The effect of surface treatment and thermocycling on the shear bond strength of orthodontic brackets to the Y-TZP zirconia ceramics: A systematic review

ABSTRACT Introduction: Various surface pre-treatment methods have been adapted to optimize the bonding between the zirconia ceramics and the orthodontic brackets. Objective: This review is aimed at systematically analyzing the relevant data available in the literature, to find out the most effective and durable bonding protocol. Methods: Database search was conducted in PubMed, Scopus, and ScienceDirect, during September 2020. The review was conducted according to the PRISMA guidelines. Results: Based on the inclusion criteria, 19 articles were selected for qualitative analysis. Meta-analysis could not be performed due to the heterogeneity of the methodology among the studies. Most of the studies scored medium risk of bias. Compared to the untreated surface, surface pretreatments like sandblasting and lasers were advantageous. Primers and universal adhesive were mostly used as an adjunct to the mechanical pretreatment of the zirconia surface. In most studies, thermocycling seemed to lower the shear bond strength (SBS) of the orthodontic brackets. Conclusion: Based on this qualitative review, surface pretreatments with lasers and sandblasting can be suggested to optimize the bracket bond strength. To clarify this finding, meta-analysis is anticipated. Hence, high heterogeneity of the included studies demands standardization of the methodology.


INTRODUCTION
With the increasing demand for esthetics and the introduction of the CAD/CAM devices, zirconia ceramics have gained much popularity in modern Dentistry. Zirconia claimed superiority in terms of mechanical properties, biocompatibility, precision and esthetics. 1,2 The yttrium-stabilized tetragonal zirconia polycrystal (Y-TZP) is the form most commonly used in Dentistry. The material is known for the phase transformation from tetragonal to monoclinic due to stressful conditions, achieving fracture resistance. 3 It has been used extensively to make inlays, onlays, crowns, post and core systems, and frameworks for porcelain fused to zirconia restorations. 4 Hence, with the increasing use of zirconia restorative materials, it is becoming more common for orthodontists to bond orthodontic brackets to a Y-TZP surface. However, due to the chemical inertness and resistance to acid-etching, bonding composite resin to the zirconia surface can be challenging. Thus, various surface treatment methods -such as mechanical, chemical, or combined -have been introduced to improve the bond strength of zirconia ceramics.
The in-vitro shear bond strength (SBS) is the most commonly used test, due to its simplicity and resemblance to the shear force exerted during mastication. In addition, aiming at testing Ahmed T, Fareen N, Alam MK -The effect of surface treatment and thermocycling on the shear bond strength of orthodontic brackets to the Y-TZP zirconia ceramics: A systematic review 5 the durability of these methods, the material can be exposed to water at a certain temperature, simulating an oral environment for a determined period - a procedure known as thermocycling.
Previously, reviews were mostly based on the bond strength between the luting cement and the zirconium surface; overlooking the impact of orthodontic brackets. [5][6][7] Therefore, this study is aimed to perform a systematic review of the in-vitro studies that reported the effect of different surface conditioning and aging on the shear bond strength of orthodontic brackets to the Y-TZP zirconia ceramics.

SEARCH STRATEGY
The review was conducted by following the Preferred Reporting   6 The electronic databases PubMed, Scopus, and ScienceDirect were searched, using the following keywords: orthodontic brackets, shear bond strength, zirconium, ceramic surface, and thermocycling. Additionally, Google Scholar was searched with the same keywords. The references of the selected studies were also checked for any missing relevant studies. All searches were performed in September 2020. The search was confined to the original articles written in English language, published between the years 2001 to 2020. All the duplicates were resolved by the Endnote X7 software (Thompson Reuters, Philadelphia, PA, USA). The search strategy is summarized in Table 1.

STUDY SELECTION
Two independent reviewers screened the titles and abstracts of all the studies found. The articles were selected according to the following inclusion and exclusion criteria:

INCLUSION CRITERIA
1) Studies that bonded orthodontic brackets to Y-TZP surface.
2) Studies that conditioned the surface before bonding.        reported by the articles was ticked with "Y" (yes) for the particular section. In case any parameter was missing, was marked with an "N" (no). Articles reporting only one (1) to three (3) of the items were considered as having a high risk of bias; four (4) to five (5) items, as medium risk of bias; and six (6) to seven (7) items, as low risk of bias. 9,10 Again, the interexaminer agreement was analyzed by Cohen's kappa statistics, and the third reviewer's opinion was requested in any event of disagreement.

LITERATURE SEARCH
A total of 109 studies was identified (Table 1). After the removal of the duplicates, 91 articles remained. Upon careful screening of the titles and abstracts, 48 articles were further excluded. The kappa value for interexaminers agreement was k = 0.816 (p < 0.001). Forty three full-text articles were screened thoroughly for eligibility and 24 articles were excluded for valid reasons (Supplementary Table 1 ). Finally, 19 articles were selected for the review. The PRISMA flow diagram of the study selection procedure is presented in Figure 1.  For the qualitative analysis, the inter-reviewers reliability of the extracted data was confirmed (k = 0.89, p < 0.001). Three types of brackets were studied. Metallic brackets were the most common; 11 2,14,20 and two studies combined zirconia primer. 22,27 Lasers were used alone in four studies; 12,[16][17][18] and as an adjunct with MDP primer and silane primer in one study each. 18,25 One study also combined femtosecond laser and sandblasting to treat Y-TZP surface. 17 Variability was observed in laser settings like power output (40 mW to 3 W); mean energy settings (50mJ to 300mJ), distance (60μm to 10mm), and the application time (5 seconds to 2 minutes). Except for two studies, 18,28 no study was found to treat the Y-TZP surface solely with primers before orthodontic bonding. The trade names and chemical composition of the primers used to treat Y-TZP surface are listed in Table 3.  Two studies did not thermocycled their samples. 16,17 Different protocols of thermocycling were observed between the studies;

RISK OF BIAS
Based on the criteria applied for the quality assessment of the selected studies; thirteen (13) studies scored medium risk of bias, three (3) studies scored low risk of bias, and the remaining three (3) studies scored high risk of bias. Sample size calculation was surprisingly absent (except for one selected study).
Half of the studies had no control groups and surface treatment. Bonding was done by the same operator in only 47.4% of cases. The frequency of the rest of the parameters was sufficient (Fig 2). Maxillary central incisor metallic bracket was mostly studied. 12,14,[16][17][18]20,25 On reviewing the studies that compared the On 37% phosphoric acid etching, metallic brackets exhibited significantly better SBS than ceramic brackets. 24 In a study, 37% phosphoric acid was combined with silane primer to enhance the SBS of the ceramic brackets; but the result was not significant. 26 In comparison to phosphoric acid etching, the hydrofluoric acid etching did not improve the SBS in both metallic and ceramic brackets. 26  Mechanical surface treatments such as sandblasting and lasers also have been studied. 12,[16][17][18]25,29 Lasers produce surface roughness by a process called ablation, which involves micro explosions and vaporization. 6 Both Er:YAG and Nd:YAG laser irradiation of zirconia surface resulted in inferior SBS, compared to sandblasting. 12,15,18,25 The Er:YAG lasers alone failed to achieve Reynolds's optimal SBS range of 5.9 - 7.8 MPa. 12,18,30 Yet, in combination with silane primer, the SBS was raised to 6.9 MPa. 25 The additional chemical reaction and wetting ability of the silane may have contributed to this finding. Besides, at high power output (above 200 mJ) the laser generates high heat, which can be detrimental to the surrounding zirconia surface. 31 The femtosecond lasers, at 200mW power output and 60μm inter-groove distance, generated SBS (5.68 MPa) closer to the optimal level. 15 In separate studies, the femtosecond laser-treated zirconia surface had better SBS compared to both alumina (Al 2 O 3 ) and silica (SiO 2 ) coated sandblasting. 16,17 Better SBS was achieved when the femtosecond laser was combined with sandblasting, but not significantly greater than the femtosecond laser alone. 17 Thereby, additional surface preparation as an adjunct to femtosecond laser can be avoided to save time, cost and patient discomfort. Besides, the laser has no reports of thermal damage due to surface irradiation. 32 Ahmed T, Fareen N, Alam MK -The effect of surface treatment and thermocycling on the shear bond strength of orthodontic brackets to the Y-TZP zirconia ceramics: A systematic review 24 Following thermocycling, there was a noticeable reduction in SBS irrespective of the surface pre-treatment methods. 2,14,19,20 Biodegradation of the treated Y-TZP surface, bonded brackets, and adhesives may have contributed to this evidence.
Hence, the assessment of the bond strength in a simulated clinical environment (i.e., in-vivo experiment) is necessary.
Thermocycling is an artificial aging procedure to test the longterm effect of bond strength. According to a study, 10,000 cycles of thermocycling are equivalent to one year of usage in the oral cavity. 33 Application of MDP and MDP containing silane primers resulted in durable and optimal SBS on both forms of sandblasted Y-TZP, even after 10,000 cycles of thermocycling. 2,20 Conversely, in the case of universal adhesives, the SBS was stable up to 2,000 cycles, but degraded significantly after 10,000 cycles. 2,20,27 Among lasers, the effect of thermocycling on the Er:YAG and Nd:YAG laser irradiated surfaces were tested.
The SBS on Er:YAG laser-treated surface was found to be just clinically acceptable within the range of 500-2,000 cycles. 12,25 In the case of Nd:YAG laser, the SBS was below an acceptable level after thermocycling. 12 Surprisingly, the effect of thermocycling on the femtosecond laser irradiation could not be found. The validation risk of the bias tool utilized in this study could not be confirmed. This fact should be regarded as an important limitation of the study, but the contents of this quality assessment tool seemed to be more justifiable and relevant to the methodology of the selected studies. The absence of meta-analysis is another limitation reflecting the heterogeneity of the studies.

CONCLUSION
As this review is solely based on the qualitative analysis of the laboratory-based in-vitro findings, the results should be interpreted with caution. To answer the research question of this review more precisely, quantitative analysis is deemed necessary. Therefore, standardization of the study protocol is necessary. However, certain points may be advised, in light of this comprehensive review: a) Hydrofluoric acid etching can be avoided to treat the Y-TZP surface, as it did not remarkably improve the SBS, considering the damage to the ceramic surface. e) Use of primers, particularly the MDP and the MDP containing silane primers as an adjunct to the mechanical pretreatments may be justified. There is a concern over the longevity of the universal adhesive.