Direct bonding of orthodontic brackets to enamel is an essential part of fixed orthodontic treatment. The bonding material forms an interface between the bracket and enamel and must provide sufficient adhesion to withstand the forces encountered during orthodontic treatment while allowing safe removal of the bracket at the completion of treatment. The development of light-cured orthodontic bonding materials has provided clinicians with several commercially available options for bracket bonding. However, the characteristics of the bonding material may influence the strength and behavior of the bracket–adhesive–enamel interface. [1]

The bonding material is an important consideration in achieving reliable bracket adhesion. Different commercially available materials may possess differences in their formulation and bonding characteristics, which may affect their ability to maintain adhesion to enamel under mechanical loading. Owens and Miller demonstrated that the shear bond strength and site of bond failure could differ according to the bonding material used for orthodontic bracket attachment, with visible light-cured composites showing greater shear bond strength than the resin-modified glass ionomer cement evaluated in their study. [1] Such findings provide a basis for evaluating bonding materials under controlled experimental conditions before their clinical application.

Shear bond strength (SBS) is commonly used as an in-vitro measure of the resistance of the bracket–adhesive–enamel interface to debonding forces. Comparative studies of orthodontic bonding systems have used SBS to evaluate the mechanical performance of different materials under standardized laboratory conditions. [2,3] The use of extracted teeth provides a controlled model for such evaluation, while maintaining the enamel surface as the substrate for bracket bonding. However, bond-strength measurements can be influenced by variations in specimen preparation, bracket design, bonding protocol, storage conditions and testing methodology. Fox et al. emphasized that substantial variation exists in orthodontic bond-strength testing methods and that standardization is necessary to allow meaningful comparison of different bonding materials. [4]

Assessment of the bonding material should not be limited to the force required to debond the bracket. The pattern of adhesive failure is also important because the amount of adhesive remaining on the enamel surface after bracket removal provides information regarding the location of bond failure. The Adhesive Remnant Index (ARI) is used to record the amount of adhesive remaining on the tooth following debonding and has been incorporated with SBS evaluation in studies of orthodontic bonding materials. 

The sample comprising of 45 extracted maxillary premolars was collected from the patients who reported to the Department of Orthodontics, Institute of Dental Sciences, Bareilly, Uttar Pradesh, India, requiring orthodontic treatment. An informed and written consent from the patients and their guardians was obtained. A total of 45 maxillary premolars were selected from extracted teeth from the patients undergoing orthodontic treatment. The teeth were cleaned with tap water to remove soft tissue and debris and placed in 0.1% thymol solution (an antimicrobial solution for inhibition of bacterial growth) for one week and then stored in distilled water until use [9]. The criteria for tooth selection were as follows: 1. The teeth with intact crown, unattrited and free from hypoplastic areas, cracks, gross irregularities, decays and fractures. 2. Teeth without pre-treatment with chemical agents, such as hydrogen peroxide, formalin, or fluoride. Forty-five samples of selected maxillary premolars were randomly divided into three groups (Group 1–3), each consisting of fifteen teeth. Each tooth was placed in a metal mold and roots were embedded in self-curing acrylic resin (width = 15 mm; breadth = 7 mm; height = 18 mm) up to 1 mm apical to cementoenamel junction. The long axis of the tooth was kept parallel to the long axis of the metal mold. Crowns were kept exposed to facilitate surface treatment and bracket bonding on buccal surface [10]. Bonding Procedure Before bonding, the facial surface of each premolar was cleaned for 10 seconds with a mixture of water and fluoride-free pumice in a rubber polishing cup using a low-speed contra-angle handpiece. The enamel surface was water rinsed to remove pumice or debris and then dried with an oil-free compressed air [11]. Instructions of the respective manufacturers were carefully followed for the bonding of the brackets using the three different bonding adhesives: Transbond XT (3M Unitek), Enlight (Ormco), and Shofu Beautifil II. The buccal surface of the teeth was etched with 37% phosphoric acid gel for 30 seconds, followed by thorough washing and drying until a characteristic frosty white etched area was observed [12,13]. The respective adhesive material was applied according to the manufacturer's instructions. The adhesive was placed on the bracket base. The bracket was placed firmly on the tooth to the desired position and angulation. The excess composite resin was removed carefully from the bracket margin with a dental probe to eliminate any increase in nominal base area and the brackets were then light cured using a light-curing unit according to the manufacturer's recommended curing time. All 45 teeth in the three groups used the same type and design of orthodontic bracket and the same standardized bonding technique, with the bonding adhesive being the only variable among the groups. After bonding, all samples were stored in distilled water at room temperature for 24 hours and subsequently tested in shear mode on a universal testing machine at a crosshead speed of 1 mm/minute connected to a computer that recorded the results of each test [Table/Fig-3]. The specimens were secured in the lower jaw of the machine so that the bonded bracket base was parallel to the shear force direction. Force applying chisel was secured in the upper jaw and specimens were stressed in an occlusogingival direction at the bracket-tooth interface with the help of chisel [Table/Fig-4]. The upper jaw was moved at a crosshead speed of 1 mm/minute [14]. The maximum load necessary to debond or initiate bracket-tooth interface fracture was recorded in Newtons and then converted into Megapascals as a ratio of Newtons to surface area of the bracket [15]. Bond strength in MPa = Force (in Newton) / Surface area of bracket (in mm²). Adhesive Remnant Index After shear mode testing, enamel surfaces of teeth were inspected independently by one evaluator. An optical microscope (stereomicroscope) at ×10 magnification was used to determine the Adhesive Remnant Index (ARI) score at the site of bond failure. This scale ranges from 0 to 3 [16]. • Score 0: No adhesive remaining on the tooth in the bonding area. • Score 1: Less than half the bonded area covered by the adhesive. • Score 2: More than half the bonded area covered by the adhesive. • Score 3: All adhesive remaining on the entire bonded area. Statistical Analysis SBS data were summarized as Mean ± SD and compared by one-way analysis of variance (ANOVA), and the significance of mean difference between the groups was determined by Tukey's post-hoc test. The ARI scores were summarized in number and percentage and compared by chi-square (χ²) test. A two-tailed p-value less than 0.05 (p < 0.05) was considered statistically significant. All analyses were performed using SPSS software.

Three groups of direct-bonded maxillary premolar brackets using different bonding adhesives were included in the study. Fifteen specimens were taken in each group. The identification of the bonding adhesives is shown in [Table/Fig-1].

A total of 45 maxillary premolars were selected from extracted teeth from the patients undergoing orthodontic

 [table/Fig-2a-c]: (a) Clarity advanced ceramic bracket ; coated with Transbond XT  (b) Clarity advanced ceramic bracket; coated with Ormco Enlight light cure  (c) Clarity advanced ceramic bracket ; coated with Shofu Beautifil II composite

 [table/Fig-3]: Universal testing machine with mounted specimen.

 [table/Fig-4]: Stressed specimen at the bracket-tooth interface with the help of chisel

The present study determines and compares the shear bond strength of orthodontic brackets bonded with three different bonding adhesives. The range, mean (± SD), median and coefficient of variation of SBS (MPa) of the three groups are summarized in [Table/Fig-5].

The mean SBS of Group 1 (Transbond XT) was the highest, followed by Group 2 (ORMCO Enlight), whereas Group 3 (Shofu Beautifil II) showed the lowest mean SBS (Group 3 < Group 2 < Group 1). Further, the coefficient of variation (%) showed that SBS of Group 1 was least variable (6.78%), followed by Group 2 (11.40%), whereas Group 3 showed the highest variability (13.14) [Table/Fig-5].

Evaluating the effect of groups on SBS, one-way analysis of variance (ANOVA) revealed a significant effect of bonding adhesives on SBS (F = 103.49, p < 0.001) [Table/Fig-5].

Further, comparison of the mean SBS among the three groups using Tukey’s test showed the following results [Table/Fig-6].

Table/Fig-5]: Shear bond strength of three groups.

Min: Minimum, Max: Maximum, SD: Standard deviation, COV: Coefficient of variation, F: ANOVA F valu

[Table/Fig-6]: Comparison of mean shear bond strength (MPa) between groups by Tukey's test.
CI: confidence interval, p > 0.05 (not significant), **p < 0.01 (moderate significant), ***p < 0.001 (highly significant)

  [Table/Fig-7]: Comparison of frequency (%) of ARI scores among three groups.
   **p<0.01 (highly significant.

Adhesive Remnant Index Score

The Adhesive Remnant Index (ARI) scores of the three groups are summarized in [Table/Fig-7]. The frequency (%) of ARI scores 0, 1, 2 and 3 was recorded for each group. The frequency of score 0 was highest for Group 3, whereas scores 1 and 2 were highest for Group 1, and score 3 was highest for Group 2 [Table/Fig-7].

Chi-square (χ²) test was used to compare the frequency (%) of ARI scores among the three groups. It showed a significant difference in ARI scores among the groups (χ2=16.94,p < 0.01).

The ARI scores were summarized as number and percentage and compared by chi-square (χ²) test. A two-tailed p-value less than 0.05 (p < 0.05) was considered statistically significant. All analyses were performed using SPSS software.

Orthodontic adhesives play an important role in the successful bonding of brackets during fixed orthodontic treatment. An ideal orthodontic adhesive should provide adequate bond strength to withstand the forces encountered during treatment while allowing safe and predictable debonding with minimal damage to the enamel surface. Different adhesive systems may vary in their composition, filler content, viscosity, polymerization characteristics and bonding mechanism, which may consequently influence their shear bond strength (SBS) [1,2]. Several factors such as the method of enamel conditioning, composition of the adhesive, bracket base design, bonding procedure, polymerization technique and method of debonding may influence the forces required for bracket failure [4,9,11–15]. In the present study, the same bracket type, bracket base design, enamel conditioning procedure, bonding protocol, storage conditions and testing conditions were used for all three groups to minimize the influence of variables other than the adhesive system. Therefore, the differences in SBS observed in the present study may primarily be attributed to the different adhesive materials used. In the present study, when comparing the mean SBS of the three adhesive systems, it was observed that Group 1 (Transbond XT) demonstrated the highest mean SBS (26.85 ± 1.82 MPa), followed by Group 2 (Ormco Enlight) (17.72 ± 2.02 MPa), whereas Group 3 (Shofu Beautifil II) demonstrated the lowest mean SBS (17.35 ± 2.28 MPa). The statistically significant difference observed among the groups indicates that the type of orthodontic adhesive influenced the SBS of the bonded brackets. The higher SBS observed with Transbond XT may be attributed to differences in adhesive composition, filler characteristics and polymerization behaviour, which may contribute to effective bonding between the conditioned enamel surface and the bracket base. Differences in the composition and configuration of orthodontic adhesive systems have previously been shown to influence bracket bond strength [1,2]. Owens and Miller compared different visible light-cured orthodontic adhesives and reported differences in shear bond strength among the materials evaluated [1]. The present study showed that the SBS of Transbond XT was significantly higher than that of Ormco Enlight and Shofu Beautifil II, whereas no statistically significant difference was observed between Ormco Enlight and Shofu Beautifil II. These findings indicate that, under the experimental conditions of the present study, Transbond XT demonstrated greater resistance to shear forces, whereas Ormco Enlight and Shofu Beautifil II showed comparatively similar SBS. Differences in SBS among orthodontic adhesive systems have also been reported in previous investigations [3,5]. The coefficient of variation also differed among the three adhesive groups. Transbond XT demonstrated the lowest coefficient of variation (6.78%), followed by Ormco Enlight (11.40%), whereas Shofu Beautifil II (13.14%) demonstrated the highest coefficient of variation. The lower coefficient of variation observed with Transbond XT indicates comparatively less variability in the SBS measurements and suggests more consistent bonding performance among the specimens tested. In contrast, the higher coefficient of variation observed with Shofu Beautifil II indicates greater variability in the individual SBS measurements within this group. The bonding performance of an orthodontic adhesive is also influenced by the condition of the enamel surface. Variations in acid concentration and etching time may produce differences in the enamel etching pattern and consequently affect bracket bond strength [12,13]. Similarly, contamination of the enamel surface with water or saliva can influence the SBS of orthodontic brackets bonded with different adhesive systems [14]. In the present study, enamel conditioning and bonding procedures were standardized for all groups to minimize the influence of these factors. The bracket base and loading conditions are also important factors affecting measured bond strength. Since an identical bracket type and bracket base configuration were used for all three groups in the present study, the influence of bracket base morphology and mechanical interlocking was minimized. Previous studies have demonstrated that bracket base characteristics and mechanical interlocking can influence measured bond strength [11]. Similarly, the loading mode during SBS testing can influence the measured bond strength of orthodontic brackets [9]. Therefore, identical testing conditions were maintained for all specimens in the present study. The testing methodology itself is an important consideration when comparing orthodontic adhesives. Fox et al. emphasized that variations in experimental design and bond strength testing methodology can influence the results obtained in orthodontic bonding studies [4]. Therefore, standardized specimen preparation, bonding procedures and testing conditions were used in the present study to allow comparison of the three adhesive systems. Adhesive Remnant Index When an orthodontic bracket is subjected to debonding forces, failure may occur at different sites, including the bracket–adhesive interface, enamel–adhesive interface or within the adhesive itself. The Adhesive Remnant Index (ARI) is used to assess the amount of adhesive remaining on the enamel surface following bracket removal and provides information regarding the pattern of bond failure [16]. In the present study, differences were observed in the distribution of ARI scores among the three adhesive groups. Group 3 (Shofu Beautifil II) showed the highest frequency of score 0, Group 1 (Transbond XT) showed the highest frequencies of scores 1 and 2, whereas Group 2 (Ormco Enlight) showed the highest frequency of score 3. The statistically significant difference in ARI distribution among the three groups (χ² = 16.94, p < 0.01) indicates that the type of adhesive may influence not only the magnitude of SBS but also the pattern of bond failure. Differences in ARI scores may be associated with differences in the relative strength of the adhesive–enamel and adhesive–bracket interfaces. Previous investigations comparing orthodontic adhesive systems have also demonstrated variations in ARI following bracket debonding [1,3]. The distribution of ARI scores indicates variation in the amount of adhesive remaining on the enamel surface following bracket removal. A higher ARI score represents a greater amount of adhesive remaining on the enamel surface. Therefore, the higher frequency of score 3 observed with Ormco Enlight indicates greater adhesive retention on the enamel surface in this group. In contrast, the higher frequency of score 0 observed with Shofu Beautifil II indicates that no adhesive remained on the enamel surface in a greater proportion of specimens in this group. However, ARI findings should be interpreted together with SBS because the amount of residual adhesive may provide information regarding the pattern of bond failure and the potential risk of enamel damage during debonding. The present study demonstrated differences among the three adhesive systems in both shear bond strength and adhesive remnant distribution. Transbond XT demonstrated the highest SBS, whereas Ormco Enlight showed the highest frequency of ARI score 3 and Shofu Beautifil II showed the highest frequency of ARI score 0. These findings indicate that different adhesive systems may exhibit different combinations of bond strength and adhesive remnant patterns. The SBS values obtained in all three groups indicate adequate bond strength under the controlled in-vitro conditions of the present study. However, a higher bond strength does not necessarily indicate a clinically superior adhesive because the pattern of bond failure and the amount of adhesive remaining after debonding are also important considerations. The clinical performance of orthodontic adhesives may be influenced by several intraoral factors that cannot be completely reproduced under laboratory conditions. Saliva contamination, variations in enamel morphology, masticatory forces, thermal changes, moisture, repeated loading and individual patient factors may affect bracket failure clinically [4,9,14]. Therefore, the SBS values obtained in an in-vitro study should be interpreted cautiously and should not be considered a direct representation of long-term clinical performance. Overall, the present study demonstrated that the type of orthodontic adhesive influenced both the shear bond strength and the adhesive remnant pattern of bonded orthodontic brackets. The findings provide comparative information regarding the bonding performance and adhesive remnant characteristics of the three adhesive systems under standardized experimental conditions LIMITATION The present study has certain limitations. The relatively small sample size and in-vitro experimental design may limit the generalizability of the findings to clinical conditions. In addition, only three bonding adhesive systems were evaluated and the specimens were subjected to standardized laboratory conditions rather than the complex environment of the oral cavity. Further studies involving a larger sample size, additional adhesive systems and clinically relevant conditions such as thermocycling and cyclic loading are recommended.

Within the conditions of the present in-vitro study, the shear bond strength of orthodontic brackets was evaluated in relation to the type of adhesive system used. The findings provide comparative information regarding the bonding performance and adhesive remnant characteristics of the three adhesive systems under standardized experimental conditions.