ArF Excimer Laser Irradiation ofHuman Dentin

F. Sanchez, 1* A.J. Espan a Tost, 2 and J.L. Morenza 1

1Departament de Fısica Aplicada i Electronica, Universitat de Barcelona,Barcelona, Spain

2Facultat d’Odontologia, Universitat de Barcelona, L’Hospitalet de Llobregat, Spain

Background and Objective: The use of excimer lasers for treat-ment of dental hard tissues has considerable potential becausethe combined characteristics of low wavelength and short pulseresult in limited heat diffusion and, therefore, tissue ablationwithout the problems of collateral damage. To date, there arerelatively few published studies concerning the effects of ex-cimer laser irradiation on dental hard tissues. Thus the presentstudy was conducted to examine the morphological changes intooth dentin subsequent to ArF excimer laser irradiation.Study Design/Materials and Methods: The morphologic changesinduced in normal, nondiseased human dentin following irra-diation by an ArF excimer laser at fluences ranging from 1 to 4J/cm2 and the number of laser pulses ranging from 50 to 1,000were evaluated by scanning electron microscopy.Results: Two modes of ablation, photochemical at low fluencesand thermal at high fluences, were observed. A fluence of 1 J/cm2 when combined with 50 or 100 pulses produced a uniformablation of the dentin surface without signs of tissue melting. Atfluences >1.5 J/cm2, the thermal mode of ablation was more ef-ficient at removing intertubular dentin than peritubular den-tin. Further, when compared to the lower fluences, the highersettings produced a rougher ablation crater surface. Addition-ally, the higher fluences produced surface melting with eachpulse and sealing of exposed dentinal tubules after irradiationwith 100–300 laser pulses.Conclusions: The photochemical and thermal mechanisms oftooth dentin ablation were identified based on significant dif-ferences in tissue morphology following laser irradiation. Therates of tissue ablation and the observed morphologic changesindicate that the ArF excimer laser could be useful for cariesremoval and sealing of exposed dentinal tubules. Lasers Surg.Med. 21:474–479, 1997. © 1997 Wiley-Liss, Inc.

Key words: dentin; ArF excimer laser; ablation; scanning electron microscopy(SEM)

INTRODUCTION

Laser irradiation is a powerful tool for thesurface modification, drilling, and cutting of bothbiological and nonbiological materials. Laser ap-plications for the treatment of soft tissue prob-lems, such as incision and/or excision of lesions,

Contract grant sponsor: CICYT; Contract grant number:Mat94-0264; Contract grant sponsor: Generalitat de Cata-lunya.

*Correspondence to: F. Sanchez, Universitat de Barcelona,Departament de Fısica Aplicada i Electronica, Avda. Diago-nal 647, E-08028, Barcelona, Spain.

Accepted 2 April 1997

Lasers in Surgery and Medicine 21:474–479 (1997)

© 1997 Wiley-Liss, Inc.

are relatively common and often result in a lesscomplicated convalescence. However, successfulapplications of lasers to dental hard tissue prob-lems have been limited by the detrimental effectsof irradiation, such as heat-induced surface crack-ing of enamel, dentin, and cementum and thermalnecrosis of the pulp.

Numerous studies report the effects of differ-ent lasers on tooth enamel and dentin. Both con-tinuous and pulsed lasers, ranging from picosec-ond to millisecond pulses, and wavelengths rang-ing from the ultraviolet, such as the excimerlasers [1–7], to the medium infrared, such as theCO2 laser [8–9], have been used in these studies.Among lasers with pulse durations in the nano-second range, the ultraviolet ArF excimer is at-tractive because of its very short 193 nm wave-length. At this wavelength, the absorption coeffi-cient is very high for mineralized tissues. Giventhis combination of wavelength and short pulseduration, the energy produced by the ArF excimerlaser is efficiently absorbed by only the most su-perficial layers of tissue. Consequently, due to theresulting low levels of heat diffusion, progressiveablation of the dental hard tissue can be effectedwithout appreciable increases in pulpal tempera-ture.

There is considerable interest in dentistry fo-cused on the possibility of being able to substitutelaser irradiation for the traditional mechanicalmeans of caries removal. The ablation rate forcarious regions, both in enamel and dentin, isgreater than in nondiseased regions. Thus intheory, caries elimination could be accomplishedwithout significant collateral damage to nondis-eased tooth structure. Additionally, laser irradia-tion could effectively seal exposed dentinal tu-bules and thereby reduce the permeability of thedentinal layer. In this regard, several investiga-tors have reported the effects of dental hard tissueablation using the ArF excimer laser. For ex-ample, Neev et al. [2] examined changes in themorphology of both dentin and enamel and intooth temperature using thermography under dif-fering conditions of laser fluence and laser fre-quency. The authors noted the sealing of dentinaltubules and suggested that ablation rates werecomparable to those obtained with other nanosec-ond pulsed lasers. Stabholz et al. [3] irradiateddentin to study the sealing of dentinal tubules.They noted melting of the mineral phase aroundthe dentinal tubules, but sealing of the tubuleopenings was not observed. However, and possi-bly more important, large heat-induced cracks

were seen in the irradiated tissue. Arima andMatsumoto [4], using low fluence, irradiated non-diseased and carious regions of both dentin andenamel and found higher ablation rates in thecarious regions. Increases in tooth temperaturewere low and a high percentage of the exposeddentinal tubules was sealed in the nondiseaseddentin, but not in the carious dentin. Collectively,these studies do not offer a complete impression ofthe effects of the ArF excimer laser irradiation ontooth dentin and enamel. In this report, we pre-sent our first results concerning the irradiation ofnondiseased human tooth dentin by an ArF ex-cimer laser. The induced changes in dentinal mor-phology after irradiation at different fluences us-ing differing numbers of pulses are reported.

MATERIALS AND METHODS

Slices of tooth dentin, 3 mm thick, were ob-tained from freshly extracted, caries-free humanmolar teeth by transversal cuts above the pulpcavity. Prior to laser irradiation, the dentinalslices (specimens) were immersed in ethanol dur-ing 10–15 minutes to remove the debris. The in-dividual specimens were glued to a holder andplaced in a XYZ manipulator. They were irradi-ated with an ArF excimer laser beam with a pulseduration of 23 ns and a wavelength of 193 nm. Arectangular mask selected the homogeneous cen-tral part of the beam and was imaged with a lenssystem on the specimen target. Spots with dimen-sions ∼0.4 × 1.0 mm2 were obtained. Laser fluencewas determined from the pulse energy measuredwith a pyroelectric energy meter and the spotarea, and it ranged between 1 and 4 J/cm2. Therepetition rate was 5 Hz and the number of beampulses were 50, 100, 300, and 1,000. Scanningelectron microscopy (SEM) was used to determinethe changes in dentin morphology and the abla-tion crater depth.

RESULTS

Dentin irradiated at a fluence of 1 J/cm2 us-ing 50 or 100 pulses exhibited a consistent anduniform pattern of ablation. The same rate of ab-lation was observed for both intertubular andperitubular dentin in most of the target area (Fig.1). Despite the near threshold conditions, the ab-lation rate was relatively high, e.g., 0.36 mm perpulse in the crater shown in Figure 1. Under theseconditions, the crater walls were noticeable sharp

Laser Irradiation of Human Dentin 475

and well defined. There were no indications ofmelting of the mineral phase of the dentin, norwas there a noticeable transition between the ir-radiated and nonirradiated areas of tissue. How-ever, the peritubular dentin of some tubules ex-hibited less of an ablation effect than did the in-tertubular dentin that resulted in elevations or‘‘hills.’’ Examination of the elevated areas athigher magnification revealed a partial sealing ofthe exposed tubules due to melting and resolidi-fication of the mineral phase, although no signs ofmelting were observed in other target areas. Thepercentage of surface area in which a nonhomo-geneous ablation was observed increased with flu-ence and the number of pulses.

At fluences close to 1.5 J/cm2 and after 300pulses, a different crater morphology is observed.There are local areas that were uniformly ablated(similar to Fig. 1), but ablation was nonhomoge-neous in most of the target area (Fig. 2). The peri-tubular dentin around a high number of tubuleswas more resistant to irradiation than the sur-rounding intertubular dentin, and it resulted inthe formation of hills (Fig. 2a). It is noticeablethat the partial closure of the openings of thesetubules is due to melting and resolidification ofthe peritubular dentin. These hills eventually as-sume a distinct cone-like configuration (Fig. 2b).The cones exhibit either totally or partially closedtubules at the vertex, whereas the cone base ap-pears to cover one to several tubules. Two tubulescan be observed on the lateral surface of the larg-est cone at the right side of Figure 2b.

At higher fluences, intertubular dentin wasmore efficiently ablated than peritubular dentin

in the whole target area, as can be seen in Figure3, which shows a dentinal surface irradiated with50 pulses at 2.6 J/cm2. There appears to be a hillformation around individual tubules, in contrastto the larger more inclusive cones that form atfluences close to 1.5 J/cm2. Individual tubulesshow a decrease in diameter and their marginsclearly exhibit evidence of melting and resolidifi-cation (Fig. 3b). Examination of the crater wallsalso revealed evidence of melting and resolidifica-tion. By increasing the number of pulses from 50to 100–300 resulted in sealing of practically alldentinal tubules (Fig. 4). Although the sealing ef-ficiency appears to increase with the number ofpulses, large cracks appear in the crater wall after1,000 pulses (Fig. 5). The combination of high flu-ence and number of pulses produced well-definedablation craters of 300–500 mm in depth. Such

Fig. 1. SEM view (45°) of tooth dentin irradiated at 1.2 J/cm2

with 50 pulses.

Fig. 2. SEM views (45°) of dentin irradiated at 1.5 J/cm2 with300 pulses: (a) area showing the early stage of hill develop-ment; (b) area showing cones. Note the open tubules on thesurface of the largest cone.

476 Sanchez et al.

craters exhibited a distinct cubic shape withnearly vertical walls.

DISCUSSION

The present study suggests that both photo-chemical and thermal effects occur when humantooth dentin is irradiated with an ArF excimerlaser. Photons at 193 nm have a high energy (6.3eV) that is sufficient to ablate both intertubularand peritubular dentin by direct breaking ofchemical bonds. At low fluence, photochemical ef-fects are dominant and, as demonstrated in thisstudy, the dentinal tubules are not sealed becauseperitubular dentin does not melt. Also, due to the

generalized effect of the laser at low fluence onboth types of dentin, the floor of the ablation cra-ter is relatively flat. A roughened surface topog-raphy was noted only in a few regions around tu-bules in which peritubular dentin had been lessefficiently ablated. With continued irradiation,the rate of ablation in these regions was reducedbecause of the increase in surface area and, insecond order, of reflectivity, a combination thatresulted in a reduction in the effective fluence.This process, usually called dilution of fluence[10], is well known as one of the causes of coneformation by cumulative laser irradiation of tar-gets of heterogeneous and even homogeneous

Fig. 3. SEM microphotographs of dentin irradiated at 2.6J/cm2 with 50 pulses: (a) 45° view of the border between ir-radiated dentin (top of photograph) and nonirradiated dentin;(b) 0° view of the crater floor. Note the signs of melting in-volving the peritubular dentin that produces a partial sealingof the tubules opening.

Fig. 4. SEM view (0°) of dentin subsequent to irradiation of2.6 J/cm2 with 100 pulses. Note that only the tubule openingof the upper left part remains unsealed.

Fig. 5. SEM view (45°) of the wall and floor of the ablationcrater following irradiation at 3.1 J/cm2 with 1,000 laserpulses. Note the heat-induced large crack.

Laser Irradiation of Human Dentin 477

compositions [11]. Although the cone formationsshown in Figure 2 have their origin in the non-uniform ablation, their development is not essen-tially determined by the heterogeneous chemicalcomposition of dentin.

Irradiation of dentin using low fluence levels(photochemical ablation) does not induce meltingand therefore the temperature increase is lowerthan that achieved when using high fluence set-tings for thermal ablation. In addition, it is to benoted that the rate of ablation at low fluence isonly slightly less than the thermal ablation rate,and a greater surface area could be irradiated asthe low fluence settings require a less focused la-ser beam. Consequently, the volume of tissue ab-lated per pulse may be comparable or evengreater than that removed by thermal ablationmode using a high fluence.

The process of thermal ablation becomesmore obvious as the fluence increases and, in thecase of tooth dentin, is manifested by the forma-tion of distinctive hills associated with the den-tinal tubules. The bases of the hills do not extendon other tubules, and their development is likelydue to the less efficient ablation of the hypermin-eralized peritubular dentin compared to that ofthe less mineralized intertubular dentin.

At the higher fluence settings, the dentinaltubules are sealed as a result of melting and re-solidification of the peritubular dentin. However,thermal effects that produce the sealing also re-sult in at least one detrimental effect, i.e., theoccurrence of heat-induced cracking. In our case,cracking could be favored by a partial dehydrationproduced by the immersion in ethanol. Due to thecracking effect, it is not possible to seal all den-tinal tubules, although the high percentage ofthose that are sealed undoubtedly has an effect ondentin permeability. It may be of potential clinicalsignificance to note that sealing of dentinal tu-bules can be achieved after a relatively few num-ber of pulses using a wide range of fluences.

Finally, it should be noted that the combina-tion of low wavelength and short duration of thepulsed laser used in this study can generate tar-get surface temperatures as high as several thou-sand Kelvin. However, such temperatures last foronly microseconds after the pulsed exposure andare confined to only the most superficial regions ofthe target surface. Rapid melting of the targetsurface is then compatible with the low tempera-ture increases measured after the pulsed expo-sure. Thus the effects of a high energy plasma onthe floor and walls of the ablation crater may be

minimal, unlike what has been suggested by Neevet al. [2,6] and Stabholz et al. [3,7].

In conclusion, this investigation on thechanges in human tooth dentin following irradia-tion with an ArF excimer laser has provided in-formation about the distinctive differences inmorphology resulting from photochemical abla-tion at low fluences and thermal ablation at flu-ences >2 J/cm2. Photochemical ablation at the lowfluence settings resulted in craters of homoge-neous morphology, a consistent rate of tissue ab-lation, and no indications of melting of the min-eral phase of the dentin specimens. At higher flu-ence, the process of thermal ablation was lesseffective at removing the hypermineralized peri-tubular dentin than intertubular dentin and pro-duced a crater floor characterized by a rough sur-face topography. Further, melting and resolidifi-cation of the peritubular dentin was noted, whichresulted in sealing of the tubules. Based on theresults of this study, it would appear that the ArFexcimer laser could be useful for both the removalof dental caries and sealing of dentinal tubules.

ACKNOWLEDGMENTS

The authors acknowledge the collaborationof the Scientific and Technical Services of Univer-sitat de Barcelona. This work is part of a researchprogram financed by CICYT of the Spanish Gov-ernment (Project No. MAT94-0264) and Generali-tat de Catalunya.

REFERENCES

1. Frentzen M, Koort HJ, Tack C. Bearbeitung von zah-nhartgeweben mit einem UV-laser unter spektrosko-pischer kontrolle. Dtsch Zahnarztl Z 1990; 45:199–201.

2. Neev J, Liaw LHL, Raney DV, Fujishige JT, Ho PD,Berns MW. Selectivity, efficiency, and surface character-istics of hard dental tissues ablated with ArF pulsed ex-cimer lasers. Lasers Surg Med 1991; 11:499–510.

3. Stabholz A, Neev J, Liaw LHL, Stabholz A, Khayat A,Torabinejad M. Effect of ArF-193 nm excimer laser onhuman dentinal tubules. Oral Surg Oral Med Oral Pathol1993; 75;90–94.

4. Arima M, Matsumoto K. Effects of ArF:excimer laser ir-radiation on human enamel and dentin. Lasers Surg Med1993; 13:97–105.

5. Moos JP, Patel BCM, Pearson GJ, Arthur G, Lawes RA.Krypton fluoride excimer laser ablation of tooth tissues:Precision tissue machining. Biomaterials 1994; 15:1013–1018.

6. Neev J, Stabholtz A, Liaw LHL, Torabinejad M, FujishigeJT, Ho PD, Berns MW. Scanning electron microscopy and

478 Sanchez et al.

thermal characteristics of dentin ablated by a short pulseXeCl excimer laser. Lasers Surg Med 1993; 13:353–362.

7. Stabholz A, Neev J, Liaw LHL, Stabholz A, Khayat A,Torabinejad M. Sealing of human dentinal tubules by XeCl308-nm excimer laser. J Endodontics 1993; 19:267–271.

8. Miserendino L. The laser apicoectomy: Endodontic appli-cation of the CO2 laser in apical surgery. Oral Surg OralMed Oral Pathol 1988; 66:615–619.

9. Pashley EL, Horner JA, Liu M, Kim S, Pashley DH. Ef-

fects of CO2 laser energy on dentin permeability. J End-odontics 1992; 18:257–262.

10. Dyer PE, Jenkins SD, Sidhu J. Development and origin ofconical structures on XeCl laser ablated polyimide. ApplPhys Lett 1986; 49:453–455.

11. Foltyn SR. Surface modification of materials by cumula-tive laser irradiation. In: Chrisey DB, Hubler GK, eds.‘‘Pulsed Laser Deposition of Thin Films.’’ New York:Wiley & Sons, 1994, pp 89–113.

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