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Title
Influence of output energy and pulse repetition
rate of the Er:YAG laser on dentin ablation.
Author(s)
Igarashi, A; Kato, J; Takase, Y; Hirai, Y
Journal
Photomedicine and laser surgery, 26(3): 189-195
URL
http://hdl.handle.net/10130/3560
Right
This is a copy of an article published in the
Photomedicine and laser surgery ?2008 copyright
Mary Ann Liebert, Inc.; Photomedicine and laser
surgery is available online at:
© Mary Ann Liebert, Inc. Pp. 189–195
DOI: 10.1089/pho.2007.2123
Influence of Output Energy and Pulse Repetition Rate of
the Er:YAG Laser on Dentin Ablation
Akihiro Igarashi, D.D.S., Ph.D., Junji Kato, D.D.S., Ph.D., Yasuaki Takase, D.D.S., Ph.D.,
and Yoshito Hirai, D.D.S., Ph.D.
Abstract
Objective:
We sought to improve the efficiency of dentin ablation with the Er:YAG laser by investigating the
effects of output energy and pulse repetition rate on ablation.
Background Data:
The Er:YAG laser is superior to other lasers in ablating dental hard tissues. However, the
factors affecting the efficiency of ablation with an Er:YAG laser remain unclear.
Methods:
Fifty bovine root dentin plates were irradiated with an Er:YAG laser at an output power of 1.0 W,
1.5 W, or 2.0 W under a water spray while moving the plate at 1 mm/sec. After irradiation, the depth and
vol-ume of each ablated site were measured by laser microscopy and the ablated surfaces were examined by
scan-ning electron microscopy.
Results:
The output power showed a strong positive correlation with the depth and volume of ablation. The
output energy had much more pronounced effects on the depth and volume of ablation compared to the pulse
repetition rate. The shape of the ablated site varied with the output power, and no cracking or vitrification was
observed under the irradiated dentin. The most effective parameters for dentin ablation were an output power
of 2.0 W, with an output energy of 80 mJ/pulse at 25 pulses per second (pps) or 100 mJ/pulse at 20 pps.
Conclusion:
These findings suggest that the output energy is the main factor affecting the efficiency of dentin
ablation with an Er:YAG laser. We propose that the efficiency of dentin ablation can be improved by choosing
an optimal combination of output energy and repetition rate.
189 Introduction
T
HE ERBIUM:YTTRIUM ALUMINUM GARNET (Er:YAG) laser, atechnology developed by Zharikov et al.1 in 1974, has a
wavelength of 2.94 m and superior water absorption effi-ciency. The laser reacts with hydroxyl groups in water mol-ecules and causes explosions at the molecular level.2 When
dentin is irradiated by the Er:YAG laser, the laser energy is absorbed by water inside the dentin tissues and vaporiza-tion of the water occurs, with a rapid increase in the water’s volume that induces a small explosion that removes the hard tissue.3–5 This ablation mechanism does not damage dental
pulp and the relatively low heating level almost completely eliminates the risk of thermal damage, a common problem during ablation with other lasers.3,6,7 Due to these
advan-tages, the Er:YAG laser is increasingly being applied in clin-ical practice.8,9 Lasers produce far less sound than rotating
cutting devices and do not produce sonic dissonance during cutting. Lasers also generate only minimal vibration during
cutting, and there is little discomfort or unpleasant sensation for the patient.8–10 Thus the Er:YAG laser is a favorable
op-tion for treatment of adults and children who may fear a visit to the dentist.8,9
There have been calls for improvement in the ablation ef-ficiency of the Er:YAG laser, because the laser ablates tooth structures more slowly than conventional rotating cutting devices.5,8,11 The goal is to increase the output energy of
Er:YAG irradiation while reducing the pulse repetition rate.12–15 However, the optimal conditions for improving
ab-lation efficiency require reconsideration, since recent im-provements in laser devices enable laser irradiation with higher pulse repetition rates. The output energy and pulse repetition rate influence the depth and volume of ablation, but only a few studies have considered the effects of these variables on ablation efficiency.15,16 Therefore, we examined
the dental ablation efficiency of the Er:YAG laser by inves-tigating the effects of varying the output energy and pulse repetition rate on the depth and volume of ablation of dentin.
Department of Operative Dentistry, Tokyo Dental College, Chiba, Japan.
-Materials and Methods
Specimen preparation and irradiation conditions
Preparation of dentin plates. Fifty lower anterior bovine teeth were used as specimens in the study. Each tooth was cut at the crown cervix and then divided at the root in a lin-gual-buccal direction. Next, the root lateral faces were indi-vidually polished with a grinder (400 and 800 grit) and wa-terproof abrasive paper (800 and 1200 grit). After grinding, each specimen was cut one-third of the way into the root apex side to prepare a dentin plate in which the dentinal tubules ran in almost the same direction.
Laser device. A prototype Er:YAG laser device (J. Morita Mfg. Co., Kyoto, Japan) was used in the study. The laser pro-duces light at a wavelength of 2.94 m with oscillated pulse waves with a pulse duration of 250 s at output energies ranging from 30 to 350 mJ/pulse, and with an adjustable pulse repetition rate (1, 3.3, 5, 10, 20, 25, or 30 pulses per sec-ond [pps]) (Table 1). Given the limits of the output energy (Table 1), the output energies differed for each pulse repeti-tion rate. The Er:YAG laser was fitted with a contact tip (600
m in diameter) for hard tissues (C600F; J. Morita Mfg. Co.). Irradiation method and conditions. The fifty dentin plates were irradiated on a movable table (S-46 Controller; Chuo Precision Machine Co., Tokyo, Japan) to obtain homogenous irradiation. The plate was fixed to the table with the grind-ing face oriented horizontally to the tabletop, and the con-tact tip was positioned 0.05 mm from the plate surface (dis-tance was measured using a height gauge; Futaba Co., Tokyo, Japan) to minimize diffusion of the laser energy un-der the water spray. The plate was moved at 1 mm/sec, which is a clinically applicable speed that allows sufficient irradiation of the dentin.
Irradiation conditions with different combinations of out-put energy and pulse repetition rate were used at a constant output power, as shown in Table 2. A total of ten irradiation conditions were used: four at an output power of 1.0 W (1.0 W group), four at 1.5 W (1.5 W group), and two at 2.0 W (2.0 W group). The output energy was set at 150 mJ/pulse based on the results of a preliminary experiment, in which an output energy of 150 mJ/pulse produced a crack in the dentin surface. The pulse repetition rate ranged from 10–30 pps, since a rate of at least 10 pps is required to ensure con-tinuous ablation of the dentin, based on results of the pre-liminary experiment. Pulse repetition rates of 30 pps had to be excluded from the 2.0 W group to stay within the upper
limit of output energy specified for the prototype device. The output energy delivered at the contact tip was measured be-fore irradiation using a LabMaster Ultima (Coherent, Santa Clara, CA, USA) and corrected when necessary. Water was infused at a rate of 2 mL/min. Five different dentin plates were irradiated under each irradiation condition. Before ir-radiation, the target site on each plate was examined at 10 and 20 magnification with a stereoscopic microscope (YSTB-2E; Nikon Co., Tokyo, Japan) to confirm the absence of natural cracks.
Measurement of ablation depth and volume
After laser irradiation, the specimens were washed with an ultrasonic washer for 5 min to prevent accumulation of ablated fragments at the ablated site. Subsequently, the spec-imens were dried and examined under a laser microscope (VK-8500; Keyence Co., Osaka, Japan) to measure the depth and volume of ablation. Ablation depth was measured at the center of each laser-irradiated area and is reported for each plate as the mean depth of six irradiated areas, the maximum number of areas formed on one plate. Ablation volume was determined in a region with a radius of approximately 300
m around the center of each laser-irradiated area, and the
reported volume is the mean of six irradiated areas on one plate. In practice, ablation depth was measured as the dis-tance of a virtual line from the surface of the specimen to the deepest point after creation of cross-sections of the ablated region. The measurement approach is shown in Fig. 1. The volume was measured over a diameter of 600 m using three-dimensional images created from these results, and was defined as the ablation volume.
Statistical analysis
A Spearman rank correlation test was used to evaluate cor-relations between output power and ablation depth, output power, and ablation volume, and depth and volume of ab-lation (with p 0.05 indicating a significant correab-lation). Sta-tistical analyses were performed within the 1.0 W (four ir-radiation conditions), 1.5 W (four irir-radiation conditions), and 2.0 W (two irradiation conditions) groups to determine how the output energies and pulse repetition rates
influ-IGARASHI ET AL. 190
TABLE 1. CHARACTERISTICS OF THE LASER DEVICE
Laser type Er:YAG laser
Wavelength 2.94 µm
Pulse duration 250 µs
Limit of output energy at 180 mJ/pulse (10 pps) each pulse repetition rate 100 mJ/pulse (20 pps) 80 mJ/pulse (25 pps) 50 mJ/pulse (30 pps) Pulse repetition rates 1, 3.3, 5, 10, 20, 25, 30 pps
TABLE 2. IRRADIATION CONDITIONS
Energy per Energy
Output pulse and Power density
power pulse repetition density per pulse (W) rate (mJ/pulse, pps) (W/cm2) (J/cm2) 1.0 W 33, 30 354 12 1.0 W 40, 25 354 14 1.0 W 50, 20 354 18 1.0 W 100, 10 354 35 1.5 W 50, 30 530 18 1.5 W 60, 25 530 21 1.5 W 75, 20 530 26 1.5 W 150, 10 530 53 1.5 W 80, 25 708 28 1.5 W 100, 20 708 35
enced the depth and volume of ablation at a constant out-put power. These comparisons were conducted by one-di-mensional analysis of variance (ANOVA) and the Scheffe method, with p 0.05 considered to indicate significance. StatView ver. 5.0 (SAS Institute, Cary, NC, USA) was used for all analyses.
Scanning electron microscopy
After measurements by laser microscopy, the specimens were dried, cut in a direction vertical to the ablated surfaces, dehydrated with an alcohol series using a conventional method, freeze-dried, and examined with a scanning elec-tron microscope (SEM) to determine the shapes of the sec-tions and ablated surfaces. Examination was performed with Au-Pd deposition using a JSM-6340F scanning electron mi-croscope (Japan Electron Co., Tokyo, Japan). SEM pho-tographs were taken at magnifications of 100 to determine the shape of the ablated site and at 1000 to examine the ir-radiated surface.
RESULTS
Depth and volume of ablation
Ablation depths and volumes are shown in Tables 3 and 4, respectively. The depth and volume tended to increase at higher output energies and lower pulse repetition rates in each group. Table 5 shows the statistical differences in abla-tion volumes among irradiaabla-tion condiabla-tions according to the Scheffe test. The ablation volume was significantly greater in the group irradiated at high output power and high en-ergy density.
Statistical analysis
Linear regression analysis of the relationship between the depth and volume of ablation gave the equation y 2.9196x – 1.5506 with a correlation coefficient of 0.93, indicating a strong positive correlation (Fig. 2). A similar analysis of the relationship between output power and ablation depth gave the equation y 3.2043x – 0.91 with a correlation coefficient TABLE 3. ABLATION DEPTH UNDER EACH
IRRADIATION CONDITION
Output Irradiation Mean
power condition (102 m) (W) (mJ/pulse, pps) SD n 1.0 2.33 0.65 n 20 33, 30 1.60 0.06 n 5 40, 25 2.26 0.03 n 5 50, 20 2.42 0.10 n 5 100, 10 2.77 0.16 n 5 1.5 3.84 0.49 n 20 50, 30 3.12 0.18 n 5 60, 25 3.18 0.05 n 5 75, 20 3.34 0.04 n 5 150, 10 4.25 0.17 n 5 2.0 5.56 0.89 n 10 80, 25 4.45 0.03 n 5 100, 20 5.51 0.46 n 5
FIG. 1. Images and measurements from laser microscopy and three-dimensional imaging.
TABLE 4. ABLATION VOLUME UNDER EACH
IRRADIATION CONDITION
Output Irradiation Mean
power condition (107 m3) (W) (mJ/pulse, pps) SD n 1.0 5.43 1.76 n 20 33, 30 3.71 0.27 n 5 40, 25 5..64 0.49 n 5 50, 20 6.80 0.98 n 5 100, 10 7.22 0.26 n 5 1.5 9.01 1.88 n 20 50, 30 6.85 0.86 n 5 60, 25 8.63 0.38 n 5 75, 20 8.33 5.58 n 5 150, 10 11.78 1.37 n 5 2.0 15.57 2.52 n 10 80, 25 12.58 0.83 n 5 100, 20 15.89 0.43 n 5
of 0.88, also indicating a strong positive correlation (Fig. 3). Analysis of the relationship between output power and ab-lation volume gave the equation y 9.7107x – 4.705 with a correlation coefficient of 0.87, also indicating a strong posi-tive correlation (Fig. 4).
Morphologic examination of the surface and cut section by SEM
The irradiated dentin had a rough surface with opened denti-nal tubules, but without cracking, vitrification, or charring. Sec-tions cut under different irradiation condiSec-tions could be roughly classified into three shapes: a box type (Fig. 5), a polyg-onal type (Fig. 6), and a bowl type (Fig. 7). The sections were mainly of the box type in the 1.0-W group, of the polygonal type in the 1.5-W group, and of the bowl type in the 2.0-W group (Table 6). No cracking or vitrification attributable to the laser radiation was observed just under the irradiated dentin in SEM images taken at 1000 magnification.
Discussion
More than 15 years have passed since the Er:YAG laser was introduced for cavity preparation in dentistry.3,12 Over this
pe-riod, a number of problems have emerged with regard to the
efficiency with which the laser cuts enamel and dentin.17 Most
attempts to increase the output energy of the laser have been motivated by the need for improved ablation efficiency,18–21
but little is known about the effects of variable irradiation con-ditions on ablation efficiency. Therefore, we assessed the ef-fects of output power, output energy, and pulse repetition rate on the amount and depth of ablation of dentin to identify the clinically optimal ablation conditions.
The output power was positively correlated with the depth and volume of ablation. These results are consistent with conventional studies12,13 that have reported increased
ablation efficiency with an increased energy level per unit time (output power) in the irradiated dentin. We also in-vestigated how the same output power in irradiation with variable combinations of output energy and pulse repetition rate influenced the efficiency of ablation. In the 1.0-W group, ablation depth and volume both increased as the output en-ergy increased. This suggests that the depth and volume of ablation increase under irradiation at larger output energies delivered at lower pulse repetition rates. Only two signifi-cant differences were found in comparisons among the four irradiation conditions: between 33 mJ/pulse, 30 pps and 50 mJ/pulse, 10 pps; and between 33 mJ/pulse, 30 pps and 100 mJ/pulse, 10 pps. These findings show that the ablation
IGARASHI ET AL. 192
TABLE 5. STATISTICAL DIFFERENCES IN ABLATION VOLUMES AMONG VARIOUS IRRADIATION CONDITIONS
mJ/pulse, pps 33, 10 40, 25 50, 20 100, 10 50, 30 60, 25 75, 20 150, 10 80, 25 100, 20 33, 10 * * * * * * * * 40, 25 * * * 50, 20 * * * * * * 100, 10 * * * * 50, 30 * * * * 60, 25 * * * * * 75, 20 * * * * * 150, 10 * * * * * * * * 80, 25 * * * * * * * * 100, 20 * * * * * * * * * *p 0.05.
FIG. 2. Relationship between the depth and volume of ab-lation.
FIG. 3. Relationship between output power and ablation depth. — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —
depth and volume increase markedly when the output en-ergy exceeds 50 mJ/pulse at an output of 1.0 W.
The ablation depth and volume in the 1.5-W group also tended to increase as the output energy increased and the pulse repetition rate decreased. However, the only signifi-cant difference in depth and volume occurred for the 150 mJ/pulse, 20 pps irradiation condition compared to the other three conditions. Thus it is unclear how the output energy and pulse repetition rate influence the depth and volume of ablation in the 1.5-W group for irradiation delivered at 50–75 mJ/pulse. At an output energy of 150 mJ/pulse, the depth and volume clearly increased and ablation was improved. Only two irradiation conditions, 80 mJ/pulse, 25 mJ and 100 mJ/pulse, 20 pps, were examined in the 2.0-W group, and the ablation depth and volume both differed significantly be-tween these conditions. At higher output power, a slight in-crease in output energy led to conspicuous changes in the depth and volume, and ablation of dentin occurred more eas-ily with increased output energy.
Collectively, these findings confirm that the output power is important for improving ablation efficiency, and that the ablation efficiency is influenced by the combination of pa-rameters selected for the output power (i.e., the combination of the output energy and pulse repetition rate).15,16
The ablated sites were divided into box, polygonal, and bowl types. In the box type, there was little cutting and the cutting depth was shallow. The polygonal type had an in-creased cutting depth compared to the box type, and the cut-ting depth and amount of cutcut-ting were largest in the bowl type. Changes in output power resulted in a change in the ablated shape from a box type to either a polygonal type or a bowl type, and at a fixed output power the ablated shape was influenced by the output energy, but not by the pulse repetition rate. Even at a high pulse repetition rate of 20 or 30 pps, the ablation efficiency did not improve if the output energy remained below a certain level. In contrast, at a low pulse rate, output energies of all levels had a major influence on the ablated shape. Thus the output energy had the main influence on ablation of hard tissue under our experimental conditions.
When the Er:YAG laser is used for cavity preparation, a high efficiency of dentin ablation, reduced pulp damage, and
reduced damage to dentin are required for achieving strong adhesion in composite restoration. Keller et al.3,8,12
recom-mended that Er:YAG laser irradiation for cavity preparation should be performed at a high output energy and a low fre-quency (pulse repetition rate) to obtain high ablation effi-ciency without pulp damage, and also commented that high-frequency laser irradiation may cause pulp damage due to accumulation of heat produced by the laser;19–21 therefore,
conditions of 150–300 mJ/pulse with a frequency of 1–2 Hz for dentin and 200–400 mJ/pulse with a frequency of 2–4 Hz for enamel were proposed for use of an Er:YAG laser in
FIG. 4. Relationship between output power and ablation volume.
FIG. 5. Morphologic presentation of a cut section by SEM, showing a typical box type ablation at 33 mJ/pulse and 30 pps in the 1.0-W group. The box type was only observed in the 1.0-W group. The dentin was ablated along the dentinal tubules. There was no cracking or charring just under the ir-radiated dentin.
FIG. 6. Morphologic presentation of a cut section by SEM, showing a typical polygonal type ablation at 60 mJ/pulse and 25 pps in the 1.5-W group. The polygonal type was ob-served in the 1.0-W and 1.5-W groups. The dentin was sharply ablated along the dentinal tubules. There was no cracking or charring just under the irradiated dentin.
dentin ablation in clinical practice.8 Many other studies have
supported these conclusions in reporting use of the Er:YAG laser for cavity preparation.6,11,14,15,19–21 However, for use of
the Er:YAG laser in the contact mode, an output energy of 100–150 mJ/pulse at a pulse repetition rate of 10 pps for dentin ablation and 200–300 mJ/pulse at 10 pps for enamel ablation have been proposed.5,7,9,17,22 Jayawardena et al.23
suggested that the pulses nature of laser energy may be transmitted through the dentinal tubules in the form of pho-tomechanical shocks that disturb the network of blood ves-sels in the outermost area of the pulp; therefore, a high out-put energy may cause more pulp damage compared to a low output energy.
Délme et al.16 commented that to obtain a more retentive
surface without cohesive microfractures, it is advisable to ap-ply an output energy of less than 200 mJ for dentin and enamel when using the Er:YAG laser,16,24,25 since a higher
output energy produces vitrification of the dentin sur-face.16,25 A rough surface may contribute to adhesion of resin
to dentin, but the presence of fragments and microfractures has adverse effects on the adhesion of resin by decreasing the bonding quality.16,24–26 Hence, there are many opinions
on the appropriate conditions of Er:YAG laser irradiation for dentin or enamel ablation, including differences regarding the type of Er:YAG laser and various laser parameters: the
amount of energy per pulse, the pulse repetition rate, the pulse duration, distance from the laser to the experimental surface, the time of irradiation (exposure of the surface to the laser), and use of a contact or non-contact mode.16,25,27
In this study, we used the Er:YAG laser in the contact mode (a contact tip 0.6 mm in diameter) with a water spray at a rate of 2 mL/min and different combinations of output energy and pulse repetition rate. The laser-irradiated dentin plate was moved at a speed of 1 mm/sec. Output powers of 2.0 W (100 mJ/pulse, 20 pps and 80 mJ/pulse, 25 pps) and 1.5 W (150 mJ/pulse, 10 pps) yielded high-efficiency dentin ablation under our experimental conditions. Cutting effi-ciency was particularly improved in the 2-W group by in-creasing the pulse repetition rate at a low output energy, re-sulting in little damage to the dental pulp through the impact of irradiation on the dentinal tubules, compared to irradia-tion at high output energy. Moreover, these condiirradia-tions pro-duced a rough dentine surface with opened dentinal tubules without cracks or vitrification. The damage to the dentin or pulp caused by accumulation of heat produced by raising the pulse repetition rate may be prevented by supplying an appropriate volume of water during laser irradiation.28
Therefore, irradiation using a low output energy and a high repetition rate may give better adhesion of resin to irradi-ated dentin compared with irradiation at high output en-ergy.
Finally, we note that similar cutting depths of dentin at the same output energies and pulse repetition rates (100 mJ/pulse at 10 pps and 150 mJ/pulse at 10 pps) were ob-tained in our study and in that of Délme et al.,16 even though
the irradiation method (contact or non-contact mode, irradi-ation spot size, and scanning speed during irradiirradi-ation) dif-fered between the two studies. Therefore, it seems likely that the power density or energy density was similar in these studies, although these data were not given in Délme et al. Based on our results, we emphasize the importance of choos-ing the correct irradiation parameters in uschoos-ing an Er:YAG laser for cavity preparation.
Conclusion
In this study we used a prototype Er:YAG laser to inves-tigate the effects of irradiation conditions such as output en-ergy and pulse repetition rate on the depth and volume of ablation. Our findings support the following conclusions: (1) the output power showed a strong positive correlation with the depth and volume of ablation; (2) the output energy had much stronger effects on the depth and volume of ablation compared to the repetition rate; (3) the shape of the ablated site varied with the output power, and no cracking or vitri-fication attributable to the laser radiation was observed just
IGARASHI ET AL. 194
TABLE 6. SHAPE OF THE ABLATED FACES
Output power 1.0 W 1.5 W 2.0 W Irradiation condition 33, 30 40, 25 50, 20 100, 10 50, 30 60, 25 75, 20 150, 10 80, 25 100, 20 (mJ/pulse, pps) Box • • Polygonal • • • • • Bowl • • •
FIG. 7. Morphologic presentation of a cut section by SEM, showing a typical bowl type ablation at 100 mJ/pulse and 20 pps in the 2.0-W group. The bowl type was observed in the 1.5-W and 2.0-W groups. The dentin was deeply ablated along the dentinal tubules. No cracking, charring, or vitrifi-cation was observed just under the irradiated dentin.
under the irradiated dentin; and (4) among the various irra-diation conditions investigated in this study, the optimal condition for ablation of dentin is an output of 2.0 W, with an output energy of 80 mJ/pulse at a pulse repetition rate of 25 pps or 100 mJ/pulse at 20 pps.
Acknowledgments
We would like to thank Mr. Katsumi Tadokoro (Oral Health Science Center, Tokyo Dental College) for his tech-nical advice and our colleagues for their advice. We also thank J. Morita Mfg. Co. for providing the laser devices.
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Dr. Akihiro Igarashi Department of Operative Dentistry Tokyo Dental College 1-2-2, Masago, Mihama-ku Chiba, 261-8502, Japan E-mail: igarasia@tdc.ac.jp
