Temperature rise during experimental light-activated bleaching

ORIGINAL ARTICLE

Temperature rise during experimental light-activatedbleaching

Eva Klaric & Mario Rakic & Ivan Sever & Zrinka Tarle

Received: 8 March 2013 /Accepted: 4 June 2013# Springer-Verlag London 2013

Abstract The purpose of this study was to evaluate the sur-face and intrapulpal temperatures after treatments with differ-ent bleaching gels subjected to different types of light activa-tion. A K-type thermocouple and infrared thermometer wereused to measure the temperature increase during the 15- or 30-min treatment period. Light-emitting diode with a center wave-length of 405 nm (LED405), organic light-emitting diode(OLED), and femtosecond laser were tested and compared toZOOM2. The tooth surface was treated with five bleachingagents and Vaseline which served as a control.The generalizedestimating equation (GEE) model was applied for testingthe differences in temperature increase. The ZOOM2 lightsource led to the largest increase in mean pulpal and toothsurface temperatures of 21.1 and 22.8 °C, followed byfocused femtosecond laser which increased the pulpal andsurface temperatures by up to 15.7 and 16.8 °C. Treat-ments with unfocused femtosecond laser, LED405, andOLED induced significantly lower mean temperature increases(p<0.001 for each comparison with ZOOM2 and focusedfemtosecond laser), both in the pulp chamber (up to 2.7, 2.5,and 1.4 °C) and at the tooth surface (up to 3.2, 3.4, and 1.8 °C).Significant differences between pulp chamber and toothsurface measurements were obtained for all types ofbleaching gel, during treatments with ZOOM2 (p<0.001),LED405 (p<0.001), and unfocused (p<0.001) and focusedfemtosecond laser (p≤0.002). Different bleaching agents orVaseline can serve as an isolating layer. Focused femtosecond

laser and ZOOM2 produced large temperature increases inthe pulp chamber and at the tooth surface. Caution isadvised when using these types of light activation, whileLED405, OLED, and unfocused femtosecond laser couldbe safely used.

Keywords Light . Tooth bleaching . Temperature

List of abbreviationsLED Light-emitting diodeOLED Organic light-emitting diodePAC Plasma archQTH Quartz–tungsten–halogenIR InfraredHP Hydrogen peroxideCP Carbamide peroxide

Introduction

Tooth whitening is becoming one of the most popular estheticand corrective treatments for discolored teeth. Bleaching canbe performed internally on nonvital teeth or externally on vitalteeth by applying hydrogen peroxide, sodium perborate, orcarbamide peroxide, the most common agents used forbleaching [1–3]. These agents are found effective for toothbleaching, but have many side effects like changes in the toothstructure, microleakage of restorations made either after orbefore the whitening procedure [4, 5], external root resorption,postoperative sensitivity, and pulpal irritation [6].

In light-activated tooth bleaching procedures, there is agreat concern about the heat generated by the light source,whichmay cause pulp irritation or severe damage like necrosis.For acceleration or more effective tooth whitening, differentlight sources may be used, such as quartz–tungsten–halogen

E. Klaric (*) : Z. TarleDepartment of Endodontics and Restorative Dentistry,School of Dental Medicine, University of Zagreb,Gunduliceva 5, 10000 Zagreb, Croatiae-mail: [email protected]

M. RakicInstitute of Physics, Bijenicka cesta 46, Zagreb, Croatia

I. SeverInstitute for Tourism, Vrhovec 5, Zagreb, Croatia

Lasers Med SciDOI 10.1007/s10103-013-1366-6

lamp (QTH), plasma lamps, light-emitting diode (LED), orhalogen or laser light. When the bleaching agent is activatedunder the influence of light, some amount of light is absorbedand the resulting energy converted into heat. This can beobserved as a possible side effect during this type of toothbleaching. Therefore, light sources may present photothermaleffects which are then associated with the chemical effect ofthe bleaching materials. Some authors consider this a majormechanism of action of all light-activated bleaching proce-dures [7, 8]. Zach and Cohen [9] concluded that a rise intemperature of 5.5 °C causes irreversible pulp damage in15 % of the teeth, while a temperature rise of 11.2 °C causesnecrosis in 60 % of the teeth. Temperature rise causes coagu-lation of the protoplasm and expansion of the liquid in the pulpand dentin tubules, and increases flow of liquids from dentintubules [10]. There is still a controversy surrounding the effectof bleaching gel. Some authors argue that the whitening gelcan improve light absorption while promoting the conversionof light energy into heat [8, 11, 12], which may result incritically high temperature values, unsafe for pulpal vitality.On the other hand, others claim that bleaching gel serves as aprotective isolating layer, making this type of bleaching pro-cedure safe for pulpal health [13].

In the present study, new light sources were used: LEDlight with center wavelength of 405 nm (in further text,LED405), OLED, and femtosecond laser. OLED is an or-ganic light-emitting diode in which the emissive electrolu-minescent layer is a film of organic compound that emitslight in response to an electric current [14]. OLEDs are usedin large-area light-emitting elements for general illuminationand were never tested as a type of light used for light-activated bleaching. A femtosecond pulse laser is a laser thatemits pulses with a duration of a hundred femtoseconds andaverage power of 1 W. The whole system of femtosecondlaser used in this study consists of a pump laser at 532 nm(Millenia, Spectra Physics, USA) and Tsunami oscillator (Ti:sapphire laser) which generates femtosecond pulses. The roleof the pump laser is to excite the crystal Ti:sapphire in theoscillator which ultimately generates femtosecond pulses. Thewavelength of the femtosecond pulses can be changed in thearea of 700–950 nm. We used the central wavelength of about770 nm, with full width at half maximum of 10 nm. Bothfocused and unfocused modes were tested. As a control lightsource, ZOOM2 was used. This whitening system utilizes amercury metal halide light. The emitted violet light has awavelength of 350–400 nm. It has an infrared filter helpingtominimize the amount of heat the patient's teeth are subjectedto during the treatment.

The purpose of the present study was to evaluate thesurface and intrapulpal temperature variation after bleachingtreatment with different gels of hydrogen peroxide (in furthertext, HP) and carbamide peroxide (in further text, CP)subjected to different sources of light activation.

Null hypotheses: (1) Different sources of light activation(LED405, OLED, femtosecond laser, and ZOOM2) with andwithout different gels of HP and CP or Vaseline gel (control) donot significantly increase the surface and pulp chamber temper-atures. (2) There is no difference in the temperature increasebetween the experimental light sources and ZOOM2 (control).(3) The presence of the bleaching gel or Vaseline has no effecton the temperature increase. (4) There is no difference in thetemperature increase between the five bleaching gels and Vas-eline (control). (5) There is no difference between the surfaceand intrapulpal temperatures during the bleaching treatment.

Materials and methods

In this study, the crowns of human teeth were submitted todifferent HP or CP bleaching agents (Table 1) activated bydifferent light sources (Table 2). A K-type thermocouple withdigital thermometer (Termopar Digital Multimeter, TektronixDMM 916, USA) was used to measure the temperatureincrease in the pulp chamber while an infrared thermometer(NIMEX NI8010, USA) was used for surface measure-ments of extracted upper incisors and canines.

Preparation of teeth

Sixty extracted maxillary central incisors and canines wereselected. They were free of dental caries, calcifications, andrestorations with similar pulp chamber morphology. Theteeth were cleaned and stored in 1 % chloramine solution atthe room temperature immediately after extraction. The useof extracted human teeth was approved by the ResearchEthics Committee of the School of Dental Medicine, Uni-versity of Zagreb, Croatia. The root portions of the teethwere sectioned with a slow-speed diamond saw (Isomet,Buehler Ltd., Lake Bluff, USA) approximately 2 mm belowthe cementoenamel junction perpendicular to the long axis ofthe teeth, and stored in deionized water. An opening wasmade into the pulpal chamber, enlarged with #6 and #8 GatesGlidden burs (Dentsply Maillefer, Tulsa, USA) from theradicular portions of the teeth so that a thermocouple wirecould be inserted. The pulpal chamber was cleaned of rem-nant pulpal tissues. The labial surfaces of the teeth werethoroughly pumiced for 2 min, rinsed in distilled water, anddried with clean, compressed air. Intrapulpal and surfacetemperature measurements were divided into six groups(n=10), depending on the light source treatment. In eachgroup, five maxillary incisors and five maxillary canineswere used. Each experimental group was treated with oneof the following: 25 and 38 % HP gel and 10, 16, and 30 %CP gel. Measurements observed without light source treat-ment referred only to the applications of these bleachinggels. However, treatments with light sources also included

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neutral Vaseline, which served as a control, and measure-ments without bleaching gel application to the pulp or sur-face. In total, the analysis included 40 different combinationsof light sources and bleaching gels.

Temperature measurements

The distance between the emitting tip of the light source andthe tooth surface was set to 10 mm. Bleaching gels weremixed according to the manufacturer's instructions and wereapplied with a brush to the tooth surface, in a layer approx-imately 2 mm thick. Before the application of a new type ofbleaching gel, the old gel was removed from the surface.Application time for all light units was 30 min except forZOOM2 (only 15 min). Before the bleaching treatment, asilicon thermal paste (FujiPoly, Milton Keynes, UK) wasintroduced in the pulp chamber, in order to improve thermalconductance. The tip of a digital K-type thermocouple wasthen introduced to the pulp chamber as closely as possible tothe external surface of the tooth, and the specimen wasproperly fixed. The surface temperature was measured usingthe digital infrared thermometer by pointing it towards thelabial surface of the tooth. The applications of each lightsource were performed on each specimen at room temperature

(24 °C) with or without the presence of the bleaching gel,waiting for a sufficient time between applications for thespecimens to return to room temperature. Room temperaturewas checked by referring to the thermocouple reading. Thetemperatures were recorded at five time points: before lightactivation and after 5, 10, 15, and 30 min of light activation.

Data analysis

Descriptive statistics were obtained to describe the mainfeatures of the temperature increase within the underlyingexperimental groups. Temperatures obtained were recordedas discrete measurements, and inspection of the data indicatedthat assumptions of normality and homogeneity were not met,which was also confirmed by Shapiro–Wilk and Box–Coxtests. Furthermore, due to the longitudinal nature of the data,i.e., repeated measurements obtained during the treatment,and detected nonlinearity in temperature change, the general-ized estimating equations (GEE) approach was selected toassess the differences in the mean temperature increase amongobserved light source/bleaching gel combinations. Poissonregression with a log link and autoregressive working corre-lation matrix were used as a part of GEE model settings.Logarithm of time was used as an offset variable meaning

Table 1 Summarized bleaching products (data given by manufacturer) including ingredients, application, active bleaching agent, and percentageconcentration of hydrogen or carbamide peroxide

Product Manufacturer Ingredients Application Active bleachingagent

Percent

ZOOM2 Discus Dental, CulverCity, USA

Water, poloxomer 497, glycerin,propylene glycol, potassium nitrate,potassium hydroxide, mentha piperita,eugenol, ferrous gluconate, hydrogenperoxide (25 %)

In office Hydrogen peroxide 25

Boost Ultradent, South Jordan,UT, USA

Propylene glycol, hydrogen peroxide(38 %), 1,1 % flouride, 3 % potassiumnitrate

In office Hydrogen peroxide 38

Viva Style 30 Ivoclar Vivadent, Schaan,Liechtenstein

Propylene glycol, carbamide peroxide(30 %), aqua, carbomer, peppermint oil

Home bleaching(individual mouthguard)

Carbamide peroxide 30


Glycerol, carbamide peroxide (16 %),carbomer, and peppermint oil




Glycerol, carbamide peroxide (10 %),carbomer, and peppermint oil



Vaseline Pliva, Zagreb, Croatia Petroleum jelly – – 100

Table 2 Light sources evaluated in the study (data given by manufacturer)

Product Manufacturer Type of light Power Applicationtime (min)

LED LED Engin Inc, San Jose, USA LED λ 405 nm 400 mW/cm2 15, 30

PPML OLED KIT engineering prototype PPML, Pescara, Italy OLED λ 400–760 nm 200 mW/cm2 15, 30

Millenia Millenia, Spectra Physics, USA Femtosecond laser λ 770 nm 800 mW/cm2 15, 30

ZOOM2 Discus Dental, Culver City, USA Mercury metal halidelight λ 350–400 nm

2000 mW/cm2 15

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that the mean temperature increase was modeled on a perminute basis, accounting for the average slope of increase(growth rate) in each time interval.

The significance level was set at 0.01. p values were adjustedfor multiple comparisons according to the Benjamini–Hochbergmethod, controlling the false discovery rate. Analysis wasperformed using SAS 8.2 (SAS Institute Inc., NC, USA).

Results

The largest temperature increase was observed during treat-ment with the ZOOM2 light source despite a shorter treatmentduration compared to other light sources (15 vs. 30 min)(Fig. 1). Treatment with the ZOOM2 light source, withoutthe application of bleaching gel, increased the baseline tem-perature (24 °C) of the pulp chamber and tooth surface by 21.1and 22.8 °C, on average (Table 3). Application of different gel

types or Vaseline lowered the mean increase to approximately16 °C in the pulp chamber, while such insulating effect wasnot observed at the tooth surface where the mean temperatureincrease remained at approximately 23 °C. A somewhat lowerincrease was observed during treatments with focused femto-second laser (15.7 and 16.8 °C without bleaching agentapplied to the pulp and tooth surface). Application ofdifferent bleaching agents or Vaseline lowered the increaseto approximately 9 °C in the pulp and 10 °C at the toothsurface. Usage of other light sources demonstrated consid-erably lower temperature increases which were below 3 °C(in the pulp) and 4 °C (at the tooth surface) for LED405 andunfocused femtosecond laser and below 2 °C for OLED.

Effect of bleaching agent

The application of bleaching gel provided an insulatingeffect during ZOOM2 (in the pulp chamber only), LED405

Fig. 1 Average temperature increases from baseline to the end of the treatment for no gel, Vaseline, and 25 % hydrogen peroxide application

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(at the tooth surface only), and during both focused andunfocused femtosecond laser treatment. Temperature increaseduring treatments with these light sources was significantlyaffected by the gel application (Fig. 2). Results of the GEEmodel revealed that applications of bleaching gel duringZOOM2 treatment resulted in approximately 1.3-fold de-crease in the mean temperature observed in the pulp chamber

compared to no gel application (p<0.001 for each comparisonof no gel application with bleaching gels tested). Mean pulpaltemperature increases, adjusted for the average slope (speed)of increase in each time interval, per minute of ZOOM2treatment were 2.2 °C without gel application and 1.6 °Cotherwise, while tooth surface temperature increased by2.3 °C, regardless of bleaching gel application (thus not

Table 3 Mean temperature in-crease for observed light source/bleaching gel combinations

Means and standard deviations arepresented in the table. ZOOM2results included measurementsobserved during the first 15 min ofexperiment while results for otherlight sources included additionalmeasurement at the end of a 30-min treatment period

Light source/bleaching gel combination Mean pulp chambertemperature increase (°C)

Mean surface enameltemperature increase (°C)

ZOOM2, without gel 21.1 (3.55) 22.8 (4.52)

ZOOM2, Vaseline 15.4 (2.93) 22.5 (4.13)

ZOOM2, 25 % HP 15.5 (2.81) 22.4 (4.06)

ZOOM2, 38 % HP 15.5 (3.05) 22.4 (3.94)

ZOOM2, 30 % CP 15.5 (2.93) 22.5 (3.96)

ZOOM2, 16 % CP 15.6 (2.75) 22.5 (4.00)

ZOOM2, 10 % CP 15.6 (2.93) 22.7 (3.84)

LED405, without gel 2.5 (1.13) 3.4 (2.01)

LED405, Vaseline 1.9 (0.78) 2.5 (1.60)

LED405, 25 % HP 1.9 (0.80) 2.6 (1.63)

LED405, 38 % HP 1.8 (0.90) 2.6 (1.76)

LED405, 30 % CP 1.8 (0.82) 2.5 (1.57)

LED405, 16 % CP 1.9 (0.80) 2.5 (1.62)

LED405, 10 % CP 1.8 (0.81) 2.5 (1.54)

OLED, without gel 1.4 (0.50) 1.8 (0.93)

OLED, Vaseline 1.2 (0.38) 1.4 (0.81)

OLED, 25 % HP 1.2 (0.38) 1.6 (0.85)

OLED, 38 % HP 1.2 (0.36) 1.5 (0.75)

OLED, 30 % CP 1.2 (0.38) 1.6 (0.90)

OLED, 16 % CP 1.2 (0.38) 1.4 (0.75)

OLED, 10 % CP 1.2 (0.38) 1.6 (0.96)

Femtosecond laser—unfocused, without gel 2.7 (1.62) 3.2 (2.09)

Femtosecond laser—unfocused, Vaseline 1.4 (0.55) 1.9 (1.10)

Femtosecond laser—unfocused, 25 % HP 1.4 (0.55) 2.1 (1.50)

Femtosecond laser—unfocused, 38 % HP 1.4 (0.53) 2.0 (1.42)

Femtosecond laser—unfocused, 30 % CP 1.4 (0.54) 2.1 (1.45)



Femtosecond laser—focused, without gel 15.7 (3.05) 16.8 (4.03)

Femtosecond laser—focused, Vaseline 8.7 (2.04) 9.5 (3.07)

Femtosecond laser—focused, 25 % HP 8.7 (2.04) 9.4 (2.78)

Femtosecond laser—focused, 38 % HP 8.8 (2.13) 9.6 (2.95)

Femtosecond laser—focused, 30 % CP 8.7 (2.10) 9.5 (2.86)



Without light source, 25 % HP 0.6 (0.50) 0.7 (0.65)

Without light source, 38 % HP 0.6 (0.54) 0.9 (0.77)

Without light source, 30 % CP 0.5 (0.51) 0.9 (0.88)



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demonstrating the insulating effect of the bleaching gel). Fur-thermore, applications of bleaching agent during LED405 treat-ment of the tooth surface resulted in approximately 1.3-fold

decrease in the mean temperature compared to no gel applica-tion (p=0.005 for comparison of no gel application with 38 %HP, p=0.004 otherwise). Mean temperature increases per

Fig. 2 Results summary: Significant differences in the mean temperature increase among light sources (presented by different letters in a column)and bleaching agents (presented by different letters in a row)

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minute of LED405 treatment were approximately 0.2 °C for allgel types tested vs. 0.3 °C in the case of no gel application to thetooth surface. Applications of bleaching gel during focusedfemtosecond laser treatment of both the pulp and tooth surfaceresulted in approximately 1.8-fold decrease in the mean tem-perature compared to the treatment without gel application(p<0.001 for each comparison of no gel application withbleaching gels tested) with the mean pulpal and tooth surfacetemperature increases of 0.6 °C/min for all gel types tested and1.0 °C in the case of no gel application. Treatment with unfo-cused femtosecond laser, without application of bleaching gel,increased the temperature by 0.2 °C/min (in the pulp) or slightlymore (at the tooth surface), revealing 1.4- to 1.8-fold increase inthe mean temperature when compared to the gel application(p≤0.005 for each comparison of no gel application withbleaching gels tested).

Effect of light source

Analysis revealed significant effect of the type of the lightsource on the temperature increase both in the pulp and at thetooth surface (Fig. 2). The largest increase was obtainedduring treatment with ZOOM2 light source and resulted inapproximately 1.5- to 3-fold increase in the mean pulpal andtooth surface temperatures compared to focused femtosec-ond laser and more than 8-fold increase compared to otherlight sources (p<0.001 for each comparison betweenZOOM2 and other light sources), with differences amplifiedby the bleaching gel application. Focused femtosecond laserrevealed a significantly higher mean temperature increasethan any other light source observed during the 30-mintreatment period, for all gel types tested (p<0.001 for eachcomparison of focused femtosecond laser with other lightsources). Using this light source resulted in more than a 3-foldincrease in the mean pulpal and tooth surface temperatures (upto 8.6 and 7.4 when compared to OLED). A comparison ofLED405 with unfocused femtosecond laser did not reveal asignificant temperature increase except in the case of Vaseline(p=0.002 in the pulp and p<0.001 at the tooth surface) and38 % HP application (to the tooth surface only; p=0.003)when LED405 resulted in 1.2- to 1.3-fold increase in the meantemperature. Furthermore, treatments with OLED resulted inlower temperature increase in the pulp chamber in comparisonto LED405, regardless of applied bleaching gel (p<0.001 forall comparisons) as well as in comparison to unfocused fem-tosecond laser, but only in the case of no gel application(p<0.001). However, tooth surface measurements revealedsignificantly lower mean temperature increases during OLEDtreatment compared to both unfocused femtosecond laser andLED405 treatments (p<0.01 for all comparisons). In general,OLED provided the lowest mean temperature increase amongobserved light sources (approximately 0.1 °C/min), regardlessof applied bleaching gel.

Comparison of pulp chamber and tooth surfacemeasurements

Significant differences between pulp chamber and toothsurface measurements were obtained for all comparisons,i.e., for all types of bleaching gel tested, during treatmentswith ZOOM2 (p<0.001), LED405 (p<0.001), and unfocused(p<0.001) and focused femtosecond laser (p≤0.002). Treat-ment with OLED revealed significant differences only duringno gel (p=0.004), 25 % HP (p=0.004), and 30 % CP(p=0.005) application. The mean temperature increase wasgreater for the tooth surface measurements, demonstratingapproximately 1.1- to 1.6-fold increase compared to the pulpchamber measurements. If none of the light sources wereused, the pulp chamber and tooth surface measurements didnot reveal significant differences in the temperature increase,regardless of the applied bleaching agent.

Discussion

This in vitro study analyzed the surface and pulpal temperaturereadings on the upper anterior teeth during simulated vital toothbleaching using experimental LED405, OLED, focused andunfocused femtosecond laser, and ZOOM2 light source. Theeffects of various lights used to accelerate the in-surgery powerbleaching procedures on the surface and pulp chamber tem-perature rises have been investigated previously [8, 11–13, 15].Zach and Cohen found in a monkey model that a 5.5 °Ctemperature rise was likely to cause irreversible pulpal damage[9]. Baldissara et al. demonstrated that an intrapulpal temper-ature rise between 8.9 and 14.7 °C did not produce pulpalpathology in humans, and the cause of the postoperative sen-sitivity was the result of modification of the dentine [16]. In astudy by Torres et al., the low-power (100–500 mW) infrareddiode lasers did not cause a temperature increase to the criticallevel of 5.5 °C like halogen light [17]. Our study confirmedthese results as low-power sources like LED405 (400mW/cm2)and OLED (200mW/cm2) did not exceed the critical thresholdof 5.5 °C, contrary to the high-power sources like ZOOM2(2,000 mW/cm2) and femtosecond laser (800 mW/cm2).Treatments with ZOOM2 increased the temperature aboveall aforementioned thresholds, despite the bleaching gel orVaseline application. The focused femtosecond laser alsoexceeded these thresholds, but was within limits set byBaldissara et al. in the case of bleaching gels or Vaselineapplication. The null hypothesis that light activation duringthe bleaching treatment does not generate heat was rejectedas it led to the significantly higher temperature increasecompared to the bleaching without light activation. Further-more, the hypothesis that ZOOM2 and experimental lightsources (LED405, OLED, and femtosecond laser) have thesame effect on temperature increase was also rejected.

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Bleaching with ZOOM2 produced the highest temperatureincrease.

In a study by Luk et al. [18], the application of infraredlaser, CO2 laser, halogen lamp, and argon laser in combina-tion with 35 % HP gel and 10 % CP gel caused significanttemperature increases at the outer and inner tooth surfaces.The IR and CO2 laser lights caused the highest tooth tem-perature increases. According to Baik et al. [15], the PACand the QTH light used in a bleach mode induced greaterintrapulpal temperature rise than the argon ion laser. Michidaat al. [19] concluded that the mean temperature in the groupactivated with Nd:YAG laser (4.3 °C) was significantlyhigher than in the group activated with halogen light (1.8 °C)or LED light (0.9 °C). Vandewalle et al. [20] indicated insig-nificant differences in the emission of heat produced by LEDand QTH, whereas Asmussen and Peutzfeldt [21], Torres et al.[17], Coelho et al. [22], and Kabbach et al. [23] concluded thatQTH induced a higher pulp chamber temperature than LED.Furthermore, in the study by Eldeniz et al. [8], the diode laserinduced significantly higher temperature increases than anyother curing unit (11.7 °C), while the LED unit produced thesmallest temperature change (6.0 °C). Other studies [24, 25]also indicated that the LED unit releases the least heat duringthe polymerization process. These results are in accordancewith our findings for LED405 and OLED.

According to the existing literature, the use of the lightactivation with whitening gel generates higher intrapulpaltemperatures. This suggests the photothermal effect of lightactivation when used in conjunction with the bleaching gel[11, 26–29]. Application of the ZOOM light source and 25%HP produced a greater temperature rise than did the lightalone [28]. According to Baik et al., increase in the pulpchamber temperatures when argon ion laser in a bleach modewas used in conjunction with the bleaching gel may beattributed to the freshness of the bleaching agent and theincorporation of a heat-enhancing colorant to the bleachinggel [15]. However, when using some light sources, gelserved as an isolating layer. This was found in the study bySulieman [13] where the increase in the surface temperaturereadings for diode laser ranged from 37 °C (1 W) to 86 °C(3 W) with no bleaching gel present. Pulp chamber temper-ature increases ranged from 4 °C (1 W) to 16 °C (3 W). Thepresence of the bleaching gel reduced temperature increasesobserved at the tooth surface and within the pulp. In our study,significant differences between pulp chamber and tooth sur-face measurements were also obtained, so the null hypothesisthat there is no difference between the pulpal and surfacetemperature increases was rejected. Furthermore, applicationof all bleaching agents or Vaseline lowered the mean pulpaltemperature increase for ZOOM2 and unfocused and focusedfemtosecond laser. This was not found for LED405 andOLED. Applications of bleaching gel at the tooth surfacehad a significant effect on the mean temperature increase

while using LED405 and unfocused and focused femtosecondlaser, which was not demonstrated for ZOOM2 and OLED.Thus, the null hypothesis that the use of bleaching gel orVaseline with light activation does not promote a significantlylower temperature increase was completely rejected forfocused and unfocused femtosecond laser where the presenceof any type of bleaching gel or Vaseline served as an isolatinglayer during the bleaching treatment, demonstrated in bothsurface and pulpal temperature measurements.

Kabbach et al. found that there were no statistically signifi-cant differences in the pulp and surface temperature increasesbetween the groups using different gels of HP [23]. Suliemanet al. also noted that the presence of HP and water in the gelcould provide a further cooling effect upon evaporation of thesecomponents from the tooth surface during light-activatedbleaching [13]. Obviously, this matter needs further investiga-tion. Bleaching agents containing HP resulted in less transmis-sion of energy to the dental structures, consequently loweringthe values of temperature transmitted to the pulp tissue andleading to less postoperative sensitivity, which was not con-firmed in our study. Differences in measured temperature riseamong HP and CP bleaching agents used were insignificant.The null hypothesis of no difference in the temperature risebetween the five whitening gels used in this study and Vaseline(control) could not be rejected.

It is also important to notice that the tooth pulp is subjectedto the risk of thermal trauma from a variety of dental pro-cedures including: the use of ultrasonic devices [30], thethermal removal of orthodontic brackets [31], the curing ofresins with LED and QTH lights [32], and during different in-office whitening procedures like laser tooth whitening [11].Intact pulpal circulation is able to dissipate some of the appliedheat before pulpal cells are damaged [33]. Baik et al. haveinvestigated pulp chamber temperature changes during light-activated vital tooth bleaching using a model that simulatedblood flow through the tooth [15]. Our in vitro model was notable to replicate this, so it is possible that the very high pulpchamber temperature rises recorded for the ZOOM2 and fo-cused femtosecond laser in reality could be lower. Theintrapulpal and surface temperatures also depend on the lightapplication period. Usually, longer light irradiation produces ahigher temperature rise becausemore light energy is convertedinto heat [34], but ZOOM2 produced greater intrapulpal andsurface temperatures during the 15-min treatment period thanany other light source during the 30 min of treatment.

This study also had a limitation concerning simulation ofthe conditions in vivo like blood circulation or natural move-ment of pulpal fluid which are important for heat transfer anddeposition. Also, histological evaluations were not included.Further study is needed to investigate the potential pulpaldamage using LED405, OLED, and femtosecond laser aspossible light sources for teeth bleaching and to providesafety parameters for the use of these devices. It is necessary

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to know how the light source interacts with different types ofbleaching gel and to determine certain time limits for the morevulnerable groups of teeth like maxillary and mandibularycentral incisors. Their often thinner enamel and dentine com-pared to the other usually treated teeth during bleaching likecanines or premolars lead to amore exposed pulp chamber. Thegreater dentine thickness protects the pulp better against pene-tration of the bleaching agent and its negative side effects [11].

Conclusion

In the present study, the mechanisms of bleaching proceduresinduced by light activation had a significant effect on theintrapulpal and surface temperature values. The bleachingactivation with LED405, OLED, and unfocused femtosecondlaser promoted less temperature variation on the tooth surfaceas well as inside the pulp chamber than ZOOM2 and focusedfemtosecond laser. Caution is advised during the usage of thelatter two light sources, particularly if used over extended timeperiods. In conclusion, this study has shown that the bleachinggel or Vaseline in some cases served as a protective insulatinglayer against the surface and pulpal temperature increases.However, the temperature values measured in this study can-not be directly applied to temperature changes in vivo. Thereason is that the experimental setup of this study did notconsider heat conduction within the tooth during in situbleaching material activation due to the effect of blood circu-lation in the pulp chamber. Further studies with differentconcentrations of bleaching agent and presence of light acti-vation along with the simulation of blood circulation arenecessary to characterize the precise intrapulpal and surfacetemperature rise.

Acknowledgments This study was supported by the Ministry ofScience, Education and Sports, Republic of Croatia (project no. 065-0352851-0410), and Croatian Science Foundation (Evaluation of newbioactive materials and procedures in restorative dental medicine).

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Lasers Med Sci

http://dx.doi.org/10.1111/j.1708-8240.2008.00190.x

http://dx.doi.org/10.3109/00016357.2011.600704

http://dx.doi.org/10.1007/s00784-010-0439-1

http://dx.doi.org/10.1007/s00784-010-0439-1

http://dx.doi.org/10.1111/j.1365-2842.2007.01798.x

http://dx.doi.org/10.1089/pho.2007.2221

http://dx.doi.org/10.1089/pho.2007.2221

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http://dx.doi.org/10.1007/s10103-008-0546-2

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Temperature rise during experimental light-activated bleaching