The European Journal of Orthodontics Advance Access originally published online on June 13, 2006
The European Journal of Orthodontics 2006 28(5):450-456; doi:10.1093/ejo/cjl010
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Effects of surface conditioning on bond strength of metal brackets to all-ceramic surfaces
inasi Saraç**
-Türk*
* Departments of Orthodontics, University of Ondokuz May
s, Turkey
** Departments of Prosthodontics, Faculty of Dentistry, University of Ondokuz Mays, Turkey
Address for correspondence Tamer Türk, Department of Orthodontics, Faculty of Dentistry, University of Ondokuz Mays, 55139 Kurupelit-Samsun, Turkey. E-mail: turkset{at}superonline.com
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Summary |
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The aim of this study was to determine the effectiveness of bonding brackets to ceramic restorations. Sixty feldspathic and 60 lithium disilicate ceramic specimens were randomly divided into six groups. Shear bond strength (SBS) and bond failure types were examined with six surface-conditioning methods: silane application to glazed surface, air particle abrasion (APA) with 25- and 50-µm aluminium trioxide (Al2O3), etching with 9.6 per cent hydrofluoric acid (HFA), and roughening with 40- and 63-µm diamond burs. Silane was applied to all roughened surfaces. Metal brackets were bonded with light cure composite, then stored in distilled water for 1 week and thermocycled (x500 at 5-55°C for 30 seconds). The ceramic surfaces were examined with a stereomicroscope at a magnification of x10 to determine the amount of composite resin remaining using the adhesive remnant index.
The lowest SBS values were obtained with HFA for feldspathic (5.39 MPa) and lithium disilicate (11.11 MPa) ceramics; these values were significantly different from those of the other groups. The highest SBS values were found with 63-µm diamond burs for feldspathic (26.38 MPa) and lithium disilicate (28.20 MPa) ceramics, and were not significantly different from 40-µm diamond burs for feldspathic and lithium disilicate ceramics (26.04 and 24.26 MPa, respectively). Roughening with 25- and 50-µm Al2O3 particles showed modest SBS for lithium disilicate (22.60 and 26.15 MPa, respectively) and for feldspathic ceramics (17.90 and 14.66 MPa, respectively). Adhesive failures between the ceramic and composite resin were noted in all groups. Damage to the porcelain surfaces was not observed.
The SBS values were above the optimal range, except for feldspathic ceramic treated with HFA and silane. With all surface-conditioning methods, lithium disilicate ceramic displayed higher SBS than feldspathic ceramic.
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Introduction |
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TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionReferences |
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In recent years there has been an increase in the number of adults seeking orthodontic treatment. Therefore, the orthodontist is often faced with the challenge of effectively bonding orthodontic brackets to ceramic restorations in adult patients.
All-ceramic dental materials are gaining popularity due to their superior biocompatibility and aesthetic appeal (Albakry et al., 2004
). Furthermore, all-ceramic materials demonstrate a great deal of diversity due to recent advances in restorative material technology (Wen et al., 1999; Guazzato et al., 2002).
Pre-treatment of ceramic surfaces is necessary to obtain sufficient strength to bond orthodontic brackets to all-ceramic restorations. Several options have been described which are generally combinations of various mechanical and chemical conditioning methods, such as bonding to glazed ceramic with a coupling agent (silane), deglazing the ceramic by roughening the surface [diamond burs; air particle abrasion (APA) with aluminium oxide], and chemical preparation of the ceramic with acids, such as phosphoric, hydrofluoric, acidulated phosphate fluoride (Eustaquio et al., 1988; Kao et al., 1988; Smith et al., 1988; Winchester, 1991; Zachrisson and Büyükylmaz, 1993; Whitlock et al., 1994; Zelos et al., 1994; Barbosa et al., 1995; Major et al., 1995; Zachrisson et al., 1996; Cochran et al., 1997; Gillis and Redlich, 1998; Bourke and Rock, 1999; Sant'Anna et al., 2002; Harari et al., 2003; Pannes et al., 2003; Özcan et al., 2004).
It has been shown that silane coupling agents appear to enhance the bond strength by increasing the chemical bond between the resin composite and ceramic material (Wood et al., 1986; Kao and Johnston, 1991; Cochran et al., 1997; Chung et al., 1999; Huang and Kao, 2001; Kocadereli et al., 2001; Schmage et al., 2003). The silica of the ceramic is chemically joined with the acrylic group of the composite resin through silanization (Zachrisson et al., 1996; Schmage et al., 2003).
It has been demonstrated that silane enhances the bonding of brackets to glazed ceramic surfaces, but that the bond strengths achieved might not be adequate for clinical orthodontics (Newman et al., 1984; Eustaquio et al., 1988; Zelos et al., 1994; Barbosa et al., 1995; Nebbe and Stein, 1996; Sant'Anna et al., 2002; Pannes et al., 2003). In general, the silane coupling agent is applied with chemical and mechanical roughening procedures (Wood et al., 1986; Major et al., 1995; Gillis and Redlich, 1998; Chung et al., 1999; Huang and Kao, 2001; Kocadereli et al., 2001; Schmage et al., 2003; Özcan et al., 2004).
Etching the ceramic surfaces with acids followed by the application of a ceramic primer and a bonding agent are advised procedures (Zachrisson and Büyükylmaz, 1993; Cochran et al., 1997; Bourke and Rock, 1999; Chung et al., 1999; Huang and Kao, 2001; Özcan et al., 2004). However, the harmful effect of hydrofluoric acid (HFA) on the soft tissues has been highlighted (Barbosa et al., 1995; Bourke and Rock, 1999; Schmage et al., 2003; Özcan et al., 2004).
Mechanical roughening with diamond burs and APA has been shown to provoke crack initiation on the ceramic surface (Peterson et al., 1998). Damage to the ceramic due to roughening during surface conditioning should be minimized since the restorations ordinarily remain in the mouth following orthodontic treatment (Schmage et al., 2003). However, in order to obtain a viable bond between the orthodontic bracket and the ceramic surface, mechanical or chemical roughening is inevitable (Wood et al., 1986; Kao et al., 1988; Barbosa et al., 1995; Gillis and Redlich, 1998; Huang and Kao, 2001; Kocadereli et al., 2001; Harari et al., 2003; Pannes et al., 2003; Schmage et al., 2003; Özcan et al., 2004).
Only a limited number of studies exist concerning the bond strength of orthodontic brackets to all-ceramic restorations, and in most of these, feldspathic ceramic was mainly used (Pannes et al., 2003; Schmage et al., 2003; Özcan et al., 2004). Furthermore, insufficient information exists concerning the bond strength of other all-ceramic materials to orthodontic brackets.
The objectives of this study were to observe the outcomes of six different surface-conditioning methods on the shear bond strength (SBS) of metal orthodontic brackets to two different all-ceramic restorative materials (feldspathic and lithium disilicate) and to evaluate the mode of failure after debonding.
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Materials and methods |
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TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionReferences |
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Sixty feldspathic (Vitadur Alpha; Vita Zahnfabrik, Bad Säckingen, Germany) and 60 lithium disilicate (Empress 2; Ivoclar Vivadent, Schaan, Liechtenstein) ceramic specimens with a diameter of 6 mm and a thickness of 3 mm were fabricated and glazed according to the manufacturers' recommendations. The specimens were embedded in autopolymerizing acrylic resin blocks (Meliodent; Heraeus Kulzer Ltd, Newbury, Berkshire, UK) with their glazed surfaces facing upwards. For each all-ceramic material, the specimens were randomly divided into six groups, each containing 10 specimens and six different surface-conditioning methods were used. The groups and the surface-conditioning methods are shown in Table 1.
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The sample size was based upon previous studies. In the majority of these studies the sample size ranged from 5 to 10 specimens (Kern and Thompson, 1994
; Whitlock et al., 1994; Zelos et al., 1994; Major et al., 1995; Nebbe and Stein, 1996; Gillis and Redlich, 1998; Bourke and Rock, 1999; Huang and Kao, 2001; Kocadereli et al., 2001; Pannes et al., 2003; Schmage et al., 2003; Özcan et al., 2004).
Silane (Bond Enhancer; Pulpdent, Watertown, Massachusetts, USA) was applied to the specimens in the first group without any roughening procedures. In the second group, APA was performed using 25 µm aluminium trioxide (Al2O3) with an air abrasion device (Bego TopTec; Bego, Germany) at a distance of approximately 10 mm and a pressure of 2.5 bars for 4 seconds. In the third group, APA was carried out using 50 µm Al2O3 under the same conditions. In the fourth group, the ceramic surfaces were etched with 9.6 per cent HFA gel (Porcelain Etch Gel; Pulpdent) for 2 minutes. In the fifth and sixth groups, mechanical roughening was performed with fine (63 µm, Medin, Nové M
sto na Morav, Czech Republic) and extra-fine (40 µm, Medin) diamond burs. The cylindrical diamond burs, with their shafts parallel to the specimens, were rotated at 40 000 rpm. After chemical and mechanical roughening, the specimens were washed and rinsed thoroughly to remove the debris and then air-dried. Subsequently, silane and the adhesive primer (TransbondTM XT; 3M Unitek, Monrovia, California, USA) were applied to all roughened specimens. The light cure adhesive paste (TransbondTM XT; 3M Unitek) was applied to the mesh base of a maxillary central incisor bracket (Gemini bracket; 3M Unitek). Subsequently, the bracket was seated and positioned manually on the ceramic surface. Excess composite was carefully removed from the periphery of the bracket base with an explorer. The surface-conditioning methods and the placement of the brackets were performed by one operator (TT). The adhesive paste was cured for a total of 20 seconds from two directions using a visible light-curing unit (Hilux 200; Benlioglu Dental Inc., Ankara, Turkey) with an output of 600 mW/cm2. All specimens were stored in distilled water at 37 ± 2°C for 1 week. The specimens were thermocycled in a custom-made device (Nova Inc., Konya, Turkey) 500 times between 5°C and 55°C with a dwelling time of 30 seconds. The shear bond test was performed with a universal testing device (Lloyd LRX; Lloyd Instruments Ltd, Fareham, Hants, UK) at a crosshead speed of 1 mm/minute. The bond strengths were calculated in megapascals (MPa).
The ceramic surfaces were examined with a stereomicroscope (Stemi 2000-C; Carl Zeiss, Göttingen, Germany) at a magnification of x10 to determine the amount of composite resin remaining according to the adhesive remnant index (Årtun and Bergland, 1984) and to assess the damage to the ceramic which may have occurred during shear bond testing.
To evaluate the effect of surface-conditioning methods on the ceramic surfaces, six additional feldspathic and six lithium disilicate ceramic specimens were prepared and glazed. The surfaces of five specimens of each ceramic were then conditioned with the same experimental protocol described above. The intact glazed and the five roughened samples for each ceramic were gold sputtered with a sputter coater (S150B; Edwards, Crawley, Sussex, UK) and examined under a field emission scanning electron microscope (SEM, JSM-6335F; Jeol, Tokyo, Japan) at 15.0 kV. The SEM photomicrographs were taken at x500 magnification for visual inspection.
Two-way analysis of variance for (2 x 2) x 10 factorial design was performed to determine significant differences among porcelain surface conditioning, porcelain types, and their interactions. All treatment combination means for SBS values were compared using the Tukey multiple comparison test (
= 0.05).
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Results |
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Mean SBS, minimum and maximum values, and standard deviations for each group, except the first group due to debonding of the brackets during thermocycling, are given in Table 2. The main effects were significant differences for the conditioning methods and ceramic types on the SBS values (P < 0.05; Table 3). There was also a significant interaction between the conditioning methods and ceramic. The results of the Tukey multiple comparison test to compare the mean SBS values are given in Table 2.
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The lowest SBS was with HFA for the feldspathic ceramic (5.39 MPa) which was not significantly different from HFA for the lithium disilicate ceramic (11.11 MPa). These values were, however, significantly different from the values of the other groups with one exception: the SBS of HFA in the lithium disilicate ceramic was not significantly different from the SBS (14.66 MPa) of APA with 50-µm Al2O3 for the feldspathic ceramic.
The highest SBS values were obtained using the fine diamond bur with the feldspathic and lithium disilicate ceramics (26.38 and 28.20 MPa, respectively) and were not significantly different from the SBS obtained with the extra-fine diamond bur for the feldspathic and lithium disilicate ceramics (26.04 and 24.26 MPa, respectively).
APA with Al2O3 particles showed, in general, modest SBS for both ceramics. The SBS values obtained using APA with 25- and 50-µm Al2O3 particles (22.60 and 26.15 MPa, respectively) and lithium disilicate ceramic were not significantly different from those obtained with the diamond burs. The SBS obtained with APA and 25- and 50-µm Al2O3 particles (17.90 and 14.66 MPa, respectively) for the feldspathic ceramic showed a significant difference from the SBS obtained with the diamond burs. Roughening by APA with 25 µm Al2O3 particles of both ceramics was not significantly different from each other.
The modes of bond failure for the brackets after different surface-conditioning methods are given in Table 4. Adhesive failures between the ceramic and composite resin were observed in all groups. Cracks or fractures of the ceramic surfaces were not observed.
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The scanning electron photomicrographs of feldspathic and lithium disilicate ceramic surfaces conditioned using different methods are presented in Figures 1 and 2, respectively. The glazed surfaces of the two ceramics had a smooth appearance (Figures 1A and 2A). APA with 25- and 50-µm Al2O3 particles demonstrated mild roughening of the surface (Figures 1B, 2B and 1C, 2C, respectively). HFA etching produced minimal change and did not appear to alter the glazed porcelain surfaces (Figures 1D and 2D). Roughening with extra-fine and fine diamond burs showed deep grooves (Figures 1E, 2E and 1F, 2F, respectively).
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Discussion |
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The aim of this study was the evaluation of the effectiveness of different surface-conditioning methods on the SBS of metal orthodontic brackets to two different all-ceramic restorative materials (feldspathic and lithium disilicate). Clinically adequate bond strength for a metal orthodontic bracket to enamel should range from 6 to 8 MPa (Reynolds, 1975
). All SBS values in the present study were above this optimal range, rendering them clinically acceptable, except for feldspathic ceramic treated with HFA and silane.
Samples coated with silane, but not exposed to chemical or mechanical roughening, were considered as the control group but demonstrated bond failures during thermocycling. Barbosa et al. (1995) reported the premature loss of brackets bonded to glazed ceramic surfaces coated with silane after 7 days of water immersion. They explained that this premature loss was due to the high solubility of silane in water. Likewise, all specimens were stored in distilled water for 1 week in the present investigation. Furthermore, the relationship between silane and the glazed surface might be affected by the glaze composition. The glaze materials containing high alumina, as used in the present study, affect the chemical reaction between silane and ceramic. Silane will not enhance the bond to ceramic that contains only a small amount of silica (Kern and Thompson, 1994; Zachrisson et al., 1996). The premature loss of brackets confirms that bonding to glazed surfaces coated with silane does not provide adequate bond strength and that silane coating should be combined with surface roughening (Wood et al., 1986; Kao et al., 1988; Smith et al., 1988; Zachrisson and Büyükylmaz, 1993; Barbosa et al., 1995; Zachrisson et al., 1996; Huang and Kao, 2001; Kocadereli et al., 2001; Harari et al., 2003; Pannes et al., 2003; Özcan et al., 2004).
In the present study the silane application was combined with mechanical or chemical roughening to increase SBS. Silane application following surface roughening provides a statistically significant increase in SBS (Wood et al., 1986; Kao et al., 1988; Smith et al., 1988; Kao and Johnston, 1991; Whitlock et al., 1994; Cochran et al., 1997; Chung et al., 1999; Huang and Kao, 2001; Kocadereli et al., 2001; Schmage et al., 2003). Silane presents a chemical link between the dental ceramic and the composite resin, and the organic portion of the molecule enhances the wettability of the ceramic surface, thereby displaying a closer micromechanical bond (Lu et al., 1992).
Chemical roughening with 9.6 per cent HFA showed the lowest SBS for both ceramic groups. However, HFA has been found to be effective for improving bond strengths in other studies (Huang and Kao, 2001; Harari et al., 2003). HFA is applied to increase micromechanical retention creating surface pits by preferential dissolution of the glass phase from the ceramic matrix and to acidify the porcelain surface before silane application (al Edris et al., 1990; Major et al., 1995). The high aluminium oxide containing glaze and the increasing strength of porcelain makes it more resistant to chemical attack and reduces the effect of HFA etching (Zachrisson et al., 1996).
No significant differences were found in this study for SBS between the two APA groups. The SBS achieved with APA was higher than that produced by HFA. However, there was no statistically significant difference between chemical etching with HFA in the lithium disilicate ceramic or APA with 50 µm Al2O3 in the feldspathic ceramic. There is disagreement concerning the effectiveness of APA with Al2O3 particles in the literature: APA with Al2O3 particles was found to be more effective than chemical etching with HFA (Schmage et al., 2003). However, in some studies no significant difference between APA and chemical etching was observed (Gillis and Redlich, 1998). Harari et al. (2003) found that application of HFA was more effective than microetching with Al2O3 particles.
Roughening with diamond burs showed significantly higher SBS than chemical etching and APA. However, there was no difference between fine and extra-fine diamond burs. Barbosa et al. (1995) stated that roughening with coarse diamond burs resulted in higher SBS when compared with other groups, i.e. glazed and deglazed surfaces with sandpaper disks. However, the differences were not observed among the groups, i.e. roughening with a diamond bur, chemical etching with HFA, and APA with Al2O3 particles (Sant'Anna et al., 2002). In another study, roughening with diamond burs without silane application showed lower bond strength than chemical etching with HFA with silane and APA with Al2O3 particles with silane (Schmage et al., 2003).
With all surface-conditioning methods, lithium disilicate ceramic, in general, showed a higher SBS than feldspathic ceramic. The processing methods and the molecular structures of the two all-ceramic systems resulted in the differences. Lithium disilicate ceramic is processed by heat-press techniques and has more homogeneous and larger molecules (Oh et al., 2000). This structural difference could explain the variations between the bond strengths of the two ceramic systems.
The SEM photomicrographs of the two ceramics etched with 9.6 per cent HFA revealed different surface morphologies. Feldspathic ceramic displayed fewer pits and more unchanged glazed surfaces than the lithium disilicate ceramic. For the two ceramics abraded with Al2O3, loss of the glazed surface and mild roughening were seen. Uniform peeling or an erosive appearance with shallow penetration and undercuts was observed when compared with chemical etching. The two ceramics roughened with diamond burs showed similar surface morphology: uniform peeling or an erosive appearance with deeper grooves, and additional undercuts were observed when compared with chemical etching and APA.
These different microscopic appearances corroborate the SBS values. The bond strength gradually increased due to the gradual increase in roughening of the ceramic surface. Although roughening of the ceramic surface results in a higher bond strength, removal of the glaze by grinding diminishes the transverse strength of the porcelain to half of that when the glaze is present (Anusavice, 1996). Cracks created during roughening lead to porcelain damage during debonding (Peterson et al., 1998).
For all samples, adhesive failures between the ceramic and composite resin were seen. This type of adhesive failure demonstrated that the bond strength between the composite and the bracket, and the cohesive strength of the composite was stronger than the bond strength between the composite and ceramic. Adhesive failures at the ceramic/composite interface are preferred to avoid ceramic fractures during debonding (Smith et al., 1988). It has been reported that if bond strengths between the ceramic and the composite resin are higher than 13 MPa, cohesive failures are observed in the ceramic (Thurmond et al., 1994). In the present study most of the groups had values higher than 13 MPa which resulted in adhesive failures. Ceramic fractures or cracks were not observed. These findings agree with the results of Harari et al. (2003), who observed adhesive failure for HFA and APA groups. This observation is clinically important: no macroscopic damage to the ceramic surface is an indication of long-term integrity of the restoration (Harari et al., 2003).
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Conclusion |
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TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionReferences |
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- SBS values were found above the optimal range (6-8 MPa), except for feldspathic ceramic treated with HFA and silane.
- With all surface-conditioning methods, lithium disilicate ceramic, in general, showed a higher SBS than feldspathic ceramic.
- Although the SBS for feldspathic ceramic was below the optimal range, the SBS for lithium disilicate ceramic was above this range for HFA. For lithium disilicate ceramics HFA might be used for adequate bond strength. Thus, possible surface damage which may be observed after mechanical roughening may be prevented.
- With feldspathic porcelain, 25 µm Al2O3 particles resulted in minimal damage to the porcelain surface, and could be used as it provided sufficient bond strength.
- For all samples, adhesive failures between the ceramic and the composite resin were seen. No ceramic fractures or cracks were observed.
- The results of this study cannot solely be associated with the surface-conditioning methods; other factors influencing cohesive fractures of ceramics, such as bonding agent, ceramic type, bracket type, and debonding technique, should be taken into consideration.
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References |
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