INTRODUCTION

In clinical practice the orthodontist may have to bond brackets to porcelain crowns. This procedure has been considered a challenging task. The traditional technique of bracket bonding to enamel surfaces is not sufficient to provide adequate bond strength to porcelain surfaces because of the physical properties of glazed surfaces and the chemical properties of bonding resins.1

In order to improve bond strength, several protocols were addressed in the literature which aimed to alter the porcelain surfaces either mechanically or chemically. Mechanical alteration included the use of diamond burs,2 green stones,3 abrasive disks,4 sandblasting,5 and laser irradiation.1 On the other hand, chemical alteration of the porcelain surface could be performed by either etching with strong acid, such as hydrofluoric acid,6,7 or with the application of a silane coupling agent.7–10 Silane forms a chemical bond with both the porcelain and the resin, forming a bridge between the two materials.11–14 Self-etching primers and 4-META resins have also been described7 for porcelain surface preparation. An alternative technique for increasing bond strength could be inferred from restorative dentistry. A solvent-free, resin-based primer (Embrace First-Coat, EFC) was introduced and used in porcelain repair. This primer eliminates the use of silane coupling agent. However, insufficient information was found regarding its effect on bond strength.

In clinical situations brackets could be subjected to cyclic stresses caused by mastication, occlusion, and orthodontic appliances.15–17 Although these cyclic stresses could be of lower magnitude than the static bond strength of the bracket, they could lead to failure of the bracket during the treatment period, a condition defined as fatigue. Fatigue is the phenomenon through which failure is induced by subjecting the material or structure to repeated subcritical loads.18,19 Since it is very important to simulate the oral environment condition in the in vitro bond strength studies, it is advantageous to evaluate the effects of cyclic loading on bond strength. Therefore, the present study was conducted to evaluate and compare the cyclic shear bond strengths (CSBSs) and static shear bond strengths (SSBSs) of metal orthodontic brackets bonded to porcelain surfaces using different conditioning protocols.

MATERIALS AND METHODS

A total of 100 feldspathic porcelain disks (Vita, Bad Säckingen, Germany), 2 mm thick and 8 mm in diameter, were fabricated. The specimens were divided equally into four groups with 25 disks each. Lower incisor standard twin edgewise metal orthodontic brackets (American Orthodontics, Sheboygan, Wisc) with an average base area of 9 mm² were used in this study. Porcelain surfaces were conditioned by etching with 9.6% hydrofluoric acid (HFA; Porcelain Etch Gel, Pulpdent Co, Watertown, Mass) or by sandblasting with aluminum oxide abrasive powder. Then, either Silane Bond Enhancer (Pulpdent Co) or EFC Primer (Pulpdent Co) was used. The brackets were bonded to conditioned porcelain surfaces using Transbond XT adhesive paste (3M Unitek, Monrovia, Calif). A description of the adhesive systems used in this study is presented in Table 1.

Table 1 Materials Used in the Study

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CSBS Testing

CSBS testing was carried out for 15 specimens from each group. Each sample was mounted in the testing machine (LRX-plus; Lloyd Instruments Ltd), as previously mentioned in the evaluation of SSBS. The samples underwent cyclic loading by means of a monobeveled steel chisel that was attached to the upper movable compartment of the machine. The load was applied at the base of bracket in the occlusogingival direction. The load profile was in the form of a sine wave at a rate of 1 Hz. Compressive shear fatigue test for 5000 load cycles were determined by testing according to the staircase (up-and-down) method.18 The first specimen was tested at 3 MPa stress, which is considered half of the lower limit of the clinically acceptable bond strength.20,21 Then, in each succeeding test the load was increased or decreased by a fixed amount of 0.6 MPa (10% of the lower limit of the clinically acceptable bond strength) according to whether the previous load resulted in a failure or no failure. Then the mean (X) and standard deviation (SD) of CSBS of each group were calculated from the following equations:

where X0  =  the lowest stress level considered in the analysis; d  =  the stress increment; N  =  the sum of the number of failures; A  =  ∑ in_i (the sum of in_i); i  =  the number of stress level at which failure occurs (the lowest stress level at which a failure occurs is denoted by i  =  0, the next at i  =  1, etc); n_i  =  the number of failures that occurs at each stress level; and B  =  ∑ i²n_i (the sum of i²n_i). In X formula, the positive sign is used when the analysis is based on nonfailures, and the negative sign is used when failures are considered.

The mode of bond failure was assessed according to the amount of adhesive left on the porcelain surface utilizing the adhesive remnant index (ARI). The ARI ranges from 0 (no adhesive left on the porcelain surface) to 3 (all adhesive left on the porcelain surface). Less than 50% of the adhesive left on porcelain surface yields a score of 1, while more than 50% of the adhesive left on the porcelain surface yields a score of 2.

RESULTS

The means and SDs of the SSBS and CSBS and the results of the LSD post hoc and t-tests are provided in Table 2. A graphical presentation of these values is shown in Figure 1. In general, there were significant differences in SSBS and CSBS between all studied groups (P < .0001).

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Table 2 Mean Static and Cyclic Shear Bond Strengths, Standard Deviations, and Results of Least Significant Difference (LSD) Post Hoc and t-Tests of the Studied Groups^a

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EFC and silane exhibited comparable SSBS values after either sandblasting or etching with HFA (P > .05). Sandblasting prior to application of either primer significantly increased SSBS values (P < .05). However, no significant difference was found in the bond strengths utilizing either HFA and EFC or sandblasting and silane (P > .05).

With regard to CSBS, sandblasting and EFC revealed the highest significant CSBS value, followed by sandblasting and silane (P < .05). On the other hand, etching with HFA prior to application of either EFC or silane yielded the lowest CSBS values (P < .05); however, no significant difference was found between them (P > .05). In addition, the results of the t-test showed a highly significant difference between the SSBS and CSBS values (P < .001).

The frequency distribution of ARI scores of both EFC and silane after etching or sandblasting are presented in Table 3. In general, failure was mainly adhesive in all groups. The results of the chi-square test indicated that there were no statistically significant differences between the studied groups of SSBS and CSBS tests (χ²  =  4.57, P  =  .6001; and χ²  =  1.88, P  =  .930).

Table 3 Frequency Distribution and Results of Chi-Square Analysis of Adhesive Remnant Index (ARI) of the Studied Groups^a

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DISCUSSION

The present study was conducted to evaluate the bond strengths of metal orthodontic brackets bonded to porcelain surfaces using either EFC or silane after etching with HFA or sandblasting. Since it is advantageous to simulate the oral environment condition in the in vitro bond strength studies, both CSBSs and SSBSs were evaluated. In addition, bond strengths were assessed after thermocycling.

The results of the current study reveal that EFC provided higher, but not significant, SSBS values compared to silane after etching with HFA or sandblasting. On the other hand, sandblasting of porcelain surfaces significantly enhanced the SSBS. This finding was in agreement with those of other studies.2,7 This could be explained by the fact that sandblasting increased the surface roughness more than did etching with HFA, which subsequently improved the mechanical retention of the adhesives. Unfortunately, mechanical alteration of porcelain surfaces leads to irreversible damage to porcelain glaze and could result in a higher incidence of porcelain fracture during debonding.22,23 However, sandblasting with aluminum oxide particles may be safer than utilizing burs or stones, since the procedure is more uniform and less aggressive.7 In spite of the positive effect of sandblasting on SSBS, the use of HFA with EFC provided nonsignificant SSBS values in comparison to utilization of sandblasting and silane. This finding could be attributed to the fact that EFC may have had a stronger potential to wet the porcelain surface and flow into the pores of the conditioned porcelain, compared with silane. In addition, EFC is a solvent-free primer, while silane contains solvent (organic solvents). If residual solvent was left behind, it may have negatively affected bond strength. Ikeda et al.24 reported that complete evaporation of solvents is difficult to achieve, even with thorough air-drying. However, HFA should be utilized with great care to avoid soft tissue trauma, as it is very corrosive.6,7,13

CSBS provides more realistic and valuable information (compared to SSBS) about the material's long-term performance in the clinical situation. In other words, CSBS could be used as a predictive indicator for its behavior in the oral environment.16–19 In the present study, the staircase method was used to determine CSBS. In this method the data are concentrated around the mean stress; hence, the number of specimens is smaller than required with other techniques, and subsequently this method is less time consuming.18 In spite of the relatively low magnitude of the cyclic loads, this method could lead to microcracks and structural failure, a phenomena commonly known as fatigue.18,19 Several factors could affect fatigue, such as stress concentration, corrosion, temperature, overload, microstructure, and residual stresses.25 In the current study, utilization of either EFC or silane after etching provided comparable CSBS values.

Accordingly, the teeth could withstand the stresses of mastication and orthodontic appliances (fatigue resistance) at the same level. Sandblasting of porcelain surfaces significantly improved the CSBS. Furthermore, the use of EFC after sandblasting provided significantly higher CSBS values compared to silane. Hence, this bonding protocol could provide more positive bracket durability in the oral environment. This could be explained by the fact that EFC formed a stronger chemical bond with both the porcelain and the resin, with less microcrack formation and structural failure than is associated with silane when it is subjected to cyclic stresses. This may be related to the differences in composition and bonding affinity between them. EFC is a light-cured resin with no bisphenol A. On the other hand, silane is an ethyl alcohol preparation with other organic solvents. In addition, the possibility of the presence of defects as a result of residual solvent with the use of silane may cause stress concentration and hence affect the CSBS.26

In addition, the results of the present study revealed that CSBS values were significantly lower than SSBS values (P < .001). This finding was in harmony with those of other studies.15,16,19 This could explain why bracket failure occurs in the oral environment when teeth are subjected to forces of low magnitude. Andreasen and Stieg26 reported a significant decrease (48% to 52%) in in vivo shear bond strength when compared with in vitro bond strength. They attributed that decrease to mechanical and masticatory stresses affecting the bonds in the oral environment in addition to other factors, such as moisture contamination during bonding, intraoral thermal fluctuation, and the constant bathing effects of saliva. Therefore, it is advantageous to evaluate the effect of cyclic loading on the bond strength of the adhesive systems utilized in orthodontic practice.

The ARI scores showed that bond failure predominantly occurred between the porcelain and the adhesive, as the majority of the adhesive remained on the bracket bases in all groups. These results were in harmony with those of other authours.13 However, no significant differences were found in ARI scores in all groups using either EFC or silane with or without sandblasting.

CONCLUSIONS

• Both EFC and silane showed comparable SSBS and CSBS.

• Sandblasting with aluminum oxide abrasive powder provided higher bond strengths than did etching with HFA.

• The use of EFC after sandblasting exhibited the highest CSBS.

• Cyclic loading significantly decreased bond strength.