Evaluation of photopolymerization efficacy and temperature rise of a composite resin using a blue diode laser (445 nm) Kouros P, Dionysopoulos D, Deligianni A, Strakas D, Sfeikos T, Tolidis K. Evaluation of photopolymerization efficacy and temperature rise of a composite resin using a blue diode laser (445 nm). Eur J Oral Sci 2020; 00: 1–7. © 2020 Eur J Oral Sci The purpose of this study was to evaluate the photopolymerization efficacy of a diode laser (445 nm) for use with a composite containing camphorquinone and to estimate the safety of the method related to the temperature rise. Five cylindrical composite specimens were prepared for each thickness: 1, 2, and 3 mm. Three light- curing modes were investigated: a light emitting diode (LED) unit and a diode laser (445 nm) with output powers at 0.7 W or 3 W. Evaluation of the polymerization efficacy was based on Vickers hardness measurements, and the highest temperatures at the bottom of the specimens were recorded using a K-type thermocouple. The highest microhardness was observed after the diode laser curing operating at 3 W. A comparison of the microhardness of the 0.7 W laser cured specimens with the LED cured specimens showed a statistically significant difference in favor of the laser curing. Laser curing operating at 3 W resulted in extremely high temperatures. Laser curing at 0.7 W resulted in statistically significantly higher maximum temper- atures than did LED curing for both 1 mm thick (52.9°C against 45.4°C) and 3 mm thick (43.6°C against 40.9°C) specimens. Diode laser (445 nm) may be an alternative for photopolymerization of composite materials and may result in a higher degree of conversion and depth of cure of composites than what has been seen with LED curing units when they emit at the same energy density. Pantelis Kouros 1 , Dimitrios Dionysopoulos 1 , Areti Deligianni 2 , Dimitris Strakas 1 , Thrasyvoulos Sfeikos 1 , Kosmas Tolidis 1 1 Department of Operative Dentistry, School of Dentistry, Aristotle University of Thessaloniki, Thessaloniki, Greece; 2 Centre Dentaire de Dentego, Tourcoing, France Dimitrios Dionysopoulos, Department of Operative Dentistry, Faculty of Dentistry, School of Health Sciences, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece E-mail: ddionys@dent.auth.gr Key words: blue diode laser; light emitting diode; polymerization efficiency; surface microhardness; temperature rise Accepted for publication September 2020 Dental composite resins are very popular restorative materials due to their good esthetics and long clinical survival. During tooth restorations, composite resins should be applied in incremental layers up to 2 mm in thickness to ensure sufficient polymerization (1) in order to achieve appropriate physical and mechanical proper- ties (2) and biocompatibility (3). The degree of conver- sion (DC), or polymerization rate, of a composite resin is the conversion ratio of the carbon double bonds (C=C) into the reacted ones (C-C). The DC is directly related to a composite’s mechanical properties and hydrolytic sta- bility in the oral environment. A complete conversion of carbon double bonds in a composite is unattainable due to the three-dimensional network development that func- tions as an obstacle for unreacted molecular chains to approach and react. In vitro and laboratory experiments indicate that it is possible to obtain a DC of up to 80%; however, for under intra-oral clinical conditions, the DC will be 70% or less, basically depending on the intensity of the light source, duration of irradiation, distance of the composite surface from the tip of the device, and depth of measurement (4). Experimental measurements of the DC may be based on direct or indirect methods. Direct methods, such as Raman spectroscopy, Fourier-transformed infrared spectroscopy, and mid- and near- infrared spec- troscopy, are directly quantifying reacted to unreacted double carbon bonds, while indirect methods are based on the measurement of mechanical and/or physical properties, which are proportionally related to the DC. Indirect methods most commonly include dilatometry, measurements of optical properties, surface hardness, or elastic modulus. Despite some evident drawbacks, indirect methods are widely used in research and are expected to give reliable results (5). The most commonly used photoinitiator system is camphorquinone (CQ) combined with a tertiary amine. CQ is responding to wavelengths between 400 and 500 nm with a peak at k max = 468 nm (6,7). Various light sources have been used for photopolymerization of composite resins, such as quartz-tungsten-halogen lamps, plasma arc lamps, light-emitting diodes (LED), or lasers (mostly argon and diodes), each one bearing different characteristics, such as wavelength range, Eur J Oral Sci 2020; 1–7 DOI: 10.1111/eos.12742 Printed in Singapore. All rights reserved © 2020 Eur J Oral Sci European Journal of Oral Sciences