INTRODUCTION

During orthodontic treatment, the bonding between bracket base and enamel surface must be strong enough to withstand stresses and shear forces. Failure of the bonding technique, lack of retention of the bracket base, and masticatory forces also contribute to displacement of orthodontic accessories, which creates a common problem in clinical orthodontics. These problems cause disturbances, delay the treatment, and increase costs.1

Once detached, a bracket can be replaced with a new or reconditioned one during rebonding. The reconditioning process basically consists of removing the bonding agent remnants from the bracket base, thus allowing the brackets to be reused without causing damage to the retention mesh while preserving its retentive characteristics.2 This procedure is very useful when using conventional brackets and becomes more desirable when using new-generation self-ligating brackets. These devices have been introduced based on the advantages they offer in orthodontic treatment. They are able to reduce unwanted friction,3 eliminate the requirement for elastomeric ligatures,4 and ensure more certain archwire engagement,5 and offer faster archwire removal and ligation.6 The authors7 of recent systematic reviews reported that self-ligating brackets do not differ from conventional brackets with regard to the patient's subjective pain experience or with regard to the efficiency of orthodontic treatment. Their advantages over conventional brackets appear to be related only to the differences they offer in terms of mandibular incisor proclination (1.5° less in self-ligating systems) and shortened chair time.8

Self-ligating brackets have a mechanical device built into the bracket to close the slot9; this device carries a much higher cost than that associated with conventional brackets. Therefore, it might be useful if the orthodontist could manage the in-office reconditioning processes without affecting the shear bond strength (SBS) of these brackets. To date the literature contains many published articles1,10,11 that involved testing the SBS of new vs reconditioned brackets, but there are no studies that have involved evaluation of the effect of the reconditioning process on the SBS of the new-generation self-ligating orthodontic brackets.

Accordingly, the aim of the present investigation was to measure and compare the SBS and adhesive remnant index (ARI) scores of three different new and reconditioned self-ligating brackets. The null hypothesis of the study was that there are no significant differences in SBS values and debond locations among the various groups.

MATERIALS AND METHODS

This study was approved institutionally by the Dipartimento di Discipline Odontostomatologiche, Reparto di Ortognatodonzia (University of Pavia, Pavia, Italy).

Before preparing the specimens, 12 scanning electron microscope photographs were taken using a scanning electron microscope (JSM-6480LV, JEOL Ltd, Tokyo, Japan) to observe differences in bracket bases. For each new and reconditioned bracket, microphotographs were taken in the deepest parts (recessions) of the bracket base (magnification 2500×).

Next, 120 freshly extracted bovine permanent mandibular incisors were collected from a local slaughterhouse and stored for 1 week in a solution of 0.1% (wt/vol) thymol. The criteria for tooth selection included an intact buccal enamel with no cracks caused by extraction and no caries. The teeth were cleansed of soft tissue and embedded in cold-curing, fast-setting acrylic (Leocryl, Leone, Sesto Fiorentino, Italy). Metal rings (15 mm in diameter) were filled with the acrylic resin and allowed to cure, thus encasing each specimen while leaving the buccal surface of the enamel exposed. Each tooth was oriented in such a fashion that its labial surface was parallel to the shearing force. The teeth were randomly divided into six groups of 20 specimens because power calculation of the study showed that this specimen number was correct for the expression of significant results.

Three different orthodontic stainless-steel maxillary central incisor self-ligating brackets were tested: Smart Clip® (3M, Monrovia, Calif), Quick® (Forestadent, Pforzheim, Germany), and Damon3MX® (Ormco, Glendora, Calif). Materials were tested following the guidelines of Fox et al.12 on bond strength testing in orthodontics. In groups 1, 3, and 5, new brackets were bonded. In groups 2, 4, and 6, brackets were bonded, detached, reconditioned, and then rebonded. The reconditioning process was carried out using a sandblasting unit (Basic Master®, Renfert, Hilzingen, Germany) with 50-µm aluminum-oxide abrasive powder. The distance between the bracket base and the hand-piece head was fixed at a 10-mm distance. Each bracket base was sandblasted for 20–40 seconds under five-bar (72.5-psi) line pressure.10

For all the groups, before bonding the labial surface of each incisor was cleaned for 10 seconds with a mixture of water and fluoride-free pumice in a rubber polishing cup with a slow-speed hand piece. The enamel surface was rinsed with water to remove pumice or debris and then dried with an oil-free airstream.

The teeth were etched with 37% phosphoric acid gel (Orthophosphoric acid gel®, 3M Unitek) for 30 seconds, followed by thorough washing and drying. A thin layer of primer (Ortho Solo®; Ormco) was applied on the etched enamel, and the brackets were then bonded with a resin (Transbond XT®, 3M Unitek) near the center of the facial surface of the teeth. Sufficient pressure was applied to express excess adhesive, which was removed from the margins of the bracket base with a scaler before polymerization. The brackets were then light-cured with a visible light-curing unit (Ortholux XT®, 3M Unitek) for 10 seconds on the mesial side and for 10 seconds on the distal side (total cure time  =  20 seconds). After bonding, all samples were stored in distilled water at room temperature for 24 hours and were then tested in a shear mode on a universal testing machine (Model 3343, Instron Industrial Products, Grove City, Pa). The specimens were secured in the lower jaw of the machine so that the bonded bracket base was parallel to the shear force direction.

The specimens were stressed in an occluso-gingival direction at a crosshead speed of 1 mm/min, as in previous studies.13–15 The maximum load necessary to debond or initiate bracket fracture was recorded in Newtons and then converted into megapascals as a ratio of Newtons to surface area of the bracket. After bond failure, the bracket bases and the enamel surfaces were examined under an optical microscope (Stereomicroscope SR, Zeiss, Oberkochen, Germany) at 20× magnification. The ARI was used to assess the amount of adhesive left on the enamel surface,16 defining bond failure site among the enamel, the adhesive, and the bracket base. This scale ranges from 0 to 3. A score of 0 indicates no adhesive remaining on the tooth; a score of 1, less than half of the adhesive remaining on the tooth; a score of 2, more than half of the adhesive remaining on the tooth; and a score of 3, all adhesive remaining on the tooth with a distinctive mesh imprint remaining.

Statistical analysis was performed with Stata 7.0 software (Stata, College Station, Tex). Descriptive statistics, including the mean, standard deviation, median, and minimum and maximum values, were calculated for all groups.

The normality of the data was calculated using the Kolmogorov-Smirnov test. Analysis of variance (ANOVA) was applied to determine whether significant differences in debond strength values existed among the groups. The Tukey's test was used as post hoc. The chi-square test was used to determine significant differences in the ARI scores among the different groups. Significance for all statistical tests was predetermined at P < .05.

RESULTS

Descriptive statistics for the SBS (in Newtons [N]) of the different brackets are illustrated in Table 1 and Figure 1. ANOVA showed the presence of significant differences among the various groups (P < .0001). Post hoc testing showed that groups 1, 4, and 5 showed significantly higher SBS values than did the other groups (P  =  .023). The reconditioning process lowered the SBS of Smart Clip and Damon3MX brackets (P  =  .006). On the contrary, Quick brackets showed significantly higher strength values after reconditioning (P < .001).

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Table 1 Descriptive Statistics of the Four Groups Tested (in Newtons [N])

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The results for the ARI scores are presented in Table 2. The chi-square test revealed a higher frequency of an ARI score of “2” for Smart Clip and Damon3MX brackets, both before and after the reconditioning procedure. Quick brackets showed a significantly higher frequency of an ARI score of “1” both before and after the reconditioning process. For all of the three brackets tested, we detected no significant differences in ARI scores between new and reconditioned groups.

Table 2 Frequency of Distribution of Adhesive Remnant Index (ARI) Scores (%)

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DISCUSSION

The null hypothesis of the study was rejected. New Smart Clip and Damon3MX brackets showed significantly higher SBS values compared with reconditioned ones, whereas new Quick brackets demonstrated significantly lower SBS values than were obtained after the reconditioning process. This is probably due to the different mesh pad design of the brackets tested (Figure 2). Previous investigations17,18 indicated a strong relationship between the base of orthodontic brackets and the retention capability because the morphology of the base design may improve the penetration of the adhesive material.19 Moreover, when evaluating scanning electron microphotographs of the recessions of the bracket base (Figure 3), the Quick bracket (Figure 3C) showed a smoother surface pattern than did the Smart Clip (Figure 3A) and Damon3MX (Figure 3E) brackets. After the reconditioning procedure minimal changes were observed for Smart Clip (Figure 3B) and Damon3MX (Figure 3F) brackets, whereas with Quick brackets a rougher surface was observed (Figures 3D).

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The base of the Quick bracket differ from those of the other two tested brackets because grooves are equipped with hook-style undercuts that should improve resin adhesion on a bracket base. A possible explanation for the results could be that during the reconditioning procedure, the pressure of the sandblaster (five bars) and the dimension of the aluminum-oxide abrasive powder (diameter: 50 µm) are sufficient to remove excess adhesive from the undercuts of the brackets, changing the surface pattern and allowing increased base roughness and higher bond strength. In fact, before the reconditioning process Quick brackets showed lower SBS values than the other two new groups. After the reconditioning process, the Smart Clip and Damon3MX brackets showed lower SBS values, whereas the Quick reconditioned brackets exhibited higher SBS values. Therefore, the reconditioning process seems to increase the SBS values of brackets with wide grooves and undercuts, whereas brackets with narrow microretentions show lower SBS values. A possible explanation could be that the higher bonding area in brackets with macroretentions is improved by the roughening created by the reconditioning process. On the other hand, the microretention of brackets with narrower grooves seems to be spoiled by the reconditioning procedure.

The SBS of new and reconditioned brackets is a subject of great interest in orthodontic research. Reconditioning of debonded brackets aims to reduce the costs of replacing new orthodontic accessories. Several in-office reconditioning methods have been introduced.10,11,20 These processes include mechanical methods (hand pieces with rotary burs, chair-side sandblasting), thermal methods (direct flaming or heating in a furnace), and a combination of the two (direct flaming to burn off the composite, followed by sandblasting and electropolishing).10 Quick et al.11 compared different reconditioning methods and demonstrated that sandblasting for a period of 15 seconds using 50-µm aluminum-oxide granules at a pressure of approximately 4.5 bar was adequate to remove the residual composite without compromising bond strength, confirming that sandblasting is the simplest, most efficient manner of immediately reconditioning debonded brackets. Sandblasting can be performed in the dental office, which reduces the costs and working time.21 In-office bracket methods have been shown10,20,22 to cause no damage to the multistranded structure of the mesh.

The adverse effects presented by the reuse of brackets have been examined in past orthodontic literature. Aspects covered include the tensile strength of the metal,23 corrosion,24 and the propensity for the release of metal ions that can stain the teeth or induce a hypersensitive reaction in the oral tissue.25 More recently, Huang et al.26 evaluated ion release from new and reconditioned orthodontic brackets. Reconditioned brackets released more ions than did the new brackets, but the total ion release averaged over the period did not exceed the recommended daily intake. With regard to chromium release, other authors27 reported lower values in reconditioned brackets than in new ones.

Reconditioned conventional orthodontic brackets have also been tested clinically,28 and no significant differences were detected in the total bond failure rate of new vs reconditioned brackets, both in upper and lower arches and both in anterior and posterior segments.

The mean SBS values found in the present investigation vary from 105.93 N to 217.14 N. Reynolds29 evaluated tensile bond strength and suggested that a minimum bond strength of 6 to 8 MPa was adequate for most clinical orthodontic needs. However, direct comparisons with Reynolds' values are not possible because in the present investigation we evaluated shear strength and not tensile strength.

Moreover, in the present study brackets were bonded onto bovine enamel. Previous studies have shown that bovine and human enamel are similar in their physical properties, composition, and bond strengths, and, therefore, bovine enamel has been reported to be a reliable substitute for human enamel in bonding studies.30

Finally, in the present investigation ARI scores were analyzed. The scores were calculated using an optic microscope under 20× magnification. Smart Clip and Damon3MX brackets showed a significantly higher frequency of ARI scores of “2,” whereas Quick brackets showed a higher frequency of ARI scores of “1.” For all of the three brackets tested no significant differences in ARI scores were found between new and reconditioned groups. This finding is in agreement with those of previous studies31 that evaluated the ARI scores of new vs reconditioned brackets. An ARI score “0” means a higher adhesion of the bonding system, more to the bracket base than to the tooth, upon removal. Thus, less time is involved in removing adhesive from the tooth. In contrast, an ARI score of “3” indicates failure between the bracket and adhesive, with less risk of enamel fracture during the debonding process.32 The results of the present investigation demonstrated a higher frequency of ARI scores of “1” and “2,” showing a mixed adhesion modality.

CONCLUSIONS

• Smart Clip and Damon3MX reconditioned brackets showed significantly lower SBS than did new ones. On the contrary, Quick reconditioned brackets showed significantly higher SBS than did new brackets.

• Both new and reconditioned brackets showed SBS values that are adequate for clinical orthodontic needs.

• Smart Clip and Damon3MX brackets showed a higher frequency of ARI scores of “2,” whereas Quick brackets showed a higher frequency of ARI scores of “1.” For all three of the brackets tested there were no significant differences in ARI scores between new and reconditioned groups.