Comparison of Bonding Technique with Two Different Light Cure Systems and Effect of Two Different Bonding Primers on the Shear Bond Strength Of Orthodontic Brackets: An In Vitro Study

Meenakshi Jayant Gavankar^1*, Vasant Pawar¹, Murlidhar Sastri¹, Nimish Arun Wajekar², Snehal Satish Pathak³ and Mukul Somnath Tambe⁴

^*Correspondence: Meenakshi Jayant Gavankar, Department of Orthodontics and Dentofacial Orthopaedics, S.M.B.T Dental College and hospital, India, Email:

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Introduction

One of the most significant developments in the field of orthodontics over the past sixty years has been the successful bonding of brackets to teeth. The foundation of this procedure was laid by Buonocore, et al [1]. In 1955 who etched enamel surface using phosphoric acid and thus made possible the bonding of brackets to the tooth surface. Direct bonding saves chair time as it does not require prior band selection and fitting, has the ability to maintain good oral hygiene, improve esthetics, and make easier attachment to crowded and partially erupted teeth.

Since molar teeth are subjected to higher masticatory impact, especially lower molars, it would be convenient to devise methods capable of increasing the efficiency of their traditional bonding. These methods may include variation in bondable molar tube material, design, bonding materials and etching techniques. For achieving successful bonding, the bonding agent must penetrate the enamel surface; have easy clinical use, dimensional stability and enough bond strength.

In the late 1970s, a new kind of composite was marketed, whose composition comprised a substance called camphoroquinone, which when exposed to light interacts with an activating agent (tertiary amine) to form free radicals which, in turn converts the resin monomers into polymers, thereby hardening the material. These materials were called light curing composites. Light intensity is another important factor for the preservation of the margins of the resin applied and polymerized. The resins have superior physical properties and better marginal integrity when polymerized by the gradual technique when compared with polymerization initiated with maximum intensity. LEDs have cold light emitted by a semiconductor, with a low light spectrum between 460 and 480 nm. Since camphoroquinone, the photoinitiator most used in resins, works by absorbing visible light ranging from 350 to 550 nm with peak absorption around 470 nm, the values emitted by the LEDs seem to be favorable. The ideal light spectrum for composite polymerization should range from 450 nm to 470 nm. With the advent of new high intensity LED for curing of orthodontic brackets, clinical bonding success can be achieved with shorter curing time [2].

Another aspect is the use of newer bonding agent i.e. Assure Universal Bonding Resin. Assure universal bonding resin is a relatively new product with fluoridereleasing properties. This bonding agent has been reinforced with resin cement, has hydrophilic properties, does not need to be photoactivated and has the capacity to bond to light-cured 4 or dual-cured adhesives. The Assure hydrophilic resin system (Reliance Orthodontic Products, Inc., USA) has been evaluated under wet conditions in some cases and proper bond strength values have been reported under such conditions. It has been claimed that the bond strength of Assure adhesive agent is not affected by contamination with saliva [3]. Moisture-insensitive primers and hydrophilic resin systems (Assure, Reliance Orthodontic Products) that have been recently introduced claim to provide comparable bond strengths in a wet environment. A distinct advantage of hydrophilic adhesives is in the treatment of impacted canines [4]. The motive of the study was to compare the shear bond strength of orthodontic brackets using such high intensity LED and comparing with the standard LED by using two different bonding primers.

Materials and Methods

One hundred twenty Maxillary premolars that had been extracted for orthodontic treatment was used in this study. After extraction, teeth were cleaned of soft tissue, polished with non -fluoridated pumice and rubber prophylactic cups at low speed for 10 second, and immersed in distilled water in a sealed container for 1-3 months until testing. All samples were embedded vertically in different colour cold-cure acrylic for identification of groups, by using metal ring moulds. The samples were randomly divided in 6 groups of 20 specimens each.

The buccal surface of each tooth was etched with a 37% phosphoric acid gel for 30 second; the teeth were rinsed and dried thoroughly with an oil-free air source before bonding. In Group 1In each of 20 teeth embedded in Green colour acrylic, a layer of Transbond XT primer was applied on the etched the brackets were then bonded with Transbond XT (3M Unitek) and light cured with high intensity LED for 1 second, according to the manufacturer’s instruction. In Group 2In each of 20 teeth embedded in Red colour acrylic, a layer of Assure universal bonding resin (Reliance) was applied on the teeth; the brackets were then bonded with Transbond XT (3M Unitek) and light cured with high intensity LED for 1 second, according to manufacturer’s instruction. In Group 3-20 teeth embedded in Orange colour acrylic were bonded with same procedure as in group 1 and the teeth were light cured for 5 seconds. In Group 4-20 teeth embedded in Blue colour acrylic were bonded with same procedure as in group 2 and the teeth were light cured for 5 seconds. In group 5 in each of 20 teeth embedded in Clear acrylic, a layer of Transbond XT Primer was applied on the etched area, the brackets was then bonded with Transbond XT (3M Unitek) and light cured with standard LED for 20 seconds, according to manufacturer’s instruction. In group 6 in each of 20 teeth embedded in Pink colour acrylic, a layer of Assure Universal bonding resin (Reliance) was applied on the teeth; the brackets were bonded with Transbond XT (3M Unitek) and light cured with standard LED for 20 second, according to manufacturer’s instruction.

Immediately after bonding, all specimens were stored in distilled water at 37°C for 24 hours. The shear bonding test was performed using a universal testing machine. The specimens were subjected to stress from a vertical direction, at a crosshead speed of 0.5 mm/min. The maximum shear force necessary to debond each bracket was recorded in Newton and then converted into Mega pascal (MPa).

Statistical analysis

SPSS for Windows, Version 21.0. Chicago, SPSS Inc. software was used to analyse the data.

Statistical analysis was done by using tools of descriptive statistics such as Mean, and SD for representing quantitative data. Student t test/ Unpaired‘t’ test used to compare between mean shear bond strength of two groups independent of each other. ANOVA F test was applied to compare significant difference among means of all groups. Post hoc data analysis which follows ANOVA F test was done by using Turkeys multiple comparison tests was also used. Post hoc test analyses multiple pair wise individual intergroup comparison. Probability p<0.05, considered as significant as alpha error set at 5% with confidence interval of 95% set in the study. Power of the study was set at 80% with beta error set at 20%.

Results

The purpose of this in-vitro study was to compare the shear bond strength of orthodontic brackets using High intensity LED and comparing with the standard LED by using two different bonding primers at different time intervals. For Group I (Transbond XT-1 second with High intensity LED) mean shear bond strength was 2.89 ± 1.02MPa. For Group II (Assure Resin-1 second with High intensity LED) mean shear bond strength was 2.71 ± 0.89 MPa. For Group III (Transbond XT-5 seconds with High intensity LED) mean shear bond strength was 3.69 ± 1.86 MPa. For Group IV (Assure Resin – 5 seconds with Standard LED) mean shear bond strength was 3.55 ± 1.30 MPa. For Group V (Transbond XT – 20 seconds with Standard LED) mean shear bond strength was 9.96 ± 3.60 MPa, Group VI (Assure Resin-20 seconds with Standard LED) have mean shear bond strength was 7.55 ± 1.69 MPa. On comparison of mean shear bond strength among all groups using ANOVA F TEST, there was highly statistical significant difference (p<0.001) found among all groups (Table1). Group V (Transbond XT-20 seconds) have the highest shear bond strength followed by Group VI (Assure resin -20 seconds). Lower the exposure time, lower was shear bond strength values.

Table 1: Comparison of shear bond strength of all groups using ANOVA F test.

On comparison of shear bond strength of high intensity LED with standard light emitting diode at different curing times, standard light with 20 seconds had statistical significantly (p<0.001) greater shear bond strength as compared to high intensity at 1 second and 5 second. But no statistical significant difference was observed between 1 sec and 5 sec for High intensity LED (Table 2). On comparison of Assure Universal Bonding Resin with a conventional bonding primer (Transbond XT) on shear bond strength of orthodontic brackets, Transbond XT had higher shear bond strength as compared to Assure Bond Resin but the difference was not of statistical significance (p>0.05) for High Intensity at 1 second and 5 second. Transbond XT had higher shear bond strength as compared to Assure Bond Resin but the difference was of statistical significance (p<0.05) for standard light at 20 seconds (Table 3).

Table 2: Comparison of shear bond strength of high intensity LED with standard light emitting diode at different curing times using ANOVA F test followed by Tukey’s post hoc test.

Table 3: Comparison of assure universal bonding resin with a conventional bonding primer on the shear bond strength of Orthodontic brackets.

Discussion

Optimum shear bond strength between a bracket and the enamel surface is necessary for orthodontic treatment. Favorable shear bond strength is in a range of 5.9 to 7.8 MPa to withstand oral and occlusal forces during treatment. At the same time, it should be easy to debond the bracket at the end of treatment without inflicting any damages on the enamel. Currently, research is on to develop time-conserving and tooth-friendly enamel conditioning systems for bracket bonding. In the present study, we evaluated the effects of all well-known enamel conditioning techniques for bracket bonding in terms of Shear Bond Strength. Phosphoric acid etching is the most common technique used in preparing the tooth for bonding procedure. However, acid etching has been implicated in decalcification and loss of enamel. Although the enamel-etching technique is a useful and accepted orthodontic procedure for bonding orthodontic brackets, it needs to be improved to establish clinically useful bond strengths while minimizing the amount of enamel loss. Etching of enamel with 37% gel formulations of phosphoric acid is a routine part of orthodontic practice. In the current study, all six groups were etched prior to bonding as research has shown that air abrasion is not an acceptable replacement for etching prior to bonding4. After the implementation of acid etching by Buonocore and enamel bonding by Newman in 1965, the adhesive technologies were established in dentistry and also in orthodontics.

A thin layer of a primer has to be applied to the enamel in order to increase the bond strength between the tooth surface and the composite resin and to enhance the resistance against marginal microleakage. A primer can also increase etched enamel damping and etched enamel retention. Furthermore, a thin layer of primer makes it possible to remove the brackets more easily. However, several studies also demonstrate that the use of a primer has no influence on shear bond strength (SBS) [5]. Newer bonding agent which is used in this study (Assure adhesive resin) is composed of biphenyl dimethacrylate (<35%), hydroxyethyl methacrylate (<20%), and acetone (<80%). It has been formulated to improve adhesion to normal and abnormal enamel surfaces, hypocalcified dentin, and surfaces with fluorosis and carious lesions, and can bond to rough metallic surfaces and composite resin restorations without any need for the application of extra primers.

In a study by Eslami, et al. [6] application of Assure adhesive resin to bond stainless steel brackets to enamel yielded adequate bond strength under dry conditions (14.18 MPa) and under contamination with saliva (13.32 MPa). Furthermore, Webster, et al [7] reported the Assure system to show more tolerance against saliva contamination. Again, Schaneweldt et al [4] concluded that the bond strength of Assure and MIP primers are not affected by saliva contamination Similarly, Nemeth, et al [8] in 2006 reported that bond strength of Assure to enamel contaminated with saliva is better than other materials. Conversely, in a study by Rix, et al no clinically significant differences were observed in the shear bond strength values of brackets bonded to enamel with the use of Assure adhesive resin [9].

In a study by Mashallah Khanehmasjedi, et al [3], application of Assure adhesive resin under dry and wet (contamination with saliva) conditions did not result in significant changes in the shear bond strength values of orthodontic brackets to enamel but, the bond strength (9.29 MPa) was higher than the minimum bond strength (5.90 MPa) necessary for bonding orthodontic brackets to enamel. In the current study, bond strength values of the brackets bonded to enamel with the use of Assure adhesive resin under dry conditions (8.83 MPa) were lower than those in Mashallah Khanehmasjedi et al 3 study. In the study by Rix, et al. reported higher bond strength values for Transbond XT specimens; although adequate bond strength of brackets to the enamel was noted in their study when bonding with Assure in dry and wet conditions; similar to our finding. TransbondTM XT (3M Unitek, Monorvia, California, USA) adhesive system is one of the standard adhesive systems in orthodontics & was chosen in this study as it yields good clinical outcomes and is widely used by researchers.

In the recent times, LED curing lights [(Dent Mark LED curing light, Wavelength: -1200-2000mw/cm2) and (woodpecker, light intensity 2300-2500mw/cm2)] have been commercially available for bonding the brackets. As it has a high-intensity light wave, so might cure the adhesive materials within 1 second. No data is published in relation to this curing technique. Therefore, in this in vitro study two LED curing units were examined and compared for checking Shear bond strength at different emission intensities and at different curing times using a Universal Testing machine.

From this study it appeared that Group C (Standard LED -20 sec) showed highly significant difference in mean shear bond strength (mean=8.75) when compared with Group B (High intensity LED-5 sec) and Group A (High intensity LED-1 sec), which was contradictory to study conducted [10]. Who observed light curing units did not influence the Shear bond strength of orthodontic brackets, but the orthodontic material influence bracket adhesion. Bracket bonding is one of the most timeconsuming procedures in orthodontics. For this reason, reducing the time required for light curing of composites would increase process efficiency and provide patients with greater comfort. Light-Emitting Diode (LED) based units are the most commonly used curing devices and have been reported to achieve satisfactory results with significantly reduced light-curing times of 10 and 8 seconds.

However, studies that have reported a decrease in curing times were conducted with LED devices with a light intensity of 1000mW/cm2, which might suggest that devices with an intensity of about 3200mW/cm2 could further reduce the light-curing times. The present study was conducted to assess the effects of reducing the curing time using High-power LED devices on the adhesion strength of metal brackets in-vitro. When different curing times were compared it resulted that Group C (Standard LED-20 sec) had the highest mean shear bond strength (mean=8.75) which is in agreement to study conducted by Layene Figueiredo Almeida, et al [11]. Which reported reducing exposure time significantly decreased mean values of SBS, even with the use of high power LED. Nonetheless, we need to be careful when extrapolating these data directly to clinical practice because in vitro results might not directly reproduce the situation in the oral cavity. Therefore, further clinical studies should be undertaken to confirm these in vitro results.

Conclusion

On comparison of High Intensity LED with standard light emitting diode at different curing times, Standard light emitting diode showed superior mean shear bond strength at 20 seconds. Transbond XT primers showed highest mean shear bond strength than assure universal bonding resin.

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