Photomedicine and Laser Surgery Volume 31, Number 12, 2013 ª Mary Ann Liebert, Inc. Pp. 619–625 DOI: 10.1089/pho.2013.3489
Effect of Er:YAG Laser Irradiation on Bonding Property of Zirconia Ceramics to Resin Cement Yihua Lin, DDS,1 Xiaomeng Song, DDS, PhD,2 Yaming Chen, DDS, PhD,3 Qingping Zhu, DDS,4 and Wei Zhang, DDS, PhD 4
Abstract
Objective: This study aimed to investigate whether or not an erbium: yttrium-aluminum-garnet (Er:YAG) laser could improve the bonding property of zirconia ceramics to resin cement. Background data: Surface treatments can improve the bonding properties of dental ceramics. However, little is known about the effect of Er:YAG laser irradiated on zirconia ceramics. Materials and Methods: Specimens of zirconia ceramic pieces were made, and randomly divided into 11 groups according to surface treatments, including one control group (no treatment), one air abrasion group, and nine Er:YAG laser groups. The laser groups were subdivided by applying different energy intensities (100, 200, or 300 mJ) and irradiation times (5, 10, or 15 sec). After surface treatments, ceramic pieces had their surface morphology observed, and their surface roughness was measured. All specimens were bonded to resin cement. Shear bond strength was measured after the bonded specimens were stored in water for 24 h, and additionally aged by thermocycling. Statistical analyses were performed using one way analysis of variance (ANOVA) and Tukey’s test for shear bond strength, and Dunnett’s t test for surface roughness, with a = 0.05. Results: Er:YAG laser irradiation changed the morphological characteristics of zirconia ceramics. Higher energy intensities (200, 300 mJ) could roughen the ceramics, but also caused surface cracks. There were no significant differences in the bond strength between the control group and the laser groups treated with different energy intensities or irradiation times. Air abrasion with alumina particles induced highest surface roughness and shear bond strength. Conclusions: Er:YAG laser irradiation cannot improve the bonding property of zirconia ceramics to resin cement. Enhancing irradiation intensities and extending irradiation time have no benefit on the bond of the ceramics, and might cause material defect.
Introduction
A
s one of the new ceramic systems, zirconia ceramic has become popular in recent years because of its high strength and toughness, good aesthetic quality, and chemical durability.1–3 With the development of computer-aided design/computer aided manufacturing (CAD/CAM), zirconia ceramic is applied to complicated dental restorations such as inlays, veneers, all-ceramic crowns, and fixed partial denture (FPD) frameworks.4 A durable and stable bond between ceramics and resin cements is fundamental for the longterm performance of all ceramic indirect restorations. The adhesion of dental restorative materials is usually established by the formation of chemical bond and micromechanical
interlocking.5 Chemical bond describes intermolecular forces at the interface. Micromechanical interlocking attributes adhesion to interpenetration of components of the two surfaces, and it is one of the key mechanisms for an adequate bond between ceramics and resin cements.6 Ceramic surface conditioning methods have been shown to have great influence on the bond strength of full ceramic restorations.7 However, the composition and physical properties of high-strength zirconium oxide ceramics differ substantially from those of silica-based ceramics. The absence of glassy phase and silicon dioxide makes them resistant to etching by hydrofluoric acid, and not amenable to silanization.8 For this reason, alternative methods for treating the inner surfaces of this material prior to bonding are necessary.
1
Department of Stomatology, The Chinese Medicine Hospital of Xiangshan in Zhejiang Province, Ningbo, PR China. Research Institute of Stomatology, Nanjing Medical University, Department of Oral and Maxillofacial Surgery, Stomatological Hospital of Jiangsu Province, Nanjing, PR China. 3 Research Institute of Stomatology, Nanjing Medical University, Department of General Dentistry, Stomatological Hospital of Jiangsu Province, Nanjing, PR China. 4 Research Institute of Stomatology, Nanjing Medical University, Department of Oral Special Consultation, Stomatological Hospital of Jiangsu Province, Nanjing, PR China. 2
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620 Surface treatment of using the Rocatec system or tribochemical silica coating allows for chemical bond to a silane coupling agent and to resin cement.9 Air abrasion with aluminum oxide (Al2O3) particles (under suitable treatment condition) was shown to increase the bond strength of zirconia ceramics by improving micromechanical interlocking.10 To date, a surface pretreatment of air abrasion with Al2O3 particles, in combination with the composite resin cement containing a modified phosphate monomer10methacryloyloxydecyldihydrogen phosphate (MDP), has been identified as a key factor in achieving a stable and durable bond for zirconia-based ceramics,11–13 and is widely used in in-clinic bonding of zirconia ceramic restorations. Although air abrasion increases the area available for bonding and improves the wettability of ceramic materials,14 flaws created by air abrasion may function as crack initiators in ceramic materials, compromising their mechanical properties and long-term performance.15 Recently, lasers have become widely used in medicine and dentistry. Among kinds of lasers, the erbium: yttrium-aluminumgarnet (Er:YAG) laser has been proposed for different clinical dentistry applications, including carious dentin removal, cavity preparation, surface conditioning, and as a surface treatment method for indirect restorations.16–18 Many studies have investigated that laser irradiation can be used as an alternative to acid etching of enamel or dentin for bonding dental materials to the tooth surface.19,20 Research has described the effects of an Er:YAG laser, as a surface treatment method, on the material characteristics of dental ceramics.21,22 Akin et al.21 concluded that roughening the surface of yttria-stabilized tetragonal zirconia polycrysta (Y-TZP) ceramic by an Er:YAG laser increased the shear bond strength of ceramic to dentin and reduced the microleakage. Another study,22 however, showed the opposite conclusion; that Er:YAG laser treatment of the zirconia surface did not result in a durable resin cement/ceramic bond. Although these studies investigated the effect of Er:YAG laser irradiation on zirconia ceramics, there is little information on the evaluation of the effect of different energy intensities and irradiation time of Er:YAG laser on the bond of zirconia ceramics. Therefore, the aim of this study was to investigate the effect of Er:YAG laser with different energy intensities and irradiation time on the bonding property of zirconia ceramics to resin cement. The two null hypotheses were that: first, Er:YAG laser irradiation could improve the bonding property of zirconia ceramics, and second, enhancing irradiation intensities and extending irradiation time could induce higher bond strength of zirconia ceramics.
LIN ET AL. ferent surface treatments. After grinding with 800 grit silicone carbide, the superficial area to be further treated was delineated with adhesive tape and then treated using different methods. The 11 groups were as follows: One control group: no surface treatment was used. One air abrasion group: treated with air abrasion with alumina particles (Vita, Germany), 110 lm AluminiumOxide for 15 sec under 0.2 MPa pressure, at the distance of 10 mm. Nine laser groups: Irradiated with a k of 2940 nm Er:YAG laser (Fotona; Ljubljana, Slovenia). The ceramic surfaces were coated with graphite prior to laser irradiation to increase the absorption of energy.22 A 1300 lm diameter straight-type sapphire tip was used perpendicularly to the surface at a distance of 1 mm. The surfaces were lased using a fine water spray during freehand operation. The water spray was supplied by the laser equipment. The pulse repetition rate was set at 10 Hz, and medium-short pulse (MSP) mode pulse width (100 ls) was used. The nine groups of laser were subdivided by applying different energy intensities and irradiation times. Table 1 showed the different parameters of laser irradiation of the nine groups. After surface treatments, the adhesive tape was removed and the ceramic pieces were ultrasonically cleaned in 96% isopropanol for 3 min and then in distilled water for 5 min, dried by air, with 21 specimens in each group. Morphological study using scanning electron microscopy (SEM) Following the surface treatments, one ceramic piece was selected randomly from each group and sputtered with a gold alloy conductive layer and the surface modality was observed by a scanning electron microscope ( JEOL 6400, Japan ElectronOptics Ltd., Tokyo, Japan). Images from each surface were registered at · 3000 magnification. The microscope operated at an accelerating voltage of 20 kV with a working distance between 12 and 37 mm. Surface roughness measurement The average surface roughness (Ra, lm) of the treated ceramic pieces was measured with a surface roughness tester
Table 1. Combinations of the Different Parameters of Er:YAG Laser Irradiation Used in Nine Laser Groups
Materials and Methods Surface treatments A dental zirconia ceramic (Kavo Everest ZS blank; KaVo Dental GmbH, Bismarckring, Germany) was obtained from the manufacturer and made into 231 pieces (10 · 10 · 2 mm) by using the Kavo Everest CAD/CAM system. These ceramic pieces were cleaned by acetone and refined water for 15 min to remove the factors that inhibit adhesion, dried naturally in the atmosphere. Then, each of the pieces was embedded in an acrylic resin block with one surface of the ceramic piece uncovered for bonding. The specimens were randomly divided into 11 groups (n = 21) according to dif-
Groups 1 2 3 4 5 6 7 8 9
Energy intensities (mJ)
Pulse repetition rate (Hz)
Power (W)
Irradiation time (sec)
100 100 100 200 200 200 300 300 300
10 10 10 10 10 10 10 10 10
1 1 1 2 2 2 3 3 3
5 10 15 5 10 15 5 10 15
Er:YAG LASER ON BOND OF ZIRCONIA CERAMICS (Surftest402 Analyzer Mitutoyo Corporation, Tokyo, Japan). Three tracings at different locations on each specimen were obtained, and a mean value was calculated. The average of these mean values was used as the score of surface roughness for each group. Shear bond strength test A resin cement (Clearfil SA Cement, Kuraray Co Ltd, Osaka, Japan) was bonded to ceramic specimens of all groups using a procedure described in detail in a previous report.10 Teflon tubes with an internal diameter of 4 and 3 mm in height were placed on the treated ceramic specimens and filled with the resin cement. The resin cement was light polymerized for 40 sec with a dental curing light (Demi, Kerr Corporation, Middleton, WI) from the top and each side of the specimen to ensure an optimal polymerization. The resin cement was bonded to the ceramic specimens and then the Teflon tube was gently removed from the specimen. All bonded specimens of each group (n = 20) were stored in 37C distilled water for 24 h, and half of them (n = 10) were additionally subjected to thermocycling TC (20,000 · ) in water (5/55C) during 666 h (*28 days), using a thermal cycling machine (Rika-Kogyo, Hachioji, Japan). The shear bond strength was tested using a universal testing machine (InstronBluehill, 4301, USA) after the water storage and thermocycling. The specimen was fixed in a machine jig and shear force, at a speed of 1.0 mm/min,23 was applied to the bonding interface until a fracture occurred. The direction of the shear force was parallel with the bonding interface. The shear bond strength was recorded automatically. Statistical analysis Statistical analysis was performed using SPSS statistics system for Windows (SPSS/PC, Vers. 17.0; SPSS, Chicago, IL). Following the Bartlett test, the data of the roughness of ceramic surface were analyzed by Dunnett’s t test. The data of shear bond strength of different groups were analyzed by one way analysis of variance (ANOVA) and Tukey’s tests for pairwise comparisons among groups. The shear bond strength of the specimens before and after thermocycling was analyzed by paired t test. All the statistical significance was established at the 5% level. Fracture mode Following the shear bond strength test, all bonding fractured faces of the specimens were evaluated with an optical microscope (Olympus U-SPT; Olympus Optical Co., Tokyo, Japan) at · 100 magnification. The failure modes were evaluated and classified into one of the following types: type A, adhesive failure between ceramics and cement; type B, adhesive failure between ceramics and cement combined with cohesive failure in cement; and type C, cohesive failure in cement. Results Morphological study by SEM The surface modality of the ceramic pieces after different treatments observed by SEM is shown in Fig. 1. Three
621 treatments induced different surface modality. The control group showed a more flat surface than did the other two groups. Air abrasion caused the rougher surface with irregularities on the surface of the ceramic, there was no obvious discernible defect on the surface when compared with the untreated surface. As to laser groups, pit structure and enlarged crystal particles of ceramic material were seen on the ceramic surface. The surface modality of the ceramics treated with different irradiation time (5, 10, 15 sec) but same output power, was similar. However, different output power (100, 200, 300 mJ) induced different surface modality. In the 100 mJ group, there were no obvious material defects, whereas the cracks and flaws on surfaces irradiated with 300 mJ could easily be detected. Surface roughness The average surface roughness of the treated ceramic specimens is recorded in Table 2. Air abrasion induced the highest roughness value among all groups ( p = 0.000). Higher energy intensity (200, 300 mJ) of laser irradiation showed rougher surface than did the control group ( p < 0.05). There were no significant differences of surface roughness among the laser groups treated with different irradiation time (5, 10, 15 sec) under the same energy intensity ( p > 0.05). Shear bond strength The mean value of shear bond strength and standard deviations (SD) of different groups after 24 h water storage and thermocycling was presented in Table 3. The air abrasion group showed significantly higher shear bond strength than other groups before and after thermocycling ( p = 0.000). The shear bond strength of all laser groups was not significantly different from that of the control group after both 24 h water storage and thermocycling ( p > 0.05). No significant differences were found among the laser groups treated with different energy intensities or irradiation time ( p > 0.05). After thermocycling, the shear bond strength of all groups had no obvious decrease ( p > 0.05). Fracture mode Frequency of failure mode after shear bond strength test of each group is shown in Table 4. The results indicated that although various patterns of failure were observed among the tested groups, the frequency of failure mode of the three groups was different. Failure mode of type A was frequently observed in the control group and laser groups, whereas failure modes of type B and type C, but not type A were observed in the air abrasion group. Few cement residuals were observed on the bonding fracture face of control group and laser groups, whereas many were observed on that of air abrasion group. Discussion Parameters of the surface treatments Excessive air abrasion induces chipping or a high loss of ceramic material, compromising the mechanical properties and long-term performance of the ceramic restorations.24 In this study, we treated ceramics with 110 lm aluminium
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LIN ET AL.
FIG. 1. Representative surface modality of the zirconia ceramic after different surface treatments (scanning electron microscopic [SEM] photograph, · 3000 magnification, white bar 10 lm). (a) Control (untreated). (b) Air abrasion with 110 lm alumina particles. (c) Er:YAG laser irradiation with 100 mJ for 5 sec. (d) Er:YAG laser irradiation with 100 mJ for 10 sec. (e) Er:YAG laser irradiation with 100 mJ for 15 sec. (f) Er:YAG laser irradiation with 200 mJ for 5 sec. (g) Er:YAG laser irradiation with 200 mJ for 10 sec. (h) Er:YAG laser irradiation with 200 mJ for 15 sec. (i) Er:YAG laser irradiation with 300 mJ for 5 sec. (j) Er:YAG laser irradiation with 300 mJ for 10 sec. (k) Er:YAG laser irradiation with 300 mJ for 15 sec.
oxide for 15 sec under 0.2 MPa pressure, which was widely used as the surface treatment for ceramic restorations before bond.23 Different studies used laser treatments with different parameters, and their results of surface roughness and bond strength were inconsistent. Akin et al.21 used 150 mJ for laser irradiating, and they reported that this parameter could be
Table 2. Surface Roughness Value of Zirconia Ceramics After Different Treatments (n = 20) Surface roughness (Ra, lm) Surface treatment
Mean(SD)
Control (untreated) Air abrasion 100 mJ for 5 sec 100 mJ for 10 sec 100 mJ for 15 sec 200 mJ for 5 sec 200 mJ for 10 sec 200 mJ for 15 sec 300 mJ for 5 sec 300 mJ for 10 sec 300 mJ for 15 sec
0.38 0.90 0.43 0.43 0.47 0.55 0.56 0.59 0.59 0.64 0.66
(0.06) (0.16) (0.08) (0.09) (0.10) (0.12) (0.13) (0.11) (0.12) (0.12) (0.15)
The control is the reference level.
Dunnett’s t test, p value 0.000 0.919 0.943 0.406 0.013 0.010 0.001 0.001 0.000 0.000
performed before luting of Y-TZP ceramics. However, Foxton et al.22 performed Er:YAG laser with 200 mJ, and they found that this parameter did not enhance the strength of the bond. Furthermore, stronger laser output power (400 or 500 mJ) was proven to damage the ceramic materials.1,6 Table 3. Shear Bond Strength Value (Mean [SD]) of Zirconia Ceramics to Resin Cement After Different Treatments (n = 20) Shear bond strength (MPa) Surface treatment Control (untreated) Air abrasion 100 mJ for 5 sec 100 mJ for 10 sec 100 mJ for 15 sec 200 mJ for 5 sec 200 mJ for 10 sec 200 mJ for 15 sec 300 mJ for 5 sec 300 mJ for 10 sec 300 mJ for 15 sec
24h water storage (n = 10) 3.87 12.03 4.26 3.66 3.83 4.60 5.37 4.82 4.88 5.74 5.61
(1.17)a (2.58)b (0.90)a (1.03)a (0.88)a (1.57)a (1.37)a (1.46)a (1.73)a (1.86)a (1.47)a
Thermocycling (n = 10) 3.81 12.16 3.90 3.26 4.09 4.30 4.26 4.19 4.58 5.41 5.52
(1.07)a (3.38)b (1.11)a (0.90)a (1.28)a (1.47)a (1.07)a (1.28)a (1.47)a (1.46)a (1.51)a
Groups with different letters are statistically significantly different ( p < 0.001).
Er:YAG LASER ON BOND OF ZIRCONIA CERAMICS
623 probably the result of the different composition of the ceramic material and its reflectance.
Table 4. Frequency of Failure Mode After the Shear Bond Strength Test Surface treatment Control ( untreated ) Air abrasion 100 mJ for 5 sec 100 mJ for 10 sec 100 mJ for 15 sec 200 mJ for 5 sec 200 mJ for 10 sec 200 mJ for 15 sec 300 mJ for 5 sec 300 mJ for 10 sec 300 mJ for 15 sec
Type A
Type B
Type C
14 0 13 15 13 14 10 11 13 9 10
6 16 7 5 7 6 10 8 7 10 10
0 4 0 0 0 0 0 1 0 1 0
Type A, adhesive failure between ceramics and cement; type B, cohesive failure in cement combined with adhesive failure between ceramics and cement; Type C, cohesive failure in cement (n = 20).
Therefore, in this study, we chose the lower output powers of 100, 200, and 300 mJ to irradiate ceramics for 5, 10, and 15 sec, respectively, to compare the effect of different irradiation parameters on the surface properties and bond efficiency of zirconia ceramics. Surface modality and roughness after different treatments Three surface treatments were applied in this study and the surface modality and roughness of the materials were strongly affected by the treatments. The group of air abrasion showed totally different morphological characteristics from that of control and laser groups. Although some microcracks could be seen on the surface of air abrasion specimens, compared with the control group, no obvious discernible defect was found. The air abrasion group also induced the highest roughness among all groups. Different energy intensities and irradiation times of Er:YAG laser were tested in this investigation. The results of SEM examination clearly indicated that this laser altered the external surfaces of zirconia ceramics. Irradiation at 200 mJ induced significant enlargement of crystal particles and some melting of the materials, and irradiation at 300 mJ even caused some cracks and loss of the materials. These results were consistent with a previous study,25 which indicated that irradiation with 200 mJ provided mild surface alterations, whereas higher laser power settings (400 and 600 mJ) caused excessive material deterioration, making them unsuitable as surface treatments for zirconia surfaces. Gokce et al.18 also suggested that during laser treatment, local temperature changes caused by heating and cooling phases created internal tensions that could damage the material. The surface roughness varied with the change of irradiation intensities. Higher irradiation intensities (200, 300 mJ) induced rougher surface than did the control group, whereas the effect of irradiation time (5, 10, 15 sec) on the roughness was not obvious. A previous study26 showed a similar result; that Er:YAG laser treated surface was rougher than the control group (untreated). However, in another work,6 the authors observed that even at a higher energy intensity (500 mJ), Er:YAG laser was not able to cause adequate roughness of the ceramic’s surface. These differences are
Evaluation of shear bond strength before and after thermocycling Shear stresses are believed to be major stresses inducing to bonding failure of restorative materials,27 and shear bond strength test is a widely used method for evaluating bonding effectiveness.28 In this study, shear bond strength test was used to investigate the bonding property of zirconia ceramics to Clearfil SA Cement, which is a resin cement modified with phosphate monomer10 MDP. The resin cement containing MDP monomer has been shown to induce stable bond efficiency, according to previous studies.29 According to the results of this study, air abrasion significantly increased the shear bond strength of the ceramics compared with other groups, and no obvious material defect was found on the ceramic surface. However, in some studies,15,30,31 air abrasion with Al2O3 particles was not proven to enhance the bond strength of the ZrO2, and even may cause adverse effects on mechanical properties of zirconia such as flexural strength and reliability. Different treatment methods and test methods may cause different results. The parameters of the sandblasting treatment should be controlled for avoiding material loss and defect. Various studies have proven that to dentin and enamel structures, the macroscopic and microscopic irregularities produced by Er:YAG laser irradiation constitute a mechanism of adhesion32,33 and increase the bond strength. However, in this study, the shear bond strength of the ceramics treated by Er:YAG laser irradiation was not significantly increased compared with the control group. Although higher energy intensity (200, 300 mJ) caused rougher surface, their shear bond strength was not significantly higher than that of the control group. This might be because the increased surface roughness was not enough to induce significant higher bond strength. These results were in agreement with those of Foxton,22 who concluded that Er:YAG laser can increase the surface area of zirconia ceramics, but cannot improve the bond strength. Although there were no significant differences in bond strength between the control group and the laser groups ( £ 300 mJ output power), the results indicated that the bond strength might be significantly enhanced by laser irradiation with even stronger output power ( > 300 mJ). However, laser irradiation with stronger output power would cause material defects, according to the results of this study. Extending irradiation time was expected to increase the surface roughness and shear bond strength of the ceramics; however, it did not work in this study. The principal effect of laser energy is the conversion of light energy into heat, and the most important interaction between the laser and substrate is the absorption of the laser energy by the substrate.34 The pigmentation of the surface and its water content along with other surface characteristics determine the amount of energy that is absorbed by the irradiated surface.35 In dentinal surfaces, the incident energy is absorbed by water molecules present in dentin crystalline structures and organic components. The laser produces microexplosions during hard tissue ablation, resulting in macroscopic and microscopic irregularities that may create a surface for adhesion.32 Nevertheless, zirconia ceramic is a
624 water-free material, and has an opaque coloration, which might influence the absorption of laser energy. Furthermore, the mechanical properties of zirconia ceramics might be negatively affected by changes in temperature, which might induce phase transformation.1 However, some studies suggested the opposite results, showing that Er:YAG laser could increase the bond strength of zirconia ceramics.18,21 Different composition of the ceramic material might be the reason for these different results. To simulate oral conditions, thermocycling was considered to be clinically relevant to aging parameters, as this factor may contribute to the degradation of cement–ceramic bonds over time.36 In this study, the shear bond strength of all groups had no significant decrease after thermocycling, which demonstrated that the resin cement containing MDP monomer could offer stable bond efficiency to zirconia ceramics no matter which surface treatment was used. Cavalcanti et al. reported that air-abraded and laser-irradiated specimens presented higher bond strength with the BisGMA-based resin cement than with the MDP-based cement. However, in their study, only the immediate bond strength (24 h after the polymerization of the resin cements) was tested, and different results might have been found if the specimens had been submitted to an aging protocol. Therefore, the long-term bond efficient should be tested in future studies. Failure mode of bond The quality of the bond should not be assessed based on bond strength data alone. The bond failure mode analyses should provide important information about bonding effectiveness.37 Cohesive fracture and mixed (cohesive fracture combines with adhesive failure) patterns are clinically preferable to the total adhesive type of failure, as the latter is usually associated with low bond strength values.38 In this study, compared with the control group and the laser groups, bond failure of the air abrasion group was mainly a mixed pattern. Cohesive failure in resin cement was also found in this group, which means that the bond strength between cement and ceramics is higher than the cohesion strength of the cement material. The control group and the laser groups had similar situations in bond failure modes, with the main failure modes of adhesive failure and mixed patterns. The micromechanical interlocking of the control group and the laser groups was not strong enough to offer higher bond strength. The findings of this study required the rejection of the null hypothesis. Er:YAG laser irradiation could not improve the bonding property of zirconia ceramics to resin cement. And enhancing irradiation intensities and extending irradiation time could not induce higher bond strength of zirconia ceramics. However, in vitro studies do not replace clinical studies, and their outcomes should be interpreted with caution.39 In addition, conclusions drawn from one zirconia ceramic system may not be applicable to other commercial systems. Whether or not the superficial flaws produced by laser irradiation will propagate through the material, and reduce the strength of the ceramics, is still not clear. Future studies are necessary to evaluate the effect of Er:YAG laser irradiation on the bonding property of other zirconia ceramic types. The material strength and compositional changes of
LIN ET AL. ceramics materials following laser irradiation should also be analyzed. Conclusions Within the limitations of this study, the following conclusions were drawn: Er:YAG laser irradiation can roughen the surface of zirconia ceramics, but cannot increase the shear bond strength of the ceramics to resin cement. Enhancing irradiation intensities and extending irradiation time cannot induce higher bond strength of zirconia ceramics, and may cause material defect. Air abrasion with alumina particles can obviously improve bonding property of zirconia ceramics to resin cement; however, the treatment parameters should be controlled for avoiding material loss and defect. Er:YAG laser irradiation is not recommended as a surface pretreatment before the bonding of clinic zirconia restorations. Acknowledgments This study was supported by the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Science and Technology Development Foundation (General Program) of Nanjing Medical University (2010NJMU110), and the project of Xiangshan science and technology plans (2013C6008). The authors thank Kuraray Co. for supplying the materials for this study. Author Disclosure Statement No competing financial interests exist. References 1. Cavalcanti, A.N., Foxton, R.M., Watson, T.F., Oliveira, M.T., Giannini, M., and Marchi, G.M. (2009). Bond strength of resin cements to a zirconia ceramic with different surface treatments. Oper. Dent. 34, 280–287. 2. Al-Amleh, B., Lyons, K., and Swain, M. (2010). Clinical trials in zirconia: a systematic review. J. Oral. Rehabil. 37, 641–652. 3. Denry, I., and Kelly, J.R. (2008). State of the art of zirconia for dental applications. Dent. Mater. 24, 299–307. 4. Alghazzawi, T.F., Lemons, J., Liu, P.R., Essig, M.E., and Janowski, G.M. (2012). The failure load of CAD/CAM generated zirconia and glass-ceramic laminate veneers with different preparation designs. J. Prosthet. Dent. 108, 386–393. 5. Martinez–Insua, A., Da Silva, D.L., Rivera, F.G., and Santana–Penin, U.A. (2000). Differences in bonding to acid-etched or Er:YAG laser treated enamel and dentin surfaces. J. Prosthet. Dent. 84, 280–288. 6. Shiu, P., De Souza–Zaroni, W.C., Eduardo Cde. P., and Youssef, M.N. (2007). Effect of feldspathic ceramic surface treatments on bond strength to resin cement. Photomed. Laser Surg. 25, 291–296. 7. Casucci, A., Monticelli, F., Goracci, C., et al. (2011). Effect of surface pre-treatments on the zirconia ceramic-resin cement microtensile bond strength. Dent. Mater. 27, 1024–1030. 8. Borges, G.A., Sophr, A.M., de Goes, M.F., Sobrinho, L.C., and Chan, D.C. (2003). Effect of etching and airborne particle abrasion on the microstructure of different dental ceramics. J. Prosthet. Dent. 89, 479–488. 9. Piascik, J.R., Swift, E.J., Thompson, J.Y., Grego, S., and Stoner, B.R. (2009). Surface modification for enhanced silanation of zirconia ceramics. Dent. Mater. 25, 1116–1121.
Er:YAG LASER ON BOND OF ZIRCONIA CERAMICS 10. Kara, H.B., Ozturk, A.N., Aykent, F., Koc, O., and Ozturk, B. (2011). The effect of different surface treatments on roughness and bond strength in low fusing ceramics. Lasers Med. Sci. 26, 599–604. 11. Kern, M., Barloi, A., and Yang, B. (2009). Surface conditioning influences zirconia ceramic bonding. J. Dent. Res. 88, 817–822. 12. Wolfart, M., Lehmann, F., Wolfart, S., and Kern, M. (2007). Durability of the resin bond strength to zirconia ceramic after using different surface conditioning methods. Dent. Mater. 23, 45–50. 13. Yoshida, K., Tsuo, Y., and Astuta, M. (2006). Bonding of dual-cured resin cement to zirconia ceramic using phosphate acid ester monomer and zirconate coupler. J. Biomed. Mater. Res. 77, 28–33. 14. Kumbuloglu, O., Lassila, L.V., User, A., and Vallittu, P.K. (2006). Bonding of resin composite luting cements to zirconium oxide by two air-particle abrasion methods. Oper. Dent. 31, 248–255. 15. Usumez A, Hamdemirci N, Koroglu BY, Simsek I, Parlar O, Sari T. (2013). Bond strength of resin cement to zirconia ceramic with different surface treatments. Lasers Med Sci. 28, 259–266. 16. Bader, C., and Krejci, I. (2006). Indications and limitations of Er:YAG laser applications in dentistry. Am. J. Dent. 19, 178–186. 17. Delme, K.I., and De Moor, R.J. (2007). Scanning electron microscopic evaluation of enamel and dentin surfaces after Er:YAG laser preparation and laser conditioning. Photomed. Laser Surg. 25, 393–401. 18. Gokce, B., Ozpinar, B., Dundar, M., Comlekoglu, E., Sen, B.H., and Gungor, M.A. (2007). Bond strengths of allceramics: acid vs laser etching. Oper. Dent. 32, 173–178. 19. Baygin, O., Korkmaz, F.M., Tuzuner, T., and Tanriver, M. (2012). The effect of different enamel surface treatments on the microleakage of fissure sealants. Lasers Med. Sci. 27, 153–160. 20. Hamamci, N., Akkurt, A., and Bas xaran, G. (2010). In vitro evaluation of microleakage under orthodontic brackets using two different laser etching, self etching and acid etching methods. Lasers Med. Sci. 25, 811–816. 21. Akin, H., Tugut, F., Akin, G.E., Guney, U., and Mutaf, B. (2012). Effect of Er:YAG laser application on the shear bond strength and microleakage between resin cements and YTZP ceramics. Lasers Med. Sci. 27, 333–338. 22. Foxton, R.M., Cavalcanti, A.N., Nakajima, M. et al. (2011). Durability of resin cement bond to aluminium oxide and zirconia ceramics after air abrasion and laser treatment. J. Prosthodont. 20, 84–92. 23. Zhang, W., Masumi, S.I., Song, X.M. (2010). Bonding property of two resin-reinforced glass-ionomer cements to zirconia ceramic. Quintessence. Int. 41, e132–140. 24. Zhang, Y., Lawn, B.R., Rekow, E.D., and Thompson, V.P. (2004). Effect of sandblasting on the long-term performance of dental ceramics. J. Biomed. Mater. Res. B. Appl. Biomater. 71, 381–386. 25. Cavalcanti, A.N., Pilecki, P., Foxton, R.M., et al. (2009). Evaluation of the surface roughness and morphologic features of Y-TZP ceramics after different surface treatments. Photomed. Laser Surg. 27, 473–479.
625 26. Subasi, M.G., and Inan, O. (2012). Influence of surface treatments and resin cement selection on bonding to zirconia. Lasers. Med. Sci. [Epub ahead of print]. 27. Beloica, M., Goracci, C., Carvalho, C.A. et al. (2010). Microtensile vs microshear bond strength of all-in-one adhesives to unground enamel. J. Adhes. Dent. 12, 427–433. 28. Valandro, L.F., ozcan, M., Amaral, R., Vanderlei, A., and Bottino, M.A. (2008). Effect of testing methods on the bond strength of resin to zirconia-alumina ceramic: microtensile versus shear test. Dent. Mater. J. 27, 849–855. 29. Valandro, L.F., Della Bona, A., Bottino, M.A., and Neisser, M.P. (2005). The effect of ceramic surface treatment on bonding to densely sintered alumina ceramic. J. Prosthet. Dent. 93, 253–259. 30. Fischer, J., Stawarczyk, B., Sailer, I., and Ha¨mmerle, C.H. (2010). Shear bond strength between veneering ceramics and ceria-stabilized zirconia/alumina. J. Prosthet. Dent. 103, 267–274. 31. Akin, H., Ozkurt, Z., Kirmali, O., Kazazoglu, E., and Ozdemir, A.K. (2011). Shear bond strength of resin cement to zirconia ceramic after aluminum oxide sandblasting and various laser treatments. Photomed. Laser Surg. 29, 797–802. 32. Aranha, A.C., Turbino, M.L., Powell, G.L., and Eduardo Cde, P. (2005). Assessing microleakage of class V resin composite restorations after Er:YAG laser and bur preparation. Lasers Surg. Med. 37, 172–177. 33. Yazici, A.R., Agarwal, I., and Campillo–Funollet, M. (2013). Effect of laser preparation on bond strength of a self-adhesive flowable resin. Lasers Med. Sci. 28, 343–347. 34. van As, G. (2004). Erbium lasers in dentistry. Dent. Clin. North. Am. 48, 1017–1059. 35. Coluzzi, D.J. (2004). Fundamentals of dental lasers: science and instruments. Dent. Clin. North. Am. 48, 751–770. 36. D’Amario, M., Campidoglio, M., Morresi, A.L., Luciani, L., Marchetti, E., and Baldi, M. (2010). Effect of thermocycling on the bond strength between dual-cured resin cements and zirconium-oxide ceramics. J. Oral. Sci. 52, 425–430. 37. Della, B.A., Anusavice, K.J., and Mecholsky, JJ. Jr. (2003). Failure analysis of resin composite bonded to ceramic. Dent. Mater. 19, 693–699. 38. Toledano, M., Osorio, R., Osorio, E., et al. (2007). Durability of resin-dentin bonds: effects of direct/indirect exposure and storage media. Dent. Mater. 23, 885–892. 39. Phark, J.H., Duarte, S. Jr., Blatz, M., and Sadan, A. (2009). An in vitro evaluation of the long–term resin bond to a new densely sintered high-purity zirconium-oxide ceramic surface. J. Prosthet. Dent. 101, 29–38.
Address correspondence to: Wei Zhang Research Institute of Stomatology Nanjing Medical University 136 Hanzhong Rd. Nanjing 210029 PR China E-mail: [email protected]
Effect of Er:YAG Laser Irradiation on Bonding Property of Zirconia Ceramics to Resin Cement Yihua Lin, DDS,1 Xiaomeng Song, DDS, PhD,2 Yaming Chen, DDS, PhD,3 Qingping Zhu, DDS,4 and Wei Zhang, DDS, PhD 4
Abstract
Objective: This study aimed to investigate whether or not an erbium: yttrium-aluminum-garnet (Er:YAG) laser could improve the bonding property of zirconia ceramics to resin cement. Background data: Surface treatments can improve the bonding properties of dental ceramics. However, little is known about the effect of Er:YAG laser irradiated on zirconia ceramics. Materials and Methods: Specimens of zirconia ceramic pieces were made, and randomly divided into 11 groups according to surface treatments, including one control group (no treatment), one air abrasion group, and nine Er:YAG laser groups. The laser groups were subdivided by applying different energy intensities (100, 200, or 300 mJ) and irradiation times (5, 10, or 15 sec). After surface treatments, ceramic pieces had their surface morphology observed, and their surface roughness was measured. All specimens were bonded to resin cement. Shear bond strength was measured after the bonded specimens were stored in water for 24 h, and additionally aged by thermocycling. Statistical analyses were performed using one way analysis of variance (ANOVA) and Tukey’s test for shear bond strength, and Dunnett’s t test for surface roughness, with a = 0.05. Results: Er:YAG laser irradiation changed the morphological characteristics of zirconia ceramics. Higher energy intensities (200, 300 mJ) could roughen the ceramics, but also caused surface cracks. There were no significant differences in the bond strength between the control group and the laser groups treated with different energy intensities or irradiation times. Air abrasion with alumina particles induced highest surface roughness and shear bond strength. Conclusions: Er:YAG laser irradiation cannot improve the bonding property of zirconia ceramics to resin cement. Enhancing irradiation intensities and extending irradiation time have no benefit on the bond of the ceramics, and might cause material defect.
Introduction
A
s one of the new ceramic systems, zirconia ceramic has become popular in recent years because of its high strength and toughness, good aesthetic quality, and chemical durability.1–3 With the development of computer-aided design/computer aided manufacturing (CAD/CAM), zirconia ceramic is applied to complicated dental restorations such as inlays, veneers, all-ceramic crowns, and fixed partial denture (FPD) frameworks.4 A durable and stable bond between ceramics and resin cements is fundamental for the longterm performance of all ceramic indirect restorations. The adhesion of dental restorative materials is usually established by the formation of chemical bond and micromechanical
interlocking.5 Chemical bond describes intermolecular forces at the interface. Micromechanical interlocking attributes adhesion to interpenetration of components of the two surfaces, and it is one of the key mechanisms for an adequate bond between ceramics and resin cements.6 Ceramic surface conditioning methods have been shown to have great influence on the bond strength of full ceramic restorations.7 However, the composition and physical properties of high-strength zirconium oxide ceramics differ substantially from those of silica-based ceramics. The absence of glassy phase and silicon dioxide makes them resistant to etching by hydrofluoric acid, and not amenable to silanization.8 For this reason, alternative methods for treating the inner surfaces of this material prior to bonding are necessary.
1
Department of Stomatology, The Chinese Medicine Hospital of Xiangshan in Zhejiang Province, Ningbo, PR China. Research Institute of Stomatology, Nanjing Medical University, Department of Oral and Maxillofacial Surgery, Stomatological Hospital of Jiangsu Province, Nanjing, PR China. 3 Research Institute of Stomatology, Nanjing Medical University, Department of General Dentistry, Stomatological Hospital of Jiangsu Province, Nanjing, PR China. 4 Research Institute of Stomatology, Nanjing Medical University, Department of Oral Special Consultation, Stomatological Hospital of Jiangsu Province, Nanjing, PR China. 2
619
620 Surface treatment of using the Rocatec system or tribochemical silica coating allows for chemical bond to a silane coupling agent and to resin cement.9 Air abrasion with aluminum oxide (Al2O3) particles (under suitable treatment condition) was shown to increase the bond strength of zirconia ceramics by improving micromechanical interlocking.10 To date, a surface pretreatment of air abrasion with Al2O3 particles, in combination with the composite resin cement containing a modified phosphate monomer10methacryloyloxydecyldihydrogen phosphate (MDP), has been identified as a key factor in achieving a stable and durable bond for zirconia-based ceramics,11–13 and is widely used in in-clinic bonding of zirconia ceramic restorations. Although air abrasion increases the area available for bonding and improves the wettability of ceramic materials,14 flaws created by air abrasion may function as crack initiators in ceramic materials, compromising their mechanical properties and long-term performance.15 Recently, lasers have become widely used in medicine and dentistry. Among kinds of lasers, the erbium: yttrium-aluminumgarnet (Er:YAG) laser has been proposed for different clinical dentistry applications, including carious dentin removal, cavity preparation, surface conditioning, and as a surface treatment method for indirect restorations.16–18 Many studies have investigated that laser irradiation can be used as an alternative to acid etching of enamel or dentin for bonding dental materials to the tooth surface.19,20 Research has described the effects of an Er:YAG laser, as a surface treatment method, on the material characteristics of dental ceramics.21,22 Akin et al.21 concluded that roughening the surface of yttria-stabilized tetragonal zirconia polycrysta (Y-TZP) ceramic by an Er:YAG laser increased the shear bond strength of ceramic to dentin and reduced the microleakage. Another study,22 however, showed the opposite conclusion; that Er:YAG laser treatment of the zirconia surface did not result in a durable resin cement/ceramic bond. Although these studies investigated the effect of Er:YAG laser irradiation on zirconia ceramics, there is little information on the evaluation of the effect of different energy intensities and irradiation time of Er:YAG laser on the bond of zirconia ceramics. Therefore, the aim of this study was to investigate the effect of Er:YAG laser with different energy intensities and irradiation time on the bonding property of zirconia ceramics to resin cement. The two null hypotheses were that: first, Er:YAG laser irradiation could improve the bonding property of zirconia ceramics, and second, enhancing irradiation intensities and extending irradiation time could induce higher bond strength of zirconia ceramics.
LIN ET AL. ferent surface treatments. After grinding with 800 grit silicone carbide, the superficial area to be further treated was delineated with adhesive tape and then treated using different methods. The 11 groups were as follows: One control group: no surface treatment was used. One air abrasion group: treated with air abrasion with alumina particles (Vita, Germany), 110 lm AluminiumOxide for 15 sec under 0.2 MPa pressure, at the distance of 10 mm. Nine laser groups: Irradiated with a k of 2940 nm Er:YAG laser (Fotona; Ljubljana, Slovenia). The ceramic surfaces were coated with graphite prior to laser irradiation to increase the absorption of energy.22 A 1300 lm diameter straight-type sapphire tip was used perpendicularly to the surface at a distance of 1 mm. The surfaces were lased using a fine water spray during freehand operation. The water spray was supplied by the laser equipment. The pulse repetition rate was set at 10 Hz, and medium-short pulse (MSP) mode pulse width (100 ls) was used. The nine groups of laser were subdivided by applying different energy intensities and irradiation times. Table 1 showed the different parameters of laser irradiation of the nine groups. After surface treatments, the adhesive tape was removed and the ceramic pieces were ultrasonically cleaned in 96% isopropanol for 3 min and then in distilled water for 5 min, dried by air, with 21 specimens in each group. Morphological study using scanning electron microscopy (SEM) Following the surface treatments, one ceramic piece was selected randomly from each group and sputtered with a gold alloy conductive layer and the surface modality was observed by a scanning electron microscope ( JEOL 6400, Japan ElectronOptics Ltd., Tokyo, Japan). Images from each surface were registered at · 3000 magnification. The microscope operated at an accelerating voltage of 20 kV with a working distance between 12 and 37 mm. Surface roughness measurement The average surface roughness (Ra, lm) of the treated ceramic pieces was measured with a surface roughness tester
Table 1. Combinations of the Different Parameters of Er:YAG Laser Irradiation Used in Nine Laser Groups
Materials and Methods Surface treatments A dental zirconia ceramic (Kavo Everest ZS blank; KaVo Dental GmbH, Bismarckring, Germany) was obtained from the manufacturer and made into 231 pieces (10 · 10 · 2 mm) by using the Kavo Everest CAD/CAM system. These ceramic pieces were cleaned by acetone and refined water for 15 min to remove the factors that inhibit adhesion, dried naturally in the atmosphere. Then, each of the pieces was embedded in an acrylic resin block with one surface of the ceramic piece uncovered for bonding. The specimens were randomly divided into 11 groups (n = 21) according to dif-
Groups 1 2 3 4 5 6 7 8 9
Energy intensities (mJ)
Pulse repetition rate (Hz)
Power (W)
Irradiation time (sec)
100 100 100 200 200 200 300 300 300
10 10 10 10 10 10 10 10 10
1 1 1 2 2 2 3 3 3
5 10 15 5 10 15 5 10 15
Er:YAG LASER ON BOND OF ZIRCONIA CERAMICS (Surftest402 Analyzer Mitutoyo Corporation, Tokyo, Japan). Three tracings at different locations on each specimen were obtained, and a mean value was calculated. The average of these mean values was used as the score of surface roughness for each group. Shear bond strength test A resin cement (Clearfil SA Cement, Kuraray Co Ltd, Osaka, Japan) was bonded to ceramic specimens of all groups using a procedure described in detail in a previous report.10 Teflon tubes with an internal diameter of 4 and 3 mm in height were placed on the treated ceramic specimens and filled with the resin cement. The resin cement was light polymerized for 40 sec with a dental curing light (Demi, Kerr Corporation, Middleton, WI) from the top and each side of the specimen to ensure an optimal polymerization. The resin cement was bonded to the ceramic specimens and then the Teflon tube was gently removed from the specimen. All bonded specimens of each group (n = 20) were stored in 37C distilled water for 24 h, and half of them (n = 10) were additionally subjected to thermocycling TC (20,000 · ) in water (5/55C) during 666 h (*28 days), using a thermal cycling machine (Rika-Kogyo, Hachioji, Japan). The shear bond strength was tested using a universal testing machine (InstronBluehill, 4301, USA) after the water storage and thermocycling. The specimen was fixed in a machine jig and shear force, at a speed of 1.0 mm/min,23 was applied to the bonding interface until a fracture occurred. The direction of the shear force was parallel with the bonding interface. The shear bond strength was recorded automatically. Statistical analysis Statistical analysis was performed using SPSS statistics system for Windows (SPSS/PC, Vers. 17.0; SPSS, Chicago, IL). Following the Bartlett test, the data of the roughness of ceramic surface were analyzed by Dunnett’s t test. The data of shear bond strength of different groups were analyzed by one way analysis of variance (ANOVA) and Tukey’s tests for pairwise comparisons among groups. The shear bond strength of the specimens before and after thermocycling was analyzed by paired t test. All the statistical significance was established at the 5% level. Fracture mode Following the shear bond strength test, all bonding fractured faces of the specimens were evaluated with an optical microscope (Olympus U-SPT; Olympus Optical Co., Tokyo, Japan) at · 100 magnification. The failure modes were evaluated and classified into one of the following types: type A, adhesive failure between ceramics and cement; type B, adhesive failure between ceramics and cement combined with cohesive failure in cement; and type C, cohesive failure in cement. Results Morphological study by SEM The surface modality of the ceramic pieces after different treatments observed by SEM is shown in Fig. 1. Three
621 treatments induced different surface modality. The control group showed a more flat surface than did the other two groups. Air abrasion caused the rougher surface with irregularities on the surface of the ceramic, there was no obvious discernible defect on the surface when compared with the untreated surface. As to laser groups, pit structure and enlarged crystal particles of ceramic material were seen on the ceramic surface. The surface modality of the ceramics treated with different irradiation time (5, 10, 15 sec) but same output power, was similar. However, different output power (100, 200, 300 mJ) induced different surface modality. In the 100 mJ group, there were no obvious material defects, whereas the cracks and flaws on surfaces irradiated with 300 mJ could easily be detected. Surface roughness The average surface roughness of the treated ceramic specimens is recorded in Table 2. Air abrasion induced the highest roughness value among all groups ( p = 0.000). Higher energy intensity (200, 300 mJ) of laser irradiation showed rougher surface than did the control group ( p < 0.05). There were no significant differences of surface roughness among the laser groups treated with different irradiation time (5, 10, 15 sec) under the same energy intensity ( p > 0.05). Shear bond strength The mean value of shear bond strength and standard deviations (SD) of different groups after 24 h water storage and thermocycling was presented in Table 3. The air abrasion group showed significantly higher shear bond strength than other groups before and after thermocycling ( p = 0.000). The shear bond strength of all laser groups was not significantly different from that of the control group after both 24 h water storage and thermocycling ( p > 0.05). No significant differences were found among the laser groups treated with different energy intensities or irradiation time ( p > 0.05). After thermocycling, the shear bond strength of all groups had no obvious decrease ( p > 0.05). Fracture mode Frequency of failure mode after shear bond strength test of each group is shown in Table 4. The results indicated that although various patterns of failure were observed among the tested groups, the frequency of failure mode of the three groups was different. Failure mode of type A was frequently observed in the control group and laser groups, whereas failure modes of type B and type C, but not type A were observed in the air abrasion group. Few cement residuals were observed on the bonding fracture face of control group and laser groups, whereas many were observed on that of air abrasion group. Discussion Parameters of the surface treatments Excessive air abrasion induces chipping or a high loss of ceramic material, compromising the mechanical properties and long-term performance of the ceramic restorations.24 In this study, we treated ceramics with 110 lm aluminium
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LIN ET AL.
FIG. 1. Representative surface modality of the zirconia ceramic after different surface treatments (scanning electron microscopic [SEM] photograph, · 3000 magnification, white bar 10 lm). (a) Control (untreated). (b) Air abrasion with 110 lm alumina particles. (c) Er:YAG laser irradiation with 100 mJ for 5 sec. (d) Er:YAG laser irradiation with 100 mJ for 10 sec. (e) Er:YAG laser irradiation with 100 mJ for 15 sec. (f) Er:YAG laser irradiation with 200 mJ for 5 sec. (g) Er:YAG laser irradiation with 200 mJ for 10 sec. (h) Er:YAG laser irradiation with 200 mJ for 15 sec. (i) Er:YAG laser irradiation with 300 mJ for 5 sec. (j) Er:YAG laser irradiation with 300 mJ for 10 sec. (k) Er:YAG laser irradiation with 300 mJ for 15 sec.
oxide for 15 sec under 0.2 MPa pressure, which was widely used as the surface treatment for ceramic restorations before bond.23 Different studies used laser treatments with different parameters, and their results of surface roughness and bond strength were inconsistent. Akin et al.21 used 150 mJ for laser irradiating, and they reported that this parameter could be
Table 2. Surface Roughness Value of Zirconia Ceramics After Different Treatments (n = 20) Surface roughness (Ra, lm) Surface treatment
Mean(SD)
Control (untreated) Air abrasion 100 mJ for 5 sec 100 mJ for 10 sec 100 mJ for 15 sec 200 mJ for 5 sec 200 mJ for 10 sec 200 mJ for 15 sec 300 mJ for 5 sec 300 mJ for 10 sec 300 mJ for 15 sec
0.38 0.90 0.43 0.43 0.47 0.55 0.56 0.59 0.59 0.64 0.66
(0.06) (0.16) (0.08) (0.09) (0.10) (0.12) (0.13) (0.11) (0.12) (0.12) (0.15)
The control is the reference level.
Dunnett’s t test, p value 0.000 0.919 0.943 0.406 0.013 0.010 0.001 0.001 0.000 0.000
performed before luting of Y-TZP ceramics. However, Foxton et al.22 performed Er:YAG laser with 200 mJ, and they found that this parameter did not enhance the strength of the bond. Furthermore, stronger laser output power (400 or 500 mJ) was proven to damage the ceramic materials.1,6 Table 3. Shear Bond Strength Value (Mean [SD]) of Zirconia Ceramics to Resin Cement After Different Treatments (n = 20) Shear bond strength (MPa) Surface treatment Control (untreated) Air abrasion 100 mJ for 5 sec 100 mJ for 10 sec 100 mJ for 15 sec 200 mJ for 5 sec 200 mJ for 10 sec 200 mJ for 15 sec 300 mJ for 5 sec 300 mJ for 10 sec 300 mJ for 15 sec
24h water storage (n = 10) 3.87 12.03 4.26 3.66 3.83 4.60 5.37 4.82 4.88 5.74 5.61
(1.17)a (2.58)b (0.90)a (1.03)a (0.88)a (1.57)a (1.37)a (1.46)a (1.73)a (1.86)a (1.47)a
Thermocycling (n = 10) 3.81 12.16 3.90 3.26 4.09 4.30 4.26 4.19 4.58 5.41 5.52
(1.07)a (3.38)b (1.11)a (0.90)a (1.28)a (1.47)a (1.07)a (1.28)a (1.47)a (1.46)a (1.51)a
Groups with different letters are statistically significantly different ( p < 0.001).
Er:YAG LASER ON BOND OF ZIRCONIA CERAMICS
623 probably the result of the different composition of the ceramic material and its reflectance.
Table 4. Frequency of Failure Mode After the Shear Bond Strength Test Surface treatment Control ( untreated ) Air abrasion 100 mJ for 5 sec 100 mJ for 10 sec 100 mJ for 15 sec 200 mJ for 5 sec 200 mJ for 10 sec 200 mJ for 15 sec 300 mJ for 5 sec 300 mJ for 10 sec 300 mJ for 15 sec
Type A
Type B
Type C
14 0 13 15 13 14 10 11 13 9 10
6 16 7 5 7 6 10 8 7 10 10
0 4 0 0 0 0 0 1 0 1 0
Type A, adhesive failure between ceramics and cement; type B, cohesive failure in cement combined with adhesive failure between ceramics and cement; Type C, cohesive failure in cement (n = 20).
Therefore, in this study, we chose the lower output powers of 100, 200, and 300 mJ to irradiate ceramics for 5, 10, and 15 sec, respectively, to compare the effect of different irradiation parameters on the surface properties and bond efficiency of zirconia ceramics. Surface modality and roughness after different treatments Three surface treatments were applied in this study and the surface modality and roughness of the materials were strongly affected by the treatments. The group of air abrasion showed totally different morphological characteristics from that of control and laser groups. Although some microcracks could be seen on the surface of air abrasion specimens, compared with the control group, no obvious discernible defect was found. The air abrasion group also induced the highest roughness among all groups. Different energy intensities and irradiation times of Er:YAG laser were tested in this investigation. The results of SEM examination clearly indicated that this laser altered the external surfaces of zirconia ceramics. Irradiation at 200 mJ induced significant enlargement of crystal particles and some melting of the materials, and irradiation at 300 mJ even caused some cracks and loss of the materials. These results were consistent with a previous study,25 which indicated that irradiation with 200 mJ provided mild surface alterations, whereas higher laser power settings (400 and 600 mJ) caused excessive material deterioration, making them unsuitable as surface treatments for zirconia surfaces. Gokce et al.18 also suggested that during laser treatment, local temperature changes caused by heating and cooling phases created internal tensions that could damage the material. The surface roughness varied with the change of irradiation intensities. Higher irradiation intensities (200, 300 mJ) induced rougher surface than did the control group, whereas the effect of irradiation time (5, 10, 15 sec) on the roughness was not obvious. A previous study26 showed a similar result; that Er:YAG laser treated surface was rougher than the control group (untreated). However, in another work,6 the authors observed that even at a higher energy intensity (500 mJ), Er:YAG laser was not able to cause adequate roughness of the ceramic’s surface. These differences are
Evaluation of shear bond strength before and after thermocycling Shear stresses are believed to be major stresses inducing to bonding failure of restorative materials,27 and shear bond strength test is a widely used method for evaluating bonding effectiveness.28 In this study, shear bond strength test was used to investigate the bonding property of zirconia ceramics to Clearfil SA Cement, which is a resin cement modified with phosphate monomer10 MDP. The resin cement containing MDP monomer has been shown to induce stable bond efficiency, according to previous studies.29 According to the results of this study, air abrasion significantly increased the shear bond strength of the ceramics compared with other groups, and no obvious material defect was found on the ceramic surface. However, in some studies,15,30,31 air abrasion with Al2O3 particles was not proven to enhance the bond strength of the ZrO2, and even may cause adverse effects on mechanical properties of zirconia such as flexural strength and reliability. Different treatment methods and test methods may cause different results. The parameters of the sandblasting treatment should be controlled for avoiding material loss and defect. Various studies have proven that to dentin and enamel structures, the macroscopic and microscopic irregularities produced by Er:YAG laser irradiation constitute a mechanism of adhesion32,33 and increase the bond strength. However, in this study, the shear bond strength of the ceramics treated by Er:YAG laser irradiation was not significantly increased compared with the control group. Although higher energy intensity (200, 300 mJ) caused rougher surface, their shear bond strength was not significantly higher than that of the control group. This might be because the increased surface roughness was not enough to induce significant higher bond strength. These results were in agreement with those of Foxton,22 who concluded that Er:YAG laser can increase the surface area of zirconia ceramics, but cannot improve the bond strength. Although there were no significant differences in bond strength between the control group and the laser groups ( £ 300 mJ output power), the results indicated that the bond strength might be significantly enhanced by laser irradiation with even stronger output power ( > 300 mJ). However, laser irradiation with stronger output power would cause material defects, according to the results of this study. Extending irradiation time was expected to increase the surface roughness and shear bond strength of the ceramics; however, it did not work in this study. The principal effect of laser energy is the conversion of light energy into heat, and the most important interaction between the laser and substrate is the absorption of the laser energy by the substrate.34 The pigmentation of the surface and its water content along with other surface characteristics determine the amount of energy that is absorbed by the irradiated surface.35 In dentinal surfaces, the incident energy is absorbed by water molecules present in dentin crystalline structures and organic components. The laser produces microexplosions during hard tissue ablation, resulting in macroscopic and microscopic irregularities that may create a surface for adhesion.32 Nevertheless, zirconia ceramic is a
624 water-free material, and has an opaque coloration, which might influence the absorption of laser energy. Furthermore, the mechanical properties of zirconia ceramics might be negatively affected by changes in temperature, which might induce phase transformation.1 However, some studies suggested the opposite results, showing that Er:YAG laser could increase the bond strength of zirconia ceramics.18,21 Different composition of the ceramic material might be the reason for these different results. To simulate oral conditions, thermocycling was considered to be clinically relevant to aging parameters, as this factor may contribute to the degradation of cement–ceramic bonds over time.36 In this study, the shear bond strength of all groups had no significant decrease after thermocycling, which demonstrated that the resin cement containing MDP monomer could offer stable bond efficiency to zirconia ceramics no matter which surface treatment was used. Cavalcanti et al. reported that air-abraded and laser-irradiated specimens presented higher bond strength with the BisGMA-based resin cement than with the MDP-based cement. However, in their study, only the immediate bond strength (24 h after the polymerization of the resin cements) was tested, and different results might have been found if the specimens had been submitted to an aging protocol. Therefore, the long-term bond efficient should be tested in future studies. Failure mode of bond The quality of the bond should not be assessed based on bond strength data alone. The bond failure mode analyses should provide important information about bonding effectiveness.37 Cohesive fracture and mixed (cohesive fracture combines with adhesive failure) patterns are clinically preferable to the total adhesive type of failure, as the latter is usually associated with low bond strength values.38 In this study, compared with the control group and the laser groups, bond failure of the air abrasion group was mainly a mixed pattern. Cohesive failure in resin cement was also found in this group, which means that the bond strength between cement and ceramics is higher than the cohesion strength of the cement material. The control group and the laser groups had similar situations in bond failure modes, with the main failure modes of adhesive failure and mixed patterns. The micromechanical interlocking of the control group and the laser groups was not strong enough to offer higher bond strength. The findings of this study required the rejection of the null hypothesis. Er:YAG laser irradiation could not improve the bonding property of zirconia ceramics to resin cement. And enhancing irradiation intensities and extending irradiation time could not induce higher bond strength of zirconia ceramics. However, in vitro studies do not replace clinical studies, and their outcomes should be interpreted with caution.39 In addition, conclusions drawn from one zirconia ceramic system may not be applicable to other commercial systems. Whether or not the superficial flaws produced by laser irradiation will propagate through the material, and reduce the strength of the ceramics, is still not clear. Future studies are necessary to evaluate the effect of Er:YAG laser irradiation on the bonding property of other zirconia ceramic types. The material strength and compositional changes of
LIN ET AL. ceramics materials following laser irradiation should also be analyzed. Conclusions Within the limitations of this study, the following conclusions were drawn: Er:YAG laser irradiation can roughen the surface of zirconia ceramics, but cannot increase the shear bond strength of the ceramics to resin cement. Enhancing irradiation intensities and extending irradiation time cannot induce higher bond strength of zirconia ceramics, and may cause material defect. Air abrasion with alumina particles can obviously improve bonding property of zirconia ceramics to resin cement; however, the treatment parameters should be controlled for avoiding material loss and defect. Er:YAG laser irradiation is not recommended as a surface pretreatment before the bonding of clinic zirconia restorations. Acknowledgments This study was supported by the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Science and Technology Development Foundation (General Program) of Nanjing Medical University (2010NJMU110), and the project of Xiangshan science and technology plans (2013C6008). The authors thank Kuraray Co. for supplying the materials for this study. Author Disclosure Statement No competing financial interests exist. References 1. Cavalcanti, A.N., Foxton, R.M., Watson, T.F., Oliveira, M.T., Giannini, M., and Marchi, G.M. (2009). Bond strength of resin cements to a zirconia ceramic with different surface treatments. Oper. Dent. 34, 280–287. 2. Al-Amleh, B., Lyons, K., and Swain, M. (2010). Clinical trials in zirconia: a systematic review. J. Oral. Rehabil. 37, 641–652. 3. Denry, I., and Kelly, J.R. (2008). State of the art of zirconia for dental applications. Dent. Mater. 24, 299–307. 4. Alghazzawi, T.F., Lemons, J., Liu, P.R., Essig, M.E., and Janowski, G.M. (2012). The failure load of CAD/CAM generated zirconia and glass-ceramic laminate veneers with different preparation designs. J. Prosthet. Dent. 108, 386–393. 5. Martinez–Insua, A., Da Silva, D.L., Rivera, F.G., and Santana–Penin, U.A. (2000). Differences in bonding to acid-etched or Er:YAG laser treated enamel and dentin surfaces. J. Prosthet. Dent. 84, 280–288. 6. Shiu, P., De Souza–Zaroni, W.C., Eduardo Cde. P., and Youssef, M.N. (2007). Effect of feldspathic ceramic surface treatments on bond strength to resin cement. Photomed. Laser Surg. 25, 291–296. 7. Casucci, A., Monticelli, F., Goracci, C., et al. (2011). Effect of surface pre-treatments on the zirconia ceramic-resin cement microtensile bond strength. Dent. Mater. 27, 1024–1030. 8. Borges, G.A., Sophr, A.M., de Goes, M.F., Sobrinho, L.C., and Chan, D.C. (2003). Effect of etching and airborne particle abrasion on the microstructure of different dental ceramics. J. Prosthet. Dent. 89, 479–488. 9. Piascik, J.R., Swift, E.J., Thompson, J.Y., Grego, S., and Stoner, B.R. (2009). Surface modification for enhanced silanation of zirconia ceramics. Dent. Mater. 25, 1116–1121.
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Address correspondence to: Wei Zhang Research Institute of Stomatology Nanjing Medical University 136 Hanzhong Rd. Nanjing 210029 PR China E-mail: [email protected]
