Evaluation of subsurface dentin microhardness after Er: yttrium-aluminum-garnet andNd: yttrium-aluminum-garnet laser irradiationLeily Macedo Firoozmand, Andressa Pereira da Silva, and Carlos Rocha Gomes Torres Citation: Journal of Laser Applications 19, 185 (2007); doi: 10.2351/1.2756857 View online: http://dx.doi.org/10.2351/1.2756857 View Table of Contents: http://scitation.aip.org/content/lia/journal/jla/19/3?ver=pdfcov Published by the Laser Institute of America Articles you may be interested in Laser alloying of Al with mixed Ni, Ti and SiC powders J. Laser Appl. 22, 121 (2010); 10.2351/1.3516482 Evaluation of the shear bond strength of a composite resin in sound, demineralized and hypermineralized bovinedentin after irradiation with Er:YAG and Nd:YAG lasers, employing a self-etching adhesive system J. Laser Appl. 22, 99 (2010); 10.2351/1.3493489 Effect of Nd:Yttrium-aluminum-garnet laser radiation on Ti6Al4V alloy properties for biomedical applications J. Laser Appl. 20, 209 (2008); 10.2351/1.2995762 Use of carbon dioxide ( CO 2 ) laser radiation in exposure of implants (stage II): A metallography evaluation indogs J. Laser Appl. 14, 248 (2002); 10.2351/1.1516414 Microstructure and microhardness study of laser bent Al-2024-T3 J. Laser Appl. 13, 32 (2001); 10.2351/1.1340339

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Evaluation of subsurface dentin microhardness after Er:yttrium-aluminum-garnet and Nd: yttrium-aluminum-garnet laser irradiation

Leily Macedo Firoozmand,a�,b� Andressa Pereira da Silva, and Carlos Rocha Gomes TorresDepartment of Restorative Dentistry, São José dos Campos School of Dentistry, São Paulo State,Brazil

�Received 18 September 2006; accepted for publication 22 November 2006;published 24 July 2007�

Background and objectives: To assess the microhardness of dentin subsurface after Er:yttrium-aluminum-garnet �YAG� and Nd:YAG laser irradiation. Study design/materials andmethods: Twenty-four bovine incisors, without pulp, were used. The vestibular surface was wornout until the dentin was reached and divided in mesial and distal regions. The samples were dividedinto two groups: GI-distal, irradiated by Er:YAG laser, and GII-distal, irradiated by Nd:YAG laser.The mesial area was protected so as to not receive the laser irradiation. The measurements weremade on Vickers digital microhardmeter. Results: For GI—there was no significant statisticaldifference, CI�−4.59 to 0.78�, between the values of irradiated �55.61±4.38� and unirradiated�57.51±4.00� areas. For GII—the values were higher for the irradiated �62.21±6.48� compared tothe unirradiated �57.82±5.42� area, CI�1.65 to 7.13�. Conclusions: There was an increase of dentinmicrohardness when the Nd:YAG was used, but the Er:YAG did not cause significant alterationsin dentin microhardness. © 2007 Laser Institute of America.

Key words: laser irradiation, surface treatment, dentin

I. INTRODUCTION

Through the years, increasing use of laser in the differentareas of dentistry, such as, operative dentistry, pediatric, pe-riodontic, and endodontic could be observed. From the lasersused and studied, the Er: yttrium-aluminum-garnet �YAG� isindicated for the removal of decayed tissue and cavitypreparations1,2 because of the emitted wavelength being ab-sorbed by water.3 The Nd:YAG is indicated for soft oraltissue, surgical, and periodontal procedures and can also beused for final removal of decayed tissue through vaporizationand sealing of the dentinal tubules without causing pulpaldamage.4

The laser when used on dental structure can cause alter-ations in the enamel5,6 as well as the dentin.6–9 The Er:YAGlaser treatment can cause chemical alterations on microareasof dentine surface, depending on the time duration of thelaser pulse used.10

Studies with the Er:YAG laser demonstrate that thewavelength is highly absorbed by the dental hard tissue com-ponents, as well as the water and the hydroxyapatite. Theelectromagnetic energy absorbed causes the instant heatingof water content that transforms into vapor, causing an in-crease of pressure that leads to the breaking and falling ofhard tissue, in a process know as ablation bymicroexplosion.11,12 Morphologically, the dentine surface ir-

radiated with Er:YAG is irregular, with open dentinal tu-bules and the absence of the smear layer.13,14

When used on normal or demineralized dentin theNd:YAG laser also causes alterations on the mineral contentof the dentine surface,15 principally on the organic compo-nents of the dentin specially the collagen.16 The dentine re-crystallization, with consequent obliteration of some tubules,was also reported by Dederich et al.17

Treating the dental structures with laser, it is also impor-tant to verify how the enamel, dentin, and pulpal alterationscan influence the quality and longevity of the restorations. Inthis way, countless authors have studied the influence of laseruse on microleakage,18 adhesion resistance,19 and dentinhypersensitivity.20

It is assumed that the modifications produced by differ-ent lasers on dentinal substrate, whether they be the ablationproduced by Er:YAG laser or the fusion caused by Nd:YAGlaser can result in alterations in the hardness of the surfaceand the subsurface of substrate, causing an uneven distribu-tion of tensions in this area, which could be one of the directcauses of variations observed in the adhesion study.

Therefore, the purpose of this work is to evaluate thealteration on the hardness of subsuperficial dentine whentreated with Er:YAG and Nd:YAG lasers. The hypothesistested was that there would be no difference in the meanhardness values of dentine when treated with Er:YAG andNd:YAG lasers.

II. MATERIAL AND METHODS

In this study 24 samples of freshly extracted intacterupted bovine incisors were used. They were stored in dis-

a�Author to whom correspondence should be addressed; present address: R.Emílio de Menezes, 304- Monte Castelo- cep.12215-020, S. J. Campos-SP,Brazil.

b�Electronic mail: leilyfiroozmand@hotmail.com

JOURNAL OF LASER APPLICATIONS VOLUME 19, NUMBER 3 AUGUST 2007

1042-346X/2007/19�3�/185/4/$23.00 © 2007 Laser Institute of America185

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tilled water and frozen at −18 °C until they were utilized.21

The roots were sectioned at the third cervical with carborun-dum disk dividing the teeth into coronal and root part. Thecoronal pulp was removed with dentinal curet �Duflex LucasNo. 86, SSWhite, Rio de Janeiro, RJ, Brazil� and endodonticH files �H 20, 25, 30, Maillefer� and the pulp chamber wasirrigated with distilled water and gently dried with air. Anopening was made on the lingual side of these teeth with around diamond bur until exposure of the pulp chamber.

The lingual opening and pulp chamber were filled withstick wax �Epoxiglass Ind Com, Diadema, SP, Brazil� toavoid the penetration of acrylic resin when the tooth wassetup. The teeth were positioned in a silicone matrix with thevestibular surface facing the top and they were embedded inclear fast-curing acrylic resin �Clássico Artigos Odontológi-cos Ind. Bras., Brazil�. The opening of the pulp chamber wasunobstructed to allow the measurement, with a caliper, of theremaining dentin thickness. This corresponded to mediumdentin depth, where the bovine dentin is similar to the humandentin.22

The tooth surfaces were worn with SiC grit 80 paper,mounted on a trimming machine �Kohl Bach, Motores Elétri-cos, Jaraguá do Sul, SC, Brazil�, using water for cooling, forthe removal of the enamel and the exposure of the mediumdentin depth. The exposed tooth surfaces were sequentiallysmoothed with sandpaper grit 100, 180, 240, 320, 400, and600 for 30 s using a polishing machine �Dp-10 PanambraIndustrial e Técnica AS, Brazil� at 600 rpm under watercooling. During this procedure, a caliper was used to con-tinuously check, the dentin thickness until it reached the me-dium depth of bovine dentin. This standard thickness corre-sponded to the medium dentin of each tooth, this portionbeing most similar to the human dentin.22

A test area was delimited on the distal surface of allsamples, using Teflon adhesive tape with rectangular cuts of3 mm width by 2 mm length. This area was selected to betreated with laser. The opposite area, corresponding to themesial half’s were protected with Teflon tape to be later ana-lyzed �Fig. 1�.

Er:YAG laser Key III �Kavo, Alemanha� was employedwith wavelength of 2.94 �m, and special tip for cavitypreparation No. 2060, using the “no contact” mode. TheNd:YAG laser used was Pulse Master 600 IQ �American

Dental Technology, USA� with wavelength of 1.064 �m andoptical fiber of 320 �m, using the no contact mode.

The samples were divided into two groups of 12 sampleseach. Group 1—the distal region of bovine dentin wastreated with Er:YAG laser with 40 mJ energy per pulse, fre-quency of 6 Hz, at 12 mm of distance, under constant watercooling, resulting in an energy density of 12,83 J /cm2.Group 2—the distal region of bovine dentin was treated withNd:YAG laser with 60 mJ energy per pulse, frequency of10 Hz, resulting in an energy density of 74,72 J /cm2. Themesial half’s remained protected by the Teflon tape and werenot irradiated, being used as the control group.

After the treatment with the lasers, the surfaces wereworn with sandpaper grit 1200 and 4000 �Carbureto de silí-cio, Norton, São Paulo, Brazil� for 30 s each in a polishingmachine �Dp-10 Panambra Industrial e Técnica AS, Brazil�at 600 rpm under water cooling. This procedure was done tobring back the surface smoothness and allow the hardnessmeasurement of the dentin subsurface. This was due to thefact that the measurement of surface was made impossible7,8

because of the formation of irregularities after treatment withlaser.

The samples were put in a Hardness Digital Vickers ma-chine �FM-700 Future-Tech Equilam, Diadema, SP, Brazil�with pyramidal Vickers diamond. Three Hardness indenta-tions were made with 50 g load for 15 s on the distal region�area treated with laser� and three on the mesial region �con-trol area�.

Each sample was divided into four quadrants with astrap scalpel and in each quadrant three indentations weremade. From these 12 indentations, the mean hardness of eachsample was calculated.

The hardness data were registered and for each regionand the averages were calculated. The data was submitted tothe T-student test, with 5% significance level.

III. RESULTS

The hardness values obtained are represented by the dis-persion figure �dot pot�, Fig. 2.

The absence of atypical values is observed in Fig. 2. Thevalues show a symmetric distribution around the mean valueand the dispersion is the same in all the experimental condi-tions.

FIG. 1. Schematic draw of bovine tooth spitted up and divided into mesialand distal portion.

FIG. 2. Dispersion diagram of the hardness values �HV� column �dot plot�obtained from 12 bovine teeth, about their mean value, for treated anduntreated laser areas.

186 J. Laser Appl., Vol. 19, No. 3, August 2007 Firoozmand, da Silva, and Torres

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In Table I the comparison of mean values carried out bythe T-Student test is observed.

Studying the Nd:YAG it was observed that the dentinarea treated presented a hardness mean value higher than theuntreated dentin; while, using the Er:YAG laser no signifi-cant statistical difference between the treated and untreatedgroups was observed.

IV. DISCUSSION

Asmussem and Peutzfeldt,23 Hossain et al.,7 and Hossainet al.8 verified the need to study the restorative materials andthe subsurface of the dental structures.

It is known that the laser energy is absorbed and spread,promoting an interaction with the adjacent tissue. Studiesdemonstrate that the high interaction of the Er:YAG laserwith the hard dental tissue and the emission of the ray in apulsed mode would allow irradiation at a sufficiently lowenergy level, to remove enamel and dentin without excessiveincrease of temperature.12 The Er:YAG lasers can penetrateinto deeper areas than the ones that have suffered ablation,interacting with cellular and neuronal elements. This can bethe mechanism of pain reduction on cavities ablationed withEr:YAG laser.24 The Nd:YAG laser is highly absorbed bypigmented dental tissue and can also affect the dental sub-surface.

Studying the dentin tissue, areas with different degreesof mineralization can be observed25 which could cause varia-tions in the dentin structure hardness. However, in this study,in order to minimize the effects due to dental structure, weevaluated equal depths of opposite dentin regions �mesial/distal� of sound bovine teeth, with and without laser irradia-tion.

The dentin hardness alteration is an aspect to be studiedas this can influence dentin adhesion to the restorative mate-rials, microleakage, and other factors. Kawabata et al.26 veri-fied that teeth irradiated with Nd:YAG laser caused fusionof the structure, sealing the pit, fissures, and other irregulari-ties. The obliteration of dentin tubules could explain whyfrom all the methods used by Yonaga et al.,20 the Nd:YAGwas the most effective for the treatment of hypersensitivityin cervical dentin, avoiding its recurrence.

Schaller et al.15 reported that treatment with Nd:YAGwould influence dentin permeability leading to an increase insuperficial acid resistance.

This study showed that dentin area irradiated byNd:YAG laser presented hardness values higher than theunirradiated area �Table I�. The results agreed with the find-ings of White and Adams,27 who verified that Nd:YAG laserused before and after acid-etching caused a hardness in-crease.

White and Goodis,16 analyzed the chemical compositionof normal and demineralized dentin after Nd:YAG laser ir-radiation and verified that the Nd:YAG laser caused alter-ations in phosphate and collagen concentrations and was ca-pable of removing the organic component of the dentin,especially the collagen. These studies agree with Sazaket al.,28 who reported that Nd:YAG irradiation causedchanges in mineral contents of enamel and dentin surface.Brucoli et al.,8 observed that Nd:YAG laser was able to alterradiographic image of the dentin, producing more radio-paque images of the irradiated dentin.

The recrystallization of dentin was observed by Deder-ich et al.,17 and a consequent obliteration of some tubules. Inthis way, the modification of the mineral composition, thereduction of permeability, the recrystallization and fusion ofthe dentin reported in literature, may be associated to theincrease of dental tissue hardness.

The Er:YAG laser causes ablations on the dentin tissuedue to microexplosions, breaking and removing the hard tis-sue; expressive alterations being expected on the remainingsubstrate.12 Nevertheless, in our study, the region irradiatedwith the Er:YAG laser did not present statistically signifi-cant difference in hardness compared to a non irradiated area�Table I�.

The results of this study are in agreement with the find-ings of Hossain et al.7 who reported that the Er:YAG laseron dentin induced minimal thermal changes in dental tissuecompositions; Ca/P ratio and Knoop hardness of the radiatedcavity floor were almost the same as the bur cavities.

Dunn et al.19 observed that the application of theEr:YAG laser induced formation of a surface with splinesand fissures on the subsurface below the normal depth ofresin penetration when analyzed in a scanning electron mi-croscope. Examining the surface of normal bovine dentinafter the Er:YAG laser irradiation Wigdor et al.,29 Visuriet al.,1 and Kawabata et al.26 describe a morphological aspectcompatible with higher permeability due to the presence ofopen dentin tubules without a smear layer and some charredpoints. Another observed characteristic was the superficialirregularity, presenting differences in level, which was inagreement with the Armengol et al.13 findings.

An effect observed when the Nd:YAG was used was anincrease of acid resistance of the substrate due the fusion ofits components.15 These observations can be related to theincrease of hardness demonstrated in this study. On the otherhand, Kameyama et al.30 concluded that the dentin treatedwith the Er:YAG laser presented small or negligible resis-tance to the acid demineralization solution. Camerlingoet al.10 reported a strong modification of the collagen ali-phatic chains.

TABLE I. Confidence interval �95%� and T-Student test of samples, for thecomparison of hardness mean values �HV�, of 12 bovine teeth.

187J. Laser Appl., Vol. 19, No. 3, August 2007 Firoozmand, da Silva, and Torres

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The results of this study showed a difference betweenEr:YAG and Nd:YAG laser devices on normal bovine den-tin hardness, that can be explained by the morphological al-teration of the mineral content caused by the interaction ofthese lasers on the dental tissue.

V. CONCLUSION

Within the limits of this study, the null hypothesis thatthere would be no difference in hardness of dentin irradiatedwith Nd:YAG was rejected, because the Nd:YAG laser pro-moted an increase of dentin hardness values. However, thenull hypothesis referring to the surface irradiated withEr:YAG was accepted, as this laser did not promote alter-ation in dentin hardness values.

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