d e n t a l m a t e r i a l s 3 5 ( 2 0 1 9 ) 414–421 Available online at www.sciencedirect.com ScienceDirect jo urnal homepage: www.intl.elsevierhealth.com/journals/dema Reporting of light irradiation conditions in 300 laboratory studies of resin-composites a,∗ b,c b David C. Watts , Christina Kaiser , Catherine O’Neill , b,∗∗ Richard Bengt Price a School of Medical Sciences and Photon Science Institute, University of Manchester, UK b Dalhousie University, Faculty of Dentistry, Dental Clinical Sciences, Halifax, Nova Scotia, Canada c Bonn-Rhine-Sieg University of Applied Sciences, Department of Natural Sciences, Rheinbach, Germany a r t i c l e i n f o a b s t r a c t Article history: Objective. To evaluate how the light delivered to resin-composites was described in recent Received 29 September 2018 articles. Received in revised form Method. PubMed was searched for 300 articles published between January 2017 and May 10 December 2018 2018 with keywords relating to photocuring of dental materials. The articles examined a Accepted 10 December 2018 wide range of resin-composite properties and performance. For each article, the information provided about the light curing unit (LCU), the light curing conditions and the characteristics and quantity of the light used in the study were recorded. Specifically, the type of LCU used; Keywords: the irradiance; how the irradiance was measured; the exposure times; whether the light Light curing energy (radiant exposure) received by the specimen was determined, or if only the light Resins output at the LCU tip was measured; whether the distance between the tip of the LCU and Bulk fill the specimen was reported; and whether the emission spectrum from the LCU was reported. Bond strength Where possible, the resin manufacturer’s minimum energy requirement (MER: the product of Light measurement the recommended minimum exposure time and irradiance) was compared to the radiant Research reproducibility and exposure delivered to the specimen. replicability Results. Of the 300 articles examined, 217 were published in 2017 and 83 in 2018. Of these articles, 130 (43%) were found in open access journals, and 170 (57%) were in subscription- based journals. The name of the LCU used was not provided in 31 articles, 14 articles did not provide the exposure time, and 227 articles did not report the distance to the specimen. An irradiance value was reported in 231 articles, but this was the irradiance received by the specimen in only 48 instances. The emission spectrum from the LCU was reported in 2 15 articles. There was a large range in the radiant exposures from below 10 J/cm to greater 2 than 100 J/cm . ∗ Corresponding author at: University of Manchester, School of Medical Sciences and Photon Science Institute, Coupland 3 Building, Oxford Road, Manchester M13 9PL, UK. ∗∗ Corresponding author at: Dalhousie University, Faculty of Dentistry, Dental Clinical Sciences, 5981 University Ave Halifax, Nova Scotia, B3H 4R2, Canada. E-mail addresses: [email protected] (D.C. Watts), [email protected] (R.B. Price). https://doi.org/10.1016/j.dental.2018.12.001 0109-5641/© 2018 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved. d e n t a l m a t e r i a l s 3 5 ( 2 0 1 9 ) 414–421 415 Significance. The majority of articles from 2017 and early 2018 did not include sufficient description of the characteristics and quantity of the light received by the resin-composite specimens to allow the study to be replicated. It is recommended that future articles should report: (1) the identity of the LCU used; (2) the radiant exposure received by the specimen 2 (J/cm ); and (3) appropriate reference to the emission spectrum from the LCU. © 2018 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved. the resin-composites that they light-cure [3]. They often hold 1. Introduction the LCU tip at some distance away from the resin-composite [3], and the LCU often moves or is held at an angle to the sur- The use of light-cured resins has revolutionized modern den- face of the restoration, thus decreasing the radiant exposure tistry. With the agreement of many nations to phase down received by the resin [10–12]. the use of amalgam as part of the Minamata agreement In contrast, Type II studies are concerned with the behavior [1,2], the light curing unit (LCU) will become an indispensable of the material, particularly mechanical strength properties. piece of equipment in almost every dental office. However, When evaluating such resin-composite properties, different despite the routine use of the LCU both in dental offices non-clinical specimen shapes and sizes are used. Rectangular and laboratory studies, the importance of the characteris- cross-section bars are used for 3-point flexure strength mea- tics (wavelength and the beam profile) of the light from the surements. Similar bars with a small central crack are used LCU and the quantity of the light received by the specimen in fracture-toughness studies, and short solid cylinders are (radiant exposure) are poorly understood and described [3–6]. used for compressive creep experiments. In these cases, the Within a resin-composite the degree of conversion, which maximum values for the investigated properties of the resin- describes the extent to which monomer has been converted composite are being examined, and it is deemed appropriate (cured) into a polymer, is related to both the spectral radiant to irradiate the resin-composites from multiple directions power (W/nm) and the radiant exposure received by the resin- (e.g., from the top, bottom and sides) with longer irradiation 2 composite (J/cm ) — the latter value is calculated from the times than would be the case clinically. In such cases, it is not 2 product of the irradiance (W/cm ) received and the exposure necessary to match “clinical reality”, but it is useful to know duration. In a preliminary study [7] that examined 100 arti- the emission spectrum of the light and the radiant exposure cles from 2010 to 2012, it was reported that it was impossible (characteristics and quantity of the light) that was delivered to determine how much light energy (radiant exposure) was to the specimen. delivered to the resin-composites in 46 (46%) articles. It was It has been reported that many studies cannot be replicated also reported that 40 of the 54 articles where the light energy or reproduced, especially among different laboratories [13–15]. could be quantified, delivered more than twice the minimum One reason may be that a study can only be replicated, or the radiant exposure recommended by the resin manufacturers results reproduced if the conditions under which the speci- to photocure the resin specimens. They considered that such mens have been fabricated and stored before measurement radiant exposure far exceeded the MER values recommended have been reported, but this information is rarely provided. by the manufacturers, and so did not reflect “clinical reality”. In an attempt to address such issues, the Academy of Dental Since long-term (5 + years) randomized clinical trials are often Materials has produced a range of guidelines for investiga- unfeasible before a product is released on the market [8,9], tors to follow so that, in principle, the experiments could be well-designed laboratory studies are an essential means to replicated [16]. In addition, the International System of Units (SI) evaluate photocured resin materials. We can divide these lab- nomenclature should be used by authors so that all readers oratory studies into two main categories that we will call Type know what is being described. I and Type II. In radiometry, the radiant power (radiant flux) describes Type I studies are primarily concerned with the aspects the energy emitted or received per unit time. The SI unit for and outcomes of the photocuring process itself. These stud- radiant power is a Watt which is defined as one Joule per ies include measurements (some in real time) of the light second. The irradiance is the radiant power that is received, transmission, depth of cure (DoC), degree of conversion (DC), not emitted, by a surface divided by the area. The SI unit for cell toxicity, shrinkage kinetics, early modulus development, 2 irradiance is W/m [6,17], although in dentistry it is usually shrinkage stress (within a defined host environment) and 2 expressed as mW/cm . The irradiance value that is frequently cuspal deflection. In such studies, the characteristics (wave- reported is often not the irradiance received by the specimen length) and quantity (radiant exposure) of light received by the but is instead the radiant exitance from the LCU. Although the resin-composite specimen should be reported. These values radiant exitance may be the same as the irradiance at 0 mm should also correspond to what a dentist could be expected distance from the light tip, this may not describe what the to deliver in “clinical reality”. This may, or may not, be the specimen actually receives because the specimens are often same as what the manufacturer recommends because many several millimeters away from the LCU tip. Furthermore, an dentists deliver a lower radiant exposure than the minimum irradiance value cannot describe how powerful a curing light that is recommended by the resin-composite manufacturer is because the area of the tip is part of the calculation and a [7,10,11]. Dentists will often use the same exposure time for all 416 d e n t a l m a t e r i a l s 3 5 ( 2 0 1 9 ) 414–421 curing light can deliver a high irradiance, but yet not be very free radicals more slowly and are less efficient than Type I powerful.