Microhardness of Nd:YAG Laser Irradiated Enamel Surfaces


Massuji KURAMOTO Jr.
Edmir MATSON
Míriam L. TURBINO
Rogério A. MARQUES

Department of Dentistry, Faculty of Dentistry, University of São Paulo, São Paulo, SP, Brazil


Correspondence: Massuji Kuramoto Junior, Av. Lineu Prestes 2270, 05508-900 São Paulo, SP, Brasil. Fax: +55-11-818-7841. e-mail: kuramoto@fo.usp.br


Braz Dent J (2001) 12(1): 31-33 ISSN 0103-6440

INTRODUCTION | MATERIAL AND METHODS | RESULTS | DISCUSSION | RESUMO | REFERENCES


Several studies have been conducted in order to verify effects of laser irradiation on dental structures. However, different settings and methods employed have led to inconclusive results. The aim of this study was to evaluate the effects of several settings of Nd:YAG laser irradiation on enamel microhardness. Fifteen groups of 10 human third molars each were submitted to Vickers microhardness indentations prior to and after laser irradiation. The data were submitted to the t-test for dependent samples (P<0.05) for statistical evaluation. Lower total delivered energy (under 30 J) resulted in no statistically significant difference of microhardness between irradiated and non-irradiated samples, whereas higher energy settings decreased enamel surface microhardness significantly. This can be due to the significant temperature rise during irradiation. Thus, we conclude that laser irradiation, under the conditions of this study, is not suitable for improving enamel properties, because of decreased microhardness of irradiated samples.

Key Words: Nd:YAG, laser, enamel, microhardness.


INTRODUCTION

Although declining, dental caries are still the most prevalent disease during childhood and adolescence. Among artificial methods for the protection of dental structures, lasers have been tested in order to improve dental enamel properties under acidic conditions. Authors report an increase of acid resistance of neodymium:yttrium, aluminum, garnet (Nd:YAG) laser irradiated enamel (1). Tagomori and Iwase (2) reported that Nd:YAG laser irradiation leads to ultrastructural alterations of enamel, as a result of melting and resolidifying of lased enamel, which would be more resistant to an acid environment.

Enamel microhardness seems to be related to enamel mineral content (3), and plays a role in enamel demineralization inhibition (4), as well as in erosion inhibition (5).

Dental literature is not conclusive about Nd:YAG laser irradiation effects on dental enamel physical properties, because of the various laser irradiation settings
used in different studies, leading to conflicting results. Some authors claim that laser irradiation increases microhardness (6), whereas others report similar (7) or lower (2,8) microhardness of irradiated samples. The aim of this in vitro study is to analyze the effects of Nd:YAG laser irradiation under several combinations of repetition rates and energies per pulse on permanent mature enamel surface microhardness, and determine, if any, the most favorable settings concerning enamel microhardness.


MATERIAL AND METHODS

One hundred and fifty erupted human third molars, provided by the Restorative Dentistry Department of the University of São Paulo, were used in this study. Teeth were caries- and cracks-free and were kept in 0.4% sodium azide from the time of extraction to the procedures of this study, a period not greater that 1 week. The teeth were randomly divided into 15 groups of 10 teeth each according to irradiation settings (10 Hz, 30 mJ; 10 Hz, 100 mJ; 10 Hz, 200 mJ; 10 Hz, 320 mJ; 30 Hz, 30 mJ; 30 Hz, 100 mJ; 30 Hz, 200 mJ; 30 Hz, 320 mJ; 50 Hz, 30 mJ; 50 Hz, 100 mJ; 50 Hz, 200 mJ; 70 Hz, 30 mJ; 70 Hz, 100 mJ; 100 Hz, 30 mJ; 100 Hz, 100 mJ). The teeth were embedded in acrylic resin (Sampl-Kwick, Buehler, Lake Bluff, IL, USA) and ground flat on buccal surfaces using 240-, 600- and 4000-grit silicon carbide paper (Carbimet, Buehler, Lake Bluff, IL, USA) in order to obtain flat enamel surfaces. Each enamel surface had a 2-mm diameter circular area isolated on which 4 indentations were made for a Vickers microhardness test (HMV 2000, Shimadzu, Kyoto, Japan) with a 100-g load. These surfaces were then submitted to Nd:YAG laser irradiation under repetition rates and energy per pulse settings as cited above for 10 s. Laser irradiation was performed with a commercial laser system (Pulsemaster 1000, American Dental Technologies, Corpus Christi, TX, USA) with a 400-µm diameter optic fiber, kept in contact with tooth structure during irradiation, which was performed in movements to cover all experimental areas. After laser irradiation, the surfaces were submitted to 4 new indentations, on the same areas, and compared statistically to initial ones with the t-test for dependent samples (P<0.05).

This study was approved by the University Ethics Committee (protocol 31/ 99).


RESULTS

Mean microhardness values, submitted to the t-test for dependent samples (P<0.05), are reported in Table 1. Settings that gave a total delivered energy ranging from 3 to 21 Joules caused no statistically significant difference in enamel microhardness. However, when total delivered energy was greater than 30 Joules, independent of repetition rates and energy per pulse settings, irradiated samples showed a statistically significant decrease in microhardness compared to initial values.


DISCUSSION

Studies concerning the effects of laser irradiation on enamel microhardness are conflicting. Some authors report that laser irradiation increases microhardness (4,6) whereas others report similar (7) or lower (2,8) microhardness values of irradiated samples. These conflicting results are due to the different laser settings and methods used in these studies, leading to inconclusive results. This study tried to determine ideal settings for Nd:YAG laser irradiation on human dental enamel in terms of microhardness, analyzing a wide variation of total delivered energy (ranging from 3 to 100 J). In this study, irradiated samples showed results either similar to or lower than controls. Lower delivered energy (ranging from 3 to 21 Joules) led to non-significant statistical differences of enamel surface microhardness. Higher delivered energy settings (from 30 to 100 Joules) led to a statistically significant decrease of enamel microhardness. In this study, laser irradiation showed no beneficial effects on enamel microhardness.

There is consent in dental literature that Nd:YAG laser irradiation causes a rise in temperature. Irradiated samples evaluated by scanning electron microscopy show melted and resolidified surfaces (9,10). Cernavin (11) reported evidence of heat-induced cracking and crazing in irradiated enamel surfaces. Once the decomposition of dental enamel by heat was reported (12), there was concern about surface changes which may occur at the site of laser irradiation where extremely high temperatures have been found.

Tagomori and Iwase (2) associated the decrease of enamel microhardness to the presence of deep cracks and to enamel surface fragility of laser irradiated enamel. These cracks were also reported by Tagomori and Morioka (1), Corpas-Pastor et al. (13) and Ariyaratnam et al. (14). Our findings are in agreement with these studies because enamel brittleness can be related to decreasing microhardness.

Results obtained in this study suggest that laser irradiation is not effective in improving enamel microhardness, and an increased total delivered energy may actually reduce microhardness. The reported cracks and voids on irradiated enamel (13,14), that could allow penetration of bacterial products into deeper layers of enamel, must be considered when using laser assisted caries prevention. Lased enamel surfaces show either the same or worse properties than non-irradiated ones, which seems to be related to heat induced alterations on enamel.


RESUMO

Kuramoto Jr. M, Matson E, Turbino ML, Marques RA. Microdureza de superfícies de esmalte irradiadas por laser. Braz Dent J 2001;12(1):31-33.

Diversos estudos têm sido efetuados para se verificar os efeitos da irradiação laser em estruturas dentais. Entretanto, uma grande variabilidade de parâmetros e métodos conduzem a resultados pouco conclusivos. O objetivo deste estudo foi avaliar os efeitos de diversos parâmetros de irradiação por um sistema Neodimio:Itrio, Aluminio, Granada sobre a microdureza de superfícies de esmalte dental. Quinze grupos de 10 terceiros-molares humanos foram submetidos ao teste de microdureza Vickers antes e após a irradiação. Os dados obtidos foram submetidos à analise estatística por meio do teste t para amostras dependentes (p<0,05). Nos grupos submetidos a menores energias totais (abaixo de 30 Joules), não foi verificada diferença estatisticamente significante entre as amostras irradiadas ou não. Nos grupos em que foi aplicada uma energia total mais alta, foi verificada uma diminuição na microdureza superficial do esmalte, cuja diferença para os grupos não-irradiados foi estatisticamente
significante. Conclui-se que a irradiação laser, nas condições deste estudo, não é aplicavel para a melhora nas propriedades do esmalte dental, em função da redução de microdureza nas amostras irradiadas.

Unitermos: Nd:YAG, laser, esmalte, microdureza.


REFERENCES

1. Tagomori S, Morioka T. Combined effects of laser and fluoride on acid resistance of human dental enamel. Caries Res 1989;23:225-231.

2. Tagomori S, Iwase T. Ultrastructural change of enamel exposed to a normal pulsed Nd:YAG laser. Caries Res 1995;29:513-520.

3. Featherstone JD, Ten Cate JM, Shariati M, Arends J. Comparison of artificial caries-like lesions by quantitative microradiography and microhardness profiles. Caries Res 1983;17:385-391.

4. Florin R, Herrmann C, Bernhardt W. Mikrohärtemessungen an laserbearbeiteten Zahnoberflächen. Stomatol DDR 1990;40:49-51.

5. Attin T, Koidl U, Buchalla W, Schaller HG, Kielbassa AM, Hellwig E. Correlation of microhardness and wear in differently eroded bovine dental enamel. Archs Oral Biol 1997;42:243-250.

6. Marquez F, Quintana E, Roca I, Salgado J. Physical-mechanical effects of Nd:YAG laser on the surface of sound dental enamel. Biomaterials 1993;14:313-316.

7. Shimizu Y. Study of histostructure of fused human enamel by laser irradiation. Nippon Hotetsu Shika Gakkai Zasshi 1989;33:1212-1225.

8. Jennett E, Motamedi M, Rastegar S, Frederickson C, Arcoria C, Powers JM. Dye-enhanced ablation of enamel by pulsed lasers. J Dent Res 1994;73:1841-1847.

9. Moritz A, Gutknecht N, Schoop U, Goharkhay K, Wernisch J, Sperr W. Alternatives in enamel conditioning: a comparison of conventional and innovative methods. J Clin Laser Med Surg 1996;14:133-136.

10. Hess JA. Subsurface morphologic changes of Nd:YAG laser-etched enamel. Lasers Surg Med 1997;21:193-197.

11. Cernavin I. A comparison of the effects of Nd:YAG and Ho:YAG laser irradiation on dentine and enamel. Aust Dent J 1995;40:79-84.

12. Holcomb DW, Young RA. Thermal decomposition of human tooth enamel. Calcif Tissue Int 1980;31:189-201.

13. Corpas-Pastor L, Moreno JV, Garrido JDLG, Muriel VP, Moore K, Elias A. Comparing the tensile strength of brackets adhered to laser-etched enamel vs. acid-etched enamel. J Am Dent Assoc 1997;128:732-737.

14. Ariyaratnam MT, Wilson MA, Mackie IC, Blinkhorn AS. A comparison of surface roughness and composite/enamel bond strength of human enamel following the application of the Nd:YAG laser and etching with phosphoric acid. Dent Mater 1997;13:51-55.


Accepted September 20, 2000
Braz Dent J 12(1) 2001


BACK TO CONTENTS