Temperature rise induced by various light curing units through human dentin

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Dental Materials Journal 2009; 28(3): 253–260 Original Paper Temperature rise induced by various light curing units through human dentin Arife DOGAN 1 , Ihsan HUBBEZOGLU 2 , Orhan Murat DOGAN 3 , Giray BOLAYIR 3 and Hakan DEMIR 3 1 Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey 2 Department of Endodontics, Faculty of Dentistry, Cumhuriyet University, Sivas, Turkey 3 Department of Prosthodontics, Faculty of Dentistry, Cumhuriyet University, Sivas, Turkey Corresponding author, Arife DOGAN; E-mail: [email protected]; [email protected] This study investigated temperature rises caused by different light curing units (LCUs) in dentin of different thicknesses. The different LCUs tested in this study were namely: quartz-tungsten-halogen (QTH) (Heliolux DLX) LCU, plasma arc (PAC) (Apollo 95E Elite) LCU, and light emitting diode (LED) (Mini LED) in standard curing mode as well as pulse and soft-start modes. One hundred and forty dentin disks of 0.5, 1, 1.5, and 2 mm thickness were prepared from mandibular molars (n=7). Temperatures were recorded using a L-type thermocouple in direct contact with the light guide tip. For all curing units/ modes, dentin thickness was inversely proportional to temperature rise and that QTH light gave significantly higher values compared to PAC and LED in all the test conditions. The highest temperature rise was observed under 0.5-mm-thick dentin disk with QTH, whereas the lowest temperature rise was registered with LED light in pulse mode under 2-mm-thick dentin. Keywords: Light curing units, Temperature rise, Dentin Received Jul 14, 2008: Accepted Sep 29, 2008 INTRODUCTION In recent developments for visible light curing, the use of visible light to cure dental materials has widely expanded to cover a vast array of products including composite resins 1) . With light-cured dental restorative materials, they set via an additional polymerization process due to exposure to light of a requisite wavelength and intensity, whereby the latter initiates the generation of free radicals to propagate the polymerization 2) . For many years, quartz-tungsten-halogen light curing units (QTH LCUs) have been used to polymerize resin composites. Typically, these light sources need to emit 500 to 800 mW/cm 2 of light for 30 to 40 seconds to polymerize composites at a depth of 2 mm 3) . The main radiant output from a conventional QTH source is infra-red energy, which is absorbed by resin composites and which results in increased molecular vibration and heat generation. Thus, conventional QTH LCUs require heat- absorbing filters to reduce the passage of infra-red energy from the light source to the tooth 4-6) . However, the LCU’s bulb, reflector, and filter degrade over time due to high operating temperatures, and unfiltered infra-red energy can result in heat generation in the pulp chamber 3,7-11) . Of late, the light emitting diode (LED) technology has been introduced to the dental profession as an alternative to conventional halogen technology. As opposed to the hot filaments used in halogen bulbs, LEDs use doped semiconductor junctions (p-n junctions) to generate and emit light. Under proper forward bias conditions, the injected electrons and holes recombine at a p-n junction and thereby emit light; in the case of gallium nitride LEDs, blue light is emitted. Additionally, a small polymer lens is included in front of the p-n junction to produce a partially collimated light 3,8,11) . On the comparison between LED and QTH LCUs, previous studies have reported on a slew of advantages that the former wield over the latter. It has been reported that LEDs are more efficient than QTH lamps in converting energy to light and that the wavelength of light emitted by LEDs closely matches the absorption spectrum of camphorquinone (CQ), a photoinitiator widely used in light-activated dental resins 3,8-10,12,13) . Unlike QTH light sources, LEDs do not generate infra-red rays — which means that they have a constant light output and a longer lamp life expectancy 3,5,7,10) . Heating of irradiated objects by LED lights is also expected to be minimal 14) . Furthermore, most of the available LED LCUs offer a user-programmable curing time ranging from 5 to 60 seconds. This provision arguably augers well for the trend toward shorter curing times in the polymerization of dental composites 15) . The plasma arc (PAC) curing light is also designed for high-speed curing of composite filling materials in direct resin restorations. A high-energy, high-pressure ionized gas in the presence of an electrical current is used to create a high-temperature

Transcript of Temperature rise induced by various light curing units through human dentin

Dental Materials Journal 2009; 28(3): 253–260

Original Paper�Temperature rise induced by various light curing units through human dentin�

Arife DOGAN1, Ihsan HUBBEZOGLU2, Orhan Murat DOGAN3, Giray BOLAYIR3 and Hakan DEMIR3

1Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey2Department of Endodontics, Faculty of Dentistry, Cumhuriyet University, Sivas, Turkey3Department of Prosthodontics, Faculty of Dentistry, Cumhuriyet University, Sivas, TurkeyCorresponding author, Arife DOGAN; E-mail: [email protected]; [email protected]

�This�study�investigated�temperature�rises�caused�by�different�light�curing�units�(LCUs)�in�dentin�of�different�thicknesses.��The�different�LCUs�tested�in�this�study�were�namely:�quartz-tungsten-halogen�(QTH)�(Heliolux�DLX)�LCU,�plasma�arc�(PAC)�(Apollo�95E�Elite)�LCU,�and�light�emitting�diode�(LED)�(Mini�LED)�in�standard�curing�mode�as�well�as�pulse�and�soft-start�modes.��One�hundred�and�forty�dentin�disks�of�0.5,�1,�1.5,�and�2�mm�thickness�were�prepared�from�mandibular�molars�(n=7).��Temperatures�were�recorded�using�a�L-type�thermocouple�in�direct�contact�with�the�light�guide�tip.� �For�all�curing�units/modes, dentin thickness was inversely proportional to temperature rise and that QTH light gave significantly higher values compared�to�PAC�and�LED�in�all�the�test�conditions.��The�highest�temperature�rise�was�observed�under�0.5-mm-thick�dentin�disk�with�QTH,�whereas�the�lowest�temperature�rise�was�registered�with�LED�light�in�pulse�mode�under�2-mm-thick�dentin.

Keywords:�Light�curing�units,�Temperature�rise,�Dentin�

Received Jul 14, 2008: Accepted Sep 29, 2008

INTRODUCTION

In� recent� developments� for� visible� light� curing,� the�use� of� visible� light� to� cure� dental� materials� has�widely� expanded� to� cover� a� vast� array� of� products�including�composite�resins1).� �With�light-cured�dental�restorative� materials,� they� set� via� an� additional�polymerization� process� due� to� exposure� to� light� of� a�requisite� wavelength� and� intensity,� whereby� the�latter� initiates� the� generation� of� free� radicals� to�propagate�the�polymerization2).

For� many� years,� quartz-tungsten-halogen� light�curing� units� (QTH� LCUs)� have� been� used� to�polymerize� resin� composites.� � Typically,� these� light�sources�need� to� emit�500� to�800�mW/cm2�of� light� for�30�to�40�seconds�to�polymerize�composites�at�a�depth�of� 2� mm3).� � The� main� radiant� output� from� a�conventional� QTH� source� is� infra-red� energy,� which�is�absorbed�by�resin�composites�and�which�results�in�increased� molecular� vibration� and� heat� generation.��Thus,� conventional� QTH� LCUs� require� heat-absorbing filters to reduce the passage of infra-red energy�from�the�light�source�to�the�tooth4-6).��However,�the LCU’s bulb, reflector, and filter degrade over time due to high operating temperatures, and unfiltered infra-red�energy�can�result� in�heat�generation�in�the�pulp�chamber3,7-11).

Of�late,�the�light�emitting�diode�(LED)�technology�has� been� introduced� to� the� dental� profession� as� an�alternative� to� conventional� halogen� technology.� � As�opposed to the hot filaments used in halogen bulbs,

LEDs� use� doped� semiconductor� junctions� (p-n�junctions)� to�generate�and�emit� light.� �Under�proper�forward� bias� conditions,� the� injected� electrons� and�holes� recombine� at� a� p-n� junction� and� thereby� emit�light;� in� the�case�of�gallium�nitride�LEDs,�blue� light�is� emitted.� � Additionally,� a� small� polymer� lens� is�included� in� front� of� the� p-n� junction� to� produce� a�partially�collimated�light3,8,11).

On� the� comparison� between� LED� and� QTH�LCUs,� previous� studies� have� reported� on� a� slew� of�advantages� that� the� former�wield�over� the� latter.� � It�has been reported that LEDs are more efficient than QTH� lamps� in� converting� energy� to� light� and� that�the� wavelength� of� light� emitted� by� LEDs� closely�matches�the�absorption�spectrum�of�camphorquinone�(CQ),� a� photoinitiator� widely� used� in� light-activated�dental� resins3,8-10,12,13).� � Unlike� QTH� light� sources,�LEDs�do�not�generate�infra-red�rays�—�which�means�that� they� have� a� constant� light� output� and� a� longer�lamp� life� expectancy3,5,7,10).� � Heating� of� irradiated�objects� by� LED� lights� is� also� expected� to� be�minimal14).� �Furthermore,�most�of� the�available�LED�LCUs�offer�a�user-programmable�curing�time�ranging�from�5�to�60�seconds.��This�provision�arguably�augers�well�for�the�trend�toward�shorter�curing�times�in�the�polymerization�of�dental�composites15).

The� plasma� arc� (PAC)� curing� light� is� also�designed for high-speed curing of composite filling materials�in�direct�resin�restorations.��A�high-energy,�high-pressure� ionized� gas� in� the� presence� of� an�electrical�current�is�used�to�create�a�high-temperature�

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light�source�strong�enough�to�increase�the�curing�rate�of�resin�composites16).��With�the�development�of�xenon�PAC�lights,�much�higher�light� intensities�(2000�mW/cm2)� are� currently� employed� to� polymerize� dental�composites� within� a� curing� time� of� merely� 3�seconds17,18).

On� the� other� hand,� rapid� polymerization� may�result� in� the� formation� of� short� polymer� chains� and�shorten� the� pre-gel� phase� such� that� the� material� is�unable� to� adequately� absorb� polymerization�contraction�stresses2).� �With�a�view�to�controlling�the�effect� of� polymerization� shrinkage� stemming� from�polymerization� contraction� stresses19),� recent�developments� have� focused� on� various� irradiations�protocols� such� as� soft-start� polymerization� or� pulse�curing.� �Soft-start�polymerization�is�characterized�by�using� an� initial� low� power� intensity� of� the� curing�light� followed� by� a� higher� power� intensity12,19,20).� � As�for pulse curing, it is initiated by a short flash of light� followed� by� a� relaxing� time� of� several� minutes�before final cure is performed10).� � These� techniques�allow the composite resin to flow from the tooth surface,� reduce� stress,� and� potentially� improve� the�marginal� integrity� of� the� restoration10,12,14,21).��However,� if� soft-start� mode� were� to� be� used� for� the�polymerization�of�resin�composites�in�order�to�realize�these� advantages,� prolonged� curing� time� is�necessary15),� and� this� may� lead� to� increased� pulpal�temperatures.

The� increase� in� pulpal� temperature� has� been�attributed� to� two� main� sources:� radiation� energy�emitted�by�the�LCU�and�the�polymerization�exotherm�of� resin� composites2,4,16,17,20,22,23).� � It� has� been� reported�that� photopolymerization� with� high� energy� output�curing units caused significantly higher pulp chamber temperature�changes�as�compared�to�the�conventional�curing�units16,19,24-26).

Besides the influential role of light curing units on� the� effect� of� heat� experienced� by� the� pulp,� some�factors�such�as�dentin�thickness�and�proximity�of�the�material� to� the� pulp� may� also� play� a� role2,4).��Damaging�effects�of�temperature�increase�on�the�pulp�during� resin� composite� polymerization� has� been�widely� investigated.� � However,� the� increased� power�of recently developed LCUs (≥1500 mW/cm2)� has�renewed�concerns�about�the�biological�safety�of�these�curing� units,� especially� when� they� are� used� in� deep�cavities�with�minimal�remaining�dentin�thickness.

To� date,� thermal� emission� from� these� recently�developed,�high-power�LCUs�has�not�been�thoroughly�and�fully�evaluated.� �To�gain�a�better�understanding�on� the� response� of� dentin� to� these� latest� changes� in�light� intensity,�this�study�examined�the�temperature�rise� in� dentin� of� different� thicknesses� caused� by�halogen,�LED,�and�PAC�LCUs.��Temperature�changes�associated� with� various� curing� modes� of� LED� LCU�were�also�compared.

MATERIALS AND METHODS

Preparation of dentin disksDentin�disks�of�0.5,�1,�1.5,�and�2�mm�thickness�were�prepared� from� freshly� extracted,� caries-free�mandibular molars embedded in Teflon molds with epoxy�resin.��These�disks�were�obtained�by�sectioning�perpendicular� to� the� long� axis� of� the� tooth� with� a�water-cooled� saw� (Isomet,� Buehler� Ltd.,� Lake� Bluff,�IL,� USA)� and� kept� in� distilled� water� until� use.� � For�each� dentin� thickness,� 35� specimens� were� obtained�for�the�different�curing�light�protocols�(n=7).��A�mylar�strip�was�placed�over�each�dentin�disk�and�the�latter�was� irradiated� from� the� top� through� the� strip� using�one�of�the�LCUs�tested.

Light curing units (LCUs)The�light�curing�units� (LCUs)�selected�for�this�study�included� a� conventional� quartz-tungsten-halogen�(QTH;� Heliolux� DLX,� Ivoclar-Vivadent,� Schaan,�Liechtenstein)� LCU,� a� plasma� arc� (PAC;� Apollo� 95E�Elite,� Dental� Medical� Technology� Systems� Inc.,�Orange,� CA,� USA)� LCU,� and� a� blue� light� emitting�diode� (LED;� Mini� LED,� Satelec,� Merignac,� France)�LCU.��Besides�the�standard�mode�of�these�LCUs,�the�soft-start� and� pulse� modes� of� LED� LCU� were� also�evaluated.� � Table� 1� shows� the� details� of� the� LCUs�and�their�curing�modes.

Temperature measurementThermal�emission�of�the�LCUs�was�measured�using�a�type�L� thermocouple� (Fe-Const.�Elimko�Co.,�Ankara,�Turkey)� connected� to� a� data� logger� (E-680,� Elimko�Co.,�Ankara,�Turkey).��E-680�series�of�universal�data�loggers/scanners� were� advanced,� new-generation�microcontroller-based� industrial� instruments�compatible� with� IEC� 668� standards.� � Data� were�collected�using�a�data�logging�software�(Data�Logger,�5.1�ver.,�Elimko�Co.,�Ankara,�Turkey)�and�stored�in�a�centrally�located�PC.

To� standardize� the� temperature� rise�measurements,� an� apparatus� was� specially� devised�for this study. It comprised two Teflon mold cylinders constructed from polytetrafluoroethylene. The movable top Teflon mold cylinder included a central�aperture�(6�mm�diameter,�2�mm�depth)�which�was� located� exactly� over� the� test� specimen.� � It� was�available� in� different� sizes� (8� mm� diameter,� 0.5,� 1,�1.5,�and�2�mm�depth)�so�as�to� form�the� lateral�walls�of�the�dentin�disks�placed�at�the�bottom�(Fig.�1).��The�light� tip� of� the� LCU� was� positioned� at� the� center� of�the Teflon mold and in direct contact with the mold, thus�standardizing�the�distance�between�the�light�tip�of� the� LCU� and� the� dentin� disks� in� each� test�condition. Apart from the movable top Teflon mold, the� entire� apparatus� was� surrounded� by� another�Teflon mold cylinder which provided an entrance for

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the� thermocouple� wire� —� via� a� hole� (1� mm� in�diameter) drilled through this Teflon mold — just beneath�the�center�of�the�dentin�disk.

During� each� temperature� measurement,� the�initial� temperature� was� recorded� after� the�temperature� stabilized� at� 20� ±� 0.1°C� followed� by�registering� the� temperature� peak.� � This� peak� was�registered�even�after�the�curing�light�was�turned�off.��The initial temperature was deducted from the final one� to� obtain� the� temperature� rise� value.� � All�measurements� were� performed� by� the� same�investigator.

Statistical analysisTemperature� rise� data� were� analyzed� using� a� SPSS�statistical�software�program�(Version�10.0,�SPSS�Inc.,�Chicago,�USA).� �The�data�were�subjected�to�two-way�analysis� of� variance� (ANOVA)� among� the� four�

different� dentin� thicknesses� as� well� as� among� the�curing light protocols. Where significant differences were� present,� Tukey’s� post hoc� test� was� applied� to�examine pairwise differences at a significant level of 0.05.

RESULTS

Table� 2� and� Fig.� 2� present� the� mean� temperature�rise values. Two-way ANOVA revealed significant differences� in� temperature� rise� values� (F=1195.94)�based� on� curing� light� protocol� (p=0.000)� and� dentin�thickness�(p=0.000).� �Groups�with�a�dentin�thickness�of� 2� mm� exhibited� the� lowest� mean� values� in�temperature�rise,�whereas�the�halogen�light�gave�the�highest�values�under�all�test�conditions.��Tukey’s�test�revealed significant differences in temperature rise among� all� the� LCUs� for� dentin� disks� with� the� same�thickness� (p<0.05),� except� between� QTH-� and� PAC-irradiated�groups�with�2-mm-thick�dentin�(p>0.05).

For� all� the� test� conditions,� LED� light� produced�the� lowest� temperature� rise.� � Examining� different�modes�of�LED�revealed�that�soft-start�mode�produced�the� highest� temperature� rise� and� that� the�temperature� rise� values� produced� by� this�polymerization technique were significantly higher than� those� of� standard� mode� in� all� the� specimens�with� the� same� dentin� thickness� (p<0.05)� (Table� 2).��While there were no statistically significant differences� between� standard� mode� and� pulse� mode�in� 2-mm-thick� and� 1.5-mm-thick� dentin� (p>0.05),�significantly lower temperature rise values were recorded� for�pulse�mode�compared� to�standard�mode�in�1-mm-thick�and�0.5-mm-thick�dentin�(p<0.05).

Table 1 Details of light curing units and their polymerization modes and profiles

Unit Light�type Diameter�of��light�tip

Wavelength�of�emission Mode Output�and�

total�time Profile

Heliolux�DLX Halogen 8�mm 400-500�nm� Standard 750�mW/cm2

(40�seconds)Continuous�energy�output�for�40s

Apollo95�E�Elite PAC 7.2�mm 460-490�nm Standard� 1600�mW/cm2

(3�seconds)Continuous�energy�output�for�3s

Mini�LED LED 7.5�mm 420-480�nm Standard *1100�mW/cm2

(10�seconds)Continuous�energy�output�for�10s

Mini�LED LED 7.5�mm 420-480�nm Pulse 1100�mW/cm2

(10�seconds)10 successive 1-second flashes at full�power�pulse�activation�mode,�with� a� rest� period� of� 250ms�between flashes

Mini�LED LED 7.5�mm 420-480�nm Soft�start 0��to�1100�mW/cm2

+1100�mW/cm2

(20�seconds)

Exponential� energy� output�automatically� increased� to� full�energy� within� 10s� +10� seconds�full�energy

*Light�intensity�as�given�by�manufacturers.

Fig.�1� Apparatus�for�measuring�temperature�changes.

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Regardless� of� the� curing� light� and/or� curing�mode� used,� the� mean� value� of� temperature� rise�increased� as� dentin� thickness� decreased� (p<0.05).��Temperature�changes�were�found�to�be�different�from�each� other� in� all� conditions� (p<0.05).� � The� highest�mean� value� in� temperature� rise� was� recorded� with�halogen� light� in� standard� mode� in� 0.5-mm-thick�

dentin (1.98±0.05˚C), whereas the lowest value was recorded� with� LED� in� pulse� mode� in� 2-mm-thick�dentin (0.32±0.03˚C).

Figures� 3–7� show� the� changes� in� temperature�rise� with� dentin� thickness� for� the� different� curing�units/modes.� � The� slopes� (i.e., the coefficients of change)�as�well�as�the�R2�values�were�given�in�Table�

Table 2 Mean temperature rise values (˚C) for each dentin thickness and curing unit/mode evaluated

Dentin�thicknessUnit Curing�mode 2�mm 1.5�mm 1�mm 0.5�mm

Halogen Standard 0.94�(0.04)a 1.23�(0.07) 1.63�(0.04) 1.98�(0.05)PAC Standard 0.90�(0.02)a 1.13�(0.04) 1.41�(0.03) 1.87�(0.04)LED Standard 0.36�(0.03)b 0.45�(0.02)c 0.64�(0.04) 0.95�(0.05)LED Pulse 0.32�(0.02)b 0.41�(0.02)c 0.56�(0.02) 0.82�(0.02)LED Soft�start 0.56�(0.04) 0.70�(0.04) 0.96�(0.04) 1.30�(0.05)

n=7�specimens�per�experimental�condition.Standard�deviations�are�shown�in�parentheses.By�two-way�ANOVA:�F=1195.94,�p=0.000,�p<0.05The�comparison�between�different�curing�units�for�the�same�dentin�thickness�and�the�comparison�among�different�dentin�thicknesses�with�the�same�curing�unit�are�shown�in�vertical�columns�and�horizontal�rows�respectively.� �Values�with�the�same� superscript� letter� are� not� statistically� different� at� p>0.05; others not superscripted differ significantly among themselves�at�p<0.05�by�Tukey’s�test.

Fig. 2 Results of temperature rise (˚C) for the experimental�groups.

Fig.�4� Temperature� rise� with� dentin� thickness� for� PAC�LCU.

Fig.�5� Temperature� rise� with� dentin� thickness� for�standard�mode�of�LED�LCU.

Fig.�3� Temperature� rise� with� dentin� thickness� for� QTH�LCU.

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3.� � Halogen� and� PAC� had� the� highest� slopes� for� the�change� in� temperature� rise� with� dentin� thickness.��This�was�most�probably�because�these�LCUs�emitted�highly� energetic� photons,� which� when� absorbed� by�dentin� decayed� into� large� populations� of� infra-red�photons.� � The� infra-red� photons,� in� turn,� increased�the�temperature.

DISCUSSION

Thermal�emission�from�light�curing�units�has�been�a�

matter� of� concern� to� the� dental� profession.� � It� has�been� stated� that� the� decisive� factor� for� temperature�rise� during� light-activated� polymerization� of� resin�composites�is�the�energy�absorbed�during�irradiation,�and� that� the� exothermic� composite� polymerization�process� is� only� of� secondary� importance� toward�temperature� rise27).� � Therefore,� the� factor� of�restorative� materials� was� excluded� from� the�experimental� design� and� that� this� study� directly�investigated� the� effect� of� recently� developed,� high-energy�LCUs�on�temperature�rise�in�dentin.� �To�this�end,� dentin� disks� of� different� thicknesses� were� used�to� simulate� different� clinical� conditions.� � Since�thermocouples� have� been� described� as� reliable� tools�for� temperature� measurements� in� published�literature25),� a� type� L� thermocouple� was� selected� for�this� study� and� that� it� operated� with� a� starting�temperature� of� 20.0� ±� 0.1°C� as� described� in� other�studies4,5).

Studies� have� shown� that� a� temperature� rise� of��5 – 6˚C in pulp tissue caused necrosis in 15% cases, a rise of 11.2˚C in 60% cases, and a rise of 11.8˚C in 100% cases24,28).� � In� the� current� study,� the� values�obtained� did� not� reach� these� limits� of� pulpal�temperature,�given�that�the�highest�temperature�rise�was� 1.98� ±� 0.05°C� in� 0.5-mm-thick� dentin� with� the�use� of� QTH� LCU.� � Therefore,� the� current� results�suggested� that� the� use� of� high-power� curing� units/modes�could�be�safe�even�in�deep�cavities.

Although contradictory findings have been reported�with�regard�to�the�effects�of�different�LCUs�on� temperature� rise,� most� studies� concurred� that�temperature rise is significantly higher when resin composites� were� cured� using� the� QTH� light� as�compared� to� those� obtained� with� the� PAC� and/or�LED�LCUs3,19,24-26).��In�the�current�study,�temperature�rise in dentin ranged between 0.32 and 1.98˚C, which was� lower� than� the� data� given� in� published�literature.� � This� could� be� attributed� to� the�experimental� design� of� the� current� study,� which�assessed�only�the�effect�of�LCUs�on�temperature�rise,�deliberately� sidestepping� the� reaction� temperatures�arising� from� the� polymerization� exotherm� of� resin�composites� —� which� was� also� included� in� other�studies.� � Although� there� was� a� tendency� toward�higher� temperature�values�with� the�PAC�LCU�in�all�the�test�conditions,�the�highest�temperature�rise�was�measured� with� the� QTH� LCU� as� compared� to� the�values� obtained� with� LED� or� PAC� LCU� (p<0.05).��However,� an� exception� was� noted� in� 2-mm-thick�dentin groups, where no significant difference in temperature� rise� was� observed� between� QTH� and�PAC�LCUs�(p>0.05).��In�comparing�between�PAC�and�LED LCUs, PAC curing led to significantly higher temperatures�than�those�induced�by�LED,�which�was�most� probably� due� to� the� higher� light� intensity� of�PAC�(1600�mW/cm2�for�PAC�versus�1100�mW/cm2�for�

Fig.�6� Temperature� rise� with� dentin� thickness� for� pulse�mode�of�LED�LCU.

Fig.�7� Temperature� rise� with� dentin� thickness� for� soft-start�mode�of�LED�LCU.

Table 3 Coefficients of temperature change with dentin thickness� according� to� different� light� curing�units/modes

Unit Curing�mode Slope R2

Halogen Standard 0.70 0.99

PAC Standard 0.64 0.97

LED Standard 0.39 0.94

LED Pulse 0.33 0.95

LED Soft�start 0.49 0.97

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LED).Yap� and� Soh19)� have� concluded� that� LED� curing�

lights emitted significantly less heat from their light guides� than� QTH� lights.� � This� assertion� could� be�accepted, given that the filtered emission spectrum of a�QTH�lamp�is�broader�than�that�of�a�LED�and�that�it�includes�wavelengths�of�light�capable�of�generating�a� higher� quantity� of� heat6).� � According� to� Lloyd� et al.29), the most significant factor that increased pulpal temperature� during� light� curing� is� the� energy�absorbed� from� irradiation.� � This� might� well� explain�the�observation� that� irradiation�with�QTH�LCU�was�associated� with� higher� temperature� rise� in�comparison�with�PAC�or�LED�LCU.� �The�QTH�LCU�used�in�this�experiment�had�a�lower�light�intensity�of�750� mW/cm2� but� produced� a� higher� total� energy� (30�J/cm2)�due�to� its�40-second�curing�time,�as�compared�with� PAC� (4.8� J/cm2� for� 3� seconds)� and� LED� (11��J/cm2�for�10�seconds).� �This�result�seemed�to�suggest�that temperature rise was influenced by total energy rather�than�light�intensity�in�the�case�of�QTH�LCU.

According� to� our� results,� it� could� be� concluded�that� in�order� to�attain� temperature� rises� lower� than�the� critical� value� which� causes� pulpal� injury,� the�high-power� LED� LCU� seemed� to� be� a� plausible�option.� � Among� the� three� different� modes� of� LED�used,� the� soft-start� mode� registered� the� highest�temperature� rise� (p<0.05)� in� all� the� test� conditions.��The� difference� in� total� energy� produced� by� these�three� modes� of� LED� could� have� also� accounted� for�the�different�temperature�changes.� �The�total�energy�produced�with�soft-start�mode�was�22�J/cm2,�whereas�it�was�11�J/cm2�for�the�other�two�modes.��From�these�results,� it�could�be�suggested�that�the�standard�and/or�pulse�modes�of�LED�LCU�might�reduce�the�risk�of�pulpal� injury� even� in� the� case� of� thin� dentin� tissue.��This�was�because�the�standard�and/or�pulse�modes�of�LED� LCU� produced� a� lower� temperature� rise� not�only� when� compared� with� the� soft-start� mode,� but�also� against� the� standard� curing� mode� of� QTH� and�PAC� LCUs� used� in� this� study.� � In� some� clinical�conditions,� the�use�of� soft-start�polymerization� could�be� a� solution� for� decreasing� stress� during�polymerization,� thereby� improving� the� marginal�sealing�of�the�restoration�and�reducing�microleakage.��However,� this� curing� mode� requires� a� longer� curing�time�and�consequently�raises� the�temperature�of� the�resin� composite� and� the� surrounding� dentin21,30).��Therefore,� especially� in� deep� cavities,� this� technique�should�be�used�with�caution.

In� this� study,� another� temperature� rise-determining�factor�that�was�taken�into�consideration�was� the� thickness� of� residual� dentin.� � According� to�Loney� and� Price31),� a� thicker� dentin� substrate� can�reduce� the� temperature� because� of� low� thermal�conductibility� of� this� substrate.� � The� present� study�confirmed this assertion. For all the light curing

units/modes� tested,� there� was� an� exponential�increase� in� temperature� rise� as� dentin� thickness�decreased� and� that� all� the� differences� were�significant.

Figures� 3–7� show� the� changes� in� temperature�with� dentin� thickness� for� the� different� curing� light�protocols,� whilst� Table� 3� gives� the� slopes� (i.e.,�coefficients of change, indicating how fast the temperature�changed�with�dentin�thickness)�and�the�R2�values.��The�last�column�of�Table�3�shows�that�the�lowest� R2� value� is� 0.94,� hence� indicating� the� high�reproducibility� of� data.� � Further,� Table� 3� revealed�that� the� slope� for� pulse� mode� of� LED� ranked� the�lowest,� whereas� that� of� QTH� LCU� ranked� the�highest.� � Photons� emitted� from� the� LED� source� are�not� energetic,� which� thus� explained� why� the� slopes�for�all�the�curing�modes�of�LED�were�lower�compared�to� those� of� QTH� and� PAC� LCUs.� � In� comparing�among� the� curing� modes� of� LED,� intermittent�irradiation�(i.e.,�pulse�mode�of�LED)�naturally�yielded�a� smaller� slope� (0.33)� than� in� the� continuous� case�(i.e.,� standard�mode� of�LED,� slope:� 0.39).� �Moreover,�extension� of� curing� time� from� 10� seconds� to� 20�seconds� increased� the� slope� from� 0.39� to� 0.49.� � This�was�because�the�energy�deposited�on�dentin�could�not�diffuse�out�easily�from�the�material,�and�thus�created�a�steeper�slope�for�the�soft-start�mode.

PAC� LCU� probably� emitted� more� energetic�photons� than� blue� LED,� causing� higher� energy�deposition.� � QTH� LCU� also� emitted� highly� energetic�photons,� thus� yielding� the� steepest� slope� of� 0.70� as�seen� from� Table� 3.� � However,� the� output� energy� of�PAC�was�1600�mW/cm2�while�that�of�QTH�LCU�was�750� mW/cm2� (Table� 1).� � In� other� words,� not� all�photons�were�absorbed�by�dentin.� �This�was�because�the absorption coefficient of a material usually depends� on� the� wavelength� of� light,� which� means�that� materials� preferentially� absorb� certain�wavelengths�of�light.

Another� point� is� that� not� all� absorbed� light�energy�directly�converts�into�heat.��Depending�on�the�excitation� modes� of� the� absorber,� some� excited�photons�decay�into�some�other�forms�of�photons�with�lower frequencies. If the photon final state after decay�is�in�the�infra-red�region,�it�will�usually�result�in�higher�temperature�rise�since�infra-red�absorption�causes� thermal� agitation.� � In� the� case� of� QTH� and�PAC� LCUs,� photons� with� high� frequencies� were�absorbed� according� to� Einstein’s� equation� (i.e.,�Energy�E=hv,�where�h� is�Planck’s�constant,�and�v� is�frequency).��Therefore,�they�degraded�into�low�energy�photons with increased population, finally causing thermal�agitation.

In� summary,� the� current� study� indicated� that�the� three� commonly� used� dental� photocuring� light�sources�—�namely,�QTH,�PAC,�and�LED�LCUs�—�did�not� reach� the� critical� temperature� rise� which� would�

Dent Mater J 2009; 28(3): 253–260 259

cause�pulpal�injury,�as�reported�by�Zach�and�Cohen28).��However,�the�extrapolation�of�temperatures�obtained�in� this� in vitro� study� cannot� be� directly� applied� to�temperature�changes�in situ�because�of�these�clinical�factors: blood circulation in the pulp chamber, fluid motion� in� the� dentinal� tubules,� as� well� as� the�dynamics� of� surrounding� periodontal� tissue16).� � In�addition,�the�experimental�setup�of�this�study�did�not�consider� heat� conduction� through� composite� resin�polymerization�—�which�could�be�a�weighty�factor�in�clinical�conditions.� �Therefore,� further� in vivo�and� in vitro� investigations�are�needed�to�assess�the�effect�of�exothermic� reaction� of� resin� composites� on�temperature�changes�in�dentin�when�different�curing�methods�are�used.

CONCLUSIONS

Within�the� limitations�of� this� in vitro� study,� it�could�be�concluded�that:

1.� Using� the� irradiation� times� as� recommended�by�the�manufacturers,�the�QTH�LCU�produced�significantly higher temperature rises than did LED�and�PAC�LCUs�in�all�the�test�conditions.

2.� The� standard� and� pulse� modes� of� LED� LCU�showed�lower�mean�values�in�temperature�rise�than�the�soft-start�mode.

3.� Dentin�thickness�was�inversely�proportional�to�temperature�rise.

ACKNOWLEDGMENTS

Special�thanks�to�Prof.�Güngör�Gündüz,�Department�of� Chemical� Engineering,� Middle� East� Technical�University,� for� his� invaluable� contributions� in�explaining the correlation coefficient of the relationship� between� dentin� thickness� and�temperature�rise.

This work was supported by Scientific Research Unit� of� Cumhuriyet� University� (CUBAP),� Project�Number:�DIS-043.

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