INTRODUCTION

White spot lesions displayed on the enamel surface are a result of organic acids, produced by cariogenic bacteria housed in retained areas of dental plaque. Full orthodontic treatment frequently spans approximately a 2-year period, and at this time 50% of patients display some form of clinical white spots.1 Lesions of significant depth (75 μm) can develop in as little as 4 weeks, a shorter time period than many orthodontic appointment intervals of 6 to 10 weeks.23 Decalcified lesions are not only unesthetic, but they may become irreversible and lead to cavitated lesions. For these reasons, white spot lesions are of great concern to orthodontics, and advancements in orthodontic adhesive materials serve as one possible avenue to prevent this occurrence.

Amorphous calcium phosphate (ACP) has the properties of both a preventive and restorative material that justify its use in dental cements, sealants, composites, and more recently, orthodontic adhesives. ACP-filled composite resins have been shown to recover 71% of the lost mineral content of decalcified teeth.4 One ACP-containing adhesive, Aegis Ortho (The Bosworth Co, Skokie, Ill), has been marketed for use as a light-cured orthodontic adhesive with similar properties to previously used resins. The premise behind this product is that during a carious attack around orthodontic brackets, at or below pH 5.8, hydroxyapatite (HAP) is leached from the enamel surface. At this low pH, ACP is capable of being broken down and releasing supersaturating levels of Ca⁺² and PO₄ ions.5 These concentrations are conducive to the formation of HAP, which in turn can be used by the tooth for remineralization.6 This elevated state can be maintained for a considerable time, offering a promising antagonist to demineralization, and can promote the prevention of future white spots throughout orthodontic treatment.

ACP-containing orthodontic adhesives are claimed to reduce the prevalence of white spots around orthodontic appliances. However, to even be considered for clinical use, they must first provide satisfactory shear bond strength. Therefore, the aim of this study was to determine whether Aegis Ortho adhesive provided an acceptable level of shear bond strength to function as an orthodontic adhesive. This product was compared with the commonly used orthodontic adhesives/cements, Transbond XT and Fuji Ortho LC. Transbond XT adhesive, a composite resin, was chosen as one of the controls due to its popularity and common use in other bond strength studies.78 Similar to the claims made by the ACP-adhesive manufacturer, the use of resin-modified glass ionomer cement (Fuji Ortho LC) has been said to be more effective in preventing white spot lesions than conventional composite resins.9 Using this cement as another comparative group allows comparison of adhesives marketed for the purpose of white spot prevention. The efficacy of the ACP-containing adhesive in preventing demineralization was not the focus of this study, but if proven to have adequate bond strength, this feature will be explored in future research.

MATERIALS AND METHODS

Sixty extracted human premolars, absent of restorations and caries, were collected and stored in distilled water at room temperature. The teeth remained in distilled water at all times except when brackets were being bonded and debonded. The teeth were randomly divided into three groups, all containing 20 specimens. Group 1 was bonded using Transbond XT (3M/Unitek Corp, Monrovia, Calif), group 2 was bonded with Aegis Ortho (Bosworth), and group 3 used Fuji Ortho LC (GC America Inc, Alsip, Ill). The direct bonding protocol provided by the manufacturer was followed for each individual adhesive.

Prior to bonding, the teeth were sectioned 2 to 3 mm below the cementoenamel junction using a separating disk and a lab hand piece to help minimize heat. Before bonding, all teeth were cleaned using coarse, oil-free pumice with a rubber prophylaxis cup, rinsed with water, and dried for 10 seconds using an air-water syringe. Etchant (Transbond XT etching gel, containing 35% phosphoric acid from 3M/Unitek) was then applied to the cleaned area of the tooth for 15 seconds, rinsed for 10 seconds, and dried with an air-water syringe for 20 seconds. This etchant was used for all three groups to provide consistency in tooth preparation. In the instance of group 3, the tooth was left moist to remain in compliance with the recommended bonding protocol of the manufacturer. Universal bicuspid brackets, Victory Series (3M/Unitek), with a 0.022-inch slot and 0° of tip and torque were used in this experiment. This bracket is a stainless steel miniature mesh twin bracket containing a 10 mm² base surface area. All bonding was performed by a single operator. Published guidelines for bond strength testing were followed when possible to allow for past and future data comparisons.10

RESULTS

The mean, standard deviation, minimum/maximum, and range of shear bond strengths for the three groups are displayed in Table 1. ANOVA and the post hoc Tukey test revealed that group 1 was significantly different from groups 2 and 3 (P < .001). Groups 2 and 3 were not significantly different from each other (P > .05).

Table 1.  Shear Bond Strength

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In evaluating the usefulness of a bonding material, the lower values in a distribution of bond strengths are frequently more important clinically compared to mean bond strength since these lower values are more likely to result in clinical debonds.11 The Weibull analysis is a survival analysis tool that has been used in orthodontic research to evaluate the distribution of bond strengths. The Weibull curves for the three groups are displayed in Figure 1. Additionally, the characteristic bond strength, Weibull modulus, and shear bond strengths at 10% and 90% probability of failure are displayed in Table 2.

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Table 2.  Weibull Modulus and Characteristic Strength Results

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The ARI scores are displayed in Table 3. A statistical difference was found with a Kruskal-Wallis test (P < .001). Mann-Whitney U-tests showed groups 1 and 3 were not significantly different (P = .057). Group 2 displayed a significantly greater ARI score compared to groups 1 and 2 (P < .01).

Table 3.  Adhesive Remnant Index (ARI) Scores by Group

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DISCUSSION

Tavas and Watts reported that shear/peel strengths of direct bonded adhesives should develop to 4 kg in 5 minutes and 6 kg in 24 hours.12 For the brackets used in this study, these force values correspond to 3.9 and 5.9 MPa, respectively. Reynolds stated that successful clinical bonding has been recorded with adhesives giving in vitro tensile bond strengths of 4.9 MPa.13 The shear bond strengths for groups 1, 2, and 3 were 15.2 ± 3.6, 6.6 ± 1.5, and 8.3 ± 2.8 MPa, respectively. All three groups possessed mean shear bond strengths above the amount recommended by Tavas and Watts to perform as an orthodontic bracket adhesive. The ACP-containing adhesive presented with the lowest mean shear bond strength; however, the statistical analysis showed that there was no significant difference between this adhesive and the resin-modified glass ionomer cement which has been used in orthodontic practice for several years.

Results for groups 1 and 3 concur with many other studies investigating the bond strength of these adhesives. Transbond XT displayed a similar mean bond strength and ARI score when compared with previous publications that used the same bracket type.711 Additionally, as shown in the present study, Transbond XT has been found to have a superior bond strength to Fuji Ortho LC in the majority of studies that have investigated them together.14–17 Regardless of ranking, these same studies concluded that Fuji Ortho LC exhibited sufficient bond strength to be used as an orthodontic adhesive. Only a few articles have investigated the incorporation of calcium phosphates into orthodontic adhesives. Dunn18 examined the bond strength of the same amorphous calcium phosphate adhesive used in this study and observed that brackets failed at a significantly lower force level when compared to a traditional composite resin adhesive, as was found in the current study. Kawabata et al19 investigated the bond strength of a resin cement containing varying amounts of α-tricalcium phosphate and concluded the formulations possessed a clinically acceptable bond strength.

As mentioned previously, the mean shear bond strength may not be the most useful indicator of performance for evaluating orthodontic adhesives. Of greater significance to the clinician are the weaker values in the collection, because these represent instances which may result in the possibility of early clinical bond failure. This aspect is evaluated in the Weibull analysis in Figure 1 and Table 2. Group 2 showed the lowest characteristic strength and bond strength at 10% and 90% probability of failure. Consistent with the mean shear bond strength data, group 3 was not much greater in these parameters than group 2 and was far less than group 1.

Although derived from in vitro data, Figure 1 indicates groups 2 and 3 have a greater than 10-fold probability of failing at the critical 4.9–5.9 MPa bond strengths mentioned above. Similarly, Table 2 shows the bond strengths at a 10% probability of failure for groups 2 and 3 are lower than those recommended bond strength values. This suggests brackets bonded with the ACP-containing adhesive (or resin-modified glass ionomer) may have a greater chance of clinically debonding. Clinical studies with Fuji Ortho LC have been inconclusive in this regard. Gaworski et al20 found a greater percentage of debonds with Fuji Ortho LC compared to a resin adhesive, while others found similar clinical debond rates between Fuji Ortho LC and several resin adhesives.21–23 No clinical studies have examined the survival rate of brackets bonded with ACP-containing adhesives. It should be stressed, however, that if this adhesive is found to have remineralization capabilities, it may be acceptable to some clinicians and patients to have a slightly greater chance of debonding if it prevents white spots.

One aspect uncovered in the Weibull analysis and statistical analysis was the consistency of bond strength with the ACP-containing adhesive. This is evidenced by the lowest standard deviation (1.5 MPa) and greatest Weibull modulus (4.9) being for group 2 compared to those for group 1 (3.6 and 4.5, respectively) and group 3 (2.8 and 3.4, respectively). The greater the Weibull modulus, the more reliable the data or the closer the bond strengths are grouped together. It should be noted, however, that the bond strength range for the ACP-containing adhesive was lower than that of the other groups, perhaps due to its lesser maximum bond strength, and this may partially account for its low standard deviation and high Weibull modulus. Nonetheless, the ACP-containing adhesive produced a consistent bond, a desirable aspect to clinicians.

In orthodontics, less adhesive left on the tooth after debonding corresponds to less work and time spent by the orthodontist in removing it. A lower ARI score is favorable in this situation and all three groups presented with a majority of specimens in the 0–1 range, thus a majority of the adhesive was removed with the bracket during the process of debonding. Group 2 did have a significantly (P < .05) greater ARI score compared to the other two groups. Therefore, the clinician would have to spend a little more time removing the ACP-containing adhesive from the tooth. Additionally, this difference in ARI scores suggests the ACP-containing adhesive exhibited more of a cohesive failure compared to the composite resin and resin-modified glass ionomer adhesives.

Several different modalities have been employed to combat the problem of demineralization or white spot formation during orthodontic treatment. Derks et al24 concluded that the most effective means of preventing white spots is through the use of a high fluoride (1500– 5000 ppm) gel and toothpaste used daily during orthodontic treatment. Another study indicated daily brushing with a fluoridated dentifrice along with daily rinsing with fluoride will help prevent demineralization.25 Both of these studies are relevant if compliance is not an issue. However, that is not always the case. A compliance rate of only 13% was achieved from patients when they were asked to use fluoride mouth rinse daily during orthodontic treatment.26 Glass ionomer cement has been scrutinized in numerous studies and the results for and against its use have been presented.2728 This has prompted companies to develop new adhesives.

Being an in vitro bond strength study, caution is advised in extrapolating the results of the present study to the clinical situation. Clinically, the adhesives are subject to stresses, temperature fluctuations, variable electrolytes, microorganisms, and other factors that may affect performance. Kawabata et al19 showed α-tricalcium phosphate incorporated resins, thermal cycled to degrade the adhesives as a simulation for clinical use, exhibited less loss of bond strength compared to other filled resins. To determine whether bonding with ACP-containing orthodontic adhesives, designed to release ions at low pH, would be clinically durable is worthy of further investigation.

ACP-containing adhesive may be a great adjunct in the prevention of white spot lesions especially in cases where compliance is lacking. This study suggests that ACP-containing adhesive can provide similar bond strengths to at least one orthodontic adhesive that is already on the market and minimally meet the bond strength recommendations of Tavas and Watts.12 Future studies, examining the efficacy of these adhesives in preventing white spot lesions, appear warranted.

CONCLUSIONS

• There was no significant difference in shear bond strength between Fuji Ortho LC and Aegis Ortho, both of which were significantly less than Transbond XT.

• The Weibull analysis showed the ACP-containing adhesive possessed a distribution of bond strengths that suggests its use may result in a greater tendency for early debonds. However, this should be factored with its positive claims, albeit not proven, of preventing white spots.

• Transbond XT and Fuji Ortho LC displayed a significantly different ARI score compared to Aegis Ortho; however, in all three groups, most of the adhesive was removed with the bracket.