Robin W. Mills

Light-emitting diode (LED) polymerisation of dental composites : Flexural properties and polymerisation potential

The clinical performance of light polymerised dental composites is greatly influenced by the quality of the light-curing unit (LCU) used. Commonly used halogen LCUs have some specific drawbacks such as decreasing of the light output with time. This may result in low degree of monomer conversion of the composites with negative clinical implications. Previous studies have shown that blue-light-emitting diode (LED) LCUs have the potential to polymerise dental composites without having the drawbacks of halogen LCUs. Despite the relatively low irradiance of current LED LCUs, their efficiency is close to that of conventional halogen LCUs with more than twice the irradiance. This phenomenon has not been explained fully yet. Hence, more tests of the LED LCUs effectiveness and of the mechanical properties of oral biomaterials processed with LED LCUs need to be carried out. This study investigates the flexural properties of three different composites with three different shades, which were polymerised with either a commercial halogen LCU or an LED LCU, respectively. In most cases no significant differences in flexural strength and modulus between composites polymerised with a halogen LCU or an LED LCU, respectively, were found. A simple model for the curing effectiveness based on the convolution absorption spectrum of the camphorquinone photoinitiator present in composites and the emission spectra of the LCUs is presented.

Polymerization and light-induced heat of dental composites cured with LED and halogen technology

Most commercial light curing units (LCUs) for dental applications use conventional halogen bulbs. Commercial LCUs using light emitting diodes (LEDs) have recently become established on the market, even though some aspects of their performance have not been fully investigated. Temperature rise of dental composites during the light-induced polymerization is considered to be a potential hazard for the pulp of the tooth. This study, therefore, investigated the temperature rise in three different composites (Z100, Durafill, Solitaire2) in two shades (A2, A4) polymerized for 40s with two LED LCUs (Freelight, custom-made LED LCU prototype) and two halogen LCUs (Trilight, Translux). The Trilight was used in the standard and soft-start mode. The temperature rise within the composites were recorded for 60s with a thermocouple and also observed with a high-resolution infrared (HRIR) camera. The factors LCU (p < 0.0001), composite (p < 0.0001) and shade (p = 0.0014) had statistically significant influences on the temperature rise. All composites cured with the halogen LCUs reached at a depth of 2 mm, a statistically significant higher temperature (p < 0.0001) than those cured with the LED LCUs. Only one composite showed a statistically significant lower temperature rise for the halogen LCUs at the 95% confidence level, when the soft-start mode was used instead of the standard mode. In general, the composites with the lighter shade (A2) reached higher temperatures than the darker shade (A4), if the LED LCUs were used. When the halogen LCUs were used, the situation was reversed, the composites with the darker shade (A4) reaching higher temperatures than the lighter shade (A2). This study showed that a HRIR camera represents a powerful tool for the observation of temperature propagation on small samples. This study also showed that LED LCUs represent a viable alternative to halogen LCUs for the light polymerization of dental composites because of a generally lower temperature increase within the composite.

Photoinitiator dependent composite depth of cure and Knoop hardness with halogen and LED light curing units.

Light curing units (LCUs) are used for the polymerization of dental composites. Recent trends in light curing technology include replacing the halogen LCUs with LCUs using light emitting diodes (LEDs) reducing curing times and varying the LCUs light output within a curing cycle. This study investigated the time dependence of the Knoop hardness and depth of cure of dental composites polymerized with a halogen LCU (Trilight) and two LED LCUs (the commercial Freelight and custom-made LED LCU prototype). The halogen LCU was used in the soft-start (exponential increase of output power) and standard mode. Four dental composites (Z100, Spectrum, Definite, Solitaire2) were selected, two of them (Definite, Solitaire2) contain co-initiators in addition to the standard photoinitiator camphorquinone. The depth of cure obtained with the Trilight in the standard mode was statistically significantly greater (p < 0.05) than that obtained with the LED LCUs for all materials and curing times. The custom made LED LCU prototype (LED63) achieved a statistically significantly greater depth of cure than the commercial LED LCU Freelight for all materials and curing times. There was no statistical difference in Knoop hardness at the 95% confidence level at the surface of the 2 mm thick sample between the LED63 or Trilight (standard mode) for the composite Z100 for all times, and for Spectrum for 20s and 40s curing time. The composites containing co-initiators showed statistically significantly smaller hardness values at the top and bottom of the samples if LED LCUs were used instead of halogen LCUs. The experiment revealed that the depth of cure test does not and the Knoop hardness test does discriminate between LCUs, used for the polymerization of composites containing photoinitiators in addition to camphorquinone.

High power light emitting diode (LED) arrays versus halogen light polymerization of oral biomaterials: Barcol hardness, compressive strength and radiometric properties.

The clinical performance of light polymerized dental composites is greatly influenced by the quality of the light curing unit (LCU) used. Commonly used halogen LCUs have some specific drawbacks such as decreasing light output with time. This may result in a low degree of monomer conversion of the composites with negative clinical implications. Previous studies have shown that blue light emitting diode (LED) LCUs have the potential to polymerize dental composites without having the drawbacks of halogen LCUs. Since these studies were carried out LED technology has advanced significantly and commercial LED LCUs are now becoming available. This study investigates the Barcol hardness as a function of depth, and the compressive strength of dental composites that had been polymerized for 40 or 20s with two high power LED LCU prototypes, a commercial LED LCU, and a commercial halogen LCU. In addition the radiometric properties of the LCUs were characterized. The two high power prototype LED LCUs and the halogen LCU showed a satisfactory and similar hardness-depth performance whereas the hardness of the materials polymerized with the commercial LED LCU rapidly decreased with sample depth and reduced polymerization time (20 s). There were statistically significant differences in the overall compressive strengths of composites polymerized with different LCUs at the 95% significance level (p = 0.0016) with the two high power LED LCU prototypes and the halogen LCU forming a statistically homogenous group. In conclusion, LED LCU polymerization technology can reach the performance level of halogen LCUs. One of the first commercial LED LCUs however lacked the power reserves of the high power LED LCU prototypes.

A brief history of LED photopolymerization

OBJECTIVES The majority of modern resin-based oral restorative biomaterials are cured via photopolymerization processes. A variety of light sources are available for this light curing of dental materials, such as composites or fissure sealants. Quartz-tungsten-halogen (QTH) light curing units (LCUs) have dominated light curing of dental materials for decades and are now almost entirely replaced by modern light emitting diode light curing units (LED LCUs). Exactly 50 years ago, visible LEDs were invented. Nevertheless, it was not before the 1990s that LEDs were seriously considered by scientists or manufactures of commercial LCUs as light sources to photopolymerize dental composites and other dental materials. The objective of this review paper is to give an overview of the scientific development and state-of-the-art of LED photopolymerization of oral biomaterials. METHODS The materials science of LED LCU devices and dental materials photopolymerized with LED LCU, as well as advantages and limits of LED photopolymerization of oral biomaterials, are discussed. This is mainly based on a review of the most frequently cited scientific papers in international peer reviewed journals. The developments of commercial LED LCUs as well as aspects of their clinical use are considered in this review. RESULTS The development of LED LCUs has progressed in steps and was made possible by (i) the invention of visible light emitting diodes 50 years ago; (ii) the introduction of high brightness blue light emitting GaN LEDs in 1994; and (iii) the creation of the first blue LED LCUs for the photopolymerization of oral biomaterials. The proof of concept of LED LCUs had to be demonstrated by the satisfactory performance of resin based restorative dental materials photopolymerized by these devices, before LED photopolymerization was generally accepted. Hallmarks of LED LCUs include a unique light emission spectrum, high curing efficiency, long life, low energy consumption and compact device form factor. SIGNIFICANCE By understanding the physical principles of LEDs, the development of LED LCUs, their strengths and limitations and the specific benefits of LED photopolymerization will be better appreciated.

Light energy attenuation through orthodontic ceramic brackets at different irradiation times

This ex vivo study evaluated the total light energy (TLE) transmission (J cm) through three types of ceramic brackets. TLE values were obtained at three discrete time intervals of 5, 10 and 20 s for each type of bracket alone. This was repeatedwith the addition of orthodontic adhesive. In the adhesive cases, an additional Vickers hardness (VH) of the bottom surface of the adhesive (the surface that would have been adjacent to the tooth in vivo) was measured to complement the work. It is well known that the VH correlates well with both the degree of conversion of the double bonds within the adhesive and therefore the strength. A poor degree of conversion potentially also increases the level of leached monomers. Three types of brackets were used, two polycrystalline and one monocrystalline. Such was the attention to detail in this work to reduce bias that the same bracket, a maxillary right central incisor, was used and even then, these were randomly assigned. The results demonstrated the monocrystalline bracket transmitted a greater amount of the light energy useful for the photopolymerization than either of the polycrystalline brackets. This in turn, was reflected in the positive correlation between light transmission andVH values in the adhesive. The authors rightly conclude that the exposure time may need to be adjusted for ceramic brackets to ensure an adequate TLE is received by the adhesive for effective photopolymerization. The next step points to a clinical study to see whether these findings have significance clinically. When measuring TLE, it is worth noting how it is calculated. Light can be delivered in pulsed, modulated or ‘continuouswave’ form, or indeed a combination of all three. For example, if the first 5 s was pulsed followed by a continuous wave emission, the 5 s exposure would be 100% pulsed, whereas in the 20 s case, pulsing would represent 25% of the exposure time. In a personal communication in 1996 with Lord Porter, the 1967 Nobel Laureate in Chemistry for Flash Photolysis, Porter, in answer to a question about TLE, answered; ‘Pulsed light will give a different result from continuous wave because the free radical concentration will be higher in the pulse case and termination by bimolecular processes will be more common and the reaction yield will be less’. I understand in this study, the light emission was constant throughout so this variable could be discounted. I also understand the resolution of the MARC®-RC detector in capturing data was 60 ms so this too was not an issue with the light source in this work. With different light sources these factors could affect the way the TLE is delivered and in theory as Porter describes, identical TLEs could give slightly differing photopolymerization yields, i.e. degrees of conversion. In summary, this work was well-designed with very good attention to detail and a useful and constructive foundation for a clinical follow-up.