Shear Bond Strength of Orthodontic Brackets Bonded to Zirconium Crowns.

BACKGROUND: An increasing demand for esthetic restorations has resulted in an increased use of all-ceramic restorations, such as zirconium. However, one of the challenges the orthodontist must be willing to face is how to increase bond strength between the brackets and various ceramic restorations.Bond strength can beaffected bybracket type, by the material that bracketsaremade of, and their base surface design or retention mode. ​: Aim: of this study was to perform a comparative analysis of the shear bond strength (SBS) of metallic and ceramic orthodontic brackets bonded to all-zirconium ceramic surfaces used for prosthetic restorations, and also to evaluate the fracture mode of these two types of orthodontic brackets.
MATERIAL AND METHODS: Twenty samples/semi-crowns of all-zirconium ceramic, on which orthodontic brackets were bonded, 10 metallic and 10 ceramic polycrystalline brackets, were prepared for this research. SBS has been testedby Universal Testing Machine, with a load applied using a knife edged rod moving at a fixed rate of 1 mm/min, until failure occurred. The force required to debond the brackets was recorded in Newton, then SBS was calculated to MPa. In addition, the samples were analyzed using a digital camera magnifier to determine Adhesive Remnant Index (ARI). Statistical data were processed using t-test, and the level of significance was set at α = 0.05.
RESULTS: Higher shear bond strength values were observed in metallic brackets bonded to zirconium crowns compared tothoseof ceramic brackets, with a significant difference. During the test, two of the ceramic brackets were partially or totally damaged.
CONCLUSION: Metallic brackets, compared to ceramic polycrystalline brackets, seemed tocreate stronger adhesion with all-zirconium surfaces due to their better retention mode. Also, ceramic brackets showed higher fragility during debonding.

Introduction

The demand for esthetic restorations has resulted in an increased use of all-ceramic restorations. Today, various types of all-ceramic crowns are in use such as zirconium crowns. ()As the number of adults seeking orthodontic treatment is increasing, clinicians often have to bond orthodontic brackets to teeth that have different types of porcelain restorations. As a result, there is a higher degree of failure compared to bonding to enamel, and different challengesin adult orthodontics. (, ). Bond strength is mostly affected by the porcelain type, surface conditioning, bracket material and retention mode, properties of the bonding adhesive, the light-curing source, as well as the skill of the clinician. (, )

After the huge development of the CAD/CAM technology, zirconiumhas become one of the most interesting materials to be examined and used in almost entire dental field. (, )Zirconium is a widely used core for all-ceramic crowns due to its high strength and esthetic appearance. This is accomplished when a veneer is layered with ceramic powder onto the zirconium core. Since fracturing of the veneer is frequently reported in the posterior teeth, because of the strong masticator forces (, ),anincreased use of monolithic zirconium crowns without veneersis encouraged. (, ) In orthodontic patients with monolithic zirconium crowns, the orthodontic bracket should be bonded directly onto the zirconium surface ().

Various mechanical or chemical methods other than etching protocol on the enamel must be followed in different types of ceramics, as well as in zirconium, in order to avoid difficulties in treating adults with fixed orthodontic appliances.It has been recommended that methods that provide proper bond strengthwith less roughening should be used in order to avoid micro-cracks of ceramic surfaces (-). Thereis an extreme danger in handling with hydrofluoric acid (HFA), which should be considered, because it can be very noxious to gums and other soft tissues andit can quickly destroy the corneas of the eyes (, ).

Another factor affecting the bond strength can be the material that brackets are made of, and their base surface design or retention mode. According to various studies, the bond strength of ceramic brackets seems to be higher than that of metallic brackets because of the stronger adhesion they obtain. Another reason for the higher bond strength of ceramic brackets can be due to their light transmittance allowing better photo-polymerization and reduced stresses in adhesive-bracket interface. (, -)Consequently, the question arises which brackets are more adequate for bonding them to full-zirconium ceramic surfaces, taking into consideration that few studies have examined the bond strength between orthodontic brackets and zirconium prostheses, and the effects of bracket material and their retention mode.

The objective of this study was to perform a comparative analysis of the shear bond strength (SBS) of metallic and ceramic orthodontic brackets bonded to all-zirconium ceramic surfaces used for prosthetic restorations, in order to examine the effect of the material that brackets are made of and their retention mode or base surface design. Also, ouraim was to evaluate the fracture mode of these two types of orthodontic brackets.

Material and methods

Twenty samples/semi-crowns of all-zirconium ceramic (Copran Zr-i Monolith, White Peaks Dental Solutions GmbH&Co.KG, Wesel, Essen, Germany), on which orthodontic brackets were bonded, 10 metallic (Mini 2000 Ormco Corp., Glendora, California, USA) and 10 ceramic polycrystalline brackets (Glam Forestadent, Bernhard Forster GmbH, Pforzheim, Germany) were prepared for this research.

In order not to affect the effectiveness of the base surface design or retention mode of orthodontic brackets (Figure 1), the bonding protocol was simplified. No mechanical roughening or hydrofluoric acid was applied, neither silane nor other primers. Bonding was conducted only after application of phosphoric acid for 120 seconds. All brackets were bonded by the same operatorwith a two-component (primer and adhesive)composite resin-based bonding system (Tranbond XT, 3M/Unitek, Monrovia, CA, USA). The adhesive was light cured for 40s, using light-emitting diode(Ledition, Ivoclar Vivadent AG, Schaan, Lichtenstein). After polymerization, the specimens were put in water bath for 24 hours.

Figure 1

Base surface design or retention mode of orthodontic brackets: a) metallic and b) ceramic

As presented in Figure 2, SBS has been tested using Universal Testing Machine (Erichsen 0-2000 N, ISO 7500-1:1, AM Erichsen GmbH&Co.KG, Hemer-Sundwig, Germany), with a load applied parallel to the buccal surface of the restoration, using knife edged rod moving at fixed rate of 1 mm/min, until failure occurred. The force required to debond the brackets was recorded in Newton, subsequently, SBS was calculated to MPa.

Figure 2

SBS testing of orthodontic brackets bonded to zirconium

In order to evaluate the type of bond failure at the bracket-adhesive interface in each test group, the samples were analyzed using a Digital Microscope (Dino-Lite, ANMO Electronics Corp., Taipei City, Taiwan), to determine Adhesive Remnant Index (, ). The measurements were conducted, using scores from 1 to 5:

1 - All adhesive remaining on the ceramic crown surface with the impression of the bracket base;

2 - More than 90% of the adhesive remaining on the ceramic crown surface;

3 - Less than 90%, but more than 10% of the adhesive remaining on the surface;

4 - Less than 10% of the adhesive remaining on the ceramic crown surface;

5 - No adhesive remaining on the ceramic crown surface.

This research was conducted as a pilot studyat the School of Dental Medicine and at the Faculty of Mechanical Engineering and Naval Architecture, Laboratory for testing mechanical properties, University of Zagreb, Croatia.

Statistical analysis: The Kolmogorov-Smirnov test was applied to ascertain that the data had a normal distribution of SBS. The hypothesis of equality of the average of SBS by the type of braces was tested by t-test for independent samples (Independent Samples Test). Because of the small sample, the Kolmogorov-Smirnov test tends to accept the hypothesis of normality, test of the hypothesis of equality of the average of SBS by the type of braces has also been processed by applying the Mann-Whitney test for independent samples (Mann-Whitney test). The level of significance was set at α = 0.05. Data processing was carried out using the software package Statistica 10.

Results

The results of the study showed that numerical values of the force necessary to debond metallic brackets (sum of 10 tests = 707,97N) of the zirconium crowns werehigher than those of ceramic brackets (sum of 10 tests = 597,70N), with a significant difference.

The estimates of parameters of SBS by type brackets are listed in Table 1 and shown in Figure 3 and 4. For metal brackets, SBS is in the range from 3.26 to 13.90, with a mean of 7.35 and standard deviation of 3:41 MPa. For ceramic brackets, these values are respectively 2:34 to 7:15, 4.66 and 1.78 MPa.

Table 1

Descriptive statistics of SBS by the type of bracket (MPa)

Type of bracket	N	Mean	Standard Deviation	Minimum	Maximum
Metalic	10	7.35	3.41	3.26	13.90
Ceramic	10	4.66	1.78	2.34	7.15

Figure 3

Means, standard errors (SE) and 95% confidence interval SBS-a bythe type of bracket

Figure 4

Median, interquartile range and range of SBS-a by the type of bracket

According to Kolmogorov-Smirnov test, theshear bond strength (SBS) is in normal distribution (Kolmogorov-Smirnov Z = 0.898, p = 0.395). The Levene's test for equality of variances (F = 1.73, p = 0.205) confirmed the homogeneity of variances for the SBS of metallic and ceramic brackets, which is a prerequisite to test the hypothesis for equality of meansfor the SBS according to type of bracket. The result of t-test for independent samples (t-test for equality of means) allows acceptance of the alternative hypothesis (t = 2.22, df = 18, p = 0.040), which in turn confirms the significantly higher value for the SBS of metallic brackets then of ceramic brackets. The acceptance of the normality of distribution for the SBS according to the Kolmogorov-Smirnov test can be queried. Testing of the hypothesis for equality of means for the SBS according to the type of brackets, was confirmed with the nonparametric Mann-Whitney test and the results (U = 25.00, p = 0.059) also confirmed the results of t-test, but with 0.9% higher than the required error of 5%. In addition, practically insignificant deviation from the stated mistake, it is possible to argue that SBS of metallic brackets is significantly higher thanthat ofceramic polycrystalline brackets.

ARI distribution by type of brackets is shown in Figure 5, as it isevidentthat distributions are almost identical: the difference is only in the 4th category and for one case in the 5th category

Figure 5

ARI distributions by type of brackets

During the test, two of the ceramic brackets were partially or totally damaged.

Discussion

According to numerous studies, the bond strength of ceramic brackets seems to be higher compared to the strength of metallic brackets due to a stronger adhesion to ceramics and light transmission, which leads to a higher degree of polymerization and stress reduction on the adhesive-bracket joint. (, ) However, our study shows that this is not the case for orthodontic brackets bonded to zirconium ceramic crowns. It seems that mechanical coupling is greater than chemical coupling of the brackets with zircon ceramic surface, and the base surface design or retention mode of orthodontic brackets plays a determinant role in their bond strength.

In this research, ARI scores indicated that, in both groups, there was a combined frequency of bond failure at the bracket-adhesive interface and at the adhesive-ceramic interface. These results are similar to other reported findings (, ).

The evaluation of the fracture mode of the two types of orthodontic brackets is in accordance with other studies, which confirmed the fact that ceramic brackets show higher fragility during debonding.

In addition, it should be considered that there are limitations of in vitro studies, and that there can be differences among in vivo and in vitro results due to the complexity of the oral cavity. Further research using different combinations of influencing factors is needed.

Conclusion

According to the results obtained from this research, we conclude that metallic brackets compared with ceramic polycrystalline brackets, seem to create stronger adhesion with all-zirconium surfaces due to their better base surface design or retention mode. Also, ceramic brackets show higher fragility during debonding.