ABSORPTION COEFFICIENTS OF DENTAL ENAMEL AT CO2 LASER WAVELENGTHS

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1 ABSORPTION COEFFICIENTS OF DENTAL ENAMEL AT CO2 LASER WAVELENGTHS G. Duplain, R. Boulay, P. Belanger, S. Smith, P. Simard To cite this version: G. Duplain, R. Boulay, P. Belanger, S. Smith, P. Simard. ABSORPTION COEFFICIENTS OF DENTAL ENAMEL AT CO2 LASER WAVELENGTHS. Journal de Physique Colloques, 1987, 48 (C7), pp.c7-279-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1987 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C7, supplément au n 12, Tome 48, décembre 1987 C7-279 ABSORPTION COEFFICIENTS OF DENTAL ENAMEL AT C0 2 LASER WAVELENGTHS G. DUPLAIN, R. BOULAY, P.A. BELANGER*, S. SMITH** and P. SIMARD** Institut National d'optique, C.P. 9970, Sainte-Foy, G1V 4C5, * Laboratoire de Recherches en Optique et Laser (LROL), Département de Physique, Université Laval, Québec, G1K 7P4, **Ecole de Médecine Dentaire, Université Laval, Québec G1K 7P4, ABSTRACT Optical properties of dental enamel at C0 2 laser wavelengths have been determined using reflectance method. A strong wavelength dependence of the absorption coefficients was observed in the spectral region which corresponds to a well known absorption band of hydroxyapatite, the main constituent of dental enamel. INTRODUCTION Applications of the CO2 laser in dentistry have proved promising. These applications include CO2 laser treatment of teeth (increase in the resistance of dental enamel to demineralization), fusion of a biocompatible dental material, and treatment that may increase the fluoride uptake of enamel. An increased resistance to demineralization has been measured for CO2 laser irradiated enamel '>». Fowler and Kuroda'' 8 present a study of the changes in enamel due to CO2 laser irradiation. Compositional, structural, and physical changes of dental enamel due to CO2 laser irradiation that enable better resistance to demineralization are now being identified. However, as Fowler and Kuroda 8 indicate, further studies are needed to determine the experimental parameters that would optimize the appropriate enamel changes. Also, identification of precise experimental conditions is essential in order to obtain reproducible results. Melcer et al., in their review of various experiments, indicate the problem of the diversity of experimental results. The infrared spectra of tooth enamel' have an absorption band in a spectral region which corresponds to CO2 laser emission. It is very likely that optical properties of dental enamel have a strong wavelength dependence in that region. A study of the optical properties of dental enamel have many implications. It will be shown that there is a CO2 laser wavelength at which the absorption coefficient reaches a maximum. This fact could be important for surface treatment of teeth where the radiation-matter interaction occurs at the first layers of the material. Any physical or chemical changes of the enamel due to CO2 laser irradiation, which produces a specific property (like increasing resistance to demineralization), will depend on the value of the absorption coefficient. DETERMINATION OF OPTICAL PROPERTIES OF DENTAL ENAMEL The absorption coefficients a of dental enamel at CO2 laser wavelengths were calculated with the optical constants. The strong absorption of dental enamel in the spectral region of interest promoted the use of the reflectance vs angle of incidence method to determine the optical constants. The equations of reflectance for a semiinfinite medium with radiation polarized parallel (R ) and perpendicular (Rj_) to the plane of incidence are given by Humphreys-Owens^. A graphical inversion method and Article published online by EDP Sciences and available at

3 C7-280 JOURNAL DE PHYSIQUE a numerical method, which calculates the best fit, were used to obtain n and k from the reflectance measurements at different angles of incidence. A detailed description of the experimental apparatus, the experimental procedure, the samples preparation, and the numerical analyses used to determine the optical constants of dental enamel are described in ~u~lainll. RESULTS Fig. 1 shows the near-normal-incidence reflectance of dental enamel at various C02 laser wavelengths. The reflectance varies between 10% and 50%, depending on the wavelength. These variations in reflectance can be assigned to vibrational modes, specifically the PO4 stretching modes12, of hydroxyapatite (Ca10(P04)6(OH)2), the main constituent of dental enamel. The absorption coefficients of dental enamel at C02 laser wavelengths are presented in Table I. All of the absorption coefficients were derived from reflectance measurements taken with parallel polarized C02 laser radiation. The wavelength and the wavenumber emission is also represented by its corresponding vibrationrotation line of the C02 molecule, with the lines grouped in four spectral bands: 9R, 9P, 10R, and 10P band. The data are presented graphically in Figure 2. TABLE I ABSORPTION COKFPICI~S OF DENTAL ENAHEL, WAVELEN(;TH (X). WAvl9KMBER (v) AND ABSORPTION COEFPICIENT (a) SPECTPA SPECTRAL LIME A(llm) v(cm-l) a (llm-l) LINE A (vm) v (cm'') (llm-') DISCUSSION Fig. 2 shows the wavelength dependence of the absorption coefficient a of dental enamel. The dashed curve represents the theoretical values of absorption coefficients that could be calculated from theoretical model13. The absorption coef f i- cients have not been measured experimentally between the 9P and 10R band, or more precisely between 9.75 and 10 pm ( cm-l), where theoretical model predicts a maximum value, since the gain of the C02 laser we used is generally too weak in this spectral region to permit a laser emission. However, it is possible to obtain C02 laser emission in this spectral region with the use of a proper isotopic species of C02 gas. It will then be possible to determine the optimum C02 laser wavelength that gives a maximum value of the absorption coefficient of dental enamel. For instance, an increase in absorption in the 9P band of the C02 laser can be measured, since a is much higher in the 9P band than in the 9R band. At 9.68 pm (1033 cm-l) the attenuation is about sixteen times (e3.3/e.52) greater than that at 10.6 pm (943 cm-l), and a factor of about 30 times can be expected with the use of the proper isotopic species of C02 gas. This comparison is important since 10.6 pm is the standard wavelength of the C02 laser because of its highest gain at this wavelength. The literature on the experimental use of the C02 laser in dentistry shows this laser was generally used at 10.6 vm. It is very likely that any specific treatment of dental enamel with the C02 laser, as those mentioned previously, will be dependent on 6

4 the C02 laser wavelength. In fact, in their study, Nelson et al. have shown a wavelength dependence of the C02 laser inhibition of dental caries at four different wavelengths. It is now possible to define the optimal conditions for the use of C02 laser in these treatments. WAVELENGTH ( pm) WAVENUMBER (cm-i) % % WAVENUMBER (cm-i) Fig.1 Dental enamel reflectance vs C02 Fig. 2 Absorption coefficients of dental laser wavelengths (wavenumbers) for enamel at C02 laser wavelengths. an angle of incidence of 15 degrees Dashed line represents theoretiand radiation polarized parallel to cal values that could be calcuthe plane of incidence. Longitu- lated from theoretical model. dinal axis of the tooth is parallel to the plane of incidence. Arrows indicate vibrational modes of hydroxyapatite. CONCLUSION The use of the C02 laser for treatments in dentistry have proved promising. Optical properties, more precisely the absorption coefficients of dental enamel at C02 laser wavelengths, were determined in order to define the optimum parameters of the C02 laser for specific treatments. A strong wavelength dependence of the absorption coefficient was obtained in the spectral region studied which corresponds to a well-known absorption band of hydroxyapatite. The attenuation is much higher in the 9P band emission of the C02 laser than at 10.6 m, the standard wavelength used for the C02 laser. The absorption could be optimized with the use of the proper C02 laser wavelength emission. The results of this study may contribute to better use of the C02 laser in dentistry. Fowler and Kuroda's recent paper8 on the properties of C02 lasers in reducing caries and their use at particular wavelength and the paper of Nelson et a1.6 on the C02 laser wavelength dependence in C02 laser inhibition of dental caries make our results more pertinent. ACKNOWLEDGEMENT This project was funded by federal (NSERC) and provincial (FCAR) research agencies. 1. R.H. STERN, R.F. SOGNNAES, "Laser Inhibition of Dental Caries Suggested by First Test in Vivo", Journal of the American Dental Association 85, 1087 (1972). 2. L. STEWART, G.L. POWELL, S. WRIGHT, "Hydroxyapatite Attached by Laser: A Potential Sealent for Pits and Fissures", Operative Dentistry, 10, 2 (1985). 3. R. BOEHM, T. BAECHLER, J. WEBSTER, S. JANKE, "Laser Processes in Preventive Dentistry", Optical Engineering 16, 493 (1977).

5 C7-282 JOURNAL DE PHYSIQUE 4. R.H. STERN, H. EASTGATE, W.J. MAUTNER, C. MORGAN, "The Laser in Dentistry: Potential Clinical Applications", Optics and Laser Technology, 22, February J. MELCER, M.T. CHAUMETTE, F. MELCER, J. DEJARDIN, R. HASSON, R. MERARD, Y. PINAUDEAU, R. WEILL. "Treatment of Dental Decay by C02 Laser Beam: Preliminary Results", Lasers in Surgery and Medicine 4, 311 (1984). 6. D.G.A. NELSON, M. SHARIATI, R. GLENA, C.P. SHIELDS, J.D.B. FEATHERSTONE, "Effect of Pulsed Low Energy I'nfrared Laser Irradiation on Artificial Caries-Like Lesion Formation", Caries Research 20, 289 (1986). 7. S. KURODA, B.O. FOWLER, "Compositional, Structural and Phase Changes in In Vitro Laser-Irradiated Human Tooth Enamel", Calcified Tissue International 36, 361 (1984). 8. B.O. FOWLER, S. KURODA, "Changes in Heated and in Laser-Irradiated Human Tooth Enamel and Their Probable Effects on Solubility", Calcified Tissue International 38, 197 (1986). 9. J. MELCER, F. MELCER, R. MERARD, R. HASSON, C. FRECHE, J. GAUTIER, "Utilisation du laser en odontologie", Innovation et technologie en mgdecine 2, 67 (1981). 10. S.P.F. HUMPHREYS-OWENS, "Comparison of Reflection Methods for Measuring Optical Constants without Polarimetric Analysis, and Proposal for New Methods Based on the Brewster Angle", Proceedings of the Physical Society 77, 949 (1961). 11. G. DUPLAIN, R. BOULAY, P.A. BELANGER, "Complex Index of Refraction of Dental Enamel at C02 Laser Wavelengths", accepted for publication in Applied Optics. 12. B.O. FOWLER, "Infrared Studies of Apatites. I. Vibrational Assignments for Calcium, Strontium and Barium Hydroxyapatites Utilizing Isotopic Substitution", Inorganic Chemistry 13, 194 (1974). 13. F. WOOTEN, Optical Properties of Solids. Academic Press, New York, 1972.