Laser Radiation Bracket Debonding

Laser Radiation Bracket Debonding Taťjana Dostálováa∗, Helena Jelínkováb, Jan Šulcb Petr Korandab, Michal Němecb, Jaroslav Racekc, Mitsunobu Miyagid Charles University, 2nd Medical School Department of Paediatric Stomatology, 150 06 Prague 5, Czech Republic b Faculty of Nuclear Sciences and Physical Engineering Czech Technical University, Břehová 7, 115 19 Prague 1, Czech Republic c Charles University, 1st Medical Faculty Department of Stomatology, Kateřinská 32, 120 00 Prague 2, Czech Republic d Sendai National College of Technology 4-16-1 Ayashi Chuo, Aoba-ku, Sendai, 989-3128, Japan a

ABSTRACT Ceramic brackets are an aesthetic substitute for conventional stainless steel brackets in orthodontic patients. However, ceramic brackets are more brittle and have higher bond strengths, which can lead to bracket breakage and enamel damage during classical type of debonding. This study examined the possibility of laser radiation ceramic brackets removing as well as the possible damage of a surface structure of hard dental tissue after this procedure. Two types of lasers were used for the experiments – a laser diode LIMO HLU20F400 generating a wavelength of 808 nm with the maximum output power 20W at the end of the fiber (core diameter 400 µm, numerical aperture 0.22). As a second source, a diode-pumped Tm:YAP laser system generating a wavelength of 1.9 µm, with up to 3.8 W maximum output power was chosen. For the investigation, extracted incisors with ceramic brackets were used. In both cases, laser radiation was applied for 0.5 minute at a maximum power of 1 W. Temperature changes of the irradiated tissue was registered by camera Electrophysics PV320. After the interaction experiment, the photo-documentation was prepared by the stereomicroscope Nikon SMZ 2T, Japan. The surface tissue analysis was processed in “low vacuum” (30 Pa) regime without desiccation. This technique was used to record back–scattered electron images. Selecting the appropriate laser, resin, and bracket combination can minimize risks of enamel degradation and make debonding more safe. Keywords: dentistry, orthodontics, ceramic bracket debonding, diode laser, Tm:YAP laser

1. INTRODUCTION In the past years various techniques have been investigated for brackets debonding. They include debonding pliers, hand sealers, and ultrasonic and rotary instruments [1-4]. The methods differ according to the bracket material which can be metal or ceramics. Conventional techniques of removing metal orthodontic brackets are not so easy to apply due to the difference in fracture toughness of the bracket, composite resin, and enamel. The incidence of bracket fracture (when conventional debonding techniques as recommended by manufacturers are employed) ranges from 10% to 35% [5]. One of the removal techniques is applying the heat generated by laser radiation to the brackets, namely CO2, KrF, XeCl, or Nd: YAG lasers [6, 7]. The current study has demonstrated the feasibility of continuous-wave Tm:YAP laser debonding of ceramic orthodontic brackets for those purposes and evaluated the enamel and bracket damage caused by laser debonding. The aim of this study was, at first, to find a simple and reliable method which can ensure laser radiation brackets removal without any resulting enamel structure changes. For that reason, a cw Tm:YAP (λ=1.98 µm) and diode (λ= 0.808 µm) ∗

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Lasers in Dentistry XIV, edited by Peter Rechmann, Daniel Fried, Proc. of SPIE Vol. 6843, 684304, (2008) 1605-7422/08/$18 · doi: 10.1117/12.762256 Proc. of SPIE Vol. 6843 684304-1 2008 SPIE Digital Library -- Subscriber Archive Copy

lasers were used to remove ceramic brackets. During the debonding procedure a very important thing is the quantitative control of the amount of thermal energy delivered to the ceramic bracket which may overheat the tooth [8]. Therefore the second goal of this investigation was the temperature increase measurement by a thermocouple placed inside the tooth as well as by thermo camera images.

2. MATERIALS AND METHODS 2.1. Laser sources The radiation sources were two laser systems working in continuous free running regime: the GaAlAs laser diode and diode-pumped Tm:YAP laser system. 2.1.1. Diode GaAlAs laser The near infrared radiation source was GaAlAs laser diode (LIMO HLU20F400) generating radiation at 808 nm. The maximum output power 20W was obtained at the end of the fiber (core diameter 400 µm, numerical aperture 0.22). The generated radiation was unpolarized. 2.1.2. Diode-pumped Tm:YAP laser 9

The second laser system was a longitudinally diode-pumped Tm:YAP laser operating at 1980 nm . For pumping of this laser, a fiber-coupled diode LIMO HLU30F400-790 generating radiation at 790 nm was used. The maximum output power 3.8 W was obtained behind the output coupler of the laser. 2.2. Tissue material Our study was conducted on ten human premolars of adolescent patients (age 11 to 15) extracted for orthodontic reasons. Ceramic brackets Facination 2 (Dentaurum, Germany) were bonded on these teeth by the ConTec LC adhesive (Dentaurum, Germany). The enamel surface was cleaned with non-fluoride polishing paste, rinsed in a stream of water, and dried in a stream of oil-free air. An etching agent ConTec Etch (Dentaurum, Germany) was then applied for 15 seconds on labial enamel surface and rinsed in a stream of water for 10 seconds. After air drying, the enamel showed chalky white hue. On the prepared etched surfaces ConTec Primer was applied, slightly dispersed with air and polymerized for 20 seconds by halogen lamp ELIPAR Free-Light 2 (3M ESPE, Germany). The ceramic brackets were coated with sufficient amount of ConTec LC adhesive, and each bracket was polymerized in frontal light stream with halogen lamp for 20 seconds. To identify the temperature rise (with a thermocouple), holes were prepared inside the teeth opposite the bracket. The teeth with bonded brackets were placed in physiological solution and stored at temperature of 7°C. 2.3. Analysis methods and measuring instruments Before and after the treatment, the brackets and the bracketed teeth were photographed by an optical microscope (Nikon SMZ-2T, Japan) equipped with a Mitsubishi CCD color video camera (CCD-100, Japan) connected to the computer. The power used in the particular irradiation was measured with the power meter Molectron EPM 2000e with probe PM3.The temperature changes during the bracket irradiation were recorded by NiCr-Ni thermocouple and GMH 3210 digital thermometer. Simultaneously, the tooth surface temperature spatial distribution (progress and attenuation) was monitored by a thermal imager-infrared camera Optilas – Electrophysics PV320L2E. After the treatment the teeth were stored in saline solution. The surface of affected tissues (i.e., the place where the bracket was stuck upon) was analyzed by the scanning electron microscope JSM 5510 LV Jeol, Japan. The teeth were processed in “low vacuum” (30 Pa) regime without desiccation. This technique was used to record back-scattered electron images. 2.4. Experimental procedure The output radiation from the specific laser was directed towards the brackets. The power used for the respective irradiation was measured by the power meter Molectron EPM 2000e with probe PM3, the time of irradiation being 30 s, 60 s, and 90 s in the case of Tm:YAP irradiation and 60 s for GaAlAs diode. The bracket with the tooth sample was heated by laser light, and after the chosen time interval the bracket was removed mechanically from the tooth surface. Two configurations of measurement were investigated – irradiation without and with water cooling of the tooth tissue.

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During this procedure, the temperature inside the tooth was recorded by the NiCr-Ni thermocouple and GMH 3210 digital thermometer. Simultaneously, the tooth surface temperature spatial distribution was monitored by thermal imager PV320L2E. The irradiation condition and the results obtained are summarized in Tables 1 and 2.

3. RESULTS 3.1 Brackets debonding by GaAlAs laser radiation Irradiation of the bracket by near infrared laser radiation (808 nm - GaAlAs diode) had no effect on the brackets retention adherence. It was not possible to release them even after irradiation at a power of 10 W for 60 s. The explanation followed from the brackets absorption measurements. The 808 nm radiation is absorbed neither by the brackets ceramic material nor by the glue; therefore, the bracket connection with the tooth is not destroyed. From the temperature measurements it is seen that radiation is transmitted through the tooth and it heats the thermocouple (Fig. 3a). The used power values and irradiation time intervals are summarized in Table 1.

Power [W]

Irradiation duration [s]

Cooling

Temp. rise [°C]

Laser diode 0.808 µm

1 2 10

60 60 60

no no no

18 29 114

Result – bracket removal

Laser system

Table 1 – Power values and irradiation time intervals for case of brackets GaAlAs diode irradiation.

0 0 0

3.1 Brackets debonding by Tm:YAP laser radiation Tm:YAP laser (1.98 µm) radiation penetrates into the brackets and also ConTec LC adhesive glue absorption band. Therefore, the heat from the laser radiation was concentrated into these components, and after 60 s the bracket was removed. The temperature accumulation is shown in Fig. 1.

Power [W]

Irradiation duration [s]

Cooling

Temp. rise [°C]

Tm:YAP laser 1.98 µm

1 1 1 1 2

30 60 60 90 60

no no yes yes yes

31 2 5 9


Result – bracket removal

Laser system

Table 2 – Power values and irradiation time intervals for brackets Tm:YAP laser irradiation.

0 OK OK OK OK

I W, 60 s, no cooling

I W, 90 s, cooling 2 W, 60 s, cooling

:ure rise

[(

30

30

60 Time Es]

Fig. 1. Temperature rise during irradiation of bracket by Tm:YAP laser radiation (1W, 90 s).

From the values in Tab.2 and from the graph in Fig.1 it is seen that if cooling is not applied during the debonding treatment, the temperature rise up of a root damage temperature. That is why the following measurements concentrated only on cases with cooling. The results are seen in Fig.2.

(a)

(b)

(c)

(d)

Fig.2. Examples of two teeth after irradiation - photographs taken by optical microscope (Nikon SMZ-2T, Japan) (a). The same tooth investigated after irradiation by scan (JSM 5510 LV Jeol, Japan) (b-d).

4. DISCUSSION The goal of this study was to find a source of radiation which can help effectively to remove the adherent bracket from the tooth enamel. The results of our investigation generally agree with previous studies, substantiating the fact that lasers can be used effectively to thermally soften the adhesive resin for removal of ceramic brackets7. Due to our investigation it has been found that it is possible to use laser radiation for easier bracket removal. The applied radiation must have the wavelength which has maximal absorption in the bracket and bonding agent material, and minimal


absorption in the tooth. If these criteria are fulfilled, the radiation is an efficient helper in the debonding, and no thermal damage to the tooth appears after the procedure. During the thermal debonding procedure, a temperature gradient exists between the tooth surface and the pulp. The gradient of temperature rise from the tooth surface through the enamel and dentin does not follow a linear relationship during transient heating10. Our experiments have shown that the 808 nm radiation is absorbed partly in bracket but is not absorbed in the agent material, and almost all energy is concentrated in the tooth tissue (i.e., in this case the thermocouple is heated mainly – see Fig.3a). Therefore the debonding is not effective even after 60 s at a power of 10 W. Irradiation by the continuously running Tm:YAP laser gave good results in the brackets debonding. From the detailed investigation by the thermal imager it was found that 1.98 µm radiation is mainly concentrated in the bracket and bonding agent material (Fig.3b) and, therefore, debonding is successful. The issue was the temperature rise inside the tooth. A number of experiments without and with water cooling and with various doses of irradiation resulted in the optimal debonding conditions obtained. The results (irradiation by 1.98 µm, 1W, 60 s) are shown in Fig.3b. The temperature rise in this case was ~ 2°C, and the enamel was clean with the rest of adhesive resin. The laser energy degraded adhesive resin by thermal softening. When the irradiation time was longer (90 s), the temperature increase was recorded to be ~ 6°C and the enamel below the bracket started to melt. The enamel surface was overheated and also dental pulp was destroyed. After 60 s irradiation by the Tm:YAP laser with a power of 2 W, the temperature increase was measured to be ~ 9°C, and changes in the enamel structure were observable. The enamel surface was covered with a thin layer of resin. Laser debonding was easier, and only slight diffusion could be seen.

Fig. 3. Temperature image of tooth with bracket irradiated by GaAlAs diode for 60 s (left) and by Tm:YAP laser with a power of 1 W for 60 s.

5. CONCLUSION Ceramic brackets debonding by the two various laser radiations were investigated. The continuously running semiconductor GaAlAs laser with a wavelength of 0.808 µm, and the diode pumped solid state Tm:YAP laser generating in a range of 1.98 µm were used, and the effects of removing ceramic brackets after irradiation were investigated. It has been found that the near infrared radiation from the GaAlAs laser is transmitted through the bracket and bonding agent, and heat generated by this radiation is concentrated into the tooth resulting in unacceptable increase tooth temperature but in no effect for debonding. The mid-infrared 1.98 µm radiation from the Tm:YAP laser with the output power 1W acting 60 s and water cooling offers the optimum dose for brackets debonding. From the SEM measurement the minimum damage of enamel for this case was found.

ACKNOWLEDGEMENT This research has been supported by the Grant of the Czech Ministry of Education No.MSM6840770022 "Laser systems, radiation and modern optical applications".

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N.R.Gorback, “Heat removal of ceramic brackets”, J. Clin. Orthod.125, pp.42-47, 1991.


2.

M.E. Vukovich, D.P. Wood, T.D. Daley, “Heat generated by grinding during removal of ceramic brackets”, Am. J Orthod. Dentofac. Orthop. 9, p. 505- 512, 1990. 3. K.V.Krell, J..M.Cwrey, S.E.Bishara, “Orthodontic bracket removal using conventional and ultrasonic debonding techniques, enamel loss, and time requirements”, Am. J. Orthod. Dentofac Orthop, 103, p. 358 366, 1993. 4. S.E.Bishara, D.E.Fehr, “ Comparisons of the effectiveness of pliers with narrow and wide blades in debonding ceramic brackets”, Am. J. Orthod. Dentofac Orthop 103, p. 253- 257, 1993. 5. S.E.Bishara, I.S.Trulove, “ Comparison of different debonding techniques of ceramic brackets: an in vitro study, part I- background and methods”, Am. J. Orthod. Dentofac. Onhop. 93, p. 145-53, 1990. 6. R.M.Tocchio, P.T.Williams, F.J.Mayor, K.G.Standing, “ Laser debonding of ceramic orthodontic brackets”, Am. J. Orthod. Dentofac. Orthop.,103, p. 155 - 163, 1993. 7. J.L. Rickabaugh, R.D.Marangoni, K.K. McCaffrey, “Ceramic debonding with the carbon dioxide laser.”, Am. J. Orthod Dentofac Orthop.,110, 88-93, 1993. 8. L.Zach, G.Cohen, “ Pulp response to externally applied heat”, Oral Surg. Oral Med. Oral Pathol. 19, p. 515530, 1965. 9. P. Černý, J. Šulc, H.Jelínková, ”Continuously tuneable diode-pumped Tm:YAP laser”, Solid State Lasers and Amplifiers II, A.Sennaroglu, Editors, Proceedings of SPIE Vol.6190, p.54-60, 2006. 10. F.A. Rueggeberg, P. Lockwood, “Thermal debracketing of orthodontic resins” Am J Orthod Dentofac Orthop. 98, p. 56-65,1990.
