Effects of CO 2 laser debonding of a ceramic bracket on the mechanical properties of enamel

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1 Original Article Effects of CO 2 laser debonding of a ceramic bracket on the mechanical properties of enamel Masahiro Iijima a ; Yoshitaka Yasuda b ; Takeshi Muguruma c ; Itaru Mizoguchi d ABSTRACT Objective: To investigate the effects of CO 2 laser debonding of a ceramic bracket on the mechanical properties of tooth enamel. Materials and Methods: Fifty-three human premolars were used in this study. The temperature changes of cross-sectioned specimens during laser irradiation were monitored with an infrared thermographic microscope system. Different laser output settings (3, 4, 5, and 6 W) were compared. The shear bond strength of brackets after laser irradiation was measured for specimens bonded with a conventional etch and rinse adhesive or with a self-etching adhesive, and the adhesive remnant index score was calculated. The hardness and elastic modulus of crosssectioned enamel after laser irradiation were investigated by the nanoindentation test. Data were compared by one-way and two-way analysis of variance, followed by the Scheffé test. Results: The temperature of enamel increased by about 200uC under CO 2 laser irradiation with a relatively high output (5 and 6 W), and a temperature increase of about 100uC to 150uC was seen under laser irradiation with a low output (3 and 4 W). The bracket shear bond strength decreased under all laser irradiation conditions. The hardness and elastic modulus of enamel were not affected by CO 2 laser debonding. Conclusion: CO 2 laser debonding may not cause iatrogenic damage to enamel. (Angle Orthod. 2010;80: ) KEY WORDS: Laser debonding; Enamel; Infrared thermography; Shear bond strength; Nanoindentation test INTRODUCTION Most ceramic brackets are made of polycrystalline alumina, single-crystal alumina, or zirconia. 1 A major clinical concern when using ceramic brackets is the risk of enamel damage (fracture) at debonding, 2 since ceramic brackets show a high bracket bond strength 2,3 and are probably difficult to deform because of their characteristic brittleness, with a low ductility and high elastic modulus compared with metal brackets. 1 To avoid enamel fracture, different debonding techniques have been suggested, such as ultrasonic debonding, electrothermal debonding, and the use of specially designed debonding instruments. 4,5 In addition, lasers have been used experimentally to debond ceramic brackets. Laser energy can degrade the adhesive resin as a result of the effects of thermal softening, thermal ablation, and photoablation. 6 It is possible that this heat may propagate to the tooth structure and eventually lead to pulp damage. 2 Although an increase in the pulpal cavity temperature of 5.5uC might cause pulpal necrosis, 7 it has been reported that appropriate laser irradiation decreases the bracket bond strength without a remarkable temperature increase in the pulpal cavity. 2,8 On the other hand, although organic components such as protein and free water comprise only a minor part of mature enamel, they are crucial to its development and are important in understanding its structural organization and physical properties. 9 Previa Associate Professor, Division of Orthodontics and Dentofacial Orthopedics, Department of Oral Growth and Development, Health Sciences University of Hokkaido, Hokkaido, Japan. b Private Practice, Nishinomiya, Japan. c Instructor, Division of Orthodontics and Dentofacial Orthopedics, Department of Oral Growth and Development, Health Sciences University of Hokkaido, Hokkaido, Japan. d Professor, Division of Orthodontics and Dentofacial Orthopedics, Department of Oral Growth and Development, Health Sciences University of Hokkaido, Hokkaido, Japan. Corresponding author: Dr Masahiro Iijima, Division of Orthodontics and Dentofacial Orthopedics, Department of Oral Growth and Development, Health Sciences University of Hokkaido, 1757, Kanazawa, Ishikari-tobetsu, Hokkaido Japan ( iijima@hoku-iryo-u.ac.jp) Accepted: May Submitted: April G 2010 by The EH Angle Education and Research Foundation, Inc. DOI: /

2 1030 IIJIMA, YASUDA, MUGURUMA, MIZOGUCHI ously, He and Swain 10 investigated the effects of heat treatment (300uC) on the mechanical properties of enamel using the nanoindentation test and found that the hardness and elastic modulus of heat-treated enamel were influenced dramatically: the protein matrix and water within burnt enamel were destroyed and removed. The embrittlement of enamel may cause iatrogenic damage to enamel, such as fracture and the creation of other surface defects. No previous studies have studied the effect of laser irradiation on the mechanical properties of enamel. The purposes of this study were to investigate the effects of CO 2 laser debonding of a ceramic bracket on the mechanical properties of tooth enamel, such as the hardness and elastic modulus, by the nanoindentation test, while monitoring the temperature change with an infrared thermographic microscope system, and to measure the bracket bond strength. MATERIALS AND METHODS Materials Fifty-three noncarious human premolars were used in this study. The teeth had been extracted for orthodontic reasons with the patients informed consent. Three premolars were used to assess the temperature change during laser debonding by infrared thermography. The remaining teeth were randomly divided into 10 groups of five specimens each for measurements of shear bond strength (SBS) and the nanoindentation test. Selection criteria included the absence of any visible decalcification or cracking of the enamel surface as viewed under a stereoscopic microscope (SMZ 1500, Nikon, Tokyo, Japan) at a magnification of 310. The buccal surfaces of all teeth were cleaned using nonfluoridated pumice. The teeth were also polished using a rubber cup, thoroughly washed, and dried using a moisture-free air source. Assessment of the Change in Temperature Single-crystal brackets (Inspire ice, Ormco, Glendora, Calif) were bonded to the buccal surfaces of premolars with a conventional etch and rinse adhesive system (Transbond XT, 3M Unitek, Monrovia, Calif) according to the manufacturer s instructions. The specimens were stored in artificial saliva at 37uC for 24 hours and then cut horizontally with a slow-speed water-cooled diamond saw at the height of the bracket slot so that they were divided into occlusal and cervical halves; the roots were cut horizontally at approximately 1 cm from the apical surface (Figure 1). The specimen was fixed on the specimen stage of an infrared thermographic microscope system (FSV-GX7000, Apiste, Osaka, Japan) with insulated tape. A CO 2 Figure 1. Schematic illustration of specimen prepared for the infrared thermographic microscope analysis. laser (continuous wave) with a wavelength of 10.6 mm (Nano Laser GL-III, GC, Tokyo, Japan) and a spot diameter of 0.45 mm was applied to one spot (bracket wings) for 5 seconds. The tip of the attachment was positioned as close as possible to the bracket wings (distance: approximately 1 mm). Temperature changes at the surface of the cross-sectioned specimens during laser irradiation and 60 seconds after irradiation were measured using an infrared thermal imaging microscope. Different laser output settings (3, 4, 5, and 6 W) were compared; all experiments were carried out at room temperature. Measurement of SBS Single-crystal brackets were bonded to the buccal surfaces of premolars with either a conventional etch and rinse adhesive system (Transbond XT, 3M Unitek) or a self-etching adhesive system (Transbond Plus, 3M Unitek) according to the manufacturer s instructions. Excess bonding material was removed with an explorer. All samples were light-cured for 10 seconds. After the bonding procedures, the specimens were stored in artificial saliva at 37uC for 24 hours. The specimens were fixed with a custom-fabricated acrylic resin block using Model Repair II (Densply-Sankin, Tokyo, Japan) and the block was fixed with a universal testing machine (EZ Test, Shimadzu, Kyoto, Japan). The CO 2 laser was applied to four spots (each bracket wing) for 5 seconds each. The tip of the attachment was positioned as close as possible to the bracket wings. Different laser output settings (3, 4, 5, and 6 W) were compared for specimens bonded with a conventional etch and rinse adhesive system and those bonded with a self-etching adhesive system (n 5 5 for each system and each adhesive system, with additional sets of nonirradiated specimens serving as

3 CO 2 LASER DEBONDING OF A CERAMIC BRACKET 1031 Figure 2. Schematic illustration of cross-sectioned tooth for the nanoindentation test. controls). After the CO 2 laser was applied, a knifeedged shearing blade was secured to the crosshead with the direction of force parallel to the buccal surface and the bracket base; the brackets were then debonded at a crosshead speed of 0.5 mm/min. After bond failure, the bracket bases and enamel surfaces were examined with a stereoscopic microscope at a magnification of 310. The adhesive remnant index (ARI) scores were used to assess the amount of adhesive left on the enamel surface. ARI scores ranged from 0 to 3: 0 5 no adhesive left on the tooth surface, and the failure site was between the adhesive and the enamel; 1 5 less than half of the adhesive was left on the tooth surface; 2 5 at least half of the adhesive was left on the tooth; 3 5 all of the adhesive was left on the tooth surface, and the failure site was between the adhesive and the bracket base. Nanoindentation Test After SBS was measured, three representative specimens from each condition (a total of 30 specimens) were chosen and divided with a slow-speed water-cooled diamond saw into occlusal and cervical halves. The occlusal halves were then encapsulated in epoxy resin for the nanoindentation test. All samples were ground (600-grit sandpaper) and polished using diamond suspensions (particle sizes of 3, 1, and 0.25 mm) to obtain a suitably polished surface. All nanoindentation testing was carried out at 28uC (ENT- 1100a, Elionix, Tokyo, Japan) with a peak load of 20 mn using a Berkovich indenter. Each test consisted of three parts: 10 seconds for loading to the peak value, 1 second of holding at the peak load, and 10 seconds for unloading. The indentations were placed at depths of 1 to 1001 mm from the surface (20 locations spaced 50 mm apart) in three regions on each specimen (Figure 2). Figure 3 shows a schematic representation of the loading-unloading curve obtained by the nanoindentation testing. Linear extrapolation methods (ISO standard 14577) 11 were used for the unloading curve between 95% and 70% of the Figure 3. Schematic representation of the loading-unloading curve obtained by nanoindentation testing. maximum test force to calculate the elastic modulus. The hardness and elastic modulus were calculated by software available with the nanoindentation apparatus. Statistical Analysis Statistical comparisons were performed with Statistical Package for the Social Sciences software (SPSS Version 16.0J for Windows, Chicago, Ill). The mean SBS values for the 10 specimen groups were compared by one-way and two-way analysis of variance (ANOVA), followed by the Scheffé multiplerange test at the 5% significance level. The two factors for ANOVA were the adhesive material (etch and rinse adhesive system or self-etching adhesive system) and the laser output condition (3, 4, 5, or 6 W). The mean hardness and elastic modulus for the 10 specimen groups were compared by one-way ANOVA followed by the Scheffé multiple-range test at the 5% significance level. The chi-square test was used to evaluate the significance of differences in the ARI scores among the different groups. For the statistical analysis, ARI scores 0 and 1 were combined, as were ARI scores of 2 and 3. RESULTS Representative images obtained 5 seconds after starting CO 2 laser irradiation as registered by the infrared thermographic microscope system are shown in Figure 4. With regard to the change in the temperature of cross-sectioned enamel, irradiation at a relatively high output (5 and 6 W) resulted in an increase of about 200uC, while a low output (3 and 4 W) caused an increase of about 100uC to 150uC. Representative thermographic images at 30 seconds after irradiation by the CO 2 laser are shown in Figure 5. For all of the laser output conditions, the

4 1032 IIJIMA, YASUDA, MUGURUMA, MIZOGUCHI Figure 4. Representative images after 5 seconds of irradiation with a CO2 laser registered by the infrared thermographic microscope system. temperature of cross-sectioned enamel had fallen to room temperature by 30 seconds after irradiation. The SBS results after irradiation with a CO2 laser are shown in Figures 6 and 7. In two-way ANOVA, the adhesive material used (conventional etch and rinse adhesive system vs self-etching adhesive system; P 5.000) was a statistically significant factor. The laser output (3, 4, 5, or 6 W; P 5.000) was also a statistically significant factor. In one-way ANOVA with the Scheffe test, specimens that were bonded with the conventional etch and rinse adhesive system showed a significantly lower SBS than the control (nonirradiated) specimens under any of the laser output conditions. For specimens bonded with self-etching adhesive, irradiation at 3 W resulted in a significantly higher SBS than specimens irradiated at 4, 5, or 6 W, although the value was significantly lower than that of the control specimen. The ARI scores for the 10 groups tested are shown in Table 1. The chi-square test results indicated no significant difference in bond failure sites among the 10 groups. Mean values for hardness and the elastic modulus of cross-sectioned enamel specimens after the measurement of SBS are shown in Figures 8 and 9. Locations at 1 mm from the enamel surface for the specimens bonded with both the etch and rinse adhesive system and the self-etching adhesive system had significantly lower values for hardness and elastic modulus than other locations. The specimens that were irradiated with a CO2 laser and a control specimen showed similar values for hardness and elastic modulus in all locations, and the mean values for locations at 51 to 1001 mm from the enamel surface (hardness: GPa; elastic modulus: GPa) were similar to those obtained in previous reports using the nanoindentation test (hardness: GPa; elastic modulus: GPa). DISCUSSION In this study, the bracket SBS decreased following CO2 laser irradiation; these results agree with those of previous reports.2,6 Previous studies8,12 showed greater bond strength for bisphenol A glycidyl methacrylate resin under laser irradiation than methylmethacrylate resin. The present study compared the bracket SBS for specimens bonded with an etch and rinse adhesive system and a self-etching adhesive system, both of which contained bisphenol A glycidyl methacrylate resin. Specimens irradiated at 3 W showed a similar percentage decrease in SBS, whether they were bonded with a conventional etch and rinse adhesive system (31%) or a self-etching adhesive system (25%), compared to control (nonirradiated) specimens. However, specimens bonded with the self-etching adhesive system showed a remarkable decrease in SBS with an increase in laser output compared with specimens prepared with a conventional etch and rinse adhesive system. The main reason for this may be that the self-etching adhesive systems produce a shallower depth of resin penetration into intact enamel as a result of a milder etching effect compared to conventional etch and rinse adhesive systems13 and therefore it may be possible to deteriorate resin tags using a low laser output. This study used an infrared thermographic microscope system to investigate the change in temperature during the CO2 laser debonding procedure. Infrared thermography is preferred over the thermocouple method to monitor changes in temperature since the temperature can be analyzed over a large area, Figure 5. Representative images after 30 seconds of irradiation with a CO2 laser registered by the infrared thermographic microscope system.

5 CO 2 LASER DEBONDING OF A CERAMIC BRACKET 1033 Table 1. Frequency Distribution of ARI Scores of Tested Groups Figure 6. Means and standard deviations for SBS (in MPa) after irradiation with a CO 2 laser in four specimen groups and one nonirradiated specimen group (control) bonded with a conventional etch and rinse adhesive system. Bars with identical letters indicate that average values are not significantly different (one-way ANOVA and Scheffé multiple-range test; P,.05). although the measurement area is limited to the surface. Therefore, this study used cross-sectioned tooth specimens to monitor the temperature change during a laser debonding procedure; however, the propagation of heat in the enamel close to the surface might be slightly different from that which occurs under actual clinical conditions. Also, the measurement was performed at room temperature, which is lower than typical intraoral temperatures. In addition, measurement of the temperature of the pulpal cavity using a cross-sectioned specimen may be unreasonable because the pulpal cavity is located far from the tooth surface and the propagation behavior of heat is Figure 7. Means and standard deviations for SBS (in MPa) after irradiation with a CO 2 laser in four specimen groups and one nonirradiated specimen group bonded with a self-etching adhesive system. Bars with identical letters indicate that average values are not significantly different (one-way ANOVA and Scheffé multiplerange test; P,.05). Adhesive Conventional etch and rinse Laser Output, W ARI Scores a None (control) Self-etching None (control) a ARI scores: 0 indicates no adhesive left on tooth surface, with the failure site between the adhesive and enamel; 1, less than half of adhesive left on tooth surface; 2 at least half of the adhesive left on tooth; and 3, all adhesive left on tooth surface, with the failure site between the adhesive and the bracket base. ARI scores of 0 and 1 were combined (less than 50% of adhesive left on tooth surface), as were 2 and 3 (50% or more of adhesive left on tooth surface), for the chi-square test. The results showed no statistical difference among groups. different from the actual clinical situation. While a high laser output effectively decreases the bracket bond strength, it is important to consider the damage done to the tooth by heating. 2,7 Previous studies 2 have reported that the increase in the temperature of the pulpal cavity upon laser irradiation was lower than the temperature at which the tooth pulp is necrotized (5.5uC). However, a lower output should help to protect the tooth pulp. A recent study 14 used the nanoindentation test to examine the mechanical properties (hardness and elastic modulus) of the orthodontic wires, and the results were compared with values for hardness and elastic modulus obtained by the conventional Vickers hardness test, three-point bending test, and tension test. The study showed that different testing methods did not yield identical values for elastic modulus and hardness, although the order among the five wire products was always the same; the variations in results can be attributed to the different material volumes being sampled, the different work-hardening levels, and perhaps the effect of the oxide layer on the surface. The nanoindentation test is a suitable method for the evaluation of mechanical properties for small volumes of materials. Another recent study 15 using the nanoindentation test showed that the hardness and elastic modulus of the enamel surface region (1 and 5 mm from the enamel surface) decreased after bracket bonding with a conventional etch and rinse adhesive system, although the self-etching adhesive system showed a minimal decrease in the hardness and elastic modulus of the enamel surface region after bracket bonding. In the present study, the mean hardness and elastic modulus at 1 mm from the

6 1034 IIJIMA, YASUDA, MUGURUMA, MIZOGUCHI Figure 8. Mean values of (a) the hardness and (b) elastic modulus of cross-sectioned enamel specimens bonded with a conventional etch and rinse adhesive system. enamel surface for specimens bonded with a conventional etch and rinse adhesive system or a self-etching adhesive system were significantly lower than those at other locations, although this tendency was stronger for the specimen bonded with the conventional etch and rinse adhesive system because of its stronger etching ability. All of the specimens showed a tendency for decreasing hardness and elastic modulus values with an increase in the distance from the surface (from 51 mm), and this trend is consistent with a previous report. 10 Previous studies 16 have characterized the structural changes in enamel heated from 100uC to 700uC and found that most enamel structures become positively birefringent when heated to 400uC. Another study found that a slight contraction of the alpha-axis dimension in the crystal lattice of enamel apatite occurred at 250uC to 400uC. 17 He and Swain 10 investigated the effects of heat treatment (300uC) on the mechanical properties of enamel using the nanoindentation test and found that the hardness and elastic modulus of heat-treated enamel were dramatically affected, since the protein matrix and water within burnt enamel were destroyed and removed. The mechanical properties such as hardness and elastic modulus observed in the present study were not affected by CO 2 laser irradiation. This is believed to be a result of the fact that the crystal structure of enamel was stable even after CO 2 laser irradiation because the temperature of enamel did not reach 300uC, which was confirmed by monitoring with an infrared thermographic microscope, suggesting that CO 2 laser debonding may not cause iatrogenic damage to enamel. Figure 9. Mean values of (a) the hardness and (b) elastic modulus of cross-sectioned enamel specimens bonded with a self-etching adhesive system.

7 CO 2 LASER DEBONDING OF A CERAMIC BRACKET 1035 CONCLUSIONS N The temperature of cross-sectioned enamel increases by about 200uC under CO 2 laser irradiation with a relatively high output (5 and 6 W), while the temperature increases by about 100uC to 150uC under laser irradiation with low output (3 and 4 W). N The hardness and elastic modulus of enamel are not affected by CO 2 laser irradiation. N CO 2 laser debonding may not cause iatrogenic damage to enamel. ACKNOWLEDGMENT The authors thank Mr Michio Toishi at GC for providing expert technical assistance with the CO 2 laser bracket debonding technique. REFERENCES 1. Eliades T, Eliades G, Brantley WA. Orthodontic brackets. In: Brantley WA, Eliades T, eds. Orthodontic Materials: Scientific and Clinical Aspects. Stuttgart, Germany: Thieme; 2001: Azzeh E, Feldon PJ. Laser debonding of ceramic brackets: a comprehensive review. Am J Orthod Dentofacial Orthop. 2003;123: Odegaard J, Segner D. Shear bond strength of metal brackets compared with a new ceramic bracket. Am J Orthod Dentofacial Orthop. 1988;94: Bishara SE, Trulove TS. Comparison of different debonding techniques for ceramic brackets: an in vitro study. Part I. Background and methods. Am J Orthod Dentofacial Orthop. 1990;98: Swartz M. A history lesson: Inspire sapphire brackets. In: DeCarlo J, ed. Clinical Impressions. Glendora, CA: Ormco; 2001;10: Tocchio RM, Williams PT, Mayer FS, Standing KG. Laser debonding of ceramic orthodontic brackets. Am J Orthod Dentofacial Orthop. 1993;103: Zach L, Cohen G. Pulp response to externally applied heat. Oral Surg Oral Med Oral Path. 1965;19: Hayakawa K. Nd:YAG laser for debonding ceramic orthodontic brackets. Am J Orthod Dentofacial Orthop. 2005;128: Masa MC, Dumont ER. Built to last: the structure, function, and evolution of primate dental enamel. Evolutionary Anthropol. 1999;8: He LH, Swain MV. Influence of environment on the mechanical behavior of mature human enamel. Biomaterials. 2007;28: ISO Metallic Materials Instrumented Indentation Test for Hardness and Materials Parameters Part 1: Test Method. 1st ed. Geneva, Switzerland. International Organization for Standardization; Mimura H, Deguchi T, Obata A, Yamagishi T, Ito M. Comparison of different bonding materials for laser debonding. Am J Orthod Dentofacial Orthop. 1995;108: Fjeld M, Øgaard B. Scanning electron microscopic evaluation of enamel surfaces exposed to 3 orthodontic bonding system. Am J Orthod Dentofacial Orthop. 2006;130: Iijima M, Muguruma T, Brantley WA, Mizoguchi I. Comparison of nanoindentation, three-point bending and tension tests for orthodontic wires. Am J Orthod Dentofacial Orthop. In press. 15. Iijima M, Muguruma T, Brantley WA, Ito S, Yuasa T, Saito T, Mizoguchi I. Effect of bracket bonding on nanomechanical properties of enamel. Am J Orthod Dentofacial Orthop. In press. 16. Oho T, Morioka T. A possible mechanism of acquired acid resistance of human dental enamel by laser irradiation. Caries Res. 1990;24: Fowler BO, Kuroda S. Changes in heated and in laserirradiated human tooth enamel and their probable effects on solubility. Calcif Tissue Int. 1986;38: