Rebonding of Orthodontic BracketsPart II, an XPS and SEM Study
Objective: The hypothesis of this two-part study is that adhesive systems for bonding orthodontic brackets (ie, two self-etch primers [Transbond and M-Bond] and a conventional phosphoric acid etch [Rely-a-Bond]) would show a difference with respect to rebonded enamel surface morphology and chemical composition.
Materials and Methods: This study examined the enamel surface before and after debonding with scanning electron microscopy and the enamel surface chemical composition for the elements Ca, P, O, F, Si, and C using x-ray photoelectron spectroscopy.
Results: The etching of the two self-etch groups is less aggressive and less uniform than that of phosphoric acid. The change in the concentration of C indicated that the separation of the bracket from the enamel surface is at the resin-enamel interface for the phosphoric acid–etched adhesive and a mixed mode involving the enamel-resin-bracket interfaces for the self-etching systems. F release appears to occur for Transbond but not for M-Bond.
Conclusions: The results confirm the original hypothesis that differences in adhesive systems are manifested in less aggressive etches and less adhesive left on the enamel surface for the self-etching adhesive systems.Abstract
INTRODUCTION
The original focus of acid etching as employed for orthodontic brackets was the bond strength, but that has now changed to the bond strength after repeated bonding, the long-term stability of the bond strength, and the effect of bonding/debonding procedures on the enamel surface. The primary orthodontic goal is to return the enamel surface to its original state after removal of the orthodontic attachments.12 Examining the enamel surface morphology after conditioning using scanning electron microscopy (SEM) has shown that prismatic enamel has a preferential dissolution in which rod patterns are delineated and that the etching pattern of self-etch primers is more conservative than the etching pattern of phosphoric acid.3–5
Robinson et al6 presented the first quantitative chemical information about the Ca, P, and protein distribution in thin sections of the enamel, indicating that the physical nature of tooth enamel makes it extremely difficult to obtain detailed analytical data about variations in chemical composition. The average calcium content was 37.5% ± 1.8%, and the average P content was 17.5% ± 1.1%, with the Ca:P ratio reasonably constant at 2.1 ± 0.1. Besic et al7 used an ion microprobe to make a qualitative and quantitative determination of 21 elements in enamel specimens. Ruse et al8 performed x-ray photoelectron spectroscopic analysis on bovine enamel incisors and found that when 37% phosphoric acid was applied for 60 seconds on unground pumiced enamel, a significant decrease in C and significant increases in O, Ca, and P were measured.
The objective of this study was to investigate the morphological and chemical changes in the enamel surface after etching and debonding from the use of self-etch and total etch adhesive systems based on the hypothesis that a difference would be observed between the three adhesive systems.
MATERIALS AND METHODS
Sample
Human upper premolars were collected and stored in an aqueous solution of thymol (0.1% wt/vol). The use of extracted human teeth has exempt status as research that does not involve human subjects as defined in 45 CFR 46.102(f) under research protocol 1995-0777 from IRB 1, Office for the Protection of Human Subjects, University of Illinois at Chicago. To prepare the specimens for the x-ray photoelectron spectroscopy (XPS) and the SEM analysis, the crowns of the teeth were sectioned mesiodistally with a diamond separating disc, leaving only a thin layer of the underlying dentin.
The three adhesive systems used in this study were Rely-a-Bond fluoride-releasing, no-mix adhesive system, employing 37% liquid phosphoric acid for etching (Reliance Orthodontics Products, Inc, Itasca, Ill); Transbond XT light cure adhesive and Transbond Plus Self Etch Primer (3M Unitek, Monrovia, Calif); and M-Bond self-cured, two-part, powder/liquid resin cement with a self-etching primer (Tokuyama Dental Corporation, Tokyo, Japan). The first two systems are Bis-GMA resin based, and the third is 4-META resin based. Premolar stainless steel brackets (Mini twin; American Orthodontics, Sheboygan, Wis) were bonded to the teeth. The buccal surface of each tooth was cleaned with nonfluoride, oil-free pumice paste using a nylon brush attached to a slow-speed hand piece for 5 seconds, and the tooth was then rinsed with water for 10 seconds and dried with an oil-free air spray. The bonding of brackets followed the manufacturer's instructions for each adhesive system.
After each debonding, all visible residual adhesive was completely removed with a sharp scalar. The teeth were then pumiced, rinsed, and air dried as was done for the first bonding. The bonding procedures were repeated two times for the SEM examination and one time for the XPS analysis on the same tooth surface.
SEM was used to examine the enamel surface after etching with phosphoric acid or the self-etch primers, before bonding, and after the three debonding sequences. The phosphoric acid–etched teeth were then rinsed with water for 10 seconds and air dried. For the two self-etch primers, the specimens were cleaned with acetone in an ultrasonic cleaner for 30 seconds to remove the monomers. The specimens were coated with gold/palladium and then examined with SEM (Hitiachi S 3000 N; Hitiachi Scientific Instruments, Nissei Sangyo America Ltd, Gaithersburg, Md).
XPS analysis consisted of 12 specimens in each group using a Kratos Axis165 spectrometer (Kratos Analytical Ltd, Manchester, UK). The XPS provided a semiquantitative analysis for the elements Ca, P, O, F, Si, and C. Each specimen was analyzed four times: the untreated enamel surface, the enamel surface after etching, and the enamel surface after the first and second debonding. Using an argon ion gun, the enamel surface was exposed to a 15-minute etch cycle to remove the surface contaminants before the first chemical analysis and then a second 15-minute etch preceding the second analysis. Surface elemental compositions were calculated from the integrated peak areas using the Kratos Vision V2.2.5 computer program. For Rely-a-Bond adhesive, the same area in each specimen was used for the four analyses. The self-etch adhesive specimens were divided into a upper portion to analyze first the untreated enamel and then the same area after applying the self-etch primer. The bottom portion was used to analyze the surface following the first and the second debonding. The analysis was done without any attempt to remove the self-etch primers. XPS analysis of the adhesive and primers was also performed for the three adhesive systems.
The statistical analysis of the XPS data initially used a three-way ANOVA for the three independent variables (adhesive system, enamel surface preparation, and analysis time, 15 and 30 minutes of argon etch). The results of this analysis showed no significant difference between the 15- and 30-minute analyses. Based on this result, a two-way ANOVA was performed indicating significance for the adhesive systems, the enamel surface preparation, and their interaction. A one-way ANOVA was then performed for the two independent variables (adhesive system and enamel surface preparation) for the mass concentration percentage of Ca, P, O, F, Si, and C. If a significant difference was found, a Games-Howell test (or Scheffé post hoc test if the result of the Levene statistics test of homogeneity of variances was insignificant) was performed to detect pairwise differences.
RESULTS
The etching pattern of 37% phosphoric acid (Figure 1a) was visible for the entire specimen; however, the degree of etching was not the same in all areas. Dissolution of the prism cores and prism peripheries and areas of incomplete demineralization were observed. With Transbond (Figure 1b), the etching was less uniform across the specimen with shallow depressions, representing dissolved prism cores and other areas completely unetched. For the M-Bond adhesive system, the etching and shallow depressions (Figure 1c) were less uniform than with phosphoric acid etching but in more areas than that seen with Transbond.
Citation: The Angle Orthodontist 78, 3; 10.2319/022707-102.1
After the first debonding and after applying Rely-a-Bond (Figure 2a), the enamel surface appeared covered with the adhesive remnant, but this adhesive remnant was not visible when the surface was polished after the debonding. For Transbond (Figure 2b) and M-Bond (Figure 2c), the enamel surface was covered with adhesive remnant, but the shallow depressions that represent the etching effect were still present.
Citation: The Angle Orthodontist 78, 3; 10.2319/022707-102.1
The three-way ANOVA statistical analysis (Table 1) of the concentration of the elements Ca, P, O, Si, C, and F from XPS showed a significant effect for the adhesive systems (P ≤ .000), the enamel surface preparation (P ≤ .000), and their interaction (P ≤ .000). There was no significant difference for the two etching times (P = .134 to .750), interactions between adhesive systems and analysis times (P = .706 to .985), enamel preparations and analysis times (P = .601 to .964), and adhesive systems, enamel preparations, and analysis times (P = .912 to .999). The results of the two-way ANOVA (Table 2) showed a statistically significant effect of the adhesive system and the enamel surface preparation on the concentration of all elements (P ≤ .000) and a significant interaction between the two variables for all elements (P ≤ .000).
The results of the one-way ANOVA (Table 3) showed that within each of the three adhesive systems, there was a significant difference in the concentration of the studied element (P ≤ .000) and the different enamel surface preparations (P ≤ .000). The only exceptions were for the M-Bond adhesive, for which the concentrations of P (P = .060) and Si were not different (P = .125).
Table 4 shows the elemental percentage mass concentrations for each adhesive system following the four enamel surface preparations. Games-Howell test (or Scheffé post hoc test when indicated) showed that after etching, the C was higher in both Transbond and M-Bond than Rely-a-Bond (P ≤ .000), while Ca, O, and F were higher in Rely-a-Bond than in both Transbond and M-Bond (P ≤ .000 for each element). After the first debonding, the enamel surface for Rely-a-Bond had higher Si (P ≤ .000) and C (P = .002) than Transbond, while Ca, P, O (P ≤ .000 for each), and F (P = .016) were higher in Transbond. Also, Rely-a-Bond had higher Si (P ≤ .000) than M-Bond, while Ca, P, and O (P ≤ .000 for each) were higher in M-Bond. After the second debonding, the enamel surface for Rely-a-Bond had higher C (P ≤ .000) and Si (P ≤ .000) than the enamel surface in both Transbond and M-Bond, while Ca, P, O, and F were higher for Transbond (P ≤ .000 for Ca, P, and O and P = .005 for F) and M-Bond (P ≤ .000 for Ca and P, P = .002 for O, and P = .001 for F).
After etching the Rely-a-Bond–treated enamel, C increased (P = .001) and Ca decreased (P = .001). After the first debonding sequence, the enamel had higher Si (P ≤ .000) and C (P = .009) but lower Ca, P, and O (P ≤ .000) than the etched enamel. After the second debonding, C (P = .037) and O (P = .027) increased and Si (P = .002) decreased compared with the first debonding sequence.
The Transbond self-etching primer resulted in a significant increase in C and P (P ≤ .000) and a significant decrease in Ca, O, F (P ≤ .000), and Si (P = .002) from the untreated enamel surface. After the first debonding, Ca, O, and F (P ≤ .000) concentrations were higher and P and C (P ≤ .000) concentrations were lower than in the etched enamel. The enamel surface after the second debonding had higher C and P (P = .004 and P = .031, respectively) but lower Ca and O (P = .001) than the surface after the first debonding. The only significant change in the untreated enamel surface after the first debonding was a decrease in F (P = .036), while after the second debonding, C increased significantly (P = .014) and P also increased (P = .053), while Ca decreased (P ≤ .000).
For the M-Bond self-etching primer, there was a significant increase in C (P ≤ .000) and a significant decrease in Ca, O, and F (P ≤ .000) from the untreated enamel surface. After the first debonding, Ca, O, and F (P ≤ .000) were higher and C (P ≤ .000) was lower than the etched enamel. Following the second debonding, Ca, P, and O (P = .005, P = .009, and P = .007, respectively) were higher but C (P = .001) was lower than the enamel surface after the first debonding. The enamel surface after the first debonding had significantly increased C (P = .028) and significantly decreased O (P = .034) compared with the untreated enamel. The enamel surface after the second debonding was not different from the untreated enamel for all the elements Ca, P, O, F, Si, and C.
DISCUSSION
SEM examination showed a similar etch pattern for the two self-etching primers (Figure 1b,c) that was less uniform than the etch pattern of the phosphoric acid (Figure 1a). This is in agreement with the results of previous studies, which showed self-etch primers to produce more conservative and a decreased amount of roughness when compared with the conventional phosphoric acid etching.910
After the debonding, the enamel surface was covered with adhesive resin, which was especially true in the Rely-a-Bond group (Figure 2a). In the two self-etching groups (Figure 2b,c), although the examination showed evidence of residual adhesive resin on the surface, the etch pattern was still visible after the debonding, which may suggest less adhesive remnant on the surface than in the Rely-a-Bond group (total etch system). It may also indicate the site of separation at the bracket-adhesive resin interface. There was no clear evidence that the amount of residual adhesive on the surface was different after each of the three debonding sequences in the three groups.
XPS is a unique technique for semiquantitative analysis, but experimental variables, including the effects of ultra-high vacuum, x-ray exposure and surface charging, the inherent lack of an intrinsic energy reference, the effect of the specimen's environment on the analysis, and the undefined depth of analysis for each analyzed material, have limited its use.11
The results of the XPS analysis (Table 4) indicated that after etching the enamel with phosphoric acid (Figure 3b), Ca decreased and C increased significantly. The increase in C is due to either decreased surface mineralization and/or increased surface contamination. After treating the enamel with the self-etch primer (Transbond and M-Bond), there was a significant decrease in Ca, O, and F and a significant increase in C. The increase in the C was due to the self-etching primer resin's covering the enamel surface and blocking the other elements, hence their observed decrease in concentration.
Citation: The Angle Orthodontist 78, 3; 10.2319/022707-102.1
The elemental composition of the enamel surface after both the first and second debonding (Table 4) was comparable for Transbond (Figure 4c,d) and M-Bond (Figure 5c,d) with respect to Rely-a-Bond (Figure 3c,d) and similar to the untreated enamel (Figures 3a, 4a, and 5a). When the untreated enamel and the enamel after debonding were compared, the enamel treated with Rely-a-Bond had significantly higher C and Si and significantly lower Ca, P, O, and F. These changes indicate that the surface is covered with adhesive remnant after debonding and that the separation of the bracket from the enamel surface was at the bracket-resin interface.
Citation: The Angle Orthodontist 78, 3; 10.2319/022707-102.1
Citation: The Angle Orthodontist 78, 3; 10.2319/022707-102.1
For Transbond only, the F decreased significantly after the first debonding. After the second debonding, the enamel surface had higher C and P and lower Ca than the untreated enamel, suggesting some residual adhesive on the surface and indicating a mixed-mode separation between enamel-resin and resin-bracket interfaces.
For the M-Bond adhesive, there was a significant increase in C after the first debonding and a significant decrease in O, and there were minimal changes in the other elements, which may be due to adhesive remnant on the surface. After the second debonding, the surface was not different from the untreated enamel, again indicating a mixed-mode failure of the enamel-resin-bracket interfaces.
The individual adhesives showed a comparable C (87.17%, 83.38%, and 87.58% for Rely-a-Bond, Transbond, and M-Bond, respectively) and Si content (4.57%, 1.94%, and 2.99% for Rely-a-Bond, Transbond, and M-Bond, respectively). The increased P in the Transbond group after the second debonding is probably the result of its relatively high concentration in the Transbond Plus self-etch primer and Transbond XT adhesive, as detected by the XPS analysis of the material (24.42% and 4.00%, respectively).
The change in the F concentration after debonding varied in the three adhesives. For Rely-a-Bond, the F concentration did not change between the untreated enamel and the enamel after the first and the second debondings, and XPS analysis of a sample of the adhesive and the primer detected no F. Both Transbond plus self-etch primer and Transbond XT adhesive are fluoride releasing, and F was detected with XPS analysis for both of them (0.35% and 0.32%, respectively). There was a significant decrease in the F after the first debonding, suggesting leaching of the F since C had increased, which indicates the presence of adhesive resin on the enamel surface. For M-Bond, the F concentration did not change after debonding. F was detected by XPS analysis for the self-etching primer and the adhesive (0.15% and 0.46%, respectively), suggesting that the F remains bound to the resin and did not leach out.
CONCLUSIONS
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The original hypothesis was confirmed: differences in adhesive bonding systems were observed, in which the etchings of the two self-etch primers were less aggressive and less uniform than that of phosphoric acid etching.
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The change in C concentration indicates that the separation of the bracket from the enamel surface is at the resin-enamel interface for the phosphoric acid–etched adhesive and is a mixed mode at the enamel-resin-bracket interfaces for the self-etching systems. The self-etching adhesive systems result in less adhesive left on the enamel surface.
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F release appears to occur for Transbond but not for M-Bond, both of which contain F in the starting adhesive composition. There is no F in Rely-a-Bond.
Enamel surface after etching with (a) 37% phosphoric acid from Rely-a-Bond, (b) Transbond self-etch primer, and (c) M-bond self-etch primer at 500× magnification. Bar is 100 microns
Enamel surface after debonding and removal of all visible adhesive for (a) Rely-a-Bond, (b) Transbond, and (c) M-bond at 500× magnification. Bar is 100 microns
X-ray photoelectron spectroscopy analysis for Rely-a-Bond for (a) untreated enamel, (b) etched enamel, (c) enamel after first debonding, and (d) enamel after second debonding
X-ray photoelectron spectroscopy analysis for Transbond for (a) untreated enamel, (b) etched enamel, (c) enamel after first debonding, and (d) enamel after second debonding
X-ray photoelectron spectroscopy analysis for M-Bond for (a) untreated enamel, (b) etched enamel, (c) enamel after first debonding, and (d) enamel after second debonding
Contributor Notes
Corresponding author: Dr James L. Drummond, Department of Restorative Dentistry, University of Illinois at Chicago, 801 South Paulina Street, Chicago, IL 60612-7212 (drummond@uic.edu)