Lasers in Surgery and Medicine 12:467-470 (1992)

Laser Etching of Teeth for Orthodontic Bracket Placement: A Preliminary Clinical Study Dai P. Roberts-Harry, BDS, MSc, FDS,

M ORTH

Orthodontic Department, Universityof Bristol Dental Hospital and School, Bristol, BS1 2LY, U.K.

Current interest in the use of layers in clinical dentistry has suggested that this technique may be applicable to enamel etching in orthodontics. A pulsed NdYAG laser was used to etch the enamel surfaces of teeth in vivo prior to the bonding of orthodontic brackets with composite resin. Overall laser bonding took considerably longer, was less reliable in terms of bond strength, and produced more discomfort than conventional acid etching. The use of the ND:YAG laser for etching teeth prior to bonding orthodontic brackets is not recommended. c' 1992 Wiley-Liss, Inc. Key words: dental hard tissues, laser etching, orthodontics

INTRODUCTION

Shortly after lasers were developed, researchers began examining the feasibility of using this technology for dental procedures. Lasers have been used for a variety of purposes such as cavity preparation, endodontics, soft tissue surgery, and the treatment of aphthous ulcers. In the 1960s it was demonstrated with ruby lasers that the surface of enamel could be vaporised and cratered using a high energy beam [l-31. However, laser energy output sufficient to cause cratering of the enamel is likely t o produce irreversible pulpal damage and even pulp death [41. It is clearly important to avoid pulpal tissue damage if lasers are to be used in clinical dentistry, and several authors have addressed this matter. Stern et al. [5]investigated the effect that a pulsed ruby laser with energy levels of 60-250 Jcm had on the pulpal tissue of chimpanzee teeth. Minimal pulpal changes were noted, and they concluded that pulp changes after laser exposure were reversible and that considerably higher energy would be needed to produce irreversible changes. Later studies by Featherstone and Nelson [61 using a CO, laser suggested that the effect on enamel depended on the wavelength of the beam. Dental hard tissue absorption was best with wavelengths of 9.32-10.59 pm and fusion melting and recrystalisation of enamel are c

1992 Wiley-Liss, Inc.

confined to a 5-10 pm thin surface region. The underlying enamel, and therefore the pulp, were not affected at depths of greater than 50 p.m. Furthermore, pulsed lasers offered a means of increasing the peak power density while keeping the pulse energy constant. Launey et al. [7] found that the Nd:YAG laser energy was poorly absorbed by enamel and dentine and had a damaging effect on the dental pulp. They concluded that, as a result, these lasers were the wrong tool for the dental treatment of hard tissues. However, these findings are at variance with those of other workers who found that with low power levels, injury to the pulp is unlikely [8-171. Myers and Myers [181 performed an in vivo study demonstrating that caries could be removed relatively painlessly using the Nd:YAG laser with no detrimental effects. Therefore, from the literature, it appears that whereas laser energy can easily harm the dental pulp, if these levels are low and the optimal wavelengths chosen, little damage occurs. Regarding the physical effects of lasers on dental hard tissues, scanning electron microAccepted for publication May 12, 1992. Address reprint requests to 1)ai P. Roberts-Harry, Orthodontic Department, University of Bristol Dental Hospital and School, Lower Maudlin St., Rristol RS1 2LY, V.K.

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scopic studies of the lased surface of dental enamel have been carried out by several workers [19-221. Hess [23] compared the effects of laser and acid etching of teeth with a SEM. He used a Nd:YAG laser on a coated enamel surface with maximum energy levels of 238.8 Jcm-,. The low energy levels used were sufficient to etch the enamel surface but too low to damage the pulp. Liberman et al. [24] studied the retention of composite materials using a CO, laser. Their results showed that lasing lasting 2 seconds with an energy density of 35 J cm-’ gives an etch pattern comparable with that produced by acid etching. Based on this experimental finding, they felt the advantages of properly performed laser etching of enamel would be a decreased manipulation time, no possible damage to dentine or pulp, and a more accurate control over the etching procedure when compared to the use of acid etching. White et al. [251 found in vitro that Nd:YAG etching of enamel improves bond strength t o metal orthodontic brackets. In spite of these studies, there has t o date been little research into the in vivo use of lasers in orthodontics. The aim of this study was to determine the clinical effectiveness of the pulsed laser etching of enamel with particular regard to the speed of bonding orthodontic brackets and the resultant bond strength. MATERIALS AND METHODS

Ethical committee approval was granted for this study. Eight consecutive patients needing treatment with fixed orthodontic appliances were selected. One dental arch was etched with a dLASE 300, Nd:YAG laser (American Dental Laser, Birmingham, MI) with a wavelength of 1.06 pm using a contact probe of 320 pm diameter. Power levels of 1.25 Watts were used, pulsed at 15 pulses per second with a pulse duration of 0.1 ms. A small pilot study on extracted teeth showed that this energy level produced a macroscopically suitable etch pattern. The other arch was then conventionally acid etched. Since human enamel has a low absorption of near infra red wavelength laser light, the teeth were initially coated with a dark ink (Sumi ink KF2, Yasutom, Brisbane, CA, Japan) as suggested by Morioka et al. [26]. The ink was then dried and subsequently removed by passing the laser tip over the ink, thus “burning” it off and leaving an etch pattern on the enamel (Fig. 1).A slight smell of burning and a soft popping noise

Fig. 1. The laser in use. Black Sumi ink has been painted onto the enamel.

were detected during the process and the patients were warned in advance of this. Special safety glasses were worn to prevent any possible eye damage by the laser light. The etching sequence involved cleaning, drying, and then painting the tooth surface with ink, drying the ink, laser etching t o remove all the ink, and finally bracket placement. For the acid etch technique, the teeth were cleaned and dried before applying 30% orthophosphoric acid for 20 seconds. After washing with water, the teeth were again dried before bracket placement. For both techniques the tooth surface was carefully examined to ensure that the surface of the enamel had a typical frosted appearance prior to bonding. Brackets were cemented onto the teeth with a Bis GMA based resin (Concise Orthodontic Composite, 3M Dental Products). The time to produce an etch pattern with the laser technique was recorded from the moment the ink was applied to the first tooth until the final remnants of ink were removed from the last tooth. The length of time taken to acid etch was measured from the moment etch was first applied until the teeth had been finally dried prior to bonding. All recordings were made to the nearest quarter minute by a dental surgery assistant using a stop watch. A simple questionnaire was completed by all these patients immediately after bracket placement. The questionnaire was used t o determine how patients compared the etching techniques. The timing of any bracket failures was noted. Both the teeth and brackets were examined t o determine the failure site, that is, at the

Laser Etching of Teeth TABLE 1. Comparison of Time Taken to Etch Teeth Using the Two Techniques* Etch Patient typea 1 A L 2 L A 3 A L

Teeth etched

Etch timeb

2.50 14.00 21112 felt hot 23.00 2.50

5421I1245 532111235

2.25 19.50 21112 felt like

mild electric current was applied 4

5

6 7 8

A L L A A L A L L

532111235 432111234

2.75 16.25

16.75 mild discomfort 542111245 543211123545 2.50 2.25 4321I1234 3211123 13.25 54321112345 2.75 54321112345 30.00 432111234 19.75 like drinking

warm/hot tea A

432111234

TABLE 2. Comparison of Average Times to Etch Teeth*

Comments

432111234 532111235 532111235 432111234

2.50

*Patients comments are included in the table. “A = acid etch; L = laser etch. bMeasured to nearest quarter minute.

tooth surface, bracket surface, or a combination of both. The vitality of all the teeth was checked prior to and 6 months after etching. RESULTS

The results are shown in Tables 1 and 2. In total, 68 teeth were acid etched and 64 laser etched. The average time taken to etch one arch was 21.84 minutes using the laser compared to 2.32 minutes for acid etching. In total, 13 bracket failures occurred with laser etching (a failure rate of 20.3%).Four brackets came off the same day, two within 1 week, four within 1 month, and three within 3 months. All the bond failures occurred at the compositeenamel interface with composite retained on the bracket base. This compared with no failures where acid etching was used. Five of the eight patients complained of discomfort during laser etching with symptoms varying from mild discomfort to the teeth “feeling like warm hot tea” or “having an slight electric current passed through the teeth.” Although the response was not tooth specific, the lower incisors were the teeth most frequently affected. Despite these comments, only one patient said they would

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Laser Acid

No. teeth etched 64 68

Total etch time

Average etch time per tooth

Average time to etch 8 teeth (one arch)

152.50 20.00

2.38 0.29

19.04 2.32

*All times measured in minutes.

be unhappy to have their teeth laser etched again. No patient objected to any part of the acid etch technique and all the patients said they preferred it t o laser etching. There was no loss of vitality in any of the teeth. DISCUSSION

The tooth sensitivity reported was surprising as it is often claimed that the use of pulsed lasers even in soft tissue surgery is painless because the pulses are shorter than the threshold level for nerve stimulation. The etching process was lengthy and it is possible that the enamel stored the laser energy in the form of heat until the teeth gradually became sensitive. A longer pulse interval might have helped, but this would have increased the etch time even further and thus been counterproductive. A reduced beam energy level may also have helped, but this would have produced a poorer etch pattern. The high bracket failure rate of laser etching may conceivably have been attributable to operator error, and it is possible that with more experience this would have been reduced. Since laser etching is a slow process, the risk of contaminating the enamel surface prior to bonding is high. However, in all cases the enamel surface was carefully checked to ensure that a frosted appearance was present. The etch pattern produced by acid etching differs from that produced by the laser. Phosphoric acid preferentially dissolves the enamel crystalites of hydroxyapatite along their long axis. Tags are thus produced in the enamel that act as a mechanical lock for the composite. In addition, high energy levels are produced in the enamel making the surface highly reactive. This improves bond strength, but the effects may be transitory. The laser, in contrast, produces microscopic cracks, fissures, and craters in the surface enamel, which mechanically link with the composite. These different etch patterns may have re-

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sulted in varying bond strengths and consequently produced the high bracket failure rate with the laser etch technique. The main disadvantage of laser etching is that it undoubtedly takes much longer than acid etching. The probe tip has a diameter of 320 pm, which must be passed over the ink surface until it has been burnt off by the laser pulses. A larger probe tip might reduce the etch time but is likely t o generate more heat and consequently more discomfort than a small tip. Although the time could be reduced a little with practice, it is unlikely that this would be sufficient to make the technique clinically acceptable. CONCLUSIONS

The laser used in this study produced a macroscopic etch pattern similar to that found with acid etching. The laser was considerably slower, produced slightly more discomfort, and was substantially less reliable than acid etching. The findings of this study concur with those of Launay et al. [71 that, at present, the Nd:YAG laser cannot be recommended for use on dental enamel. ACKNOWLEDGMENTS

I thank Mr. M. Midda for suggesting the use of the laser for etching teeth, Mr. P. RentonHarper for his help and advice, and the American Dental Laser Company for the use of their machine. REFERENCES 1. Stern RH, Sognnaes RF. Laser beam effect on dental hard tissues. J Dent Res 1964; 43:873. 2. Kinersly T, Jarabak JP, Phatak NM, DeMent J. Laser effects on tissue and materials related to dentistry. J Am Dent Assoc 1965; 70:593-600. 3. Goldman L, Gray JA, Goldman J , Goldman B, Meyer R. Effect of laser beam impacts on teeth. J Am Dent Assoc 1965; 701601-606. 4. Adrian JC, Bernier JL, Sprague WG. Laser and the dental pulp. J Am Dent Assoc 1971; 83:113-117. 5. Stern RH, Renger HL, Howell FV. Laser effects on vital dental pulps. Brit Dent J 1969; 127:26-28. 6. Featherstone JDB, Nelson DGA. Laser effects on dental hard tissues. Adv Dent Res 1987; 1:21-26.

7. Launay Y, Mordon S, Cornil A, Brunetaud JM, Moschetto Y. Thermal effects of lasers on dental hard tissues. Laser Surg Med 1987; 7:473-477. 8. Adrian JC. Pulp effects on Neodynium laser a preliminary report. Oral Surg 1977; 44:301. 9. Serebro L, Segal T, Nordenberg D, Gorfil C, Bar-Lev M. Examination of pulp following laser beam irradiation. Lasers Surg Med 1987; 7:236-239. 10. Powell GL, Whisenant BK, Morton TH. Carbon dioxide laser oral safety parameters for teeth. Lasers Surg Med 1990; 10~389-392. 11. Hibst R, Keller U. Experimental studies on the application of Er:YAG laser on dental hard substances: 1 measurement of ablation rate. Lasers Surg Med 1989; 9:341344. 12. Keller U, Hibst R. Experimental studies on the application of Er:YAG laser on dental hard substances: 2 light microscopic and SEM investigations. Lasers Surg Med 1989; 9:345-351. 13. Yamamoto H, Sat0 K. Prevention of dental caries by acousto-optically Q-switched Nd:YAG laser irradiation. J Dent Res 1980; 59(2):137. 14. Myers TD, Myers DM. In vitro caries removal. CDA J 1988; May:9-10. 15. Goldman L, Goldman B, Van Lieu N. Current laser dentistry. Lasers Surg Med 1987; 6:559-562. 16. Morioka T, Morita E, Suzuki K. Temperature increment and sensory response of sound teeth of volunteers with an irradiation of Q-switched Nd-YAG laser. Hoku Eisei Gakkai Zasshi 1882; 31(5):44-50. 17. Renton-Harper P. Nd:YAG treatment of dental hypersensitivity. Proceedings of the Second Congress of International Society of Lasers in Dentistry 1990:92. 18. Myers TD, Myers DM. In vivo caries removal utilising the YAG laser. J Mich Dent Assoc 1985; 67:66. 19. Myers TD, Myers WD. The use of a laser for debridment of incipient caries. J Pros Dent 1985; 53(6):776-779. 20. Dacev B, Djulgerova E, Rajcev L, Dimitrov S1: Scanning electron microscopy of dental enamel after irradiation with YAG-neodynium laser. Stomatolgiia (Sofiia) 1987; 69(4):7-13. 21. de Raad M, Paschoud Y, Holz J. Effets du laser a CO, sur les tissus dentaires. J Biol Buccale 1988; 16:137-150. 22. Nelson DGA, Jongebloed, Featherstone JDB. Laser irradiation of human dental enamel and dentine. New Zealand Dent J 1986; 8274-77. 23. Hess JA. Scanning electron microscopic studies of laserinduced morphologic changes of a coated enamel surface. Lasers Surg Med 1990; 10:458-462. 24. Liberman R, Segal TH, Nordenberg D, Sebero LI. Adhesion of composite materials to enamel: Comparison between the use of acid and lasing as pretreatment. Lasers Surg and Med 1984; 4:323-327. 25. White JM, Goodis HE, Asbill SR, Watanabe LG. Orthodontic bracket bond strength to Nd:YAG laser etched enamel. J Dent Res 1991; 70:297 Abstract No. 252. 26. Morioka TR, Suzuki K, Tagomeri S. Effect of beam absorptive mediators on a n acid resistance of surface of enamel by Nd-YAG laser irradiation. J Dent Health 1984; 34~40-44.