Depth of cure and compressive strength of dental composites cured with blue light emitting diodes (LEDs) K.D. Jandt a, * , R.W. Mills a , G.B. Blackwell b , S.H. Ashworth c a Department of Oral and Dental Science, Dental Materials Science and Biomaterials Section, University of Bristol, Lower Maudlin Street, Bristol BS1 2LY, UK b DENTSPLY DeTrey GmbH, De-Trey-Straße 1, D-78467 Konstanz, Germany c School of Chemistry, University of Bristol, Bristol BS8 1TS, UK Received 6 May 1999; received in revised form 10 August 1999; accepted 25 August 1999 Abstract Objective: The primary objective of this pilot study was to test the hypotheses that (i) depth of cure and (ii) compressive strength of dental composites cured with either a light emitting diode (LED) based light curing unit (LCU) or a conventional halogen LCU do not differ significantly. The second objective of this study was to characterise irradiance and the emitted light spectra for both LCUs to allow comparisons between the units. Methods: Dental composite (Spectrum TPH, shades A2 and A4) was cured for 40 s with either a commercial halogen LCU or a LED LCU, respectively. The LED LCU uses 27 blue LEDs as the light source. The composites’ depth of cure was measured for 10 samples of 4 mm diameter and 8 mm depth for each shade with a penetrometer. The results were compared using a Student’s t-test. Compressive strengths were determined after 6 and 72 h, for six samples of 4 mm diameter and 6 mm depth for each shade after being polymerised for 40 s from each end of the mould. Groups were compared using a three way ANOVA. Results: The conventional halogen LCU cured composites significantly  p  0:05 deeper (6.40 mm A2, 5.19 mm A4) than did the LED LCU (5.33 mm A2, 4.27 mm A4). Both units cured the composite deeper than required by both ISO 4049 and the manufacturer. A three way ANOVA showed that there were no significant differences in the compressive strengths of samples produced with either the LED LCU or the halogen LCU  p  0:460: Significant differences in compressive strength of samples stored for 6 and 72 h  p  0:0006 and of samples of different shades  p  0:035 were found as confirmed by the three way ANOVA. The light spectra of both units differed strongly. While the halogen LCU showed a broad distribution of wavelengths with a power peak at 497 nm, the LED LCU emitted most of the generated light at 465 nm. The LED LCU produced a total irradiance of 350 mW cm -2 whereas the halogen LCU produced a total irradiance of 755 mW cm -2 . Significance: The results showed that both units provided sufficient output to exceed minimum requirements in terms of composites’ depth of cure according to ISO 4049 and the depth of cure and the composites’ compressive strength stated by the manufacturer. Compressive strengths of dental composites cured under laboratory conditions with a LED LCU were statistically equivalent to those cured with a conventional halogen LCU. With its inherent advantages, such as a constant power output over the lifetime of the diodes, LED LCUs have great potential to achieve a clinically consistent quality of composite cure.  2000 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Blue light emitting diodes; Light curing unit; Composites; Irradiance; Spectrum; Depth of cure; Mechanical properties 1. Introduction Despite their popularity, halogen technology light curing units (LCUs) used to polymerise dental materials have several drawbacks. For example, halogen bulbs have a limited effective lifetime of approximately 40–100 h [1]. In addition, the LCU’s bulb, reflector and filter degrade over time due to the high operating temperatures and the large quantity of heat which is produced during the duty cycles. This results in a reduction of the LCU’s curing effec- tiveness over time [2]. The clinical implication is that with an ageing LCU, light activated dental materials will be less well cured with poorer physical properties and an increased risk of premature failure of restorations—assuming no compensation for decreased LCU irradiance. It has been shown that many halogen LCUs used by dental practitioners do not reach the minimum power output specified by the manufacturers [2–4]. This can be caused by Dental Materials 16 (2000) 41–47 dental materials 0109-5641/00/$20.00 + 0.00  2000 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S0109-5641(99)00083-4 www.elsevier.com/locate/dental * Corresponding author. Tel.: +44-117-928-4418; fax: +44-117-928- 4780. E-mail address: k.jandt@bris.ac.uk (K.D. Jandt).