on composite's bottom surface. Previous studies had also found similar results 12,21,24,25 . During photopolymerization, there is a considerable reduction of irradiance in deeper regions through the bulk of the composite, due to absorption and scattering of light by the resin matrix and particles filler 25 .…”

“…Since the energy density was kept constant for the three LED LCUs, we could suggest the influence of the light's wavelength on the results. Previous studies report that not only the irradiance but also the wavelength of the emitted light has a direct effect on the degree of polymerization of resin composites 12,21 . According to Nomoto 11 , the polymerization of composites is affected by light wavelength, 470 nm being the most efficient because it maximizes the camphorquinone activation 1,10,19 , which is the most commonly present photoinitiator in the composition of composites 26 .…”

“…To considerer a composite's depth of cure to be adequate, the B/T ratio should be greater than 80% 30 . On the other hand, Torno et al 21 do not consider that the B/T ratio alone is a good method to evaluate the polymerization effectiveness 21 . That is because if the composite is not properly cured but has a bottom hardness value similar to the top hardness value, the B/T ratio can be greater than 80%, and as such the deficient polymerization will be considered suitable.…”

“…However, there is little data about the performance of experimental LED LCUs with different wavelength peaks evaluated through DC, hardness and the elastic modulus of the cured composite. Moreover, authors found LED LCUs whose wavelength peak is below the optimum value for activation of camphorquinone 21,22 . The aim of this study was to evaluate the effect of different peak wavelengths of experimental LED LCUs on the hardness, elastic modulus, degree of conversion and temperature rise of a composite.…”

The Effect of Composition, Temperature and Post-Irradiation Curing of Bulk Fill Resin Composites on Polymerization Efficiency

“…Many parameters may affect the degree of polymerization of bulk fill RBCs such as their composition (photoinitiators, fillers and organic matrix) 6 , the technical characteristics of the light-curing unit (light intensity, thermal emission, wave length range, diameter of the tip) and the conditions of photo-polymerization (curing mode and exposure time) 22 , the post-irradiation period 21 , the temperature 23,24 and the incremental thickness of the material 25 . As statistical analysis revealed the existence of significant differences among the tested bulk fill RBCs with regard to microhardness, the formulated Ho1, which states that there were no differences in microhardness among the RBC materials of the same measurement depth, had to be rejected.…”