Orthodontic bracket bonding with a plasma-arc light and resin-reinforced glass ionomer cement

ORIGINAL ARTICLE

Orthodontic bracket bonding with a plasma-arc light and resin-reinforced glass ionomer cement Haruo Ishikawa, DDS, MSD, PhD,a Akira Komori, DDS, PhD,b Iori Kojima, DDS,c and Fumihito Ando, DDS, PhDd Tokyo, Japan Developments in light-curing technology have led to the introduction of a plasma-arc light-curing unit that delivers high-intensity output for faster curing. The purposes of this study were to determine the shear bond strengths of light-cured resin-reinforced glass ionomer cement cured with a plasma-arc light-curing unit and to evaluate the durability of the resultant bond strength with thermal cycling. Comparisons were made between light-cured resin-reinforced glass ionomer cement and light-cured composite resin. Two light-curing units were used in this study: a plasma-arc light-curing unit and a conventional light-curing unit. The mean shear bond strengths of light-cured resin-reinforced glass ionomer cement with the plasma-arc and the conventional lightcuring units were 20.3 MPa and 26.0 MPa, respectively. An analysis of variance showed no statistically significant differences between the plasma-arc and the conventional light-curing units. Light-cured resinreinforced glass ionomer cement and light-cured composite resin demonstrated similar bond strengths and exhibited no statistical differences. There was no statistical difference in bond strength between the teeth that were thermal cycled and those that were not. Failure sites for the brackets bonded with light-cured resinreinforced glass ionomer cement appeared to be predominantly at the bracket-adhesive interface. The SDs of light-cured composite resin were high for both light-curing units. Whereas the coefficients of variation for lightcured resin-reinforced glass ionomer cement ranged from 20% to 30%, those of light-cured composite resin ranged from 40% to 60%. The bond strength of light-cured resin-reinforced glass ionomer cement cured with either a conventional light-curing unit or a plasma-arc light-curing unit surpassed the clinically required threshold. The plasma-arc light-curing unit may be an advantageous alternative to the conventional light-curing unit for orthodontic bracket bonding with both light-cured resin-reinforced glass ionomer cement and light-cured composite resin. (Am J Orthod Dentofacial Orthop 2001;120:58-63)

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rthodontic brackets have been bonded with light-cured resin routinely since photoactivated restorative materials were developed. A primary advantage associated with a light-cured bonding system is the rapid polymerization that occurs with exposure to visible light, which produces on-command polymerization. This characteristic enables the bracket to be precisely placed on the tooth within a controlled working time. The archwire can also be immediately engaged in the brackets. Light-cured bonding systems can be divided into 3 categories: light-cured composite resins (LCR), lightcured resin-reinforced glass ionomer cements (LCGIC),

From the Department of Orthodontics, Nippon Dental University, Tokyo, Japan. aPrivate Practice, Tokyo, Japan; Former Professor and Chair. bAssistant Professor. cInstructor and Research Fellow. dInstructor and Research Fellow. Reprint requests to: Akira Komori, Department of Orthodontics, Nippon Dental University, 2-3-16 Fujimi, Chiyoda-ku, Tokyo 102-8158, Japan; e-mail, [email protected]. Submitted, September 2000; revised and accepted, December 2000. Copyright © 2001 by the American Association of Orthodontists. 0889-5406/2001/$35.00 + 0 8/1/115148 doi:10.1067/mod.2001.115148

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and light-cured compomers. A recent article reported that LCGIC exhibited all the qualities needed to bond brackets without the use of phosphoric acid etching.1 LCR and LCGIC subjected to immediate light exposure with conventional light-curing units showed similar tensile and shear bond strengths.2 The curing reaction of LCGIC is a dual process; a conventional acid-base reaction occurs on mixing the material, and photochemical polymerization occurs on exposure to visible light. Whereas the acid-base reaction progresses gradually on mixing, the photochemical polymerization is initiated, and essentially completed, with exposure to visible light. The two parts of the process would desirably occur interactively. Developments in light-curing units have led to the introduction of a plasma-arc light-curing unit that delivers high-intensity output and exhibits accelerated light-curing times of 3 seconds per site.3 This rapidcuring feature saves considerable chairside time when compared with conventional light-curing units. It is not known however if the dual process of LCGIC is achieved and, additionally, if the LCGIC bond strength that results from plasma-arc light-curing is reduced. In previous reports, the use of a plasma-arc light-curing

Ishikawa et al 59

American Journal of Orthodontics and Dentofacial Orthopedics Volume 120, Number 1

unit for orthodontic bracket bonding was discussed, but the effects of high-intensity exposure on bond strength were not examined.4,5 The purposes of this study were to determine the shear bond strengths of LCGIC cured with a plasmaarc light-curing unit and to assess the amount of residual adhesives on the corresponding enamel surfaces. In addition, the durability of the bond formed between the bracket and the enamel was evaluated with thermal cycling. Comparisons were made between LCGIC and LCR. MATERIAL AND METHODS

One hundred twenty freshly extracted bovine mandibular incisors were obtained after the animals were killed. The crowns of the extracted teeth were intact with no signs of enamel fractures or caries noted under direct inspection. To test each adhesive, 120 teeth were divided randomly into 8 groups of 15 teeth each, corresponding to 2 light-curing units and 2 bonding agents, with or without thermal cycling. To standardize the enamel surface characteristics, the buccal surface of each crown was ground and finished with a polishing device, under running water. Progressively finer polishing of the enamel surface was performed with 120-, 600-, 1500- and 2400-grit waterproof abrasive paper. Two bonding adhesives were used in this study: Fuji ORTHO LC (GC, Tokyo, Japan) and Light-Bond (Reliance Orthodontic Products, Itasca, Ill). Fuji ORTHO LC is an LCGIC and was developed for use in orthodontic bracket bonding procedures. Light-Bond is an LCR and was used as a subject for comparison. Stainless steel orthodontic brackets with a mesh backing (350-0104; Ormco, Glendora, Calif) were selected for use in this study. The same operator (A.K.) carried out the bonding procedures and bond testing according to a standardized technique. For the LCGIC groups, the enamel surfaces were pretreated with an application of 10% polyacrylic acid (Ortho Conditioner; GC) for 20 seconds and then air-water rinsed. Powder and liquid were measured with an electronic balance. For the LCR groups, the enamel surfaces were acid-etched with a solution of phosphoric acid (Etching Agent EL24; Reliance Orthodontic Products) for 30 seconds. All specimens were rinsed thoroughly with an air-water spray and left wet. Just before bracket placement, each specimen was dried lightly with a 3-way syringe. The manufacturers’ recommendations were followed for the mixing and handling of Fuji ORTHO LC and LightBond. Mixed adhesive of Fuji ORTHO LC was left on a cold slab to control flow during the bonding procedure. After bracket placement, the excess bonding material was carefully removed with a scalpel.

A

B Fig 1. Light-curing units used in this study. A, Apollo 95E, plasma-arc light-curing unit with xenon arc lamp. B, NEW LIGHT VL-II, conventional light-curing unit with halogen lamp.

Two light-curing units, as seen in Figure 1, were used in this study: Apollo (Apollo 95E; Dental/Medical Diagnostics, Woodland Hills, Calif) and VL-II (NEW LIGHT VL-II; GC). In the groups subjected to Apollo, each bracket was exposed for 3 seconds at the incisal and gingival margins for a total of 6 seconds. VL-II is a conventional light-curing unit and was used as a subject for comparison. In the groups subjected to VL-II, each bracket was exposed for 20 seconds at the incisal and gingival margins for a total of 40 seconds. After light exposure, all bonded specimens were stored at 37°C for 24 hours at 100% humidity. At this time, half the groups were measured for shear bond strengths and the remaining groups were subjected to thermal cycling before bond testing. Thermal cycling with 2000 repetitions between 5°C and 55°C, with a 30-second dwell time in each bath, was performed to simulate accelerated aging by thermally induced stresses with a thermocycling apparatus. After completion of the thermal cycling, shear bond strength was measured.

60 Ishikawa et al

American Journal of Orthodontics and Dentofacial Orthopedics July 2001

Fig 2. Box plots show distribution of shear bond strength. Boundaries of boxes represent 25% and 75% of values, line in box represents median value, and lines connected to boxes by vertical bars indicate 10th and 90th percentiles.

Each specimen was mounted in a measuring device (AUTOGRAPH AGS-50A; Shimadzu, Kyoto, Japan) and loaded with a cross-head speed of 1 mm/minute. A custom chisel-shaped rod was used so that the force was exerted adjacent and parallel to the bracket base and applied to the bond interface, which also enabled measurement of shear stress failure.2 The force required to dislodge the bracket was recorded. To control systematic errors, the test results were measured in random order. The amount of residual adhesive was classified with the adhesive remnant index (ARI) developed by Årtun and Bergland,6 which consists of a 4-point scale with scores ranging from 0 to 3. A score of 0 indicates that no adhesive is left on the tooth, a score of 1 indicates that less than half the adhesive is left on the tooth, a score of 2 indicates that more than half the adhesive is left on the tooth, and a score of 3 indicates that all the adhesive is left on the tooth, including a distinct impression of the bracket mesh. The ARI score was assessed by the same operator (A.K.). The surfaces of the debonded teeth were inspected with an optical stereomicroscope. Means and SDs of shear bond strengths were calculated for each group. The data were subjected to an analysis of variance (ANOVA) to determine if any significant differences existed between the bond strengths of each group (P < .05). A Wilcoxon rank sum test was

carried out to determine if there were any significant differences between the ARI scores of LCGIC and LCR. RESULTS

Shear bond strength is represented in Table I and illustrated with box plots in Figure 2. The results of a 3-way ANOVA are expressed in Table II. The mean shear bond strengths of every group were more than 20 MPa. The mean shear bond strengths of LCGIC subjected to Apollo and VL-II were 20.3 MPa and 26.0 MPa, respectively, representing a reduction of approximately 20% for the Apollo group. The mean shear bond strengths of LCR subjected to Apollo and VL-II were 23.8 MPa and 20.4 MPa, respectively, representing a reduction of approximately 10% for the VL-II group. The ANOVA showed no statistically significant differences between Apollo and VL-II. LCGIC and LCR demonstrated similar bond strengths and indicated no statistical differences. There were no statistical differences in bond strengths between the groups that were thermal cycled and those that were not. The SDs of LCR were high for both light-curing units. The SDs of LCGIC were not as high when compared with those of the LCR groups. For example, whereas the coefficients of variation for LCGIC ranged from 20% to 30%, those of LCR ranged from 40% to 60%.

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American Journal of Orthodontics and Dentofacial Orthopedics Volume 120, Number 1

Table III illustrates the distribution of ARI scores for the 120 sites tested. LCGIC demonstrated higher ARI scores than did LCR with both light-curing units. A Wilcoxon rank sum test revealed a statistical difference between LCGIC and LCR for ARI scores (P < .01). No damage to the enamel surface was observed after debonding in any of the groups. DISCUSSION

Some reports have emphasized that conventional glass ionomer cements are not recommended for routine clinical orthodontic bracket bonding because of the weakness of the mechanical bond strength.7-10 Many variables, such as enamel type, testing device, bracket base design, storage media of teeth, method and direction of debonding, and cross-head speed, may affect the bond strength data in these investigations. Because there is no consensus on the materials and methods for orthodontic bond strength tests, evaluations of bonding agents should be based on both laboratory tests and clinical trials. McCourt et al11 stated that the shear bond strength of the adhesive to the bracket material was higher than 10 MPa. Miguel et al12 also reported that conventional glass ionomer cements showed a statistically significant higher failure rate (50.89%) in clinical trials, compared with composite resin (7.96%). However, shear bond strengths of the LCGIC, with the use of conventional light-curing units, after 24 hours and 24 hours with thermal cycling were 25.8 ± 3.8 MPa and 24.6 ± 4.9 MPa, respectively. There were no statistically significant differences between the shear bond strengths of LCR and LCGIC cured with a conventional light-curing unit. Fowler13 reported that there was no significant difference of failure rate between chemically cured composite resin and LCGIC. These observations suggest that LCGIC has the potential to resist forces that consistently change during the course of orthodontic treatment. The ARI as an in vitro assessment of the amount of adhesive is reproducible.14 In this study, the failure sites for brackets bonded with LCGIC appeared to be primarily at the bracket-adhesive interface as compared with LCR, resulting in a statistically significant difference between LCGIC and LCR for ARI scores. Because the majority of LCGIC remains on the tooth surface, the bond between LCGIC and the tooth is stronger than the bond strength recorded. If a higher bond strength is required, changes in bracket base design could be considered to enhance the bond. Several in vitro studies have reported that failure sites for LCGIC bonds occur at the enamel-adhesive interface, with the majority of adhesive attached to the bracket base.1,15 This may be due to bracket types and the surface condition of the enamel.

Table I. Shear

bond strengths in megapascals 24 h

Mean Apollo LCGIC LCR VL-II LCGIC LCR

Thermal cycling

Coefficient of variation SD (%)

Mean

SD

Coefficient of variation (%)

20.3 23.8

5.1 13.9

25.1 58.4

20.1 20.7

5.8 8.6

28.9 41.5

26.0 20.4

5.6 12.1

21.5 59.3

23.4 22.1

5.4 10.0

23.1 45.2

Sum of squares

F value

P value

14.145 90.828 32.865 226.875 4.720 10.920 97.921 8884.441

0.178 1.145 0.414 2.86 0.06 0.138 1.234

.6736 .2869 .5211 .0936 .8077 .7113 .2689

Table II. Results

of analysis of variance Degrees of freedom

Bonding agents (A) 1 Light curing units (B) 1 24 h/thermal cycling (C) 1 AB 1 AC 1 BC 1 ABC 1 Error 112

Table III. Distribution

of adhesive remnant index scores

24 h

Apollo LCGIC LCR VL-II LCGIC LCR

Thermal cycling

0

1

2

3

0

1

2

3

0 3

4 4

10 7

1 1

1 6

2 6

10 3

2 0

0 13

7 1

6 0

2 1

1 12

6 3

5 0

3 0

The curing phenomenon of LCGIC involves 2 reactions; a conventional acid-base reaction occurs on mixing the material, and a photochemical polymerization occurs with exposure to visible light. The set material has interpenetrating matrices formed by the ionic matrix from the acid-base reaction and the polymerization matrix from the photochemical reaction.16 This dual process is desirable. When a plasma-arc light-curing unit is used for bracket bonding with LCGIC, the dual process of LCGIC may not be synchronized because the duration of light exposure is extraordinarily short, which creates an adverse reaction in the bonding process. In this study however there was no statistically significant difference between a conventional light-curing unit and a plasma-arc light-curing unit. Curing with a plasma-arc

62 Ishikawa et al

light-curing unit did not contribute to a reduction in the bond strength of LCGIC. The initial set of LCGIC was caused by a polymerization of the resin component. The curing process of the glass ionomer component may be independent of light initiation. Because the bond strength of LCGIC is not affected by the duration of light exposure, adhesion to the tooth surface, promoted by the glass ionomer component, may be achieved before completion of polymer formation. Enamel fracture and abrasion associated with debonding are influenced by the degree of resin penetration and by the amount of decalcification that results from phosphoric acid etching. Bonding agents that rely on phosphoric acid etching are more likely to require enamel alteration. Therefore, many investigators have explored alternatives that would reduce the risk of enamel decalcification. Although the application of topical fluoride after etching and the use of a reduced concentration of phosphoric acid may decrease the risk of decalcification, they are unable to completely prevent enamel decalcification.17-19 Resin-reinforced glass ionomer cements such as LCGIC may be an advantageous solution for bracket bonding without the necessity of phosphoric acid etching. These resin-reinforced glass ionomer cements require only a 10% solution of polyacrylic acid as an enamel pretreatment. Pretreatment with polyacrylic acid accomplishes 2 objectives; it removes surface contaminants and alters the surface energy by diffusion of the acid and exchange of ions.20,21 These actions are termed “conditioning,” which should be distinguished from acid etching, which prepares the enamel surface before bonding with composite resin. Conditioning produces an uncontaminated surface without evidence of a prism-like etching pattern.10 Because conditioning facilitates a chemical bond between the glass ionomer and the enamel, conditioning with a 10% polyacrylic acid should be performed before bracket bonding with a resin-reinforced glass ionomer cement.22 The SDs of LCR were high for both light-curing units. Under certain clinical conditions where higher forces were in effect, some bracket bonds failed. The SDs of the LCGIC groups are not as high as those of the LCR groups. The coefficients of variation of LCGIC ranged from 21.5% to 28.9%. Powers et al23 stated that the goal in bond testing should be to achieve a coefficient of variation in the range of 20% to 30%. The coefficients of variation of LCGIC were within this range. In comparison, LCR may be technique sensitive because the SDs of LCR showed a wide variation. The main advantage of using a plasma-arc lightcuring unit is the reduction in chair time. With the conventional light-curing unit, it takes 40 seconds to bond

American Journal of Orthodontics and Dentofacial Orthopedics July 2001

each bracket; therefore, approximately 14 minutes are required for light exposure from premolar to premolar in both arches. In contrast, approximately 2 minutes are required for curing with the plasma-arc light-curing unit. Thus, a light-cured bonding system with a plasma-arc light-curing unit is a significant timesaving option in clinical practice. In addition, the combination of LCGIC and a plasma-arc light-curing unit not only saves time but also aids in the prevention of enamel loss. CONCLUSIONS

1. LCGIC and LCR exhibited similar bond strengths and showed no statistical differences. 2. No significant reduction of bond strength was observed in LCGIC cured with a plasma-arc lightcuring unit as compared with LCGIC cured with a conventional light-curing unit. 3. There was no statistical difference in bond strengths between teeth that were thermal cycled and those that were not. 4. LCGIC showed a lower coefficient of variation than did LCR. 5. The bond strength of LCGIC cured with either a conventional light-curing unit or a plasma-arc light-curing unit surpassed the clinically required threshold. 6. The plasma-arc light-curing unit can be recommended as an advantageous alternative to the conventional light-curing unit for orthodontic bracket bonding because it significantly reduces the light-curing time without affecting the shear bond strength. REFERENCES 1. Cacciafesta V, Jost-Brinkmann P-G, Süβenberger U, Miethke RR. Effects of saliva and water contamination on the enamel shear bond strength of a light-cured glass ionomer cement. Am J Orthod Dentofacial Orthop 1998;113:402-7. 2. Komori A, Ishikawa H. The effect of delayed light exposure on bond strength: light-cured resin-reinforced glass ionomer cement vs light-cured resin. Am J Orthod Dentofacial Orthop 1999;116:139-45. 3. Clinical Research Associates. New resin curing lights, high intensity vs. multi-mode intensity. Newsletter 1999;23:1-6. 4. Cacciafesta V, Sfondrini MF, Sfondrini G. A xenon arc light-curing unit for bonding and bleaching. J Clin Orthod 2000;34:94-6. 5. Silverman E, Cohen M. Bonding with a plasma-arc curing light and resin-modified glass ionomer. J Clin Orthod 2000;34:233-5. 6. Årtun J, Bergland S. Clinical trials with crystal growth conditioning as an alternative to acid-etch enamel pretreatment. Am J Orthod 1984;85:333-40. 7. Fajen VB, Duncanson MG, Nanda RS, Currier GF, Angolkar PV. An in vitro evaluation of bond strength of three glass ionomer cements. Am J Orthod Dentofacial Orthop 1990;97:316-22. 8. Øen JO, Gjerdet NR, Wisth PJ. Glass ionomer cements used as bonding materials for metal orthodontic brackets. An in vitro study. Eur J Orthod 1991;13:187-91. 9. Moseley HC, Horrocks EN, Pearson GJ, Davies EH. Effects of

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cyclic stressing on attachment bond strength using glass ionomer cement and composite resin. Br J Orthod 1995;22:23-7 Komori A, Ishikawa H. Evaluation of a resin-reinforced glass ionomer cement for use as an orthodontic bonding agent. Angle Orthod 1997;67:189-96. McCourt JW, Cooley RL, Barnwell S. Bond strength of lightcure fluoride-releasing base-liners as orthodontic bracket adhesives. Am J Orthod Dentofacial Orthop 1991;100:47-52. Miguel JAM, Almeida MA, Chevitarese O. Clinical comparison between a glass ionomer cement and a composite for direct bonding of orthodontic brackets. Am J Orthod Dentofacial Orthop 1995;107:484-7. Fowler PV. A twelve-month clinical trial comparing the bracket failure rates of light-cured resin-modified glass ionomer adhesive and acid-etch chemical-cured composite. Aust J Orthod 1998;15:186-90. Bradburn G, Pender N. An in vitro study of the bond strength of two light-cured composites used in the direct bonding of orthodontic brackets to molars. Am J Orthod Dentofacial Orthop 1992;102:418-26. Bishara SE, Olsen ME, Damon P, Jakobsen JR. Evaluation of a new light-cured orthodontic bonding adhesive. Am J Orthod Dentofacial Orthop 1998;114:80-7.

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16. Wilson AD. Resin-modified glass-ionomer cements. Int J Prosthodont 1990;3:425-9. 17. O’Reilly MM, Featherstone JDB. Demineralization and remineralization around orthodontic appliances: an in vivo study. Am J Orthod Dentofacial Orthop 1987;92:33-40. 18. Büyükyilmaz T, Øgaard B, Dahm S. The effect on the tensile bond strength of orthodontic brackets of titanium tetrafluoride (TiF4) application after acid etching. Am J Orthod Dentofacial Orthop 1995;108:256-61. 19. Wang WN, Yeh CL, Fang BD, Sun KT, Arvystas MG. Effect of H3PO4 concentration on bond strength. Angle Orthod 1994;64: 377-82. 20. Akinmade AO, Nicholson JW. Glass-ionomer cements as adhesives. Part I: Fundamental aspects and their clinical relevance. J Mater Sci, Mater Med 1993;4:95-101. 21. Mount GJ. Glass-ionomer cements: past, present and future. Oper Dent 1994;19:82-90. 22. Attin T, Buchalla W, Hellwig E. Influence of enamel conditioning on bond strength of resin-modified glass ionomer restorative materials and polyacid-modified composites. J Prosthet Dent 1996;76:29-33. 23. Powers JM, Kim H-B, Turner DS. Orthodontic adhesives and bond strength testing. Semin Orthod 1997;3:147-56.

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