The European Journal of Orthodontics Advance Access published December 29, 2014 European Journal of Orthodontics, 2014, 1–6 doi:10.1093/ejo/cju080
Original article
Debonding and adhesive remnant cleanup: an in vitro comparison of bond quality, adhesive remnant cleanup, and orthodontic acceptance of a flash-free product Thorsten Grünheid*, Geoffrey N. Sudit**, and Brent E. Larson* *Division of Orthodontics, University of Minnesota School of Dentistry, Minneapolis, MN, and **Private Practice, Austin, TX, USA Correspondence to: Thorsten Grünheid, Division of Orthodontics, University of Minnesota School of Dentistry, 515 Delaware Street S.E., Minneapolis, MN 55455, USA. E-mail: [email protected]
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SUMMARY Background/objectives: A new flash-free adhesive promises to eliminate the need to clean up excess adhesive upon orthodontic bracket bonding. This study evaluated this new adhesive with regard to microleakage at the enamel–bracket interface, amount of adhesive remaining on the tooth after bracket debonding, time required for adhesive remnant cleanup, and clinical practitioners’ preference in comparison to a conventional adhesive. Materials/methods: A total of 184 bovine incisors were bonded with ceramic brackets using either the flash-free adhesive (APC Flash-Free Adhesive Coated Appliance System, 3M Unitek [3M], Monrovia, California, USA) or a conventional adhesive (APCII Adhesive Coated Appliance System, 3M). Twenty-four of the teeth were scanned using microcomputed tomography to quantify microleakage into the adhesive layer. Twenty orthodontists debonded the brackets, removed the remaining adhesive, and then completed a survey regarding their preference for one of the two adhesives. The adhesive remnant was quantified and the time required for its removal recorded. Differences between the adhesives were tested for statistical significance. Results: For both adhesives, the microleakage was minimal with no significant differences between the two adhesives. The adhesive remnant was significantly larger for the flash-free adhesive, whereas there was no significant difference in adhesive cleanup time. Fourteen out of the 20 orthodontists preferred the flash-free adhesive over the conventional adhesive. Limitations: In vitro testing cannot replicate the actual clinical situation during in vivo debonding. Conclusions: With regard to bond quality and adhesive remnant cleanup, the new flash-free adhesive performs just as well as the conventional adhesive, and, of the two products, is the one preferred by most orthodontists.
Introduction New orthodontic products are continuously introduced to clinicians seeking practical and efficient solutions for their practice. One such product that strives for the title of a ‘practical and efficient solution’ is a new flash-free adhesive for bracket bonding. In the course of
bracket bonding, flash, i.e. excess adhesive, typically flows around the bracket base onto the enamel surface as pressure is applied to the bracket. This flash needs to be removed prior to adhesive cure in order to prevent the adhesive from causing mechanical irritation to the gingiva (1) and to decrease the incidence of plaque accumulation and subsequent enamel demineralization (2). The flash-free adhesive,
© The Author 2014. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: [email protected]
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Materials and methods Approval to conduct this study was obtained from the Institutional Review Board at the University of Minnesota. Informed consent was obtained from all participating orthodontists.
Specimen preparation A total of 184 permanent bovine incisors were collected from a local slaughterhouse, washed in running water, and stored in a 0.1 per cent aqueous solution of thymol at room temperature (6). Selection criteria included sound buccal enamel and no damage due to the extraction process. The teeth were freed from remnants of the periodontal ligament, and the buccal enamel surfaces were ground flat with an abrasive wheel on a dental model trimmer and pumiced using a muslin buff wheel on a dental laboratory polishing lathe to eliminate the characteristic undulations on the labial surface of bovine incisors (7, 8). Care was taken not to perforate the enamel. Twenty-four teeth were randomly selected and prepared for microcomputed tomography. The teeth were divided into two
equal-sized groups and bonded using the flash-free adhesive in group 1 and the conventional adhesive in group 2 as follows. Each buccal tooth surface was cleaned and polished using a fluoride-free prophylaxis paste (Topex Prep&Polish, Sultan Healthcare, Hackensack, New Jersey, USA) on a rubber cup attached to a low-speed handpiece for 5 seconds, rinsed with water, and dried using moisture-free air. Each buccal surface was etched with 35 per cent orthophosphoric acid (Temrex, Freeport, New York, USA) for 30 seconds, rinsed thoroughly with water to ensure complete removal of the etchant, air-dried until it appeared dull and frosty, and primed using a light cure adhesive primer (Transbond XT Primer, 3M Unitek [3M], Monrovia, California, USA) following the manufacturer’s instructions. The teeth were then bonded with adhesive pre-coated ceramic maxillary central incisor brackets (Clarity Advanced, 3M) using a system with a flash-free adhesive (APC Flash-Free Adhesive Coated Appliance System, 3M) in group 1 and a system with a conventional adhesive (APCII Adhesive Coated Appliance System, 3M) in group 2. These systems use identical brackets and differ only with regard to the adhesive coated on the brackets. All brackets were bonded under a constant pressure of 3 N, which was calibrated with a pressure gauge (Correx, Haag-Streit, Bern, Switzerland). Excessive adhesive around brackets pre-coated with the conventional adhesive was removed with a sharp scaler. The adhesive was light-cured through the bracket for 3 seconds with a new light-emitting diode polymerization device (Ortholux Luminous Curing Light, 3M). The distance between the exit window and the bracket was maintained at less than 5 mm in order to obtain optimum polymerization. Two teeth, one for each adhesive, were prepared as positive controls. These teeth had a thin adhesive tape extending between the enamel surface and the bracket base, which was removed after bracket bonding to form a crevice. All teeth that had been prepared for microcomputed tomography, with the exception of two teeth that were used as negative controls, were submerged in a 50 per cent aqueous solution of silver nitrate (Sigma Aldrich, St. Louis, Missouri, USA) at room temperature for 24 hours to allow detection of microleakage into the adhesives. The remaining 164 teeth were mounted in self-cure orthodontic acrylic resin in 20 sets of eight teeth, each set consisting of two central incisors and six lateral incisors to simulate a dental arch (Figure 1). The teeth were embedded in acrylic to just below the cementoenamel junction and then bonded with adhesive pre-coated ceramic maxillary central incisor brackets using the system with the flash-free adhesive on one side and the system with the conventional adhesive on the other side as detailed above. Side allocation was randomized for each set of teeth. Two brackets were bonded to each central incisor, while one bracket was bonded to each lateral incisor amounting to 10 brackets bonded to each set of teeth. Thus, a total of 100 brackets of the APC Flash-Free Adhesive Coated Appliance System (3M) and 100 brackets of the APCII Adhesive Coated Appliance System (3M) were bonded to the simulated dental arches. After completion of the bonding procedure, the specimens were stored in distilled water at 37°C for 24 hours to allow bond maturation (9).
Microcomputed tomography The teeth prepared for microcomputed tomography were scanned in an XT H 225 microcomputed tomography system (Nikon Metrology, Brighton, Michigan, USA) at spatial resolutions ranging from 1.6 to 5.0 μm. The scan parameters used were 90 kV, 90 µA, 708 ms exposure time, 720 projections. Each scan projection was performed four times and then averaged to improve the signal-to-noise ratio.
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which is contained within a nonwoven form-fitting fibre mesh on the bracket base, promises to eliminate the need to clean up flash upon bonding. When a flash-free adhesive-coated bracket is placed on a tooth, the adhesive is designed to spread out and conform to the tooth surface, making uniform and consistent contact with no flash to clean up. For orthodontic treatment with fixed appliances to be successful, a high quality bond is required for the brackets to remain attached to the tooth for the entire length of treatment. The bond must have as few voids as possible because voids in the adhesive layer may reduce bond strength, lead to bond failure, and/or facilitate formation of white spot lesions (3). At the completion of orthodontic treatment, the brackets are removed from the enamel surface. When a bonded bracket is removed, failure at one of three sites must occur: between the bonding material and the bracket, within the bonding material itself, or between the bonding material and the enamel surface. If a strong bond to the enamel has been achieved, failure at the enamel surface on debonding is undesirable, because the bonding material may tear the enamel surface as it pulls away from it. For this reason, the interface between the bonding material and the bracket is the site of failure preferred by most orthodontists (4), and it is considered ideal if the adhesive remains on the tooth surface after debonding (5). Obviously, the remaining adhesive needs to be removed from the tooth surface. The time spent on removal of the remaining adhesive after debonding is a large contributor to what tends to be one of the longest appointments throughout orthodontic treatment. Longer appointments require more of a patient’s valuable time and are more expensive for the clinician. Bond quality, failure mode upon debonding, and ease of adhesive remnant cleanup after debonding are important factors for clinicians when choosing an adhesive for orthodontic bracket bonding. The clinicians’ preference and the acceptance in the orthodontic community will ultimately determine the success of a new adhesive. For these reasons, the aims of this study were to assess the quality of the bond at the enamel–bracket interface, the amount of adhesive remaining on the tooth surface after bracket debonding, the time required for adhesive remnant cleanup, and clinical practitioners’ preference for a new flash-free adhesive in comparison to a conventional adhesive currently in use. It was hypothesized that there were no significant differences between adhesives with regard to these qualities.
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T. Grünheid et al.
Each orthodontist was asked to remove the remaining adhesive from the tooth surfaces using new a tungsten carbide finishing bur (H 283-21-012, Brasseler, Savannah, Georgia, USA) in a low-speed handpiece (12). Adhesive removal was considered complete when the tooth surface felt smooth and appeared free of composite upon visual inspection under a dental operating light. Adhesive removal was timed to the nearest second using a digital stopwatch and verified under ×2.5 magnification using dental loupes. Once the adhesive remnant cleanup was completed, the orthodontists were asked to complete a specifically-designed survey aimed at determining their preference for one of the two adhesives (Appendix 1) and to rate their debonding and adhesive removal experiences with the two adhesives on a five-point Likert scale ranging from ‘very unpleasant’ to ‘very pleasant.’ A pilot survey was conducted on 10 orthodontic dental assistants to test the clarity of the questions and validate the survey instrument.
Statistical analysis
Figure 1. (A) Bovine incisors mounted in self-cure orthodontic acrylic resin to simulate a dental arch. (B) Debonding procedure.
Debonding and adhesive remnant cleanup The sets of teeth mounted in orthodontic acrylic were affixed to manikin heads. Twenty orthodontists were asked to each debond the teeth using a purpose-designed instrument (Unitek Self-Ligating Bracket Debonding Instrument, 3M) following the manufacturer’s instructions (Figure 1). Each orthodontist was blinded as to the adhesive type being told only that one side was ‘Product A’ while the other side was ‘Product B.’ Once the brackets were debonded, a single calibrated operator scored the adhesive remnant index (ARI) (11) blinded to the groups under ×2.5 magnification using dental loupes (Orascoptic, Middleton, Wisconsin, USA) as follows: 0 = no adhesive left on the tooth; 1 = less than half of the adhesive left on the tooth; 2 = more than half of the adhesive left on the tooth; and 3 = all adhesive left on the tooth. Intra-operator agreement was assessed by re-scoring 5 specifically bonded and debonded arches after a washout period of 3 weeks.
Results Quality of the bond at the enamel–bracket interface Examples of microcomputed tomography scans of brackets bonded with the flash-free and the conventional adhesive are shown in Figure 2. In all scans, the flash-free adhesive showed a smooth, non-textured surface with the adhesive spread out, conformed to the enamel surface. In contrast, the conventional adhesive showed a ruffled surface with more irregular transition from adhesive to the enamel surface. The volumes of silver nitrate penetration into the adhesives are presented in Table 1. For both adhesives tested, the mikroleakage was minimal with slightly more silver nitrate penetrating into the conventional adhesive. However, this difference was not statistically significant (P = 0.074).
Amount of adhesive remaining on the tooth surface after bracket debonding The weighted kappa coefficient for intra-operator agreement of ARI scoring was 0.952 indicating excellent agreement (13). No enamel tear-outs were observed. Percentages of ARI scores after debonding are shown in Table 2. In 94 per cent of the brackets bonded with the flash-free adhesive, all or most of the adhesive remained on the tooth after bracket removal indicating failure at the bracket–adhesive interface. This failure mode, as indicated by ARI scores 2 and 3, occurred in 64 per cent of the brackets bonded with the conventional adhesive. Statistical testing revealed that the amount of adhesive
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All scans were reconstructed using dedicated software (CT Pro 3D, Nikon Metrology) and then scrutinized, slice-by-slice, for silver nitrate microleakage into the adhesive layer, as indicated by presence of the radiopaque dye between the bracket base and the enamel surface. The amount of silver nitrate was quantified as follows. For each image, the adhesive layer was selected as volume of interest (VOI). The VOIs were segmented to discriminate silver nitrate from background (SkyScan CT-analyzer Version 1.1, Bruker microCT, Kontich, Belgium). Optimum thresholds for the segmentation were visually determined by gradual variation and comparison of the outcome with silver nitrate deposits on the bracket surface in the original scan (10). In a segmented image, only voxels with a linear attenuation value above the threshold, i.e. those representing silver nitrate, kept their original gray value, while voxels with a linear attenuation value below the threshold were made transparent. The volume of silver nitrate microleakage was then calculated using the number of voxels and the scan resolution.
Mean values, standard deviations, and coefficients of variation of the time required for adhesive remnant cleanup were calculated for each adhesive. Differences in adhesive removal time and amount of silver nitrate microleakage between the types of adhesives were tested for statistical significance using a Mann–Whitney rank sum test after the data had been tested for normality (Kolmogorov–Smirnov test). A weighted kappa coefficient was calculated to assess intra-operator agreement of ARI scoring. Differences in ARI between the adhesives were tested for statistical significance using a Cochran–Armitage test for trend. Pearson’s correlation coefficients were calculated, separately for each adhesive, to evaluate the association between the ARI and the time required for adhesive remnant cleanup. Differences in the clinical practitioners’ debonding and adhesive removal experiences between the adhesives were tested for statistical significance using a CochranArmitage test for trend. Statistical analyses were performed using SAS 9.4 for Windows (SAS Institute Inc., Cary, North Carolina, USA) with P-values of less than 0.05 considered statistically significant.
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Figure 2. Examples of microcomputed tomography scans used to quantify mikroleakage of silver nitrate into the adhesive layer of brackets bonded with the flash-free (left) and the conventional adhesive (right). The profile sections show the amount of flash with each type of adhesive as well as radiopaque silver nitrate deposits on the outer surface of the brackets.
remaining on the tooth after bracket debonding was significantly greater (P 0.05).
Clinical practitioners’ preference Percentages of Likert scores of clinical practitioners’ debonding experience are shown in Table 4. Eighty per cent of the clinicians rated their debonding experience with the flash-free adhesive as ‘somewhat pleasant’ or ‘very pleasant,’ compared to 60 per cent with the conventional adhesive. Despite more positive ratings for the flash-free adhesive than for the conventional adhesive, the difference was not statistically significant (P = 0.179). Percentages of Likert scores of clinical practitioners’ adhesive removal experience are shown in Table 5. Seventy-five per cent of the clinicians rated their adhesive removal experience with the flashfree adhesive as ‘somewhat pleasant’ or ‘very pleasant,’ whereas 50 per cent of the clinicians rated their adhesive removal experience in Table 4. Clinical practitioners’ debonding experience with the adhesives tested.
Table 2. Adhesive remnant index (ARI) after debonding.
Flash-free adhesive
Conventional adhesive Occurrence
Flash-free adhesive
Conventional adhesive
Experience
Occurrence
ARI score
Occurrence
Percentage
Occurrence
Percentage
0
0
0
0
0 1 2 3
2 4 20 74
2 4 20 74
11 25 36 28
11 25 36 28
Very unpleasant Somewhat unpleasant Neutral Somewhat pleasant Very pleasant
2
10
4
20
2 7
10 35
4 7
20 35
9
45
5
25
0 = no adhesive left on tooth, 1 = less than half of the adhesive left on tooth, 2 = more than half of the adhesive left on tooth, and 3 = all adhesive left on tooth.
Table 3. Time required for adhesive remnant cleanup per quadrant. Flash-free adhesive
Conventional adhesive
Time (s)
Range (s)
COV (%)
Time (s)
Range (s)
118 (82)
21–290
69.94
134 (95)
30–351
COV (%) 71.00
Results are mean values (standard deviations). COV, Coefficient of variation. No statistically significant differences between groups (Mann–Whitney rank sum test, P > 0.05).
Percentage
Percentage
Table 5. Clinical practitioners’ adhesive removal experience with the adhesives tested. Flash-free adhesive
Conventional adhesive
Experience
Occurrence
Occurrence
Percentage
Very unpleasant Somewhat unpleasant Neutral Somewhat pleasant Very pleasant
0 1
0 5
1 3
5 15
4 7
20 35
6 4
30 20
8
40
6
30
Percentage
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Flash-free adhesive
After averaging the ARI data at the arch level, the linear association between ARI and adhesive removal time was estimated using Pearson’s r, separately by type of adhesive. For the conventional adhesive the correlation was relatively high (r = 0.521) and statistically significant (P=0.019), whereas for the flash-free adhesive it was lower (r = 0.077) and not statistically significant (P = 0.748).
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T. Grünheid et al. those two categories with the conventional adhesive. Despite more positive ratings for the flash-free adhesive than for the conventional adhesive, the difference was not statistically significant (P = 0.154). Fourteen out of the 20 orthodontists preferred the flash-free adhesive over the conventional adhesive for the following reasons (more than one reason per clinician possible): easier to remove (×8); less force needed for bracket debonding (×4); softer material (×3); faster to remove (×2); less pressure needed on handpiece for adhesive removal (×2); easier to see (×1); more predictable debonding (×1). The other six orthodontists preferred the conventional adhesive over the flash-free adhesive for the following reasons (more than one reason per clinician possible): less adhesive remaining on the tooth (×4); easier to remove (×2); faster to remove (×1).
Discussion
Quality of the bond at the enamel–bracket interface In dentistry, microleakage is defined as seeping and leaking of fluids and bacteria at the enamel–composite interface (17). Microleakage around fillings can increase the likelihood of postoperative sensitivity and recurrent caries (17) while microleakage around orthodontic brackets can cause formation of white spot lesions (3).Using microcomputed tomography, this study found only very minimal microleakage for both adhesives tested, which suggests that a very high quality of the bond can be achieved with both the flash-free and the conventional adhesive. These findings are in agreement with earlier studies that showed little or no microleakage at the adhesive– enamel interface with 3M’s Transbond XT light cure adhesive (18, 19), which is essentially the adhesive of the APCII Adhesive Coated Appliance System used in this study.
Amount of adhesive remaining on the tooth surface after bracket debonding Upon bracket removal, the flash-free adhesive left more adhesive on the tooth surface than the conventional adhesive. The ARI scores indicate that, in general, bond failure occurred either at the bracket– adhesive interface or within the adhesive, and that the tooth–adhesive interface was not typically the site of failure. While not absolute, ARI scores give a good indication of where the bond failure occurs. In comparison to the conventional adhesive, the flash-free adhesive failed more reliably and predictably at the bracket–adhesive interface or, more likely, the bracket–mesh interface. The nonwoven mesh at the bracket base, which contains the flash-free adhesive, is a new design feature of the APC Flash-Free Adhesive Coated Appliance System. Although the exact design of the mesh and the fracture
Time required for adhesive remnant cleanup There was no significant difference in time required for adhesive remnant cleanup after debonding between the flash-free adhesive and the conventional adhesive; however, there was a trend towards shorter cleanup times with the flash-free adhesive. This may appear surprising because the flash-free adhesive left more adhesive on the enamel surface after bracket debonding than the conventional adhesive. In fact, while a greater amount of conventional adhesive remaining on the tooth typically required more time to be removed, as indicated by the statistically significant positive correlation between ARI and removal time; this was not the case with the flash-free adhesive. We assume that the different consistency of the flash-free adhesive, at least in part, explains the ease of removal. The difference in consistency is most likely caused by a lower filler content compared to the conventional adhesive. Although a decrease in adhesive remnant removal time may seem desirable to both practitioners and patients, lower filler content may also result in decreased bond strength and resulting higher clinical failure rates (21, 22). Currently, no studies are available on the bond strength of the flash-free adhesive used in this study; however, a few studies have compared the bond strength of the conventional adhesive used in this study to more flowable, less filled adhesives. One such study found 3M’s Transbond XT adhesive to have a higher flexural modulus and shear bond strength (SBS) and lower contraction stress than the more flowable adhesives to which it was compared (23). Another study demonstrated that it had higher SBS values than the less-filled composites; and interestingly, significantly more adhesive remained on the enamel surface with the less-filled adhesives compared to the Transbond XT adhesive (24). These findings agree with the present results and our assumption that the flash-free adhesive is less filled than the conventional adhesive. If bond strength and clinical failure rate remain acceptable, the consistency of the flash-free adhesive should make bracket debonding and adhesive remnant cleanup easier, both adding to a more positive experience for the clinical practitioner.
Clinical practitioners’ preference The majority of orthodontists blinded preferred the flash-free adhesive over the conventional adhesive because of its ease and speed of removal, and because less force was needed for bracket debonding. Interestingly, four practitioners who preferred the conventional adhesive explained their preference with the fact that less adhesive remained on the enamel surface after debonding. These practitioners may have considered a possible decrease in adhesive removal time, which comes with a smaller amount of adhesive remnant, worth a higher risk of enamel tear-outs. Overall, however, these findings suggest that clinical practitioners prefer an adhesive material that allows
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Conventional adhesives for orthodontic bracket bonding have traditionally required the removal of excess adhesive around the bracket base prior to adhesive cure. In an effort to improve and optimize the process of bracket bonding, a new flash-free adhesive has been introduced to the market, which promises to eliminate the need to clean up flash upon bonding. This study evaluated this new adhesive with regard to microleakage at the enamel–bracket interface, amount of adhesive remaining on the tooth surface after bracket debonding, time required for adhesive remnant cleanup, and clinical practitioners’ preference in an in vitro setting. In vitro studies using nonhuman dental tissues are very commonly used to evaluate dental materials (14, 15). This work used bovine teeth as a model for human teeth. A number of previous studies have validated this model and have shown that bovine incisors correspond remarkably well to human teeth (7, 16).
mechanism is the manufacturer’s trade secret, we hypothesize that fracture at the bracket–mesh interface is facilitated by a lower material density at that site. To our knowledge, no other studies have yet been completed on the flash-free adhesive; however, very similar ARI scores have been reported for the conventional adhesive used in this study. Very recently, Sharma et al. (20) showed that the majority of ARI scores after removal of brackets bonded with Transbond XT were either 2 or 3, suggesting that most or all adhesive remained on the tooth after debonding. Our findings suggest that even more adhesive is left on the enamel surface after debonding when the flash-free adhesive is used. While this may be beneficial to orthodontic patients as it minimizes the risk of enamel tear-outs (4), more material remains on the tooth surface that requires cleanup.
6 easy bracket debonding and that can be removed from the enamel surface quickly and easily when left behind. A less filled adhesive, such as the flash-free adhesive, meets these criteria and can make for easier debonding and faster cleanup. In contrast, an adhesive with higher filler content may have increased SBS, which may result in fewer bond failures during treatment. Although direct extrapolation of the results of this in vitro study to the clinical situation is not possible, the findings suggest that the flash-free adhesive performs satisfactorily with regard to bond quality, failure mode, and adhesive remnant cleanup. A study on the clinical application of the flash-free adhesive would be the next logical progression in this line of research. If such an in vivo study confirms that bond strength and failure rate are clinical acceptable, the use of the flash-free adhesive may have advantages over the use of conventional adhesives. These advantages of the flash-free adhesive include potential savings in chair time because it eliminates the need to clean up flash upon bonding, and possible decreases in plaque accumulation and subsequent enamel demineralization caused by its smoother surface.
Conclusions
Funding 3M Unitek research grant (T.G.).
Acknowledgements The authors are grateful to Dr Philippe Gaillard for performing the statistical analyses.
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• Both the flash-free and the conventional adhesive show a very high quality of the bond with very minimal microleakage at the enamel–bracket interface. • The flash-free adhesive fails more reliably and predictably at the bracket–adhesive interface and leaves a significantly greater amount of adhesive remnant on the tooth surface after bracket debonding. • There is a noticeable, however not statistically significant, trend towards shorter adhesive remnant cleanup times with the flashfree adhesive, despite a larger amount of adhesive remaining on the tooth after bracket debonding. • When blinded, the majority of orthodontists prefer the flash-free product over the conventional product because it is easier and faster to remove, and less force is needed for bracket debonding and adhesive removal.
European Journal of Orthodontics, 2014
Original article
Debonding and adhesive remnant cleanup: an in vitro comparison of bond quality, adhesive remnant cleanup, and orthodontic acceptance of a flash-free product Thorsten Grünheid*, Geoffrey N. Sudit**, and Brent E. Larson* *Division of Orthodontics, University of Minnesota School of Dentistry, Minneapolis, MN, and **Private Practice, Austin, TX, USA Correspondence to: Thorsten Grünheid, Division of Orthodontics, University of Minnesota School of Dentistry, 515 Delaware Street S.E., Minneapolis, MN 55455, USA. E-mail: [email protected]
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SUMMARY Background/objectives: A new flash-free adhesive promises to eliminate the need to clean up excess adhesive upon orthodontic bracket bonding. This study evaluated this new adhesive with regard to microleakage at the enamel–bracket interface, amount of adhesive remaining on the tooth after bracket debonding, time required for adhesive remnant cleanup, and clinical practitioners’ preference in comparison to a conventional adhesive. Materials/methods: A total of 184 bovine incisors were bonded with ceramic brackets using either the flash-free adhesive (APC Flash-Free Adhesive Coated Appliance System, 3M Unitek [3M], Monrovia, California, USA) or a conventional adhesive (APCII Adhesive Coated Appliance System, 3M). Twenty-four of the teeth were scanned using microcomputed tomography to quantify microleakage into the adhesive layer. Twenty orthodontists debonded the brackets, removed the remaining adhesive, and then completed a survey regarding their preference for one of the two adhesives. The adhesive remnant was quantified and the time required for its removal recorded. Differences between the adhesives were tested for statistical significance. Results: For both adhesives, the microleakage was minimal with no significant differences between the two adhesives. The adhesive remnant was significantly larger for the flash-free adhesive, whereas there was no significant difference in adhesive cleanup time. Fourteen out of the 20 orthodontists preferred the flash-free adhesive over the conventional adhesive. Limitations: In vitro testing cannot replicate the actual clinical situation during in vivo debonding. Conclusions: With regard to bond quality and adhesive remnant cleanup, the new flash-free adhesive performs just as well as the conventional adhesive, and, of the two products, is the one preferred by most orthodontists.
Introduction New orthodontic products are continuously introduced to clinicians seeking practical and efficient solutions for their practice. One such product that strives for the title of a ‘practical and efficient solution’ is a new flash-free adhesive for bracket bonding. In the course of
bracket bonding, flash, i.e. excess adhesive, typically flows around the bracket base onto the enamel surface as pressure is applied to the bracket. This flash needs to be removed prior to adhesive cure in order to prevent the adhesive from causing mechanical irritation to the gingiva (1) and to decrease the incidence of plaque accumulation and subsequent enamel demineralization (2). The flash-free adhesive,
© The Author 2014. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: [email protected]
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Materials and methods Approval to conduct this study was obtained from the Institutional Review Board at the University of Minnesota. Informed consent was obtained from all participating orthodontists.
Specimen preparation A total of 184 permanent bovine incisors were collected from a local slaughterhouse, washed in running water, and stored in a 0.1 per cent aqueous solution of thymol at room temperature (6). Selection criteria included sound buccal enamel and no damage due to the extraction process. The teeth were freed from remnants of the periodontal ligament, and the buccal enamel surfaces were ground flat with an abrasive wheel on a dental model trimmer and pumiced using a muslin buff wheel on a dental laboratory polishing lathe to eliminate the characteristic undulations on the labial surface of bovine incisors (7, 8). Care was taken not to perforate the enamel. Twenty-four teeth were randomly selected and prepared for microcomputed tomography. The teeth were divided into two
equal-sized groups and bonded using the flash-free adhesive in group 1 and the conventional adhesive in group 2 as follows. Each buccal tooth surface was cleaned and polished using a fluoride-free prophylaxis paste (Topex Prep&Polish, Sultan Healthcare, Hackensack, New Jersey, USA) on a rubber cup attached to a low-speed handpiece for 5 seconds, rinsed with water, and dried using moisture-free air. Each buccal surface was etched with 35 per cent orthophosphoric acid (Temrex, Freeport, New York, USA) for 30 seconds, rinsed thoroughly with water to ensure complete removal of the etchant, air-dried until it appeared dull and frosty, and primed using a light cure adhesive primer (Transbond XT Primer, 3M Unitek [3M], Monrovia, California, USA) following the manufacturer’s instructions. The teeth were then bonded with adhesive pre-coated ceramic maxillary central incisor brackets (Clarity Advanced, 3M) using a system with a flash-free adhesive (APC Flash-Free Adhesive Coated Appliance System, 3M) in group 1 and a system with a conventional adhesive (APCII Adhesive Coated Appliance System, 3M) in group 2. These systems use identical brackets and differ only with regard to the adhesive coated on the brackets. All brackets were bonded under a constant pressure of 3 N, which was calibrated with a pressure gauge (Correx, Haag-Streit, Bern, Switzerland). Excessive adhesive around brackets pre-coated with the conventional adhesive was removed with a sharp scaler. The adhesive was light-cured through the bracket for 3 seconds with a new light-emitting diode polymerization device (Ortholux Luminous Curing Light, 3M). The distance between the exit window and the bracket was maintained at less than 5 mm in order to obtain optimum polymerization. Two teeth, one for each adhesive, were prepared as positive controls. These teeth had a thin adhesive tape extending between the enamel surface and the bracket base, which was removed after bracket bonding to form a crevice. All teeth that had been prepared for microcomputed tomography, with the exception of two teeth that were used as negative controls, were submerged in a 50 per cent aqueous solution of silver nitrate (Sigma Aldrich, St. Louis, Missouri, USA) at room temperature for 24 hours to allow detection of microleakage into the adhesives. The remaining 164 teeth were mounted in self-cure orthodontic acrylic resin in 20 sets of eight teeth, each set consisting of two central incisors and six lateral incisors to simulate a dental arch (Figure 1). The teeth were embedded in acrylic to just below the cementoenamel junction and then bonded with adhesive pre-coated ceramic maxillary central incisor brackets using the system with the flash-free adhesive on one side and the system with the conventional adhesive on the other side as detailed above. Side allocation was randomized for each set of teeth. Two brackets were bonded to each central incisor, while one bracket was bonded to each lateral incisor amounting to 10 brackets bonded to each set of teeth. Thus, a total of 100 brackets of the APC Flash-Free Adhesive Coated Appliance System (3M) and 100 brackets of the APCII Adhesive Coated Appliance System (3M) were bonded to the simulated dental arches. After completion of the bonding procedure, the specimens were stored in distilled water at 37°C for 24 hours to allow bond maturation (9).
Microcomputed tomography The teeth prepared for microcomputed tomography were scanned in an XT H 225 microcomputed tomography system (Nikon Metrology, Brighton, Michigan, USA) at spatial resolutions ranging from 1.6 to 5.0 μm. The scan parameters used were 90 kV, 90 µA, 708 ms exposure time, 720 projections. Each scan projection was performed four times and then averaged to improve the signal-to-noise ratio.
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which is contained within a nonwoven form-fitting fibre mesh on the bracket base, promises to eliminate the need to clean up flash upon bonding. When a flash-free adhesive-coated bracket is placed on a tooth, the adhesive is designed to spread out and conform to the tooth surface, making uniform and consistent contact with no flash to clean up. For orthodontic treatment with fixed appliances to be successful, a high quality bond is required for the brackets to remain attached to the tooth for the entire length of treatment. The bond must have as few voids as possible because voids in the adhesive layer may reduce bond strength, lead to bond failure, and/or facilitate formation of white spot lesions (3). At the completion of orthodontic treatment, the brackets are removed from the enamel surface. When a bonded bracket is removed, failure at one of three sites must occur: between the bonding material and the bracket, within the bonding material itself, or between the bonding material and the enamel surface. If a strong bond to the enamel has been achieved, failure at the enamel surface on debonding is undesirable, because the bonding material may tear the enamel surface as it pulls away from it. For this reason, the interface between the bonding material and the bracket is the site of failure preferred by most orthodontists (4), and it is considered ideal if the adhesive remains on the tooth surface after debonding (5). Obviously, the remaining adhesive needs to be removed from the tooth surface. The time spent on removal of the remaining adhesive after debonding is a large contributor to what tends to be one of the longest appointments throughout orthodontic treatment. Longer appointments require more of a patient’s valuable time and are more expensive for the clinician. Bond quality, failure mode upon debonding, and ease of adhesive remnant cleanup after debonding are important factors for clinicians when choosing an adhesive for orthodontic bracket bonding. The clinicians’ preference and the acceptance in the orthodontic community will ultimately determine the success of a new adhesive. For these reasons, the aims of this study were to assess the quality of the bond at the enamel–bracket interface, the amount of adhesive remaining on the tooth surface after bracket debonding, the time required for adhesive remnant cleanup, and clinical practitioners’ preference for a new flash-free adhesive in comparison to a conventional adhesive currently in use. It was hypothesized that there were no significant differences between adhesives with regard to these qualities.
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Each orthodontist was asked to remove the remaining adhesive from the tooth surfaces using new a tungsten carbide finishing bur (H 283-21-012, Brasseler, Savannah, Georgia, USA) in a low-speed handpiece (12). Adhesive removal was considered complete when the tooth surface felt smooth and appeared free of composite upon visual inspection under a dental operating light. Adhesive removal was timed to the nearest second using a digital stopwatch and verified under ×2.5 magnification using dental loupes. Once the adhesive remnant cleanup was completed, the orthodontists were asked to complete a specifically-designed survey aimed at determining their preference for one of the two adhesives (Appendix 1) and to rate their debonding and adhesive removal experiences with the two adhesives on a five-point Likert scale ranging from ‘very unpleasant’ to ‘very pleasant.’ A pilot survey was conducted on 10 orthodontic dental assistants to test the clarity of the questions and validate the survey instrument.
Statistical analysis
Figure 1. (A) Bovine incisors mounted in self-cure orthodontic acrylic resin to simulate a dental arch. (B) Debonding procedure.
Debonding and adhesive remnant cleanup The sets of teeth mounted in orthodontic acrylic were affixed to manikin heads. Twenty orthodontists were asked to each debond the teeth using a purpose-designed instrument (Unitek Self-Ligating Bracket Debonding Instrument, 3M) following the manufacturer’s instructions (Figure 1). Each orthodontist was blinded as to the adhesive type being told only that one side was ‘Product A’ while the other side was ‘Product B.’ Once the brackets were debonded, a single calibrated operator scored the adhesive remnant index (ARI) (11) blinded to the groups under ×2.5 magnification using dental loupes (Orascoptic, Middleton, Wisconsin, USA) as follows: 0 = no adhesive left on the tooth; 1 = less than half of the adhesive left on the tooth; 2 = more than half of the adhesive left on the tooth; and 3 = all adhesive left on the tooth. Intra-operator agreement was assessed by re-scoring 5 specifically bonded and debonded arches after a washout period of 3 weeks.
Results Quality of the bond at the enamel–bracket interface Examples of microcomputed tomography scans of brackets bonded with the flash-free and the conventional adhesive are shown in Figure 2. In all scans, the flash-free adhesive showed a smooth, non-textured surface with the adhesive spread out, conformed to the enamel surface. In contrast, the conventional adhesive showed a ruffled surface with more irregular transition from adhesive to the enamel surface. The volumes of silver nitrate penetration into the adhesives are presented in Table 1. For both adhesives tested, the mikroleakage was minimal with slightly more silver nitrate penetrating into the conventional adhesive. However, this difference was not statistically significant (P = 0.074).
Amount of adhesive remaining on the tooth surface after bracket debonding The weighted kappa coefficient for intra-operator agreement of ARI scoring was 0.952 indicating excellent agreement (13). No enamel tear-outs were observed. Percentages of ARI scores after debonding are shown in Table 2. In 94 per cent of the brackets bonded with the flash-free adhesive, all or most of the adhesive remained on the tooth after bracket removal indicating failure at the bracket–adhesive interface. This failure mode, as indicated by ARI scores 2 and 3, occurred in 64 per cent of the brackets bonded with the conventional adhesive. Statistical testing revealed that the amount of adhesive
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All scans were reconstructed using dedicated software (CT Pro 3D, Nikon Metrology) and then scrutinized, slice-by-slice, for silver nitrate microleakage into the adhesive layer, as indicated by presence of the radiopaque dye between the bracket base and the enamel surface. The amount of silver nitrate was quantified as follows. For each image, the adhesive layer was selected as volume of interest (VOI). The VOIs were segmented to discriminate silver nitrate from background (SkyScan CT-analyzer Version 1.1, Bruker microCT, Kontich, Belgium). Optimum thresholds for the segmentation were visually determined by gradual variation and comparison of the outcome with silver nitrate deposits on the bracket surface in the original scan (10). In a segmented image, only voxels with a linear attenuation value above the threshold, i.e. those representing silver nitrate, kept their original gray value, while voxels with a linear attenuation value below the threshold were made transparent. The volume of silver nitrate microleakage was then calculated using the number of voxels and the scan resolution.
Mean values, standard deviations, and coefficients of variation of the time required for adhesive remnant cleanup were calculated for each adhesive. Differences in adhesive removal time and amount of silver nitrate microleakage between the types of adhesives were tested for statistical significance using a Mann–Whitney rank sum test after the data had been tested for normality (Kolmogorov–Smirnov test). A weighted kappa coefficient was calculated to assess intra-operator agreement of ARI scoring. Differences in ARI between the adhesives were tested for statistical significance using a Cochran–Armitage test for trend. Pearson’s correlation coefficients were calculated, separately for each adhesive, to evaluate the association between the ARI and the time required for adhesive remnant cleanup. Differences in the clinical practitioners’ debonding and adhesive removal experiences between the adhesives were tested for statistical significance using a CochranArmitage test for trend. Statistical analyses were performed using SAS 9.4 for Windows (SAS Institute Inc., Cary, North Carolina, USA) with P-values of less than 0.05 considered statistically significant.
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Figure 2. Examples of microcomputed tomography scans used to quantify mikroleakage of silver nitrate into the adhesive layer of brackets bonded with the flash-free (left) and the conventional adhesive (right). The profile sections show the amount of flash with each type of adhesive as well as radiopaque silver nitrate deposits on the outer surface of the brackets.
remaining on the tooth after bracket debonding was significantly greater (P 0.05).
Clinical practitioners’ preference Percentages of Likert scores of clinical practitioners’ debonding experience are shown in Table 4. Eighty per cent of the clinicians rated their debonding experience with the flash-free adhesive as ‘somewhat pleasant’ or ‘very pleasant,’ compared to 60 per cent with the conventional adhesive. Despite more positive ratings for the flash-free adhesive than for the conventional adhesive, the difference was not statistically significant (P = 0.179). Percentages of Likert scores of clinical practitioners’ adhesive removal experience are shown in Table 5. Seventy-five per cent of the clinicians rated their adhesive removal experience with the flashfree adhesive as ‘somewhat pleasant’ or ‘very pleasant,’ whereas 50 per cent of the clinicians rated their adhesive removal experience in Table 4. Clinical practitioners’ debonding experience with the adhesives tested.
Table 2. Adhesive remnant index (ARI) after debonding.
Flash-free adhesive
Conventional adhesive Occurrence
Flash-free adhesive
Conventional adhesive
Experience
Occurrence
ARI score
Occurrence
Percentage
Occurrence
Percentage
0
0
0
0
0 1 2 3
2 4 20 74
2 4 20 74
11 25 36 28
11 25 36 28
Very unpleasant Somewhat unpleasant Neutral Somewhat pleasant Very pleasant
2
10
4
20
2 7
10 35
4 7
20 35
9
45
5
25
0 = no adhesive left on tooth, 1 = less than half of the adhesive left on tooth, 2 = more than half of the adhesive left on tooth, and 3 = all adhesive left on tooth.
Table 3. Time required for adhesive remnant cleanup per quadrant. Flash-free adhesive
Conventional adhesive
Time (s)
Range (s)
COV (%)
Time (s)
Range (s)
118 (82)
21–290
69.94
134 (95)
30–351
COV (%) 71.00
Results are mean values (standard deviations). COV, Coefficient of variation. No statistically significant differences between groups (Mann–Whitney rank sum test, P > 0.05).
Percentage
Percentage
Table 5. Clinical practitioners’ adhesive removal experience with the adhesives tested. Flash-free adhesive
Conventional adhesive
Experience
Occurrence
Occurrence
Percentage
Very unpleasant Somewhat unpleasant Neutral Somewhat pleasant Very pleasant
0 1
0 5
1 3
5 15
4 7
20 35
6 4
30 20
8
40
6
30
Percentage
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Flash-free adhesive
After averaging the ARI data at the arch level, the linear association between ARI and adhesive removal time was estimated using Pearson’s r, separately by type of adhesive. For the conventional adhesive the correlation was relatively high (r = 0.521) and statistically significant (P=0.019), whereas for the flash-free adhesive it was lower (r = 0.077) and not statistically significant (P = 0.748).
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T. Grünheid et al. those two categories with the conventional adhesive. Despite more positive ratings for the flash-free adhesive than for the conventional adhesive, the difference was not statistically significant (P = 0.154). Fourteen out of the 20 orthodontists preferred the flash-free adhesive over the conventional adhesive for the following reasons (more than one reason per clinician possible): easier to remove (×8); less force needed for bracket debonding (×4); softer material (×3); faster to remove (×2); less pressure needed on handpiece for adhesive removal (×2); easier to see (×1); more predictable debonding (×1). The other six orthodontists preferred the conventional adhesive over the flash-free adhesive for the following reasons (more than one reason per clinician possible): less adhesive remaining on the tooth (×4); easier to remove (×2); faster to remove (×1).
Discussion
Quality of the bond at the enamel–bracket interface In dentistry, microleakage is defined as seeping and leaking of fluids and bacteria at the enamel–composite interface (17). Microleakage around fillings can increase the likelihood of postoperative sensitivity and recurrent caries (17) while microleakage around orthodontic brackets can cause formation of white spot lesions (3).Using microcomputed tomography, this study found only very minimal microleakage for both adhesives tested, which suggests that a very high quality of the bond can be achieved with both the flash-free and the conventional adhesive. These findings are in agreement with earlier studies that showed little or no microleakage at the adhesive– enamel interface with 3M’s Transbond XT light cure adhesive (18, 19), which is essentially the adhesive of the APCII Adhesive Coated Appliance System used in this study.
Amount of adhesive remaining on the tooth surface after bracket debonding Upon bracket removal, the flash-free adhesive left more adhesive on the tooth surface than the conventional adhesive. The ARI scores indicate that, in general, bond failure occurred either at the bracket– adhesive interface or within the adhesive, and that the tooth–adhesive interface was not typically the site of failure. While not absolute, ARI scores give a good indication of where the bond failure occurs. In comparison to the conventional adhesive, the flash-free adhesive failed more reliably and predictably at the bracket–adhesive interface or, more likely, the bracket–mesh interface. The nonwoven mesh at the bracket base, which contains the flash-free adhesive, is a new design feature of the APC Flash-Free Adhesive Coated Appliance System. Although the exact design of the mesh and the fracture
Time required for adhesive remnant cleanup There was no significant difference in time required for adhesive remnant cleanup after debonding between the flash-free adhesive and the conventional adhesive; however, there was a trend towards shorter cleanup times with the flash-free adhesive. This may appear surprising because the flash-free adhesive left more adhesive on the enamel surface after bracket debonding than the conventional adhesive. In fact, while a greater amount of conventional adhesive remaining on the tooth typically required more time to be removed, as indicated by the statistically significant positive correlation between ARI and removal time; this was not the case with the flash-free adhesive. We assume that the different consistency of the flash-free adhesive, at least in part, explains the ease of removal. The difference in consistency is most likely caused by a lower filler content compared to the conventional adhesive. Although a decrease in adhesive remnant removal time may seem desirable to both practitioners and patients, lower filler content may also result in decreased bond strength and resulting higher clinical failure rates (21, 22). Currently, no studies are available on the bond strength of the flash-free adhesive used in this study; however, a few studies have compared the bond strength of the conventional adhesive used in this study to more flowable, less filled adhesives. One such study found 3M’s Transbond XT adhesive to have a higher flexural modulus and shear bond strength (SBS) and lower contraction stress than the more flowable adhesives to which it was compared (23). Another study demonstrated that it had higher SBS values than the less-filled composites; and interestingly, significantly more adhesive remained on the enamel surface with the less-filled adhesives compared to the Transbond XT adhesive (24). These findings agree with the present results and our assumption that the flash-free adhesive is less filled than the conventional adhesive. If bond strength and clinical failure rate remain acceptable, the consistency of the flash-free adhesive should make bracket debonding and adhesive remnant cleanup easier, both adding to a more positive experience for the clinical practitioner.
Clinical practitioners’ preference The majority of orthodontists blinded preferred the flash-free adhesive over the conventional adhesive because of its ease and speed of removal, and because less force was needed for bracket debonding. Interestingly, four practitioners who preferred the conventional adhesive explained their preference with the fact that less adhesive remained on the enamel surface after debonding. These practitioners may have considered a possible decrease in adhesive removal time, which comes with a smaller amount of adhesive remnant, worth a higher risk of enamel tear-outs. Overall, however, these findings suggest that clinical practitioners prefer an adhesive material that allows
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Conventional adhesives for orthodontic bracket bonding have traditionally required the removal of excess adhesive around the bracket base prior to adhesive cure. In an effort to improve and optimize the process of bracket bonding, a new flash-free adhesive has been introduced to the market, which promises to eliminate the need to clean up flash upon bonding. This study evaluated this new adhesive with regard to microleakage at the enamel–bracket interface, amount of adhesive remaining on the tooth surface after bracket debonding, time required for adhesive remnant cleanup, and clinical practitioners’ preference in an in vitro setting. In vitro studies using nonhuman dental tissues are very commonly used to evaluate dental materials (14, 15). This work used bovine teeth as a model for human teeth. A number of previous studies have validated this model and have shown that bovine incisors correspond remarkably well to human teeth (7, 16).
mechanism is the manufacturer’s trade secret, we hypothesize that fracture at the bracket–mesh interface is facilitated by a lower material density at that site. To our knowledge, no other studies have yet been completed on the flash-free adhesive; however, very similar ARI scores have been reported for the conventional adhesive used in this study. Very recently, Sharma et al. (20) showed that the majority of ARI scores after removal of brackets bonded with Transbond XT were either 2 or 3, suggesting that most or all adhesive remained on the tooth after debonding. Our findings suggest that even more adhesive is left on the enamel surface after debonding when the flash-free adhesive is used. While this may be beneficial to orthodontic patients as it minimizes the risk of enamel tear-outs (4), more material remains on the tooth surface that requires cleanup.
6 easy bracket debonding and that can be removed from the enamel surface quickly and easily when left behind. A less filled adhesive, such as the flash-free adhesive, meets these criteria and can make for easier debonding and faster cleanup. In contrast, an adhesive with higher filler content may have increased SBS, which may result in fewer bond failures during treatment. Although direct extrapolation of the results of this in vitro study to the clinical situation is not possible, the findings suggest that the flash-free adhesive performs satisfactorily with regard to bond quality, failure mode, and adhesive remnant cleanup. A study on the clinical application of the flash-free adhesive would be the next logical progression in this line of research. If such an in vivo study confirms that bond strength and failure rate are clinical acceptable, the use of the flash-free adhesive may have advantages over the use of conventional adhesives. These advantages of the flash-free adhesive include potential savings in chair time because it eliminates the need to clean up flash upon bonding, and possible decreases in plaque accumulation and subsequent enamel demineralization caused by its smoother surface.
Conclusions
Funding 3M Unitek research grant (T.G.).
Acknowledgements The authors are grateful to Dr Philippe Gaillard for performing the statistical analyses.
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• Both the flash-free and the conventional adhesive show a very high quality of the bond with very minimal microleakage at the enamel–bracket interface. • The flash-free adhesive fails more reliably and predictably at the bracket–adhesive interface and leaves a significantly greater amount of adhesive remnant on the tooth surface after bracket debonding. • There is a noticeable, however not statistically significant, trend towards shorter adhesive remnant cleanup times with the flashfree adhesive, despite a larger amount of adhesive remaining on the tooth after bracket debonding. • When blinded, the majority of orthodontists prefer the flash-free product over the conventional product because it is easier and faster to remove, and less force is needed for bracket debonding and adhesive removal.
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