Indian Journal of Dental Research

ORIGINAL RESEARCH
Year
: 2012  |  Volume : 23  |  Issue : 6  |  Page : 747--752

Influence of adhesion promoters and curing-light sources on the shear bond strength of orthodontic brackets


Claudia Tavares Machado, Boniek Castillo Dutra Borges, Gustavo Jales Rodrigues Araujo, Alex Jose Souza dos Santos, Fabio Roberto Dametto, Fabio Henrique de Sa Leitao Pinheiro 
 Department of Dentistry at Potiguar University (Laureate International Universities), Natal, RN, Brazil

Correspondence Address:
Boniek Castillo Dutra Borges
Department of Dentistry at Potiguar University (Laureate International Universities), Natal, RN
Brazil

Abstract

Context: The effect of different curing units on bond strength of orthodontic brackets is still unclear when utilizing nanofilled composites in comparison with traditional Transbond-XT. Aim: To evaluate the influence of two adhesive promoters and two curing-light units on the shear bond strength (SBS) of orthodontic brackets. Settings and Design: The factors under study were adhesive promoters (nanofilled composite - Filtek-Z350 flowable restorative and conventional orthodontic adhesive - Transbond XT) and curing-light units (halogen lamp - Ultralux and LED device - Radii-Call). Material and Methods: Forty lower bovine incisors were utilized. The teeth were distributed in four groups (n = 10) according to the combination between adhesive promoters and curing-light units. Scotchbond Multipurpose-Plus and Transbond-XT primer were used to bond Filtek-Z350 Flowable Restorative and Transbond-XT, respectively. After storage in distilled water for 24 h, the brackets were subjected to SBS test at a speed of 0.5 mm/min until bracket debonding. The Adhesive Remnant Index (ARI) was assigned at fractured specimens. Statistical analysis used: Analysis of variance and Tukey test were utilized. The Kruskal-Wallis test was used to compare ARI scores between the groups (p<0.05). Results: There was statistically significant difference between the adhesive promoters tested. Transbond-XT showed higher SBS means than Filtek-Z350. There was no statistically significant difference between both curing-light units tested in this study, neither between ARI scores. Conclusions: The conventional orthodontic adhesive presented higher bond strength than the nanofilled composite, although both materials interacted similarly to the teeth. The curing-light devices tested did not influence on bond strength of orthodontic brackets.



How to cite this article:
Machado CT, Dutra Borges BC, Rodrigues Araujo GJ, Souza dos Santos AJ, Dametto FR, de Sa Leitao Pinheiro FH. Influence of adhesion promoters and curing-light sources on the shear bond strength of orthodontic brackets.Indian J Dent Res 2012;23:747-752


How to cite this URL:
Machado CT, Dutra Borges BC, Rodrigues Araujo GJ, Souza dos Santos AJ, Dametto FR, de Sa Leitao Pinheiro FH. Influence of adhesion promoters and curing-light sources on the shear bond strength of orthodontic brackets. Indian J Dent Res [serial online] 2012 [cited 2019 Dec 13 ];23:747-752
Available from: http://www.ijdr.in/text.asp?2012/23/6/747/111252


Full Text

In orthodontics, cemented bands have slowly been replaced with bonded orthodontic brackets since the introduction of the acid-etch technique in 1955 by Buonocore.[1] Nowadays, there is a growing tendency to even replace the molar bands with bonded tubes, thus reducing substantially the risk of enamel demineralization and periodontal problems.[2] However, one must bear in mind that composite resins used in orthodontics can also contribute to plaque accumulation around brackets,[3] especially when the excess or flush has not been removed thoroughly.

Nanofilled adhesive materials have been regarded as an excellent choice to achieve a superior finishing and polishing of composite restorations. It has also been claimed that this material helps to reduce long-term abrasion and marginal leakage.[4] Because of so many advantages, it seems reasonable to investigate whether nanofilled composites are also capable to withstand orthodontic forces. If so, they shall be considered a potential candidate to substitute the current conventional orthodontic composites.

Two authors[5],[6] have made comparisons of the shear bond strength (SBS) between traditional and nanofilled composites. Although less resistant than traditional orthodontic adhesives, nanofilled adhesives were capable of withstanding force levels similar to those produced during orthodontic treatment with fixed appliances. However, halogen lamp was the only source of light activation used in both these studies.

The halogen lamp is the most frequently used light source for polymerization of resin-based dental materials. It has some disadvantages such as heat generation, short lifetime of the lamp, and degradation of the filters. Light-emitting diode (LED) curing units have been investigated in an attempt to overcome these problems. Advantages attributed to LED are the coincidence of its peak irradiance with camphorquinone (a photoinitiator agent commonly found in composite resin formulations used in dentistry), less heat generation, and increased lifetime.[7]

The degrees of conversion and hardness with a traditional orthodontic resin (Transbond XT) were found to be quite similar when photoactivated by either LED or halogen lamp[8]. However, the effect of these different curing units on bond strength of orthodontic brackets is still unclear when utilizing nanofilled composites.

The present study aimed to investigate the SBS of orthodontic brackets when nanofilled and conventional resin-based orthodontic fixative materials were photoactivated by either an LED or a Halogen lamp. The following null hypothesis (Ho) was formulated: there is no statistically significant difference in the SBS of brackets bonded with either type of curing-light source regardless of the type of composite resin.

 Materials and Methods



Experimental design and ethics

The SBS of orthodontic brackets was assessed in order to verify the influence of the following factors: 1. Adhesive promoter at two levels - a conventional orthodontic fixation system (Transbond XT), and a nanofilled composite resin (Z350 Flowable Restorative); 2. Curing-light unit at two levels - a conventional halogen lamp (Ultralux, Dabi Atlante), and an LED source (Radii-Cal, SDI). [Table 1] and [Table 2] show further detail on the adhesive materials and curing-light sources used in this study.{Table 1}{Table 2}

The local research ethics board informed that the utilization of bovine teeth for research purposes no longer requires formal approval.

Sample preparation

Lower central incisors without major enamel defects or staining were obtained from authorized slaughter houses. Forty newly extracted teeth were selected, cleaned, and stored individually in a 0.5% thymol solution at room temperature. This solution was changed every 7 days.

Approximately 20 × 20 mm polyvinyl chloride (PVC) rings were fabricated, and filled up with liquid polystyrene resin. The roots were completely embedded into the resin right in the center of each ring, leaving only the crowns exposed. In order to standardize the tooth inclination, a geometry acrylic triangle was used. The aim was to guarantee that the middle third of each tooth buccal surface formed a 90-degree angle with the flat surface of the cured polystyrene resin base. Following this, the PVC rings were detached from the resin, and the specimens stored in distilled water.

To render the buccal surfaces smoother and eliminate any minor defects, water sandpaper sheets in a decreasing order of granulation (600, 400, 320, and 260) were used. Dental prophylaxis with pumice slurry and rubber cups was carried out on the buccal surface of each tooth for 10 s. After this, the teeth were rinsed with a water spray for 30 s, and dried with oil-free compressed air for the same amount of time. The rubber cups were substituted with new ones at every 10 teeth.

The area that was going to receive the bracket was acid-etched for 15 s with a 37% phosphoric acid gel (Condac 37, FGM) applied with a syringe. The teeth were rinsed for 30 s, and dried for 20 s until obtaining a chalky appearance. The selection of the type of sealant, adhesive paste, and curing light unit varied according to the experimental group to which the specimen was randomly allocated. To avoid performance bias, a combinatory analysis of all possible random sequences of four alphabetic letters (A, B, C, and D) was generated, each of them representing an experimental group: Group A (Scotchbond Multipurpose Plus + Filtek Z350 Flowable Restorative + Halogen light), Group B (Scotchbond Multipurpose Plus + Filtek Z350 Flowable Restorative + LED device), Group C (Transbond XT adhesive primer + Transbond XT adhesive paste + Halogen light) and Group D (Transbond XT adhesive primer + Transbond XT adhesive paste + LED device). Each group was comprised of 10 specimens.

The hydrophobic resin of Scotchbond Multipurpose Plus (3M ESPE) was spread on the acid-etched surface of teeth in groups A and B by using a fine disposable brush (KG Sorensen, Brazil), and light cured for 10 s immediately thereafter. The Transbond XT adhesive primer was spread on the acid-etched surface of teeth in groups C and D by using a brush-like applicator.

Both Filtek Z350 Flowable Restorative and Transbond XT adhesive paste were placed in the center of the bracket mesh, and homogeneously distributed with a composite spatula. The amount of material used on each bracket was standardized with the help of an improvised dosimeter. The bracket base area was 14.85 mm2.

The brackets (Dental Morelli, batch number 1333903, Sorocaba, Brazil) were held by a bracket forceps (Starlet, São Paulo, SP), and positioned in the middle of the buccal surface of the crowns. Using the other end of the forceps, a pressure was exerted for 5 s over the brackets with the aim of removing the excess of composite between the bracket mesh and the enamel. On each tooth, the composite flush was cleaned away with a regular dental probe (SSWhite, Juiz de Fora, MG, Brazil) for no more than 20 s. The remaining composite resin was light cured for 40 s, being 20 s mesially and 20 s distally. The curing light systems varied according to the group: Halogen light (Ultralux, Dabi Atlante) for Groups A and B, and LED light (Radi Call-SDI) for Groups C and D. Following bracket bonding, the specimens were stored at room temperature into dark plastic recipients containing distilled water for 24 h before undertaking the shearing bond strength test. The span of 24 h between bracket bonding and performance of the SBS test is supported by the findings of Chanda and Stein[9] in 1996.

SBS analysis

The SBS test was performed at Shimadzu universal testing machine at a speed of 0.5 mm/min until bracket debonding. This machine was calibrated for every 5 tests. The samples were adapted into a metal clamp in a position favoring the application of a shearing movement parallel to the buccal surface of the crowns. A loop made of stainless steel rectangular wire (.019"×.025") was attached to the machine at one end, and encased under the lower wings of the brackets to deliver the shearing stress. Shear strength values were obtained in newtons (N), and then converted into Mega Pascal (MPa).

Adhesive remnant index measurement

After debonding, any remaining adhesive was assessed and scored according to the modified Adhesive Remnant Index (ARI),[10] following the scores published elsewhere. [11],[12] All teeth were photographed by the Nikon Digital Camera D60 (Nikon Corp., Tokyo, Japan, serial number 3387562) coupled to an F2.8 EX DG Macro Lens of 105 mm (Sigma, Kanagawa, Japan, serial number 3066557), and to the Electronic Flash Macro EM-140 DG NA-iTTL (Sigma, Kanagawa, Japan, serial number 2058775). The camera was adjusted to manual mode, shutter speed of 1/200 s, aperture of f/64, and ISO sensitivity set to 200. The zoom lens was used in manual focus, full focus mode, with the diaphragm positioned at #32, and a scale distance of 0.0313 while the manual mode was programmed for Flash with an intensity of 1/32. Images were obtained of fractured specimens and processed using the Microsoft Office Picture Manager Software in an attempt to facilitate the identification of the entire enamel surface covered by the remaining composite/adhesive. Brightness was decreased by 10%, and contrast was increased by 100%.

Analysis of the images was made by a calibrated examiner. The scoring criteria of the index were as follows [Figure 1], [Figure 2], [Figure 3], and [Figure 4]:{Figure 1}{Figure 2}{Figure 3}{Figure 4}

1 = All of the composite, with an impression of the bracket base remaining on the tooth;

2 = More than 90% of the composite remained on the tooth;

3 = More than 10% but less than 90% of the composite remained on the tooth;

4 = Less than 10% of the composite remained on the tooth;

5 = No composite remained on the tooth;

Statistical analysis

Descriptive statistics data including the mean, standard deviation, and confidence intervals were calculated for each of the groups tested. Two-way analysis of variance (ANOVA) and Tukey multi-comparison tests were used to compare SBS between the groups. The Kruskal-Wallis test was used to determine significant differences in the ARI scores between the experimental groups. Significance for all statistical tests was set at P<0.05. All data, except for the Kruskal-Wallis test, which was performed by Assistant Beta version 7.5, were entered and analyzed by SPSS (Statistical Package for the Social Sciences, SPSS Inc, Chicago, IL, version 9.0 for Microsoft Windows).

 Results



The Tukey test indicated that there was a statistically significant difference between both adhesive materials tested (p<0.001). No statistically significant difference was observed between the curing devices utilized (p>0.05). Multiple comparisons by the Tukey test are shown in [Table 3].{Table 3}

Transbond XT groups photoactivated either by halogen or LED lamps presented higher SBS means than all groups utilizing Filtek Z350 Flowable Restorative. On the other hand, samples of Transbond XT and Filtek Z350 Flowable Restorative photoactivated by the halogen lamp showed similar results to those photoactivated by the LED device.

The ARI scores are listed in [Table 4]. Kruskal-Wallis test results showed no differences between the experimental groups.{Table 4}

 Discussion



The null hypothesis tested in this study was partially accepted. Orthodontic brackets bonded with Transbond XT system presented statistically significant higher SBS than those bonded using Filtek Z350 Flowable Restorative. However, there were not statistically significant differences between curing-light devices.

Bracket bond strength results of a resin-based adhesive can be influenced by several factors. First and foremost, bond strength performance is closely related to the composition and characteristics of the adhesive material itself as well as the complicated interactions among the various materials at the different interfaces. [13] Moreover, polymerization efficiency has been directly related to improvements in bond strength of resinous fixative orthodontic agents. [8] Although in the present study polymerization efficiency of the materials tested was not assessed through the degree of conversion (DC) and hardness, it was demonstrated that halogen lamp and LED units can provide comparable results in terms of DC and hardness. Mirabella et al [14] and Krishnaswamy and Sunitha [15] have also reported no differences in SBS means between samples cured either by a halogen or an LED lamp analyzing SBS of conventional orthodontic adhesive promoters.

In the absence of a standardized and widely used bond strength assessment protocol, an intra-study comparison with a control group seems to be a sensible approach. [16] In view of the fact that Transbond XT system (the control group) showed higher SBS means than Filtek Z350 Flowable Restorative, one might presume that a better interaction between Transbond XT components and the acid-etched enamel occurred. However, results of the ARI scores showed that a similar quantity of adhesive/composite remained on the enamel in all groups tested [Table 4]. Consequently, such interaction between the adhesive promoters and the enamel might have occurred equally. A possible explanation for this result lies in the fact that bovine teeth used in this study had their buccal surfaces abraded by sandpaper sheets (materials and methods), thus exposing a different prismatic arrangement that favored composite bonding. Possibly, a higher cohesive strength presented for the Transbond XT adhesive paste in comparison to Filtek Z350 Flowable Restorative would explain the differences achieved by the bond strength test. Further works are needed to confirm the aforementioned theory.

Although Transbond XT system provided higher bond strength means than Filtek Z350 Flowable Restorative (nano-composite) in the present study, both materials yielded SBS values capable of resisting orthodontic forces. According to Reynolds,,[17] a minimum bond strength of 5.9-7.8 MPa is necessary to guarantee a bracket survival compatible with the demands of orthodontic treatment. Considering that the SBS produced by Filtek Z350 Flowable Restorative is within the range capable of fulfilling the requirements of orthodontic mechanics, this material seems to be a versatile option in the orthodontic clinic as it can be used in both orthodontics and restorative dentistry. In addition, its lower SBS values shall contribute to decreasing iatrogenic conditions caused during debonding such as enamel fractures and crazes.

Based on a study published by Hannig et al, [18] the smoothness associated with nanofilled composites should also be considered an advantage of these materials. Such characteristic will presumably render the enamel surface around the brackets less prone to plaque accumulation or staining.

Bonding orthodontic brackets to etched teeth using light-cured systems has become a standard clinical practice[13] instigating the investigation of adhesive protocols that can improve clinical outcomes. However, essential factors such as stress arising from an activated archwire coupled with occlusal loads, critical pH, and temperature variation cannot be replicated in a laboratorial setting.[19] Therefore, randomized clinical-controlled studies are needed to confirm the results obtained in this in vitro investigation.

The null hypothesis could be partially accepted. The lamp devices tested provided no difference in shear bond strength (SBS) means. Transbond XT conventional orthodontic adhesive presented higher shear bond strengths than the nano-composite Filtek Z350 Flowable Restorative. Both materials presented similar Adhesive Remnant Indexes.

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