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Effects of high-speed curing devices on shear bond strength and microleakage of orthodontic brackets
ORIGINAL ARTICLE
Effects of high-speed curing devices on shear bond strength and microleakage of orthodontic brackets Jeffrey W. James, DDS, MS,a Barbara H. Miller, DDS, MS,b Jeryl D. English, DDS, MS,c Larry P. Tadlock, DDS, MS,d and Peter H. Buschang, MA, PhDe Dallas, Tex This study evaluated the shear-peel bond strength and mode of bond failure of 3 curing devices (plasma arc light, argon laser, and conventional halogen light) and 2 orthodontic bracket adhesives with different filler contents (Transbond XT and Adhesive Precoated [APC]). Observations of microleakage were also reported. Ninety human adolescent premolars were randomly divided into 6 groups, and standardized brackets were bonded according to the manufacturers’ recommendations. The plasma arc light produced significantly (P ⫽ .006) higher bond strength than did the halogen light or the argon laser when Transbond was used. When APC was used, the plasma arc light and the halogen light produced similar results, and they both produced significantly (P ⫽ .015) higher bond strengths than did the argon laser. Overall, the APC showed substantially less variation in bond strength than did the Transbond. Although all curing methods showed significant microleakage (P ⬍ .001), differences among the 3 curing lights occurred only when APC was used. Microscopic evaluations demonstrated that 95% of the specimens failed for adhesion at the bracket or tooth surface; the argon laser produced the highest adhesive remnant index scores. On the basis of bond strength and microleakage results, the plasma arc light was comparable with or superior to the other curing devices, depending on the adhesive used. (Am J Orthod Dentofacial Orthop 2003;123:555-61)
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isible light-cured (VLC) adhesives have become increasingly more popular for bonding orthodontic attachments because they offer several advantages over chemically cured adhesives, including ease of use, extended working time, improved bracket placement, easier cleanup of excess adhesive, and better physical properties because air is not incorporated during mixing.1-3 The disadvantage of VLC adhesives is the time required to expose the adhesives to the curing device. The most common initiator used in VLC adhesives is camphoroquinone, Financial support provided by Baylor College of Dentistry; supplies donated by 3M Unitek. a Private practice, Palestine, Tex. b Associate professor, Department of Biomaterials Sciences, Baylor College of Dentistry, Dallas, Tex. c Associate professor and chairman, Department of Orthodontics, Baylor College of Dentistry, Dallas, Tex. d Assistant clinical professor, Department of Orthodontics, Baylor College of Dentistry, Dallas, Tex. e Professor and director of orthodontic research, Department of Orthodontics, Baylor College of Dentistry, Dallas, Tex. Reprint requests to: Dr Peter H. Buschang, Department of Orthodontics, Baylor College of Dentistry, The Texas A & M University System Health Science Center, 3302 Gaston Ave, Dallas, TX 75246; e-mail, phbuschang@ tambcd.edu. Submitted, September 2001; revised and accepted, May 2002. Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 ⫹ 0 doi:10.1016/S0889-5406(02)00019-7
which is sensitive to light in the blue region (450 to 500 nm) of the visible light spectrum, with the peak activity centered around 480 nm.4 Because of their unique characteristics, argon lasers and plasma arc lights have the potential to dramatically reduce the curing time of dental composites. The argon laser produces a highly concentrated, coherent beam of light centered around a wavelength of 480 nm, which is optimal for the activation of most dental composites. Likewise, the plasma arc light reduces curing time because it produces much more intensive light than does the halogen light.5 To improve practice efficiency, many orthodontists use argon lasers, plasma arc lights, and other high-speed curing devices, even though all aspects of their efficacy for curing orthodontic adhesives have not been fully investigated. Studies in restorative dentistry have demonstrated that curing composites with lasers and high-intensity lights causes increased polymerization shrinkage and marginal leakage (microleakage).6-11 Polymerization shrinkage also varies from composite to composite and depends on the percentage of filler, the diluent, and the percentage of monomer conversion in the specific composite resin.6,12 In addition, Miyazaki et al13 showed that polymerization shrinkage increases as the filler content decreases. Finally, several studies have 555
556 James et al
demonstrated that laser curing of restorative composites provides bond strengths comparable with those of conventional halogen lights.14,15 There has been increasing interest in lasers and high-intensity lights, such as the plasma arc, to cure adhesives, primarily because they cure the adhesive faster than do conventional halogen lights.14-20 By speeding up the curing process, these lights save chair time for doctor and patient.14 A few studies have shown that curing orthodontic adhesives with an argon laser or a plasma arc light produces bond strength comparable with that of conventional halogen lights.5,16-20 However, more documentation is needed, including determining whether the argon laser and the plasma arc light produce increased microleakage around the orthodontic brackets, which could increase the patient’s risk of decalcification. Furthermore, the effect of the filler content of the orthodontic adhesive on bond strength and microleakage should be more thoroughly evaluated. The purpose of this study was to use standardized test procedures to evaluate the shear-peel bond strength, the mode of bond failure, and the microleakage of 3 curing devices (argon laser, plasma arc light, and conventional halogen light) and 2 orthodontic bracket adhesives with different filler contents.21,22 MATERIAL AND METHODS
Ninety human adolescent premolars extracted for orthodontic purposes were collected over a 4-month period. The teeth were debrided and examined for caries, preexisting fractures, and restorations. The teeth were disinfected in 10% formalin for 2 weeks and then stored in deionized distilled water for the rest of the experiment. The teeth were randomly assigned to 6 subsamples with 15 teeth per subsample. The principal investigator (J.W.J.) bonded the samples during 2 sessions over a 24-hour period. Stainless steel twin premolar brackets (.018-in Victory Series, 3M Unitek, Monrovia, Calif) were used for all teeth. The brackets were bonded with 1 of 3 curing devices (the Ortholux XT halogen light [3M Unitek], the ILT Argon Laser [ILT Systems, Salt Lake City, Utah], and the Plasma Arc unit [American Dental Technologies, Inc, Corpus Christi, Tex]) and 2 orthodontic adhesives (Transbond XT and APC [Adhesive Precoated], 3M Unitek). According to the manufacturers, the Transbond XT and APC are approximately 77% and 80% quartz (silica) filler, respectively. Before bonding, the teeth were prepared according to the following protocol: (1) polished for 10 seconds with a rubber prophy cup, with oil-free and fluoride-free pumice and water, (2) rinsed for 10 seconds with an
American Journal of Orthodontics and Dentofacial Orthopedics May 2003
air-water syringe, (3) dried for 5 seconds with an air-water syringe, (4) acid-etched for 30 seconds with 37% phosphoric acid gel (3M Unitek), (5) rinsed for 10 seconds with an air-water syringe, (6) dried for 5 seconds with an air-water syringe, and (7) coated with primer (Transbond XT, 3M Unitek) that was thinned with a puff of air from the air-water syringe. The adhesive-loaded bracket was pushed to fully contact the tooth, and the flash was removed. The adhesives were cured according to the light manufacturers’ guidelines as follows: halogen light (772 mW/ cm2), 10 seconds mesially and distally (20 seconds total); argon laser (238 mW/cm2), 5 seconds mesially and distally (10 seconds total); and plasma arc unit (1407 mW/cm2), 3 seconds mesially and 2 seconds distally (5 seconds total). During the bonding procedure, the Cure Rite radiometer (Dentsply Caulk, Milford, Del) was used to record 21 light intensities for each curing device, and the average intensity was calculated. Light intensities were recorded before bonding, after every 2 samples (30 samples per light), and then 5 more times at 1-minute intervals. Five days after bonding, the teeth were thermal cycled for approximately 24 hours (1300 cycles). Each thermal cycle consisted of immersing the teeth for 30 seconds in water at 10°C, immediately followed by 30 seconds in water at 55°C. Next, the teeth were embedded in acrylic to approximately the level of the cementoenamel junction. To ensure that all the brackets were mounted in the same orientation relative to the acrylic cylinder, the teeth were suspended from a .017 ⫻ .025-in stainless steel archwire tied in the bracket slot. This mounting procedure ensured consistency for the point of force application and direction of the debonding force. The specimens were coded to blind the examiner to the adhesive and curing method used. The specimens were then placed in a container of methylene blue dye and deionized water for 6 days. After staining, the brackets were debonded with the Instron universal testing machine (Instron Corp, Canton, Mass). The specimens were placed in a jig so that the bracket base was parallel to the debonding force. A shear debonding force was applied to the bracket base in an occlusal-gingival direction at a crosshead speed of 0.1 mm/min. The area of the bracket base was measured with the University of Texas Health Science Center San Antonio Image Tool program (available on web site) to calculate megapascals (MPa). Debonded specimens were randomly examined at 50⫻ magnification with an optical microscope (Stereomicroscope SR, Zeiss, Oberkochen, Germany) to evaluate the microleakage and the mode of bond failure. Microleakage was determined by measuring the
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Table I.
Differences in light intensity, duration of curing, and total light energy among curing devices
Curing method Argon laser Halogen Plasma arc
Mean intensity (mW/cm2)*
SD (mW/cm2)
Range (mW/cm2)
Duration of curing (sec)
Total light energy (mW/cm2/sec)*†
238.1 771.9 1407.2
5.3 19.8 69.5
228.0-246.0 740.0-802.0 1251.0-1516.0
10 20 5
2381 15,438 7035
*P ⬍ .001. † Total light energy ⫽ mean intensity ⫻ duration of curing. Table II.
Differences in shear bond strength between adhesives and among curing devices Curing method Argon laser
Adhesive Transbond XT APC Difference P value
Halogen
Plasma arc
Method differences
Mean (MPa)
SD
Mean (MPa)
SD
Mean (MPa)
SD
P value
Post-hoc comparisons
4.4 4.2 0.2 .662
1.8 1.0 0.8 —
5.0 5.3 0.3 .631
1.6 1.0 0.6 —
6.6 5.0 1.6 .004
1.7 1.0 0.7 —
.006 .015 — —
P ⬎ (H ⫽ L) (P ⫽ H) ⬎ L — —
L, Argon laser; H, halogen; P, plasma arc.
deepest dye penetration on the tooth surface perpendicular to the bracket margin and rounding to the nearest .02 mm. Also, observations were made of the bracket surface to verify the observations of the tooth surface. Mode of bond failure was determined on the basis of the amount of adhesive remaining on the tooth and bracket pad and was expressed as a percentage of the total bonded area. Adhesive remnant index (ARI) scores23 were assigned to each specimen. An ARI score of 0 indicated that no adhesive was left on the tooth in the bonded area; 1 indicated that less than half of the adhesive was left on the tooth; 2 indicated that more than half was left on the tooth; and 3 indicated that all the adhesive remained on the tooth, with a distinct impression of the bracket mesh. The data were statistically analyzed for normality and homogeneity of variances. A 2-way analysis of variance (ANOVA) showed a significant interaction between the independent variables, making it necessary to evaluate separately the curing method and the adhesive. The curing methods were evaluated with a 1-way ANOVA followed by a post hoc least squared differences test. The adhesives were evaluated with an independent samples t test, and the ARI scores were analyzed with a Pearson chi-square test. To evaluate reliability, 15 specimens were randomly selected and reexamined. For the ARI scores, there were no differences between the original and the
replicate values. For the microleakage values, there were no systematic differences between the original and the replicate values with a method error of .075 mm (ME ⫽ 公⌺d2/2n). RESULTS
There were significant differences (P⬍ .001) in light intensity and total light energy among the 3 curing devices (Table I). The coefficients of variation for the lights were similar. The 1-way ANOVA revealed significant differences in shear-peel bond strength between curing methods for both adhesives (Table II). Although all curing methods showed significant microleakage (P ⬍.001), there was wide variation among curing devices and adhesives. The 1-way ANOVA showed significant differences in the microleakage for each curing method with APC, but not with Transbond (Table III). There was no significant correlation between shear-peel bond strength and microleakage r ⫽ .06; P ⫽ .61). The chi-square test showed significant differences in the ARI scores of the 3 curing devices (Table IV). Only 4 of the specimens showed small areas of cohesive failure. There were no incidences of enamel fracture. There were no significant differences in the ARI scores of the 2 adhesives (Table V).
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Table III.
American Journal of Orthodontics and Dentofacial Orthopedics May 2003
Differences in microleakage between adhesives and among curing devices Curing method Argon laser
Halogen
Plasma arc
Method differences
Mean (mm)
SD
Mean (mm)
SD
Mean (mm)
SD
P value
Post-hoc comparisons
.26 0.29 0.03 .674
0.11 0.24 0.13 —
0.31 0.35 0.04 .530
0.20 0.16 0.04 —
0.47 0.15 0.32 .004
0.36 0.11 0.25 —
.084 .013 — —
— (H ⫽ L) ⬎ P — —
Adhesive Transbond XT APC Difference P value
L, Argon laser; H, halogen; P, plasma arc.
Absolute and relative frequency of ARI scores for curing devices
Table IV.
ARI scores (%) Curing method Argon laser Halogen Plasma arc
1
2
3
Sample size (n)
4 (14.2) 13 (46.4) 8 (27.5)
9 (32.1) 8 (28.5) 13 (44.8)
15 (53.6) 7 (25.0) 8 (27.5)
28 28 29
2 ⫽ 10.1; df ⫽ 4; P ⫽ .038. 1, Less than 50% of adhesive on tooth; 2, more than 50% of adhesive on tooth; 3, 100% of adhesive on tooth.
Absolute and relative frequency of ARI scores for adhesives
Table V.
ARI scores (%) Adhesive Transbond XT APC
1
2
3
Sample size (n)
11 (26.2) 14 (32.6)
15 (35.7) 15 (34.9)
16 (38.1) 14 (32.6)
42 43
2 ⫽ 0.482; df ⫽ 2; P ⫽ .786.
DISCUSSION
In the present study, the in vitro shear-peel bond strengths of the adhesives were comparable with or less than previously reported for similar materials and methods.5,16,20,24,25 The bond strengths might have been lower because the samples were stressed by thermal cycling for approximately 24 hours (1300 cycles); samples in most previous studies of orthodontic adhesives were not thermal cycled. Studies in restorative dentistry have shown that thermal cycling of samples can decrease the bond strength by 20% to 70%.26,27 The brackets were also debonded with a slower crosshead speed (0.1 mm/min) than used in some previous studies. This crosshead speed (recommended by Fox et al21) and the similarly low speed of 0.5 mm/min (recommended by Eliades and Brantley22) are lower than what might be used clinically. The
higher impact velocity in vivo allows the viscoelastic properties of the adhesive to affect the bond strength. Lower crosshead speeds might better remove the influence of the material’s strain rate sensitivity, which might be expected to decrease bond strength. In addition, it has been shown that orthodontic adhesive bond strength depends on the type of tooth used; premolars show lower bond strength than mandibular first molars.28 In this study, adolescent permanent teeth were used, and it has been shown that the bond strengths of younger permanent teeth are less than those of older permanent teeth.29 The results of this study showed that the plasma arc light produced mean bond strengths that were comparable with or greater than those produced by the halogen light, depending on the type of adhesive. With Transbond, the plasma arc light yielded significantly higher bond strengths than did the halogen light or the argon laser. With APC, the plasma arc light and the halogen light produced similar bond strengths, and both yielded significantly higher bond strengths than did the argon laser. Previous studies have shown that curing APC and Transbond with a plasma arc light produces bond strength comparable with that produced by a halogen light.5,16 The plasma arc light might be expected to yield adequate bond strength for orthodontic adhesives even at the lower extreme values (Fig). Reference publications have suggested that adhesivebracket systems should be able to withstand a stress of 6 to 8 MPa30; however, many variables affect these numbers in vitro and in vivo, including thermal cycling. This study clearly showed that the argon laser produced the lowest bond strength with either adhesive. In addition, the argon laser produced the lowest extreme values with both adhesives; this could lead to an increase in the number of bond failures. Whereas the argon laser emits light over a narrower band of wavelengths in the blue-green spectrum (457.9 to 514.5 nm) than do conventional lights,18 it also
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Fig. Range of variation (5th to 95th percentiles) in shear bond strength for curing devices.
collimates the light, which results in a more consistent intensity over any given distance.31 However, even though the narrower wavelength and collimation are beneficial, the total light energy (intensity of the light times the duration of exposure) appears to be a more important factor when determining the degree of polymerization. The plasma arc light produced an intensity approximately 2 times greater than the halogen light and about 6 times greater than the argon laser. However, the halogen light yielded a total light energy approximately 2 times greater than the plasma arc light and about 6.5 times greater than the argon laser. Greater total light energy generally results in increased fracture toughness and greater flexural strength, which usually translate into greater bond strength.32-34 Because the argon laser cures at a considerably lower intensity (238.1 mW/cm2) than the plasma arc or halogen light, it might require curing times longer than the recommended 10 seconds to provide comparable total light energy. However, in this study, in an effort to follow the procedures and use the variables that would most likely be used by a clinical practitioner, we used the curing times recommended by the light manufacturers. Increasing the curing time to provide a comparable total light energy would eliminate the time-saving aspect and increase heat and therefore the possibility of tissue damage. In this study, the bond strengths of the 2 adhesives depended on the curing method, and the variation in bond strength was substantially different for the 2 adhesives. Differences in bond strengths between the 2 adhesives were restricted to the plasma arc light, which showed significantly higher bond strength for Transbond than for APC. Similarly, Bishara et al25 showed that uncoated metal brackets bonded with Transbond XT had significantly higher shear bond strength than brackets precoated with APC. There appears to be a
critical material-to-light interaction that requires further investigation. Because the APC had less variation in bond strength than Transbond with all curing devices (Fig), it is probably less technique-sensitive than Transbond. The manufacturer applied the APC to the bracket base, whereas the investigator applied the Transbond. Because the APC required fewer steps in the bonding procedure, there was a decreased risk for contamination, which might have decreased the variation. All curing methods showed significant microleakage, with amounts ranging from approximately 5% to 15% of the bracket width. There was no significant difference in microleakage between the curing methods when Transbond was used. With APC, the halogen light and the argon laser produced significantly more microleakage (approximately 5%) than did the plasma arc light. These results contradict studies in restorative dentistry that demonstrate greater polymerization shrinkage and marginal leakage with higher intensity lights.6-11 In restorative dentistry, composite is placed in a cavity preparation, and rapid curing can create excessive shrinkage and gap formation along the composite-preparation interface. In contrast, orthodontic adhesive layers are very thin; there is adhesive at the edges of the bracket to absorb some shrinkage, and, because the bracket is free floating, the shrinkage can pull the bracket closer to the enamel.5 Therefore, in orthodontic applications, polymerization shrinkage caused by high-intensity curing is probably less of a concern than it is in restorative dentistry. There was no significant difference in microleakage between the 2 adhesives except when the plasma arc light was used; then the Transbond had significantly more microleakage than did the APC. This finding is consistent with restorative dentistry studies that show that the filler content of the composite affects the polymerization shrinkage,6,12 and decreased filler con-
560 James et al
tent causes greater polymerization shrinkage.13 The Transbond had approximately 3% less filler than did the APC; this might have contributed to the increased microleakage. However, when the plasma arc light was used, the Transbond still had significantly higher bond strength than did the APC. Whereas the adhesive with the most microleakage might be expected to have the lowest bond strength, the results showed no correlation between microleakage and bond strength. In addition, the effects of filler content on microleakage were present only when the highest intensity light (plasma arc) was used. Once again, there appears to be an important material-to-light interaction. Microscopic evaluation of bond failure showed that 95% of the specimens failed for adhesion at either the bracket or the tooth surface. This agrees with other studies that demonstrated adhesive rather than cohesive failures.5,16,18 When we used the argon laser, most specimens had 100% of the adhesive remaining on the tooth surface, in accordance with the findings of Talbot et al.18 When the plasma arc light and the halogen light were used, approximately 75% of the specimens had less than 100% of the adhesive on the enamel; this is a clinical advantage because there is less adhesive to remove after debonding. The location of the bond failure is clearly affected by the curing method, and further investigations are needed to explain the differences. The bond strength and microleakage results in this study support the use of the plasma arc light to cure orthodontic adhesives. Because the plasma arc light requires approximately 25% of the amount of time that the halogen light requires, it more efficiently produces adequate bond strength. The brackets in an arch can be cured in about 1 minute 15 seconds with the plasma arc light versus 5 minutes with the halogen light. Therefore, curing with the plasma arc light saves considerable time for doctor and patient. The cost of the plasma arc light is approximately 3 to 4 times more than that of the halogen light, but half that of an argon laser. In a busy orthodontic practice, the initial extra cost of the plasma arc light can pay for itself by saving chair time. CONCLUSIONS 1. Curing orthodontic adhesives with a plasma arc light
for 5 seconds produces shear-peel bond strength that is comparable with or greater than that produced by the halogen light or the argon laser, depending on the type of adhesive used. In addition, the plasma arc light saves time for doctor and patient.
American Journal of Orthodontics and Dentofacial Orthopedics May 2003
2. The APC showed substantially less variation in bond strength than did the Transbond, suggesting that it is a less technique sensitive. 3. There was a significant difference in the location of the bond failures (ARI scores) among the 3 curing methods. When the plasma arc light or the halogen light was used, less adhesive remained on the enamel surface, making cleanup easier. 4. Whereas all curing methods showed evidence of microleakage, curing the adhesive with a highintensity light did not cause a significant increase in microleakage. Further investigation is needed in the area of microleakage around orthodontic brackets. The authors thank Phil Ford and Becky Fellows for helping with data collection and Julie Bradshaw for helping with manuscript preparation. REFERENCES 1. Pollack BF, Blitzer MH. The advantages of visible light curing resins. NY Dent J 1982;48:228-30. 2. Tavas MA, Watts DC. A visible light-activated direct bonding material: an in vitro comparative study. Br J Orthod 1984;11: 33-7. 3. Greenlaw R, Way DC, Galil KA. An in vitro evaluation of a visible light-cured resin as an alternative to conventional resin bonding systems. Am J Orthod Dentofacial Orthop 1989;96:21420. 4. Cook WD. Spectral distribution of dental photopolymerization sources. J Dent Res 1982;61:1436-8. 5. Oesterle LJ, Newman SM, Shellhart WC. Rapid curing of bonding composite with a xenon plasma arc light. Am J Orthod Dentofacial Orthop 2001;119:610-6. 6. Burgess JO, DeGoes M, Walker R, Ripps AH. An evaluation of four light-curing units comparing soft and hard curing. Pract Periodont Aesthet Dent 1999;11:125-32. 7. Davidson-Kaban SS, Davidson CL, Feilzer AJ, deGee AJ, Erdilek N. The effect of curing light variations on bulk curing and wall-to-wall quality of two types and various shades of resin composites. Dent Mater 1997;13:344-52. 8. Mehl A, Hickel R, Kunzelmann KH. Physical properties and gap formation of light-cured composites with and without “softstartpolymerization.” J Dent 1997;25:321-30. 9. Feilzer AJ, Dooren LH, deGee AJ, Davidson CL. Influence of light intensity on polymerization shrinkage and integrity of restoration-cavity interface. Eur J Oral Sci 1995;103:322-6. 10. Unterbrink GL, Muessner R. Influence of light intensity on two restorative systems. J Dent 1994;23:183-9. 11. Uno S, Asmussen A. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991;99:440-4. 12. Masutani S, Matsuzaki T, Akiyama Y. Study on light cured composite resins: consideration of the continuous volumetric shrinkage of resins during light irradiation. Jpn J Conserv Dent 1989;32:1605-11. 13. Miyazaki M, Hinoura K, Onose H, Moore BK. Effect of filler content of light-cured composites on bond strength to bovine dentine. J Dent 1991;19:301-3. 14. Shanthala BM, Munshi AK. Laser vs visible-light cured composite resin: an in vitro shear bond study. J Clin Pediatr Dent 1995;19:121-5.
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15. Hinoura K, Masashi M, Onose H. Influence of argon curing on resin bond strength. Am J Dent 1993;6:69-71. 16. Sfondrini MF, Cacciafesta V, Pistorio A, Sfondrini G. Effects of conventional and high-intensity light-curing on enamel shear bond strength of composite resin and resin-modified glassionomer. Am J Orthod Dentofacial Orthop 2001;119:30-5. 17. Lalani N, Foley TF, Voth R, Banting D, Mamandras A. Polymerization shrinkage with the argon laser: curing time and shear bond strength. Angle Orthod 2000;70:28-33. 18. Talbot TQ, Blankenau RJ, Zobitz ME, Weaver AL, Lohse CM, Rebellato J. Effect of argon laser irradiation on shear bond strength of orthodontic brackets: an in vitro study. Am J Orthod Dentofacial Orthop 2000;118:274-9. 19. Weinberger SJ, Foley TF, McConnell RJ, Wright GZ. Bond strengths of two ceramic brackets using argon laser, light, and chemically cured resin systems. Angle Orthod 1997;67:173-8. 20. Kurchak M, Desantos B, Powers J, Turner D. Argon laser for light-curing adhesives. J Clin Orthod 1997;31:371-4. 21. Fox NA, McCabe JF, Buckley JG. A critique of bond strength testing in orthodontics. Br J Orthod 1994;21:33-43. 22. Eliades T, Brantley WA. The inappropriateness of conventional orthodontic bond strength assessment protocols. Eur J Orthod 2000;22:13-23. 23. Artun J, Berglund S. Clinical trials with crystal growth conditioning as an alternative to acid-etch enamel pretreatment. Am J Orthod 1984;85:333-40. 24. Bishara SE, VonWald L, Laffoon JF, Warren JJ. Effect of a self-etch primer/adhesive on the shear bond strength of orthodontic brackets. Am J Orthod Dentofacial Orthop 2001;119:621-4.
25. Bishara SE, Olsen M, VonWald L. Comparisons of shear bond strength of precoated and uncoated brackets. Am J Orthod Dentofacial Orthop 1997;112:617-21. 26. Nolden R. Bonding of restorative materials to dentine: the present status in the Federal Republic of Germany. Int Dent J 1985;35:166-72. 27. Diaz-Arnold AM, Aquilino SA. An evaluation of the bond strengths of four organosilane materials in response to thermal stress. J Prosthet Dent 1989;62:257-60. 28. Hobson RS, McCabe JF, Hogg SD. Bond strength to surface enamel for different tooth types. Dent Mater 2001;17:184-9. 29. Sheen DH, Wang WN, Tarng TH. Bond strength of younger and older permanent teeth with various etching times. Angle Orthod 1993;63:225-9. 30. Brantley WA, Eliades T. Orthodontic materials: scientific and clinical aspects. New York: Thieme; 2001. p. 111. 31. Blankenau RJ, Kelsey WP, Powell GL, Shearer GO, Barkmeier WW, Cavel WT. Degree of composite resin polymerization with visible light and argon laser. Am J Dent 1991;4:40-2. 32. Lovell LG, Newman SM, Bowman SN. The effects of light intensity, temperature, and comonomer composition on the polymerization behavior of dimethacrylate dental resins. J Dent Res 1999;78:1469-76. 33. Rueggeberg FA, Caughman WF, Curtis JW. Effect of light intensity and exposure duration on cure of resin composite. Oper Dent 1994;19:26-32. 34. Miyazaki M, Oshida Y, Moore BK, Onose H. Effect of light exposure on fracture toughness and flexural strength of light cured composites. Dent Mater 1996;12:328-32.
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Effects of high-speed curing devices on shear bond strength and microleakage of orthodontic brackets Jeffrey W. James, DDS, MS,a Barbara H. Miller, DDS, MS,b Jeryl D. English, DDS, MS,c Larry P. Tadlock, DDS, MS,d and Peter H. Buschang, MA, PhDe Dallas, Tex This study evaluated the shear-peel bond strength and mode of bond failure of 3 curing devices (plasma arc light, argon laser, and conventional halogen light) and 2 orthodontic bracket adhesives with different filler contents (Transbond XT and Adhesive Precoated [APC]). Observations of microleakage were also reported. Ninety human adolescent premolars were randomly divided into 6 groups, and standardized brackets were bonded according to the manufacturers’ recommendations. The plasma arc light produced significantly (P ⫽ .006) higher bond strength than did the halogen light or the argon laser when Transbond was used. When APC was used, the plasma arc light and the halogen light produced similar results, and they both produced significantly (P ⫽ .015) higher bond strengths than did the argon laser. Overall, the APC showed substantially less variation in bond strength than did the Transbond. Although all curing methods showed significant microleakage (P ⬍ .001), differences among the 3 curing lights occurred only when APC was used. Microscopic evaluations demonstrated that 95% of the specimens failed for adhesion at the bracket or tooth surface; the argon laser produced the highest adhesive remnant index scores. On the basis of bond strength and microleakage results, the plasma arc light was comparable with or superior to the other curing devices, depending on the adhesive used. (Am J Orthod Dentofacial Orthop 2003;123:555-61)
V
isible light-cured (VLC) adhesives have become increasingly more popular for bonding orthodontic attachments because they offer several advantages over chemically cured adhesives, including ease of use, extended working time, improved bracket placement, easier cleanup of excess adhesive, and better physical properties because air is not incorporated during mixing.1-3 The disadvantage of VLC adhesives is the time required to expose the adhesives to the curing device. The most common initiator used in VLC adhesives is camphoroquinone, Financial support provided by Baylor College of Dentistry; supplies donated by 3M Unitek. a Private practice, Palestine, Tex. b Associate professor, Department of Biomaterials Sciences, Baylor College of Dentistry, Dallas, Tex. c Associate professor and chairman, Department of Orthodontics, Baylor College of Dentistry, Dallas, Tex. d Assistant clinical professor, Department of Orthodontics, Baylor College of Dentistry, Dallas, Tex. e Professor and director of orthodontic research, Department of Orthodontics, Baylor College of Dentistry, Dallas, Tex. Reprint requests to: Dr Peter H. Buschang, Department of Orthodontics, Baylor College of Dentistry, The Texas A & M University System Health Science Center, 3302 Gaston Ave, Dallas, TX 75246; e-mail, phbuschang@ tambcd.edu. Submitted, September 2001; revised and accepted, May 2002. Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 ⫹ 0 doi:10.1016/S0889-5406(02)00019-7
which is sensitive to light in the blue region (450 to 500 nm) of the visible light spectrum, with the peak activity centered around 480 nm.4 Because of their unique characteristics, argon lasers and plasma arc lights have the potential to dramatically reduce the curing time of dental composites. The argon laser produces a highly concentrated, coherent beam of light centered around a wavelength of 480 nm, which is optimal for the activation of most dental composites. Likewise, the plasma arc light reduces curing time because it produces much more intensive light than does the halogen light.5 To improve practice efficiency, many orthodontists use argon lasers, plasma arc lights, and other high-speed curing devices, even though all aspects of their efficacy for curing orthodontic adhesives have not been fully investigated. Studies in restorative dentistry have demonstrated that curing composites with lasers and high-intensity lights causes increased polymerization shrinkage and marginal leakage (microleakage).6-11 Polymerization shrinkage also varies from composite to composite and depends on the percentage of filler, the diluent, and the percentage of monomer conversion in the specific composite resin.6,12 In addition, Miyazaki et al13 showed that polymerization shrinkage increases as the filler content decreases. Finally, several studies have 555
556 James et al
demonstrated that laser curing of restorative composites provides bond strengths comparable with those of conventional halogen lights.14,15 There has been increasing interest in lasers and high-intensity lights, such as the plasma arc, to cure adhesives, primarily because they cure the adhesive faster than do conventional halogen lights.14-20 By speeding up the curing process, these lights save chair time for doctor and patient.14 A few studies have shown that curing orthodontic adhesives with an argon laser or a plasma arc light produces bond strength comparable with that of conventional halogen lights.5,16-20 However, more documentation is needed, including determining whether the argon laser and the plasma arc light produce increased microleakage around the orthodontic brackets, which could increase the patient’s risk of decalcification. Furthermore, the effect of the filler content of the orthodontic adhesive on bond strength and microleakage should be more thoroughly evaluated. The purpose of this study was to use standardized test procedures to evaluate the shear-peel bond strength, the mode of bond failure, and the microleakage of 3 curing devices (argon laser, plasma arc light, and conventional halogen light) and 2 orthodontic bracket adhesives with different filler contents.21,22 MATERIAL AND METHODS
Ninety human adolescent premolars extracted for orthodontic purposes were collected over a 4-month period. The teeth were debrided and examined for caries, preexisting fractures, and restorations. The teeth were disinfected in 10% formalin for 2 weeks and then stored in deionized distilled water for the rest of the experiment. The teeth were randomly assigned to 6 subsamples with 15 teeth per subsample. The principal investigator (J.W.J.) bonded the samples during 2 sessions over a 24-hour period. Stainless steel twin premolar brackets (.018-in Victory Series, 3M Unitek, Monrovia, Calif) were used for all teeth. The brackets were bonded with 1 of 3 curing devices (the Ortholux XT halogen light [3M Unitek], the ILT Argon Laser [ILT Systems, Salt Lake City, Utah], and the Plasma Arc unit [American Dental Technologies, Inc, Corpus Christi, Tex]) and 2 orthodontic adhesives (Transbond XT and APC [Adhesive Precoated], 3M Unitek). According to the manufacturers, the Transbond XT and APC are approximately 77% and 80% quartz (silica) filler, respectively. Before bonding, the teeth were prepared according to the following protocol: (1) polished for 10 seconds with a rubber prophy cup, with oil-free and fluoride-free pumice and water, (2) rinsed for 10 seconds with an
American Journal of Orthodontics and Dentofacial Orthopedics May 2003
air-water syringe, (3) dried for 5 seconds with an air-water syringe, (4) acid-etched for 30 seconds with 37% phosphoric acid gel (3M Unitek), (5) rinsed for 10 seconds with an air-water syringe, (6) dried for 5 seconds with an air-water syringe, and (7) coated with primer (Transbond XT, 3M Unitek) that was thinned with a puff of air from the air-water syringe. The adhesive-loaded bracket was pushed to fully contact the tooth, and the flash was removed. The adhesives were cured according to the light manufacturers’ guidelines as follows: halogen light (772 mW/ cm2), 10 seconds mesially and distally (20 seconds total); argon laser (238 mW/cm2), 5 seconds mesially and distally (10 seconds total); and plasma arc unit (1407 mW/cm2), 3 seconds mesially and 2 seconds distally (5 seconds total). During the bonding procedure, the Cure Rite radiometer (Dentsply Caulk, Milford, Del) was used to record 21 light intensities for each curing device, and the average intensity was calculated. Light intensities were recorded before bonding, after every 2 samples (30 samples per light), and then 5 more times at 1-minute intervals. Five days after bonding, the teeth were thermal cycled for approximately 24 hours (1300 cycles). Each thermal cycle consisted of immersing the teeth for 30 seconds in water at 10°C, immediately followed by 30 seconds in water at 55°C. Next, the teeth were embedded in acrylic to approximately the level of the cementoenamel junction. To ensure that all the brackets were mounted in the same orientation relative to the acrylic cylinder, the teeth were suspended from a .017 ⫻ .025-in stainless steel archwire tied in the bracket slot. This mounting procedure ensured consistency for the point of force application and direction of the debonding force. The specimens were coded to blind the examiner to the adhesive and curing method used. The specimens were then placed in a container of methylene blue dye and deionized water for 6 days. After staining, the brackets were debonded with the Instron universal testing machine (Instron Corp, Canton, Mass). The specimens were placed in a jig so that the bracket base was parallel to the debonding force. A shear debonding force was applied to the bracket base in an occlusal-gingival direction at a crosshead speed of 0.1 mm/min. The area of the bracket base was measured with the University of Texas Health Science Center San Antonio Image Tool program (available on web site) to calculate megapascals (MPa). Debonded specimens were randomly examined at 50⫻ magnification with an optical microscope (Stereomicroscope SR, Zeiss, Oberkochen, Germany) to evaluate the microleakage and the mode of bond failure. Microleakage was determined by measuring the
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Table I.
Differences in light intensity, duration of curing, and total light energy among curing devices
Curing method Argon laser Halogen Plasma arc
Mean intensity (mW/cm2)*
SD (mW/cm2)
Range (mW/cm2)
Duration of curing (sec)
Total light energy (mW/cm2/sec)*†
238.1 771.9 1407.2
5.3 19.8 69.5
228.0-246.0 740.0-802.0 1251.0-1516.0
10 20 5
2381 15,438 7035
*P ⬍ .001. † Total light energy ⫽ mean intensity ⫻ duration of curing. Table II.
Differences in shear bond strength between adhesives and among curing devices Curing method Argon laser
Adhesive Transbond XT APC Difference P value
Halogen
Plasma arc
Method differences
Mean (MPa)
SD
Mean (MPa)
SD
Mean (MPa)
SD
P value
Post-hoc comparisons
4.4 4.2 0.2 .662
1.8 1.0 0.8 —
5.0 5.3 0.3 .631
1.6 1.0 0.6 —
6.6 5.0 1.6 .004
1.7 1.0 0.7 —
.006 .015 — —
P ⬎ (H ⫽ L) (P ⫽ H) ⬎ L — —
L, Argon laser; H, halogen; P, plasma arc.
deepest dye penetration on the tooth surface perpendicular to the bracket margin and rounding to the nearest .02 mm. Also, observations were made of the bracket surface to verify the observations of the tooth surface. Mode of bond failure was determined on the basis of the amount of adhesive remaining on the tooth and bracket pad and was expressed as a percentage of the total bonded area. Adhesive remnant index (ARI) scores23 were assigned to each specimen. An ARI score of 0 indicated that no adhesive was left on the tooth in the bonded area; 1 indicated that less than half of the adhesive was left on the tooth; 2 indicated that more than half was left on the tooth; and 3 indicated that all the adhesive remained on the tooth, with a distinct impression of the bracket mesh. The data were statistically analyzed for normality and homogeneity of variances. A 2-way analysis of variance (ANOVA) showed a significant interaction between the independent variables, making it necessary to evaluate separately the curing method and the adhesive. The curing methods were evaluated with a 1-way ANOVA followed by a post hoc least squared differences test. The adhesives were evaluated with an independent samples t test, and the ARI scores were analyzed with a Pearson chi-square test. To evaluate reliability, 15 specimens were randomly selected and reexamined. For the ARI scores, there were no differences between the original and the
replicate values. For the microleakage values, there were no systematic differences between the original and the replicate values with a method error of .075 mm (ME ⫽ 公⌺d2/2n). RESULTS
There were significant differences (P⬍ .001) in light intensity and total light energy among the 3 curing devices (Table I). The coefficients of variation for the lights were similar. The 1-way ANOVA revealed significant differences in shear-peel bond strength between curing methods for both adhesives (Table II). Although all curing methods showed significant microleakage (P ⬍.001), there was wide variation among curing devices and adhesives. The 1-way ANOVA showed significant differences in the microleakage for each curing method with APC, but not with Transbond (Table III). There was no significant correlation between shear-peel bond strength and microleakage r ⫽ .06; P ⫽ .61). The chi-square test showed significant differences in the ARI scores of the 3 curing devices (Table IV). Only 4 of the specimens showed small areas of cohesive failure. There were no incidences of enamel fracture. There were no significant differences in the ARI scores of the 2 adhesives (Table V).
558 James et al
Table III.
American Journal of Orthodontics and Dentofacial Orthopedics May 2003
Differences in microleakage between adhesives and among curing devices Curing method Argon laser
Halogen
Plasma arc
Method differences
Mean (mm)
SD
Mean (mm)
SD
Mean (mm)
SD
P value
Post-hoc comparisons
.26 0.29 0.03 .674
0.11 0.24 0.13 —
0.31 0.35 0.04 .530
0.20 0.16 0.04 —
0.47 0.15 0.32 .004
0.36 0.11 0.25 —
.084 .013 — —
— (H ⫽ L) ⬎ P — —
Adhesive Transbond XT APC Difference P value
L, Argon laser; H, halogen; P, plasma arc.
Absolute and relative frequency of ARI scores for curing devices
Table IV.
ARI scores (%) Curing method Argon laser Halogen Plasma arc
1
2
3
Sample size (n)
4 (14.2) 13 (46.4) 8 (27.5)
9 (32.1) 8 (28.5) 13 (44.8)
15 (53.6) 7 (25.0) 8 (27.5)
28 28 29
2 ⫽ 10.1; df ⫽ 4; P ⫽ .038. 1, Less than 50% of adhesive on tooth; 2, more than 50% of adhesive on tooth; 3, 100% of adhesive on tooth.
Absolute and relative frequency of ARI scores for adhesives
Table V.
ARI scores (%) Adhesive Transbond XT APC
1
2
3
Sample size (n)
11 (26.2) 14 (32.6)
15 (35.7) 15 (34.9)
16 (38.1) 14 (32.6)
42 43
2 ⫽ 0.482; df ⫽ 2; P ⫽ .786.
DISCUSSION
In the present study, the in vitro shear-peel bond strengths of the adhesives were comparable with or less than previously reported for similar materials and methods.5,16,20,24,25 The bond strengths might have been lower because the samples were stressed by thermal cycling for approximately 24 hours (1300 cycles); samples in most previous studies of orthodontic adhesives were not thermal cycled. Studies in restorative dentistry have shown that thermal cycling of samples can decrease the bond strength by 20% to 70%.26,27 The brackets were also debonded with a slower crosshead speed (0.1 mm/min) than used in some previous studies. This crosshead speed (recommended by Fox et al21) and the similarly low speed of 0.5 mm/min (recommended by Eliades and Brantley22) are lower than what might be used clinically. The
higher impact velocity in vivo allows the viscoelastic properties of the adhesive to affect the bond strength. Lower crosshead speeds might better remove the influence of the material’s strain rate sensitivity, which might be expected to decrease bond strength. In addition, it has been shown that orthodontic adhesive bond strength depends on the type of tooth used; premolars show lower bond strength than mandibular first molars.28 In this study, adolescent permanent teeth were used, and it has been shown that the bond strengths of younger permanent teeth are less than those of older permanent teeth.29 The results of this study showed that the plasma arc light produced mean bond strengths that were comparable with or greater than those produced by the halogen light, depending on the type of adhesive. With Transbond, the plasma arc light yielded significantly higher bond strengths than did the halogen light or the argon laser. With APC, the plasma arc light and the halogen light produced similar bond strengths, and both yielded significantly higher bond strengths than did the argon laser. Previous studies have shown that curing APC and Transbond with a plasma arc light produces bond strength comparable with that produced by a halogen light.5,16 The plasma arc light might be expected to yield adequate bond strength for orthodontic adhesives even at the lower extreme values (Fig). Reference publications have suggested that adhesivebracket systems should be able to withstand a stress of 6 to 8 MPa30; however, many variables affect these numbers in vitro and in vivo, including thermal cycling. This study clearly showed that the argon laser produced the lowest bond strength with either adhesive. In addition, the argon laser produced the lowest extreme values with both adhesives; this could lead to an increase in the number of bond failures. Whereas the argon laser emits light over a narrower band of wavelengths in the blue-green spectrum (457.9 to 514.5 nm) than do conventional lights,18 it also
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James et al 559
Fig. Range of variation (5th to 95th percentiles) in shear bond strength for curing devices.
collimates the light, which results in a more consistent intensity over any given distance.31 However, even though the narrower wavelength and collimation are beneficial, the total light energy (intensity of the light times the duration of exposure) appears to be a more important factor when determining the degree of polymerization. The plasma arc light produced an intensity approximately 2 times greater than the halogen light and about 6 times greater than the argon laser. However, the halogen light yielded a total light energy approximately 2 times greater than the plasma arc light and about 6.5 times greater than the argon laser. Greater total light energy generally results in increased fracture toughness and greater flexural strength, which usually translate into greater bond strength.32-34 Because the argon laser cures at a considerably lower intensity (238.1 mW/cm2) than the plasma arc or halogen light, it might require curing times longer than the recommended 10 seconds to provide comparable total light energy. However, in this study, in an effort to follow the procedures and use the variables that would most likely be used by a clinical practitioner, we used the curing times recommended by the light manufacturers. Increasing the curing time to provide a comparable total light energy would eliminate the time-saving aspect and increase heat and therefore the possibility of tissue damage. In this study, the bond strengths of the 2 adhesives depended on the curing method, and the variation in bond strength was substantially different for the 2 adhesives. Differences in bond strengths between the 2 adhesives were restricted to the plasma arc light, which showed significantly higher bond strength for Transbond than for APC. Similarly, Bishara et al25 showed that uncoated metal brackets bonded with Transbond XT had significantly higher shear bond strength than brackets precoated with APC. There appears to be a
critical material-to-light interaction that requires further investigation. Because the APC had less variation in bond strength than Transbond with all curing devices (Fig), it is probably less technique-sensitive than Transbond. The manufacturer applied the APC to the bracket base, whereas the investigator applied the Transbond. Because the APC required fewer steps in the bonding procedure, there was a decreased risk for contamination, which might have decreased the variation. All curing methods showed significant microleakage, with amounts ranging from approximately 5% to 15% of the bracket width. There was no significant difference in microleakage between the curing methods when Transbond was used. With APC, the halogen light and the argon laser produced significantly more microleakage (approximately 5%) than did the plasma arc light. These results contradict studies in restorative dentistry that demonstrate greater polymerization shrinkage and marginal leakage with higher intensity lights.6-11 In restorative dentistry, composite is placed in a cavity preparation, and rapid curing can create excessive shrinkage and gap formation along the composite-preparation interface. In contrast, orthodontic adhesive layers are very thin; there is adhesive at the edges of the bracket to absorb some shrinkage, and, because the bracket is free floating, the shrinkage can pull the bracket closer to the enamel.5 Therefore, in orthodontic applications, polymerization shrinkage caused by high-intensity curing is probably less of a concern than it is in restorative dentistry. There was no significant difference in microleakage between the 2 adhesives except when the plasma arc light was used; then the Transbond had significantly more microleakage than did the APC. This finding is consistent with restorative dentistry studies that show that the filler content of the composite affects the polymerization shrinkage,6,12 and decreased filler con-
560 James et al
tent causes greater polymerization shrinkage.13 The Transbond had approximately 3% less filler than did the APC; this might have contributed to the increased microleakage. However, when the plasma arc light was used, the Transbond still had significantly higher bond strength than did the APC. Whereas the adhesive with the most microleakage might be expected to have the lowest bond strength, the results showed no correlation between microleakage and bond strength. In addition, the effects of filler content on microleakage were present only when the highest intensity light (plasma arc) was used. Once again, there appears to be an important material-to-light interaction. Microscopic evaluation of bond failure showed that 95% of the specimens failed for adhesion at either the bracket or the tooth surface. This agrees with other studies that demonstrated adhesive rather than cohesive failures.5,16,18 When we used the argon laser, most specimens had 100% of the adhesive remaining on the tooth surface, in accordance with the findings of Talbot et al.18 When the plasma arc light and the halogen light were used, approximately 75% of the specimens had less than 100% of the adhesive on the enamel; this is a clinical advantage because there is less adhesive to remove after debonding. The location of the bond failure is clearly affected by the curing method, and further investigations are needed to explain the differences. The bond strength and microleakage results in this study support the use of the plasma arc light to cure orthodontic adhesives. Because the plasma arc light requires approximately 25% of the amount of time that the halogen light requires, it more efficiently produces adequate bond strength. The brackets in an arch can be cured in about 1 minute 15 seconds with the plasma arc light versus 5 minutes with the halogen light. Therefore, curing with the plasma arc light saves considerable time for doctor and patient. The cost of the plasma arc light is approximately 3 to 4 times more than that of the halogen light, but half that of an argon laser. In a busy orthodontic practice, the initial extra cost of the plasma arc light can pay for itself by saving chair time. CONCLUSIONS 1. Curing orthodontic adhesives with a plasma arc light
for 5 seconds produces shear-peel bond strength that is comparable with or greater than that produced by the halogen light or the argon laser, depending on the type of adhesive used. In addition, the plasma arc light saves time for doctor and patient.
American Journal of Orthodontics and Dentofacial Orthopedics May 2003
2. The APC showed substantially less variation in bond strength than did the Transbond, suggesting that it is a less technique sensitive. 3. There was a significant difference in the location of the bond failures (ARI scores) among the 3 curing methods. When the plasma arc light or the halogen light was used, less adhesive remained on the enamel surface, making cleanup easier. 4. Whereas all curing methods showed evidence of microleakage, curing the adhesive with a highintensity light did not cause a significant increase in microleakage. Further investigation is needed in the area of microleakage around orthodontic brackets. The authors thank Phil Ford and Becky Fellows for helping with data collection and Julie Bradshaw for helping with manuscript preparation. REFERENCES 1. Pollack BF, Blitzer MH. The advantages of visible light curing resins. NY Dent J 1982;48:228-30. 2. Tavas MA, Watts DC. A visible light-activated direct bonding material: an in vitro comparative study. Br J Orthod 1984;11: 33-7. 3. Greenlaw R, Way DC, Galil KA. An in vitro evaluation of a visible light-cured resin as an alternative to conventional resin bonding systems. Am J Orthod Dentofacial Orthop 1989;96:21420. 4. Cook WD. Spectral distribution of dental photopolymerization sources. J Dent Res 1982;61:1436-8. 5. Oesterle LJ, Newman SM, Shellhart WC. Rapid curing of bonding composite with a xenon plasma arc light. Am J Orthod Dentofacial Orthop 2001;119:610-6. 6. Burgess JO, DeGoes M, Walker R, Ripps AH. An evaluation of four light-curing units comparing soft and hard curing. Pract Periodont Aesthet Dent 1999;11:125-32. 7. Davidson-Kaban SS, Davidson CL, Feilzer AJ, deGee AJ, Erdilek N. The effect of curing light variations on bulk curing and wall-to-wall quality of two types and various shades of resin composites. Dent Mater 1997;13:344-52. 8. Mehl A, Hickel R, Kunzelmann KH. Physical properties and gap formation of light-cured composites with and without “softstartpolymerization.” J Dent 1997;25:321-30. 9. Feilzer AJ, Dooren LH, deGee AJ, Davidson CL. Influence of light intensity on polymerization shrinkage and integrity of restoration-cavity interface. Eur J Oral Sci 1995;103:322-6. 10. Unterbrink GL, Muessner R. Influence of light intensity on two restorative systems. J Dent 1994;23:183-9. 11. Uno S, Asmussen A. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991;99:440-4. 12. Masutani S, Matsuzaki T, Akiyama Y. Study on light cured composite resins: consideration of the continuous volumetric shrinkage of resins during light irradiation. Jpn J Conserv Dent 1989;32:1605-11. 13. Miyazaki M, Hinoura K, Onose H, Moore BK. Effect of filler content of light-cured composites on bond strength to bovine dentine. J Dent 1991;19:301-3. 14. Shanthala BM, Munshi AK. Laser vs visible-light cured composite resin: an in vitro shear bond study. J Clin Pediatr Dent 1995;19:121-5.
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15. Hinoura K, Masashi M, Onose H. Influence of argon curing on resin bond strength. Am J Dent 1993;6:69-71. 16. Sfondrini MF, Cacciafesta V, Pistorio A, Sfondrini G. Effects of conventional and high-intensity light-curing on enamel shear bond strength of composite resin and resin-modified glassionomer. Am J Orthod Dentofacial Orthop 2001;119:30-5. 17. Lalani N, Foley TF, Voth R, Banting D, Mamandras A. Polymerization shrinkage with the argon laser: curing time and shear bond strength. Angle Orthod 2000;70:28-33. 18. Talbot TQ, Blankenau RJ, Zobitz ME, Weaver AL, Lohse CM, Rebellato J. Effect of argon laser irradiation on shear bond strength of orthodontic brackets: an in vitro study. Am J Orthod Dentofacial Orthop 2000;118:274-9. 19. Weinberger SJ, Foley TF, McConnell RJ, Wright GZ. Bond strengths of two ceramic brackets using argon laser, light, and chemically cured resin systems. Angle Orthod 1997;67:173-8. 20. Kurchak M, Desantos B, Powers J, Turner D. Argon laser for light-curing adhesives. J Clin Orthod 1997;31:371-4. 21. Fox NA, McCabe JF, Buckley JG. A critique of bond strength testing in orthodontics. Br J Orthod 1994;21:33-43. 22. Eliades T, Brantley WA. The inappropriateness of conventional orthodontic bond strength assessment protocols. Eur J Orthod 2000;22:13-23. 23. Artun J, Berglund S. Clinical trials with crystal growth conditioning as an alternative to acid-etch enamel pretreatment. Am J Orthod 1984;85:333-40. 24. Bishara SE, VonWald L, Laffoon JF, Warren JJ. Effect of a self-etch primer/adhesive on the shear bond strength of orthodontic brackets. Am J Orthod Dentofacial Orthop 2001;119:621-4.
25. Bishara SE, Olsen M, VonWald L. Comparisons of shear bond strength of precoated and uncoated brackets. Am J Orthod Dentofacial Orthop 1997;112:617-21. 26. Nolden R. Bonding of restorative materials to dentine: the present status in the Federal Republic of Germany. Int Dent J 1985;35:166-72. 27. Diaz-Arnold AM, Aquilino SA. An evaluation of the bond strengths of four organosilane materials in response to thermal stress. J Prosthet Dent 1989;62:257-60. 28. Hobson RS, McCabe JF, Hogg SD. Bond strength to surface enamel for different tooth types. Dent Mater 2001;17:184-9. 29. Sheen DH, Wang WN, Tarng TH. Bond strength of younger and older permanent teeth with various etching times. Angle Orthod 1993;63:225-9. 30. Brantley WA, Eliades T. Orthodontic materials: scientific and clinical aspects. New York: Thieme; 2001. p. 111. 31. Blankenau RJ, Kelsey WP, Powell GL, Shearer GO, Barkmeier WW, Cavel WT. Degree of composite resin polymerization with visible light and argon laser. Am J Dent 1991;4:40-2. 32. Lovell LG, Newman SM, Bowman SN. The effects of light intensity, temperature, and comonomer composition on the polymerization behavior of dimethacrylate dental resins. J Dent Res 1999;78:1469-76. 33. Rueggeberg FA, Caughman WF, Curtis JW. Effect of light intensity and exposure duration on cure of resin composite. Oper Dent 1994;19:26-32. 34. Miyazaki M, Oshida Y, Moore BK, Onose H. Effect of light exposure on fracture toughness and flexural strength of light cured composites. Dent Mater 1996;12:328-32.
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