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Effects of a carbon dioxide laser on human root dentin
0099-2399/95/2101-0004/$03.00/0 JOURNALOF ENDODONTICS Copyright © 1995 by The American Association of Endodontists
Printed in U.S.A. VOL. 21, NO. 1, JANUARY1995
Effects of a Carbon Dioxide Laser on Human Root Dentin Russ P. Read, DOS, J. Craig Baumgartner, DDS, PhD, and Stephen M. Clark, DMD
constant. This results in the effect of the laser being contained in a very superficial layer of the target tissue with minimal effect on surrounding tissue (2). Protection of surrounding "nontarget" tissue may be of considerable importance. Bahcall demonstrated damage to the periodontal ligament of dog teeth treated with a Nd:YAG laser. This damage resulted in an increased incidence of ankylosis, cemental lysis, and bone remodeling (6). Ichesco et al. (7) found that the dentin of apically resected roots was more permeable to fluids than the dentin of nonresected roots. The coronal margin of an apical bevel, near the cementodentinal junction, has - 13,000 dentinal tubules/ mm 2 (8). These dentinal tubules can provide a pathway for leakage to or from the root canal if the root-end preparation and root-end Idling do not extend far enough coronally (9, 10). As the slope of the apical bevel increases, the root-end filling needs to be deeper to seal the resected tubules adequately. It may be impossible to make an apical preparation parallel to the long axis of the root and deep enough to include all the tubules of the resected dentin. Reducing or eliminating the permeability of resected apical dentin would seem advantageous in apical endodontic procedures. The use of laser energy may enhance the success of endodontic apical procedures by reducing leakage through resected dentinal tubules. Pinks and Beatty (11) found that apical dye penetration could be reduced by using a dentin bonding material in the rootend preparation and covering the bevel. There is concern about how periapical tissues respond to these materials and how long they will maintain their seal in the periap~cal environment. Exposing dentin to laser radiation may result in a surface that is less permeable to fluids (4, 12). After laser irradiation, the structure of the recrystaUized dentin resembles that of normal enamel (13). Miserendino (14) reported good results using a CO2 laser for an endodontic apical procedure. Zakariasen et al. (15) have reported on the ability of the CO2 laser to eliminate microorganisms from the root canal. A CO2 dental laser (Luxar LX-20, Bothel, WA) has FDA approval and is currently being used on soft tissue. This laser is highly "tunable," with watt settings from 2 to 20, 10 different modes (combinations of pulse width and pulse repetition rate), or continuous power. Bonin et al. (16) showed that different energy levels of the CO2 laser (Satelec, France) had different effects on the permeability of dentin. Tani and Kawada (5) showed that irradiating bovine dentin with a CO2 laser (Mochida Pharmaceutical Co., Japan) in defocus mode
The effects of the Luxar LX-20 CO2 dental laser on resected apical root dentin were examined using stereomicroscopy and scanning electron microscopy. The surfaces of 2-mm-thick sections of dentin from freshly extracted human teeth were exposed to CO2 laser radiation. Fluences used ranged from 2.1 to 625.0 J/cm 2. The effects of the laser energy on the dentin ranged from no visible effects, to charring, cracking, cratering, and glazing. The most dramatic effect was cracking. Cracking was evident on all specimens having any visible modification of the dentin. A prototype-curved laser tip was used and compared with a standard straight tip. The curved tip did not deliver laser energy to the dentin as efficiently as a straight tip. CO2 laser radiation did not consistently obliterate dentin tubules.
The clinical use of lasers is becoming more common in the practice of dentistry. Several dental lasers have received FDA approval for use on human soft tissue, and research continues on the applicability of lasers on dental hard tissues (1). The effect of laser irradiation on a tissue is related to the absorption bands of the tissue (2). If the radiation wavelength emitted by a laser coincides with the absorption bands of a particular tissue, then the effect of that laser energy will primarily be on the target tissue with little radiation being transmitted through the tissue. CO2 lasers operate on a wavelength of 10.6 t~m (invisible infrared) (3). Enamel, cementum, and dentin conrain hydroxyapatite, which has absorption bands in the infrared region due to phosphate, carbonate, and hydroxyl groups in the crystal structure. These absorption bands in the 9.0 through 11.0-~tm region coincide closely with the radiation produced by the CO2 laser (4). This implies that the radiation from the CO2 laser should be efficiently absorbed by dental hard tissues. This would produce an effect on the target tissue with minimal effects on surrounding tissues such as the periodontal ligament and bone. This is in contrast with the Nd:YAG laser that is dependent on the opacity of the tissue for its absorption. In translucent dentin, a dye has to be placed on the dentin to achieve adequate absorption of Nd:YAG radiation (5). Pulsed lasers provide a way to increase the peak power density while keeping the energy density or fluence 4
Vol. 21, No. 1, January 1995
Carbon Dioxide Laser
5
resulted in dosing of the tubular orifices in the presence of a smear layer. In contrast, Pashley et al. (17) showed that CO2 laser energy (Sharplan 1020, Laser Industries, Tel Aviv, Israel) actually resulted in increased permeability in coronal dentin by melting away the smear layer resulting in opening of the tubule orifices. In a study using the ArF-193 excimer laser (Lambda Physik, Goettingen, Germany) on vertical sections of dentin from the cementoenamel junction area, the laser was found to remove peritubular and intertubular dentin and "open" the dentinal tubules (18). By observing and studying the physical effects of laser energy on dentin, we can expand our understanding of the laser's effects on dental hard tissues. This understanding will aid in our ability to determine the applicability of not only the CO2 laser, but also new and different lasers yet to be developed. The added "tunability" of the laser used in this study, with its pulsed application, needs to be evaluated for its effect on human root dentin. The purpose of this investigation is to examine the physical changes produced with various modes and watt settings of the Luxar LX-20 CO2 dental laser on cross-sections of resected apical root dentin using the scanning electron microscope and stereomicroscope. MATERIALS AND METHODS
Fie 1. A, Apical dentin specimen with six lases using the standard straight ceramic tip. When viewing the specimen as a clock face, the
1:00 o'clock position was exposed to a fluence of 6.25 J/cm2; 3:00 o'clock position, 12.25 J/cm=; 5:00 o'clock position, 25 J/cm=; 7:00 o'clock position, 62.5 J/cm2; 9:00 o'clock position; 125 J/cm2; and 11:00 o'clock position, 250 J/cm 2. Note glazing and charring in the four higher fluences. Stereomicroscope, original magnification x24. B, Apical dentin specimen with six lases using the prototype curved tip. The same fluences were used in the same positions as the specimen in A (1:00 o'clock and 3:00 o'clock lases had no visible effect, and the 5:00 o'clock lase is just visible). The curved tip appeared to deliver less laser energy to the dentin when compared with the straight tip. A fluence of 62.5 J/cm 2 with the curved tip had a similar effect on the dentin as a fluence of only 12.5 J/cm 2 with the
Freshly extracted human incisors were used in the study. The teeth were sectioned ~/sing an aluminum oxide disc (Dedeco, Long Eddy, NY) on a slow-speed handpiece. Approximately 2-mm-thick cross-sections were made from the apical third of the roots and stored in sterile isotonic saline with 0.2% sodium azide to inhibit bacterial growth. This storage medium was selected because of its minimal effect on the dentin. Each cross-section was cut at 45 degrees to the long axis of the tooth to simulate a beveled root end. A smear layer was produced on the apical sides of the resected dentin sections with wet 600-grit SiC paper (3 M, St. Paul, MN). To simulate in vivo conditions of hydration with dentinal fluid, each specimen was lased "wet" in a pool of isotonic saline with the apical surface up, but not submerged. The laser was used in a single-pulse setting to facilitate evaluation of a specific amount of CO2 laser energy. Six different pulse widths (laser exposure times) at the same wattage were used on each specimen. A clockwise pattern of "lases" was used on each dentin specimen at the: h00 o'clock position, mode 1; 3:00 o'clock position, mode 4; 5:00 o'clock position, mode 7; 7:00 o'clock position, mode 8; 9:00 o'clock position, mode 9; and I h00 o'clock position, mode 10 (Fig. 1, A and B). Specimens were lased at different watt settings ranging from 2 to 15 W. However, only 1 W setting was used for the six "lases" on each specimen. Specimens were marked with a number to indicate what watt setting was used; 1, 15 W; 2, 11 W; 3, 8 W; 4, 6 W; 5, 4 W; and 6, 2 W. Watt settings higher than 15 were not used because of the extensive damage imparted to straight tip. The 9:00 o'clock lase (125 J/cm =) was in transparent dentin and produced noticeably more glaze than the 11:00 o'clock lase that had twice the energy density (250 J/cm2). Note the thick layer of cementum around the periphery of the specimen. The effect of the laser energy appeared to be similar on dentin and cementum in the 9:00 o'clock lase. Stereomicroscope, original magnification x24.
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Journal of Endodontics
the dentin. The laser modes selected were: 1, 4, 7, 8, 9, and 10. These six modes represented all the pulse widths available with the LX-20. In the single-pulse setting, mode l corresponds to a single pulse of laser energy lasting 5 ms; mode 4, l0 ms; mode 7, 20 ms; mode 8, 50 ms; mode 9, 100 ms; and mode I0, 200 ms. Therefore, each consecutive lase represented a pulse approximately twice the duration of the pulse before it. A prototype-curved tip for the laser was evaluated (Luxar). The tip had a 25-degree curve and was made from wave guide tubing. In a clinical setting, this tip would facilitate laser access to a resected root end. The tip had guides on it that permitted placing it exactly 3 m m from the dentin specimens. This is important in that the energy imparted to the target tissue is inversely related to the laser's distance from the target. The guides also made it possible to reproducibly place the tip at 90 degrees to the dentin surface for maximum energy absorption. To compare the energy output of the curved prototype tip with that of a conventional straight tip, two specimens were lased at the same watt setting and modes (Fig. 1, .4 and B). The conventional straight tip was also used at 90 degrees to the surface of the specimens at a distance of 3
FIG 2. This lase, using the curved tip, is a result of a fluence of 333 J/cm =. Note the glazing, cratedng, and multiple fractures when viewed with the stereomicroscope. Odginal magnification x65.
mm.
Specimens were examined with a stereomicroscope (Nikon SMZ-2T; Tokyo, Japan) and a scanning electron microscope (SEM) (JEOL USA, Inc., Peabody, MA). After examination with the stereomicroscope and before SEM evaluation, the dentin specimens were sputter coated with - 5 0 nm of goldpalladium using a Hummer VII (Anatech Ltd., Alexandria, VA). Magnifications from x35 to xS,000 with the SEM and x 15 and x63 with the stereomicroscope were used to evaluate the effect of the laser on the physical modification of dentin. Photomicrographs were made using Polaroid 55PN film for the SEM and Kodak Ektachrome 100 HC for the stereomicroscope.
RESULTS The effect of the laser energy on the resected apical dentin ranged from no visible effect at low energy, to charring cracking, cratering, and "glazing" of the dentin at higher energy (Figs. l (.4 and B) and 2). The most striking result of the laser energy on the dentin were the many fractures that occurred in the target areas. Cracks were apparent not only where there was a cratedng effect from higher energy hits, but also in lower energy lases (Fig. 3). Some of the fractures were up to 15 ~m in width. The areas showing cratefing had a white-glazed center surrounded by a halo of blackened or charred dentin when viewed through the stereomicroscope (Fig. 2). The higher the laser energy, the more cracking and cratefing, and the larger the surrounding charred area. The craters that appeared white and glazed through the stereomicroscope looked very porous and uneven with the SEM (Fig. 4, .4 and B). The craters had holes within them anywhere from 3 to 10 times larger than the open dentinal tubules. Open dentinal tubule orifices could be seen through these holes underneath the glazed areas (Fig. 4B). In several of the lower energy lases, the smear layer remained intact, but there was still marked cracking of the dentin (Fig. 3). When the specimens were viewed with the SEM, charred areas exhibited
FIG 3. This lase, using the curved tip, is a result of a fluence of 83 J/ cm=. Note extensive cracking of dentin even in the absence of smear layer modification. SEM, odginal magnification x200.
dentinal tubules that were more open than the surrounding unlased dentin (Fig. 4C). The curved prototype tip did not deliver as much laser energy to the dentin as the conventional straight tip. A lase of 62.5 J/cm 2 with the curved tip was comparable in effect to a lase of 12.5 J/cm 2 with the straight tip (Fig. 1, .4 and B). The pattern of the effect on dentin was irregular, with the curved tip as opposed to a more consistent round pattern with the conventional straight tip (Fig. 1, .4 and B). One specimen enabled a comparison of laser effects between transparent (sclerotic) dentin and nontransparent death. A lase of 125 J/cm 2 produced noticeably more glazing in sclerotic, transparent dentin than the adjacent lase of 250 J/cm 2 in nontransparent dentin (Fig. 1B). DISCUSSION Pashley et al. (17) showed that the halo areas around glazed craters caused by CO2 laser irradiation of coronal dentin were more permeable to the passage of phosphate-buffered saline
Carbon Dioxide Laser
Voi. 21, No. 1, January 1995
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than unlased areas. This was because of the removal of the smear plugs from the once occluded dentinal tubules. This same effect was seen in this study. The areas immediately surrounding high-energy lases had the smear layer physically modified, resulting in the dentinal tubules being open (Fig. 4, A and C). The large number of cracks and fractures seen in the present study would add to the increased permeability caused by smear layer modification. Stabholz et al. (18) found similar cracks in lateral human root dentin in a study involving an ArF-193 excimer laser. They speculated that these cracks could be caused by thermal stress, acoustic and shock waves, or by the release of super-heated plasma. The cracks seen in this study were seen in all specimens with any visible modification of the dentin. They dominated the effects of the laser on the resected apical root dentin. It would seem that the use of CO2 laser energy on resected apical root dentin will not result in a less permeable surface. It may in fact create fissures large enough to hold significant numbers of bacteria or tissue breakdown products that could cause inflammation in the adjacent tissue. The curved tip used in this study did not deliver the same amount of laser energy to the dentin surface as a standard straight ceramic tip. Some energy was probably dispersed prematurely with the curved tip. The pattern of the effect of the laser radiation on the dentin also appeared different with the curved tip when compared with a standard straight tip. The energy from the curved tip appeared scattered and inconsistent. The energy from the straight tip was more focused, and the effect on dentin was a more circular pattern with a more distinct halo immediately surrounding the lase as mentioned by Pashley et al. (Fig. 1, A and B). The use of both the stereomicroscope and the SEM allowed examination of the glazed and charred areas, with a magnified view of the physical modification of the dentin surface. The use of the stereomicroscope permitted us to see the effects of the laser energy on the dentin before any artifactual effects that might have been caused by the process of sputter-coating the dentin specimens under vacuum. There were fewer artifacts created by vacuum sputter-coating than we anticipated. This research was Supported in part by an Endodontic Graduate Student Award from the Research and Education Foundation of the American Association of Endodontists. The opinions, assertions, materials, and methodologies herein are private ones of the author and are not to be construed as official or reflecting the views of the American Association of Endodontists or ttte Research and Education Foundation. Drs. Read, Baumgartner, and Clark are affiliated with the School of Dentistry, Oregon Health Sciences University, Portland, OR. Address requests for reprints to J. Craig Baumgartner, DDS, PhD, School of Dentistry, Oregon Healt~ Sciences University, 611 S.W. Campus Drive, Porttand, OR 97201. FIG 4. A, Laser dentin, using the curved tip. Fluence of 114.6 J/cm =. This was a glazed crater. Note the porosity of the surface and the surrounding open dentinal tubules. SEM, original magnification x1,000. B, Lased dentin, using the curved tip. Fluence of 45.8 J/cm 2. Note the large hole in the center of the glazed crater and the patent dentinal tubules on the underlying surface (arrow). SEM, original magnification x1,000. C, Lased dentin using the standard straight ceramic tip. Fluence of 62.5 J/cm 2. When the specimens were viewed with the SEM, charred areas (black arrow) exhibited dentinal tubules that were more open than the surrounding unlased dentin (white arrow). SEM, original magnification x500.
References 1. Ratliff MS. Lasers in dentistry: an analysis. J Oreg Dent Assoc 1991 ;Fal1:25-30. 2. Featherstone JDB, Nelson DGA. Laser effects on dental hard tissues. Adv Dent Res 1987;1:21-6 3. Luxar Operators Manual. Botheil, Washington: Luxar, revised October 1, 1991. 4. Nelson DGA, Jongebloed WL, Featherstone JDB. Laser irradiation of human dental enamel and dentine. N Zealand Dent J 1986;82:74-7. 5. Tani Y, Kawada H. Effects of laser irradiation on dentin. I. Effect on smear layer. Dent Mater J 1987;6:127-34.
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6. Bahcall J, Howard P, Miserendino L, Walia H. Preliminary investigation of the histological effects of laser endodontic treatment on the pedradicutar tissues in dogs. J Endodon 1992;18:47-51. 7. Ichesco E, Ellison R, Corcoran J. A spectrophotometric analysis of dantinal leakage in the resected root [Abstract]. J Endodon 1986;12:129. 8. Tidmarsh BG, Arrowsmith MG. Dentinal tubules at the root ends of apicoected teeth: a scanning electron microscopic study. Int Endod J
1989;22:184-9. 9. Beatty R. The effect of reverse filling preparation design on apical leakage [Abstract 805]. J Dent Res 1986;65:259, 10. Vertucci F, Beatty R. Apical leakage associated with retrofilling techniques: a dye study. J Endedon 1986;12:331-6. 11. Pinks I, Beatty R. Effect of a dentin bonding material as a reverse filling [Abstract 806]. J Dent Res 1985;65:259. 12. Dederich DN, Zakarfasen KL, Tulip J. Scanning electron microscopic
Journal of Endodontics analysis of canal wall dentin following neodymium-yttrium-aluminum-garnet laser irradiation. J Endodon 1984;10:428-31. 13. Kantola S. Laser-induced effects on tooth structure. VII. X-ray diffraction study of dentine exposed to a CO2 laser. Acta Odont Scand 1973;31:3816. 14. Miserendino L The laser aplcoectomy: endodontic application of the CO= laser for pedapical surgery. Oral Surg 1988;66:615-9. 15. Zakariasen KL. Bactericidal action of carbon dioxide laser radiation in expedmantal dental root canals. Can J Microbiol t986;32:942-6. 16. Bonin P, Boivin R, Poulard J. Dentinal permeability of the dog canine after exposure of a cervical cavity to the beam of a CO= laser. J Endedon 1991;17:116-8. 17. Pashley EL, Homer JA, L.iu M, Kim S, Pashley DH. Effects of CO= laser energy on dentin permeability. J Endodon 1992;18:257-62. 18. Stabholz, Neev J, Liaw LH, Khayat A. Effect of ArF-193 nm excimer laser on human dentina! tubules. Oral Surg 1993;75:90-4.
You Might Be Interested Concern about the carcinogenicity of chloroform has spilled into endodontics despite the minute amounts used during root canal therapy. Recent analysis shows that the experiments on which the initial concerns were based were, to use descriptive restraint, highly suspect. To show some deleterious effect of chloroform, the original experimenters first selected a strain of mice which were known to have a naturally elevated rate of liver tumors. They then shoved a tube down the mice's throats and injected a shot of chloroform five times a week, every week, throughout their entire life time. Sure enough they got additional liver tumors. Yet if the same susceptible mice were given high levels of chloroform in their drinking water they developed practically no tumors. Based on the forced injection of chloroform into the mice stomachs, it was concluded that drinking water with 4.3 parts per billion over a lifetime would increase human risk of cancer by 1 in 100,000. Yet the experiments in which chloroform was actually administered in drinking water showed no increase in liver cellular proliferation when the chloroform concentration was 1,800,000 parts per billion (Science 264:183, 1994).
Could we possibly reintroduce the concept of risk/benefit ratio into the health decision-making process? Thanks to Paul Eleazar
Printed in U.S.A. VOL. 21, NO. 1, JANUARY1995
Effects of a Carbon Dioxide Laser on Human Root Dentin Russ P. Read, DOS, J. Craig Baumgartner, DDS, PhD, and Stephen M. Clark, DMD
constant. This results in the effect of the laser being contained in a very superficial layer of the target tissue with minimal effect on surrounding tissue (2). Protection of surrounding "nontarget" tissue may be of considerable importance. Bahcall demonstrated damage to the periodontal ligament of dog teeth treated with a Nd:YAG laser. This damage resulted in an increased incidence of ankylosis, cemental lysis, and bone remodeling (6). Ichesco et al. (7) found that the dentin of apically resected roots was more permeable to fluids than the dentin of nonresected roots. The coronal margin of an apical bevel, near the cementodentinal junction, has - 13,000 dentinal tubules/ mm 2 (8). These dentinal tubules can provide a pathway for leakage to or from the root canal if the root-end preparation and root-end Idling do not extend far enough coronally (9, 10). As the slope of the apical bevel increases, the root-end filling needs to be deeper to seal the resected tubules adequately. It may be impossible to make an apical preparation parallel to the long axis of the root and deep enough to include all the tubules of the resected dentin. Reducing or eliminating the permeability of resected apical dentin would seem advantageous in apical endodontic procedures. The use of laser energy may enhance the success of endodontic apical procedures by reducing leakage through resected dentinal tubules. Pinks and Beatty (11) found that apical dye penetration could be reduced by using a dentin bonding material in the rootend preparation and covering the bevel. There is concern about how periapical tissues respond to these materials and how long they will maintain their seal in the periap~cal environment. Exposing dentin to laser radiation may result in a surface that is less permeable to fluids (4, 12). After laser irradiation, the structure of the recrystaUized dentin resembles that of normal enamel (13). Miserendino (14) reported good results using a CO2 laser for an endodontic apical procedure. Zakariasen et al. (15) have reported on the ability of the CO2 laser to eliminate microorganisms from the root canal. A CO2 dental laser (Luxar LX-20, Bothel, WA) has FDA approval and is currently being used on soft tissue. This laser is highly "tunable," with watt settings from 2 to 20, 10 different modes (combinations of pulse width and pulse repetition rate), or continuous power. Bonin et al. (16) showed that different energy levels of the CO2 laser (Satelec, France) had different effects on the permeability of dentin. Tani and Kawada (5) showed that irradiating bovine dentin with a CO2 laser (Mochida Pharmaceutical Co., Japan) in defocus mode
The effects of the Luxar LX-20 CO2 dental laser on resected apical root dentin were examined using stereomicroscopy and scanning electron microscopy. The surfaces of 2-mm-thick sections of dentin from freshly extracted human teeth were exposed to CO2 laser radiation. Fluences used ranged from 2.1 to 625.0 J/cm 2. The effects of the laser energy on the dentin ranged from no visible effects, to charring, cracking, cratering, and glazing. The most dramatic effect was cracking. Cracking was evident on all specimens having any visible modification of the dentin. A prototype-curved laser tip was used and compared with a standard straight tip. The curved tip did not deliver laser energy to the dentin as efficiently as a straight tip. CO2 laser radiation did not consistently obliterate dentin tubules.
The clinical use of lasers is becoming more common in the practice of dentistry. Several dental lasers have received FDA approval for use on human soft tissue, and research continues on the applicability of lasers on dental hard tissues (1). The effect of laser irradiation on a tissue is related to the absorption bands of the tissue (2). If the radiation wavelength emitted by a laser coincides with the absorption bands of a particular tissue, then the effect of that laser energy will primarily be on the target tissue with little radiation being transmitted through the tissue. CO2 lasers operate on a wavelength of 10.6 t~m (invisible infrared) (3). Enamel, cementum, and dentin conrain hydroxyapatite, which has absorption bands in the infrared region due to phosphate, carbonate, and hydroxyl groups in the crystal structure. These absorption bands in the 9.0 through 11.0-~tm region coincide closely with the radiation produced by the CO2 laser (4). This implies that the radiation from the CO2 laser should be efficiently absorbed by dental hard tissues. This would produce an effect on the target tissue with minimal effects on surrounding tissues such as the periodontal ligament and bone. This is in contrast with the Nd:YAG laser that is dependent on the opacity of the tissue for its absorption. In translucent dentin, a dye has to be placed on the dentin to achieve adequate absorption of Nd:YAG radiation (5). Pulsed lasers provide a way to increase the peak power density while keeping the energy density or fluence 4
Vol. 21, No. 1, January 1995
Carbon Dioxide Laser
5
resulted in dosing of the tubular orifices in the presence of a smear layer. In contrast, Pashley et al. (17) showed that CO2 laser energy (Sharplan 1020, Laser Industries, Tel Aviv, Israel) actually resulted in increased permeability in coronal dentin by melting away the smear layer resulting in opening of the tubule orifices. In a study using the ArF-193 excimer laser (Lambda Physik, Goettingen, Germany) on vertical sections of dentin from the cementoenamel junction area, the laser was found to remove peritubular and intertubular dentin and "open" the dentinal tubules (18). By observing and studying the physical effects of laser energy on dentin, we can expand our understanding of the laser's effects on dental hard tissues. This understanding will aid in our ability to determine the applicability of not only the CO2 laser, but also new and different lasers yet to be developed. The added "tunability" of the laser used in this study, with its pulsed application, needs to be evaluated for its effect on human root dentin. The purpose of this investigation is to examine the physical changes produced with various modes and watt settings of the Luxar LX-20 CO2 dental laser on cross-sections of resected apical root dentin using the scanning electron microscope and stereomicroscope. MATERIALS AND METHODS
Fie 1. A, Apical dentin specimen with six lases using the standard straight ceramic tip. When viewing the specimen as a clock face, the
1:00 o'clock position was exposed to a fluence of 6.25 J/cm2; 3:00 o'clock position, 12.25 J/cm=; 5:00 o'clock position, 25 J/cm=; 7:00 o'clock position, 62.5 J/cm2; 9:00 o'clock position; 125 J/cm2; and 11:00 o'clock position, 250 J/cm 2. Note glazing and charring in the four higher fluences. Stereomicroscope, original magnification x24. B, Apical dentin specimen with six lases using the prototype curved tip. The same fluences were used in the same positions as the specimen in A (1:00 o'clock and 3:00 o'clock lases had no visible effect, and the 5:00 o'clock lase is just visible). The curved tip appeared to deliver less laser energy to the dentin when compared with the straight tip. A fluence of 62.5 J/cm 2 with the curved tip had a similar effect on the dentin as a fluence of only 12.5 J/cm 2 with the
Freshly extracted human incisors were used in the study. The teeth were sectioned ~/sing an aluminum oxide disc (Dedeco, Long Eddy, NY) on a slow-speed handpiece. Approximately 2-mm-thick cross-sections were made from the apical third of the roots and stored in sterile isotonic saline with 0.2% sodium azide to inhibit bacterial growth. This storage medium was selected because of its minimal effect on the dentin. Each cross-section was cut at 45 degrees to the long axis of the tooth to simulate a beveled root end. A smear layer was produced on the apical sides of the resected dentin sections with wet 600-grit SiC paper (3 M, St. Paul, MN). To simulate in vivo conditions of hydration with dentinal fluid, each specimen was lased "wet" in a pool of isotonic saline with the apical surface up, but not submerged. The laser was used in a single-pulse setting to facilitate evaluation of a specific amount of CO2 laser energy. Six different pulse widths (laser exposure times) at the same wattage were used on each specimen. A clockwise pattern of "lases" was used on each dentin specimen at the: h00 o'clock position, mode 1; 3:00 o'clock position, mode 4; 5:00 o'clock position, mode 7; 7:00 o'clock position, mode 8; 9:00 o'clock position, mode 9; and I h00 o'clock position, mode 10 (Fig. 1, A and B). Specimens were lased at different watt settings ranging from 2 to 15 W. However, only 1 W setting was used for the six "lases" on each specimen. Specimens were marked with a number to indicate what watt setting was used; 1, 15 W; 2, 11 W; 3, 8 W; 4, 6 W; 5, 4 W; and 6, 2 W. Watt settings higher than 15 were not used because of the extensive damage imparted to straight tip. The 9:00 o'clock lase (125 J/cm =) was in transparent dentin and produced noticeably more glaze than the 11:00 o'clock lase that had twice the energy density (250 J/cm2). Note the thick layer of cementum around the periphery of the specimen. The effect of the laser energy appeared to be similar on dentin and cementum in the 9:00 o'clock lase. Stereomicroscope, original magnification x24.
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Journal of Endodontics
the dentin. The laser modes selected were: 1, 4, 7, 8, 9, and 10. These six modes represented all the pulse widths available with the LX-20. In the single-pulse setting, mode l corresponds to a single pulse of laser energy lasting 5 ms; mode 4, l0 ms; mode 7, 20 ms; mode 8, 50 ms; mode 9, 100 ms; and mode I0, 200 ms. Therefore, each consecutive lase represented a pulse approximately twice the duration of the pulse before it. A prototype-curved tip for the laser was evaluated (Luxar). The tip had a 25-degree curve and was made from wave guide tubing. In a clinical setting, this tip would facilitate laser access to a resected root end. The tip had guides on it that permitted placing it exactly 3 m m from the dentin specimens. This is important in that the energy imparted to the target tissue is inversely related to the laser's distance from the target. The guides also made it possible to reproducibly place the tip at 90 degrees to the dentin surface for maximum energy absorption. To compare the energy output of the curved prototype tip with that of a conventional straight tip, two specimens were lased at the same watt setting and modes (Fig. 1, .4 and B). The conventional straight tip was also used at 90 degrees to the surface of the specimens at a distance of 3
FIG 2. This lase, using the curved tip, is a result of a fluence of 333 J/cm =. Note the glazing, cratedng, and multiple fractures when viewed with the stereomicroscope. Odginal magnification x65.
mm.
Specimens were examined with a stereomicroscope (Nikon SMZ-2T; Tokyo, Japan) and a scanning electron microscope (SEM) (JEOL USA, Inc., Peabody, MA). After examination with the stereomicroscope and before SEM evaluation, the dentin specimens were sputter coated with - 5 0 nm of goldpalladium using a Hummer VII (Anatech Ltd., Alexandria, VA). Magnifications from x35 to xS,000 with the SEM and x 15 and x63 with the stereomicroscope were used to evaluate the effect of the laser on the physical modification of dentin. Photomicrographs were made using Polaroid 55PN film for the SEM and Kodak Ektachrome 100 HC for the stereomicroscope.
RESULTS The effect of the laser energy on the resected apical dentin ranged from no visible effect at low energy, to charring cracking, cratering, and "glazing" of the dentin at higher energy (Figs. l (.4 and B) and 2). The most striking result of the laser energy on the dentin were the many fractures that occurred in the target areas. Cracks were apparent not only where there was a cratedng effect from higher energy hits, but also in lower energy lases (Fig. 3). Some of the fractures were up to 15 ~m in width. The areas showing cratefing had a white-glazed center surrounded by a halo of blackened or charred dentin when viewed through the stereomicroscope (Fig. 2). The higher the laser energy, the more cracking and cratefing, and the larger the surrounding charred area. The craters that appeared white and glazed through the stereomicroscope looked very porous and uneven with the SEM (Fig. 4, .4 and B). The craters had holes within them anywhere from 3 to 10 times larger than the open dentinal tubules. Open dentinal tubule orifices could be seen through these holes underneath the glazed areas (Fig. 4B). In several of the lower energy lases, the smear layer remained intact, but there was still marked cracking of the dentin (Fig. 3). When the specimens were viewed with the SEM, charred areas exhibited
FIG 3. This lase, using the curved tip, is a result of a fluence of 83 J/ cm=. Note extensive cracking of dentin even in the absence of smear layer modification. SEM, odginal magnification x200.
dentinal tubules that were more open than the surrounding unlased dentin (Fig. 4C). The curved prototype tip did not deliver as much laser energy to the dentin as the conventional straight tip. A lase of 62.5 J/cm 2 with the curved tip was comparable in effect to a lase of 12.5 J/cm 2 with the straight tip (Fig. 1, .4 and B). The pattern of the effect on dentin was irregular, with the curved tip as opposed to a more consistent round pattern with the conventional straight tip (Fig. 1, .4 and B). One specimen enabled a comparison of laser effects between transparent (sclerotic) dentin and nontransparent death. A lase of 125 J/cm 2 produced noticeably more glazing in sclerotic, transparent dentin than the adjacent lase of 250 J/cm 2 in nontransparent dentin (Fig. 1B). DISCUSSION Pashley et al. (17) showed that the halo areas around glazed craters caused by CO2 laser irradiation of coronal dentin were more permeable to the passage of phosphate-buffered saline
Carbon Dioxide Laser
Voi. 21, No. 1, January 1995
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than unlased areas. This was because of the removal of the smear plugs from the once occluded dentinal tubules. This same effect was seen in this study. The areas immediately surrounding high-energy lases had the smear layer physically modified, resulting in the dentinal tubules being open (Fig. 4, A and C). The large number of cracks and fractures seen in the present study would add to the increased permeability caused by smear layer modification. Stabholz et al. (18) found similar cracks in lateral human root dentin in a study involving an ArF-193 excimer laser. They speculated that these cracks could be caused by thermal stress, acoustic and shock waves, or by the release of super-heated plasma. The cracks seen in this study were seen in all specimens with any visible modification of the dentin. They dominated the effects of the laser on the resected apical root dentin. It would seem that the use of CO2 laser energy on resected apical root dentin will not result in a less permeable surface. It may in fact create fissures large enough to hold significant numbers of bacteria or tissue breakdown products that could cause inflammation in the adjacent tissue. The curved tip used in this study did not deliver the same amount of laser energy to the dentin surface as a standard straight ceramic tip. Some energy was probably dispersed prematurely with the curved tip. The pattern of the effect of the laser radiation on the dentin also appeared different with the curved tip when compared with a standard straight tip. The energy from the curved tip appeared scattered and inconsistent. The energy from the straight tip was more focused, and the effect on dentin was a more circular pattern with a more distinct halo immediately surrounding the lase as mentioned by Pashley et al. (Fig. 1, A and B). The use of both the stereomicroscope and the SEM allowed examination of the glazed and charred areas, with a magnified view of the physical modification of the dentin surface. The use of the stereomicroscope permitted us to see the effects of the laser energy on the dentin before any artifactual effects that might have been caused by the process of sputter-coating the dentin specimens under vacuum. There were fewer artifacts created by vacuum sputter-coating than we anticipated. This research was Supported in part by an Endodontic Graduate Student Award from the Research and Education Foundation of the American Association of Endodontists. The opinions, assertions, materials, and methodologies herein are private ones of the author and are not to be construed as official or reflecting the views of the American Association of Endodontists or ttte Research and Education Foundation. Drs. Read, Baumgartner, and Clark are affiliated with the School of Dentistry, Oregon Health Sciences University, Portland, OR. Address requests for reprints to J. Craig Baumgartner, DDS, PhD, School of Dentistry, Oregon Healt~ Sciences University, 611 S.W. Campus Drive, Porttand, OR 97201. FIG 4. A, Laser dentin, using the curved tip. Fluence of 114.6 J/cm =. This was a glazed crater. Note the porosity of the surface and the surrounding open dentinal tubules. SEM, original magnification x1,000. B, Lased dentin, using the curved tip. Fluence of 45.8 J/cm 2. Note the large hole in the center of the glazed crater and the patent dentinal tubules on the underlying surface (arrow). SEM, original magnification x1,000. C, Lased dentin using the standard straight ceramic tip. Fluence of 62.5 J/cm 2. When the specimens were viewed with the SEM, charred areas (black arrow) exhibited dentinal tubules that were more open than the surrounding unlased dentin (white arrow). SEM, original magnification x500.
References 1. Ratliff MS. Lasers in dentistry: an analysis. J Oreg Dent Assoc 1991 ;Fal1:25-30. 2. Featherstone JDB, Nelson DGA. Laser effects on dental hard tissues. Adv Dent Res 1987;1:21-6 3. Luxar Operators Manual. Botheil, Washington: Luxar, revised October 1, 1991. 4. Nelson DGA, Jongebloed WL, Featherstone JDB. Laser irradiation of human dental enamel and dentine. N Zealand Dent J 1986;82:74-7. 5. Tani Y, Kawada H. Effects of laser irradiation on dentin. I. Effect on smear layer. Dent Mater J 1987;6:127-34.
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6. Bahcall J, Howard P, Miserendino L, Walia H. Preliminary investigation of the histological effects of laser endodontic treatment on the pedradicutar tissues in dogs. J Endodon 1992;18:47-51. 7. Ichesco E, Ellison R, Corcoran J. A spectrophotometric analysis of dantinal leakage in the resected root [Abstract]. J Endodon 1986;12:129. 8. Tidmarsh BG, Arrowsmith MG. Dentinal tubules at the root ends of apicoected teeth: a scanning electron microscopic study. Int Endod J
1989;22:184-9. 9. Beatty R. The effect of reverse filling preparation design on apical leakage [Abstract 805]. J Dent Res 1986;65:259, 10. Vertucci F, Beatty R. Apical leakage associated with retrofilling techniques: a dye study. J Endedon 1986;12:331-6. 11. Pinks I, Beatty R. Effect of a dentin bonding material as a reverse filling [Abstract 806]. J Dent Res 1985;65:259. 12. Dederich DN, Zakarfasen KL, Tulip J. Scanning electron microscopic
Journal of Endodontics analysis of canal wall dentin following neodymium-yttrium-aluminum-garnet laser irradiation. J Endodon 1984;10:428-31. 13. Kantola S. Laser-induced effects on tooth structure. VII. X-ray diffraction study of dentine exposed to a CO2 laser. Acta Odont Scand 1973;31:3816. 14. Miserendino L The laser aplcoectomy: endodontic application of the CO= laser for pedapical surgery. Oral Surg 1988;66:615-9. 15. Zakariasen KL. Bactericidal action of carbon dioxide laser radiation in expedmantal dental root canals. Can J Microbiol t986;32:942-6. 16. Bonin P, Boivin R, Poulard J. Dentinal permeability of the dog canine after exposure of a cervical cavity to the beam of a CO= laser. J Endedon 1991;17:116-8. 17. Pashley EL, Homer JA, L.iu M, Kim S, Pashley DH. Effects of CO= laser energy on dentin permeability. J Endodon 1992;18:257-62. 18. Stabholz, Neev J, Liaw LH, Khayat A. Effect of ArF-193 nm excimer laser on human dentina! tubules. Oral Surg 1993;75:90-4.
You Might Be Interested Concern about the carcinogenicity of chloroform has spilled into endodontics despite the minute amounts used during root canal therapy. Recent analysis shows that the experiments on which the initial concerns were based were, to use descriptive restraint, highly suspect. To show some deleterious effect of chloroform, the original experimenters first selected a strain of mice which were known to have a naturally elevated rate of liver tumors. They then shoved a tube down the mice's throats and injected a shot of chloroform five times a week, every week, throughout their entire life time. Sure enough they got additional liver tumors. Yet if the same susceptible mice were given high levels of chloroform in their drinking water they developed practically no tumors. Based on the forced injection of chloroform into the mice stomachs, it was concluded that drinking water with 4.3 parts per billion over a lifetime would increase human risk of cancer by 1 in 100,000. Yet the experiments in which chloroform was actually administered in drinking water showed no increase in liver cellular proliferation when the chloroform concentration was 1,800,000 parts per billion (Science 264:183, 1994).
Could we possibly reintroduce the concept of risk/benefit ratio into the health decision-making process? Thanks to Paul Eleazar