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Journal of Dental Research http://jdr.sagepub.com/ Dye-enhanced Ablation of Enamel by Pulsed Lasers E. Jennett, M. Motamedi, S. Rastegar, C. Frederickson, C. Arcoria and J.M. Powers J DENT RES 1994 73: 1841 DOI: 10.1177/00220345940730120801 The online version of this article can be found at: http://jdr.sagepub.com/content/73/12/1841 Published by: http://www.sagepublications.com On behalf of: International and American Associations for Dental Research Additional services and information for Journal of Dental Research can be found at: Email Alerts: http://jdr.sagepub.com/cgi/alerts Subscriptions: http://jdr.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav Citations: http://jdr.sagepub.com/content/73/12/1841.refs.html Downloaded from jdr.sagepub.com by guest on July 11, 2011 For personal use only. No other uses without permission. J Dent Res 73(12):1841-1847, December, 1994 Dye-enhanced Ablation of Enamel by Pulsed Lasers E.Jennettl 2, M. Motamedil', S. Rastegar2, C. Frederickson3, C. Arcoria4, andJ.M. Powers5 'Biomedical Laser and Spectroscopy Program, Jennie Sealy Hospital, Route D56, University of Texas Medical Branch, 301 University Boulevard, Galveston, Texas 77555; 2Bioengineering Program, Texas A&M University, College Station; 3MicroFab Technologies, Plano; 4Baylor College of Dentistry, Department of Restorative Sciences, Dallas; and 5University of Texas Houston Health Science Center, Dental Branch, Department of Basic Sciences, Section Oral Biomaterials;*to whom correspondence and reprint requests should be addressed Abstract. Laser removal of dental hard tissue has been Introduction proposed as a replacement for or augmented approach to the dental handpiece. The main limitation for widespread usage Laser ablation of the tooth has been reported in many of lasers in dentistry has been inefficient ablation of dental articles since Stern and Sognnaes (1964) reported on the hard tissue, accompanied by potential laser-induced damage application of the ruby laser for removal of dental hard to the surrounding tissue. The research focuses on a novel tissue. The potential applications of lasers in dentistry have approach for enhancement of tissue ablation and been recognized by many groups, and much preliminary confinement of laser interaction to a small tissue volume by research has been conducted, including the ablation of controlled placement of an exogenous dye on the enamel dental soft and hard tissues as well as surface-processing surface. Studies were done with both pulsed alexandrite and (sealing fissures) (Stewart et al., 1985). pulsed Nd:YAG lasers, with indocyanine green and India ink, Stern and Sognnaes, in their early work, demonstrated respectively, used as photo-absorbers. These dye-enhanced that, in principle, ablation of enamel is possible. However, laser processes demonstrated the feasibility of this technique extensive damage induced in the tooth discouraged further for cavity preparation. While control studies produced little development of laser-based systems for the removal of or no appreciable crater, average preparation depth for the dental hard tissue. Since then, various continuous-wave dye-enhanced ablation was from 1 to 1.5 mm, with a diameter lasers-such as the CO2, Nd:YAG, and argon-and pulsed of approximately 0.6 mm. lasers-such as the Er:YAG, ArF excimer, ruby, CO2, and the Knoop hardness measurements show that, surrounding Nd:YAG-have been tried on dental hard tissue in an effort the crater, there is a small annular region slightly softened by to replace or complement the dental handpiece with an the laser action. SEM studies of the interior structure of the effective alternative which permits the fine removal of tooth did not show significant damage to the surrounding tissue in a controlled fashion (Willenborg, 1989; Neev et al., tissue. Temperature measurement studies indicated that the 1991; Quintana et al., 1992; Pick, 1993; Wigdor et al., 1993). pulsed nature of the laser, combined with the photo- In recent years, there has been increasing optimism absorbing dye, effectively prevented significant temperature about the development of laser-based systems that can be rise at the pulp. used for the efficient and potentially painless removal of The remarkable effectiveness of this technique in creating dental hard tissue. The currently used dental handpiece is cavity preparations and the absence of any notable collateral legendary for the discomfort and pain it causes during the damage to the surrounding tissue suggest that dye-enhanced vital tooth-preparation process. A laser that could remove pulsed-laser ablation could be used as an alternative to the dental hard tissue without producing patient trauma and dental handpiece in selected procedures. fear would be a major benefit to both dentists and patients alike. Key words. Laser Ablation, Calcified Tissue, Hardness, Most current medical lasers generate optical radiation in Pulp Temperature, Temperature Measurement Studies. the visible and near-infrared regions of the spectrum. For this range, dental hard tissue is highly translucent and Received February 10,1994; AcceptedJuly 20,1994 hence has lower optical absorption and does not allow for 1841 Downloaded from jdr.sagepub.com by guest on July 11, 2011 For personal use only. No other uses without permission. 1842 Jennett et al. J Dent Res 73(12) 1994 efficient tissue ablation (Vickers et al., 1992; Wigdor, 1993). poured into each sample cup and around the tooth, until the However, promising results have been reported about the resin was approximately 5 mm from the top of the facial potential use of dyes to enhance tissue absorption in the surface. The resin was allowed to cure for approximately 20 near-infrared range for several medical applications min. (Hillenkamp, 1989; Chuck et al., 1989; Oz et al., 1990; De The samples were removed from the sample cups and Coste et al., 1992; Brooks et al., 1992). prepared for surface grinding, which was extremely The application of photo-absorbing dyes in combination important since a flat surface is necessary for accurate with pulsed lasers causes effective tissue absorption to be Knoop hardness testing. During testing, the diamond dramatically increased and energy penetration to be indenter is loaded onto the tooth sample, and an indentation confined to a small volume. This technique confines the is made. Therefore, if the surface is uneven, an abnormally deposited laser energy spatially while delivering a short low Knoop hardness value will be obtained, because one pulse of laser beam. Less thermal build-up occurs in the end of the indenter had a greater surface area with which to surrounding tissue, creating a safer and more efficient indent. Therefore, each sample was ground individually on means of tissue removal. Theoretically, any laser with an the Buehler Polishmet Polisher. First, a Buehler 600-grit appropriate dye may be useful for these types of Carbimet Paper disk was used. The samples were ground for applications. The wavelength range (from 700 to 1100 nm) approximately one minute, or until there was a uniform flat has the advantage of being delivered via flexible, reliable, surface on the facial side of the molar. The samples were and inexpensive silica fibers. In addition, there is a very high then polished with a Mastertex Buehler Polishing Cloth and potential for the development of reliable, inexpensive, Alpha Micropolish II Alumina (1.0 gim). Polishing diode-laser-based systems which operate in this range. significantly reduced the lines on the flattened surface of The objective of this study was to establish the feasibility the tooth when examined microscopically. This simplified of dye-enhanced pulsed-laser ablation for the cavitation of the Knoop Hardness measurements. dental enamel. Studies were conducted by use of a The lasers used for the cavitation of enamel were the combination of Nd:YAG laser with India ink and pulsed Nd:YAG (1064 nm) laser (Incisive Technologies, Inc.) alexandrite laser with indocyanine green for the ablation of and alexandrite (720-790 nm) laser (1-2-3, Schwartz Electro- extracted human molars. optics, Inc.). The Nd:YAG laser is commonly used and well- known in the medical/dental community; it generates laser Materials and methods pulses of approximately 100 gsec. In these experiments, an energy of 200 mJ/pulse was utilized, resulting in an energy Ablation depths, temperature rise in the pulp, Knoop density of 160 J/cm2. The laser beam was delivered to the hardness, and SEM studies were measured and studied on surface of the tooth via a quartz fiber, resulting in a 0.4-mm the treated samples. These measurements were conducted spot diameter. For most experiments, the laser was pulsed at on the same tooth specimens so that more reliable and 1 Hz. consistent data could be obtained. Extracted human molars India ink (Black Sumi ink, Yasutomo Co.) was deposited were used as the sample specimens throughout these on the tooth surface by either a small drop through a 26- experiments. gauge needle, or by solid-state fluid-dispensing jet (Microjet, Twenty specimens were used for Knoop hardness testing MicroFab Corp., Plano, TX). When the jet was used, trains of (before and after laser irradiation), and three were used to 5 pulses (1 Hz) at approximately 100 pL per pulse were used determine average temperature rise in the pulp during laser for controllable and reproducible staining of tissue. irradiation. They were stored in a formaldehyde solution The alexandrite laser has a broad spectrum, with a peak until prepared for use. The molars were rinsed with tap at 750 nm and a pulsewidth of approximately 200 gsec, water and brushed with a soft-bristled toothbrush for composed of many spikes (Fig. 1). The raw alexandrite laser removal of excess particles. After being dried, the molars beam was focused through a convex lens to a 0.5-mm spot were sterilized with ethylene oxide gas to protect the size (diameter), with an energy of 700 mJ/pulse at the tooth investigators from any diseases and organisms that might surface, resulting in an energy density at the surface of have been present on the tooth surfaces. In preparation for approximately 350 J/cm2. Laser radiation was delivered at laser irradiation and hardness studies, the facial sides of the 0.25 Hz. These parameters were chosen as a result of molars were used for experimentation. The teeth were preliminary experiments conducted to determine the individually suspended in sample cups (Buehler Sampl- threshold for dye-enhanced ablation of the enamel. The kups) with utility wax and a wooden stirring stick, to energy levels used in the study were chosen to ensure ensure that the tops of the molars were not covered with efficient removal of the enamel tissue at magnitudes resin. sufficiently higher than the ablation threshold, which was Buehler Sampl-Kwick Powder was mixed with Buehler determined to be about 200J/cm2. Sampl-Kwick Liquid in a 2:1 ratio to form an Cavity preparations were created by use of this laser isobutylmethacrylate resin, which was then immediately with ICG (indocyanine green, Eastman Kodak Company, Downloaded from jdr.sagepub.com by guest on July 11, 2011 For personal use only. No other uses without permission. j Dent Res 73(12) 1994 Dye-enhanced Ablation ofDental Hard Tissue Rochester, NY) to facilitate ablation of enamel tissue. The dye is currently used in clinical applications such as the measurement of cardiac output. To determine the influence of dye concentration on ablation efficiency, we conducted concentration studies on the ICG-alexandrite combination. Indocyanine green was combined with distilled water at concentrations of 0.5, 1.0, and 1.5% for various experiments where ablation efficiency and the extent of tissue involvement were studied. Directly before laser irradiation, a spot of ICG (diameter approximately 1 mm) was topically applied to the surface of the tooth by means of a 26-gauge needle. This spot was located directly at the point of impact of the alexandrite laser beam. A single pulse was then delivered from the laser. This process was repeated until the target number of ablation pulses was delivered. The amount of dye placed on the surface of the tooth was not controlled in the experiments where the alexandrite laser was used. Control experiments were run without the topical o ;- O *. -6- ~--1 0 0, -14- . -50 4 .. i...- i 0 l 50 Il 100 :.............................. 150 200 Figure 1. Temporal distribution of alexandrite laser pulse. ... ... 1843 application of ICG. The diameters of the laser-induced craters as well as the residual lesions were determined by use of a reticule within average temperature rise in pulpal tissue and to assess the the microscope. The depth was measured by means of the impact of the laser beam on the pulp of the tooth. This study micrometer gauge on the microscope (Nikon Biological utilized two extracted teeth that were fitted with Microscope Labophot), focused on the surface surrounding thermocouples (0.005 in, Omega type K) inserted into the the crater. The focal depth was then changed to focus on the pulp chamber through holes drilled in the bottom of the bottom of the hole (accuracy, + 0.7 ,um). Each measurement tooth root. In these experiments, the dye was applied via was repeated three times. Based on repeated measurements, microsyringe, and the alexandrite laser was pulsed at 0.25 a maximum estimate of the error in measurement was Hz. determined to be 0.5 mm (SD). The change in focal length To examine the surface morphology of the treated was determined by the micrometer gauge on the fine-focus groups, we removed the coronal portions of the teeth (both knob of the microscope, and this change was manifested as enamel and dentin) from the remaining tooth structure a depth measurement. with a slow-speed diamond saw (Buehler, Ltd., Lake Bluff, To determine the effect of laser ablation on the IL). These coronal remnants were air-dried for 24 h and mechanical properties of surrounding tissue, we performed individually attached to mounted aluminum stubs with Knoop hardness tests on tooth samples after the grinding cyanoacrylate glue (Loctite Corp., Cleveland, OH) and silver and polishing were complete. Determination of the sample paint (Fullam, Inc., Latham, NY). Each remnant was lightly site for hardness testing was based on the appearance and coated with gold (Hummer VII, Anatech, Ltd, Alexander, orientation of the tooth surface. The Leco DM-400FT Knoop VA) to a 21-nanometer thickness, and the lased enamel Hardness tester was used for hardness measurements. Once surface was viewed under scanning electron microscopy the indentation had been made, the distance between the (JEOL JSM 35CF, Tokyo, Japan) at 130 and 600X two end-points of the indentation was measured with magnifications so thatthe differences in surface quality calibrated pointers. Calibration was done once for every among the laser-prepared cavitations could be observed. tooth so that accuracy between teeth could be ensured. Prior to laser irradiation, hardness was measured in at least five Results and Discussion different spots within the area to be irradiated. The measurements were then repeated in approximately the Rather than compare Nd:YAG with alexandrite laser, this same area after irradiation. This allowed us to determine study tested the general feasibility of dye-enhanced pulse whether the laser treatment had effected a change in laser ablation of dental hard tissue. Ablation depths were hardness on the enamel surface. Each sample was measured measured for three concentrations of the ICG for the for hardness in five different locations on the polished alexandrite laser ablation of the teeth. An energy level of surface of the tooth with a 200-g load. By measuring approximately 700 mJ/pulse delivered at the tooth surface hardness several times on the surface of each tooth, we was used in each case. Six different measurements were could obtain a more accurate average hardness value, since made for each concentration. The cavitations were typically enamel hardness is known to vary with tooth position. 0.6 mm in diameter and approximately i to 1.5 mm in depth, We conducted temperature studies to monitor the with the variation due to the differing concentrations of dye Downloaded from jdr.sagepub.com by guest on July 11, 2011 For personal use only. No other uses without permission. 1844 Jennett et al. J Decnt Res 7.3(12) 1994 ..B 0** - r r E -7 1.5- -aD.lb. T !.u. -1i / - 1- 0.5- Dil.-of 0- 1.0% 1.5% Uleal ,oft ICG Concentration Figure 3. Schematic representation of thc lasci clII cct on thc tooth surf ace. Figure 2. The el fect of ICG concentration on the total depth ol the laser cavitation (Fig. 2). The deepest crater was obtained with the 1.5% ICG the pulsed Nd:YAG laser. The role of the dye in pulsed laser concentration. These craters did not show any evidence of ablation is important in the localization of temperature and carbonization under the microscope, later verified by SEM. heat from the laser light. The combination of the short Specific to thisexperiment, one of the absorption peaks pulsed laser and the dye absorption on the tooth surl'ace of ICG lies directly in the alexandrite wavelength spectrum. appears to provide clean and efIficient ablation ofenamel. This maximizes the confinement of the laser light in the Hardness tests were performed on every tooth sample enamel at the site of ICG placement. Similar effects are seen (Table), including means and standard deviations on both with the India ink dye-enhancement in combination with the lased and non-lased sides of the tooth. Each hardness Table. Knoop hardness mneasurementsfor samples before and afterlaser irradiation Sample Number Mean Base Hardness(+ SD) Mean Hardness afterl aser(+ SD) Percent Chanige 337 + 6.69 324 + 16.97 3.9 2 337 + 21.48 307 + 2001 8.7 3 359 + 29.37 326 + 19.11 8.9 4 320 + 22.10 341 + 17.78 + 6. 5 331 + 20.74 322 + 22.87 2.7 6 332 + 29.52 306 + 26.67 7.8 7 319 + 14.90 306 + 6.38 4.0 8 335 + 5.22 314 + 6.69 6.0 9 359 + 17.92 326 + 12.50 9.1 10 327 + 24.83 319 + 16.76 2.3 11 322 + 6.28 302 + 1277 6.3 12 346 + [2.54 314 + 16.54 9.2 13 348 + 22.10 308 + 12.92 -116( 14 324 + 1620 295 + 6.31 9.2 15 325 + 19.58 299 + 18.35 7.9 16 353 + 5.63 303 + 9.04 -14.0 17 361 + 1345 311 + 8.35 -13.9 18 314 + 6.88 318 + 19.46 +1.1 19 318 + 13.01 295 + 13.46 7.5 20 327 + 11.84 334 + 13.60 + 2.0 Average 335 + 1601 314 + 14.83 -62 Downloaded from jdr.sagepub.com by guest on July 11, 2011 For personal use only. No other uses without permission. jDcnt Res 73(12)1994 Dye-enhanced Ablation of Dental Hard Tissue 1845 . . . 5.-1. I I I I I I- 4- 3- without dye 2- 1- 0- I I I I 1*I 4. I. 5t) 1()o -l() Time (seconds) 2Y00 2S0 Figure 4. Temperature measurements of the dye enhanced alexandrite showing eflective elimination of temperature rise at the pulpal cavity. Similar results were seen with the Nd.YAG laser enhanced with India ink. Figure 5. SEM of the interior surf ace of the dyc-cnhan(ccd value represented in the Table is an average of the five alexandrite laser preparations (mag. 600X) measurements taken on each tooth. Fig. 3 shows schematically the regions of ablation, an annular 'soft' region, and the region where hardness tests were performed show that the superficial surf ace of the annulus on the lased samples. A statistical analysis was performed (t surrounding the ablated enamel appears to be a test of dilfferences) on the hardness data before and after recrystalized enamel beaded surface, which could be laser irradiation. This analysis suggests that the hardness of removed with a hand-held dental instrument. The interior the enamel surrounding the irradiated zone decreased sides of the crater contain what appears to be recrystalized significantly. The 95% confidence interval for these data did molten material which has re-precipitated into the crater not include zero, and therefore, the difference between after irradiation. The SEM studies reinforce the idea that mean values was statistically significant (p = 0.00001). The irradiation with the alexandrite laser does not cause any percent change in the mean values was a decrease of 6.16% apparent thermal damage to the surrounding tissue. It is in the lased enamel as compared with the non-lased enamel. not currently known how these changes would affect However, the clinical importance of this decrease in enamel resin-bonded restorations in clinical practice. Since this hardness is uncertain. Generally, since enamel hardness can study was meant to demonstrate a new technique in hard- vary between 250 and 500 on any given tooth, a variation tissue 'optical drilling', resin-bonding studies have not yet decrease of 6.16% in a bounded area around the irradiated been conducted. Future studies with these types of dye- cavitation may not be clinically significant. Long-term laser combinations with more clinical relevance are being studies will be needed to substantiate the clinical plan ned. significance of these findings. The presence of re-precipitated crystals has been Cavitations resulting from the dye-enhanced alexandrite previously reported in laser ablation of dental hard tissue laser preparations were from about I to 1.5 mm deep. During (Featherstone and Nelson, 1987; Feuerstein et al., 1992). these preparations, temperature recordings showed no Vahl (1968) had shown with x ray diffraction that the significant change in temperature at the pulp cavity (Fig. 4). material that re-precipitated onto the surrounding surface The control test shown was a tooth in the same situation; of the irradiated crater was tricalcium phosphate or other however, no dye was applied. Without dye, no cavitation high-temperature phases. This new material is a occurred; however, there was a temperature rise of from 3 to dehydrated form of hydroxyapatite (Vahl, 1968). It is 4 C at the pulp cavity. These temperature measurements possible that the re-precipitated crystals seen with the dye give an estimate of the average temperature at the pulp enhanced alexandrite preparations are also tricalciuin cavity. Transient temperature at the pulp could have more phosphate (Kuroda and Fowler, 1984). This would explain variations, however, these experiments were conducted in a the softer area in the surrounding annulus and would Iworst case scenario', i.e., in air. Without-the surrounding indicate that temperatures above the tooth surface were heat dissipaters (blood, gingiva), as in these experiments, the extremely high. average and transient temperature increases should An SEM photograph (Fig. 6) is typical of the results theoretically be much higher than if the same experiment obtained in these dye-assisted ablations by India ink and were conducted under in vivo conditions. However, in vivo the pulsed NdLYAG laser. Application of 50 pulses generally experiments will be needed to verify this. produced cone-shaped craters about 0.5 to 1 mm deep. The SEM studies were done for both alexandrite and cut surfaces of the tooth were free of shattering or cracking, Nd:YAG experiments. Alexandrite SEM studies (Fig. 5) but there was a fine "spray" of material (presumably a Downloaded from jdr.sagepub.com by guest on July 11, 2011 For personal use only. No other uses without permission. 1846 jennctt et al. J Dent Res 73(12)1994 and superheating steam trapped within the matrix of the hydroxyapatite. This theory is based on the large disparity between the vaporization temperature of trapped water and that of hydroxyapatite. Here, when the pressure exceeds the tensile strength of the tooth, material is ablated. With these theories, one can only estimate the actual method of ablation occurring with the dye-enhanced pulsed lasers. Further work will determine the mechanism of ablation and the overall effects on the tooth. Preliminary work has begun on the effects of acoustic and plasma waves generated during dye-enhanced laser ablation of enamel. Further experiments will determine the effects of laser-induced acoustic waves on the structural integrity of the tooth. This series of experiments has shown that the use of the dye-enhanced pulsed laser for tooth cavitation is feasible and could be developed into a practical method Figure 6. SEM of the dye-enhanced Nd YAG laser preparations. for cavity preparation. We demonstrate that the pulsed The interior surl ace of this preparation is bumpy and rough, nature of the laser is primarily responsible for clean and similar to that seen in Fig 5 (mag 130X). efi icient ablation. Furthermore, preliminary studies do not indicate a significant rise in temperature within the recondensed residue of previously molten hydroxyapatite) tooth structure during such ablation. There was a small that coated the inner surface of the crater and made a very measurable decrease (about 6%) in hardness in an fine (100 pm) rim on the enamel surface surrounding the annulus surrounding the crater; however, the clinical and crater. medical importance of such a decrease is yet to be For a clinically acceptable system to be created, injury determined. Work is in progress to determine the actual must be minimized and eff'iciency of the laser drilling mechanism of ablation and optimization. Development of process maximized. The mechanism or combination of a dye-laser handpiece is currently under consideration. mechanisms contributing to ablation of enamel must be The advent of such systems would allow for easy understood. Several explanations are being explored. In integration within the dentist's office by utilization of dye-enhanced pulsed ablation, explosive vaporization of suctions and water sprays to remove excess material water may contribute. It is theorized that the water in the during the preparation process, much like those used ICG solution superheats with exposure from the laser today in conventional drilling. These results demonstrate light, explosively expands in a plume that recoils, causes clinical relevance and offer a major reason to pursue this a counterforce, and, in return, removes material from the novel approach in dental research. tooth (Hibst and Keller, 1993). Theoretically, this recoil effect would not cause thermal damage to the adjacent Acknowledgments tissue, since ablation occurs above the surface of the tooth. This work was supported in part by a grant from the Cavitations due to vaporization effects seen with other Department of Energy (DOE), and in part by NIH grant high-powered lasers can also be produced with the dye- #1R43 DE10687. enhanced pulsed laser (Keller and Hibst, 1989; Vickers et al, The authors would like to thank David Ladd of the 1992; Hibst and Keller, 1993; Venugopalan et al, 1993). If the University of Texas Houston Health Science Center, Dental rate of' vaporization becomes sufficiently high, then the Branch, for the use of the mechanics laboratory and Knoop resulting shock waves can introduce another significant hardness tester for preparation of samples and hardness ablation mechanism. As the ICG is ablated, an upwardly testing. Thanks also to Michael Coy of the Electron active force is complemented by a reactive force down onto Microscopy Center, Texas A&M University, for his expertise the tooth. The magnitude of this reactive force is dependent and advice with the SEM studies. on the mass removal rate and the speed at which the Appreciation is also extended to Dr.Edward Mallia for vaporized dye leaves the surface. The pressure exerted by providing tooth samples for experimentation and to Dr. the shock wave can be estimated by assuming that the Duncan MacFarlane for useful discussion on the theory temperature of the ablated material is the vaporization behind ablation mechanisms. temperature. When this pressure exceeds the compressive This paper is based partially on a thesis submitted to the strength of the tooth, material is ablated. graduate faculty, Texas A&M University, in partial fulfillment A third contributory mechanism could involve heating of the requirements for the Master of Science degree. Downloaded from jdr.sagepub.com by guest on July 11, 2011 For personal use only. 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