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Evaluation of shear bond strength between indirect composite veneer and zirconia coated with silicated glass nano particles.

INTRODUCTION

Need for non-metallic restorative materials along with the highest esthetic and properties such as high biocompatibility, color stability and good thermal insulation are usually the reasons for increasing the use of ceramic in dentistry [1, 2]. Regardless of how to fabricate ceramics, their surface requires pretreatment because bonding to ceramics depends on locking and mechanical stabilizing created by small retentions on its surface as well as chemical bonding. Since non-silica based Yttria partially stabilized tetragonal zirconia (Y-TZP) are resistant to etching [3], other techniques such as air-abrasion, silanization and laser irradiation are used to increase the roughness of zirconia surface [4]. Airborne-particle abrasion is the most common technique to create surface roughness which results in the formation of micro-retentions at surface [5, 6]. The more effective pretreatment is silica coating by tribochemical technique during which zirconia surfaces are first cleaned, then roughened using alumina particles and finally surfaces are abraded by silica-modified alumina particles [7, 8]. Here, the pressure of launching particles is an important parameter that shows the level of penetrating silica into the surface [9]. Recently, the results of airborne-particle abrasion treatment has been generally questioned because hitting particles cause small cracks on the surface of Y-TZP ceramic which result in decreasing the strength of the material [10, 11].

Due to the relatively high prevalence of porcelain chipping and high thermal cycles created during porcelain firing that having the potential to weaken the zirconia structure, in this study, indirect laboratory composite was used for veneering the surface of zirconia. Although the mechanical properties of indirect composites are lower than ceramics, in some clinical conditions, indirect composites can be utilized: for example, in the restorations of dental implants. Various mechanical tests have been applied to evaluate the level of bond strength between ceramic and resin among which shear bond strength test is more commonly used. The present study aimed to assess the effect of silicated glass nano-particles coating on shear bond strength between indirect composite veneer and zirconia ceramic.

MATERIALS AND METHODS

A total of 34 samples of zirconia disks (Y-TZP, Cercon, Germany) with dimensions of 7.5 x 7.5 x 3.5mm were prepared using cutting machine. The surface of the samples was ground using silicon carbide abrasive paper (400-1000grit) and prepared for sintering. After sintering, the samples were dimensionally changed and their approximate dimensions were measured as equal to 6 x 6 x 2.8 mm. All specimens were ultrasonicly cleaned by acetone, ethanol and deionized water, each for 2 min. Then the specimens were divided into two groups of 17 as random allocation: Group 1 which was only sintered (control or AS: as sintered group); group 2 which received silicated glass nano-particles coating (silica coated: SC). Using the sol-gel method and soluble salts of silicated glass, nano-particles were deposited on the zirconia substrate. The materials used for the deposition included: Tetraethyl orthosilicate (C8[H.sub.2][O.sub.4]Si), Nitric acid (HN[O.sub.3], Mw:0.2 g/mol), Triethyl phosphate (C6H15O4P, Mw:0.005 g/mol), Calcium nitrate tetrahydrate (Ca(NO3)2.4[H.sub.2]O, Mw:0.031 g/mol) and Polyethylene glycol (HOCH2CH2OH, Mw:62.07 g/mol). After deposing particles on the surface of samples by sol-gel method, sintering was performed at the temperature of about 700 [degrees]C. The obtained nano-particles were studied in terms of phase by x-ray diffraction test (XRD) (X'Pert Pro MPD- PANalytical). Then, one specimen of each group was examined to evaluate morphology and microstructure of the surface by AFM (Dualscope C26- DME).

In order to assess shear bond strength, zirconia discs were embedded in the acrylic materials such that only one of their surfaces with dimensions of 6 x 6mm was out of acrylic materials. The clear plastic hollow rings with an internal diameter and depth of 2mm were placed on samples to determine the bonding area. Then silane (Prosil-FGM-Brazil) was applied as a thin layer on specimens and according to the manufacturer's instructions, it was dried with gentle air pressure after one minute. Then, a 1mm thickness layer of laboratory composite (Gradia-GC Corp-Japan) was packed into the rings and then cured in the light polymerization device (Signum-Heraeus Kulzer-Germany) for 90 seconds. The second layer was also put on the first layer with the same

dimensions and was again cured in the light polymerization device for 90 seconds. Finally, all samples were subjected to curing for 180 seconds (Fig. 1). Samples were maintained at the room temperature for 10 minutes. A half sample of each group was immersed in water at 37[degrees]C for 24 hours and the other half was thermocycled (3000 cycles, 5-55[degrees]c, dwell time of 30s). Then shear bond strength test was performed using universal testing machine (Zwick, Germany) at a cross head speed of 0.5mm/min. By dividing the failure load recorded for each sample (N) to the bonded area ([mm.sup.2]), the bond strength (MPa) is obtained: [tau](MPa) = F(N) / A([mm.sup.2])

In the next stage, the fracture type was evaluated by scanning electron microscopy (SEM) (KYKYEM3200).

The values of shear bond strength were analyzed using SPSS version 16.0 software. To determine the difference in values of the shear bond strength in various groups, ANOVA was used. Student t-test compared the shear bond strength between pre- and post- thermocycled specimens. P values less than 0.05 were considered to indicate statistical significance.

RESULTS AND DISCUSSION

Findings:

The study results showed that the highest bond strength before thermocycling is related to the group coated with silicated glass nano-particles with the mean and standard deviation of 8.12 [+ or -] 1.78 Mpa; whereas for the group without coating, the bond strength is 3.86 [+ or -] 0.94 Mpa. The mean difference of bond strength before thermocycling was statistically significant between two groups (p-value: 0.000). The highest bond strength after thermocycling is related to the group coated with silicated glass nano-particles with a mean and standard deviation of 7.60 [+ or -] 1.90 Mpa; while for the group without coating, the bond strength is 3.39 [+ or -] 0.76 Mpa. The mean difference of bond strength before thermocycling was statistically significant between two groups (pvalue: 0.000) (Fig. 2).

The difference of bond strength is statistically significant between the group with and without coating before and after thermocycling (P < 0.05). But doing or not doing thermocycling does not significantly change the bond strength neither in the group with coating nor in the group without it. Results of X-ray analysis shows that silicated glass nano-particles coating has been formed as an amorphous layer on the zirconia surface (Fig. 3).

Evaluating the surface of samples with an Atomic Force Microscope (AFM) was conducted to study the roughness of surface and evaluate the coating creation in the nanometer scale (<100nm). The mean roughness was 14.1nm for the group without coating and 38.9nm for the group coated with silicated glass nano-particles (Fig. 4). As AFM analysis shows, the surface roughness of both groups has been remained in the nanometer range. Also, results of the mean roughness indicate that the uniform and proper coating has been created on samples using sol-gel method. In terms of comparing the mode of failure between groups (by SEM), the facture has been mixed in the group coated with nano-particles (cohesive and adhesives) and adhesive type in the group without coating (Fig. 5).

Discussion:

Several methods have been proposed to do the preparation process of ceramics' surfaces that among them, sandblasting, silica coating, applying metal primer and tin plating can be pointed out. It has been determined that using sandblasting method is able to create a suitable surface roughness on the ceramic surface. One of its disadvantages is that during the process, alumina particles penetrate to the restorative materials due to the entered velocity and pressure and even in some cases, are not also able to clean through etching by acid or ultrasonic cleaning. Due to the possibility of starting the subcritical cracks in zirconia when the restoration is thin, the system is also criticized [9, 11, 12]. Air abrasion of dental materials has the potential to remove a significant amount of material that can affect their clinical adaptation [13]. Due to zirconia resistance against acid and lack of its appropriate response against the usual methods of preparation by etching, the results of typical preparation treatments is doubtful [3]. Zirconia ceramics are not silica base, therefore a physical-chemical challenge for the durable and reliable resin bond exist while the chemical stable silane-silica bond cannot be directly created [14]. In recent years, coating techniques with silica have been applied for converting the silica free surface of zirconia into the rich surface of silica so that the chemical bond is established with silane. Coating with silica has commonly been used by Tribochemical method (eg. Rocatec or CoJet) in a way that the zirconia surface is air abraded with alumina particles coated by nano-silica leading to nano- silica penetration into the zirconia surface. Studies show that coating with silica by Tribochemical method and silanization substantially increases the bond strength between zirconia and resin materials [15-17]; however, it is still not clear that; this is because of coating with silica or the surface roughness from air abrasion. Coating with silica and subsequently silane application has been introduced for the surface preparation of soft materials such as metals, while zirconia is a dense material and mechanically tough which may be hard to coat with silica. Studies indicate that coating with silica by Tribochemical method and then, silane application has an effect similar to that of air abrasion with alumina particles to improve the resin- zirconia bond strength. As a result, the surface roughness increases by this method [18-21]. It has been reported that coating with silica by Tribochemical method does not create stable and durable resin- zirconia bond [6], probably due to the lack of strong silica binding to the zirconia surface. Several methods have been presented for coating with silica. In the present study, the sol-gel method has been used for coating. The present study aimed to investigate the effect of nano-particles coating of silicated glass on the shear bond strength of indirect composite veneer and zirconia ceramic.

In their study, Jevnikar et al. (2010) examined the effects of alumina nano-particles coating on values of bond strength of resin to zirconia ceramics before and after thermocycling [22]. Results showed that in the zirconia samples, the alumina nano-particles coating could significantly increase the resin bond strength regardless of the primary surface treatment technique. Thermocycling had no effect on values of bond strength in the groups coated with alumina nano-particles. The results are in line with the present study. In the present study, the mean shear bond strength of group coated with nano-particles before and after the thermocycling has been significantly increased compared to the control group. Although in both groups, following the thermocycling, the bond strength has been reduced but not significantly. While in a study aimed to evaluate the bond strength between indirect composite and ceramic zirconia before and after thermocycling, Komine et al concluded that the bond strength of indirect composite to zirconia is affected by the type of primer and thermocycling [23]. The present study was performed in laboratory (in vitro) and its results should be interpreted with regard to the limitations of these studies. In laboratory conditions, the applied forces are different with forces existing in clinical conditions. In the oral cavity, there are types of stresses such as thermal changes, humidity, acidity and microbial plaque changes whose simulation is difficult in laboratory conditions [24].

In the present study, a half of samples in each group were stored in water at 37[degrees]C for 24 hours (in order to simulate the oral environment) and another half were thermocycled(3000 cycles, 5-55[degrees]c). The effect of longterm storage which is very important in simulating clinical conditions was not reviewed in the present study. However, short-term bond strength tests are also important in predicting the way of performance of materials and laboratory methods in clinical conditions. SEM results indicated that in the silicated glass nanoparticles coating group, the failure mode was mixed type (cohesive and adhesives) whereas in the group without coating the fracture was the adhesive type.

Conclusion:

Considering the increased bond strength of indirect composite to coated zirconia substrate before and after thermocycling and the simplicity of surface treatment, this technique of coating with silicated glass nanoparticles by sol-gel method can be used to increase the bond strength and success rate of prosthodontic treatments.

ARTICLE INFO

Article history:

Received 21 September 2014

Received in revised form 25 November 2014

Accepted 22 December 2014

Available online 3 January 2015

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(1) Alhavaz Abdolhamid, (2) Rabiee Sayed Mahmood, (3) Soltankarimi Vahid, (4) Bagheri Mohammad Ali, (5) Dadgar Sepideh, (6) Ramezani Mojtaba, (7) Pourahmadi Sadegh

(1) Dental Materials Research Center, Assistant Professor, Department of Prosthodontics, Babol University of Medical Sciences, Babol, Iran.

(2) Materials Engineering Group, Department of Mechanical Engineering, Babol University of Technology, Babol, Iran.

(3) Assistant Professor, Department of Prosthodontics, Babol University of Medical Sciences, Babol, Iran.

(4) Assistant Professor, Department of Prosthodontics, Birjand University of Medical Sciences, Birjand, Iran.

(5) Post Graduate Student, Department of Orthodontics, Dental Faculty, Azad University of Isfahan, Isfahan, Iran.

(6) Assistant Professor, Department of Prosthodontics, Kermanshah University of Medical Sciences, Kermanshah, Iran.

(7) Assistant Professor, Department of Prosthodontics, Yasouj University of Medical Sciences, Yasouj, Iran.

Corresponding Author: Bagheri Mohammad Ali, Birjand University of Medical Sciences, Birjand, Iran,

E-mail: aliibagheri@yahoo.com

Fig. 2 Comparison of bond strength between groups.

Mean Shear bond strength (MPa)

AS      3.86
AS+TC   3.40
SC      8.12
SC+TC   7.60

Error bars: 95% CI

p < 0.001

Note: Table made from bar graph.
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Author:Abdolhamid, Alhavaz; Mahmood, Rabiee Sayed; Vahid, Soltankarimi; Ali, Bagheri Mohammad; Sepideh, Dad
Publication:Advances in Environmental Biology
Article Type:Report
Date:Nov 1, 2014
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