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Annals of Dental Research Annals of Dental Research (2011) Vol 1 (1): 54-61 © Mind Reader Publications: All Rights Reserved www.adres.yolasite.com www.journalshub.com Review Article Use of Light Curing Units in Orthodontics: A Review Goyal Amita, Jyothikiran Hb, Shivalinga BMb a Abstract Department of Orthodontics and Introduction: Because of their wide field of applications, light Dentofacial Orthopedics, Guru Nanak Dev Dental College, Sunam, Punjab, India b Department of Orthodontics and curing units are now indispensable for any orthodontist or a general dentist, and hence it is very important to be familiar with the various types of light curing units, their history, specifications, advantages and disadvantages. Dentofacial Orthopedics, J.S.S. Dental Methods: College, Mysore, Karnataka, India review, a search was conducted in the following database: PubMed To find the articles appropriate for this systematic (from1966 to March 2010) using the terms ―curing lights Correspondence to Dr. Amit Goyal, Department of Orthodontics and Dentofacial Orthopedics, Guru Nanak Dev Dental College, Sunam, Punjab, India Email: [email protected] Phone: +91-8699347476 orthodontics‖ in the search box. Eligibility of the selected studies was determined by reading the abstracts of articles identified by the search. All the articles that seemed to meet the inclusion criteria of the systematic review topic were selected and the actual articles collected. The reference lists of the retrieved articles were also hand searched for any applicable studies that may have been missed in the database searches. Article Info Received: 21 August 2011 Revised: 20 October 2011 Accepted: 21 November 2011 Conclusion: When selecting curing lights for an office, many variables need to be considered. Armed with knowledge about each curing-light category, orthodontists can evaluate their unique practice style and select the appropriate light or lights. Annals of Dental Research (2011) 1(1), 54-61 Keywords: Light curing units, Orthodontics. Copyright: © 2011 Goyal et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. 2011- A DR- 13 Light Cure in Orthodontics Goyal et al The light source first used for curing Introduction Since the discovery of the bis-GMA type of adhesive by Bowen, various methods of bonding orthodontic brackets directly to pre-conditioned tooth surfaces have been composite resins was an ultraviolet light (UV), which required 1 minute per millimeter of resin thickness for curing. The benzoin methyl ether component of composite resin was studied by Newman 1, Mizrahi and Smith2, 3, 4, sensitive to light in the 340 nm spectrum. Retief, Dreyer and Gavron5, and Miura, Because of safety concerns about the long term Nakagawa and Masuhara6. Visible-light-cured use of UV light, visible light curing (VLC) was adhesives, now used by the vast majority of orthodontists, were first described by Bassiouny7 in 1978. Materials and methods Studies satisfying the following criteria were included in this systematic review: I. Use of tungsten-quartz halogen curing lights, argon lasers, plasma arc curing lights or light emitting diodes for curing orthodontic resins. II. Cross sectional and longitudinal studies. III. Articles in English IV. Articles published from January 1966 to March 2010 To find the articles appropriate for this systematic review, a search was conducted in the following database: PubMed (from1966 to March 2010) and used the terms ―curing lights orthodontics‖ in the search box. Eligibility of the selected studies was determined by reading the abstracts of articles identified by the search. All the articles that seem to meet the inclusion criteria of the systematic review topic were selected and the actual articles collected. The reference lists of the retrieved articles were also hand searched for any applicable studies that may have been missed in the database searches. History of curing lights introduced around 19808. Compared with UVcured resins, VLC resins were found to have a greater depth of curing. The curing of VLC resins is based on the presence of camphoroquinone, which is sensitive to light in the range of 460 to 480 nm wavelength spectrum, with an optimum at 468 nm. The earliest visible light curing units used halogen bulbs. Their spectrum of radiation is continuous over the visible range, with radiation intensity increasing significantly towards the red end of the spectrum. Most tungsten-quartz halogen lights produce an energy density of approximately 400 mW/cm2, with a broad bandwidth between 400 and 520 nm. The halogen based curing lights are still popular in the market but despite their popularity, halogen bulbs have several shortcomings. These light curing units use most of their energy to heat a tungsten filament until it glows, creating light. Only 1% of the total energy input is converted into light; the remainder is generated as heat. The heat can cause blistering of expensive light filters and discoloration of reflectors. The cooling fan can be noisy and bulky. Halogen bulbs have a limited effective lifetime of approximately 40100 hours and have to be replaced thereafter. Moreover, with a recommended curing time of 40 seconds per bracket, the light-curing time for bonding both the maxillary and mandibular ADR, Vol 1, Issue 1, 2011 55 Goyal et al Light Cure in Orthodontics arches can approach 15 minutes which can be seconds of plasma irradiation cures many resin inconvenient. So, reducing the curing time composites to a hardness comparable with that became the next immediate goal for the achieved after 40 seconds with conventional developers. curing lights. The first attempt to reduce curing time In 1995, Mills et al11 proposed solid- was undertaken with argon lasers in the late state light-emitting diode (LED) technology 9 1980s . The purpose of these lamps is to for the polymerization of light-activated dental increase the output light energy to an intensity materials. Instead of hot filaments used in 2 that approaches 800 mW/cm and to narrow halogen bulbs, LEDs use junctions of doped the wavelength to approximately 470 nm. semiconductors to generate light. LEDs have a Independent research has verified that argon potential lifetime of over 10,000 hours and can lasers can reduce the amount of light exposure, be subjected to mechanical shock and vibration but to equal the bracket bond strength of a 40- with very low failure rates. The latest blue second exposure to a conventional tungsten- LEDs use indium gallium nitride technology quartz halogen light, the laser must cure the and can generate photons of a particular composite resin for 10 seconds. This is still a wavelength by varying the band gap. A wide dramatic reduction in the required exposure band gap material produces high-energy time, from 15 minutes to 4. Nevertheless, the photons near the blue region of the visible argon laser has several disadvantages. The spectrum. As current flows through the laser units themselves are relatively large and semiconductor chips, electrical energy is expensive converted directly into light, and little energy costing more than $6000; replacement bulbs are nearly $2000. is emitted as heat. This results in a stable, Plasma arc curing lights (PACL) were 10 efficient, long lasting output of blue light of introduced in mid 1990s . The light cure 440 to 480 nm. source is a xenon gas that is ionized by 2 conversion of the LEDs has allowed the electrodes with a large voltage potential, to development of cordless light-curing units that produce plasma. The emitted white light is operate silently and have a very long life. filtered to a bandwidth of 450-500 nm, and the Effect of curing times on shear bond power strength density can reach more than 2 2000mW/cm . These lights were made to be as The efficient energy Each of the curing lights produces a reliable as laser lights at a relatively lower cost different ($3000-$4000). In contrast to lasers, plasma resulting in different curing times required for arc sources do not emit distinct frequencies complete adhesive polymerization. light frequency and intensity, but, rather, continuous frequency bands. According to Lalani et al12 and Elvebak However, these bands are much narrower than et al13, at 300 mW power the argon laser has those of incandescent lights (halogen bulbs). the capability to maximally polymerize the Consequently, less filtering of undesired adhesive in as little as 5 seconds. Oesterle et frequencies is required. Because of high al14 and Signorelli et al15 found out that xenon intensity, the manufacturers say that 1 to 3 56 ADR, Vol 1, Issue 1, 2011 Light Cure in Orthodontics Goyal et al plasma arc curing lamp exposure times of 6 to 9 seconds produced shear bond strengths equal to those produced with 40 second exposures to Effect of light guides and distance of a conventional tungsten quartz halogen curing light tip from the bracket on shear bond 16 17 light, in vitro. Silta et al , Usumez et al and strength Turkkahraman and Kucukesmen18 studied that orthodontic brackets be photopolymerized for atleast 20 seconds with the light emitting diode light cure units before archwires are engaged and that this is equivalent to 40 seconds of halogen-based illumination. Based on the above data it can be concluded that with standard metal brackets, suggested curing times for a complete cure are 15 to 20 seconds on the mesial and distal of each bracket (total 40 seconds) using a halogen light, 10 seconds mesial and distal (total 20 seconds) for LED lights, 2-3 seconds mesial and distal (total 5 seconds) using an argon laser, and 3-4 seconds mesial and distal (total 6-9 seconds) with a plasma arc lamp. Recently, a new, highpowered halogen light was claimed to achieve ideal bond strength with only 6 seconds of cure time19. Ceramic brackets require half of the Bishara et al21 studied the effects of using a smaller diameter light guide on the shear bond strength of orthodontic brackets. They concluded that the use of the Mini Turbo Light Guide (diameter 4mm) did not seem to significantly influence either the shear bond strength or the bracket/adhesive/enamel failure site as compared to a standard curved light guide (diameter 11mm). However, Evans et al22 studied the effects of using Power Slot and Turbo Tip light guides and concluded that these light guides with their collimation of visible light to increase its intensity can be recommended as advantageous alternatives for curing composite resins for orthodontic bonding procedures. The distance of the light tip to the bracket can decrease bond strength. With increased distance, bond strength degrades total time for each light type, and the light more significantly with LED lights than with source should be aimed directly through the other curing lights; if access is difficult, a non- bracket20. Molar bonds require about 150% LED light source should be considered. 23 longer curing times on each the mesial and distal. Temperature rise Halogen, argon laser, and plasma arc Effect of light intensity on shear bond strength lights all generate significant heat and require a cooling fan. All of these instruments therefore 13 Elvebak et al compared the shear bond generate significant noise during and after strength produced by an argon laser at 4 operation. Although halogen lights have different power settings (100, 150, 200 and powerful fans to cool the unit, the intense heat 250 mW) and found out that brackets bonded does damage the light bulb so that the effective using the curing light of 150mW power bulb life is only about 100 hours.24 The exhibited the highest bond strength. minimal heat generated by LED lights can be easily dissipated by heat sinks, and these lights ADR, Vol 1, Issue 1, 2011 57 Goyal et al Light Cure in Orthodontics can therefore operate noiselessly. Heat from microleakage with quartz tungsten halogen curing units is transferred to teeth during (mean, 1.10 ± 1.05 mm) and the greatest bonding procedures; this can, at times, cause microleakage with plasma arc curing light discomfort to patients. It is known that any curing units (mean, 2.63 ± 1.49 mm). Serdar increase in pulpal temperature exceeding 5°C Arikan et al32 compared microleakage beneath to 6°C may result in irreversible tissue ceramic and metal brackets photopolymerized 25 showed that in vitro with light emitting diode or conventional light pulp chamber temperature increase from laser curing units and observed microleakage along units were significantly lower than that from all bonding interfaces, regardless of the type of 26 bracket or LCU used. The tested light emitting measured the temperature rise in the composite diode curing unit provided a reduction in chair samples with three different light sources, and time but caused more leakage between because of very short exposure time, they adhesive-bracket found a slight temperature rise with the high- brackets were used. However Mustafa Ulker et power plasma light. Cobb et al27 and Powell et al33 found that the type of light-curing unit al28 concluded that argon lasers should pose no (Halogen, light emitting diode, plasma arc threat to the pulp if used at recommended curing) did not significantly affect the amount energies. In fact, Powell et al25 stated that, at of microleakage at the enamel-adhesive- recommended curing times, in vitro pulp- bracket complex. chamber temperature increases from argon Design considerations damage. Powell et al the conventional curing lights. Tarle et al interface when metal lasers were significantly lower than those of The energy efficiency and minimal heat the conventional curing lights. Aslihan Uzel et generation of LED curing give them many al 29 and Siddik Malkoc et al 30 found in their characteristics that the other sources do not study that halogen light induced significantly have. Because they use minimal energy and do higher intrapulpal temperature changes than not need a cooling fan, LED lights are able to did the LED and PAC. However, orthodontic be marketed as cordless units with a bonding with light-curing units did not exceed rechargeable battery. And because they do not the critical 5.5°C value for pulpal health. have any moving parts or light filaments, they Microleakage better resist vibration and shock. 34 As there is Asli Baysal et al31 studied the no heat damage to the diodes, LED lights are microleakage under bonded lingual retainers effective for more than 10,000 hours of use using high-intensity curing lights and found with little output degradation. that little or no microleakage occurs at the Although their exact mechanisms are composite/enamel interface with high-intensity unknown, argon lasers uniquely restructure light curing units. However, high-intensity enamel-surface light curing units allow more microleakage at some demineralization resistance when used the composite/wire interface and may not be for safe synergistically with the protection imparted by for bonding of lingual retainers. Comparison of the results showed the least 58 bonding. characteristics, This protection conferring increases fluoride.35 ADR, Vol 1, Issue 1, 2011 Light Cure in Orthodontics Each of the Goyal et al various curing-light 2. Mizrahi E, Smith DC. Direct cementation of varieties has a different design. Plasma arc and orthodontic brackets to dental enamel. An argon laser curing lights are cumbersome and investigation using a zinc polycarboxylate weigh between 9 and 15 pounds. Many owners cement. Br Dent J. 1969 Oct 21;127(8):371-5. of these lights mount the light on a rolling cart 3. Mizrahi E, Smith DC. Direct attachment of or dedicate a single chair for bonding orthodontic brackets to dental enamel. 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The effect of argon laser irradiation on demineralization resistance of human enamel adjacent to orthodontic brackets: an in vitro study. Angle Orthod. 2003;73: 249–258. Citation : Goyal A, Jyothikiran H, Shivalinga BM. Use of Light Curing Units in Orthodontics: A Review. Annals of Dental Research 2011; 1 (1): 54-61. ADR, Vol 1, Issue 1, 2011 61