Composite depth of cure obtained with QTH and LED units assessed by microhardness and micro-raman spectroscopy

Abstract

This study analyzed the depth of cure of a composite assessed by microhardness and the degree of conversion as a function of the light cure unit (LCU) used. Two light cure units, one LED (Ultraled-Dabi Atlante) and one quartz-tungsten-halogen (QTH, Optilux 401-Demetron) unit were used to cure 4.0 × 4.0 mm and 5.0 mm deep composite specimens (Filtek Z250, 3M ESPE). After 24 hours storage at 37°C, Knoop microhardness and degree of conversion were measured on the irradiated surface and at each millimeter of the sample's depth. The degree of conversion was determined by using micro-Raman spectroscopy. The specimens cured with the QTH unit presented uniform decay in microhardness up to 4 mm in depth. Beyond 4 mm, the drop was abrupt. With LED photoactivation, uniform decay was observed only up to 2 mm. At higher depths, the decay in microhardness increased rapidly, especially beyond 3 mm. Depth of cure assessed by micro-Raman revealed that the degree of conversion behaved similarly to microhardness for both LCUs. A strong linear regression between microhardness and the degree of conversion, including both LCUs, was established with R2=0.980.