ISSN 1054-660X, Laser Physics, 2009, Vol. 19, No. 5, pp. 1056–1060.
LASER METHODS IN CHEMISTRY, BIOLOGY, AND MEDICINE
© Pleiades Publishing, Ltd., 2009. Original Text © Astro, Ltd., 2009.
Laser Ablation of Dental Materials Using a Microsecond Nd:YAG Laser M. L. Siniaevaa, M. N. Siniavskyb, V. P. Pashininb, Ad. A. Mamedova, V. I. Konovb, and V. V. Kononenkob a Sechenov
b Prokhorov
Medical Adademy, Moscow, Russia General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia e-mail:
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected] Received June 30, 2008; in final form, December 3, 2008
Abstract—The action of microsecond laser pulses with a wavelength of 1064 nm on dental tissues (enamel and dentin) and various dental materials used for tooth replacement and filling (ceramics, metal alloys, and composites) is studied. It is demonstrated that the ablation thresholds of all of the dental materials are significantly lower than the threshold laser fluences for the dental tissue (Ethr = 200–300 J/cm2). At the laser fluences that do not allow ablation and damage of the dental tissues, the dental materials are effectively removed at a rate of no greater than 40 µm per pulse. It is shown that the laser ablation of the materials under study involves two processes (evaporation and volume explosion) depending on the optical density. The results obtained indicate that the laser radiation with a wavelength of 1064 nm and the microsecond pulse duration is promising for dental applications, since it allows effective cleaning of the tooth surface from various dental materials in the absence of the damages of dental tissues. PACS numbers: 42.62.Be, 52.38.Mf DOI: 10.1134/S1054660X09050314
INTRODUCTION Multidisciplinary study is known to be effective in biomedicine. This work is devoted to the pulsed laser ablation that is employed in various therapeutic procedures and branches of medicine (e.g., dentistry [1–3]). The laser ablation is interpreted as the removal of substance from the target surface upon the laser heating. The heating efficiency and level are predominantly determined by two processes: absorption of electromagnetic radiation and heat redistribution due to heat conduction. These effects control the temperature of the laser heating of the surface and the heating depth of the target at the irradiation spot and, hence, the rate of material removal and the ablation threshold (i.e., the minimum energy of the laser pulse that corresponds to the etching of the surface). Note that the character of the laser ablation depends on both the physical properties of the target material and the parameters of the laser radiation. Normally, the object under study is predetermined in applied research and the main problem lies in the selection of the laser parameters for the effective ablation (wavelength λ, intensity I, pulse duration τ, and pulse repetition rate f ) [1, 4]. Note the importance of the delivery of the highpower (effective) laser radiation to the irradiation spot via optical fibers in the absence of the fiber damage. Erbium (λ ≈ 2.7–2.9 µm) and CO2 (λ = 10.6 µm) lasers have been used in dentistry, since the radiation of these sources is highly absorbed by water, which deter-
mines the optical properties of the enamel, dentin, and bone tissues (see, for example, [3–12]). However, the problem of the delivery of the corresponding highpower radiation remains unsolved, which impedes the application of laser technologies. Neodymium lasers with a wavelength of λ = 1.06 µm are free of such a disadvantage and are effectively coupled with silica optical fibers. Nevertheless, the neodymium lasers may seem to be inappropriate for the dental applications, since the radiation is weakly absorbed by dental tissues (the absorption length is several millimeters) and the precise laser processing is impossible. The purpose of this work is the analysis of the application of pulsed neodymium lasers for an important problem of the selective and effective removal of inorganic dental materials (cement, plastics, metals) whose optical properties substantially differ from those of dental tissues rather than a conventional task of the removal of dental tissues. Note that several aspects of this problem have already been studied. Alexander et al. [13] cleaned the surface of tooth enamel from the filling material after the removal of the bracket system. However, the third harmonic radiation of the neodymium laser (λ = 0.35 µm) was used. Carvalho and Valera [2] demonstrated the removal of infected tissues from the canal under the action of the Nd:YAG laser. Note also the importance of the laser-pulse duration for the laser ablation. Both long pulses (LPs) with τ =
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First objective
Nd: YAG microsecond laser
Delivery optical fiber
Sample
Avalanche photo diode Second objective
3-axis automatic coordinate table
Pulse energy meter
Fig. 1. Block diagram of the experimental laser setup.
100–1000 µs and short pulses (SPs) with τ ≤ 10–100 ns were employed. The LP procedures exhibit a relatively high efficiency, since the heat is transferred over a significant distance (l ~ (χτ)1/2, where χ is the thermal diffusivity) of several hundred microns during the laser-pulse action. The laser technology is excessively effective and, hence, is imprecise. The material adjacent to the irradiation area is substantially heated, which leads to the painful sensation upon the propagation to the dental tissues [4]. The SP regime is characterized by relatively high selectivity and precision. However, the ablation rate is no greater than 1 µm/pulse and relatively high intensities and laser-pulse energies are needed. The disadvantages of this regime are the generation of strong shock waves that lead to painful sensation and a relatively low radiation stability of optical fibers. In this work, we present a detailed comparative analysis of the ablation of dental tissues and various dental materials (filling composites and ceramics and metal alloys used for the tooth replacement) using the repetitively-pulsed radiation of the Nd:YAG laser and LASER PHYSICS
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search for the regimes of the self-controlled reliable selective removal of such materials in the absence of the dental-tissue damage. For this purpose, we use microsecond laser pulses that enable one to solve three problems as was demonstrated with alternative materials in [14, 15]: we reach a relatively high efficiency of the ablation process at the sufficient precision and provide the radiation stability of optical fibers. EXPERIMENTAL To generate laser pulses with relatively high energies and a duration of 1–2 µs, we have created a Nd:YAG laser (λ = 1064 nm) with a fiber-optic delay line. The laser exhibits either single-pulse or multipulse lasing with a pulse energy of up to 200 mJ and a repetition rate of up to 100 Hz. An objective focuses the radiation to a fiber with a diameter of 200 µm and a length of several meters. The output objective focuses the radiation on the surface of the sample. The laserspot diameter on the sample is 50–100 µm and the laser-fluence distribution over the surface at the irradiation spot is close to rectangular. A built-in thin dielec-
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Ablation rate, µm/pulse 80 Herculite Prizmafil Metal 60 Ceramics
RESULTS AND DISCUSSION
Enamel ablation threshold
40 Dentine ablation threshold
20
0
100
200 300 Laser fluence, J/cm2
Fig. 2. Plots of the ablation rates of the Herculite and Prizmafil filling composites.
tric coated glass plate in the objective reflects a small part of the radiation to the measuring unit that allows the real-time monitoring of the pulse shape and energy. The sample is fixed on a computer-controlled translation stage. Figure 1 shows the block diagram of the setup. In the experiments, 26 extracted teeth of different persons serve as targets. We also employ self-curing composites (microfills with a filler-particle size of less than 1 µm (Herculite XRV, Kerr, Germany) and microfills with a filler-particle size of about 30 µm (Prizmafil, Stomadent, Russia)), ceramics, and metal alloy [16]. We perform the multipulse irradiation of the materials at the laser fluences ranging from 10 to 200 J/cm2 and, then, measure the resulting crater depth using a Zygo New View interference microscope and a Carl Zeiss optical microscope. The surface of a sample is mechanically polished prior to the laser irradiation, which leads to an increase in the measurement accuracy. The samples of the filling materials are deposited as thin (500 µm) layers on glass substrates. Then, the surfaces are processed using a diamondcoated plane polisher with a grain size of less than 1 µm. At the final stage, the samples are cleaned in an ultrasonic bath and the abrasive particles are removed from the surface. Ceramic and metal-alloy plates with a size of 5 × 5 mm and a thickness of about 0.5 mm also serve as the samples.
To determine the thresholds and rates V (in µm/pulse) of the laser ablation, we perform the multipulse ( f ~ 1 Hz and the number of pulses is N ≤ 100) irradiation of the samples at the laser fluence at the irradiation spot E = Iτ = 100–350 J/cm2. For each value of E, we determine the rate of the material removal from the slope of the curve of the crater depth versus the number of pulses. The surfaces of the enamel and dentin samples are not mechanically polished prior to the irradiation. Thus, we avoid the effect of polishing (e.g., the introduction of the abrasive particles to the surface layer) on the optical properties of the samples. Figure 2 shows the results on ablation threshold Ethr and V(E). First, we find that the dental tissue is neither removed nor evaporated when E < Ethr = 200 J/cm2 (for dentin) and E < Ethr = 300 J/cm2 (for enamel) and even the multipulse irradiation does not cause the crater formation. Moreover, the dental tissues are insignificantly heated and fractures are absent if E Ethr. Second, the ablation thresholds of all of the dental materials are significantly lower (Ethr ≤ 30 J/cm2) than the threshold fluences for the dental tissue. Third, the effective removal of the dental materials at the rate V ≤ 40 µm/pulse is observed at E ≤ 100 J/cm2 when the tooth ablation and damage are impossible. Fourth, two regimes of the material removal are found. For the alloy and ceramics, the crater diameter approximately equals the diameter of the irradiation spot and the circular crater exhibits a smooth bottom (Figs. 3 and 4). In the vicinity of the crater boundary, we also observe the redeposition of the evaporated material of the target and the melt drops that are ejected from the ablation area under the action of the reactive force of vapors. Such a scenario is typical of the ablation of most materials. An interesting feature corresponds to the irradiation of the composite materials. In this case, we observe extremely high (up to 60 µm/pulse) linear rates of the material removal, which are significantly greater than the rates for the ablation of the ceramics and alloy. The microscopic analysis of the irradiation area (Fig. 4) shows that the crater sizes can be several times greater than the diameter of the irradiation spot and that the craters can be irregular. In our opinion, this is due to the fact that the pulsed ablation of the polymers at a radiation wavelength of 1.06 µm does not obey the conventional scenario that involves the evaporation of materials from the radiation-heated surface. The experimental results indicate that the volume explosion provides the material removal. In this case, relatively high transparency and inhomogeneity of the composite material lead to the localization of the heating area deep in the sample rather than at the surface. The heating area can be localized, for example, at the LASER PHYSICS
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100 µm
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Fig. 3. Optical images of the craters resulting from the surface irradiation of (left-hand panel) dental ceramics and (right-hand panel) metal.
100 µm
100 µm
Fig. 4. Typical microphotographs of the surface damage for the irradiation of the Herculite filling material.
grain interfaces, where the absorbance can be higher. Dumore and Fried [17] report on a similar effect for the ablation of ceramics with the radiation of the neodymium laser. Such a volume heating can lead to a sharp increase in the mechanical damages inside the sample and the subsequent explosive ejection. The crater shape that correlates with composite grains (Fig. 4) proves the volume explosion. Note that such a regime of the material removal is promising for the laser cleaning that does not necessitate the formation of craters with smooth and predictable profiles. Note also that the observed rates of the removal of all of the materials under study are higher than the rates corresponding to the action of the nanosecond laser pulses at comparable levels of E. In addition, we deliver the laser radiation via the optical fiber and observe a relatively high radiation stability of the fibers for microsecond laser pulses.
(≤40 µm/pulse) ablation of various dental materials (filling composites, ceramics, and metal alloy) and avoid the ablation of dentin and enamel.
CONCLUSIONS The experimental results make it possible to determine the main features of the ablation of dental materials with the microsecond pulses of the Nd:YAG laser. In a relatively wide range of fluences for such a laser (E ≤ 100 J/cm2), we both implement the effective
When a nonpulpless tooth is refilled, the application of a dental drilling machine does not allow the precise removal of the filling material, which is important for the procedure performed in the vicinity of the bottom of the pulp camera, since a thermal burn or an opening of the pulp cavity can lead to the pulp inflammation.
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Several procedures remain unchanged in modern dentistry in spite of the advent of new technologies. For example, in the prosthetic dentistry involving the repeated replacement, the removal of various fixed elements is implemented with the aid of mechanical impact tools, which can lead to dental traumas and dislocations or using dental drilling machines, which reduce the risk of traumatic injury but necessitate longer processing. In the therapeutic dentistry, the removal of old filling materials (e.g., from root canals) is also implemented using hand tools or drilling machines: in the former case, the procedures are difficult and ineffective and, in the latter case, the trauma risk is relatively high and the walls of root canals can be perforated. The ultrasonic systems are less traumatic but their efficiency is relatively low.
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These facts indicate the topicality of the development of new dental technologies that are nontraumatic, easy-to-implement, and effective. The application of the laser ablation of dental materials provides a decrease in the damage of dental tissues and painful sensation in various dental manipulations and makes the manipulations easier, so that the quality of treatment becomes higher. Thus, the laser ablation is promising for practical dentistry. The further development of the ablation of dental materials with microsecond pulses of the Nd:YAG laser can be a significant contribution to the progress of new dental technologies. REFERENCES 1. H. P. Berlien and G. J. Müller, Applied Laser Medicine (Springer, Berlin, Heidelberg, New York, 2003; Mir, Ioscow, 1997). 2. C. A. T. Carvalho and M. C. Valera, “Effects of Nd:YAG and Er:YAG Lasers on the Sealing of Root Canal Fillings,” J. Clin. Laser. Med. Surg. 20, 215–219 (2005). 3. M. Curti, J. Rocca, and M. Bertrand, “Morpho-Structural Aspects of Er:YAG-Preapared Class V Cavites,” J. Clin. Laser. Med. Surg. 22, 119–123 (2004). 4. A. M. Prokhorov, V. I. Konov, I. Ursu and I. Mihailescu, Laser Radiation Interaction with Metals (Nauka, Moscow, 1988; Editura Academiei, Bucharest, 1988). 5. D. Fried, N. Ashouri, T. Breunig, and R. Shori, “Mechanism of Water Augmentation during IR Laser Ablation of Dental Enamel”, J. Laser. Surg. Med. 31, 186–193 (2002). 6. K. Matsumoto, X. Wang, C. Zhang, and J. Kinoshita, “Effect Novel Er:YAG Laser in Caries Removal and Cavity Preparation: A Clinical Observation,” Photomed Laser Surg. 25, 8–13 (2007). 7. F. Schwarz, A. Sculean, and M. Brecard, “In Vivo and InVitro Effects of an Er:YAG Laser, a GaAlAs Diode Laser, and Scalling and Root Planning of Periodontally Diseased Root Surfaces: A Comparative Histologic Study”, J. Laser Surg. Med. 32, 359–366 (2003).
8. T. M. Marraccini, L. Bachmann, H. A. Wigdor, J. T. Walsh, Jr., M. L. Turbino. A. Stabholtz, D. M. Zezell, “Enamel and Dentin Irradiation with 9.6 µm CO2 and 2.94 µm Er:YAG Lasers: Bond Strength Evaluation,” Laser Phys. Lett. 3, 96–101 (2006). 9. P. C. G. Silva, S. T. Porto-Neto, R. F. Z. Lizarelli, and V. S. Bagnato, “Orthodontic Brackets Removal under Shear and Tensile Bond Strength Resistance Tests—a Comparative Test between Light Sources,” Laser Phys. Lett. 5, 220–226 (2008). 10. H. Jelinkova, T. Dostalova, M. Necaronmec, P. Koranda, M. Miyagi, K. Iwai, Y.-W. Shi, and Y. Matsuura, “FreeRunning and Q-Switched Er:YAG Laser Dental Cavity and Composite Resin Restoration,” Laser Phys. Lett. 4, 835–839 (2007). 11. D. A. M. P. Malta, M. A. M. Kreidler, G. E. Villa, M. F. de Andrade, C. R. Fontana, and R. F. Z. Lizarelli, “Bond Strength of Adhesive Restorations to Er:YAG Laser-Treated Dentin,” Laser Phys. Lett. 4, 153–156 (2007). 12. T. M. Marraccini, L. Bachmann, H. A. Wigdor, J. T. Walsh, Jr., A. Stabholtz, and D. M. Zezell, “Morphological Evaluation of Enamel and Dentin Irradiated with 9.6 µm CO2 and 2.94 µm Er:YAG Lasers,” Laser Phys. Lett. 2, 551–555 (2005). 13. R. Alexander, J. Xie, and D. Fried, “Selective Removal of Residual Composite from Dental Enamel Surfaces using the Third Harmonic of a Q-Switched Nd:YAG Laser,” J. Laser Surg. Med. 30, 240–245 (2002). 14. V. I. Konov, “Laser Microtechnology,” Smestre d’ete 1993, presented at the Institute of Applied Physics, (Univ. of Berne, 1993), p. 150. 15. S. V. Garnov, V. I. Konov, T. V. Kononenko, V. P. Pashinin, and M. N. Sinyavsky, “Microsecond Laser Material Processing at 1.06 µm,” Laser Phys. 14, 910–915 (2004). 16. V. A. Popkov and O. V. Nesterova, Stomatological Science of Materials (MEDpress-inform, Moscow, 2006). 17. T. Dumore and D. Fried, “Selective Ablation of Orthodontic Composite by using Sub-Microsecond IR Laser Pulses with Optical Feedback,” J. Clin. Laser Med. Surg. 27, 103–110 (2000).
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