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The effect of heat treatment and surface neutralization on bond strength of orthodontic brackets to lithium disilicate glass-ceramic
journal of the mechanical behavior of biomedical materials 103 (2020) 103605
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Journal of the Mechanical Behavior of Biomedical Materials journal homepage: http://www.elsevier.com/locate/jmbbm
The effect of heat treatment and surface neutralization on bond strength of orthodontic brackets to lithium disilicate glass-ceramic Samer M. Alaqeel Dental Health Department, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh, 11433, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords: Dental ceramics Surface conditioning Heat treatment Neutralization Surface texture Bond strength
Purpose: The study evaluated and compared the effect of pre-etching heat treatment and post-etching surface neutralization on the surface texture parameters and initial adhesion strength of orthodontic brackets bonded to lithium disilicate glass-ceramic using water-based and resin-based cement. Materials and methods: A total of 120 samples were fabricated by duplicating the buccal surface of the maxillary premolar. The samples were randomly assigned to two groups: the cementing surface of group 1 samples was heat-treated, and that of group 2 samples was left untreated. The samples of each group were further divided into 4 subgroups (n ¼ 15) according to the use of neutralization and the type of cement used for bonding. The surface texture parameters after etching were determined using a non-contact surface profilometer, and the bond strength was determined by a universal material tester. The results were analyzed by analysis of variance and the Scheffe post hoc test. Results: The samples that were heat-treated showed statistically significant higher bond strength in all the sub groups, and the acid-neutralized samples showed higher bond strength using both types of cement; however, the increase was statistically significant only in resin-based cement-bonded samples. Resin-based cement-bonded samples showed higher bond strength than water-based cement-bonded samples. Conclusion: Pre-etching heat treatment and post-etching acid neutralization of the cementing surface of lithium disilicate glass-ceramic significantly improve the surface texture and initial bond strength to orthodontic brackets.
1. Introduction
et al., 2013). In particular, air-particle abrasion is used for non-silica containing ceramics such as zirconia (Ozcan et al., 2013). An alterna tive, chemico-mechanical technique used for these ceramics is tri bochemical silica-coating (Sarac et al., 2011). Bonding brackets to silica-containing glass-ceramic surfaces is a challenging procedure, and it is vital for predictable orthodontic treat ments (Kilponen et al., 2019). Stronger bonding provides greater anchorage on glass-ceramic surfaces and predictable and controlled € tooth movement (Ozcan and Volpato, 2015). In this context, ceramic surface conditioning is the collective procedure aimed to alter the sur face free energy, involving acid etching and silane application. Both of these procedures improve the wettability of the cementing surface (Ramakrishnaiah et al., 2018a). Acid etching creates a rough surface by selectively dissolving the glassy phase. Thus, this procedure not only increases the surface area available for cement bonding but also exposes the hydroxyl groups required for chemical bonding with the silane coupling agent (Matinlinna and Vallittu, 2007a,b; Qeblawi et al., 2010; Lung and Matinlinna, 2012; Ramakrishnaiah et al., 2016).
Orthodontic brackets are bonded to natural enamel or artificial (metal and ceramic) restorations by a cementing medium, after careful conditioning of the cementing surfaces (Asiry et al., 2018). The need to bond orthodontic brackets to different dental restorative materials is increasing with the number of adult patients seeking orthodontic treatment. Thus, to strengthen the bond, different surface conditioning € protocols are followed for different restorative materials (Ozcan and Volpato, 2015). In the case of glass-ceramics containing silica, the most common technique is etching with hydrofluoric acid (HF) and the application of silane coupling agents. However, the acidic and corrosive nature of HF may cause tissue trauma during application. Hence, a proper protocol must be followed to isolate the area, apply the HF with a disposable microbrush, and rinse the excess acid into a polyethylene cup with water spray. Air-particle abrasion by aluminum trioxide (Al2O3) and surface roughening by diamond burs are other mechanical tech niques that have resulted in crack initiation on ceramic surfaces (Ozcan E-mail addresses: [email protected], [email protected].
https://doi.org/10.1016/j.jmbbm.2019.103605 Received 25 November 2019; Received in revised form 22 December 2019; Accepted 23 December 2019 Available online 27 December 2019 1751-6161/© 2019 Elsevier Ltd. All rights reserved.
S.M. Alaqeel
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
Silane coupling agents are activated hybrid inorganic-organic func tional monomers that are capable of bonding two dissimilar materials (Matinlinna et al., 2005; Matinlinna and Lassila, 2010, 2011). Meth acryloxypropyltrimethoxysilane (MPS) is the most commonly used silane primer in dentistry. It contains a non-hydrolyzable methacrylate group and a hydrolyzable methoxy group. The methoxy group reacts with the exposed hydroxyl groups (-OH) of the ceramic surface, and the methacrylate group polymerizes with the unset resin cement (Mat inlinna et al., 2005; Durgesh et al., 2015). Hence, silanes are chemically bifunctional and, when applied on silica-containing glass-ceramics, form durable siloxane (-Si-O-Si-) bonds (Matinlinna et al., 2006a,b; Kilponen et al., 2019). The durability of the chemical bond with silane coupling agents depends on the properties of the etched ceramic surface. It has been shown that the etching procedure forms fluorosilicate precipitates of Na, K, Ca (Canay et al., 2001; Ramakrishnaiah et al., 2018b). However, the acid precipitates may remain in deep pores, degrading the ceramic surface and lowering the surface pH (Sato et al., 2013). The low pH and the presence of fluorosilicate precipitates may affect the bond strength; hence, it is recommended to rinse the ceramic cementing surface thor oughly before applying a silane coupling agent (Canay et al., 2001). Furthermore, manufacturers recommend using a neutralizing agent on the etched ceramic surface to reduce the pH and prevent further degradation of the cementing surface, because the acidic pH interferes with the resin cement polymerization, compromising the bonding (Ramakrishnaiah et al., 2018b). Though the neutralizing agent does not have a direct effect on bonding, it promotes it by neutralizing the etched surface and removing inorganic precipitates. Thereby, the etched sur face is clean and ready for chemical bonding with a silane coupling agent exposing OH ions. Bracket debonding during the treatment is another major concern because it increases treatment chairside time, it damages the enamel and increases the overall treatment cost. Though some studies reported a debonding incidence of 0.6–9.6%, these low levels of debonding are attributed to surface conditioning and the use of adhesive resin cements, and a bond failure as high as 28.3% has been reported (Almosa and Zafar, 2018). Several studied examined the surface conditioning in terms of the effect on the bond strength of acid etching (acid concen tration, etching time) and of the application of a primer (Matinlinna et al., 2004, 2006; Ramakrishnaiah et al., 2016, 2018a, 2018c; Asiry et al., 2018). Sarac et al. studied the effect of surface conditioning methods on different all-ceramic materials and reported a significant increase in bond strength (Sarac et al., 2011). In the current study, we aimed at evaluating the effect of applying heat treatment and a neutralizing agent on a ceramic cementing surface on the initial bond strength of water- and resin-based cement. The null hypothesis tested was that these procedures do not have an effect on the ceramic surface texture and initial bond strength of an orthodontic bracket bonded to it.
immediately afterward. Instead, the cementing surface of samples in group 2 was etched without pre-heat treatment. Etching was carried out in a ventilated laboratory with all safety measures to avoid acid hazard, and, after etching, the samples were thoroughly rinsed with running water for 30 s to remove excess acid. To characterize the surface topography, the surface texture param eters of a sample selected randomly from each group were measured using a non-contact surface profilometer (Bruker Contour GT-K™, Bruker, Berlin, Germany). The sample was placed on the sample holder so that the measuring surface was perpendicular to the laser light. The surface texture parameters were acquired using the nanolens on a randomly selected area measuring 1.261 mm � 0.946 mm x 500 mm in the X, Y, and Z-direction respectively. The resolution was 100 points/ mm, and the images were captured at optical zoom 3 X. The parameters measured were the surface roughness (Sa), maximum surface peak height (Sp) and valley or pit depth (Sv), the sum of these two (Sz), the root mean square value of the sampling area (Sq), the skewness or roughness symmetry (Ssk), and the kurtosis of the topographic height distribution (Sku). The samples of each group were further divided into 4 subgroups (1a–d and 2a–d) of n ¼ 15 to evaluate the effect of the neutralizing agent on the initial bond strength and the difference in initial bond strength between resin- and water-based cement. The subgrouping and surface treatment protocols are presented in Fig. 1. For subgroups 1a, 1b and 2a, 2b, the samples were neutralized (IPS Ceramic Neutralizing Powder™, Ivoclar Vivadent, Schaan, Liechtenstein), ultrasonically cleaned, and treated with silane (Mono bond Plus™, Ivoclar-Vivadent, Schaan, Liechtenstein) as detailed in our previous study (Ramakrishnaiah et al., 2018b). For subgroups 1c, 1d and 2c, 2d, the samples were treated with silane without neutralization. For subgroups 1a, 1c and 2a, 2c), the standard premolar bracket (TP orthodontics Inc™. Indiana, USA) was bonded using a resin-based cement (Transbond XT™, 3M Unitek, Monrovia, CA, USA). For subgroups 1b, 1d and 2b, 2d, the brackets were bonded using a water-based cement (GC Fuji Ortho LC™, GC Corporation, Tokyo, Japan). The cement was mixed according to the manufacturer in structions, applied to the standard premolar bracket, accurately posi tioned onto the ceramic cementing surface, and pressed firmly. The excess cement from the edges was carefully removed with plastic in struments. Then, it was light-cured for 20 s using the Elipar™ Free Light 2 lamp (3M ESPE, Seefeld, Germany) with wavelength 420–540 nm and blue light output power of 1505 mW/cm2 (MARC™ system, Blue Light Analytics, Halifax, Canada), as detailed in our previous reports (Ram akrishnaiah et al., 2018a, 2018c). 2.2. Shear bond strength test Shear bond (adhesion) strength (SBS) was determined by shear bond test (ISO 10,477:2004) using a universal material testing machine (Interface type 8500/8800; Instron™, Canton, MA, USA). The samples were mounted on the instrument using a custom metal holder; the tip of the holder was positioned at the ceramic-bracket interface, and a controlled force was applied at a crosshead speed of 1 mm/min. The SBS was recorded by the software built-in in the testing machine. The SBS was obtained in Newton (N) and converted to megapascal (MPa) units.
2. Materials and methods 2.1. Sample preparation and grouping Lithium disilicate glass-ceramic was used for the current study (IPS emax Ceram™, Ivoclar Vivadent, Schaan, Liechtenstein), 120 samples were fabricated as facets reproducing the buccal surface of the maxillary first premolar. The samples were glazed and mounted using an autopolymerizing acrylic resin as a base, with the cementing surface above it. The facets were polished to remove any remnants of acrylic resin and cleaned in distilled water for 10 min using an ultrasonic cleaner (Qantex 140™, L and R, Kearny, NJ, USA). The samples were assigned randomly to two main groups containing 60 samples. For the samples in group 1, the cementing surface was first heat-treated by hot air. A steady stream of air at 60 � C was directed perpendicular to the cementing surface from a distance of 1 cm for 90 s, and etching was performed for 20 s with 5% HF (IPS Ceramic etching gel™, Ivoclar Vivadent, Schaan, Liechtenstein)
2.3. Analysis of failure mode The failure mode of the ceramic samples was investigated. The cementing surface was examined under an optical microscope at 20 X magnification (Nikon™, SM2-10, Tokyo, Japan) and the samples were categorized into 4 groups according to the amount of cement remaining on the ceramic surface, using the adhesive remnant index (ARI) (Arthur and Bergland, 1984): 2
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Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
Fig. 1. Flow chart of the sample grouping.
“0” “1” “2” “3”
– no cement left on the ceramic surface. – less than half of the cement left on the ceramic surface. – more than half of the cement left on the ceramic surface. – all cement left on the ceramic surface.
The samples of subgroup 1a (heat-treated, neutralized, and bonded with resin-based cement) had the highest SBS, and the lowest SBS value was recorded in subgroup 2d (non-heat-treated, non-neutralized, and bonded with water-based cement). Table 1 shows the comparison of the mean SBS of the heat- and nonheat-treated ceramic samples. The heat-treated samples showed statis tically significant higher SBS values than the non-heat-treated ones, irrespective of neutralization and type of cement used. Table 2 shows the comparison of the mean SBS of the neutralized and non-neutralized ceramic samples. As seen, the treatment with a neutralizing agent after etching had a significant effect on the SBS. The neutralized samples had higher SBS than the non-neutralized ones in all subgroups except subgroups 1b and 1d. In other words, the surface neutralization had the least (statistically insignificant) effect on the SBS of brackets bonded with glass ionomer cement. Table 3 shows the comparison of the mean SBS of samples bonded with resin and water-based cement. The resin cement showed higher SBS values in all subgroups. The highest mean SBS was for heat-treated and surface neutralized samples. Instead, the lowest mean SBS was found in subgroup 2d, where the samples were non-heat-treated, non-neutral ized, and bonded with water-based cement.
2.4. Statistical analysis The results of the SBS test were tabulated and analyzed using the SPSS (statistical package for social sciences) software version 22.0 (SPSS Inc., Chicago, IL, USA). Analysis of variance (ANOVA) and pairwise comparison was carried out using the Scheffe post hoc test at a signifi cance level of p < 0.05. 3. Results Figs. 2 and 3 show the surface texture parameters of the samples that were heat-treated and non-heat-treated, respectively, before applying the etchant. As seen, the parameters differed in the two cases. The sur face roughness, which is considered the most critical parameter for adhesion strength, was higher in the heat-treated samples than in the non-heat-treated ones.
Fig. 2. Surface texture parameters of the heat-treated ceramic sample. 3
S.M. Alaqeel
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
Fig. 3. Surface texture parameters of the non-heat-treated ceramic sample.
Table 4 and Table 5 report the ARI score for heat-treated and nonheat-treated samples, respectively. None of the samples showed adhe sive failure. Most of the samples in group 1 had ARI score 3, and fewer 2, whereas in group 2 the samples were equally distributed in ARI scores 1, 2 and 3.
Table 1 Comparison of the mean SBS of heat-treated and non-heat-treated ceramic samples. Subgroup
Mean SBS
SD
P value
1a 2a 1b 2b 1c 2c 1d 2d
15.2 10.77 8.34 6.65 13.6 9.3 7.61 5.36
1.33 1.16 1.35 1.24 1.32 1.07 1.16 0.71
<.00001*
4. Discussion
.0013*
The current study investigated the effect of the application of dry heat on the texture of the ceramic cementing surface and the initial bond strength (i.e., not considering aging and storage conditions of the sam ples) of orthodontic brackets. Additionally, it evaluated the effect of a neutralizing agent on their bond strength. The neutralizing agent is known to be effective in reducing the surface pH of acid-etched glassceramic cementing surfaces and to arrest the etching by neutralizing the acid retained within deep porosities (Ramakrishnaiah et al., 2018b). To do this, the bond strength of two different types of cement commonly used in clinical practice, water- and resin-based, were compared in the presence and absence of heat treatment and of a neutralizing agent. The results exhibited an increase in the mean SBS of orthodontic brackets bonded to ceramic surfaces when these were heat-treated and surface-neutralized, using both water-based and resin-based cements. Hence, the null hypothesis is rejected. Regarding SBS, this is a static macro-test method in which the ad hesive strength of two materials is determined by the application of a shear load at the adhesive interface. Because of its simplicity, it is the most commonly used method to measure adhesion (Braga et al., 2010). However, SBS is not an actual shear test, because it also applies tensile forces on the samples during loading, the test also has some shortcom ings. The parameters that influence the SBS test are i) the substrate area
<.00001* <.00001*
SD is the standard deviation. * indicates statistical significance (p < 0.05). Table 2 Comparison of the mean SBS of neutralized and non-neutralized ceramic samples. Subgroup
Mean SBS
SD
P value
1a 1c 1b 1d 2a 2c 2b 2d
15.2 13.06 8.34 7.61 10.77 9.3 6.65 5.36
1.33 1.32 1.35 1.16 1.16 1.07 1.24 0.71
0.00014* 0.125 0.0012* 0.0015*
SD is the standard deviation. * indicates statistical significance (p < 0.05). Table 3 Comparison of the mean SBS of resin-based and water-based cement. Subgroup
Mean SBS
SD
P value
1a 1b 1c 1d 2a 2b 2c 2d
15.2 8.34 13.06 7.61 10.77 6.65 9.3 5.36
1.33 1.35 1.32 1.16 1.16 1.24 1.07 0.71
<.00001*
Table 4 Failure modes of the heat-treated samples according to the ARI score. ARI score (N ¼ 15)
<.00001* <.00001*
0 1 2 3
<.00001*
SD is the standard deviation. * indicates statistical significance (p < 0.05).
Heat-treated ceramic samples subgroups 1a
1b
1c
1d
0 1 3 11
0 2 6 7
0 0 5 10
0 2 6 7
*Number indicates the total number of specimens failed with respective ARI score. For each subgroup, the number of samples failed with the corresponding ARI score is indicated. 4
S.M. Alaqeel
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
a vital step to remove the traces of acid from deep pores after etching (Ramakrishnaiah et al., 2018b). The excess acid, if not removed or neutralized, continues the etching process, removing more of the glass phase and generating wide pores (Ramakrishnaiah et al., 2016). The entrapped acid may also interfere with the cement polymerization, and both can lead to a weak ceramic-cement bond (Ramakrishnaiah et al., 2018b). However, a previous study has shown a decrease in bond strength after surface neutralization of lithium disilicate ceramics (Saavedra et al., 2009). According to the authors, the reaction between acid traces and the neutralizing agent forms a complex of sodium fluo ride and unstable carbonic acid. This complex precipitates in deep pores and reduces the bond strength (Canay et al., 2001; Saavedra et al., 2009). Hence, after surface neutralization, ultrasonic cleaning is rec ommended to remove acid complexes and promote bonding with cement. In agreement with the previous studies, the results of the cur rent study showed a significant increase in bond strength when the acid is neutralized and no (statistically insignificant) increase in water-based cement. When the brackets were bonded using water-based cement, the SBS was lower than with resin-based cement. In fact, in resin-bonded sam ples, the bond strength was nearly twice. This could be because of the composition and higher biomechanical properties of resin-based cement with respect to water-based cement. From a clinical point of view, the findings of this study are highly significant. Neutralizing the etched ceramic surface decreases the sur face pH and enhances the bonding, but care must be taken to rinse off the excess neutralizing agent and its precipitates before the application of the silane coupling agent. Increasing the temperature of the ceramic surface accelerates the etching process and increases the surface roughness, though a suitable method to apply heat on the ceramic sur face in real, intraoral environments must be addressed. This will be done in future studies. The limitations of this study are the following: i) the effect of heat treatment and surface neutralization on initial adhesion strength were emphasized not considering artificial aging and storage; ii) the experi ment was carried out in vitro, and the elevated temperatures used will lead to tissue damage in clinical environments. Hence, further study is required to find a clinically acceptable temperature that provides favorable cementing surfaces.
Table 5 Failure modes of the non-heat-treated samples according to the ARI score. ARI score (N ¼ 15) 0 1 2 3
Non-heat-treated ceramic samples subgroups 2a
2b
2c
2d
0 2 5 8
0 3 7 5
0 3 5 7
0 4 6 5
*Number indicates the total number of specimens failed with respective ARI score. For each subgroup, the number of samples failed with the corresponding ARI score is indicated.
available for bonding, ii) the elastic modulus of the adhesive resin cement, iii) the type of loading (wire loop, point, or knife-edge), and iv) the storage conditions (Sirisha et al., 2014). In our experiments, these factors did not play a role because the test was performed in vitro, all the samples had the same size and were treated and tested in common laboratory conditions. An alternative to the static SBS test is the mac ro/micro shear fatigue test, in which the samples are subjected to cyclic and sub-critical loading to simulate clinical conditions (Sirisha et al., 2014). In the cementation of natural tooth glass-ceramic restorations and that of orthodontic brackets to glass-ceramics, etching by HF is consid ered as the most critical step (Holand et al., 2000). Because of its acidic nature, HF dissolves the weaker glass phase and creates micro porosities in its place. Thus, the acid action of HF not only generates a rough surface but also increases the area of the cementing surface required for mechanical interlocking with adhesive cement (Della Bona and Anusa vice, 2002; Zogheib et al., 2011). Additionally, HF etching contributes to the chemical bonding with a silane coupling agent by exposing hydroxyl ions (Matinlinna et al., 2006a,b; Matinlinna and Vallittu, 2007a,b). Several studies have reported an altered surface micromorphology and an increase in bond strength after acid etching (Matinlinna and Vallittu, 2007a,b; Ramakrishnaiah et al., 2016). However, the acid action itself depends on factors such as the etching time and the concentration of acid. According to a recent study, increasing the temperature of the acid and ceramic cementing surface also affected the micromorphology of the surface (Sundfeld et al., 2016; Hailan et al., 2017; Puppin-Rontani et al., 2017). In the current study, the temperature of the ceramic surface was increased before etching. The results showed an increase in surface texture, especially surface roughness. Heat serves as a catalyst and promotes acid etching by inducing structural relaxation of the glass phase and precipitation of crystal phases (Zhang et al., 2014). The degree of glass phase structural relax ation and crystal precipitation is directly proportional to the time and temperature of the heat treatment. The latter, in particular, decreases the viscosity of the ceramic because the ceramic surface melts (Hjerppe et al., 2010; Leroy, 2013; Asadzadeh et al., 2019). Because of this, 35 samples showed an ARI score of 3 in the heat-treated group, whereas they were only 25 in the non-heat-treated group, irrespective of surface neutralization and the type of cement used. In the presence of heat, the HF ions move and react faster, which results in vigorous collision with the crystal phase of the ceramic; hence, the heat accelerates etching (Sundfeld et al., 2016). However, increasing the temperature and time of the heat treatment can be disadvantageous, because it can deteriorate the structural and mechanical properties by inducing internal stress and the formation of superficial cracks (Ozdemir and Ozdogan, 2018; Asadzadeh et al., 2019). Either the etching time or the acid concentra tion must be reduced when heat is used to accelerate etching; otherwise, etching will remove the glass phase further, loosening the crystal phase and resulting in larger pores. Because larger pores offer less mechanical interlocking to cement, this will result in a weak bond (Naves et al., 2010; Ramakrishnaiah et al., 2016). Neutralization of the etched glass-ceramic surface is also considered
5. Conclusions From the current study, the following conclusions were drawn. (a) Increasing the temperature of the cementing surface of a lithium disilicate ceramic significantly increases the bond strength of orthodontic brackets bonded with the tested types of cement. (b) Neutralizing the cementing surface of a lithium disilicate ceramic after etching increases the bond strength of orthodontic brackets bonded with the tested types of cement. (c) Resin-based cement provides significantly higher bond strength than water-based cement after heat treatment and surface neutralization. Declaration of competing interest The authors declare no conflicts of interest. Acknowledgments The authors acknowledge the Deanship of Scientific Research, Col lege of Applied Medical Sciences Research Center at King Saud Uni versity for funding this research.
5
S.M. Alaqeel
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
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Almosa, N., Zafar, H., 2018. Incidence of orthodontic bracket detachment during orthodontic treatment: a systematic review. Pak. J. Med. Sci. 34, 744–750. Arthur, J., Bergland, S., 1984. Clinical trials with crystal growth conditioning as an alternative to acid-etch enamel pretreatment. Am. J. Orthod. 85, 333–340. Asadzadeh, N., Ghorbanian, F., Ahrary, F., Rajati Haghi, H., Karamad, R., Yari, A., Javan, A., 2019. Bond strength of resin cement and glass ionomer to Nd:YAG lasertreated zirconia ceramics. J. Prosthodont. 28, e881–e885. Asiry, M.A., AlShahrani, I., Alaqeel, S.M., DurgeshB, H., Ramakrishnaiah, R., 2018. Effect of two-step and one-step surface conditioning of glass ceramic on adhesion strength of orthodontic bracket and effect of thermo-cycling on adhesion strength. J. Mech. Behav. Biomed. Mater. 84, 22–27. Braga, R.R., Meira, B.C., Boaro, C.C., Xavier, T.A., 2010. Adhesion to tooth structure: a critical review of “macro” test methods. Dent. Mater. 26, e38–e49. Canay, S., Hersek, N., Ertan, E., 2001. Effect of different acid treatments on a porcelain surface. J. Oral Rehabil. 28, 95–101. Della Bona, A., Anusavice, K.J., 2002. Microstructure, composition, and etching topography of dental ceramics. Int. J. Prosthodont. (IJP) 15, 159–167. Durgesh, B.H., Hijji, S.A., AlKheraif, A.A., Ramakrishnaiah, R., Basavarajappa, S., Al Sharawy, M., Matinlinna, J.P., 2015. A novel silane system as a primer for orthodontic bonding—a pilot study. Int. J. Adhesion Adhes. 62, 101–106. Hailan, Q., Lingyan, R., Rongrong, N., Xiangfeng, M., 2017. Effect of hydrofluoric acid concentration on the surface morphology and bonding effectiveness of lithium disilicate glass ceramics to resin composites. Hua xi kou qiang yi xue za zhi 35, 593–597. Hjerppe, J., Fr€ oberg, K., Lassila, L., Vallittu, P.K., 2010. The effect of heat treatment and feldspathic glazing on some mechanical properties of zirconia. Silicon 2, 171–178. Holand, W., Schweiger, M., Frank, M., Rheinberger, V., 2000. A comparison of the microstructure and properties of the IPS Empress 2 and the IPS Empress glassceramics. J. Biomed. Mater. Res. 53, 297–303. Kilponen, L., Varrela, J., Vallittu, P.K., 2019. Priming and bonding metal, ceramic and polycarbonate brackets. Biomater. Investig. Dent. 6, 61–72. Leroy, F., 2013. Molecular driving forces. Statistical thermodynamics in biology, chemistry, physics, and nanoscience. Soft Mater. 11, 231-231. Lung, C.Y., Matinlinna, J.P., 2012. Aspects of silane coupling agents and surface conditioning in dentistry: an overview. Dent. Mater. 28, 467–477. Matinlinna, J.P., Lassila, L.V., 2010. Experimental novel silane system in adhesion promotion between dental resin and pretreated titanium. Part II: effect of long-term water storage. Silicon 2, 79–85. Matinlinna, J.P., Lassila, L.V., 2011. Enhanced resin-composite bonding to zirconia framework after pretreatment with selected silane monomers. Dent. Mater. 27, 273–280. Matinlinna, J.P., Lassila, L.V., Kangasniemi, I., Yli-Urpo, A., Vallittu, P.K., 2005. Shear bond strength of Bis-GMA resin and methacrylated dendrimer resins on silanized titanium substrate. Dent. Mater. 21, 287–296. Matinlinna, J.P., Lassila, L.V., Vallittu, P.K., 2006. The effect of a novel silane blend system on resin bond strength to silica-coated Ti substrate. J. Dent. 34, 436–443. Matinlinna, J.P., Lassila, L.V., Vallittu, P.K., 2006. Evaluation of five dental silanes on bonding a luting cement onto silica-coated titanium. J. Dent. 34, 721–726. € Matinlinna, J.P., Ozcan, M., Lassila, L.V., Vallittu, P.K., 2004. The effect of a 3-metha cryloxypropyltrimethoxysilane and vinyltriisopropoxysilane blend and tris(3trimethoxysilylpropyl)isocyanurate on the shear bond strength of composite resin to titanium metal. Dent. Mater. 20, 804–813. Matinlinna, J.P., Vallittu, P.K., 2007. Bonding of resin composites to etchable ceramic surfaces - an insight review of the chemical aspects on surface conditioning. J. Oral Rehabil. 34, 622–630.
6
Contents lists available at ScienceDirect
Journal of the Mechanical Behavior of Biomedical Materials journal homepage: http://www.elsevier.com/locate/jmbbm
The effect of heat treatment and surface neutralization on bond strength of orthodontic brackets to lithium disilicate glass-ceramic Samer M. Alaqeel Dental Health Department, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh, 11433, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords: Dental ceramics Surface conditioning Heat treatment Neutralization Surface texture Bond strength
Purpose: The study evaluated and compared the effect of pre-etching heat treatment and post-etching surface neutralization on the surface texture parameters and initial adhesion strength of orthodontic brackets bonded to lithium disilicate glass-ceramic using water-based and resin-based cement. Materials and methods: A total of 120 samples were fabricated by duplicating the buccal surface of the maxillary premolar. The samples were randomly assigned to two groups: the cementing surface of group 1 samples was heat-treated, and that of group 2 samples was left untreated. The samples of each group were further divided into 4 subgroups (n ¼ 15) according to the use of neutralization and the type of cement used for bonding. The surface texture parameters after etching were determined using a non-contact surface profilometer, and the bond strength was determined by a universal material tester. The results were analyzed by analysis of variance and the Scheffe post hoc test. Results: The samples that were heat-treated showed statistically significant higher bond strength in all the sub groups, and the acid-neutralized samples showed higher bond strength using both types of cement; however, the increase was statistically significant only in resin-based cement-bonded samples. Resin-based cement-bonded samples showed higher bond strength than water-based cement-bonded samples. Conclusion: Pre-etching heat treatment and post-etching acid neutralization of the cementing surface of lithium disilicate glass-ceramic significantly improve the surface texture and initial bond strength to orthodontic brackets.
1. Introduction
et al., 2013). In particular, air-particle abrasion is used for non-silica containing ceramics such as zirconia (Ozcan et al., 2013). An alterna tive, chemico-mechanical technique used for these ceramics is tri bochemical silica-coating (Sarac et al., 2011). Bonding brackets to silica-containing glass-ceramic surfaces is a challenging procedure, and it is vital for predictable orthodontic treat ments (Kilponen et al., 2019). Stronger bonding provides greater anchorage on glass-ceramic surfaces and predictable and controlled € tooth movement (Ozcan and Volpato, 2015). In this context, ceramic surface conditioning is the collective procedure aimed to alter the sur face free energy, involving acid etching and silane application. Both of these procedures improve the wettability of the cementing surface (Ramakrishnaiah et al., 2018a). Acid etching creates a rough surface by selectively dissolving the glassy phase. Thus, this procedure not only increases the surface area available for cement bonding but also exposes the hydroxyl groups required for chemical bonding with the silane coupling agent (Matinlinna and Vallittu, 2007a,b; Qeblawi et al., 2010; Lung and Matinlinna, 2012; Ramakrishnaiah et al., 2016).
Orthodontic brackets are bonded to natural enamel or artificial (metal and ceramic) restorations by a cementing medium, after careful conditioning of the cementing surfaces (Asiry et al., 2018). The need to bond orthodontic brackets to different dental restorative materials is increasing with the number of adult patients seeking orthodontic treatment. Thus, to strengthen the bond, different surface conditioning € protocols are followed for different restorative materials (Ozcan and Volpato, 2015). In the case of glass-ceramics containing silica, the most common technique is etching with hydrofluoric acid (HF) and the application of silane coupling agents. However, the acidic and corrosive nature of HF may cause tissue trauma during application. Hence, a proper protocol must be followed to isolate the area, apply the HF with a disposable microbrush, and rinse the excess acid into a polyethylene cup with water spray. Air-particle abrasion by aluminum trioxide (Al2O3) and surface roughening by diamond burs are other mechanical tech niques that have resulted in crack initiation on ceramic surfaces (Ozcan E-mail addresses: [email protected], [email protected].
https://doi.org/10.1016/j.jmbbm.2019.103605 Received 25 November 2019; Received in revised form 22 December 2019; Accepted 23 December 2019 Available online 27 December 2019 1751-6161/© 2019 Elsevier Ltd. All rights reserved.
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Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
Silane coupling agents are activated hybrid inorganic-organic func tional monomers that are capable of bonding two dissimilar materials (Matinlinna et al., 2005; Matinlinna and Lassila, 2010, 2011). Meth acryloxypropyltrimethoxysilane (MPS) is the most commonly used silane primer in dentistry. It contains a non-hydrolyzable methacrylate group and a hydrolyzable methoxy group. The methoxy group reacts with the exposed hydroxyl groups (-OH) of the ceramic surface, and the methacrylate group polymerizes with the unset resin cement (Mat inlinna et al., 2005; Durgesh et al., 2015). Hence, silanes are chemically bifunctional and, when applied on silica-containing glass-ceramics, form durable siloxane (-Si-O-Si-) bonds (Matinlinna et al., 2006a,b; Kilponen et al., 2019). The durability of the chemical bond with silane coupling agents depends on the properties of the etched ceramic surface. It has been shown that the etching procedure forms fluorosilicate precipitates of Na, K, Ca (Canay et al., 2001; Ramakrishnaiah et al., 2018b). However, the acid precipitates may remain in deep pores, degrading the ceramic surface and lowering the surface pH (Sato et al., 2013). The low pH and the presence of fluorosilicate precipitates may affect the bond strength; hence, it is recommended to rinse the ceramic cementing surface thor oughly before applying a silane coupling agent (Canay et al., 2001). Furthermore, manufacturers recommend using a neutralizing agent on the etched ceramic surface to reduce the pH and prevent further degradation of the cementing surface, because the acidic pH interferes with the resin cement polymerization, compromising the bonding (Ramakrishnaiah et al., 2018b). Though the neutralizing agent does not have a direct effect on bonding, it promotes it by neutralizing the etched surface and removing inorganic precipitates. Thereby, the etched sur face is clean and ready for chemical bonding with a silane coupling agent exposing OH ions. Bracket debonding during the treatment is another major concern because it increases treatment chairside time, it damages the enamel and increases the overall treatment cost. Though some studies reported a debonding incidence of 0.6–9.6%, these low levels of debonding are attributed to surface conditioning and the use of adhesive resin cements, and a bond failure as high as 28.3% has been reported (Almosa and Zafar, 2018). Several studied examined the surface conditioning in terms of the effect on the bond strength of acid etching (acid concen tration, etching time) and of the application of a primer (Matinlinna et al., 2004, 2006; Ramakrishnaiah et al., 2016, 2018a, 2018c; Asiry et al., 2018). Sarac et al. studied the effect of surface conditioning methods on different all-ceramic materials and reported a significant increase in bond strength (Sarac et al., 2011). In the current study, we aimed at evaluating the effect of applying heat treatment and a neutralizing agent on a ceramic cementing surface on the initial bond strength of water- and resin-based cement. The null hypothesis tested was that these procedures do not have an effect on the ceramic surface texture and initial bond strength of an orthodontic bracket bonded to it.
immediately afterward. Instead, the cementing surface of samples in group 2 was etched without pre-heat treatment. Etching was carried out in a ventilated laboratory with all safety measures to avoid acid hazard, and, after etching, the samples were thoroughly rinsed with running water for 30 s to remove excess acid. To characterize the surface topography, the surface texture param eters of a sample selected randomly from each group were measured using a non-contact surface profilometer (Bruker Contour GT-K™, Bruker, Berlin, Germany). The sample was placed on the sample holder so that the measuring surface was perpendicular to the laser light. The surface texture parameters were acquired using the nanolens on a randomly selected area measuring 1.261 mm � 0.946 mm x 500 mm in the X, Y, and Z-direction respectively. The resolution was 100 points/ mm, and the images were captured at optical zoom 3 X. The parameters measured were the surface roughness (Sa), maximum surface peak height (Sp) and valley or pit depth (Sv), the sum of these two (Sz), the root mean square value of the sampling area (Sq), the skewness or roughness symmetry (Ssk), and the kurtosis of the topographic height distribution (Sku). The samples of each group were further divided into 4 subgroups (1a–d and 2a–d) of n ¼ 15 to evaluate the effect of the neutralizing agent on the initial bond strength and the difference in initial bond strength between resin- and water-based cement. The subgrouping and surface treatment protocols are presented in Fig. 1. For subgroups 1a, 1b and 2a, 2b, the samples were neutralized (IPS Ceramic Neutralizing Powder™, Ivoclar Vivadent, Schaan, Liechtenstein), ultrasonically cleaned, and treated with silane (Mono bond Plus™, Ivoclar-Vivadent, Schaan, Liechtenstein) as detailed in our previous study (Ramakrishnaiah et al., 2018b). For subgroups 1c, 1d and 2c, 2d, the samples were treated with silane without neutralization. For subgroups 1a, 1c and 2a, 2c), the standard premolar bracket (TP orthodontics Inc™. Indiana, USA) was bonded using a resin-based cement (Transbond XT™, 3M Unitek, Monrovia, CA, USA). For subgroups 1b, 1d and 2b, 2d, the brackets were bonded using a water-based cement (GC Fuji Ortho LC™, GC Corporation, Tokyo, Japan). The cement was mixed according to the manufacturer in structions, applied to the standard premolar bracket, accurately posi tioned onto the ceramic cementing surface, and pressed firmly. The excess cement from the edges was carefully removed with plastic in struments. Then, it was light-cured for 20 s using the Elipar™ Free Light 2 lamp (3M ESPE, Seefeld, Germany) with wavelength 420–540 nm and blue light output power of 1505 mW/cm2 (MARC™ system, Blue Light Analytics, Halifax, Canada), as detailed in our previous reports (Ram akrishnaiah et al., 2018a, 2018c). 2.2. Shear bond strength test Shear bond (adhesion) strength (SBS) was determined by shear bond test (ISO 10,477:2004) using a universal material testing machine (Interface type 8500/8800; Instron™, Canton, MA, USA). The samples were mounted on the instrument using a custom metal holder; the tip of the holder was positioned at the ceramic-bracket interface, and a controlled force was applied at a crosshead speed of 1 mm/min. The SBS was recorded by the software built-in in the testing machine. The SBS was obtained in Newton (N) and converted to megapascal (MPa) units.
2. Materials and methods 2.1. Sample preparation and grouping Lithium disilicate glass-ceramic was used for the current study (IPS emax Ceram™, Ivoclar Vivadent, Schaan, Liechtenstein), 120 samples were fabricated as facets reproducing the buccal surface of the maxillary first premolar. The samples were glazed and mounted using an autopolymerizing acrylic resin as a base, with the cementing surface above it. The facets were polished to remove any remnants of acrylic resin and cleaned in distilled water for 10 min using an ultrasonic cleaner (Qantex 140™, L and R, Kearny, NJ, USA). The samples were assigned randomly to two main groups containing 60 samples. For the samples in group 1, the cementing surface was first heat-treated by hot air. A steady stream of air at 60 � C was directed perpendicular to the cementing surface from a distance of 1 cm for 90 s, and etching was performed for 20 s with 5% HF (IPS Ceramic etching gel™, Ivoclar Vivadent, Schaan, Liechtenstein)
2.3. Analysis of failure mode The failure mode of the ceramic samples was investigated. The cementing surface was examined under an optical microscope at 20 X magnification (Nikon™, SM2-10, Tokyo, Japan) and the samples were categorized into 4 groups according to the amount of cement remaining on the ceramic surface, using the adhesive remnant index (ARI) (Arthur and Bergland, 1984): 2
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Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
Fig. 1. Flow chart of the sample grouping.
“0” “1” “2” “3”
– no cement left on the ceramic surface. – less than half of the cement left on the ceramic surface. – more than half of the cement left on the ceramic surface. – all cement left on the ceramic surface.
The samples of subgroup 1a (heat-treated, neutralized, and bonded with resin-based cement) had the highest SBS, and the lowest SBS value was recorded in subgroup 2d (non-heat-treated, non-neutralized, and bonded with water-based cement). Table 1 shows the comparison of the mean SBS of the heat- and nonheat-treated ceramic samples. The heat-treated samples showed statis tically significant higher SBS values than the non-heat-treated ones, irrespective of neutralization and type of cement used. Table 2 shows the comparison of the mean SBS of the neutralized and non-neutralized ceramic samples. As seen, the treatment with a neutralizing agent after etching had a significant effect on the SBS. The neutralized samples had higher SBS than the non-neutralized ones in all subgroups except subgroups 1b and 1d. In other words, the surface neutralization had the least (statistically insignificant) effect on the SBS of brackets bonded with glass ionomer cement. Table 3 shows the comparison of the mean SBS of samples bonded with resin and water-based cement. The resin cement showed higher SBS values in all subgroups. The highest mean SBS was for heat-treated and surface neutralized samples. Instead, the lowest mean SBS was found in subgroup 2d, where the samples were non-heat-treated, non-neutral ized, and bonded with water-based cement.
2.4. Statistical analysis The results of the SBS test were tabulated and analyzed using the SPSS (statistical package for social sciences) software version 22.0 (SPSS Inc., Chicago, IL, USA). Analysis of variance (ANOVA) and pairwise comparison was carried out using the Scheffe post hoc test at a signifi cance level of p < 0.05. 3. Results Figs. 2 and 3 show the surface texture parameters of the samples that were heat-treated and non-heat-treated, respectively, before applying the etchant. As seen, the parameters differed in the two cases. The sur face roughness, which is considered the most critical parameter for adhesion strength, was higher in the heat-treated samples than in the non-heat-treated ones.
Fig. 2. Surface texture parameters of the heat-treated ceramic sample. 3
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Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
Fig. 3. Surface texture parameters of the non-heat-treated ceramic sample.
Table 4 and Table 5 report the ARI score for heat-treated and nonheat-treated samples, respectively. None of the samples showed adhe sive failure. Most of the samples in group 1 had ARI score 3, and fewer 2, whereas in group 2 the samples were equally distributed in ARI scores 1, 2 and 3.
Table 1 Comparison of the mean SBS of heat-treated and non-heat-treated ceramic samples. Subgroup
Mean SBS
SD
P value
1a 2a 1b 2b 1c 2c 1d 2d
15.2 10.77 8.34 6.65 13.6 9.3 7.61 5.36
1.33 1.16 1.35 1.24 1.32 1.07 1.16 0.71
<.00001*
4. Discussion
.0013*
The current study investigated the effect of the application of dry heat on the texture of the ceramic cementing surface and the initial bond strength (i.e., not considering aging and storage conditions of the sam ples) of orthodontic brackets. Additionally, it evaluated the effect of a neutralizing agent on their bond strength. The neutralizing agent is known to be effective in reducing the surface pH of acid-etched glassceramic cementing surfaces and to arrest the etching by neutralizing the acid retained within deep porosities (Ramakrishnaiah et al., 2018b). To do this, the bond strength of two different types of cement commonly used in clinical practice, water- and resin-based, were compared in the presence and absence of heat treatment and of a neutralizing agent. The results exhibited an increase in the mean SBS of orthodontic brackets bonded to ceramic surfaces when these were heat-treated and surface-neutralized, using both water-based and resin-based cements. Hence, the null hypothesis is rejected. Regarding SBS, this is a static macro-test method in which the ad hesive strength of two materials is determined by the application of a shear load at the adhesive interface. Because of its simplicity, it is the most commonly used method to measure adhesion (Braga et al., 2010). However, SBS is not an actual shear test, because it also applies tensile forces on the samples during loading, the test also has some shortcom ings. The parameters that influence the SBS test are i) the substrate area
<.00001* <.00001*
SD is the standard deviation. * indicates statistical significance (p < 0.05). Table 2 Comparison of the mean SBS of neutralized and non-neutralized ceramic samples. Subgroup
Mean SBS
SD
P value
1a 1c 1b 1d 2a 2c 2b 2d
15.2 13.06 8.34 7.61 10.77 9.3 6.65 5.36
1.33 1.32 1.35 1.16 1.16 1.07 1.24 0.71
0.00014* 0.125 0.0012* 0.0015*
SD is the standard deviation. * indicates statistical significance (p < 0.05). Table 3 Comparison of the mean SBS of resin-based and water-based cement. Subgroup
Mean SBS
SD
P value
1a 1b 1c 1d 2a 2b 2c 2d
15.2 8.34 13.06 7.61 10.77 6.65 9.3 5.36
1.33 1.35 1.32 1.16 1.16 1.24 1.07 0.71
<.00001*
Table 4 Failure modes of the heat-treated samples according to the ARI score. ARI score (N ¼ 15)
<.00001* <.00001*
0 1 2 3
<.00001*
SD is the standard deviation. * indicates statistical significance (p < 0.05).
Heat-treated ceramic samples subgroups 1a
1b
1c
1d
0 1 3 11
0 2 6 7
0 0 5 10
0 2 6 7
*Number indicates the total number of specimens failed with respective ARI score. For each subgroup, the number of samples failed with the corresponding ARI score is indicated. 4
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Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
a vital step to remove the traces of acid from deep pores after etching (Ramakrishnaiah et al., 2018b). The excess acid, if not removed or neutralized, continues the etching process, removing more of the glass phase and generating wide pores (Ramakrishnaiah et al., 2016). The entrapped acid may also interfere with the cement polymerization, and both can lead to a weak ceramic-cement bond (Ramakrishnaiah et al., 2018b). However, a previous study has shown a decrease in bond strength after surface neutralization of lithium disilicate ceramics (Saavedra et al., 2009). According to the authors, the reaction between acid traces and the neutralizing agent forms a complex of sodium fluo ride and unstable carbonic acid. This complex precipitates in deep pores and reduces the bond strength (Canay et al., 2001; Saavedra et al., 2009). Hence, after surface neutralization, ultrasonic cleaning is rec ommended to remove acid complexes and promote bonding with cement. In agreement with the previous studies, the results of the cur rent study showed a significant increase in bond strength when the acid is neutralized and no (statistically insignificant) increase in water-based cement. When the brackets were bonded using water-based cement, the SBS was lower than with resin-based cement. In fact, in resin-bonded sam ples, the bond strength was nearly twice. This could be because of the composition and higher biomechanical properties of resin-based cement with respect to water-based cement. From a clinical point of view, the findings of this study are highly significant. Neutralizing the etched ceramic surface decreases the sur face pH and enhances the bonding, but care must be taken to rinse off the excess neutralizing agent and its precipitates before the application of the silane coupling agent. Increasing the temperature of the ceramic surface accelerates the etching process and increases the surface roughness, though a suitable method to apply heat on the ceramic sur face in real, intraoral environments must be addressed. This will be done in future studies. The limitations of this study are the following: i) the effect of heat treatment and surface neutralization on initial adhesion strength were emphasized not considering artificial aging and storage; ii) the experi ment was carried out in vitro, and the elevated temperatures used will lead to tissue damage in clinical environments. Hence, further study is required to find a clinically acceptable temperature that provides favorable cementing surfaces.
Table 5 Failure modes of the non-heat-treated samples according to the ARI score. ARI score (N ¼ 15) 0 1 2 3
Non-heat-treated ceramic samples subgroups 2a
2b
2c
2d
0 2 5 8
0 3 7 5
0 3 5 7
0 4 6 5
*Number indicates the total number of specimens failed with respective ARI score. For each subgroup, the number of samples failed with the corresponding ARI score is indicated.
available for bonding, ii) the elastic modulus of the adhesive resin cement, iii) the type of loading (wire loop, point, or knife-edge), and iv) the storage conditions (Sirisha et al., 2014). In our experiments, these factors did not play a role because the test was performed in vitro, all the samples had the same size and were treated and tested in common laboratory conditions. An alternative to the static SBS test is the mac ro/micro shear fatigue test, in which the samples are subjected to cyclic and sub-critical loading to simulate clinical conditions (Sirisha et al., 2014). In the cementation of natural tooth glass-ceramic restorations and that of orthodontic brackets to glass-ceramics, etching by HF is consid ered as the most critical step (Holand et al., 2000). Because of its acidic nature, HF dissolves the weaker glass phase and creates micro porosities in its place. Thus, the acid action of HF not only generates a rough surface but also increases the area of the cementing surface required for mechanical interlocking with adhesive cement (Della Bona and Anusa vice, 2002; Zogheib et al., 2011). Additionally, HF etching contributes to the chemical bonding with a silane coupling agent by exposing hydroxyl ions (Matinlinna et al., 2006a,b; Matinlinna and Vallittu, 2007a,b). Several studies have reported an altered surface micromorphology and an increase in bond strength after acid etching (Matinlinna and Vallittu, 2007a,b; Ramakrishnaiah et al., 2016). However, the acid action itself depends on factors such as the etching time and the concentration of acid. According to a recent study, increasing the temperature of the acid and ceramic cementing surface also affected the micromorphology of the surface (Sundfeld et al., 2016; Hailan et al., 2017; Puppin-Rontani et al., 2017). In the current study, the temperature of the ceramic surface was increased before etching. The results showed an increase in surface texture, especially surface roughness. Heat serves as a catalyst and promotes acid etching by inducing structural relaxation of the glass phase and precipitation of crystal phases (Zhang et al., 2014). The degree of glass phase structural relax ation and crystal precipitation is directly proportional to the time and temperature of the heat treatment. The latter, in particular, decreases the viscosity of the ceramic because the ceramic surface melts (Hjerppe et al., 2010; Leroy, 2013; Asadzadeh et al., 2019). Because of this, 35 samples showed an ARI score of 3 in the heat-treated group, whereas they were only 25 in the non-heat-treated group, irrespective of surface neutralization and the type of cement used. In the presence of heat, the HF ions move and react faster, which results in vigorous collision with the crystal phase of the ceramic; hence, the heat accelerates etching (Sundfeld et al., 2016). However, increasing the temperature and time of the heat treatment can be disadvantageous, because it can deteriorate the structural and mechanical properties by inducing internal stress and the formation of superficial cracks (Ozdemir and Ozdogan, 2018; Asadzadeh et al., 2019). Either the etching time or the acid concentra tion must be reduced when heat is used to accelerate etching; otherwise, etching will remove the glass phase further, loosening the crystal phase and resulting in larger pores. Because larger pores offer less mechanical interlocking to cement, this will result in a weak bond (Naves et al., 2010; Ramakrishnaiah et al., 2016). Neutralization of the etched glass-ceramic surface is also considered
5. Conclusions From the current study, the following conclusions were drawn. (a) Increasing the temperature of the cementing surface of a lithium disilicate ceramic significantly increases the bond strength of orthodontic brackets bonded with the tested types of cement. (b) Neutralizing the cementing surface of a lithium disilicate ceramic after etching increases the bond strength of orthodontic brackets bonded with the tested types of cement. (c) Resin-based cement provides significantly higher bond strength than water-based cement after heat treatment and surface neutralization. Declaration of competing interest The authors declare no conflicts of interest. Acknowledgments The authors acknowledge the Deanship of Scientific Research, Col lege of Applied Medical Sciences Research Center at King Saud Uni versity for funding this research.
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S.M. Alaqeel
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103605
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