Effect of Er,Cr:YSGG Laser Treatment on Microshear Bond Strength of Zirconia to Resin Cement Before and After Sintering Amir Ghasemia / Hamid Kermanshahb / Maryam Ghavamc / Afshin Nateghifardd / Hassan Torabzadehe / Ardalan Nateghifardf / Kaveh Zolfagharnasabg / Hadi Ahmadih Purpose: To evaluate the effect of Er,Cr:YSGG laser treatment on microshear bond strength of zirconia to resin cement before and after sintering. Materials and Methods: Ninety pre-sintered yttrium-stabilized tetragonal zirconia specimens (4 × 3 × 2 mm) were divided into 6 groups (n = 15). In group C, sintered zirconia was not treated (control group). In groups AS2 and AS3, sintered zirconia blocks were irradiated by Er,Cr:YSGG using a power of 2 and 3 W, respectively. Groups PS2 and PS3 consisted of pre-sintered blocks conditioned by Er,Cr:YSGG at 2 and 3 W, respectively. In group AA, sintered zirconia was air abraded with 50-μm alumina powder. One block was made using the same preparations as mentioned above and was morphologically assessed by SEM. Microcylinders of Panavia F 2.0 were placed on the treated surface of the groups. Samples were incubated at 37°C and 98% humidity for 48 h and then subjected to microshear bond strength testing. The mode of failure was evaluated. Data were analyzed by one-way ANOVA and Tukey’s HSD test (p < 0.05). Results: There was a statistically significant difference between group AA and the others (p < 0.0001). A significant difference was also noted between groups AS3 and C (p = 0.031). Complete surface roughness was seen in group AA and the bond failure was mostly cohesive, while in laser-treated groups, the surfaces roughness was much lower vs other groups, and the mode of failure was mostly adhesive. Conclusion: Laser treatment of pre-sintered Y-TZP cannot be recommended for improving the bond. Although sandblasting of sintered Y-TZP yielded better results than the rest of the groups, 3 W power after sintering can also be effective in enhancing the bonding strength of resin cement to zirconia. Keywords: Er,Cr:YSGG laser, pre- and post-sintered zirconia, microshear bond strength, surface treatment, failure mode analysis, SEM. J Adhes Dent 2014; 16: 377–382. 10.3290/j.jad.a32444
Submitted for publication: 29.12.13; accepted for publication: 28.03.14
ecently, the application of ceramics has led to major changes in the field of dentistry.44 Among these ceramics, zirconia has found a special place with its unique structural properties such as chemical stability, biocompatibility, high compressive strength, and coeffi-
cient of thermal expansion similar to that of dental hard tissues.7 Zirconia behaves in a polycrystalline manner according to the pressure and temperature conditions.4,7 Pure zirconia exists as a monoclinic structure at room tem-
Associate Professor, Preventive Dentistry Research Center, Research Institute of Dental Sciences, Department of Restorative Dentistry, School of Dentistry, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Idea, hypothesis, contributed to discussion.
Associate Professor, Iranian Center for Endodontic Research, Research Institute of Dental Science, Department of Restorative Dentistry, School of Dentistry, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Proofread the manuscript.
Associate Professor, Dental Research Center and Laser Research Center, Department of Restorative Dentistry, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran. Experimental design, contributed to discussion.
Undergraduate Student, Department of Biological Sciences, San Jose State University, San Jose, USA. Proofread the manuscript.
Dentist in Private Practice, Tehran; Dentist, School of Dentistry, Tehran University of Medical Sciences, International Campus, Tehran, Iran. Performed a certain test.
Dentist in Private Practice, Tehran, Iran. Consulted on and performed statistical evaluation.
Associate Professor, Department of Restorative Dentistry, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran. Experimental design, contributed to discussion. Dentist in Private Practice, Tehran; PhD Student, School of Dentistry, Tehran University of Medical Sciences, International Campus, Tehran, Iran. Idea, experimental design, performed the experiments in partial fulfillment of requirements for a degree, wrote the manuscript.
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Correspondence: Afshin Nateghifard, School of Dentistry, Tehran University of Medical Sciences, International Campus, Khaniabad no, Tehran, Iran. Tel: +98-912-550-1570, Fax: +98-212-225-8618. e-mail: [email protected]
Ghasemi et al
Table 1 Materials used in this study Materials
ZirokonZahn (Steger; Ahrntal, Italy)
Main component: ZrO2 (+HfO2) 95% Y2O3 4.95%-5.26%, Al2O3 0.15%-0.35%,SiO2 0.02%, Fe2O3 max 0.01%, Na2O max 0.04%
Panavia F 2.0 (Kuraray Medical; Osaka, Japan)
10-methacryloxydecyldihydrogen phosphate (MDP) Paste A: bisphenol-A polyethoxydimethacrylate (BPEDMA), MDP, aliphatic dimethacrlyate (DMA) Paste B: composite containing Al-Ba-B-Si glass/silica
perature. At 1170°C, it transforms into the tetragonal phase. Cooling down zirconia results in its tetragonal to monoclinic transformation, which subsequently increases the volume. This generates a considerable amount of stress and leads to cracking of the material. Yttria (yttrium oxide) is commonly used to stabilize the zirconia’s structure and maintain its remarkable characteristics. Consequently, at room temperature, Y-TZP (yttria-stabilized tetragonal zirconia polycrystals) can be found in its tetragonal crystalline phase, which is metastable.4,11,28 Preparing zirconium systems for establishing a proper bond to teeth is a challenging task.17,34 Etching with HF and the application of silane on zirconia is not an effective method, because stabilized, monophased zirconia does not have a glass structure; moreover, chemical bonding with silane is also not effective.6,8,9,15,30,43 In recent years, a number of research projects have been done on obtaining adhesive methods for establishing surface roughness and chemical bonds with zirconia ceramics.1,2,3,10,19,20,24,25,29,32 One of these methods is silica coating, which increases the silica content of the surface of zirconia.7,12,24 However, the silica layer may not have a strong connection to the zirconia surface, and thus be the weak interface of the bonded system.37 Another technique is selective infiltration etching.2,10 Beside confirmatory results,2,10 there is still concern related to the effect of intergrain porosities produced by this method on the mechanical properties of zirconia.1 Despite being closely studied in the past years, this method is very difficult to execute, because it requires special instruments and materials, and has a complex general protocol that contains several stages. In addition, a number of specific primers and adhesive resins for creating chemical bonding with zirconia have been introduced.11,15,25,35 However, some researchers are still doubtful about the effectiveness of this method in forming a truly durable bond with zirconia in long-term intraoral conditions.3,32 The plasma spray and fluorination methods are some of the newest techniques in use.31 These procedures are as complicated as the aforementioned method, require specific instruments, and are very expensive.32 One of the most commonly used methods of increasing the surface roughness and creating micromechanical retention is air abrasion.4,5,11,14,21,37,39,42 A number of articles with regard to increasing the monoclinic phase after the application of air abrasion have been published.18,28 378
However, due to the strength reduction caused by such phase transformation,28 it may be a questionable technique. Another recently developed method of increasing the surface roughness utilizes lasers, which have the advantage of chair-side execution. Laser devices such as Er:YAG, CO2, and Nd:YAG have been employed by many researchers to establish the surface roughness and irregularity.4,5,11,14,37,38,39,40,42 Er,Cr:YSGG is another effective laser system that has been studied on other substrates.22,33 However, the literature contains only limited data regarding this laser system in surface treatment of Y-TZP.27 Since post-sintering treatment may increase phase transformation, which results in damage to zirconia, presintering treatment has received more attention.18,28 Moreover, pre-sintered zirconia is softer, which facilitates creating surface roughness.28 The present study was performed to evaluate the microshear bond strength of resin cement to Y-TZP after surface treatments by either air abrasion or Er:Cr;YSGG irradiation before or after sintering. The null hypothesis was that laser irradiation before or after sintering zirconia would not improve the bond strength between Y-TZP and resin cements.
MATERIALS AND METHODS Ninety specimens of yttrium-oxide–stabilized zirconium blocks (Zirkonzahn, Steger; Ahrntal, Italy) 4 mm long, 3 mm wide, and 2 mm high were prepared (Table 1). The specimens were serially polished with 400-, 600-, and 800-grit silicon carbide abrasive papers (3M ESPE; St Paul, MN, USA) to obtain standardized flat surfaces. Then the samples were divided into 6 groups based on the type of preparation and sintering phase: y Group C: The control group consisted of 15 sintered zirconia blocks which received no treatment. y Group AS2: Fifteen sintered zirconia blocks were irradiated by Er,Cr:YSGG (G6/Waterlase, Biolase Technology; San Clemente, CA, USA) laser at a power output of 2 W with a 2780-nm wavelength, pulse duration of 140 μs, and a fixed repetition rate of 50 Hz for 50 s at a distance of 1 mm. The laser beam was delivered by the 800-μm diameter MZ8 tip. The laser irradiation was performed under an air/water (50%/1%) cooling system. The Journal of Adhesive Dentistry
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y Group AS3: Fifteen sintered zirconia blocks were irradiated in a manner similar to group AS2, but with 3 W power output. y Group PS2: Fifteen pre-sintered blocks were conditioned as in group AS2 and then densely sintered. y Group PS3: Fifteen pre-sintered blocks were conditioned as in group AS3 and then densely sintered. y Group AA: Fifteen sintered zirconia blocks were air abraded using a sandblasting device (Danville Materials; San Ramon, CA, USA) with 50-μm Al2O3 particles at 3.5 to 4.1 psi and a fixed distance of 2 to 3 mm from the bonding surface for 10 s. In order to morphologically assess the surface characteristics using SEM (Hitachi S-4160; Tokyo, Japan), one zirconia block was made as described above, polished up to 3000 grit, and sputter coated with gold (E5100, Polaron Equipment; Watford, Hertfordshire, UK). Observation was carried out at 150X and 2000X magnifications for pre- and post-sintering treatments, respectively. The specimens were sintered in a special furnace (MIHM-Vogt Dental Gerätebau; Stutensee, Germany) at 1500°C for 7 h in compliance with the manufacturer’s instructions. Specimens were ultrasonically cleaned (Quantrex 90, L&R Ultrasonics; Kearny, NJ, USA) in the pre-sintering stage for groups PS2 and PS3, and for the rest of the groups in the post-sintering stage, using 96% isopropanol for 3 min and dried in oil-free air before surface treatment. Pieces of Tygon tubing (Tygon, Norton Performance Plastic; Cleveland, OH, USA) with an inner diameter of 0.7 mm and height of 1 mm were used in bonding resin cements to zirconia blocks. Panavia F2.0 resin cement (Kuraray; Tokyo, Japan) was prepared and mixed according to manufacturer’s instructions (Table 1) and the Tygon tubes were filled with resin cement using a plugger (Dentsply Maillefer; Ballaiques, Switzerland). One tube was placed on each zirconia block and then light cured (Demetron LC, SDS/Kerr; Orange, CA, USA) for 20 s at a light intensity of 600 mW/cm2. All specimens were incubated at 37°C, 98% humidity for 48 h (Model PL455G PecoPooya Electronics; Tehran, Iran). Afterwards, the Tygon tubes were carefully removed from the resulting resin cylinders using a sharp scalpel (blade No. 11) under a stereomicroscope. All samples were checked for the absence of bubbles using a stereomicroscope at 40X magnification. For measuring bond strength between zirconia blocks and resin cements, all experimental groups were subjected to microshear bond strength testing (microtensile tester, Bisco; Schaumburg, IL, USA). In order to change the design of this testing device from microtensile to microshear, one flat metal plate was made and metal cylinders (0.7 mm × 10 mm) were laser soldered along a straight line. This part was attached to the microtensile device with a cyanoacrylate adhesive (Mitreaple, Beta Kymia; Istanbul, Turkey). The zirconia blocks were attached to the microtensile device using a cyanoacrylate adhesive (Mitreaple, Beta Kymia). In order to apply the shear force, an orthodontic wire with a diameter of Vol 16, No 4, 2014
Table 2 Means and standard deviations (SD) for each group and Tukey’s HSD test results Groups
2W after sintering (AS2)
3W after sintering (AS3)
2W pre-sintering (PS2)
3W pre-sintering (PS3)
Air abrasion (AA)
Same superscript letters indicates statistical similarity (p > 0.05, Tukey’s HSD).
0.4 mm was used. Loading was applied at a crosshead speed of 0.5 mm/min and the maximum force needed for breaking the specimens was noted. The microshear bond strength of each sample was calculated by dividing the force (in N) at debonding by the surface area of the resin cylinder (mm2). The debonded surfaces of specimens were examined with a stereomicroscope (Olympus, szx-9; Orangeburg, NY, USA) at 20X magnfication. The mode of failure was classified as adhesive (debonding only at the resin cement/ceramic interface) or cohesive (rupture in the cement). Afterwards, the percentage of failed samples in each group was determined. One SEM image was taken of each failure type at 30X magnification. The data analysis was performed by SPSS 16 (SPSS for Windows; SPSS; Chicago, IL, USA), and the Kolmogorov-Smirnov test was used to determine normal distribution. Levene’s test was used to assess the equality of variances among the tested groups. For statistical analysis, one-way ANOVA was used; if this showed significant differences, Tukey’s post-hoc HSD test was conducted for individual comparisons. A p-value < 0.05 was selected as the level of statistical significance in this study.
RESULTS Table 2 shows the mean (± SD) bond strength results. The results of one-way ANOVA indicated significant differences between experimental groups (p < 0.001). The highest and lowest mean microshear bond strengths were seen in groups AA and C, respectively. There was a statistically significant difference between group AA and the rest of the groups (p < 0.0001). A significant difference was also noted between group AS3 and group C (p = 0.031), as shown by Tukey’s post-hoc multiple comparison test (Table 2). The SEM images of experimental groups are shown in Figs 1 and 2. These micrographs revealed a completely roughened surface in samples treated with Al2O3 379
Ghasemi et al Fig 1a SEM image of pre-sintering laser-treated zirconia surface, group PS2. The surface exhibits distinct irregularities with larger dimensions.
Fig 1b SEM image of pre-sintering laser-treated zirconia surface, group PS3. The surface exhibits distinct irregularities with larger dimensions.
Fig 2a SEM image of post-sintering treatment of zirconia surface, group C.
Fig 2b SEM image of post-sintering treatment of zirconia surface, group AA.
Fig 2c SEM image of post-sintering treatment of zirconia surface,group AS2.
Fig 2d Group AS3. The rougher surface of group AA in comparison with groups C, AS2, and AS3 can be seen.
Table 3 Distribution of failure type (%) by group Groups
2W after sintering (AS2)
3W after sintering (AS3)
2W pre-sintering (PS2)
3W pre-sintering (PS3)
Air abrasion (AA)
air abrasion (Fig 2b). In both pre-sintered laser-treated samples, a larger-scale continuous ablation effect was seen (Fig 1). Limited surface irregularities of smaller dimensions were also seen in both post-sintered laser treated groups (Fig 2). Most failures were adhesive (Table 3). Figure 3 depicts SEM images of the fracture pattern in each group. 380
DISCUSSION In recent years, zirconium oxide ceramics have been attracting worldwide interest, and the use of this material is on the rise.11,44 Because of their outstanding fulfillment of function and aesthetic requirements, methods of bonding zirconia to dental tissue have gained a substantial amount of attention.33 Furthermore, zirconia is chemically neutral and highly resistant to hydrofluoric acid etching. Therefore, creating the micromechanical bond is a very challenging task. 6,8,915,30,43 In order to overcome this limitation, previous studies have used different surface treatment techniques.1,3,4,5,7,8,10,11,14,18,23,24,29 Laser irradiation has been found to be a relatively safe and convenient method of roughening the surface of materials.4 With respect to several other studies that evaluated the effect of laser treatment on surface roughness, the present article also assesses the effect of two different laser power outputs on the bond between zirconia and resin cement.4,5,11,14,27,38,40,42 The widespread use of erbium lasers in dental practice has made them the laser of choice for our study. Furthermore, Eduardo et al16 The Journal of Adhesive Dentistry
Ghasemi et al Fig 3a Adhesive failure after microshear testing (mostly seen in non-sandblasted groups). The red circle and arrow show the area in the center of the Tygon tube where fracture occurred. The particles around the cylinder are the remaining debris from the resin cement on the surface of the zirconia block. Fig 3b Cohesive (seen in sandblasted group). The red circle and arrow show the area in the center of the Tygon tube where fracture occurred. The particles around the cylinder are the remaining debris from the resin cement on the surface of the zirconia block.
esin of r der ent n i l Cy cem
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reported an acceptable effect of various power outputs of Er,Cr:YSGG laser from 0.5 to 5.0 W used on glassinfiltrate alumina blocks.16 In accordance with that study, the present authors utilized Er,Cr:YSGG laser at 2 and 3 W for surface treatment of Y-TZP blocks. Although other studies concluded that other lasers, eg, CO2, might be effective for conditioning zirconia surfaces,5,41 CO2 laser treatment has also been associated with material cracking,40 and was deemed unsuitable for this study. Panavia F2.0 resin cement was used in the current study because this cement contains 10-methacryloyloxydecyl dihydrogen phosphate (MDP) functional monomer (Table 1). Several studies have shown that resin cements composed of 10-MDP monomer can establish stronger bonds, due to the ability to form chemical bonds with metal oxides such as zirconia.6,11,20 Clinically, shear stresses account for the majority of stresses involved in bond failure between teeth and restorations.36 Microshear bond strength testing is designed to reduce the dimensions of the bonding area, because as the bonding area decreases in size, the risk of structural defects and cracks arising also decreases. Therefore, this study used the microshear method for measuring bond strength.36 In a 2012 study conducted by Moon et al,28 a high proportion of monoclinic structure on zirconia surfaces that were air abraded after being sintered was observed. This finding was also seen by Liu et al23 in a study on air abrading sintered zirconia. The process of sintering after applying the surface treatment decreases the percentage of the monoclinic phase.28 The reduction of the monoclinic phase as well as flaws and cracks in ceramic during sintering can be effective in increasing the bond strength, as was also observed in samples that were air abraded before being sintered. Since post-sintering treatments reduce the structural properties of zirconia,28 the present authors decided to evaluate the effect of laser on pre-sintered zirconia. A similar idea was previously tested in other studies regarding the effect of air abrasion in the pre-sintering stage.18,28 The data of the current study showed that air abrasion resulted in the highest bond strength (52 MPa). This finding is supported by several studies;5,11,18,28 however, a few other studies reported that the effect of laser on increasing the bond strength was equal to or greater than that of air abrasion.42 Such discrepant findings may be the result of different characteristics of the lasers and air abrasion techniques employed.
esin of r der ent n i l Cy cem
The SEM images taken after surface treatments showed greater micromechanical roughening and irregularities on sandblasted zirconia samples when compared with other groups (Fig 2b), supporting the results of the microshear test. However, an increase in microshear bond strength was also found in laser-treated groups, but this increase was significant only in the sintered group treated with a power output of 3 W (group AS3) compared to the control group, as supported by the SEM images from group AS3 (greater topographical changes) and group AS2 (only slightly changed surface) vs group C. This finding is consistent with those of Akyıl et al5 using Er:YAG and Liu et al23 using CO2 laser. The evaluation of bond strength values in the pre- and post-sintering laser groups showed that preparation sequence had no significant effect on the bond strength in either group. This finding agreed with Fazi et al18 and Moon et al,28 who examined the effect of air abrasion in the same manner. The morphological changes seen on the SEM images consisted in larger irregularities in presintering than in the post-sintering laser-treated groups (Fig 1). This finding was not associated with a significant increase in bond strength. Such unexpected results could have possibly occurred due to the fact that these irregularities did not produce surface roughness sufficient to enhance the bond strength. Some authors have suggested that increasing the duration of laser treatment may possibly improve the bond strength.41 However, in this study, since the Er,Cr:YSGG laser irradiation before or after sintering did not produce a significant improvement in bond strength, the authors failed to reject the null hypothesis. The failure mode analysis showed that the fracture pattern was adhesive in most of the specimens (Table 3), which was consistent with the lower microshear bond strength in non-sandblasted groups. Cohesive fracture in Panavia provided further evidence for the apparent increase of microshear bond strength in the sandblasted group.
CONCLUSION Within the limitations of this study, it can be concluded that surface treatment of sintered Y-TZP ceramic with air abrasion is more effective than Er,Cr:YSGG laser irradiation. Er,Cr:YSGG laser at only 3 W power after sintering can be regarded as another surface treatment option for roughening the zirconia surface to establish better bond strength with resin cements. 381
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ACKNOWLEDGMENTS This work is based on the thesis submitted to school of dentistry, Tehran University of Medical Sciences, International Campus in partial fulfillment of the doctorate degree in dentistry. The authors would like to thank Dr Mahammad Javad Kharrazi Fard for completing the statistical analysis.
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Clinical relevance: For obtaining a higher bond strength between zirconia restorations and resin cement, Er,Cr:YSGG laser irradiations before sintering zirconia cannot significantly improve the bond strength as much as the conventional method of air abrasion. However, utilizing Er,Cr:YSGG at 3 W power after sintering may serve as an alternative technique for enhancing the bond strength between zirconia and resin cement.
The Journal of Adhesive Dentistry