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Comparison of bond strength between a conventional resin adhesive and a resin-modified glass ionomer adhesive: An in vitro and in vivo study
ORIGINAL ARTICLE
Comparison of bond strength between a conventional resin adhesive and a resinmodified glass ionomer adhesive: An in vitro and in vivo study Andrew Summers, DDS, MS,a Elizabeth Kao, DMD, MS,b Jeffrey Gilmore, DDS, MS,c Erdogan Gunel, PhD,d and Peter Ngan, DMDe Sioux Falls, Utah, Morgantown, WVa, and Marietta, Ohio The objectives of this study were (1) to compare the in vivo survival rates of orthodontic brackets bonded with a resin-modified glass ionomer adhesive (Fuji Ortho LC; GC America, Alsip, Ill) after conditioning with 10% polyacrylic acid and a conventional resin adhesive (Light Bond; Reliance Orthodontic Products, Itasca, Ill) bonded with 37% phosphoric acid, (2) to compare the in vitro bond shear/peel bond strength between the 2 adhesives, (3) to determine the mode of bracket failure in the in vivo and in vitro tests according to the adhesive remnant index (ARI), and (4) to compare the changes in surface morphology of enamel surface after etching or conditioning with 10% polyacrylic acid, with scanning electron microscopy. In the in vitro study, 50 extracted premolars were randomly divided into 4 groups: brackets bonded with Fuji Ortho LC or Light Bond adhesive that were debonded after either 30 minutes or 24 hours. Bond strengths were determined with a testing machine at a crosshead speed of 1 mm/min. Data were analyzed with analysis of variance and a paired Student t test. The in vivo study consisted of 398 teeth that were randomly bonded with Fuji Ortho LC or Light Bond adhesive in 22 subjects with the split-mouth technique. Bracket survival rates and distribution were followed for 1.3 years. Data were analyzed with Kaplan-Meier product-limit estimates of survivorship function. The in vitro study results showed significant differences (P ⬍ .05) among the adhesives and the debond times. Light Bond had significantly greater bond strengths than Fuji Ortho LC at 24 hours (18.46 ⫾ 2.95 MPa vs 9.56 ⫾ 1.85 MPa) and 30 minutes (16.19 ⫾ 2.04 MPa vs 6.93 ⫾ 1.93 MPa). Mean ARI scores showed that Fuji Ortho LC had significantly greater incidences of enamel/adhesive failure than Light Bond adhesive (4.9 vs 4.1). For the in vivo study, no significant differences in failure rate, sex, or location in dental arch or ARI ratings were found between the 2 adhesives. These results suggest that, compared with conventional resin, brackets bonded with resin-modified glass ionomer adhesive had significantly less shear bond strength in vitro. However, similar survival rates of the 2 materials studied after 1.3 years indicate that resin-reinforced glass ionomers can provide adequate bond strengths clinically. The weaker chemical bonding between the adhesive and the enamel might make it easier for clinicians to clean up adhesives on the enamel surface after debonding. (Am J Orthod Dentofacial Orthop 2004;126:200-6)
E
tching enamel with 37% phosphoric acid is used routinely by orthodontists to bond orthodontic brackets to enamel.1 The disadvantages of this procedure are the loss of enamel during etching,2
a
Private practice, Sioux Falls, Utah. Professor, Department of Restorative Dentistry, West Virginia University School of Dentistry. c Private practice, Marietta, Ohio. d Professor, Department of Statistics, West Virginia University School of Dentistry. e Professor and chair, Department of Orthodontics, West Virginia University School of Dentistry. Reprints requests to: Dr Peter Ngan, West Virginia University, School of Dentistry, Department of Orthodontics, Health Science Center North, P.O. Box 9480, Morgantown, WV 26506; e-mail, [email protected]. Submitted, April 2003; revised and accepted, June 2003. 0889-5406/$30.00 Copyright © 2004 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2003.06.013 b
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the necessity of strict adherence to a dry field, the multiple steps required, and the remaining resin residue that cannot be easily removed after debonding of the bracket. A resin-modified glass ionomer (RMGI) adhesive (Fuji Ortho LC; GC America, Alsip, Ill) has been introduced that can be used for bonding brackets without acid etching.5-9 However, previous studies have shown that nonetched RMGI adhesives have lower bond strength and a higher failure rate when compared with conventional acid-etch resin bonding agents. Conditioning the tooth surface with 10% polyacrylic acid does not cause as much damage to the enamel surface as etching with 37% phosphoric acid but enhances the bond strength of RMGI adhesives, at least under in vitro condition.13-15 The purposes of this study were (1) to determine the in vivo survival rate of
Summers et al 201
American Journal of Orthodontics and Dentofacial Orthopedics Volume 126, Number 2
Four test groups with number of samples, test material, and time of testing
Table II.
Group
n
Bonding material
Score
I II III IV
13 12 13 12
RMGI (Fuji Ortho LC) RMGI (Fuji Ortho LC) Resin (Light Bond) Resin (Light Bond)
Table I.
Time of testing 30 24 30 24
min h min h
Modified ARI scale and corresponding definitions
5 4 3 2 1
orthodontic brackets bonded with an RMGI adhesive after conditioning with 10% polyacrylic acid with a conventional resin adhesive bonded with 37% phosphoric acid used as the control group, (2) to determine the in vitro shear/peel bond strength of the 2 adhesives, (3) to determine the mode of failure in the in vivo and in vitro tests, and (4) to compare the changes in surface morphology of enamel surface after etching or conditioning with 10% polyacrylic acid, with scanning electron microscopy (SEM). MATERIAL AND METHODS In vitro bond strength study
Two bonding materials were tested: Fuji Ortho LC, an RMGI, and Light Bond (Reliance Orthodontic Products, Itasca, Ill), a light-cured resin. Fifty extracted, noncarious premolars were randomly divided into 4 groups, as shown in Table I. The teeth were cleaned, steam autoclaved, and stored in distilled water and 0.9% thymol. The teeth were mounted in epoxide resin (Buehler, Lake Bluff, Ill), with a surveyor used to ensure that the bonding surface was parallel to the debonding force, pumiced, and stored in distilled water. For groups I and II, the bonding procedure consisted of pumicing the tooth surface for 10 seconds with flour pumice, followed by a rinse of 10 seconds with water. The bonding surface was conditioned with 10% polyacrylic acid for 20 seconds and rinsed for 10 seconds. Each tooth was then wiped with a moist cotton roll to ensure that the bonding surface was not desiccated, and excess water was removed. Fuji Ortho LC RMGI capsule was triturated for 10 seconds and then applied to a GAC micro-arch universal orthodontic premolar bracket (GAC International, Bohemia, NY) with a base dimension of 3.12 ⫻ 3.40 mm, covering the entire base of the bracket without bubbles or voids. The bracket was applied to the tooth with a constant force, and the surrounding flash was carefully removed. The adhesive was light-cured with the Ortholux XT visible light-curing unit (3M Unitek, Monrovia, Calif) for a total of 40 seconds, with 20-second curing intervals from the mesial and distal aspects of the bracket. For Groups III and IV, the bonding procedure
Definition All of adhesive remained on bracket More than 90% of adhesive remained on bracket More than 10% but less than 90% of adhesive remained on bracket Less than 10% of adhesive remained on bracket No adhesive remained on bracket
consisted of pumicing the tooth surface for 10 seconds, followed by a rinse for 10 seconds with water. The enamel was etched for 30 seconds with 37% phosphoric acid and then rinsed for 10 seconds. The tooth was dried with a stream of air until a chalky white appearance was observed. A thin layer of Light Bond sealant was applied with a brush and light-cured for 10 seconds. A bracket was applied to the tooth with a constant force with the Light Bond adhesive. Flash was carefully removed, and the adhesive was light-cured for a total of 40 seconds, with 20-second curing on the mesial and distal aspects. The teeth were mounted in a testing ring with the facial enamel surface perpendicular to the base of the mounting ring, with a dental surveyor. Epoxide resin was used to secure the tooth in the mounting ring. Teeth from all groups were stored in an incubator with 100% humidity immediately after bonding. Debonding force (in newtons) was determined within 30 minutes after bonding in Groups I and III and 24 hours after bonding in Groups II and IV, with a testing machine (Instron, Canton, Mass) with a cross-head speed of 1 mm/min. The bracket failure interface was examined under light microscopy to determine whether the failure occurred at the enamel-adhesive or the bracket-adhesive interface. The brackets were then assessed with a modified adhesive remnant index (ARI) and scored for the amount of resin material adhering to the bracket.14 The criteria for the modified ARI scale are shown in Table II. Data were analyzed by analysis of variance (ANOVA) and paired Student t test. The facial surfaces of 3 noncarious premolars were observed with SEM to compare the effects of 37% phosphoric acid and 10% polyacrylic acid on dental enamel. The first tooth was the control, with no enamel treatment. The second tooth had the facial surface conditioned with 10% phosphoric acid for 20 seconds, followed with a rinse for 10 seconds. The third tooth had the facial surface etched with 37% phosphoric acid for 30 seconds and then rinsed for 10 seconds. The teeth were dehydrated in a series of alcohol concentrations ranging from 40% to 95% and prepared for
202 Summers et al
observation with SEM by gold sputtering of the enamel surfaces. In vivo survival distribution study
The in vivo portion of the experiment included 22 patients who received comprehensive orthodontic treatment in the Department of Orthodontics, West Virginia University School of Dentistry. The selection criteria included no decalcification on teeth, good oral hygiene, and permanent dentition. A split-arch technique was used, in which the maxillary right quadrant and the mandibular left quadrant were bonded with either Fuji Ortho LC RMGI or Light Bond resin adhesive. This was determined randomly by a coin toss. The remaining 2 quadrants were bonded with the material that was not chosen for the other 2 quadrants. Each tooth was pumiced for 10 seconds and rinsed for 10 seconds. The quadrants bonded with Fuji Ortho LC had the bonding surfaces conditioned with 10% polyacrylic acid for 20 seconds and rinsed for 10 seconds. The teeth were wiped with a moist cotton roll to ensure that the bonding surface was not desiccated, and excess water was removed. Fuji Ortho LC RMGI capsule was triturated for 10 seconds and applied to the base of an orthodontic bracket covering the entire base of the bracket, without bubbles or voids. The bracket was applied to the tooth with a constant force, and any flash was carefully removed. The adhesive was light-cured with the Ortholux XT visible light-curing unit for a total of 40 seconds, with 20-second curing intervals from the mesial and distal aspects of the bracket. The quadrants bonded with Light Bond resin adhesive had the bonding surfaces prepared with acid etching of the enamel for 30 seconds with 37% phosphoric acid and then rinsed for 10 seconds. Each tooth was dried with a stream of air until a chalky white appearance was observed. A thin layer of Light Bond sealant was applied with a brush and light-cured for 10 seconds. Light Bond adhesive was placed on the bracket and applied to the tooth with a constant force. Flash was carefully removed, and the adhesive was light-cured with the Ortholux XT visible light-curing unit for a total of 40 seconds, with a 20-second cure interval from the mesial and distal aspects. Bracket failures were noted during the study. Any failed bracket was saved, and the tooth was no longer followed in the study. The bracket failure interface was observed with light microscopy to determine the failure interface. The failed brackets were then assessed with the modified ARI and scored with respect to the amount of resin material adhering to the bracket.16
American Journal of Orthodontics and Dentofacial Orthopedics August 2004
Table III.
In vitro shear bond strength for the 4 test
groups Group I II III IV
Bonding material Fuji Ortho LC Fuji Ortho LC Light Bond Light Bond
Debond Mean SD Minimum Maximum time n (MPa) (MPa) (MPa) (MPa) 30 24 30 24
min h min h
13 12 13 12
6.93 9.56 16.19 18.46
1.93 1.85 2.04 2.95
3.43 7.10 12.65 15.44
9.83 12.42 19.14 23.47
SD, standard deviation.
Data analysis
Data in the in vitro study were analyzed by ANOVA and paired Student t test. For the in vivo data, significant differences in the bracket survival rate among the 2 materials, patient sex, location in the oral cavity, and ARI scores were determined with the Kaplan-Meier product limit survival estimates and the log-rank test at P ⱕ .05. RESULTS In vitro bond strength study
The shear force recorded in newtons on the testing machine was converted to megapascals by dividing the force by the area of the bracket base (3.12 mm ⫻ 3.40 mm ⫽ 10.608 mm2). The shear bond strengths of all test groups are shown in Table III. The control group (Light Bond debonded at 24 hours) was found to have the highest mean shear bond strength (18.46 ⫾ 2.95 MPa). This was followed by the Light Bond group debonded at 30 minutes (16.19 ⫾ 2.04 MPa). The Fuji Ortho LC group debonded at 24 hours had a mean shear bond strength of 9.56 ⫾ 1.85 MPa, and the Fuji Ortho LC group debonded at 30 minutes had the lowest mean shear bond strength (6.93 ⫾ 1.93 MPa). Analysis of variance showed significant differences in the shear bond strengths among the 4 groups (P ⬍ .05). Paired Student t tests showed a significant difference between the control group (Light Bond at 24 hours) and the 3 experimental groups: Fuji Ortho LC, 24 hours (P ⬍ .0001), Fuji Ortho LC, 30 minutes (P ⬍ .0001), and Light Bond, 30 minutes (P ⬍ .03). Significant differences were found between the Light Bond, 30 minutes, and the Fuji Ortho LC groups (P ⬍ .0001). Significant differences were also found between the Fuji Ortho LC, 24 hours, and Fuji Ortho LC, 30 minutes, groups (P ⬍ .001). Bracket failure interface study
The average ARI scores for the 4 test groups are shown in Table IV. Fuji Ortho LC, 30 minutes, and Fuji Ortho LC, 24 hours, had the highest mean ARI scores
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Table IV.
Average ARI scores for the 4 in vitro test
groups
Group I II III IV
Bonding material Fuji Ortho LC Fuji Ortho LC Light Bond Light Bond
Mean Minimum Maximum Debond ARI ARI ARI time n score SD score score 30 24 30 24
min h min h
13 12 13 12
4.92 4.67 4.31 4.08
0.28 0.65 0.75 0.67
4 3 3 3
5 5 5 5
(4.92 ⫾ 0.28 and 4.67 ⫾ 0.65, respectively). Light Bond, 30 minutes, and Light Bond, 24 hours, had ARI scores of 4.31 ⫾ 0.75 and 4.08 ⫾ 0.67, respectively. Significant differences in ARI scores were found between Light Bond, 24 hours, and Fuji Ortho LC, 30 minutes (P ⬍ .05), and also between Light Bond, 24 hours, and Fuji Ortho LC, 24 hours (P ⬍ .04). No significant differences were found between Light Bond, 24 hours, and Light Bond, 30 minutes (P ⬍ .44). Significant differences were found between Fuji Ortho LC, 30 minutes, Fuji Ortho LC, 24 hours, and Light Bond, 30 minutes (P ⬍ .01). No differences were found between Fuji Ortho LC, 30 minutes, and Fuji Ortho LC, 24 hours (P ⬍ .21). The distribution of ARI scores for the test brackets is shown in Table V. Most of the brackets bonded with Fuji Ortho LC had an ARI score of 5 (Table V). Most of the brackets bonded with Light Bond had an ARI score of 4. Light Bond had 4 brackets with a score of 3, whereas Fuji Ortho LC had 1 bracket in this category. No sample tested received a score of less than 3.
In vivo bracket survival distribution
Twenty-two patients participated in this study (9 male, 13 female). The mean observation time at final data collection was 481.4 days (1.32 years), with a maximum of 664 days (1.82 years) and a minimum of 217 days (0.59 years). A total of 199 teeth were bonded with Light Bond, and 199 teeth were bonded with Fuji Ortho LC. During the observation period, 10 brackets bonded with Light Bond adhesive failed, resulting in a failure rate of 5%. Thirteen brackets bonded with Fuji Ortho LC failed, giving a slightly higher failure rate of 6.5%. However, the Kaplan- Meier survival distribution test showed no statistically significant correlation between the type of bonding material and bracket failure rates (P ⬍ .41) (Fig 2). There was no statistically significant correlation between sex and bracket failure rates. The male subjects had a 5.2% bracket failure rate, and the female subjects had a failure rate of 6.2%. There was no statistically significant correlation between the quadrant in which the teeth were bonded and the bracket failure rate (P ⬍ .99). The failure rates were 6.0% for the maxillary right quadrant, 6.0% for the maxillary left quadrant, 5.0% for the mandibular right quadrant, and 5.0% for the mandibular left quadrant. The ARI scores for all failed brackets (Table VI) showed that Light Bond had a slightly higher mean ARI value (3.7) than Fuji Ortho LC (3.5). There were no statistically significant differences in the ARI scores between the 2 bonding materials (P ⬍ .52). Most brackets that were bonded with Light Bond had an ARI score of 5 (Table VII). There were more brackets with ARI scores of 3 and 4 in the Fuji Ortho LC groups than in the Light Bond groups.
SEM study of enamel morphology
Three extracted premolars were examined with SEM at ⫻1000 magnification. The untreated enamel showed a smooth surface (Fig 1, A). After application of 37% phosphoric acid for 30 seconds, the enamel showed surface irregularities typical of a Type I enamel etching pattern; etching of prism cores was predominant17 (Fig 1, B). Figure 1, C, shows the enamel surface after application of 10% polyacrylic acid for 20 seconds. The enamel surface exhibits minimal surface irregularities and smooth precipitate in some areas. Comparison of the enamel surfaces shows that enamel conditioned with 37% phosphoric acid produced a qualitatively rougher enamel surface than the enamel conditioned with 10% polyacrylic acid, indicating a greater loss of enamel from conditioning with 37% phosphoric acid.
DISCUSSION In vitro study
The shear/peel bond strengths of the 2 bonding materials were measured at 30 minutes and at 24 hours. This was designed to more fully simulate a clinical situation, because archwires are typically placed at the bonding appointment, when light-cured resins or cements might not have been completely polymerized.2 The bond strengths of both the composite resin and the RMGI were significantly higher at 24 hours than at 30 minutes. The reduced shear/peel bond strength immediately after bonding agrees with the findings of Bishara et al15 and is probably related to incomplete polymerization of light-cured materials.18,19 The bond strengths of the composite resin with 37% phosphoric acid at both 24 hours and 30 minutes were
204 Summers et al
Table V.
American Journal of Orthodontics and Dentofacial Orthopedics August 2004
Number of the in vitro brackets in each score category of ARI
Group
Material
I II III IV
Debonded
Fuji Ortho LC Fuji Ortho LC Light Bond Light Bond
30 24 30 24
ARI ⫽ 1
ARI ⫽ 2
ARI ⫽ 3
ARI ⫽ 4
ARI ⫽ 5
0 0 0 0
0 0 0 0
0 1 2 2
1 2 5 7
12 9 6 3
min h min h
Fig 1. A, SEM of untreated enamel surface. B, SEM of enamel conditioned with 37% phosphoric acid for 30 seconds. C, SEM of enamel conditioned with 10% polyacrylic acid for 20 seconds. For all panels, original magnification ⫻1000. Table VII. Number of in vivo brackets in each ARI score category Group Fuji Ortho LC Light Bond
Fig 2. Bracket survival distribution over time with Fuji Ortho LC and Light Bond. Table VI.
Descriptive statistics of all in vivo ARI
scores Adhesive
n
Mean
Minimum
Maximum
Fuji Ortho LC Light Bond
13 10
3.5 3.7
1 1
5 5
significantly greater than those of the RMGI with 10% polyacrylic acid. These findings agree with those of Jobalia et al,6 Meehan et al,20 Bishara et al,15 and Owens et al.21 Bonding of glass ionomers is enhanced
ARI ⫽ 1 ARI ⫽ 2 ARI ⫽ 3 ARI ⫽ 4 ARI ⫽ 5 1 0
1 1
4 2
4 0
3 7
by surface conditioning with 10% polyacrylic acid.22 The acid removes contaminants and pellicles from the substrate surface and serves as a cleaning and wetting agent to improve the bonding of cement to enamel. Shammaa et al7 reported adequate bond strength when glass ionomer (Fry Ortho LC) was bonded to enamel clinically without acid etching. However, the bond strength was less compared with the values reported in this study with 10% polyacrylic acid. Etching of enamel with 37% phosphoric acid produces resin tags to a depth of 80 m that greatly increase the mechanical retention of composite resin to the enamel.23 Tavas and Watt24 recommended that adhesive bond strength greater than 58 N is necessary for clinical use. According to this recommendation, Fuji Ortho LC adhesive, with a shear bond strength of 73.5 N (6.9 MPa) at 30 minutes, has the potential to resist forces during orthodontic treatment. However, the average force transmitted to a bracket during mastication has been reported to be between 40 and 120 N.25,26 The use of Fuji Ortho LC in areas of traumatic or heavy occlusion might be guarded.
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American Journal of Orthodontics and Dentofacial Orthopedics Volume 126, Number 2
The SEM results showed that the enamel surface conditioned with 37% phosphoric acid was qualitatively rougher than when 10% polyacrylic acid was used. The preferential etching of prism cones is typical of a type I enamel etching pattern.17 Polyacrylic acid has a larger molecular size than phosphoric acid. The smaller molecule of 37% phosphoric acid was able to penetrate to a greater depth; thus, potentially more enamel can be lost during conditioning and debonding. SEM studies show that enamel surfaces after debonding of brackets are more porous with acid etching compared with clean and smooth enamel surface without etching.3 Repumicing of the bonded surface could restore the tooth surface to its original appearance.27 In vivo investigation
Bracket survival distribution was used to analyze the in vivo data. This gives the clinician a better perspective on when failure occurred compared with studying the failure rates only at the final data collection. Our study showed no statistically significant differences between Light Bond and Fuji Ortho LC as bonding materials. The failure rates of Light Bond and Fuji Ortho LC were 5% and 6.5%, respectively. In several studies, the failure rates for brackets bonded with glass ionomer cements varied from 3.2% to 50%.28-33 The lowest failure rates for Fuji Ortho LC were reported by Fricker29 in 1998 (3.2%) and Silverman et al30 in 1995 (3.3%). In addition, several clinicians, after 3 years of using Fuji Ortho LC, experienced bonding success rates comparable to those of conventional composite resins.31,32 The findings in the present study confirm the observations that, overall, the clinical performance of Fuji Ortho LC was similar to that of composite resin. In the present study, the predominant mode of bracket failure for the Fuji Ortho LC adhesive was at the enamel-cement interface, in both in vitro and in vivo conditions. The in vitro results suggest that chemical and mechanical bonding of glass ionomer cement to the bracket is stronger than the chemical bond of glass ionomer to the enamel, even in the presence of the resin component. This agrees with the results reported by McSherry,26 that glass ionomer cement bonds better to the metal band of the bracket than to the enamel. In contrast, when acid etching was used with Light Bond materials, a higher percentage of failure occurred at the resin-bracket interface. This is probably because of the incomplete polymerization of the resin below the bracket base.19 Air entrapment behind the mesh of a bracket can also affect polymerization, because of oxygen inhibition of free radical polymerizing in light-
cured composite materials.35 Clinically, it is the authors’ experience that, during debonding, the resin adhesive remaining on the tooth is more difficult to remove than the RMGI adhesive. CONCLUSIONS
1. In vitro results showed significantly greater shear bond strengths when brackets were bonded with 37% phosphoric acid and composite resin (Light Bond) compared with RMGI (Fuji Ortho LC) bonded with 10% polyacrylic acid. 2. Significantly greater shear bond strengths can be obtained 24 hours after bonding brackets for both materials. 3. The in vivo results showed no significant difference in bracket failure rates between Fuji Ortho LC and Light Bond after 1.3 years. Clinically, Fuji Ortho LC adhesive has adequate bond strength to withstand the occlusal forces of chewing and biting. 4. The ARI study showed that the predominant bracket failure interface for Fuji Ortho LC was at the enamel-adhesive interface. The weaker chemical bonding between RMGI and the enamel might make it easier for clinicians to clean up the adhesive on the enamel surface after debonding. 5. The SEM study showed that etching with 37% phosphoric acid on dental enamel for 30 seconds produced a qualitatively rougher and more porous surface than conditioning with 10% polyacrylic acid.4,10,11,12,34 REFERENCES 1. Newman GV. Epoxy adhesives for orthodontic attachments: progress report. Am J Orthod 1965;51:901-12. 2. Thompson RE, Way DC. Enamel loss due to prophylaxis and multiple bonding/debonding of orthodontic attachments. Am J Orthod 1981;79:282-95. 3. Osorio R, Toledano M, Garc´ ia-Godoy F. Enamel surface morphology after bracket debonding. J Dent Child 1998;65:313-7. 4. Joseph VP, Rossouw E. The shear bond strengths of stainless steel and ceramic brackets used with chemically and lightactivated composite resins. Am J Orthod Dentofacial Orthop 1990;97:121-5. 5. 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. 6. Jobalia SB, Valente RM, Waldemar GR, BeGole EA, Evans CA. Bond strength of visible light-cured glass ionomer orthodontic cement. Am J Orthod Dentofacial Orthop 1997;112:205-8. 7. Shammaa I, Ngan P, Kim H, Kao E, Gladwin M, Gunel E, et al. Comparison of bracket debonding force between two conventional resin adhesives and a resin-reinforced glass ionomer cement: An in vitro and in vivo study. Angle Orthod 1999;69: 463-9. 8. Millett DT, Cattanach D, McFadzean R, Pattison J, McColl J. Laboratory evaluation of a compomer and a resin modified glass
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American Journal of Orthodontics and Dentofacial Orthopedics August 2004
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of three visible light-cured orthodontic adhesives. Angle Orthod 2000;70:352-6. Cook PA, Luther F, Youngson CC. An in vitro study of the bond strength of light-cured glass ionomer cement in the bonding of orthodontic brackets. Eur J Orthod 1996;18:199-204. Diedrich P. Enamel alterations from bracket bonding and debonding: a study with the scanning electron microscope. Am J Orthod 1981;79:500-22. Tavis MA, Watts DC. A visible light activated direct bonding material. An in vitro study. Br. J Ortho 1984;11:33-7. Reynolds JR. A review of direct orthodontic bonding. Br J Orthod 1975;2:171-8. McSherry PF. An in vitro evaluation of the tensile and shear strengths of four adhesives used in orthodontics. Eur J Orthod 1996;18:319-27. Newman GV, Facq JM. The effects of adhesive systems on tooth surfaces. Am J Orthod 1971;59:67-75. Millet DT, McCabe JF. Orthodontic bonding with glass ionomer cement—a review. Eur J Orthod 1996;18:385-99. Fricker JP. A new self-curing resin-modified glass-ionomer cement for the direct bonding of orthodontic brackets in vivo. Am J Orthod Dentofacial Orthop 1998;113:384-6. Silverman E, Cohen M, Demke RS, Silverman M. A new light-cured glass ionomer cement that bonds brackets to teeth without etching in the presence of saliva. Am J Orthod Dentofacial Orthop 1995;108:231-6. Cook PA. Direct bonding with glass ionomer cement. J Clin Orthod 1990;24:509-11. Hegarty DJ, Macfarlane TV. In vivo bracket retention comparison of a resin-modified glass ionomer cement and a resin-based bracket adhesive system after a year. Am J Orthod Dentofacial Orthop 2002;121:496-501. Gaworski M, Weinstein M, Borislow AJ, Braitman LE. Decalcification and bond failure: a comparison of a glass ionomer and a composite resin bonding system in vivo. Am J Orthod Dentofacial Orthop 1999;116:518-21. Hitmi L, Muller C, Mujajic M, Attal JP. An 18-month clinical study of bond failures with resin-modified glass ionomer cement in orthodontic practice. Am J Orthod Dentofacial Orthop 2001; 120:406-15. Maijer R, Smith DC. Variables influencing the bond strength of metal orthodontic bracket bases. Am J Orthod 1981;79:20-34.
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Comparison of bond strength between a conventional resin adhesive and a resinmodified glass ionomer adhesive: An in vitro and in vivo study Andrew Summers, DDS, MS,a Elizabeth Kao, DMD, MS,b Jeffrey Gilmore, DDS, MS,c Erdogan Gunel, PhD,d and Peter Ngan, DMDe Sioux Falls, Utah, Morgantown, WVa, and Marietta, Ohio The objectives of this study were (1) to compare the in vivo survival rates of orthodontic brackets bonded with a resin-modified glass ionomer adhesive (Fuji Ortho LC; GC America, Alsip, Ill) after conditioning with 10% polyacrylic acid and a conventional resin adhesive (Light Bond; Reliance Orthodontic Products, Itasca, Ill) bonded with 37% phosphoric acid, (2) to compare the in vitro bond shear/peel bond strength between the 2 adhesives, (3) to determine the mode of bracket failure in the in vivo and in vitro tests according to the adhesive remnant index (ARI), and (4) to compare the changes in surface morphology of enamel surface after etching or conditioning with 10% polyacrylic acid, with scanning electron microscopy. In the in vitro study, 50 extracted premolars were randomly divided into 4 groups: brackets bonded with Fuji Ortho LC or Light Bond adhesive that were debonded after either 30 minutes or 24 hours. Bond strengths were determined with a testing machine at a crosshead speed of 1 mm/min. Data were analyzed with analysis of variance and a paired Student t test. The in vivo study consisted of 398 teeth that were randomly bonded with Fuji Ortho LC or Light Bond adhesive in 22 subjects with the split-mouth technique. Bracket survival rates and distribution were followed for 1.3 years. Data were analyzed with Kaplan-Meier product-limit estimates of survivorship function. The in vitro study results showed significant differences (P ⬍ .05) among the adhesives and the debond times. Light Bond had significantly greater bond strengths than Fuji Ortho LC at 24 hours (18.46 ⫾ 2.95 MPa vs 9.56 ⫾ 1.85 MPa) and 30 minutes (16.19 ⫾ 2.04 MPa vs 6.93 ⫾ 1.93 MPa). Mean ARI scores showed that Fuji Ortho LC had significantly greater incidences of enamel/adhesive failure than Light Bond adhesive (4.9 vs 4.1). For the in vivo study, no significant differences in failure rate, sex, or location in dental arch or ARI ratings were found between the 2 adhesives. These results suggest that, compared with conventional resin, brackets bonded with resin-modified glass ionomer adhesive had significantly less shear bond strength in vitro. However, similar survival rates of the 2 materials studied after 1.3 years indicate that resin-reinforced glass ionomers can provide adequate bond strengths clinically. The weaker chemical bonding between the adhesive and the enamel might make it easier for clinicians to clean up adhesives on the enamel surface after debonding. (Am J Orthod Dentofacial Orthop 2004;126:200-6)
E
tching enamel with 37% phosphoric acid is used routinely by orthodontists to bond orthodontic brackets to enamel.1 The disadvantages of this procedure are the loss of enamel during etching,2
a
Private practice, Sioux Falls, Utah. Professor, Department of Restorative Dentistry, West Virginia University School of Dentistry. c Private practice, Marietta, Ohio. d Professor, Department of Statistics, West Virginia University School of Dentistry. e Professor and chair, Department of Orthodontics, West Virginia University School of Dentistry. Reprints requests to: Dr Peter Ngan, West Virginia University, School of Dentistry, Department of Orthodontics, Health Science Center North, P.O. Box 9480, Morgantown, WV 26506; e-mail, [email protected]. Submitted, April 2003; revised and accepted, June 2003. 0889-5406/$30.00 Copyright © 2004 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2003.06.013 b
200
the necessity of strict adherence to a dry field, the multiple steps required, and the remaining resin residue that cannot be easily removed after debonding of the bracket. A resin-modified glass ionomer (RMGI) adhesive (Fuji Ortho LC; GC America, Alsip, Ill) has been introduced that can be used for bonding brackets without acid etching.5-9 However, previous studies have shown that nonetched RMGI adhesives have lower bond strength and a higher failure rate when compared with conventional acid-etch resin bonding agents. Conditioning the tooth surface with 10% polyacrylic acid does not cause as much damage to the enamel surface as etching with 37% phosphoric acid but enhances the bond strength of RMGI adhesives, at least under in vitro condition.13-15 The purposes of this study were (1) to determine the in vivo survival rate of
Summers et al 201
American Journal of Orthodontics and Dentofacial Orthopedics Volume 126, Number 2
Four test groups with number of samples, test material, and time of testing
Table II.
Group
n
Bonding material
Score
I II III IV
13 12 13 12
RMGI (Fuji Ortho LC) RMGI (Fuji Ortho LC) Resin (Light Bond) Resin (Light Bond)
Table I.
Time of testing 30 24 30 24
min h min h
Modified ARI scale and corresponding definitions
5 4 3 2 1
orthodontic brackets bonded with an RMGI adhesive after conditioning with 10% polyacrylic acid with a conventional resin adhesive bonded with 37% phosphoric acid used as the control group, (2) to determine the in vitro shear/peel bond strength of the 2 adhesives, (3) to determine the mode of failure in the in vivo and in vitro tests, and (4) to compare the changes in surface morphology of enamel surface after etching or conditioning with 10% polyacrylic acid, with scanning electron microscopy (SEM). MATERIAL AND METHODS In vitro bond strength study
Two bonding materials were tested: Fuji Ortho LC, an RMGI, and Light Bond (Reliance Orthodontic Products, Itasca, Ill), a light-cured resin. Fifty extracted, noncarious premolars were randomly divided into 4 groups, as shown in Table I. The teeth were cleaned, steam autoclaved, and stored in distilled water and 0.9% thymol. The teeth were mounted in epoxide resin (Buehler, Lake Bluff, Ill), with a surveyor used to ensure that the bonding surface was parallel to the debonding force, pumiced, and stored in distilled water. For groups I and II, the bonding procedure consisted of pumicing the tooth surface for 10 seconds with flour pumice, followed by a rinse of 10 seconds with water. The bonding surface was conditioned with 10% polyacrylic acid for 20 seconds and rinsed for 10 seconds. Each tooth was then wiped with a moist cotton roll to ensure that the bonding surface was not desiccated, and excess water was removed. Fuji Ortho LC RMGI capsule was triturated for 10 seconds and then applied to a GAC micro-arch universal orthodontic premolar bracket (GAC International, Bohemia, NY) with a base dimension of 3.12 ⫻ 3.40 mm, covering the entire base of the bracket without bubbles or voids. The bracket was applied to the tooth with a constant force, and the surrounding flash was carefully removed. The adhesive was light-cured with the Ortholux XT visible light-curing unit (3M Unitek, Monrovia, Calif) for a total of 40 seconds, with 20-second curing intervals from the mesial and distal aspects of the bracket. For Groups III and IV, the bonding procedure
Definition All of adhesive remained on bracket More than 90% of adhesive remained on bracket More than 10% but less than 90% of adhesive remained on bracket Less than 10% of adhesive remained on bracket No adhesive remained on bracket
consisted of pumicing the tooth surface for 10 seconds, followed by a rinse for 10 seconds with water. The enamel was etched for 30 seconds with 37% phosphoric acid and then rinsed for 10 seconds. The tooth was dried with a stream of air until a chalky white appearance was observed. A thin layer of Light Bond sealant was applied with a brush and light-cured for 10 seconds. A bracket was applied to the tooth with a constant force with the Light Bond adhesive. Flash was carefully removed, and the adhesive was light-cured for a total of 40 seconds, with 20-second curing on the mesial and distal aspects. The teeth were mounted in a testing ring with the facial enamel surface perpendicular to the base of the mounting ring, with a dental surveyor. Epoxide resin was used to secure the tooth in the mounting ring. Teeth from all groups were stored in an incubator with 100% humidity immediately after bonding. Debonding force (in newtons) was determined within 30 minutes after bonding in Groups I and III and 24 hours after bonding in Groups II and IV, with a testing machine (Instron, Canton, Mass) with a cross-head speed of 1 mm/min. The bracket failure interface was examined under light microscopy to determine whether the failure occurred at the enamel-adhesive or the bracket-adhesive interface. The brackets were then assessed with a modified adhesive remnant index (ARI) and scored for the amount of resin material adhering to the bracket.14 The criteria for the modified ARI scale are shown in Table II. Data were analyzed by analysis of variance (ANOVA) and paired Student t test. The facial surfaces of 3 noncarious premolars were observed with SEM to compare the effects of 37% phosphoric acid and 10% polyacrylic acid on dental enamel. The first tooth was the control, with no enamel treatment. The second tooth had the facial surface conditioned with 10% phosphoric acid for 20 seconds, followed with a rinse for 10 seconds. The third tooth had the facial surface etched with 37% phosphoric acid for 30 seconds and then rinsed for 10 seconds. The teeth were dehydrated in a series of alcohol concentrations ranging from 40% to 95% and prepared for
202 Summers et al
observation with SEM by gold sputtering of the enamel surfaces. In vivo survival distribution study
The in vivo portion of the experiment included 22 patients who received comprehensive orthodontic treatment in the Department of Orthodontics, West Virginia University School of Dentistry. The selection criteria included no decalcification on teeth, good oral hygiene, and permanent dentition. A split-arch technique was used, in which the maxillary right quadrant and the mandibular left quadrant were bonded with either Fuji Ortho LC RMGI or Light Bond resin adhesive. This was determined randomly by a coin toss. The remaining 2 quadrants were bonded with the material that was not chosen for the other 2 quadrants. Each tooth was pumiced for 10 seconds and rinsed for 10 seconds. The quadrants bonded with Fuji Ortho LC had the bonding surfaces conditioned with 10% polyacrylic acid for 20 seconds and rinsed for 10 seconds. The teeth were wiped with a moist cotton roll to ensure that the bonding surface was not desiccated, and excess water was removed. Fuji Ortho LC RMGI capsule was triturated for 10 seconds and applied to the base of an orthodontic bracket covering the entire base of the bracket, without bubbles or voids. The bracket was applied to the tooth with a constant force, and any flash was carefully removed. The adhesive was light-cured with the Ortholux XT visible light-curing unit for a total of 40 seconds, with 20-second curing intervals from the mesial and distal aspects of the bracket. The quadrants bonded with Light Bond resin adhesive had the bonding surfaces prepared with acid etching of the enamel for 30 seconds with 37% phosphoric acid and then rinsed for 10 seconds. Each tooth was dried with a stream of air until a chalky white appearance was observed. A thin layer of Light Bond sealant was applied with a brush and light-cured for 10 seconds. Light Bond adhesive was placed on the bracket and applied to the tooth with a constant force. Flash was carefully removed, and the adhesive was light-cured with the Ortholux XT visible light-curing unit for a total of 40 seconds, with a 20-second cure interval from the mesial and distal aspects. Bracket failures were noted during the study. Any failed bracket was saved, and the tooth was no longer followed in the study. The bracket failure interface was observed with light microscopy to determine the failure interface. The failed brackets were then assessed with the modified ARI and scored with respect to the amount of resin material adhering to the bracket.16
American Journal of Orthodontics and Dentofacial Orthopedics August 2004
Table III.
In vitro shear bond strength for the 4 test
groups Group I II III IV
Bonding material Fuji Ortho LC Fuji Ortho LC Light Bond Light Bond
Debond Mean SD Minimum Maximum time n (MPa) (MPa) (MPa) (MPa) 30 24 30 24
min h min h
13 12 13 12
6.93 9.56 16.19 18.46
1.93 1.85 2.04 2.95
3.43 7.10 12.65 15.44
9.83 12.42 19.14 23.47
SD, standard deviation.
Data analysis
Data in the in vitro study were analyzed by ANOVA and paired Student t test. For the in vivo data, significant differences in the bracket survival rate among the 2 materials, patient sex, location in the oral cavity, and ARI scores were determined with the Kaplan-Meier product limit survival estimates and the log-rank test at P ⱕ .05. RESULTS In vitro bond strength study
The shear force recorded in newtons on the testing machine was converted to megapascals by dividing the force by the area of the bracket base (3.12 mm ⫻ 3.40 mm ⫽ 10.608 mm2). The shear bond strengths of all test groups are shown in Table III. The control group (Light Bond debonded at 24 hours) was found to have the highest mean shear bond strength (18.46 ⫾ 2.95 MPa). This was followed by the Light Bond group debonded at 30 minutes (16.19 ⫾ 2.04 MPa). The Fuji Ortho LC group debonded at 24 hours had a mean shear bond strength of 9.56 ⫾ 1.85 MPa, and the Fuji Ortho LC group debonded at 30 minutes had the lowest mean shear bond strength (6.93 ⫾ 1.93 MPa). Analysis of variance showed significant differences in the shear bond strengths among the 4 groups (P ⬍ .05). Paired Student t tests showed a significant difference between the control group (Light Bond at 24 hours) and the 3 experimental groups: Fuji Ortho LC, 24 hours (P ⬍ .0001), Fuji Ortho LC, 30 minutes (P ⬍ .0001), and Light Bond, 30 minutes (P ⬍ .03). Significant differences were found between the Light Bond, 30 minutes, and the Fuji Ortho LC groups (P ⬍ .0001). Significant differences were also found between the Fuji Ortho LC, 24 hours, and Fuji Ortho LC, 30 minutes, groups (P ⬍ .001). Bracket failure interface study
The average ARI scores for the 4 test groups are shown in Table IV. Fuji Ortho LC, 30 minutes, and Fuji Ortho LC, 24 hours, had the highest mean ARI scores
Summers et al 203
American Journal of Orthodontics and Dentofacial Orthopedics Volume 126, Number 2
Table IV.
Average ARI scores for the 4 in vitro test
groups
Group I II III IV
Bonding material Fuji Ortho LC Fuji Ortho LC Light Bond Light Bond
Mean Minimum Maximum Debond ARI ARI ARI time n score SD score score 30 24 30 24
min h min h
13 12 13 12
4.92 4.67 4.31 4.08
0.28 0.65 0.75 0.67
4 3 3 3
5 5 5 5
(4.92 ⫾ 0.28 and 4.67 ⫾ 0.65, respectively). Light Bond, 30 minutes, and Light Bond, 24 hours, had ARI scores of 4.31 ⫾ 0.75 and 4.08 ⫾ 0.67, respectively. Significant differences in ARI scores were found between Light Bond, 24 hours, and Fuji Ortho LC, 30 minutes (P ⬍ .05), and also between Light Bond, 24 hours, and Fuji Ortho LC, 24 hours (P ⬍ .04). No significant differences were found between Light Bond, 24 hours, and Light Bond, 30 minutes (P ⬍ .44). Significant differences were found between Fuji Ortho LC, 30 minutes, Fuji Ortho LC, 24 hours, and Light Bond, 30 minutes (P ⬍ .01). No differences were found between Fuji Ortho LC, 30 minutes, and Fuji Ortho LC, 24 hours (P ⬍ .21). The distribution of ARI scores for the test brackets is shown in Table V. Most of the brackets bonded with Fuji Ortho LC had an ARI score of 5 (Table V). Most of the brackets bonded with Light Bond had an ARI score of 4. Light Bond had 4 brackets with a score of 3, whereas Fuji Ortho LC had 1 bracket in this category. No sample tested received a score of less than 3.
In vivo bracket survival distribution
Twenty-two patients participated in this study (9 male, 13 female). The mean observation time at final data collection was 481.4 days (1.32 years), with a maximum of 664 days (1.82 years) and a minimum of 217 days (0.59 years). A total of 199 teeth were bonded with Light Bond, and 199 teeth were bonded with Fuji Ortho LC. During the observation period, 10 brackets bonded with Light Bond adhesive failed, resulting in a failure rate of 5%. Thirteen brackets bonded with Fuji Ortho LC failed, giving a slightly higher failure rate of 6.5%. However, the Kaplan- Meier survival distribution test showed no statistically significant correlation between the type of bonding material and bracket failure rates (P ⬍ .41) (Fig 2). There was no statistically significant correlation between sex and bracket failure rates. The male subjects had a 5.2% bracket failure rate, and the female subjects had a failure rate of 6.2%. There was no statistically significant correlation between the quadrant in which the teeth were bonded and the bracket failure rate (P ⬍ .99). The failure rates were 6.0% for the maxillary right quadrant, 6.0% for the maxillary left quadrant, 5.0% for the mandibular right quadrant, and 5.0% for the mandibular left quadrant. The ARI scores for all failed brackets (Table VI) showed that Light Bond had a slightly higher mean ARI value (3.7) than Fuji Ortho LC (3.5). There were no statistically significant differences in the ARI scores between the 2 bonding materials (P ⬍ .52). Most brackets that were bonded with Light Bond had an ARI score of 5 (Table VII). There were more brackets with ARI scores of 3 and 4 in the Fuji Ortho LC groups than in the Light Bond groups.
SEM study of enamel morphology
Three extracted premolars were examined with SEM at ⫻1000 magnification. The untreated enamel showed a smooth surface (Fig 1, A). After application of 37% phosphoric acid for 30 seconds, the enamel showed surface irregularities typical of a Type I enamel etching pattern; etching of prism cores was predominant17 (Fig 1, B). Figure 1, C, shows the enamel surface after application of 10% polyacrylic acid for 20 seconds. The enamel surface exhibits minimal surface irregularities and smooth precipitate in some areas. Comparison of the enamel surfaces shows that enamel conditioned with 37% phosphoric acid produced a qualitatively rougher enamel surface than the enamel conditioned with 10% polyacrylic acid, indicating a greater loss of enamel from conditioning with 37% phosphoric acid.
DISCUSSION In vitro study
The shear/peel bond strengths of the 2 bonding materials were measured at 30 minutes and at 24 hours. This was designed to more fully simulate a clinical situation, because archwires are typically placed at the bonding appointment, when light-cured resins or cements might not have been completely polymerized.2 The bond strengths of both the composite resin and the RMGI were significantly higher at 24 hours than at 30 minutes. The reduced shear/peel bond strength immediately after bonding agrees with the findings of Bishara et al15 and is probably related to incomplete polymerization of light-cured materials.18,19 The bond strengths of the composite resin with 37% phosphoric acid at both 24 hours and 30 minutes were
204 Summers et al
Table V.
American Journal of Orthodontics and Dentofacial Orthopedics August 2004
Number of the in vitro brackets in each score category of ARI
Group
Material
I II III IV
Debonded
Fuji Ortho LC Fuji Ortho LC Light Bond Light Bond
30 24 30 24
ARI ⫽ 1
ARI ⫽ 2
ARI ⫽ 3
ARI ⫽ 4
ARI ⫽ 5
0 0 0 0
0 0 0 0
0 1 2 2
1 2 5 7
12 9 6 3
min h min h
Fig 1. A, SEM of untreated enamel surface. B, SEM of enamel conditioned with 37% phosphoric acid for 30 seconds. C, SEM of enamel conditioned with 10% polyacrylic acid for 20 seconds. For all panels, original magnification ⫻1000. Table VII. Number of in vivo brackets in each ARI score category Group Fuji Ortho LC Light Bond
Fig 2. Bracket survival distribution over time with Fuji Ortho LC and Light Bond. Table VI.
Descriptive statistics of all in vivo ARI
scores Adhesive
n
Mean
Minimum
Maximum
Fuji Ortho LC Light Bond
13 10
3.5 3.7
1 1
5 5
significantly greater than those of the RMGI with 10% polyacrylic acid. These findings agree with those of Jobalia et al,6 Meehan et al,20 Bishara et al,15 and Owens et al.21 Bonding of glass ionomers is enhanced
ARI ⫽ 1 ARI ⫽ 2 ARI ⫽ 3 ARI ⫽ 4 ARI ⫽ 5 1 0
1 1
4 2
4 0
3 7
by surface conditioning with 10% polyacrylic acid.22 The acid removes contaminants and pellicles from the substrate surface and serves as a cleaning and wetting agent to improve the bonding of cement to enamel. Shammaa et al7 reported adequate bond strength when glass ionomer (Fry Ortho LC) was bonded to enamel clinically without acid etching. However, the bond strength was less compared with the values reported in this study with 10% polyacrylic acid. Etching of enamel with 37% phosphoric acid produces resin tags to a depth of 80 m that greatly increase the mechanical retention of composite resin to the enamel.23 Tavas and Watt24 recommended that adhesive bond strength greater than 58 N is necessary for clinical use. According to this recommendation, Fuji Ortho LC adhesive, with a shear bond strength of 73.5 N (6.9 MPa) at 30 minutes, has the potential to resist forces during orthodontic treatment. However, the average force transmitted to a bracket during mastication has been reported to be between 40 and 120 N.25,26 The use of Fuji Ortho LC in areas of traumatic or heavy occlusion might be guarded.
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American Journal of Orthodontics and Dentofacial Orthopedics Volume 126, Number 2
The SEM results showed that the enamel surface conditioned with 37% phosphoric acid was qualitatively rougher than when 10% polyacrylic acid was used. The preferential etching of prism cones is typical of a type I enamel etching pattern.17 Polyacrylic acid has a larger molecular size than phosphoric acid. The smaller molecule of 37% phosphoric acid was able to penetrate to a greater depth; thus, potentially more enamel can be lost during conditioning and debonding. SEM studies show that enamel surfaces after debonding of brackets are more porous with acid etching compared with clean and smooth enamel surface without etching.3 Repumicing of the bonded surface could restore the tooth surface to its original appearance.27 In vivo investigation
Bracket survival distribution was used to analyze the in vivo data. This gives the clinician a better perspective on when failure occurred compared with studying the failure rates only at the final data collection. Our study showed no statistically significant differences between Light Bond and Fuji Ortho LC as bonding materials. The failure rates of Light Bond and Fuji Ortho LC were 5% and 6.5%, respectively. In several studies, the failure rates for brackets bonded with glass ionomer cements varied from 3.2% to 50%.28-33 The lowest failure rates for Fuji Ortho LC were reported by Fricker29 in 1998 (3.2%) and Silverman et al30 in 1995 (3.3%). In addition, several clinicians, after 3 years of using Fuji Ortho LC, experienced bonding success rates comparable to those of conventional composite resins.31,32 The findings in the present study confirm the observations that, overall, the clinical performance of Fuji Ortho LC was similar to that of composite resin. In the present study, the predominant mode of bracket failure for the Fuji Ortho LC adhesive was at the enamel-cement interface, in both in vitro and in vivo conditions. The in vitro results suggest that chemical and mechanical bonding of glass ionomer cement to the bracket is stronger than the chemical bond of glass ionomer to the enamel, even in the presence of the resin component. This agrees with the results reported by McSherry,26 that glass ionomer cement bonds better to the metal band of the bracket than to the enamel. In contrast, when acid etching was used with Light Bond materials, a higher percentage of failure occurred at the resin-bracket interface. This is probably because of the incomplete polymerization of the resin below the bracket base.19 Air entrapment behind the mesh of a bracket can also affect polymerization, because of oxygen inhibition of free radical polymerizing in light-
cured composite materials.35 Clinically, it is the authors’ experience that, during debonding, the resin adhesive remaining on the tooth is more difficult to remove than the RMGI adhesive. CONCLUSIONS
1. In vitro results showed significantly greater shear bond strengths when brackets were bonded with 37% phosphoric acid and composite resin (Light Bond) compared with RMGI (Fuji Ortho LC) bonded with 10% polyacrylic acid. 2. Significantly greater shear bond strengths can be obtained 24 hours after bonding brackets for both materials. 3. The in vivo results showed no significant difference in bracket failure rates between Fuji Ortho LC and Light Bond after 1.3 years. Clinically, Fuji Ortho LC adhesive has adequate bond strength to withstand the occlusal forces of chewing and biting. 4. The ARI study showed that the predominant bracket failure interface for Fuji Ortho LC was at the enamel-adhesive interface. The weaker chemical bonding between RMGI and the enamel might make it easier for clinicians to clean up the adhesive on the enamel surface after debonding. 5. The SEM study showed that etching with 37% phosphoric acid on dental enamel for 30 seconds produced a qualitatively rougher and more porous surface than conditioning with 10% polyacrylic acid.4,10,11,12,34 REFERENCES 1. Newman GV. Epoxy adhesives for orthodontic attachments: progress report. Am J Orthod 1965;51:901-12. 2. Thompson RE, Way DC. Enamel loss due to prophylaxis and multiple bonding/debonding of orthodontic attachments. Am J Orthod 1981;79:282-95. 3. Osorio R, Toledano M, Garc´ ia-Godoy F. Enamel surface morphology after bracket debonding. J Dent Child 1998;65:313-7. 4. Joseph VP, Rossouw E. The shear bond strengths of stainless steel and ceramic brackets used with chemically and lightactivated composite resins. Am J Orthod Dentofacial Orthop 1990;97:121-5. 5. 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. 6. Jobalia SB, Valente RM, Waldemar GR, BeGole EA, Evans CA. Bond strength of visible light-cured glass ionomer orthodontic cement. Am J Orthod Dentofacial Orthop 1997;112:205-8. 7. Shammaa I, Ngan P, Kim H, Kao E, Gladwin M, Gunel E, et al. Comparison of bracket debonding force between two conventional resin adhesives and a resin-reinforced glass ionomer cement: An in vitro and in vivo study. Angle Orthod 1999;69: 463-9. 8. Millett DT, Cattanach D, McFadzean R, Pattison J, McColl J. Laboratory evaluation of a compomer and a resin modified glass
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American Journal of Orthodontics and Dentofacial Orthopedics August 2004
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