Comparison of shear bond strength of metallic orthodontic brackets bonded to zirconia models underwent different surface conditioning methods and different primer systems

INTRODUCTION

Direct bonding in orthodontics has decreased enamel decalcification, improved gingival health, and made the placement of orthodontic appliances more comfortable for orthodontists and patients.^[1] Nowadays, patients seek esthetic dentistry, so ceramic, zirconia, E.max crowns, or other types of fixed partial prosthesis meet their needs. With the increase in demand for orthodontic treatment for adult patients, orthodontists are often challenged with the problem of bonding orthodontic brackets to different types of prosthesis.^[2] Based on this evidence, numerous methods to improve bracket bonding to such restoration have been suggested, like mechanical (diamond bur, abrasive discs, air-particle abrasion, or laser), chemical (orthophosphoric acid, hydrofluoric acid [HF], maleic acid, or silane), or combinations of both methods to alter the surface characteristics of porcelain to withstanding orthodontic forces and provide sufficient bond strength.^[3,4] HF acid etching is typically utilized to improve the bracket bonding to traditional ceramics.^[5] Quentin et al.^[6] concluded that 40% HF is the most appropriate concentration for conditioning zirconia at ambient temperature because it forms the fastest and most uniform etching. Air particle abrasion is a technique in which aluminum oxide (Al₂O₃) particles, generally 50 µm, are projected to create abrasion by high air pressure on the surface of ceramic or another fixed prosthesis.^[7] During brackets bonding, the use of primer is highly recommended; many commercial porcelain or zirconia primers are available that are used to treat glazed surfaces and provide a strong bond by increasing the wettability of the ceramic or zirconia surface for bonding of adhesive material.^[8] The manufacturer claims that Assure^® Plus is a recently introduced universal adhesive with high bond strength to normal enamel as well as to irregular metal surfaces such as gold, amalgam, stainless steel, ceramic, zirconia, and e.max pontics. Assure^® Plus can be polymerized by chemical curing, light-curing, and dual-curing systems.^[9-11] As already said, providing reliable bonding between the bracket and the surface of restoration is necessary. This connection should be strong enough to prevent bonding failure by orthodontic force or by masticatory force and to protect the integrity of restoration during the deboning of brackets at the end of orthodontic treatment.^[12] The objective of this study was to compare the shear bond strength (SBS) of metal brackets bonded to glazed zirconia models using the two surface conditioning methods and two different primer materials.

MATERIAL AND METHODS

The ethical approval with Ref. no. (UoM.Dent.23/33) for this research was obtained from the Research Ethics Committee of the College of Dentistry/Mosul University.

The sample

The investigated sample included 60 Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) glazed zirconia models; each model consisted of two parts [Figure 1]; the upper part of the model is a crown of upper left central incisor with a diameter determined according to Ash and Nelson^[13] and Sangalli et al.^[14] The cylindrical base with a diameter of 10 mm in height and a radius length of 10 mm. The sample size was calculated using sample size calculation formulas by Charan et al.^[15] and based on a study done by Mehta et al.^[16]

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Figure 1:

Zirconia models.

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Criteria of sample selection

The labial surface of models was examined by a stereomicroscope (Japan/Union/ME3138) under ×10 magnification power [Figure 2] to confirm that selected models were clear of any impurities, porosities, cracks, or irregularities.^[17]

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Figure 2:

Evaluation of zirconia models by stereomicroscope.

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Surface treatment procedure

For HF groups, the middle third of the labial surface of the models was treated with 9.6% HF acid for 1 min, then rinsed for 30 s and air-dried.^[18]

For Al₂O₃ groups: The models are fixed in a special design base to ensure standardization of distance and direction between microetcher (Ortho Technology, Emergo Europa) and model surface, as shown in [Figure 3]. Then, the middle third of the labial surface of models underwent air abrasion with 50 µm Al₂O₃ particles using the microetcher at a distance of 10 mm and in a direction perpendicular to the labial surface of specimens with the pressure of 0.25 Mps for 15 s. Then, the models were rinsed thoroughly under tap water to remove Al₂O₃ particles and then air dried.^[19]

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Figure 3:

Air abrasion of models.

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Bonding the brackets

For the groups of Transbond^™ XT orthodontic primer, a thin layer of adhesive primer (Transbond XT; 3 M Unitek^®, Monrovia, CA, USA) was applied and well-distributed on the center of the middle third of the labial surface of the crown of the model and left for 30 s and then dry with oil-free air for 10 s to remove excess Then, curing started using LED light curing device for 10 s according to manufactured instruction. For the Assure^® Plus groups, a single coat of the Assure^® Plus primer was applied and well distributed on the center of the middle third of the labial surface of the crown of the model and left for 2 min, then thoroughly dry for 3–5 s and then curing started using LED light curing device for 10 s. All these procedures followed the manufactured instructions.

In all groups, the adhesive paste Transbond^™ XT (Transbond XT; 3 M Unitek^®, Monrovia, CA, USA) was applied over the base of the upper central incisor bracket (standard Edgewise 0.022 inch slot metal bracket, Dentaurum). Subsequently, the brackets were positioned at the treated center of the labial surface of the crown of the model at a distance of 4 from the incisor edge. Boons gauge was used to ensure the correct bracket position [Figure 4]. After that, the model is insulted on a customized mold and transferred to the stage of the universal testing machine. A universal testing machine applied a load of 200 g at a bracket slot for 10 s to confirm uniform adhesive thickness^[20] [Figure 5]. A sharp dental explorer removed the excess resin. Then, the adhesive was photopolymerized using an LED light curing device with a wavelength of (420–480 nm) and an illumination of (1200–1500) mw/cm². The curing light was applied for 20 seconds for the mesial side and 20 seconds for the distal side; the tip of the curing device was at a distance of 2mm from the mesial and distal edges of the bracket base.^[21] The specimens were allowed to bench rest for 30 min and then placed in a sealed container containing distilled water and stored in an incubator at 37°C for 24 h before testing.^[22]

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Figure 4:

Bracket positioning.

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Figure 5:

A stable pressure is applied to the bracket.

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SBS measurement

The SBS test was measured using the universal testing machine (GESTER, Fujian, China) at the postgraduate laboratory, College of Dentistry, University of Mosul, with a crosshead speed of 0.5 mm/min. A prefabricated holder for the specimens has been constructed to ensure proper and secure seating of the specimen so that the bracket base is parallel to the direction of the shear force [Figure 6]. The chisel-shaped blade was directed toward the tooth-–bracket interface in an occlusal-gingival direction. The necessary load to debond or initiate bracket failure was recorded in the Newton unit and converted to the MPa unit by dividing the failure load or force in the Newton unit by the surface area of the bonded bracket base (mm²).

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Figure 6:

Shear bonding strength measurement by universal testing machine.

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Adhesive remnant index (ARI) measurement

After debonding of the brackets, the labial surface of the crown of the models was examined under Stereomicroscope at ×10 magnification power (Optica, Italy) to assess the amount of adhesive material left on the model’s surfaces. The criteria that were used for measuring ARI scores were as follows:^[10]

• Score 0 = No adhesive score remnant on the labial surface of the model.

• Score 1 = Less than half of the adhesive remained on the labial surface of the model.

• Score 2 = More than half of the adhesive remained on the labial surface of the model.

• Score 3 = All of the adhesive remained on the labial surface of the model, with a distinct impression of the bracket’s mesh [Figure 7].

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Figure 7:

Adhesive remnant index score 3.

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RESULTS

Descriptive statistics and one-way analysis (ANOVA) of the SBS values of each group is shown in [Table 1]. The Al₂O₃ with Assure^® Plus group had the highest mean value and revealed a significant difference between the means of SBS values of the groups at P ≤0.05. Using Duncan’s Multiple Range test for SBS [Table 2], significant discrepancies were detected in Assure^® Plus groups and Transbond^™ XT primer groups. The highest SBS was found in Al₂O₃ groups. The mean SBS for HF groups for Transbond^™ XT adhesive and Assure^® Plus was lower than that one’s obtained by Al₂O₃. Conversely, the control groups had the lowest mean SBS values [Table 2]. The independent t-test [Table 3] revealed that there was a significant difference at P ≤ 0.05 between Assure^® Plus and Transbond^™ XT primer type in Al₂O₃ groups, and there was no significant difference among other groups.

The distribution of the ARI scores among groups is illustrated in [Table 4]. The majority of the models in control and HF had scores of 0 and score 1 (all adhesive remained on the bracket base), while most of the samples of Assure^® Plus with Al₂O₃ groups had scores of 2 and 3 (all the adhesive remained on the zirconia surface). The Kruskal–Wallis test revealed significant differences in the ARI scores among the groups at P ≤ 0.05, as shown in [Table 5].

DISCUSSION

Esthetic restorations such as zirconia crowns are highly requested for adults, new challenges are presented, like the bonding of orthodontic braces to zirconia surfaces. The ideal property of bonding material must be high enough to withstand the orthodontic forces during treatment and also allow debonding of the brackets at the end of treatment to maintain the integrity of the zirconia surface.^[3,23]

In the present study, when the zirconia models were roughened by 9.6% HF, the SBS was 4.0182 MPa for Transbond^™ XT and 4.0989 MPa for Assure^® Plus, which is lower than 5.9 and 7.8 MPa which is the reasonable clinical bond strength values of SBS that stated by Reynolds et al.^[24] The use of HF acid was investigated by several previous studies and reported that the application of HA on zirconia surface is not sufficient to provide adequate adhesion, as zirconia has a low silica content, which makes it resistant to acid etching and difficult to create porosities.^[1,25] According to Faria et al.,^[26] the HF provides no effect on the zirconia surface but provides adequate adhesive strength on glass ceramics, and this is due to differences in the composition of ceramics materials, which produce distinct topographical features after etching.

While Quentin et al.^[6] found that the used of high concentration of HF 40% is appropriate for conditioning of zirconia specimens because it leads to uniform and fast etching. Furthermore, our result is in contrast with Zhang et al.^[5] who considered HF acid as a promising surface conditioning method to promote bracket-zirconia bonding without excessive zirconia damage. However, intraoral etching by HF can be dangerous and considered toxic. Hence, alternatives to HF can be used like orthophosphoric acid, sandblasting, and carbon dioxide laser.^[27,28]

Several researchers have sandblasted zirconia specimens with Al₂O₃ particles to provide higher mechanical retention^[9,10,16,29] by increasing the surface roughness of zirconia.^[30] Farag^[31] used Al₂O₃ particle sizes for sandblasting (40, 80, and 110 µm) and observed that the use of coarser Al₂O₃ particles led to an increase in surface irregularities and then increased the surface area available for adhesive, improving the micro-mechanical retention and finally increasing the bond strength values.

The effectiveness of sandblasting in increasing the SBS between the bonding materials and the zirconia specimens is similar to the study of Ourahmoune et al.^[32] They showed that air abrasion increases surface roughness and wettability of the zirconia materials, and the contact angle increases, increasing the mechano-retention and enhancing the bond strength.

Mehta et al.^[16], in their study, concluded that bonding brackets to sandblasted zirconia surfaces with Reliance Assure Plus resulted in higher SBSs than the retention between the orthodontic attachment and the adhesive.

Kwak et al.^[1] observed that when air abrasion was done with 30 µm Al₂O₃ on the glazed zirconia, producing a randomized rough surface and providing acceptable bonding of metal bracket to glazed zirconia.

The values of SBS obtained by Assure^® Plus and Transbond^™ XT with zirconia specimens did not differ significantly in the control and HF groups. In contrast, the SBS of orthodontic brackets bonded to zirconia using Assure^® Plus was significantly higher than those bonded by Transbond^™ XT for Al₂O₃ groups because of the higher flowability of Assure^® Plus, which provides adequate SBS. The same conclusions were concluded by Amirabadi et al.^[33], wherein the adhesion of the orthodontic bracket to ceramic that bonded by Assure^® Plus was significantly superior to that when bonded with Transbond^™ XT.

In contrast to these studies, Mehta et al.^[16] reported a similar bonding strength of Assure^® Plus and Transbond^™ XT for zirconia specimens. Furthermore, Douara et al.^[29] revealed no significant differences between the use of Transbond^™ XT and Assure^® Plus on zirconia specimens.

ARI was used to determine the position and mode of adhesive failure. Several studies have advocated that it is preferable for the occurrence of adhesion failure at the tooth adhesive interface so that the resin remnants on the surface can be cleaned safely with rotary instruments.^[30,34-36]. When debonding orthodontic brackets from the enamel surface, it is important to avoid enamel damage and with minimal adhesive remaining on the teeth surface. Likewise, for all restorations, the aim is for the debonded area to have minimal cohesive damage to ceramic or zirconia and, at the same time, have minimal residual adhesive left.^[16]

The adhesive failure in the control and HF group bonded by Transbond^™ XT or Assure^® Plus had a score of 0 and 1, which was designated to adhesivezirconia interface failure. Contrarily, most of the models in the Al₂O₃ group bonded with Assure^® Plus had a score of 2 and 3, which was designated to adhesivebracket interface failure, while in Al₂O₃ group bonded with Transbond^™ XT had a score of 1 and 2, which was designated as mixtype, indicating a favorable failure mode. This result suggests that the Transbond^™ XT is a suitable adhesive for use with zirconia material.

CONCLUSION

The SBS obtained when bonding metal orthodontic brackets using the Transbond^™ XT adhesive or Assure^® Plus with air abrasion by Al₂O₃ particles were satisfactory for zirconia restoration. On the other hand, inadequate SBS values were achieved when using HF treatment of zirconia surface, so acid etching of zirconia models by HF had no significant effect on SBS of metal brackets bonded to zirconia specimens. According to the ARI result, Transbond^™ XT is a suitable adhesive for use with zirconia material.