Precision ablation of dental enamel using a subpicosecond pulsed laser

Transcription

1 ADRF RESEARCH REPORT Australian Dental Journal 2003;48:(4): Precision ablation of dental enamel using a subpicosecond pulsed laser AV Rode,* EG Gamaly,* B Luther-Davies,* BT Taylor, M Graessel, JM Dawes, A Chan, RM Abstract In this study we report the use of ultra-short-pulsed near-infrared lasers for precision laser ablation of freshly extracted human teeth. The laser wavelength was ~800nm, with pulsewidths of 95 and 150fs, and pulse repetition rates of 1kHz. The laser beam was focused to an approximate diameter of 50µm and was scanned over the tooth surface. The rise in the intrapulpal temperature was monitored by embedded thermocouples, and was shown to remain below 5 C when the tooth was air-cooled during laser treatment. The surface preparation of the ablated teeth, observed by optical and electron microscopy, showed no apparent cracking or heat effects, and the hardness and Raman spectra of the laser-treated enamel were not distinguishable from those of native enamel. This study indicates the potential for ultra-short-pulsed lasers to effect precision ablation of dental enamel. Key words: Laser treatment, subpicosecond laser ablation, temperature monitoring, dental enamel. (Accepted for publication 15 May 2003.) INTRODUCTION The promise that lasers offer to both dentists and patients of simple, painless dental treatments has stimulated continuing research into their use for removal of dental hard tissue. When patients were surveyed on their interest in laser treatments they responded positively. 1 However, in practice slow material removal rates and unacceptable local tissue damage have inhibited the clinical use of lasers in many hard tissue applications. Studies of the ablation of enamel and dentine, using pulsed lasers such as *Research School of Physical Science and Engineering, Australian National University, Canberra, Australian Capital Territory. Centre for Lasers and Applications, Department of Physics, Macquarie University, New South Wales. Private Dental Practice, Caringbah, New South Wales. Centre for Atom Optics and Ultrafast Spectroscopy, Swinburne University of Technology, Hawthorn, Victoria. Nd:YAG or Ho:YAG, have shown evidence of cracking, melting, charring, fissuring or crazing of tooth surfaces, and inefficient material removal. 2 Typically, treatment by a conventional pulsed laser source (pulsewidths of nanoseconds to microseconds) leads to cracking and uncontrolled material removal, resulting in poor surface preparation. 3-5 The poor absorption of most laser radiation by tooth structures requires high laser power, and causes significant excess heat deposition in the tooth, with potential for intrapulpal damage. More recently, Er:YAG 6,7 lasers operating at a wavelength of ~2.9µm, used in conjunction with water spray to enhance the absorption of the laser radiation by the tissue and simultaneously cool the treated area, have offered more efficient material removal rates, but with reports of collateral damage to the tooth surface. 7 The development of high-average-power, high-repetition-rate, subpicosecond lasers has refocused interest in laser treatments, due to the highly efficient tissue ablation and minimal collateral damage offered by using such lasers in the appropriate parameter regime. 3-5,8-11 Conventional (ns-µs) pulsed lasers tend to produce a local temperature rise and possibly a shock wave in the tooth, and thermal effects in the bulk dental tissue surrounding the treated spot lead to collateral tissue damage and may cause pain for the patient. By decreasing the laser pulse duration to the subpicosecond time regime and increasing the peak laser intensity, it is possible to induce a different mechanism of laser-tissue removal. In this electrostatic ablation mechanism, 12 the laser light is absorbed in a small volume of material and electrons and then ions are ejected rapidly, reducing the heat that is conducted to the surrounding tissue. Scaling laws enable the calculation of threshold laser parameters for ablation of various materials. The change in ablation mechanism from thermally induced in the conventional long pulse regime to the electrostatic mechanism, results in distinct changes in tooth morphology, substantially reduced collateral damage to the tissue, and a decrease in the energy fluence required for Australian Dental Journal 2003;48:4. 233

2 significant material removal. 3-5 For laser pulsewidths of more than 1ps, ablation was less efficient, and surface melting was observed, whereas ablation efficiency was maximized for ultrashort pulses of 130fs to 1ps and the surface showed less evidence of melting. 5 In this study, we present the results of ablation of dental enamel using ultra-short-pulsed lasers. The ablation threshold and the ablation rate were measured and the accompanying intrapulpal temperature rise in the tooth was monitored. The surface preparation of the tooth was observed, and the collateral damage in the bulk of the ablated teeth was characterized using optical and electron microscopy, hardness measurements and Raman spectroscopy. In conclusion, we give recommendations for precise laser machining of dental enamel by subpicosecond laser pulses. MATERIALS AND METHODS Ten healthy human premolars were used for this study. The teeth were first extracted from patients undergoing unrelated orthodontic treatment, and were sterilized in 10 per cent sodium hypochlorite solution for 10 minutes and then stored in formalin. Teeth used for intrapulpal temperature measurements were sliced longitudinally in half using a diamond-bladed saw. The pulp was removed to permit the insertion of miniature thermocouples that were located at the odontoblast layer with superglue, and the area was sealed using epoxy resin. The thermal conductivity of the epoxy was of the order of 0.8Wm -1 K -1, and considering the small quantities used, the resulting thermal resistance was not likely to affect the maximum intrapulpal temperature measured. The temperature was monitored every 0.2s with a digital thermometer, accurate to 0.1 C. Some sliced teeth were clamped together and the sliced edge was laser treated, to allow post-treatment examination of the targeted tissue. Two Ti:Sapphire laser systems operating at 1kHz pulse repetition rate were used in the experiments. The first was a 780-nm 150-fs laser system (Clark-MXR CPA-2001, Ann Arbor, Michigan, USA) at the Australian National University with an average laser power W. The second laser (SpectraPhysicsTsunami and Spitfire, Mountainview, Colorado, USA) at Swinburne University of Technology generated 95fs pulses at 805nm and average powers of 0.8-1W. The energy stability in both lasers was typically within 2 per cent. The laser pulses were focused onto the tooth surface using a 250mm focal length scanning lens to a focal spot of ~50µm diameter (focal spot area cm -2 ), providing a maximum laser intensity of around W/cm 2 with 150-fs pulses and W/cm 2 with 95-fs pulses (maximum laser fluences 21J/cm 2 and 36.5J/cm 2 respectively). Inserting neutral density filters into the beam varied the laser intensity. There was substantial scatter in the ablation rate measurements, up to ±50 per cent at a fixed laser power, which was attributed to the roughness of the enamel surface and variation between the teeth specimens. An important condition for the ultra-short-pulse interaction mode is the temporal pulse profile. Each laser had a low-peakpower pre-pulse of nanosecond duration, before the high-peak-power 100fs main pulse. The intensity contrast ratio between the main pulse and the pre-pulse has been measured to be >10 4 for both lasers using an autocorrelator to accurately derive the temporal pulse shape over a wide dynamic range of intensity. In each case, the laser beam was propagated in air, focused and scanned over a 13 1mm 2 or a 23 2mm 2 area on the tooth enamel with a dual-axis galvanometer beam scanning system forming a square crater. The ablated volume was determined by measuring the crater depth after a period of s ( laser pulses). The scanning speed was rather slow, of the order of a few Hz. Focusing a microscope objective to the top of the tooth surface and subsequently to the floor of the ablated crater with a calibrated microscrew gave an estimate of the crater depth with an average uncertainty of 0.1µm. These measurements were verified with nano-profilometer scans and with scanning electron micrographs (SEM) of the walls of the crater. The ablation threshold values were calculated from the experimental data of the ablated depth versus the laser fluence. At least two teeth were treated with each set of laser parameters. Vickers hardness tests were performed (Leitz Miniload, Leitz, Wetzler, Germany) to determine the effect of the ultrashort pulsed laser ablation on the mechanical properties of the surrounding dental tissue. Measurements were conducted on the healthy (untreated) enamel of three teeth, and also on the base of the laser-ablated crater of these teeth. A weighted diamond stylus was allowed to impinge on the enamel surface, and the resulting indentation was measured with an optical microscope and calibrated graticule, to estimate the hardness. Because the measurements were affected by the roughened surface of the teeth, the results showed considerable scatter, and an average of at least four measurements in different spots was taken for each tooth. Thus, the hardness of the craters prepared by the laser ablation could be compared with the hardness of healthy enamel. An additional test of the properties of the lasertreated and healthy (untreated) enamel was made using a Raman microprobe spectrometer (Renishaw, Wottonunder-Edge, Gloucestershire, UK). Using a focused helium cadmium excitation laser with an optical microscope, this instrument yields the characteristic Raman spectra for a small region of a given sample. The Raman spectra of the various samples allow a comparison of the crystalline properties of the enamel in different regions of the tooth surface. The background level in all spectra recorded was high, due to scattered light from the rough enamel surface, but the observed spectral peaks are characteristic of the chemical and crystalline structure of the samples. To compare the crater features of short-pulse and long-pulse ablation, we performed a few ablations 234 Australian Dental Journal 2003;48:4.

3 the crater edge (Fig 1). By contrast, the enamel ablated by the 60ps pulses showed a darkened ring of thermally damaged tissue about 0.3mm from the crater (Fig 2). This observation was supported by the SEM images of the 150fs and 60ps laser ablated craters (Fig 3). The right hand images in Fig 3 show enamel droplets on the surface of the tooth next to the squares, with a raised enamel surface near the edge of the crater due to heat deposition in the bulk of the tooth. The floor of the ablated craters appeared melted and uneven. In sharp contrast to the picosecond laser results, the enamel ablated by 95 and 150fs pulses (seen in the left images of Fig 3) showed no collateral thermal damage at all. The floor of the fs-laser-ablated craters was also smoother, with sharper edges and corners than those ablated with 60ps pulses. Fig 1. Optical microscope image of 2mm 2 x0.25mm craters on the tooth surface ablated with 150-fs laser pulses at 1kHz repetition rate. Average power ~1W, ablation time two minutes and intrapulpal temperature rise ~3 C. using a non-commercial Q-switched mode-locked Nd:YAG laser. The 60-ps laser pulses at 1.064µm followed in trains of ~ ns duration (~17 pulses per train) with the train repetition rate 500Hz-2kHz ( pulses per second), the average laser power was 3-8W respectively ( µJ/pulse). The laser was focused to a 25µm spot (laser intensity ~ W/cm 2, fluence ~50J/cm 2 ), and scanned over the same size area on the tooth surface. The laser repetition rate was chosen to maintain the intrapulpal temperature rise during the 10s ablation below 4 C. RESULTS Optical and SEM images The ablated areas were examined with an optical microscope and a scanning electron microscope. The images of the craters ablated with sub-picosecond lasers showed the absence of any collateral damage at Fig 2. Optical microscope image of 2mm 2 crater on the tooth surface ablated with 60ps laser pulses at 1kHz repetition rate. Average power ~1W, ablation time two minutes and intrapulpal temperature rise ~3 C. Ablation rate measurements The ablation rates were measured over the intensity range W/cm 2 with both 95fs and 150fs pulses (Fig 4). The ablation threshold fluence F th was found to be 2.2±0.1J/cm 2 for both 95fs and 150fs pulses. There was no visible damage to the enamel surface at laser fluences below this threshold value. The ablation rates increased linearly above the threshold level for the available laser fluences up to 36.5J/cm 2. There was no detectable difference, within the accuracy of measurements, in the ablation rate with 95fs and 150fs laser pulses. The most efficient ablation was observed at the maximum laser fluence (intensity) of 36.5J/cm 2 ( W/cm 2 ), which yielded 1.8µm of ablated depth per 1mJ pulse. This is equivalent to a drilling rate of mm 3 /s for our 1kHz lasers. This value is in reasonable agreement with the results of Neev et al. 3 By comparison, a typical drilling rate for a dental drill is 1mm 3 /s. 13 However, mechanical drills yield drilling rates that are highly dependent on the burr used, the tooth condition and the operator s judgement. Intrapulpal temperature measurements After laser irradiation for 200s duration, the average rise in the intrapulpal temperature measured with seven teeth was 10 C, with variations from tooth to tooth, dependent on the distance from the ablated area to the tip of the thermocouple placed in the pulp cavity. When the teeth were cooled by air blown from a distance of 10mm at various flow rates from 5 to 20L/min (Fig 5), the temperature was observed to saturate at a level of less than 5 C. Since Zach and Cohen 14 showed that with a rise of intrapulpal temperature of 5.5 C, 15 per cent of the tooth pulp may suffer irreversible thermal damage, this value is considered critical. Micro-Raman spectroscopy Raman spectra were recorded over the wavenumber range of 0 to 1800cm -1 for the laser-treated and untreated enamel on two teeth (Fig 6). The lack of surface polish affected the measurements as it increased the background scattered light. The rising background Australian Dental Journal 2003;48:4. 235

4 Fig 3. Scanning electron microscopy images of the edges and floors of 0.3mm deep craters in enamel ablated with 150-fs laser (left) and 60-ps laser (right). There is no collateral damage around the fs-laser ablated cavity; the edges are sharp and the crater floor is smooth. With the 60-ps laser there is a thermally modified area around the crater and occasionally some cracks. The bars are 10µm. level was an artefact due to the high scattering from the unpolished enamel surface. However, peaks at 960, 1100, 580 and 470cm -1 could be identified. These correspond to the characteristic n 1, n 2 and n 4 vibrations of the phosphate (PO 4 3- ) groups in hydroxyapatite. 15 It is notable that the peaks occured in both the untreated and laser-treated teeth, although with slightly different ratios. No other peaks were observed, which suggested that the crystalline structure of the hydroxyapatite in the native enamel was unaffected by the laser treatment. Hardness tests The Vickers hardness tester was used to measure the hardness of an untreated area and a laser-treated area of enamel from each of three teeth. Accurate hardness measurements should be performed on smooth polished surfaces, but in this case, polishing was deemed to possibly affect the surface quality. Although the laser-ablated enamel surface was slightly roughened, the untreated enamel was also noted to not be microscopically smooth. This hindered the precise measurement of the indentation. An average was taken of at least four measurements from each tooth as some variation was observed. Table 1 shows the hardness results obtained for the teeth. Although there was considerable scatter in the results between teeth, there was no significant difference noted between the laser-treated and the untreated enamel hardness measurements. From these results, we conclude that the laser treatment did not significantly affect the enamel surrounding or beneath the ablated crater. For example, melting was not observed on the tooth surface following ultrashort-pulsed laser ablation. DISCUSSION Since the 1940s the mechanical drill has been accepted as the benchmark dental tool for the removal of hard tissues and caries in cavity preparation. Unfortunately, it is not possible to avoid the potential 236 Australian Dental Journal 2003;48:4.

5 Untreated enamel: Raman spectrum Ablated depth per pulse, µm Intensity (arb units) a Raman shift (wave numbers) Laser-treated enamel: Raman spectrum Laser Fluence, J/cm 2 Fig 4. Ablated depth vs. laser fluence with 95fs (stars) and 150fs (crosses) pulses. The line is the theoretical prediction from the model of Ref 12, taking the experimentally determined ablation threshold of 2.2J/cm 2 for both pulse durations. adverse thermal side effects such as pulpal injury, dentine hypersensitivity and crack formation resulting from the use of rotary instruments during tooth preparation in every case. 16,17 Since the 1960s, lasers have been investigated as a potential tool for ablation of dental hard tissues with the aim of replacing the dental drill. However, laser systems that have been investigated for hard tissue applications have led to reports of charring, cracking and damage of enamel structures 18 change in microhardness of exposed enamel 19 and excessive increase in temperature in the pulp up to 30 C 20 and the surrounding tissues. 21,22 Mid-infared lasers (Er:YAG and Er:YSGG), with pulse durations in the microsecond regime, are highly absorbed by water and hydroxyapatite. Erbium laser energy induces localized heating and water vapor expansion inside the dental tissue causing internal Intensity (arb units) b Raman shift (wave numbers) Fig 6. Raman spectra recorded for a) the untreated and b) lasertreated enamel surfaces, show the characteristic spectral peaks. The rising background is due in both cases to scattered light from the unpolished surface of the tooth, before and after laser treatment. micro-explosions and fragmentation of dental hard tissue. 6 Although they are 3-4 times slower at ablating hard tissue than a conventional dental high-speed drill, 23,24 they promote better patient comfort and minimize the loss of sound tooth structure, without the noise and the vibration of the dental rotary instruments. With the use of the cooling water spray, the intrapulpal temperature is maintained within safe limits. Hence, such lasers have gradually gained publicity, especially in treating anxious patients. 25,26 In recent years these lasers have been established as a viable alternative for the selective and precise removal of carious dental tissue. 27,28 In this regime of operation, although water spray is used to improve the removal efficiency of hard dental tissues 29 and allow greater thermal safety, collateral damage such as cracks and micro-fractures have been reported. 7 Studies have also demonstrated there is an increase, although not significant, in the Ca/P weight ratio of the underlying ablated surface after Er:YAG and Er:Cr:YSGG laser irradiation. 30,31 Since the melting point of natural apatite is about 1000 C and Fig 5. Temperature monitored by an embedded thermocouple in the pulp cavity without air-cooling and with air-cooling at various air flow rates. Table 1. Vickers hardness results for three teeth, lasertreated and healthy (untreated), averaged over at least four spots in each case Tooth number Laser-treated enamel hardness kg mm ±40 277±22 255±22 Untreated enamel hardness kg mm ±40 260±44 264±44 Australian Dental Journal 2003;48:4. 237

6 recrystallization occurs at about 700 C, 32 the increased Ca/P ratio of the surface after laser treatment might be due to the formation of tetracalcium phosphate, which occurs above 1,100 C. 33 However, in some of these studies, the laser irradiation of extracted teeth may not reflect the clinical situation, in which blood perfusion of the tissue improves thermal dissipation. These constraints and limitations in dental applications have lead to investigations of laser systems operating in the ultra-short pulsed regime (pulse duration less than 10ps) as a new approach to ablating dental hard tissues. 34 This technique is promising. However, collateral thermal effects in enamel have not been completely eliminated. 35,36 In a proposed mechanism 12 for ultrashort-pulsedlaser ablation, the electrons absorb sufficient energy during the laser pulse to escape from the target surface. Each escaped electron creates an electric field with respect to the surface. If the energy of the escaped electrons is sufficient, then the ions are pulled out of the laser-affected spot on the target surface. This electric field is the physical reason for the non-thermal mechanism of ablation with subpicosecond laser pulses. 12 Thus, energy transfer from the laser pulse to the bulk of the target material, which leads to accompanying collateral damage, is prevented. There is no melting or evaporation in a conventional equilibrium sense in the electrostatic ablation regime. Other mechanisms for laser ablation such as thermal or photo-acoustic, occur over significantly longer timescales. The ablation threshold determined from the electrostatic ablation model 12, 35 is 0.95J/cm 2, which is in reasonable agreement with the experimental value of 2.2J/cm 2. While other ablation mechanisms cannot be excluded, the agreement between the predicted and measured ablation thresholds is encouraging evidence that the electrostatic ablation mechanism dominates for the fs laser pulse ablation. Taking an intensity of W/cm 2 as typical for the ultra-short laser ablation conditions, the ablation rate is 4x10 31 cm -2 s -1. This ablation rate is almost four orders of magnitude higher than that in the thermal regime of evaporation with nanosecond laser pulses, and about 100 times higher than with 60-ps pulses. 37 Both carious dental tissue and dentine are softer than healthy enamel, so these typically ablate up to 10 times more readily. 36,38 Since the water content of both dentine and carious enamel is higher than for healthy enamel, altering the chemical content, the model of Ref 12 predicts a slightly lower threshold for ablation. The hardness, elasticity, or other properties of the tissue are not considered in the electrostatic model of ablation using ultrashort-pulsed lasers. Further studies are planned to compare the ablation rates for healthy and carious enamel and dentine. In this study, we have demonstrated subpicosecond laser ablation of dental enamel with high pulse repetition rate, and have shown that the intrapulpal temperature rise may be maintained below 5 C with simultaneous air-cooling. Note that the presence of pulp tissues can alter laser-induced temperature rise measured in the absence of the pulp 39 but we attempted to reduce this effect by embedding the thermocouple in epoxy in the pulp chamber. Considering the electrostatic ablation mechanism proposed for ultrashort pulsed laser ablation, we do not expect the intrapulpal temperature to increase significantly in our laser ablation experiments, as this mechanism is not thermal in origin. 12 However, the laser contrast ratio of 10 4 means that there is a significant energy component in the pulse that is of the order of nanoseconds in pulsewidth. This background nanosecond pulse, which does not contribute to the electrostatic ablation mechanism, appears to contribute substantially to the intrapulpal temperature rise that we measure. We suspect that nonlinear-optical pulsenarrowing techniques, for example, frequency doubling of the infrared laser pulses, would have the desired effect of reducing the intensity of the nanosecond background pulse and improving the peak-tobackground pulse contrast ratio. This should reduce the intrapulpal temperature rise, so that air cooling would not be required. Further experiments to investigate this approach are planned. The ablation rate of material removal is 1µm/pulse, or about 2x10-3 mm 3 /s, which is relatively high for laser ablation of dental tissue. Mechanical drill tissue removal rates are of the order of 1mm 3 /s, depending on the cutting tool chosen. Practical tissue removal rates with subpicosecond laser ablation might require pulse repetition rates of the order of 20-50kHz. The clinical motivation for subpicosecond laser ablation of dental tissue is potentially the precise material removal and quality of surface preparation. No collateral thermal damage is observed microscopically or by Raman spectroscopy or hardness measurements, in the prepared teeth. Improvements to the temporal pulse profile may lead to a further reduction in the measured intrapulpal temperature rise. CONCLUSIONS In conclusion, subpicosecond laser pulse ablation offers the potential to avoid or minimize the detrimental side effects of the dental drill, with extremely precise removal of hard tissue. With further clinical studies, we anticipate that the use of ultrashort-pulsed lasers will enhance the efficacy of many dental and surgical procedures. ACKNOWLEDGEMENTS The authors thank Associate Professor E Goldys and Dr H Fei for the Raman microprobe measurements. Useful suggestions by the referees are also acknowledged. The work was financed by a Research Grant from the Australian Dental Research Foundation and by Macquarie University, the Australian National University and Swinburne University. 238 Australian Dental Journal 2003;48:4.

7 REFERENCES 1. Wigdor HA. Patients perceptions of lasers in dentistry. Lasers Surg Med 1997;20: Cernavin I. A comparison of the effects of Nd:YAG and Ho:YAG laser irradiation on dentine and enamel. Aust Dent J 1995;40: Neev J, Da Silva LB, Feit MD, Perry MD, Rubenchik AM, Stuart BC. Ultrashort pulse lasers for hard tissue ablation. IEEE J Sel Top Quantum Electron 1996;2: Oraevsky AA, Da Silva LB, Rubenchik AM, et al. Plasma mediated ablation of biological tissues with nanosecond-tofemtosecond laser pulses: relative role of linear and nonlinear absorption. IEEE J Sel Top Quantum Electron 1996;2: Kim BM, Feit MD, Rubenchik AM, et al. Influence of pulse duration on ultrashort laser pulse ablation of biological tissues. J Biomed Opt 2001;6: Keller U, Hibst R. Experimental studies of the application of Er:YAG laser on dental hard substances: II. Light microscopic and SEM investigations. Lasers Surg Med 1989;9: Frentzen M, Winkelstrater C, van Benthem H, Koort HJ. The effects of pulsed ultraviolet and infra-red lasers on dental enamel. Eur J Prosthodont Restor Dent 1996;4: Feit MD, Rubenchik AM, Kim BM, Da Silva LB, Perry MD. Physical characterisation of ultrashort laser pulse drilling of biological tissue. Appl Surf Sci 1998; : Loesel FH, Niemz MH, Bille JF, Juhasz T. Laser-induced optical breakdown on hard and soft tissues and its dependence on the pulse duration: experiment and model. IEEE J Quantum Electron 1996;32: Momma C, Nolte S, Chichkov BN, von Alvensleben F, Tunnermann A. Precise laser ablation with ultrashort pulses. Appl Surf Sci 1997; : Niemz MH. Investigation and spectral analysis of the plasma induced ablation mechanism of dental hydroxyapatite. Appl Phys B 1994;58: Gamaly EG, Rode AV, Tikhonchuk VT, Luther-Davies B. Ablation of solids by femtosecond lasers: Ablation mechanism and ablation thresholds for metals and dielectrics. Phys Plasmas 2002;9: Altshuler GB, Belikov AV, Sinelnik Y. A laser-abrasive method for the cutting of enamel and dentine. Lasers Surg Med 2001;28: Zach L, Cohen G. Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol 1965;19: Tsuda H, Arends J. Raman spectroscopy in dental research: a short review of recent studies. Adv Dent Res 1997;11: Bergenholtz G. Pathogenic mechanisms in pulpal disease. J Endod 1990;16: Barnett F. Pulpal response to restorative procedures and materials. Curr Opin Dent 1992;2: Koort HJ, Frentzen M. Laser effects on dental hard tissues. In: Miserendino LJ, Pick RM, eds. Lasers in Dentistry. Chicago: Quintessence, 1995;63: Kantola S, Laine E, Tarno T. Laser induced effects on tooth structure. Acta Odont Scand 1973;31: Paghdiwala AF. Application of the Erbium:YAG laser on hard dental tissues: Evaluation of temperature changes and cutting effects. Laser Inst Am 1989;64: Anic I, Vidovic D, Luic M, Tudja M. Laser induced molar tooth pulp chamber temperature changes. Caries Res 1992;26: Paghdiwala AF, Vaidyanathan TK, Paghdiwala MF. Evaluation of erbium:yag radiation of hard dental tissues: analysis of temperature changes, depth of cuts and structural effects. Scanning Microsc 1993;7: Levy G, Koubi GF, Miserendino LJ. Cutting efficiency of a midinfrared laser on human enamel. J Endodo 1998;24: Hibst R Keller U. Experimental studies of the application of the Er:YAG laser on dental hard substances: I. Measurement of the ablation rate. Lasers Surg Med 1989;9: Keller U, Hibst R, Geurtsen W, et al. Erbium:YAG laser application in caries therapy. Evaluation of patient perception and acceptance. J Dent 1998;26: Pelagalli J, Gimbel CB, Hansen RT, Swett A, Winn DW. Investigation of the use of Er:YAG laser versus dental drill for caries removal and cavity preparation phase I. J Clin Laser Med Surg 1997;15: Hadley J, Young DA, Eversole LR, Gornbein JA. A laser-powered hydrokinetic system for caries removal and cavity preparation. J Am Dent Assoc 2000;131: Yamada Y, Hossain M, Nakamura Y, Suzuki N, Matsumoto K. Removal of carious dentine by mechanical, chemo-mechanical and Er:YAG laser in deciduous teeth. J Oral Laser Appl 2001;1: Burkes EJ, Hoke J, Gomes E, Wolbarsht M. Wet versus dry enamel ablation by Er:YAG laser. J Prosthet Dent 1992;67: Kimura Y, Yu DG, Fujita A, Yamashita A, Murakami Y, Matsumoto K. Effects of Er,Cr:YSGG irradiation on canine mandibular bone. J Periodontol 2001;723: Tokonabe H, Kouji R, Watanabe H, Nakamura Y, Matsumoto K. Morphological changes of human teeth with Er:YAG laser irradiation. J Clin Laser Med Surg1999;17: LeGeros RZ. Calcium phosphates in oral biology and medicine. Monogr Oral Sci 1991;15: Kuroda S, Fowler BO. Compositional, structural, and phase changes in in vitro laser-irradiated human tooth enamel. Calcif Tissue Int 1984;36: Rode AV, Gamaly E, Luther-Davies B, et al. Subpicosecond laser ablation of dental enamel. J Appl Phys 2002;92: Niemz MH. Cavity preparation with the Nd:YLF picosecond laser J Dent Res 1995;74: Lizarelli RFZ, Moriyama LT, Bagnato VS. Ablation rate and micromorphological aspects with Nd:YAG picosecond pulsed laser on primary teeth. Lasers Surg Med 2002;31: Lizarelli RFZ, Kurachi C, Misoguti L, Bagnato VS. A comparative study of nanosecond and picosecond laser ablation in enamel: morphological aspects. J Clin Laser Med Surg 2000;18: Ostertag M, McKinley JT, Reinisch L, Harris DM, Tolk NH. Laser ablation as a function of the primary absorber in dentin. Lasers Surg Med 1997;21: Chang JC, Wilder-Smith P. Laser-induced thermal events in empty and pulp-filled dental pulp chambers. Lasers Surg Med 1998;22: Address for correspondence/reprints: Judith Dawes Department of Physics Macquarie University New South Wales judith@ics.mq.edu.au Australian Dental Journal 2003;48:4. 239