Abstract
The objective of this study was to evaluate the effect of the exposure reciprocity law of a multi-wave light-emitting diode (LED) on the light transmittance (LT), depth of cure (DOC) and degree of conversion in-depth (DC) of a bulk fill composite. A bulk fill composite (EvoCeram® bulk fill, Ivoclar Vivadent) was photoactivated using the multi-wave LED (VALO™ Cordless, Ultradent). The LED was previously characterized using a spectrophotometer to standardize the time of exposure when using the Standard or Xtra-Power modes with the same radiant exposure of 20J/cm2. LT was evaluated through samples of the bulk fill composite every millimeter till 4 mm in-depth. DOC was evaluated according to the ISO 4049. DC of the central longitudinal cross-section from each sample of the DOC test was mapped using FT-NIR microscopy. Data were statistically analyzed according to the experimental design (α=0.05; ß=0.2). The radiant exposure in the violet wavelength range for Standard and Xtra-Power was 4.5 and 5.0 J/cm2, respectively; for the blue wavelength range the radiant exposure for Standard and Xtra-Power was 15.5 and 15.0 J/cm2, respectively. There was no statistical difference in the DOC using Standard or Xtra-Power light-curing modes, but the DOC was lower than the claimed by the manufacturer (4 mm). The DC was not significantly affected by the light-curing mode up to 4 mm in depth (p>0.05). According to exposure reciprocity law, the reduction in exposure time using the same radiant exposure did not affect the depth of cure of the bulk fill composite.
Highlights
High powered light-curing units (LCUs) have increased irradiance and claimed as it could be used for shorter exposure time, reducing the curing time needed for resin-based composites (RBCs) and the chairside procedure time [1,2]
The increase on irradiance and reduction in light exposure time needed to ensure adequately cure of RBCs is based on the statement that the degree of conversion and mechanical properties of RBCs are dependent upon the radiant exposure (J/cm2) [3], which is the product of irradiance and exposure time (s) [4]
This principle is known as the “exposure reciprocity law”, and it has been an important topic for manufacturers that claim that similar properties can be achieved by RBCs using high power LCUs with a shorter exposure time that is compensated by high irradiance
Summary
High powered light-curing units (LCUs) have increased irradiance and claimed as it could be used for shorter exposure time, reducing the curing time needed for resin-based composites (RBCs) and the chairside procedure time [1,2]. The increase on irradiance and reduction in light exposure time needed to ensure adequately cure of RBCs is based on the statement that the degree of conversion and mechanical properties of RBCs are dependent upon the radiant exposure (J/cm2) [3], which is the product of irradiance (mW/cm2) and exposure time (s) [4] This principle is known as the “exposure reciprocity law”, and it has been an important topic for manufacturers that claim that similar properties can be achieved by RBCs using high power LCUs with a shorter exposure time that is compensated by high irradiance. As the degree of conversion is directly related to the efficiency of the photoinitiator system and the amount of light absorbed by the photoinitiator [8,9], it should be expected that reciprocity should hold at a similar radiant exposure as the same total amount of efficient radicals is created by different irradiances and light exposure times
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