Journal of Orthodontics, Vol. 41, 2014, 175–180
Influence of different methods of cleaning custom bases on the shear bond strength of indirectly bonded brackets Lylian K. Kanashiro1, Julissa J. Robles-Ruı´z1, Ana L. Ciamponi1, Igor S. Medeiros2, Andre´ Tortamano1 and Joa˜o B. Paiva1 1
Department of Orthodontics and Pediatric Dentistry, School of Dentistry, University of Sao Paulo, Sao Paulo, SP, Brazil; 2Department of Biomaterials and Oral Biology, School of Dentistry, University of Sao Paulo, Sao Paulo, SP, Brazil
Objective: To determine the influence on shear bond strength and bond failure location of four cleaning methods for orthodontic bracket custom bases. Design: In vitro laboratory study. Material and methods: Eighty bovine teeth were divided at random into four groups. The bracket custom bases were cleaned with different methods: group 1 with methyl methacrylate monomer, group 2 with acetone, group 3 with 50 mm aluminium oxide particles and group 4 with detergent. The brackets were indirectly bonded onto the teeth with the Sondhi Rapid-Set self-curing adhesive. The maximum required shear bond strength to debond the brackets was recorded. The bond failure location was evaluated using the Adhesive Remnant Index (ARI). One-way analysis of variance (ANOVA) analysis (P,0.05) was used to detect significant differences in the bond strength. Kaplan–Meier survival plots and log-rank test were done to compare the survival distribution between the groups. The Kruskal–Wallis test (P,0.05) was used to evaluate the differences in the ARI scores. Results: The mean bond strengths in groups 1, 2, 3 and 4 were 23.7¡5.0, 25.3¡5.1, 25.6¡3.7 and 25.7¡4.2 MPa, respectively. There were no significant statistically differences in either the bond strength or the ARI score between the groups. Conclusion: The four custom base-cleaning methods presented the same efficiencies on indirect bond of the brackets; thus, practitioners can choose the method that works best for them. Key words: Bond strength, custom bases cleaning methods, indirect bonding, orthodontic brackets Received 11 February 2013; accepted 4 December 2013
Introduction The indirect bonding technique was initially proposed by Silverman et al. (1972). This technique involves positioning brackets on plaster models with an adhesive cement and then transferring them to the patient’s mouth by means of a tray, where they are then bonded onto the enamel surface (Silverman et al., 1972). Thomas (1979) refined this technique, creating a customized resin base that reduces material flash and facilitates cleaning. Since its introduction to orthodontics, indirect bonding has improved in both technique and materials and has become more widely accepted and used (Keim et al., 2008). When first introduced, the rates of bond failure of the indirect bonding were high (Zachrisson and Brobakken, 1978); however, in the past few years, several studies have reported laboratory bond strengths (Klocke et al., 2003; Linn et al., 2006) and clinical bond failure rates (Thiyagarajah et al., 2006; Deahl et al., 2007) comparable to those of directly bonded brackets. Address for correspondence: J. J. Robles-Ruiz, Orthodontics Master Program, Department of Orthodontics and Pediatric Dentistry, School of Dentistry, University of Sao Paulo, Avenida Professor Lineu Prestes 2227, Cidade Universita´ria, Sao Paulo, SP 05508-000, Brazil. Email: [email protected]
# 2014 British Orthodontic Society
The majority of current indirect bonding protocols are based on the method introduced by Thomas (1979), with some variations on the type of resin used to manufacture the custom bases (Klocke et al., 2003; Kalange and Thomas, 2007), the adhesives used to bond the brackets (Miles and Weyant, 2005), the conditioning applied to the enamel (Wiechmann, 2000; Cal-Neto et al., 2011) and the materials and techniques used to manufacture the transfer trays (Fortini et al., 2007; Pellan, 2007; Sondhi, 2007). There are also variations on the methods used to clean the plaster and separating medium residues or any other contamination from the custom bases prior to bonding. For a successful indirect bond, each step of the technique must be performed with great care and precision while avoiding unnecessary complexity. Several ways to clean custom bases have been described in the literature (Wiechmann, 2000; Klocke et al., 2003; Linn et al., 2006; Fortini et al., 2007; Pellan, 2007; Sondhi, 2007; Tortamano et al., 2007; Thompson
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et al., 2008; Cal-Neto et al., 2011); however, little data are available on these methods. The use of pellets soaked with acetone has been suggested to clean custom bases (Wiechmann, 2000; Cal-Neto et al., 2011); according to Wiechmann (2000), acetone would remove any traces of separating medium, as well as the inhibition layer, improving adhesion. Methyl methacrylate monomer applied to custom bases 10 min before the bonding has also been used as a cleaning procedure (Pellan, 2007); however, it has also been described as a conditioning method to improve adhesion between the custom base and the adhesive agent (Hickham, 1993; Miles, 2010). Some researchers recommend sandblasting the custom base with aluminium oxide before bonding to remove any contamination and increase the surface area, improving the bond strength (Klocke et al., 2003; Thompson et al., 2008). Thompson et al. (2008) evaluated various conditioning methods for custom bases and found that the highest bond strength was achieved when the custom base surfaces were sandblasted. Other authors prefer to wash custom bases with detergent using a brush or an ultrasonic cleaner (Sondhi, 2007; Tortamano et al., 2007). Different combinations of these methods have also been described in the literature (Miles and Weyant, 2005; Kothari, 2006; Sondhi, 2007; Miles, 2010). Currently, different methods are used to clean custom bases in the indirect bonding of brackets. This variety of methods often makes choosing one approach difficult for clinicians, who need to determine the best way to clean the custom bases of the brackets to obtain clinically acceptable bond strengths. Despite the fact that clinicians demand the simplest and most efficient indirect bonding technique, few studies have researched the preparation methods for custom bases before bonding (Thompson et al., 2008), and none have compared the different cleaning methods for custom bases. The purpose of this study was to determine the influence on shear bond strength and bond failure location of four cleaning methods for brackets custom bases that were bonded indirectly including the use of methyl methacrylate monomer, acetone, sandblasting with aluminium oxide and detergent. The null hypothesis was that there was no significant difference in bond strength neither in bond failure location between the four cleaning custom bases methods when used for indirect bonding. Material and methods The sample size calculation was based on a one-way analysis of variance (ANOVA) model, using values derived from a pilot study and the literature (Cal-Neto
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et al., 2011). It was assumed that a difference in shear bond strength of 5 MPa is clinically significant. Using this data with a standard deviation of 4.55, a 5% significance level and 80% power, it was determined that a sample size of 20 teeth per group would be sufficient to detect a statistically significant difference in shear bond strength between groups. Eighty recently extracted bovine teeth were acquired and stored in a solution of 0.1% thymol for 1 week. Then all soft tissue remnants were removed with a scaler. At that time, the teeth were polished with pumice and rubber prophylactic cups at low speed for 10 s, and stored in distilled water at room temperature until the start of the experiment. All of the teeth were free of cavities, wear, cracks, fractures or any other visible defects. To prepare the specimens, the pulp and roots were removed. Immediately after, the crowns were inserted in plastic cylinders (19.05 mm diameter and 10 mm height) with self-curing dental acrylic in such a way that the vestibular surfaces remained above the acrylic and parallel to the cylinder base. Two V-shaped grooves were made with an acrylic laboratory bur from the upper edge and at opposite sides of each specimen to allow the adequate positioning of the transfer trays within the cylinders in a later stage (Thompson et al., 2008). Casts of the specimens were made with condensation silicone (Zetaplus; Zhermack SpA, RO, Italy) and type IV dental plaster (Durone; Dentsply Ind. and Com., Petro´polis, RJ, Brazil). The plaster models were allowed to dry for 24 h. Later, a layer of separating medium diluted in water at a proportion of 1 : 1 was applied to each model and allowed to dry for 20 min. Lower central incisor metallic brackets were used (full twin standard edgewise, AbzilMR; 3M Unitek, Sa˜o Jose do Rio Preto, Brazil), and the base area was estimated at approximately 7.67 mm2. The bracket bases were sandblasted with 50 mm aluminium oxide for 1 s. A small amount of Transbond XT resin (3M Unitek, Monrovia, CA, USA) was placed on the bracket’s mesh, and the brackets were then immediately placed in the centres of the vestibular surfaces. Previous studies (Arici et al., 2005; Muguruma et al., 2010) have shown that the resin thickness affect the bond strength; thus, a force of 453.59 g was applied to the centre of each bracket aided by a Gillmore needle to standardize the resin thickness. Any excess material was removed with a dental explorer. The brackets were then light-cured for 40 s, 10 s per side, at a distance of 2–3 mm with a LED lamp (Flash Lite 1401; Discus Dental, Culter City, CA, USA) and at a light intensity of 1100 mW/cm2. The transfer trays were made with 1 mm-thick flexible thermoplastic resin sheets (FGM Dental Products,
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Joinville, SC, Brazil) in a device for vacuum adaptation; they were then cut 3 mm below the upper edges of the casts and submerged in water for 30 min. Later, the transfer trays were carefully removed from the casts, and the custom bases were light-cured for an additional 20 s. The specimens were randomly divided into four equal groups (n520), and the brackets custom bases were cleaned 10 min before bonding in accordance with the following protocols:
N N N
Group 1: Cleaning with a pellet soaked in methyl methacrylate monomer, for 5 s. Group 2: Cleaning with a pellet soaked in acetone, for 5 s. Group 3: Sandblasting with 50 mm aluminium oxide at a distance of 5 mm for 1 s using a microetcher (Danville, San Ramon, CA, USA) with 70 psi air pressure, followed by a 10-s application of oil-free compressed air. Group 4: Cleaning with detergent and a toothbrush for 30 s, rinsing with water for 30 s and drying with oil-free compressed air for 10 s.
All of the teeth were cleaned with a cup of prophylaxis and pumice, rinsed with water for 10 s and dried with oil-free compressed air. The enamel was then conditioned with 37% phosphoric acid (Ivoclar-Vivadent AG, Schaan, Liechtenstein) for 30 s, rinsed with water for 20 s and dried with oil-free compressed air until the enamel presented a white appearance. The brackets were bonded on the teeth with the selfcuring adhesive Sondhi Rapid-Set (3M Unitek) according to the manufacturer’s instructions. The transfer trays were then placed on the teeth, lightly pressed for 30 s and removed 5 min later, after polymerization had occurred. Immediately after the bonding, the teeth were stored in distilled water at room temperature before undergoing the shearing test. A mechanical testing machine (Kratos; Industrial Equipment, Cotia, SP, Brazil) with a load cell of 1 kN and a crosshead speed of 0.5 mm/min was used to determine the maximum shear strength required to debond the brackets. The specimens were placed in the machine in such a way that Table 1
Cleaning custom bases methods in indirect bonding
the edge of the chisel used to debond the brackets was as close as possible to the tooth and the bracket base interface, allowing the force to be applied parallel to the bracket base. For each specimen, the force was recorded in Newtons (N) and was divided by the surface area of the bracket base to obtain the shear bond strength in megapascals (MPa). All bonding and debonding procedures were carried out by the same operator. Once the brackets were debonded, the enamel of each tooth was examined by a second operator with an optical stereomicroscope, and the Adhesive Remnant Index (ARI) was determined to evaluate the bond failure location. The ARI score ranged between 0 and 3 as follows: 0, no adhesive is left on the tooth; 1, less than half of the adhesive is left on the tooth; 2, more than half of the adhesive is left on the tooth; and 3, all the adhesive is left on the tooth, with a clear impression of the bracket’s mesh (Artun and Bergland, 1984). The mean shear bond strength, standard deviation and minimum/maximum values were calculated for each group. The data from the bond strength were evaluated with the Shapiro–Wilk method to verify their normality. The one-way ANOVA (P,0.05) was employed to determine possible differences in the bond strength between the groups. Kaplan–Meier survival analysis was used to present the cumulative probability of failure at given levels of applied force. A log-rank test was employed to compare the groups. The non-parametric Kruskal–Wallis test was used to detect differences in the ARI scores (P,0.05). IBM SPSS Statistics 20 software (IBM Corp., Chicago, IL, USA) was used for the analysis. Results The mean shear bond strength, standard deviation and minimum/maximum values for each group are shown in Table 1. The group that was cleaned with methyl methacrylate presented the lowest mean shear bond strength (23.7¡5), while the group cleaned with detergent attained the highest mean (25.7¡4.2). The one-way ANOVA test showed that no significant differences
Mean shear bond strengths and standard deviation by group Mean*
Methyl methacrylate Acetone Aluminium oxide Detergent
20 20 20 20
181.9 193.9 196.3 197.0
23.7 25.3 25.6 25.7
5.0 5.1 3.7 4.2
15.2 15.3 14.7 17.1
32.6 32.5 30.4 32.6
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existed between the mean shear bond strengths obtained by the four cleaning methods (P50.516). The Kaplan– Meier survival plot demonstrated that the performances of the groups were not different from each other (Figure 1). A significant difference between the four groups was not observed with the log-rank test (P50.696). The distributions of ARI scores are shown in Table 2. More than 80% of the specimens of each group had scores of 0 and 1. There were no differences in ARI scores between groups (P50.217).
Discussion The mean bond strengths obtained by the four evaluated groups had values between 23.7 and 25.7 MPa. The minimum in vitro bond strength required for a reliable orthodontic bonding is still unknown and varies according to certain factors, such as the adhesive used, the bracket base design, the resistance of the enamel to acid etching, enamel integrity and the technique used by each clinician (Greenlaw et al., 1989; Pickett et al., 2001). Nevertheless, some authors have suggested that bond strength values between 5.9 and 9.7 MPa are adequate for clinical situations (Reynolds, 1975; Greenlaw et al., 1989). All of the cleaning methods evaluated in this study had mean bond strength values higher than the range suggested as clinically acceptable; even the minimum values of bond strength for the four groups (G1: 15.2, G2: 15.3, G3: 14.7 and G4: 17.1 MPa) were higher than this acceptable range. Despite some methodological differences in the design of the experiment and the materials used, we observed some similarities in previously reported bond strengths and those measured in this study (Wiechmann, 2000; Thompson et al., 2008). The group that was cleaned with aluminium oxide had a mean bond strength of 25.6¡3.7 MPa, a result that agrees with the findings of Thompson et al. (2008). That study evaluated the effect of different custom base-conditioning methods on the bond strength of the indirectly bonded brackets and measured a mean bond strength of 22.0¡8.7 MPa in the group in which the custom bases were sandblasted with 50 mm aluminium oxide for 1 s. Likewise, the mean value of bond strength obtained in the group cleaned with acetone (25.3¡5.1) was similar to that reported by Wiechmann (2000) (24.2¡6.16 MPa) for the group whose enamel was conditioned with 37% phosphoric acid and whose resin base was cleaned with acetone before bonding. There were no significant differences between the groups’ mean bond strengths and the Kaplan–Meier survival distribution curves demonstrated that the performance of the groups evaluated was not different;
Kaplan–Meier survival plot
therefore, we would expect similar clinical behaviour among the brackets following cleaning of the custom bases with any of the four evaluated methods. This finding indicates that clinicians can use the cleaning method that they prefer and what works best for them. Nevertheless, as with any in vitro study, caution must be used when attempting to extrapolate these results to a clinical setting. The ARI is used as a tool to evaluate the bond failure location in studies of bond strength. During the removal of the brackets, the bond failure can occur at the ¨ zer adhesive/enamel or the bracket/adhesive interface (O et al., 2010). In this study, most specimens had ARI scores between 0 and 1 for the bond failures, indicating that very little adhesive was left on the enamel after the removal of the brackets. Clinically, this would translate into less time required for adhesive residue cleaning;
Table 2 Scores for Adhesive Remnant Index (ARI) by group ARI scoresa* Evaluated groups
Methyl methacrylate Acetone Aluminium Oxide Detergent
10 4 8 7
10 14 10 9
0 1 1 1
0 1 1 3
a ARI indicates Adhesive Remnant Index: 0, no adhesive is left on the tooth; 1, less than half of the adhesive is left on the tooth; 2, more than half of the adhesive is left on the tooth; and 3, all the adhesive is left on the tooth, with a clear impression of the bracket’s mesh. *K54.715, P50.216.
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however, it also implies a greater risk of fracturing the ¨ zer et al. (2010) stated enamel during bracket removal. O that the favourite location for bond failures to occur is controversial. The ARI scores did not present significant differences between the four groups, consistent with the fact that there were no significant differences in bond strength. Several cleaning methods for custom bases have been described as part of the indirect bonding of brackets procedure (Wiechmann, 2000; Klocke et al., 2003; Linn et al., 2006; Fortini et al., 2007; Pellan, 2007; Sondhi, 2007; Tortamano et al., 2007; Thompson et al., 2008; Cal-Neto et al., 2011); however, it was not found in the literature any research that evaluated the influence of these methods on bond strength. As in any other orthodontic procedure, a simple, inexpensive and efficient indirect bonding technique is desirable; therefore, it is necessary to identify the most efficient cleaning method for custom bases to simplify the technique and facilitate the clinicians’ choices. Thus, the main strength of this study is that it evaluated the influence on shear bond strength and bond failure location of four cleaning methods for custom bracket bases used in the indirect bonding technique: the use of methyl methacrylate monomer, acetone, 50 mm aluminium oxide and detergent. The evaluated methods were chosen because they were the most frequently cited in the existing literature (Wiechmann, 2000; Klocke et al., 2003; Pellan, 2007; Sondhi, 2007; Tortamano et al., 2007; Thompson et al., 2008; Cal-Neto et al., 2011). In the past few years, several studies have used bovine teeth to evaluate the bond strength of indirectly bonded brackets (Wiechmann, 2000; Klocke et al., 2003; Klocke et al., 2004; Thompson et al., 2008). Studies that compared the bond strength obtained in human and bovine enamel recorded bond strength values that were comparable between both (Nakamichi et al., 1983; Fowler et al., 1992) or reported that the bovine enamel bond strength was slightly inferior to the force obtained with human enamel (Oesterle et al., 1998; Saleh and Taymour, 2003). Despite these differences, bovine enamel has been reported as a reliable substitute for human enamel in adhesion studies (Fowler et al., 1992; Oesterle et al., 1998; Saleh and Taymour, 2003). Therefore, bovine mandibular incisors were chosen as substrates for bracket bonding. The main weakness of this study is that it was undertaken in vitro, and we must be careful when applying these results to clinical situations. Ideal conditions for bracket bonding and optimum isolation of humidity exist solely in in vitro study settings. The bond failure of brackets in the oral cavity is caused by thermal fluctuations and repetitive mechanical loads, the absorption of
Cleaning custom bases methods in indirect bonding
fluids and biodegradation (Murray and Hobson, 2003; Brauchli et al., 2011). The bond strength of brackets must be able to withstand masticating forces, stress imposed by the orthodontic mechanics and patients’ abuse (Pickett et al., 2001). Therefore, in vivo studies are still required to verify the effects of each of these cleaning methods on the bond strength of indirectly bonded brackets. Another limitation relates to the calculation of bond strength because the true bracket base was not known and only the nominal area was used (Viwattanatipa et al., 2010). Although this does not affect the outcome of the study, as only one type of bracket was used, readers should be cautious when interpreting bond strength values individually. The results of this study demonstrate that cleaning custom bracket bases with methyl methacrylate monomer, acetone, 50 mm aluminium oxide particles or detergent results in similar bond strengths and bond failure locations; therefore, the null hypothesis was accepted. Clinician’s choice of cleaning agents might depend on factors such as experience with the use of a certain method, the facilities available in the office and the costs. Conclusion N The four cleaning methods for custom bases resulted in similar efficiencies for the indirect bonding of brackets regarding both bond strength and the bond failure location. N Clinicians can choose the method what works best for them, depending on their experience, the facilities available in the office and the costs.
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