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Comparison of shear bond strengths of orthodontic brackets bonded to deciduous and permanent teeth
ORIGINAL ARTICLE
Comparison of shear bond strengths of orthodontic brackets bonded to deciduous and permanent teeth Toshiya Endo,a Rieko Ozoe,b Koichi Shinkai,c Junko Shimomura,d Yoshiroh Katoh,e and Shohachi Shimookaf Niigata, Japan Introduction: This study was conducted (1) to compare the bond strengths of identical orthodontic brackets bonded to deciduous and permanent teeth with either of 2 primer/adhesive systems and (2) to evaluate the modes of bracket/adhesive failure. Methods: Thirty-four permanent teeth and 34 deciduous teeth were collected. To these freshly extracted teeth, metal brackets were bonded with 2 primer/adhesive systems. These specimens were divided into 4 groups. In group I, consisting of 17 permanent teeth, the brackets were bonded with an acid-etching adhesive system; in group II, the self-etching adhesive system was used for bonding the brackets to 17 permanent teeth; in group III, consisting of 17 deciduous teeth, the acid-etching adhesive system was used; and in group IV, the self-etching adhesive system was used for bonding the brackets to 17 deciduous teeth. Shear bond strength was measured with a universal testing machine, and the mode of bracket/adhesive failure was determined according to the adhesive remnant index. Results: The shear bond strengths of all 4 groups exceeded what is believed to be clinically sufficient strength of 6 to 8 MPa. Whether deciduous or permanent teeth, there were no significant differences in shear bond strength between the acid-etching and self-etching adhesive systems. The shear bond strengths of the brackets bonded to the deciduous teeth with either adhesive system were lower than those to the permanent teeth. Bond failure occurred at the enamel-adhesive interface more frequently in the self-etching adhesive system than in the acid-etching adhesive system. Conclusions: Bond strengths of brackets were lower with deciduous teeth but were high enough for clinical purposes. (Am J Orthod Dentofacial Orthop 2008;134: 198-202)
E
arly orthodontic treatment for various types of malocclusion is recommended to enhance dental and skeletal development before completion of the permanent dentition.1 An edgewise appliance is used for the treatment of dental (functional) anterior crossbites, maxillary incisor crowding, and impacted (unerupted) maxillary incisors in the early mixed dentition.2-4 To facilitate anterior crossbite correction and ectopic or impacted incisor alignment in early treatment, orthodontic brackets are bonded to deciduous canines and molars, as well as to permanent incisors.
From Nippon Dental University Niigata Hospital, Niigata, Japan. a Assistant professor, Orthodontic Dentistry. b Research assistant, Orthodontic Dentistry. c Associate professor, Department of Operative Dentistry. d Lecturer, Department of Pediatric Dentistry. e Professor and chairman, Department of Operative Dentistry. f Professor and chairman, Department of Pediatric Dentistry. Reprint requests to: Toshiya Endo, Orthodontic Dentistry, Nippon Dental University Niigata Hospital, 1-8 Hamaura-cho, Niigata 951-8580, Japan; e-mail, [email protected]. Submitted, March 2006; revised and accepted, May 2006. 0889-5406/$34.00 Copyright © 2008 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.05.045
198
These deciduous teeth are used as anchor teeth in orthodontic therapy with edgewise appliances.4 The bond strength of orthodontic brackets bonded to enamel surfaces with light-cured adhesives depends on several factors, including variations in the enamel surface,5-11 tooth type,12 type of etchant and its concentration and application time,13,14 types of primers and adhesive,15-17 type of bracket and its base shape and design,18 and type of curing light source and its applying conditions.19,20 The enamel surface structure of deciduous teeth is different from that of permanent teeth. The outer prismless enamel layer, more common in deciduous teeth than in permanent teeth,5-7 lacks the characteristic prism markings of enamel and has no well-developed etch pattern with well-defined prisms, resulting in insufficient resin penetration and weak bond strength.8 A study reported that the exposure of enamel crystallites is more important than well-defined etch patterns.9 The enamel crystallite diameter of deciduous teeth is relatively larger than that of permanent teeth.10 The chemical compositions of calcium and phosphorus in deciduous teeth are similar to those in permanent teeth.11
American Journal of Orthodontics and Dentofacial Orthopedics Volume 134, Number 2
A conventional adhesive system of orthodontic brackets to enamel mainly involves acid etching, rinsing, priming, and bonding steps. A new self-etching primer/adhesive system, combining the acid etching, rinsing, and priming steps, has revolutionalized the clinical procedure of orthodontic treatment. The combination of these 3 steps simplifies the clinical handling of adhesive systems and saves chair time.21 Some researchers, however, found no significant differences in bond strength between the new self-etching adhesive system and the conventional acid-etching adhesive system.17,22,23 Others showed that the bond strengths of the adhesive systems with self-etching primers were significantly lower than those with phosphoric acid etchants.15,23-25 To our knowledge, there are few reports about the characteristics of orthodontic bracket bonding to deciduous teeth,26 although extensive and detailed investigations have been made with permanent teeth. The purposes of this study were to evaluate the bond strengths of orthodontic brackets bonded to deciduous teeth with a self-etching adhesive system or a conventional acid-etching adhesive system and the adhesive remnants on the deciduous teeth after debonding the brackets, and to compare the results with those obtained when bonded to permanent teeth. MATERIAL AND METHODS
The protocol was approved by the local Committee of Ethics of Nippon Dental University Niigata Hospital, Niigata, Japan. Informed consent was obtained from all participants, who had received written or oral information about this project. Thirty-four freshly extracted permanent teeth and 34 freshly extracted deciduous teeth were collected from the participants. The criteria for tooth selection included intact buccal enamel with no pretreatment chemical agents, no cracks from extraction, no hypoplastic enamel, and no caries. These teeth were washed in water to remove contamination and stored in a solution of 0.1% thymol at 4°C until they were used in this study. The permanent teeth were divided into 2 groups of 17 (groups I and II) with equal numbers of maxillary and mandibular first and second premolars in each group. The deciduous teeth were also divided into 2 groups of 17 (groups III and IV) with equal numbers of maxillary and mandibular canines and first and second molars in each group. The division was based on the suggestion by Hobson et al12 that comparisons of bond strength values should be made with the same tooth type or appropriately stratified groups of teeth. The brackets used in this study were metal mesh-
Endo et al 199
base premolar edgewise appliances with an 0.018-in slot (Victory series, 3M Unitek, Monrovia, Calif). The average bracket base area was 9.94 mm2. The brackets were bonded to the teeth according to 1 of 2 protocols according to the manufacturer’s instructions. The acid-etching adhesive system was applied to groups I and III, and the self-etching adhesive system to groups II and IV. In groups I and III, the buccal surface of each tooth crown was cleansed with a mixture of water and fluoride-free pumice in a rubber prophylactic cup for 10 seconds. Each tooth was then rinsed with a water spray for 10 seconds and dried with an oil- and moisture-free airstream. The buccal enamel surface was etched with 35% phosphoric acid gel (3M Unitek) for 15 seconds, followed by thorough rinsing and drying. A frosted appearance indicated that etching was successful. A thin uniform layer of Transbond XT primer (3M Unitek) was applied to the etched enamel surface, and Transbond XT adhesive (3M Unitek) was applied to the bracket base. The bracket was placed on the buccal surface of the tooth and pressed firmly to express any adhesive from the margins of the bracket base. Excess adhesive was removed with an explorer before curing. Then, the bracket was light-cured with an Ortholux LED curing light (3M Unitek) for 10 seconds (5 seconds mesially, 5 seconds distally). In groups II and IV, the buccal surface of each tooth crown was cleansed and rinsed, as in groups I and III. Excess water was removed from the buccal enamel surface (but it was unnecessary to dry them thoroughly). The Transbond plus self-etching primer, with both etchant and primer combined into 1 product, was rubbed on the buccal enamel surface for 5 seconds and blown off gently with the oil- and moisture-free airstream. The brackets were then bonded with Transbond XT adhesive and light-cured with the LED curing light, as in groups I and III. The root of each tooth with a bonded bracket was cut off with a separating disk. The tooth crown was embedded in the specimen holder ring with self-curing acrylic resin, so that the buccal enamel surface was parallel to and projected above the rim of the cylindrical specimen holder ring. All specimen holder rings with embedded teeth were stored in distilled water at 37°C for 24 hours. A universal testing machine (EZ Test, Shimadzu, Kyoto, Japan) was used to determine shear bond strengths. The specimen holder rings were arranged in this machine so that a load was applied to the occlusal bracket wings with a force in the occlusogingival direction parallel to the buccal enamel surface. The force required to shear off the bracket was recorded in newtons (N) at a crosshead speed of 1.0 mm per minute. The shear bond strength was then
200 Endo et al
Table I.
American Journal of Orthodontics and Dentofacial Orthopedics August 2008
Descriptive statistics for shear bond strength and distribution of ARI scores Shear bond strength (MPa)
ARI scores
Group
n
Mean
SD
Range
0
1
2
3
I II III IV
17 17 17 17
12.18 11.69 9.17 8.49
2.64 3.96 2.14 2.01
8.13-17.11 6.56-13.23 4.27-17.46 5.55-11.37
0 7 7 7
7 6 4 6
4 2 3 2
6 2 3 2
calculated by dividing the shear force by the bracket base area. After testing the shear bond strength, the bracket bases and the enamel surfaces were examined with a stereomicroscope at 8 times magnification by 1 investigator (R.O.) to evaluate the amount of adhesive remaining on the teeth. The adhesive remnant index (ARI) was used to assess the quantity of adhesive remaining on the enamel surface.27 The ARI scores ranged from 0 to 3: 0, no adhesive remaining on the tooth; 1, less than half of the adhesive remaining on the tooth; 2, more than half of the adhesive remaining on the tooth; and 3, all adhesive remaining on the tooth with a distinct impression of the bracket base. Statistical analysis
Statistical analyses were performed with the software StatMate III (ATMS, Tokyo, Japan). Descriptive statistics including mean bond strengths, standard deviations, and ranges were calculated. Two-way analysis of variance (ANOVA) and Scheffé post-hoc tests were used to test the main effects of tooth types and primer/adhesive systems on the shear bond strength and to identify where differences occurred. One-way ANOVA and Scheffé post-hoc tests were used to compare the shear bond strengths between tooth types and between primer/adhesive systems separately. The nonparametric Kruskal-Wallis test was used to test the significant differences between groups with regard to ARI scores. All statistical tests were performed at the P ⬍0.05 level of significance. RESULTS
Descriptive statistics for the shear bond strengths of each group are shown in Table I. The results of the 2-way ANOVA are shown in Table II. The 2-way ANOVA indicated no significant interaction between the main effects of tooth types and primer/adhesive systems and no significant difference in shear bond strength between acid-etching and self-etching adhesive systems. The 2-way ANOVA and Scheffé post-hoc tests found a significant difference in shear bond strength between deciduous and permanent teeth. The overall mean shear bond
Table II.
Two-way ANOVA
Source Tooth type Primer/adhesive system Interaction Error
Sum of squares
df
Mean square
F
Significance
164.39
1
164.39
21.04
⬍0.001
5.83 0.15 500.12
1 1 64
5.83 0.15 7.81
0.75 0.02
0.39 0.89
strengths for groups I and II (11.94 MPa) were significantly greater than those for groups II and IV (8.83 MPa). The 1-way ANOVA indicated no significant differences in shear bond strength between groups I and II and between groups III and IV. The 1-way ANOVA and Scheffé post-hoc tests showed significant differences in shear bond strength between groups I and III and between groups II and IV, demonstrating that the shear bond strengths to the deciduous teeth were significantly lower than those to the permanent teeth, no matter which of the 2 adhesive systems was used. The distribution of ARI scores for each group is shown in Table I. The Kruskal-Wallis test showed significant differences in ARI scores between groups I and II and between groups I and IV. Bond failure at the enamel-adhesive interface occurred more frequently in the self-etching adhesive system than in the acidetching adhesive system. DISCUSSION
No published studies have investigated differences in shear bond strength between deciduous and permanent teeth when identical orthodontic brackets were bonded with different primer/adhesive systems. Bond strengths between 6 and 8 MPa are sufficient for successfully bonding orthodontic brackets to enamel clinically.28 Based on the results of Weibull analysis, Hobson et al29 presumed that, at this level of stress (8 MPa), less than 14% of all bonds would fail in the worst scenario (blood contamination). Our findings showed that the shear bond strengths of all 4 groups were higher than the clinically required range of 6 to 8 MPa.
Endo et al 201
American Journal of Orthodontics and Dentofacial Orthopedics Volume 134, Number 2
We also found no significant differences in shear bond strength between acid-etching and self-etching adhesive systems. These results were consistent with previous findings of some researchers, who applied acid-etching and self-etching adhesive systems to human permanent teeth17,22,23,30 and bovine teeth31,32 under conditions of dry enamel surfaces. Although self-etching primers generally produce less demineralization of enamel than do phosphoric acid etchants,24 the pH of the self-etching primer in this study, which was the same as used by Arnold et al,22 might be too acidic to obtain satisfactory bond strength for attaching brackets to enamel. Several other investigations showed that bond strengths were significantly lower when self-etching adhesive systems were used in conditions of uncontaminated (dry) enamel surfaces than when acid-etching adhesive systems were used.15,23-25,33-35 Moreover, other investigations showed that, in water-, saliva-, and blood-contaminated conditions, the self-etching adhesive system had higher bond strengths than the acid-etching adhesive system, suggesting that self-etching primers might be suitable for bonding brackets in uncooperative children in the mixed dentition.17,31 We found, between deciduous and permanent teeth, significant differences in the shear bond strengths of brackets bonded with the 2 adhesive systems. However, these findings disagreed with the those of Ergas et al,26 who found no significant differences in the shear bond strengths of brackets between deciduous molars and premolars. In our study, the significantly lower shear bond strength in the deciduous teeth might be explained by the outer prismless enamel layer and the adhesive thickness between the bracket base and the enamel surface. The outer prismless enamel layer is more common in deciduous teeth than in permanent teeth.5-7 The area on the labial surface, where the prismless enamel layer is most common, is the gingival third of the tooth crown in permanent teeth, whereas this area in deciduous teeth is the middle third, which corresponds to the position where brackets are bonded. The outer prismless enamel layer might prevent infiltration of the self-etching primer and allow shallow etching,24 resulting in insufficient penetration of the adhesive into the enamel surface. This deficient adhesive resin penetration might cause lower bond strengths in deciduous teeth than in permanent teeth. On the other hand, Pashley and Tay33 reported that the efficacy of selfetching primers did not depend on their etching aggressiveness. The base of the brackets in this study was designed to fit the premolar enamel surface configuration. The 3-dimensional surface configuration of deciduous canines is similar to that of the premolars, whereas the
configuration of the deciduous molars is more complex than that of the premolars, causing the adhesive thickness between the bracket base and the deciduous enamel surface to increase. The increased adhesive thickness due to the complex deciduous enamel surface configurations could be another factor responsible for lowering the bond strength in the deciduous teeth. This suggests the need for a special type of bracket for deciduous teeth. In this study, the self-etching adhesive system applied to the permanent and deciduous teeth had higher frequencies of bond failure at the enameladhesive interface than did the acid-etching adhesive system applied to the permanent teeth as shown in previous studies.25,31,32,35,36 Our results might have reflected insufficient penetration of adhesive resins into the self-etched enamel surface. Some studies found no significant differences in the mode of bracket/adhesive failure between acid-etching and self-etching adhesive systems in dry conditions.17,34,36,37 On the other hand, Bishara et al15 found that more adhesive remained on the teeth with the self-etching adhesive system than with the acid-etching adhesive system. There were no consistent findings as to the bracket/adhesive failure modes for the self-etching adhesive systems. Our findings that groups I and II had almost the same shear bond strengths but significant differences in the distribution of ARI scores support the observations by some investigators that the amount of residual adhesive resins might not be related to shear bond strength.34,38 CONCLUSIONS
1. The shear bond strengths of all 4 groups were higher than the clinically sufficient range of 6 to 8 MPa. 2. Regardless of whether the teeth were deciduous or permanent, there were no significant differences in shear bond strength between acid-etching and selfetching adhesive systems. 3. The shear bond strengths to the deciduous teeth were lower than those to the permanent teeth when either adhesive system was used. 4. The self-etching adhesive system applied to the permanent and deciduous teeth had higher frequencies of bond failure at the enamel-adhesive interface than the acid-etching adhesive applied to the permanent teeth. 5. The comparison of the shear bond strengths and the bracket/adhesive failure modes between the deciduous and permanent teeth suggest that deciduous teeth can be used as anchor teeth in orthodontic treatment with edgewise appliances.
202 Endo et al
American Journal of Orthodontics and Dentofacial Orthopedics August 2008
REFERENCES
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1. Bishara SE, Justus R, Graber TM. Proceedings of the workshop discussions on early treatment. Held by the College of Diplomates of the American Board of Orthodontics in Quebec City, Canada on July 13-17, 1997. Am J Orthod Dentofacial Orthop 1998;113:5-6. 2. Endo T, Mizutani Y, Ozoe R, Kubota M, Shimooka S. Effects of early treatment of maxillary incisor crowding by using two bands and four brackets (2x4) mechanotherapy. Ped Dent J 2004;14:87-94. 3. McKeown HF, Sandler J. The two by four appliance: a versatile appliance. Dent Update 2001;28:496-500. 4. Tal E, Kupietzky A. Orthodontic alignment of permanent incisors following previous trauma of a primary tooth. Pediatr Dent 2000;22:71-2. 5. Ripa LW, Gwinnett AJ, Buonocore MG. The “prismless” outer layer of deciduous and permanent enamel. Arch Oral Biol 1966;11:41-8. 6. Gwinnett AJ. Human prismless enamel and its influence on sealant penetration. Arch Oral Biol 1973;18:441-4. 7. Whittaker DK. Structural variations in the surface zone of human tooth enamel observed by scanning electron microscopy. Arch Oral Biol 1982;27:383-92. 8. Gwinnett AJ, Matsui A. A study of enamel adhesives. The physical relationship between enamel and adhesive. Arch Oral Biol 1967;12:1615-20. 9. Hobson RS, McCabe JF, Rugg-Gunn AJ. The relationship between acid-etch patterns and bond survival in vivo. Am J Orthod Dentofacial Orthop 2002;121:502-9. 10. Arends J, Jongebloed WL, Schuthof J. Crystallite diameters of enamel near the anatomical surface. An investigation of mature, deciduous and non-erupted human enamel. Caries Res 1983;17:97105. 11. Bird MJ, French EL, Woodside MR, Morrison MI, Hodge HC. Chemical analyses of deciduous enamel and dentin. J Dent Res 1940;19:413-23. 12. Hobson RS, McCabe JF, Hogg SD. Bond strength of surface enamel for different tooth types. Dent Mater 2001;17:184-9. 13. Retief DH, Busscher HJ, de Boer P, Jongebloed WL, Arends J. A laboratory evaluation of three etching solutions. Dent Mater 1986;2:202-6. 14. Legler LR, Retief DH, Bradley EL, Denys FR, Sadowsky PL. Effects of phosphoric acid concentration and etch duration on the shear bond strength of an orthodontic bonding resin to enamel. An in vitro study. Am J Orthod Dentofacial Orthop 1989;96:485-92. 15. 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. 16. Grandhi RM, Combe EC, Speidel TM. Shear bond strength of stainless steel orthodontic brackets with a moisture-insensitive primer. Am J Orthod Dentofacial Orthop 2001;119:251-5. 17. Rajagopal R, Padmanabhan S, Gnanamani J. A comparison of shear bond strength and debonding characteristics of conventional, moisture-insensitive, and self-etching primers in vitro. Angle Orthod 2004;74:264-8. 18. Sorel O, Alam RE, Chagneau F, Cathelineau G. Comparison of bond strength between simple foil mesh and laser-structured base retention brackets. Am J Orthod Dentofacial Orthop 2002;122: 260-6. 19. Cacciafesta V, Sfondrini MF, Scribante A, Boehme A, JostBrinkmann PG. Effect of light-tip distance on the shear bond strengths of composite resin. Angle Orthod 2005;75:386-91. 20. Evans LJ, Peters C, Flickinger C, Taloumis L, Dunn W. A comparison of shear bond strengths of orthodontic brackets using
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Comparison of shear bond strengths of orthodontic brackets bonded to deciduous and permanent teeth Toshiya Endo,a Rieko Ozoe,b Koichi Shinkai,c Junko Shimomura,d Yoshiroh Katoh,e and Shohachi Shimookaf Niigata, Japan Introduction: This study was conducted (1) to compare the bond strengths of identical orthodontic brackets bonded to deciduous and permanent teeth with either of 2 primer/adhesive systems and (2) to evaluate the modes of bracket/adhesive failure. Methods: Thirty-four permanent teeth and 34 deciduous teeth were collected. To these freshly extracted teeth, metal brackets were bonded with 2 primer/adhesive systems. These specimens were divided into 4 groups. In group I, consisting of 17 permanent teeth, the brackets were bonded with an acid-etching adhesive system; in group II, the self-etching adhesive system was used for bonding the brackets to 17 permanent teeth; in group III, consisting of 17 deciduous teeth, the acid-etching adhesive system was used; and in group IV, the self-etching adhesive system was used for bonding the brackets to 17 deciduous teeth. Shear bond strength was measured with a universal testing machine, and the mode of bracket/adhesive failure was determined according to the adhesive remnant index. Results: The shear bond strengths of all 4 groups exceeded what is believed to be clinically sufficient strength of 6 to 8 MPa. Whether deciduous or permanent teeth, there were no significant differences in shear bond strength between the acid-etching and self-etching adhesive systems. The shear bond strengths of the brackets bonded to the deciduous teeth with either adhesive system were lower than those to the permanent teeth. Bond failure occurred at the enamel-adhesive interface more frequently in the self-etching adhesive system than in the acid-etching adhesive system. Conclusions: Bond strengths of brackets were lower with deciduous teeth but were high enough for clinical purposes. (Am J Orthod Dentofacial Orthop 2008;134: 198-202)
E
arly orthodontic treatment for various types of malocclusion is recommended to enhance dental and skeletal development before completion of the permanent dentition.1 An edgewise appliance is used for the treatment of dental (functional) anterior crossbites, maxillary incisor crowding, and impacted (unerupted) maxillary incisors in the early mixed dentition.2-4 To facilitate anterior crossbite correction and ectopic or impacted incisor alignment in early treatment, orthodontic brackets are bonded to deciduous canines and molars, as well as to permanent incisors.
From Nippon Dental University Niigata Hospital, Niigata, Japan. a Assistant professor, Orthodontic Dentistry. b Research assistant, Orthodontic Dentistry. c Associate professor, Department of Operative Dentistry. d Lecturer, Department of Pediatric Dentistry. e Professor and chairman, Department of Operative Dentistry. f Professor and chairman, Department of Pediatric Dentistry. Reprint requests to: Toshiya Endo, Orthodontic Dentistry, Nippon Dental University Niigata Hospital, 1-8 Hamaura-cho, Niigata 951-8580, Japan; e-mail, [email protected]. Submitted, March 2006; revised and accepted, May 2006. 0889-5406/$34.00 Copyright © 2008 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.05.045
198
These deciduous teeth are used as anchor teeth in orthodontic therapy with edgewise appliances.4 The bond strength of orthodontic brackets bonded to enamel surfaces with light-cured adhesives depends on several factors, including variations in the enamel surface,5-11 tooth type,12 type of etchant and its concentration and application time,13,14 types of primers and adhesive,15-17 type of bracket and its base shape and design,18 and type of curing light source and its applying conditions.19,20 The enamel surface structure of deciduous teeth is different from that of permanent teeth. The outer prismless enamel layer, more common in deciduous teeth than in permanent teeth,5-7 lacks the characteristic prism markings of enamel and has no well-developed etch pattern with well-defined prisms, resulting in insufficient resin penetration and weak bond strength.8 A study reported that the exposure of enamel crystallites is more important than well-defined etch patterns.9 The enamel crystallite diameter of deciduous teeth is relatively larger than that of permanent teeth.10 The chemical compositions of calcium and phosphorus in deciduous teeth are similar to those in permanent teeth.11
American Journal of Orthodontics and Dentofacial Orthopedics Volume 134, Number 2
A conventional adhesive system of orthodontic brackets to enamel mainly involves acid etching, rinsing, priming, and bonding steps. A new self-etching primer/adhesive system, combining the acid etching, rinsing, and priming steps, has revolutionalized the clinical procedure of orthodontic treatment. The combination of these 3 steps simplifies the clinical handling of adhesive systems and saves chair time.21 Some researchers, however, found no significant differences in bond strength between the new self-etching adhesive system and the conventional acid-etching adhesive system.17,22,23 Others showed that the bond strengths of the adhesive systems with self-etching primers were significantly lower than those with phosphoric acid etchants.15,23-25 To our knowledge, there are few reports about the characteristics of orthodontic bracket bonding to deciduous teeth,26 although extensive and detailed investigations have been made with permanent teeth. The purposes of this study were to evaluate the bond strengths of orthodontic brackets bonded to deciduous teeth with a self-etching adhesive system or a conventional acid-etching adhesive system and the adhesive remnants on the deciduous teeth after debonding the brackets, and to compare the results with those obtained when bonded to permanent teeth. MATERIAL AND METHODS
The protocol was approved by the local Committee of Ethics of Nippon Dental University Niigata Hospital, Niigata, Japan. Informed consent was obtained from all participants, who had received written or oral information about this project. Thirty-four freshly extracted permanent teeth and 34 freshly extracted deciduous teeth were collected from the participants. The criteria for tooth selection included intact buccal enamel with no pretreatment chemical agents, no cracks from extraction, no hypoplastic enamel, and no caries. These teeth were washed in water to remove contamination and stored in a solution of 0.1% thymol at 4°C until they were used in this study. The permanent teeth were divided into 2 groups of 17 (groups I and II) with equal numbers of maxillary and mandibular first and second premolars in each group. The deciduous teeth were also divided into 2 groups of 17 (groups III and IV) with equal numbers of maxillary and mandibular canines and first and second molars in each group. The division was based on the suggestion by Hobson et al12 that comparisons of bond strength values should be made with the same tooth type or appropriately stratified groups of teeth. The brackets used in this study were metal mesh-
Endo et al 199
base premolar edgewise appliances with an 0.018-in slot (Victory series, 3M Unitek, Monrovia, Calif). The average bracket base area was 9.94 mm2. The brackets were bonded to the teeth according to 1 of 2 protocols according to the manufacturer’s instructions. The acid-etching adhesive system was applied to groups I and III, and the self-etching adhesive system to groups II and IV. In groups I and III, the buccal surface of each tooth crown was cleansed with a mixture of water and fluoride-free pumice in a rubber prophylactic cup for 10 seconds. Each tooth was then rinsed with a water spray for 10 seconds and dried with an oil- and moisture-free airstream. The buccal enamel surface was etched with 35% phosphoric acid gel (3M Unitek) for 15 seconds, followed by thorough rinsing and drying. A frosted appearance indicated that etching was successful. A thin uniform layer of Transbond XT primer (3M Unitek) was applied to the etched enamel surface, and Transbond XT adhesive (3M Unitek) was applied to the bracket base. The bracket was placed on the buccal surface of the tooth and pressed firmly to express any adhesive from the margins of the bracket base. Excess adhesive was removed with an explorer before curing. Then, the bracket was light-cured with an Ortholux LED curing light (3M Unitek) for 10 seconds (5 seconds mesially, 5 seconds distally). In groups II and IV, the buccal surface of each tooth crown was cleansed and rinsed, as in groups I and III. Excess water was removed from the buccal enamel surface (but it was unnecessary to dry them thoroughly). The Transbond plus self-etching primer, with both etchant and primer combined into 1 product, was rubbed on the buccal enamel surface for 5 seconds and blown off gently with the oil- and moisture-free airstream. The brackets were then bonded with Transbond XT adhesive and light-cured with the LED curing light, as in groups I and III. The root of each tooth with a bonded bracket was cut off with a separating disk. The tooth crown was embedded in the specimen holder ring with self-curing acrylic resin, so that the buccal enamel surface was parallel to and projected above the rim of the cylindrical specimen holder ring. All specimen holder rings with embedded teeth were stored in distilled water at 37°C for 24 hours. A universal testing machine (EZ Test, Shimadzu, Kyoto, Japan) was used to determine shear bond strengths. The specimen holder rings were arranged in this machine so that a load was applied to the occlusal bracket wings with a force in the occlusogingival direction parallel to the buccal enamel surface. The force required to shear off the bracket was recorded in newtons (N) at a crosshead speed of 1.0 mm per minute. The shear bond strength was then
200 Endo et al
Table I.
American Journal of Orthodontics and Dentofacial Orthopedics August 2008
Descriptive statistics for shear bond strength and distribution of ARI scores Shear bond strength (MPa)
ARI scores
Group
n
Mean
SD
Range
0
1
2
3
I II III IV
17 17 17 17
12.18 11.69 9.17 8.49
2.64 3.96 2.14 2.01
8.13-17.11 6.56-13.23 4.27-17.46 5.55-11.37
0 7 7 7
7 6 4 6
4 2 3 2
6 2 3 2
calculated by dividing the shear force by the bracket base area. After testing the shear bond strength, the bracket bases and the enamel surfaces were examined with a stereomicroscope at 8 times magnification by 1 investigator (R.O.) to evaluate the amount of adhesive remaining on the teeth. The adhesive remnant index (ARI) was used to assess the quantity of adhesive remaining on the enamel surface.27 The ARI scores ranged from 0 to 3: 0, no adhesive remaining on the tooth; 1, less than half of the adhesive remaining on the tooth; 2, more than half of the adhesive remaining on the tooth; and 3, all adhesive remaining on the tooth with a distinct impression of the bracket base. Statistical analysis
Statistical analyses were performed with the software StatMate III (ATMS, Tokyo, Japan). Descriptive statistics including mean bond strengths, standard deviations, and ranges were calculated. Two-way analysis of variance (ANOVA) and Scheffé post-hoc tests were used to test the main effects of tooth types and primer/adhesive systems on the shear bond strength and to identify where differences occurred. One-way ANOVA and Scheffé post-hoc tests were used to compare the shear bond strengths between tooth types and between primer/adhesive systems separately. The nonparametric Kruskal-Wallis test was used to test the significant differences between groups with regard to ARI scores. All statistical tests were performed at the P ⬍0.05 level of significance. RESULTS
Descriptive statistics for the shear bond strengths of each group are shown in Table I. The results of the 2-way ANOVA are shown in Table II. The 2-way ANOVA indicated no significant interaction between the main effects of tooth types and primer/adhesive systems and no significant difference in shear bond strength between acid-etching and self-etching adhesive systems. The 2-way ANOVA and Scheffé post-hoc tests found a significant difference in shear bond strength between deciduous and permanent teeth. The overall mean shear bond
Table II.
Two-way ANOVA
Source Tooth type Primer/adhesive system Interaction Error
Sum of squares
df
Mean square
F
Significance
164.39
1
164.39
21.04
⬍0.001
5.83 0.15 500.12
1 1 64
5.83 0.15 7.81
0.75 0.02
0.39 0.89
strengths for groups I and II (11.94 MPa) were significantly greater than those for groups II and IV (8.83 MPa). The 1-way ANOVA indicated no significant differences in shear bond strength between groups I and II and between groups III and IV. The 1-way ANOVA and Scheffé post-hoc tests showed significant differences in shear bond strength between groups I and III and between groups II and IV, demonstrating that the shear bond strengths to the deciduous teeth were significantly lower than those to the permanent teeth, no matter which of the 2 adhesive systems was used. The distribution of ARI scores for each group is shown in Table I. The Kruskal-Wallis test showed significant differences in ARI scores between groups I and II and between groups I and IV. Bond failure at the enamel-adhesive interface occurred more frequently in the self-etching adhesive system than in the acidetching adhesive system. DISCUSSION
No published studies have investigated differences in shear bond strength between deciduous and permanent teeth when identical orthodontic brackets were bonded with different primer/adhesive systems. Bond strengths between 6 and 8 MPa are sufficient for successfully bonding orthodontic brackets to enamel clinically.28 Based on the results of Weibull analysis, Hobson et al29 presumed that, at this level of stress (8 MPa), less than 14% of all bonds would fail in the worst scenario (blood contamination). Our findings showed that the shear bond strengths of all 4 groups were higher than the clinically required range of 6 to 8 MPa.
Endo et al 201
American Journal of Orthodontics and Dentofacial Orthopedics Volume 134, Number 2
We also found no significant differences in shear bond strength between acid-etching and self-etching adhesive systems. These results were consistent with previous findings of some researchers, who applied acid-etching and self-etching adhesive systems to human permanent teeth17,22,23,30 and bovine teeth31,32 under conditions of dry enamel surfaces. Although self-etching primers generally produce less demineralization of enamel than do phosphoric acid etchants,24 the pH of the self-etching primer in this study, which was the same as used by Arnold et al,22 might be too acidic to obtain satisfactory bond strength for attaching brackets to enamel. Several other investigations showed that bond strengths were significantly lower when self-etching adhesive systems were used in conditions of uncontaminated (dry) enamel surfaces than when acid-etching adhesive systems were used.15,23-25,33-35 Moreover, other investigations showed that, in water-, saliva-, and blood-contaminated conditions, the self-etching adhesive system had higher bond strengths than the acid-etching adhesive system, suggesting that self-etching primers might be suitable for bonding brackets in uncooperative children in the mixed dentition.17,31 We found, between deciduous and permanent teeth, significant differences in the shear bond strengths of brackets bonded with the 2 adhesive systems. However, these findings disagreed with the those of Ergas et al,26 who found no significant differences in the shear bond strengths of brackets between deciduous molars and premolars. In our study, the significantly lower shear bond strength in the deciduous teeth might be explained by the outer prismless enamel layer and the adhesive thickness between the bracket base and the enamel surface. The outer prismless enamel layer is more common in deciduous teeth than in permanent teeth.5-7 The area on the labial surface, where the prismless enamel layer is most common, is the gingival third of the tooth crown in permanent teeth, whereas this area in deciduous teeth is the middle third, which corresponds to the position where brackets are bonded. The outer prismless enamel layer might prevent infiltration of the self-etching primer and allow shallow etching,24 resulting in insufficient penetration of the adhesive into the enamel surface. This deficient adhesive resin penetration might cause lower bond strengths in deciduous teeth than in permanent teeth. On the other hand, Pashley and Tay33 reported that the efficacy of selfetching primers did not depend on their etching aggressiveness. The base of the brackets in this study was designed to fit the premolar enamel surface configuration. The 3-dimensional surface configuration of deciduous canines is similar to that of the premolars, whereas the
configuration of the deciduous molars is more complex than that of the premolars, causing the adhesive thickness between the bracket base and the deciduous enamel surface to increase. The increased adhesive thickness due to the complex deciduous enamel surface configurations could be another factor responsible for lowering the bond strength in the deciduous teeth. This suggests the need for a special type of bracket for deciduous teeth. In this study, the self-etching adhesive system applied to the permanent and deciduous teeth had higher frequencies of bond failure at the enameladhesive interface than did the acid-etching adhesive system applied to the permanent teeth as shown in previous studies.25,31,32,35,36 Our results might have reflected insufficient penetration of adhesive resins into the self-etched enamel surface. Some studies found no significant differences in the mode of bracket/adhesive failure between acid-etching and self-etching adhesive systems in dry conditions.17,34,36,37 On the other hand, Bishara et al15 found that more adhesive remained on the teeth with the self-etching adhesive system than with the acid-etching adhesive system. There were no consistent findings as to the bracket/adhesive failure modes for the self-etching adhesive systems. Our findings that groups I and II had almost the same shear bond strengths but significant differences in the distribution of ARI scores support the observations by some investigators that the amount of residual adhesive resins might not be related to shear bond strength.34,38 CONCLUSIONS
1. The shear bond strengths of all 4 groups were higher than the clinically sufficient range of 6 to 8 MPa. 2. Regardless of whether the teeth were deciduous or permanent, there were no significant differences in shear bond strength between acid-etching and selfetching adhesive systems. 3. The shear bond strengths to the deciduous teeth were lower than those to the permanent teeth when either adhesive system was used. 4. The self-etching adhesive system applied to the permanent and deciduous teeth had higher frequencies of bond failure at the enamel-adhesive interface than the acid-etching adhesive applied to the permanent teeth. 5. The comparison of the shear bond strengths and the bracket/adhesive failure modes between the deciduous and permanent teeth suggest that deciduous teeth can be used as anchor teeth in orthodontic treatment with edgewise appliances.
202 Endo et al
American Journal of Orthodontics and Dentofacial Orthopedics August 2008
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