Abstract
This study aimed to evaluate the effect of the composite shade and distance from the light-curing unit (LCU) tip on the irradiance reaching the bottom of composite disks and on the depth of polymerization. Composites of three shades (opaque - OXDC, bleach - BXL, and A2) were inserted into molds with 3-mm of thickness positioned over a spectrometer and photo-activated with the LCU (Bluephase) tip placed at 0 or 1 cm from the composite surface. The mean irradiance reaching the bottom of composite was recorded during the entire photo-activation (30 s). Specimens (2 x 2 x 4 mm) were polymerized and used to map the degree of conversion achieved in different depths from irradiated surface. Specimens were sectioned into slices that were positioned over the platform of the infra-red microscope connected to the spectrometer to map the conversion. The conversion was measured in eight different depths every 500-µm. Increasing the distance of LCU tip reduced the irradiance only for A2. Interposing OXDC disks resulted in lowest values of irradiance and A2 the highest one. A tendency to decrease the conversion was observed towards the bottom of specimens for all experimental conditions, and the slope was more accentuated for OXDC. Differences among shades and distances from LCU tip were evident only beyond 1.5-2.0 mm of depth. In conclusion, both composite shade and distance from LCU tip might affect the light-transmission and depth of polymerization, while the effect of last was more pronounced.
Highlights
Composite resins are largely used worldwide to restore both anterior and posterior teeth with high success rates [1,2]
The degree of conversion depends on the emission spectra of light curing units matching the absorption spectra of photo-initiators used in these materials [5,6] and on the light reaching all portions of the restoration [7,8,9]
The distance from the tip of the light guide in relation the irradiance achieved at the composite surface and can be controlled by the clinician in some situations [15,18,19]
Summary
Composite resins are largely used worldwide to restore both anterior and posterior teeth with high success rates [1,2]. The annual failure rate of posterior composite restorations is as low as 2.0% and most failures are related to the presence of recurrent caries, and to fracture of the material or tooth structure [3]. Increased annual failure rates (up to 4%) have been reported for composite restorations in anterior teeth, but in those cases re-interventions due to aesthetic reasons are accounted for [4]. The longevity of composite restoration depends on several factors, including the bonding to the tooth substrate and the mechanical strength of the material, which in turn depends on proper polymerization. Light transmission directly affects the properties of a composite, and the restoration performance
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