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Polymerization efficiency of LED curing lights.

PURPOSE
The purpose of this study was to compare the curing efficiency of three commercially available light-emitting diode (LED)-based curing lights with that of a quartz tungsten halogen (QTH) curing light by means of hardness testing. In addition, the power density (intensity) and spectral emission of each LED light was compared with the QTH curing light in both the 380- to 520-nm and the 450- to 500-nm spectral ranges.


MATERIALS AND METHODS
A polytetrafluoroethylene mold 2 mm high and 8 mm in diameter was used to prepare five depth-of-cure test specimens for each combination of exposure duration, composite type (Silux Plus [microfill], Z-100 [hybrid]), and curing light (ZAP Dual Curing Light, LumaCure, VersaLux, Optilux 401). After 24 hours, Knoop hardness measurements were made for each side of the specimen, means were calculated, and a bottom/top Knoop hardness (B/T KH) percentage was determined. A value of at least 80% was used to indicate satisfactory polymerization. A linear regression of B/T KH percentage versus exposure duration was performed, and the resulting equation was used to predict the exposure duration required to produce a B/T KH percentage of 80% for the test conditions. The power densities (power/unit area) of the LED curing lights and the QTH curing light (Optilux 401) were measured 1 mm from the target using a laboratory-grade, laser power meter in both the full visible light spectrum range (380-780 nm) and the spectral range (between 450 and 500 nm), using a combination of long- and short-wave edge filters.


RESULTS
The emission spectra of the LED lights more closely mirrored the absorption spectrum of the commonly used photoinitiator camphorquinone. Specifically, 95% of the emission spectrum of the VersaLux, 87% of the LumaCure, 84% of the ZAP LED, and 78% of the ZAP combination LED and QTH fell between 450 and 500 nm. In contrast, only 56% of the emission spectrum of the Optilux 401 halogen lamp fell within this range. However, the power density between 450 and 500 nm was at least four times greater for the halogen lamp than for the purely LED lights. As a result, the LED-based curing lights required from 39 to 61 seconds to cure a 2-mm thick hybrid resin composite and between 83 and 131 seconds to adequately cure a microfill resin composite. By comparison, the QTH light required only 21 and 42 seconds to cure the hybrid and microfill resin composites, respectively.


CLINICAL SIGNIFICANCE
The first-generation LED-based curing lights in this study required considerably longer exposure durations than the QTH curing light to adequately polymerize a hybrid and a microfill resin composite.