Knoop hardness of ten resin composites irradiated with high-power LED and quartz-tungsten-halogen lights

Abstract

This study compared a high-power light-emitting-diode (LED) curing light (FreeLight 2, 3M ESPE) with a quartz-tungsten-halogen (QTH) light (TriLight, 3M ESPE) to determine which was the better at photo-polymerising 10 resin composites. Class I preparations were prepared 4-mm deep into human teeth and filled with 10 different composites. The composites were irradiated for 50% or 100% of their recommended times using the LED light, and for 100% of their recommended times with the QTH light on either the high or medium power setting. Fifteen minutes later, the Knoop hardness of the composites was measured to a depth of 3.5 mm from the surface.

When irradiated by the LED light for their recommended curing times, the Knoop hardness of all 10 composites stayed above 80% of the maximum hardness of the composite to a depth of at least 1.5 mm; three composites maintained a Knoop hardness that was more than 80% of their maximum hardness to a depth of 3.5 mm. Repeated measurements analysis of variance indicated that all the two-way and three-way interactions between the curing light, depth, and composite were significant (p<0.01). To eliminate the choice of composite as a factor, an overall comparison of the lights was performed using the Kruskal–Wallis test and distribution free multiple comparisons of the ranked hardness values. The LED light, used for the composite manufacturer's recommended time, was ranked the best at curing the composites to a depth of 3 mm (p<0.01). The LED light used for 50% of the recommended time was not significantly different from the QTH light used for 100% of the recommended time on the high power setting.

Introduction

The most commonly used dental curing light is the quartz-tungsten-halogen (QTH) light, but light-emitting-diode (LED) curing lights are gaining in popularity. Compared to QTH lights, LEDs convert electricity into light more efficiently, produce less heat, and are more robust. LEDs also last for thousands of hours [1] in contrast to the 30–50 h lifespan of a conventional QTH light bulb [2].

Some resin composites, especially the lighter shades, use photoinitiators or co-initiators that are not as yellow [3] as a commonly used photoinitiator, camphorquinone (CQ). These photoinitiators are activated by wavelengths below the peak absorption for CQ [3]. First generation LED curing lights could not polymerise all dental resins, especially those using photoinitiators and co-initiators other than CQ [2], [4], [5], as well as a correctly functioning QTH light [2], [4], [5], [6], [7], [8], [9], [10]. This may be due to their narrow spectral bandwidth, which was close to the 468 nm absorption peak of CQ [11], [12], and their low overall irradiance [4], [10], [13].

Prototype high-power LED curing lights can polymerise some resins as well as or better than some QTH lights [2], [4], [7], [10], [12], [14], but the performance of commercially available high-power LED lights is unknown. The FreeLight 2 (3M ESPE, St. Paul MN, 55144-1000, USA) uses one high-power LED and delivers a spectral emission with a greater peak irradiance and power output (the area under the spectral emission curve) compared to previous FreeLight curing lights. Fig. 1 shows that the spectral emission from the FreeLight 2 peaks at ∼456 nm, which is shifted to the left of the 468 nm absorption peak of CQ [11], [12]. This increase in output and shift in the spectral emission to the lower wavelengths should enhance the photo-activation of resins that use photoinitiators other than CQ.

If a photo-activated resin does not receive a sufficient number of photons at the correct wavelengths, the amount of polymerisation and degree of conversion will be inadequate [11], [15], [16]. Previous studies have reported that a QTH light should deliver a minimum irradiance (power density) of 300–400 mW/cm² to adequately cure a 1.5–2-mm increment of resin composite [16], [17], [18]. These recommendations were made for QTH lights that produce a broad spectrum of wavelengths and may not apply to LED curing lights. Although most new curing lights produce more than 400 mW/cm², the wavelengths of light produced do not always match the absorption characteristics of the photoinitiators used in some resins. Consequently, inadequate polymerisation of some resins occurs when these lights are used [2], [4], [5], [7], [8], [9], [12], [14], [19], [20].

A review of recent literature that investigated the effects of different curing lights or techniques on resin polymerisation revealed several shortcomings in the methods:

• Most authors have provided only the power output (irradiance) in mW/cm²[8], [9], [15], [20], [21], [22], [23], but this gives insufficient information about the curing light. Fig. 2 shows the spectral emissions from three different curing lights. The distance from the light guide to a spectroradiometer (USB 2000 with a cosine corrector CC-3UV attachment, Ocean Optics, Dunedin, FL, 34698, USA) was adjusted so that each light delivered the same irradiance to the cosine corrector. Even though all three lights delivered the same irradiance of 400 mW/cm², Fig. 2 shows that the spectral emissions from the lights were very different. Therefore, when evaluating different curing lights, it is essential to provide their spectral emission as well as their irradiance.

• As the distance increases from the light guide, the design of the light guide has an effect on the irradiance received by the resin [13], [24], [25]. In many studies the irradiance was measured at or very close to the end of the light guide, but the samples were not irradiated at this distance and were sometimes covered by a glass slide [4], [8], [9], [14], [22], [23]. Thus, the actual irradiance received by the specimens in these studies is unknown.

• Fig. 1 shows the characteristics of the spectral emission from the LEDemetron (Kerr Corp., Orange, CA, 92867, USA), FreeLight, and FreeLight 2 curing lights when recorded using the same calibrated Ocean Optics USB 2000 spectroradiometer. Fig. 1 clearly shows that these brands of LED-curing lights have different spectral emissions. Therefore, global statements about the performance of all LED-curing lights [4], [8], [15], [20], [23], [26] should not be based on an evaluation of only one brand of light.

• Most studies have used only one example of each curing light to predict the performance of a particular brand of light [2], [4], [6], [7], [8], [9], [12], [13], [14], [15], [18], [20], [22], [23], [26], [27], [28]. Fig. 1 shows the mean of three recordings of the spectral emission from five LEDemetron, six FreeLight, and three FreeLight 2 curing lights. Within each brand there was some variation in the spectral emissions. Therefore, if just one light is used in a study, the results reported could be based on an aberrant curing light. If three examples of each curing light are used, then the results are more likely to be representative of the brand of curing light tested.

• Since different brands of composites can have very dissimilar properties [4], [5], [6], [12], [17], [19], [25], [29], the curing light may not have the same effect on all composites. However, interaction effects between curing lights and composites may not have been adequately considered in the statistical analyses [4], [8], [26], which can become complex when several lights, composites and irradiation conditions are studied.

• Using metal or plastic molds [2], [4], [6], [7], [8], [9], [12], [14], [17], [18], [20], [22], [25], [26], [28], [29] may affect the extent of polymerisation [30], [31] and produce results that would not occur in a tooth.

• A good correlation has been reported between the Knoop hardness and the degree of conversion of the monomer within the resin [32], [33]. Techniques that determine the depth of cure by scraping away the soft resin composite and then measuring the thickness of the remaining hard composite [15], [18], [34], or techniques that use a penetrometer to measure the thickness of soft composite [7], [13] may overestimate the actual depth of cure. Measuring the Knoop hardness values (KHN) is a more sensitive method to determine the depth of cure, or to distinguish between the efficacies of different light sources [4], [35].

New designs of curing lights should perform as well as or better than the curing lights they are to replace. Therefore, the hypothesis of this study is that a commercially available high-power LED light, used for each composite manufacturer's recommended curing time, can polymerise a selection of composites better than a conventional QTH light.