INTRODUCTION

Scientific evidence and clinical observations have oriented orthodontists to esthetic permanent or semipermanent retention with bonded lingual retainers and lingual retainers, which are now effective devices for long-term retention.1 Bonded lingual retainers are fabricated in various designs that consist of combinations of different wires in different sizes and different composites.1 With the introduction of sandblasting, a third generation of retainers made of 0.32-in. stainless-steel wires has been described.2 Although multistrand wires are stated to replace round wires,3 canine-to-canine retainers have advantages over flexible wire retainers as they are more hygienic, more comfortable for the tongue, and easy for the patient to notice when they come loose.2

Several factors affect the bond strength and durability of retainers, including the adhesive system used, filler size and type, composite composition, curing unit type, and exposure time. Although not evidence based in orthodontics, microleakage may also contribute to the bond strength of composites.4 In restorative dentistry, James et al5 could not demonstrate any correlation between microleakage and bond strength; however, numerous studies have shown the effects of microleakage on the durability of bonding.67

Different composites have been suggested for use in fabricating retainers, including both restorative and orthodontic bonding materials. Several adhesives were developed especially for lingual retainers, and manufacturers offer ease of application and optimal handling properties for these adhesives. These highly filled, light-cured resins are also claimed to be a better choice when longevity and durability are required.89 Recently, flowable composites, originally created for restorative dentistry by increasing the resin content of traditional microfilled composites, have been suggested as lingual retainer adhesives.10–12

The microleakage beneath composites is particularly important in orthodontics, especially for lingual retainer adhesives, as they are exposed to the oral cavity and are intended to serve in the mouth for a long period of time. So far, to our knowledge, no research reports have investigated and compared the microleakage of lingual retainer adhesives when used for canine-to-canine retention purposes.

The aim of this in vitro study is to evaluate third-generation canine-to-canine retainers with different composites from the microleakage point of view. For the purposes of this investigation, the null hypothesis assumed that different types of composites used for lingual retainers would not influence the amount of microleakage observed between enamel-composite and wire-composite interfaces.

MATERIALS AND METHODS

Forty-five noncarious human mandibular canine teeth that had been extracted for periodontal reasons were used in this study. The teeth were stored in a distilled water solution. Immediately before bonding, the teeth were cleaned with a scaler and pumice to remove soft tissue remnants, callus, and plaque. Teeth were separated into three groups of 15 teeth each. All samples were etched for 30 seconds with 37% orthophosphoric acid (3M Dental Products, St Paul, Minn), rinsed with water from a three-in-one syringe for 30 seconds, and dried with an oil-free source for 20 seconds.

Round, stainless-steel, 0.36-in.-diameter wires were used in all groups. Wires were cut in 15-mm lengths to ensure standardization. One end of the wire was bent to fit the lingual curvature of the canine. The wires were sandblasted with 50-μm aluminum oxide powder for about 5 seconds using a microetcher (Danville Engineering, Danville, Calif) at an air pressure of about 100 psi. No retention loop was prepared.

Three different commercially available composite pastes, Transbond XT (3M Unitek, Monrovia, Calif), Transbond LR (3M Unitek), and Venus Flow (Heraeus Kulzer, Dormagen, Germany), were used in this study. Materials and application procedures are shown in Table 1, and detailed descriptions of the composites used in this study are shown in Table 2. Transbond XT primer was used for Transbond XT and LR and Single Bond (3M Espe, St Paul, Minn) for Venus Flow, both according to the manufacturers' recommendations. Before composite placement, Transbond primers were applied to the etched surface as a thin, uniform coat and were not cured. However, Single Bond was cured for 10 seconds. Composite pastes were applied and cured by a light-emitting diode (Elipar Free Light 2, 3M ESPE Dental Products, St Paul, Minn) for 10 seconds according to the manufacturer's instructions.

Table 1.  Materials and Application Procedures

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Table 2.  Composites and Chemical Compositions

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To ensure stability during placement and contouring of the composite, teeth were placed over silicone putty compound, and care was taken to inspect the bonding area carefully (Figure 1). The other ends of the wires in all groups were embedded in silicone putty to provide the best fit with the tooth surface (Figure 1). While the samples were prepared, care was taken to shape the bulk of composite 4 mm in diameter and to ensure 1 mm of composite thickness over the wire.13 However, for ethical concerns, small and large samples were included in the study.

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Prior to dye penetration, the apices of the teeth were sealed with sticky wax. After that, the teeth were rinsed in tap water and air dried, and nail varnish was applied to the entire surface of the tooth except for approximately 1 mm from the restorations. To minimize dehydration of the restorations, the teeth were replaced in water as soon as the nail polish dried. The teeth were immersed in 0.5% solution of basic fuchsine for 24 hours at room temperature. After being removed from the solution, the teeth were rinsed in tap water, and the superficial dye was removed with a brush and dried.

Each restoration was sectioned in a transverse plane (parallel to the lingual retainer wire) just above the wire with a low-speed water-cooled diamond. The specimens were first evaluated under a stereomicroscope (20× magnification; SZ 40, Olympus, Tokyo, Japan) for dye penetration along the composite-enamel interface at both the mesial and distal border. Then, the lingual retainer wires were gently removed from the composite bulk, and the dye penetration between the composite-wire interface both mesially and distally was also evaluated under a stereomicroscope. Microleakage was determined by direct measurement using an electronic digital caliper, and the data were recorded to the nearest value as a range 0.5 to 5 mm.

RESULTS

The intraexaminer kappa scores for the assessment of microleakage were high, with all values greater than 0.8 (Table 3). Descriptive statistics and comparisons of mesial and distal microleakage between enamel-composite and wire-composite interfaces of three composite groups are shown in Table 4. Little or no microleakage was observed at the mesial and distal sides for three composites in two different interfaces, and these findings were not statistically significant (P > .05).

Table 3.  Intraexaminer Kappa Scores for Assessment of Microleakage

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Table 4.  Comparison of Microleakage Scores Between Mesial and Distal Sides of Composites at Two Different Interfaces by Mann-Whitney U-Test with Bonferroni Correction

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Mesial and distal microleakage scores for each specimen were pooled, and the microleakage scores for each composite and interface were obtained by calculating the mean of mesial and distal microleakage scores. Comparisons among the three composites in both the enamel-composite and wire-composite interfaces indicated no significant difference in the amount of microleakage observed (P > .05; Table 5).

Table 5.  Comparison of Microleakage Scores Between Different Composites at Two Different Interfaces by Kruskal-Wallis Test

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Thus, the null hypothesis failed to be rejected. Venus Flow composite in the enamel-composite interface and Transbond LR composite in the wire-composite interface exhibited no microleakage. Maximum values for microleakage were determined in Transbond LR (1 mm) and Venus Flow (2 mm) in the enamel-composite and wire-composite interfaces, respectively.

DISCUSSION

In restorative dentistry, microleakage is defined as seeping and leaking of fluids and bacteria between the tooth-restoration interface.14 O'Reilly and Featherstone15 and Øgaard et al16 have shown that visible white lesions can develop within 4 weeks, and according to Gladwin and Bagby,14 microleakage increases the likelihood of recurrent caries and postoperative sensitivity. From an orthodontic perspective, it is possible to interpret this fact as the likelihood of formation of white spot lesions or carries at and under the enamel-composite interface.17 It is also likely that microleakage under the composite holding the retainer wire may result in failure of the fixed retainer.

Polymerization shrinkage of the adhesive material may cause formation of microleakage, promoting micro gaps between the adhesive material and the enamel surface, which may initiate white spot lesions under the bonding area.5 For orthodontic adhesives, Sener et al18 found that high-intensity halogen curing units resulted in a more polymerization shrinkage than did the conventional quartz-tungsten halogens. This subject will be a problem when different composites or curing units used for lingual retainer. Therefore, we aimed to evaluate the amount of microleakage observed between the enamel and composite interfaces when different composites are used.

When failure rates are compared, larger round, stainless-steel wires are superior to thinner stranded wires.21920 The most common failure type is detachment at the wire and composite interface because of insufficient adhesive over the wire or unfavorable occlusal contacts, which results in abrasion of the composite.2021 With the introduction of sandblasting,2 stainless-steel wires have started to be used routinely for canine-to-canine retainers. Similarly to the wire-composite interface, the seeping and leaking of fluids and bacteria14 between the wire and composite interface will cause failure. Thus, the investigation of microleakage between wire-composite interfaces might be an important topic for the clinical success of treatments and lingual retainers.

To approximate the in vivo situations better, human mandibular canine teeth were used in this study. Human teeth have been used in only a few studies.22 Mandibular canines have not been used in previous studies despite the fact that they are the most commonly bonded teeth for mandibular retainers. For ethical considerations, the number of teeth was kept to a minimum.

No previous study appears to have assessed microleakage for orthodontic composites used in fabricating the lingual retainers. In this study, the dye penetration method was chosen to determine microleakage on the bonded specimens. This is the most commonly used method to assess microleakage of dental materials.23 It is easy to perform, fast, and economic, but the shortcoming of the technique is subjectivity of reading the specimens.24 In our study, all specimens were evaluated by the same operator at two times to evaluate measurement error. The intraexaminer kappa scores for assessment of microleakage were high, with all values greater than 0.8. It is important to note, however, that the assessment was made only at the mesial and distal aspect of each tooth. These sites were selected because they were readily identifiable on each specimen. Microleakage, however, may not be similar to other sites on a bonded tooth, although studies on restorative dentistry have assumed that one site assessment is representative of the whole tooth.25

Distal and mesial portions were evaluated separately to decide whether the access of the wire to the composite causes more leakage on the mesial side. The presence of thick round wire and the related difficulty of forming the composite as smooth as the distal portion on the mesial side is thought to be a reason for microleakage between wire composite and also for enamel composite. Descriptive statistics and comparisons of mesial and distal microleakage at the two different interfaces of three adhesives showed little or no microleakage differences, and these findings were not statistically significant. The microleakage results in this study support the use of all these composites in routine orthodontic practice.

Although Concise is the most commonly preferred composite resin used in lingual retainer studies,3 an orthodontic bonding material, Transbond XT, was chosen because light-cured adhesives are easier to form, especially in the mandibular anterior area, where visibility is critical. According to Bearn,3 Concise or Transbond would also compare favorably because of their abrasion resistance and strength. Transbond LR is a specially manufactured composite for lingual retainers. These composites are highly filled and routinely used light-cured resins in orthodontic practice and are better choices when longevity and durability are required.89 Recently, the use of flowable composites was suggested for bonding lingual retainers. These were originally manufactured for restorative dentistry by increasing the resin content of traditional microfilled composites.10–12 Flowable composites are claimed to be advantageous as no mixing is required and needle tips on the application syringes allow direct and precise composite placement. This composite is not sticky and flows toward the bulk of the material rather than away from it: no trimming and polishing are required, with reduced chair time. With these advantages of ease of forming and light curing, it is interesting that this material has not been evaluated in the literature.

Comparisons of the composites in the present study indicated no significant difference in the amount of microleakage. Ziskind et al26 indicated that, because of low filler content, the flowable composite resin presents remarkable flow characteristics compared with a restorative composite resin. As a result, enhanced wetting of the tooth surface and a low modulus of elasticity can be achieved, and we expected a reduction of microleakage because of its stress reduction by flow property. Consistent with these expectations, Venus Flow composite in the enamel-composite interface exhibited no microleakage. However, highly critical maximum leakage was seen in only one sample of flowable composite between the wire and composite interface, which may have been caused by technical error during the fabrication of the retainer.

CONCLUSIONS

• The presence of wire in the mesial portion of the lingual retainer did not increase the microleakage at the enamel-composite and wire-composite interfaces in all groups.

• There was no significant difference in the mean rank of microleakage among Transbond XT, Transbond LR, and Venus Flow composite at the enamel-composite interface and at the wire-composite interface.

• All three types of composites are comparable in terms of resistance to microleakage.