Comparison of Shear Bond Strength of a Newly Introduced Primer with Conventional Primer Using Two Types of Brackets Bonded to Different Surfaces: An In Vitro Study

Introduction

In recent times, an increasing proportion of the adult population is seeking orthodontic treatment.^1,2 However, these adults often present with multiple restorations and prostheses owing to cumulative damage from carious or missing teeth. These restorative surfaces (such as amalgam, composite resin, and porcelain) may pose challenges in bonding orthodontic attachments to these materials.^1-3

The attachment of brackets to teeth plays a key role in the early stages of orthodontic treatment. Formerly, this was achieved by banding the teeth. But the introduction of acid-etching and direct bonding has led to dramatic changes in the practice of Orthodontics.⁴ The median bond failure rate for practitioners in the United States is around 5%.⁵ Reynolds suggested that a bond strength of 5.9-7.8 MPa was adequate for most clinical orthodontic needs and to withstand masticatory forces.⁶

Minimizing bond failures is important since replacing loose brackets is inefficient, time-consuming, and costly. Consequently, researchers are continuously developing techniques and materials to achieve higher bond strength and durability in the presence of different surfaces and saliva.¹These include sandblasting, acid-etching, silane coupling, and resin priming.¹

Since self-etching primers provide the best quality and gentlest procedures for bonding orthodontic brackets, it becomes imperative to explore the efficacy of newer products in the market against traditionally established materials, especially in terms of their shear bond strength (SBS) and residual adhesive (which can create fracture and irregularities).^1,7

While Transbond XT (TXT) (3M Unitek) has long been shown to attain among the highest bond strengths (15.49 MPa) of any primer, a new versatile primer has been recently introduced named Assure Plus (AP) (Reliance Orthodontic Products) claiming that it can bond to every dental surface including normal/atypical enamel, amalgam, and porcelain.^1,7

Hence, the present study aimed to compare the SBS and adhesive remnants achieved with the newly introduced AP primer versus those achieved with the conventional TXT primer when used for bonding metal and ceramic brackets to three different surfaces [tooth enamel, composite, and porcelain-fused-to-metal (PFM) ceramic].

Materials and Methods

This in vitro study was conducted at a tertiary care hospital in Bengaluru, Karnataka, India, after obtaining ethical clearance from the Institutional Review Board (MRADC/ECIRB/2015-16). The sample size was estimated using the software GPower v. 3.1.9.2. Considering the effect size to be measured (f) at 40%, power of the study (1-β) at 80% and the margin of error (α) at 5%, the total sample size needed was calculated to be 120, with 10 samples required at each sub-group level [10 samples x 2 types of brackets x 2 priming methods x 3 groups = 120 samples].

In this study, three types of experimental models were made: Group-1 [40 extracted non-carious human maxillary 1st premolar teeth mounted on acrylic blocks], Group-2 [40 light cure composite restorations (3M Unitek, USA) embedded in acrylic blocks], and Group-3 [40 PFM crowns prepared on one extracted tooth (Vita, Germany) and partially embedded in acrylic blocks but with buccal surfaces exposed]. Each group was divided into two subgroups based on type of primer used: Subgroup-A (20 samples primed with TXT primer) and Subgroup-B (20 samples primed with AP primer). Each subgroup was further sub-divided into two categories based on type of brackets bonded: Category-M (metal brackets, n=10) and Category-C (ceramic brackets, n=10).

Group-1: Forty therapeutically extracted human maxillary 1st premolar teeth were cleaned with ultrasonic scaler, lightly pumiced, and stored in distilled water at room temperature. They were mounted on cylindrical bases (12 mm diameter x 25 mm height) made with acrylic (DPI-RR cold cure, DENTSPLY). These teeth were etched with 37% orthophosphoric acid (EAZETCH) for 15 seconds and then rinsed with water and dried, making surfaces appear frosty white.

Group-2: For the forty composite restoration samples, a cavity was prepared measuring 8 mm in diameter and 2 mm in depth with straight acrylic cutting bur on similar acrylic molds as in Group-1. A bonding agent (AdperTM Single Bond 2, 3M ESPE) was applied to it and cured with LED light for 10 seconds. Each cavity was filled with layers of composite (FiltekTM Z350 XT, 3M ESPE). To create a smooth surface, the composite was compressed with a glass slide and excess material was removed. The composite was light polymerized as per the manufacturer’s instructions (30 seconds) through the glass plate at a 90-degree angle to the top of the surfaces. Composite surfaces were treated with five side-to-side abrasive strokes under medium pressure using Horico abrasive strip (4mm).

Group-3: Crown preparation was done on one extracted premolar tooth and its elastomeric impression was recorded and poured with die material. Forty casts were made from this on which PFM crowns were fabricated. These PFM crowns were mounted in acrylic as in Group-1, but the buccal surfaces were exposed. The buccal surfaces of the crowns were roughened with a fine diamond bur, rinsed, and dried. For subgroup-3A, PFM crowns were etched with 9% hydrofluoric acid as per manufacturer’s instructions for 90 seconds, rinsed with water spray, and air dried. Once the buccal surfaces of PFM crowns appeared frosty, a silane coupling agent (UltraDent®) was applied on to the buccal surface with a mini brush tip and allowed to evaporate for 60 seconds and air dried. For subgroup-3B, PFM crowns with roughened buccal surfaces were primed with one coat of Reliance Porcelain Conditioner for 60 seconds, and then air dried.

Subgroup A: A thin uniform coat of TXT (3M Unitek) primer was applied, lightly dried with air, and cured for 10 seconds with LED light (3M ESPE Elipar, 1200 mW/ cm² ).

Subgroup B: A thin uniform coat of AP (Reliance Orthodontic Products) primer was applied, lightly dried with air, and cured for 10 seconds with LED light (3M ESPE Elipar, 1200 mW/cm² ).

Category M: Metal brackets were bonded onto the samples using Transbond XT light cure adhesive paste. The specimens were light cured for five seconds each on mesial and distal sides of each tooth after removing the excess adhesive around the brackets.

Category C: Ceramic brackets were bonded onto the samples using Transbond XT light cure adhesive paste. The specimens were light cured for five seconds through the bracket after removing the excess adhesive around the brackets.

All the samples were stored in distilled water for one week at room temperature. After one week, shear bond strength (SBS) was measured for each sample at the Centre For Nano Science And Engineering, Indian Institute of Science, Bengaluru, India. The samples were positioned in the universal testing machine with the long axis parallel to the direction of the load application. A 0.009”-diameter stainless-steel wire in the shape of a loop was fixed to the upper cross head at one end and adjusted to engage the bracket at the other end. A crosshead speed of 10 mm/minute was used and the maximum load necessary to debond the brackets was recorded. The force required to shear the bracket was recorded and SBS was calculated in Megapascals (1 MPa =1N/mm² ) by dividing the force values by the bracket base area.

The sheared surfaces and bases of the brackets were further investigated with a stereomicroscope (Lynx) at 5X magnification to assess the adhesive remnants on the samples’ surfaces. The Adhesive Remnant Index (ARI), as described by Artun and Bergland (1984), was used as a means of defining the sites of bond failures between the surfaces (tooth enamel, composite, and PFM ceramic), resin (adhesive) and the bracket base.⁸ Herein, Score 0 = no adhesive remained on the surface, Score 1 = <50% of adhesive remained on the surface, Score 2 = >50% of adhesive remained on the surface, Score 3 = all adhesive remained on the surface.⁸

Statistical analysis

Statistical Package for Social Sciences (SPSS) for Windows (Version 22.0. Released 2013, Armonk, NY: IBM Corp.) was used to perform statistical analysis.

Descriptive statistics, including mean, standard deviation (SD), minimum and maximum values, were calculated for all the three groups. The means SBS values were compared using factorial Analysis of variance (ANOVA). Inferential statistics, including one-way ANOVA test followed by Tukey’s post hoc analysis, were used to determine significant differences in the ARI scores between the groups. The level of significance was set at p <0.05. Weibull analysis (survival analysis) was employed for comparing SBS between the groups in order to predict the number of bonds likely to fail at a particular bond strength.

Results

This study analysed 120 surfaces in total, comprising of 40 teeth (Group-1), 40 composite (Group-2), and 40 PFM ceramic (Group-3) samples. Table 1 presents the inter-group comparison of mean SBS using one-way ANOVA test followed by Tukey’s post hoc analysis. Mean SBS was least in Group-1BM (tooth, AP primer, metal bracket) [6.313 MPa] and highest in Group-3BC (PFM, AP primer, ceramic bracket) [17.297 MPa]. The differences in mean SBS between the primers and brackets were statistically significant only in Group-3 (BC>BM>AC>AM, p<0.001), which showed higher SBS values compared to Group-1 and Group-2.

Table 2 presents the multiple inter-group comparisons of mean SBS using Tukey’s HSD post hoc analysis. For Group-3 (PFM crowns), Subgroup-AM (TXT primer, metal brackets) showed significantly lower mean SBS as compared to Subgroup-BM (AP primer, metal brackets) [p=0.004] and Subgroup-BC (AP primer, ceramic brackets) [p <0.001]. Similarly, for Group-3 (PFM crowns), Subgroup-AC (TXT primer, ceramic brackets) showed significantly lower mean SBS as compared to Subgroup-BM (AP primer, metal brackets) [p=0.005] and Subgroup-BC (AP primer, ceramic brackets) [p <0.001]. However, within the same primer subgroup, the mean SBS values showed no significant differences between metal and ceramic brackets (p >0.05).

Table 3 presents the inter-group comparison of mean ARI scores using one-way ANOVA test followed by Tukey’s post hoc analysis. Mean ARI score was lowest (1.4) in Groups-1AC, -2BM, and -3AC, while it was highest (1.8) in Groups-2AM and -2BC. However, the groups showed no significant differences within themselves for ARI scores. 

As per Weibull analysis (Table 4), the probability of failure (tested at clinically accepted SBS of 8 MPa) was highest in Group-1AC (90%) and lowest in Group-3BC (0%).

Discussion

Increasing prevalence of adult orthodontics necessitates improving bonding techniques to different intraoral surfaces (enamel, composite, amalgam, porcelain). Research focusing on newer materials minimizing bond failures is important since loose brackets are associated with loss of comfort, efficiency, time, and finances. Hence, the present study compared the shear bond strength (SBS) and adhesive remnants achieved with the new AP primer versus conventional TXT primer for bonding metal/ceramic brackets to three different surfaces (tooth enamel, composite, and PFM ceramic). The two primers showed no significant differences in mean SBS with enamel and composite and in mean ARI with any surface (p >0.05). However, with ceramic surface, AP primer showed a significantly higher SBS than TXT primer (BC>BM>AC>AM, p <0.001). AP primer also showed lower failure probability than TXT primer for all surfaces.

TXT was used as the conventional standard of comparison since it is already proven to possess the highest bond strength (15.49 MPa).⁷ However, the SBS values achieved with this primer were lower in the present study, likely because of the greater variety of surfaces used. Toodehzaeim et al., noted that the new AP primer provided suitable bond strengths when used for bonding orthodontic brackets to different intraoral restorative surfaces, a finding mirrored in the present research.¹

The results of this study also find support in the research work of Buyukcavus et al., who reported higher SBS in the AP group (15.05±3.72 MPa) compared to TXT group (7.52±2.89 MPa) (p <0.05), with no significant inter-group differences in the ARI scores (p >0.05).9 Naseh et al. too, compared these two primers and concluded that AP provided a higher SBS for a longer period of time compared to TXT and its bond strength was not affected by aging.10 They found that SBS of AP did not differ significantly from TXT when aged for 24 hours (p=0.280) or 3 months (p=0.114) but was significantly higher when aged for 6 months (p=0.000).10 Using Spearman correlation test, they also noted direct and significant correlation between SBS and ARI scores (r=0.834, p <0.001).¹⁰

In concordance with the current research, Jamal et al., also found that SBS of both the primers was greater on PFM crown surfaces than on teeth surfaces or acrylic crown surfaces.¹¹ SBS was seen to be highest with AP primer + ceramic brackets combination (corresponding to BC subgroup in the present study).¹¹

Even in case of rebonding of orthodontic brackets, Knaup et al., suggested that rebonding strength could be compensated using hydrophilic priming systems, with AP primer enhancing the SBS more than TXT primer at first bonding (AP 20.29±4.95 MPa vs. TXT 18.45±2.57 MPa) as well as at second bonding (AP 16.76±3.71 MPa vs. TXT 13.06±3.19 MPa).¹²

However, contrary to the present study, Sreeshna P et al., observed that TXT primer showed significantly greater SBS and higher ARI scores than the AP primer.¹³ Some conflicting results were also reported by Jamal et al., who found SBS to be lowest with AP primer + metal brackets combination (corresponding to BM subgroup in the present study).¹¹ Moreover, Schaneveldt et al., reported a higher frequency of enamel fractures at debonding with TXT primer (22.5%) than with AP primer (0%).¹⁴

Hence, AP primer provides a good balance between bond strength and residual adhesive, facilitating durable bonding with minimal damage debonding. Therefore, it can be recommended for multipurpose bonding to various surfaces like enamel, composite, and ceramic.

However, this research is limited in sample size and variety of primers and bonding materials tested. Further, large-scale studies are encouraged to explore better alternatives in this dynamic world of adult orthodontics.

Conclusion

The newly introduced Assure Plus primer showed significantly greater SBS than the conventional Transbond XT primer when used for bonding brackets to PFM crowns. Assure Plus primer also showed lower failure probability than Transbond XT primer on all the three surfaces (tooth, composite, PFM ceramic).

Ethics: The study protocol was approved by the Institutional Ethics Committee [Ref no: MRADC/ ECIRB/2015-16]. Patients were recruited into the study after obtaining their written informed consent.

Conflict of Interest

The authors declare no conflict of interest.

Source of funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Acknowledgements

Nil