1http://dx.doi.org/10.20396/bjos.v18i0.8656586 Volume 18 2019 e191449 Original Article 1 School of Dentistry, University of Federal of Piaui, Teresina, Piauí, Brazil. 2 Department of Restorative Dentistry, School of Dentistry, University of Federal of Piauí, Teresina, Piauí, Brazil 3 School of Dentistry, University of São Paulo, São Paulo, Brazil Corresponding author: Urias Silva Vasconcelos School of Dentistry, University of Federal of Piaui, Teresina, Piauí, Brazil. Street Maestro Pedro Alcântara, nº 4708, Jurema Park, Teresina, Piaui, Brazil. Phone: +55 86 99419-2442. E-mail: urias.svasconcelos@gmail.com Received: November 19, 2018 Accepted: June 02, 2019 Effects of the combination of Er: YAG laser and sandblasting on the shear bond strength between zirconia and resin cement Urias Silva Vasconcelos1,*, Thalisson Saymo de Oliveira Silva1, Daylana Pacheco da Silva1, Graciela Maria Oliveira Sipaúba1, Valdimar da Silva Valente2, Raquel Virginia Zanetti3, Carmem Dolores Vilarinho Soares de Moura2 Aim: This study was conducted to determine the bond strength between zirconia ceramic and resin luting cement according to the type of surface treatment applied. Methods: Sixty zirconia cylinders measuring 4 x 4 mm were manufactured and distributed into five experimental groups and a control group (n = 10): G1 - sandblasting with 110 μm aluminum oxide; G2 - sandblasting with 30 μm Rocatec Soft®; G3 - Er:YAG laser irradiation at 400 mJ; G4 - Er:YAG laser combined with sandblasting with aluminum oxide; G5 - Er:YAG laser combined with sandblasting with Rocatec Soft®; G6 - no treatment (Control). The zirconia cylinders were bonded in the center of composite resin cylinders (6 mm diameter) using RelyX Ultimate® cement. Shear strength was measured after thermocycling (6000 cycles and 5-55° C). Data were analyzed using one-way ANOVA and Tukey’s post-hoc with a 5% significance level. Results: Group 4 had a significantly higher shear strength than all the other groups, except Group 1. Group 3 had a significantly lower shear strength compared with Groups 1 and 4. Conclusion: Sandblasting with 110μm aluminum oxide particles after laser irradiation may be an effective zirconia surface treatment. The use of Er: YAG laser alone is not effective at increasing the bond between resin cement and zirconia. Keywords: Zirconium. Laser therapy. Shear Strength. 2 Vasconcelos et al. Introduction Zirconium oxide has been used to manufacture crowns and fixed prosthesis infra- structures1 because of its aesthetics, biocompatibility and high mechanical strength2. It has a highly crystalline structure, which means it does not bond well with resin cement using standard techniques3,4. The luting of prostheses with this type of infra- structure is often carried out with conventional cements5. Unlike many ceramics, zirconia consists mainly of crystals5. The absence of a glassy phase interferes with the effectiveness of adhesive cementation procedures, such as the use of hydrofluoric acid and silane1,3,6,7. Surface treatments and the use of mate- rials that have a chemical compatibility with zirconia have been found to improve the strength of adhesive bonding1,4,6,8. The surface treatments of zirconia include sandblasting with aluminum oxide (Al2O3), sandblasting with silica-coated (silicatization) Al2O3 8-10, the use of a phosphate mono- mer primer (MDP), plasma spraying and the use of layers of a low melting point por- celain for subsequent acid etching3,9,11,12. These treatments can increase the adhesive bond. However, few studies have assessed the durability of this bond after storage and/or thermocycling3. Sandblasting can cause flaws and defects that may compro- mise the mechanical properties and durability of crowns13,14. Alternatives to zirconia surface treatment such as irradiation with high power lasers (Er: YAG, Nd: YAG and CO2) have also been recommended11,13,15-24. This treatment can increase the shear strength of the zirconia to resin cement15,25, however, it can cause microcracks that weaken the zirconia structure and impair adhesion26. Studies using combinations of chemical and mechanical treatments of zirconia’s sur- face have produced contradictory results25,27,28, and thus further studies are required. The purpose of this study was to analyze the effects of isolated and combined sur- face treatments of zirconia on the adhesive bond. The hypothesis tested was that the surface treatments do not alter the adhesive bond strength. Materials and methods Sixty 4 x 4 mm cylinders were obtained from the milling of polycrystalline tetrago- nal zirconia stabilized with yttrium Y-TZP (ZrO2-Y2O3), and pre-sintered at 1,530ºC, following the manufacturer’s instructions (ProtMat® Advanced Materials, São Paulo, Brazil). The surface to be treated was polished with a double-sided diamond disc (Komet, São Paulo, Brazil) for 60 seconds under constant water cooling. After pol- ishing, the cylinders were cleaned using ultrasound with isopropyl alcohol (99.8%) for 5 minutes and stored in Petri dishes. Surface treatments The surfaces of the zirconia cylinders were treated using the following protocols: Group 1 – sandblasting with 110 μm aluminum oxide (Al2O3) particles (Polidental® Ind. and Comercio Ltda, São Paulo, SP, Brazil) (Zhermack Technical S25R) at a dis- tance of 10 mm perpendicular to the zirconia surface for 10 seconds and under 3 Vasconcelos et al. 2.8 bar pressure; Group 2 - sandblasting with Rocatec Soft® 30 μm (3M ESPE, St. Paul, USA) for 20 seconds, following the same protocol as Group 1; Group 3 - Er:YAG laser irradiation (Smart Plus®, Deka laser, Florence, Italy) 10 mm perpendicular to the surface of the zirconia for 15 seconds with air/water spray cooling (wavelength 2,940 nm, 400 mJ of energy intensity, power of 4.0 W, pulse 100 μs and frequency of 10 Hz); Group 4 - Er: YAG laser irradiation with the same protocol as Group 3, followed by sandblasting with aluminum oxide 110 μm as in Group 1; Group 5 - Er: -YAG laser irradiation with the same protocol as Group 3, followed by sandblasting with Rocatec Soft® 30 μm as in Group 2; Group 6 - no surface treatment, only pol- ishing and ultrasound cleaning (Control). After the treatments, the cylinders were cleaned using ultrasound with 99.8% isopro- panol for 5 minutes, dried in the open air and then stored in Petri dishes until the bonding procedure. The cylinders of groups 2 and 5 were not cleaned with ultrasound in order to avoid removal of the silica deposited on the surface. Scanning electron microscope analysis One specimen from each group was randomly selected for surface morphological analysis using a scanning electron microscope (Quanta Feg 250, FEI, USA, 5Kv). Pho- tomicrographs were captured under 500x, 1000x, 3000x and 5000x magnification. Preparation of composites cylinders Composite cylinders (Filtek Z350®, 3M ESPE, USA) 10 mm thick x 6 mm in diame- ter were made using a transparent plastic tube, photoactivated for 30 seconds on all surfaces with LED light (Bluephase®, Ivoclar Vivadent, Schaan, Liechtenstein), at 1200 mW/cm2. After photoactivation, the plastic tube was removed and the cylin- dersphotoactivated again for 40 seconds. The surface of the cylinders was polished with SofLex® polishing discs used in decreasing order of granulation and then sand- blasted with 30 μm aluminum oxide particles for 10 seconds. The composite resin cylinders were placed in a matrix 15 mm thick by 20 mm in diameter and embedded in colorless chemically activated acrylic resin (JET, Clássico, Artigos Odontológicos, São Paulo, Brazil). Bonding Procedure For the bonding of the zirconia cylinders to the composite resin cylinders, Single Bond Universal® adhesive (3M, ESPE, USA) (with 10-MDP/silane) and RelyX™ Ultimate® cement (3M, ESPE, USA) were used following the manufacturer’s recommendations. The zirconia cylinder was pressed with a finger on the composite resin cylinder and the cement excess was removed with a microbrush. Each surface of the cylinders was photoactivated for 30 seconds with an LED light Bluephase®. Thermal cycling The specimens were stored for 24 hours at 37° C in distilled water. They were then submitted to 6,000 cycles (5-55 °C) (Thermal Cycle Machine, Mst CT-3, São Carlos, Brazil) for 30 seconds in each bath and 2 seconds of transition25. 4 Vasconcelos et al. Shear bond strength The shear strength test was performed in a mechanical testing machine (Emic, model DL 2000®, Ribeirão Preto, São Paulo, SP, Brazil) with a knife-shaped device at a load of 50 Kgf and a speed of 0.5 mm/min applied at the interface of the zirconia-cement until fracture (Figure 1). The shear bonding strength in MPa was calculated by: F/A where: F was the force applied (N); and A, the area of bonding (mm²), corresponding to 12.56 mm2. Failure mode analysis A single examiner analyzed the images (Software Proscope HR, USA) of the fractured specimens using an optical microscope (Avantscope, USA) with a 50x magnification to identify whether the faults were adhesive (interface failure), cohesive (in resinous cement) or a combination of both types. Statistical Analysis The data were analyzed using SPSS Statistics software version 20.0 for Windows. The Kolmogorov-Smirnov test was applied to assess the distribution of data. Only one factor was involved in this study: the surface treatment of zirconia. Therefore, the data were analyzed using one-way ANOVA and the post hoc Tukey test to compare the means of the groups. Descriptive statistics were used to analyze the distribution of fracture types in the groups. A significance level of α=.05 was established. Results Morphological differences were observed after the surface treatments (Figure 2). The surface treated with aluminum oxide 110 μm presented roughness throughout its Figure 1. Shear bond strength test. 5 Vasconcelos et al. length (Figure 2A). Rocatec soft 30 μm promoted a smoother surface than aluminum oxide, but silica deposits were observed (Figure 2B) similar to Figure 2E. In Figure 2C, the laser created deep irregularities (scratches) on the surface; in addition, there were grooves that resulted from the polishing. The Er: YAG laser-treated surface followed by sandblasting with aluminum oxide particles 110 μm presented several lines of microcracks and areas of depression (Figure 2D). The surface of the untreated (con- trol) specimen had grooves that resulted from the polishing (Figure 2F). ANOVA showed significant differences between the groups (F = 7.429, p <0.001). Group 4 was significantly different when compared with the Control group, but there 12/16/2015 9:44:37 AM HV 5.00 kV spot 4.5 det LFD mode SE HFW 138 µm mag 3000 x 40 µm 12/16/2015 10:21:04 AM HV 5.00 kV spot 4.5 det LFD mode SE HFW 138 µm mag 3000 x 40 µm 12/16/2015 10:29:17 AM HV 5.00 kV spot 4.5 det LFD mode SE HFW 138 µm mag 3000 x 40 µm 12/16/2015 10:10:55 AM HV 5.00 kV spot 4.5 det LFD mode SE HFW 138 µm mag 3000 x 40 µm 12/16/2015 10:45:48 AM HV 5.00 kV spot 4.5 det LFD mode SE HFW 138 µm mag 3000 x 40 µm 12/16/2015 10:35:39 AM HV 5.00 kV spot 4.5 det LFD mode SE HFW 138 µm mag 3000 x 40 µm B DC FE A Figure 2. Scanning electron microscopy of treatments (3,000x magnification). 2A: sandblasting with 110 μm aluminum oxide. 2B: sandblasting with Rocatec Soft®. 2C: Er: YAG laser irradiation. 2D: Er: YAG laser irradiation plus sandblasting with aluminum oxide 110 μm. 2E: Er: YAG laser plus sandblasting with Rocatec Soft®. 2F: Control. 6 Vasconcelos et al. was no difference when compared with Group 1. Group 3 was statistically different when compared with Groups 1 and 4 (Table 1). Table 1. Mean shear strength (in MPa) of the groups after surface treatments. Groups Mean SD Group 1 (Al2O3) 19.82b,c 5.94 Group 2 (Rocatec Soft®) 15.93a,b 4.79 Group 3 (Er:YAG laser) 10.97a 3.31 Group 4 (Er:YAG laser + Al2O3) 23.12c 3.40 Group 5 (Er:YAG laser + Rocatec Soft®) 15.28a,b 5.46 Group 6 (Control) 15.65a,b 4.21 Different letters in the same column expressed a statistically significant difference (one-way ANOVA and Tukey’s post-hoc, p<.05). The most frequent type of failure was the mixed type (Groups 2, 3, 5 and 6) corre- sponding to 52% of the total, followed by cohesive and adhesive failures. There was no adhesive failure in the groups treated with sandblasting with 110 μm aluminum oxide particles. In the group treated with Er: YAG laser followed by 110 μm Al2O3 sand- blasting adhesive failures totalled 11%, indicating that these surface treatments pro- moted a better adhesive bond (Table 2). Table 2. Distribution of failure types between the groups. Groups Types of failure % Adhesive Cohesive Mixed 1 0 78 22 2 33 11 56 3 22 0 78 4 11 67 22 5 22 0 78 6 44 0 56 Discussion The results demonstrated that the null hypothesis must be rejected. This study showed that the surface treatment may influence the adhesive bond. Since zirconia is a ceramic that should not be etched with hydrofluoric acid, treatments with aluminum oxide blasting can improve the strength of the bond with the resin cement, as demon- strated by the results of this study. In this study, Er: YAG laser irradiation was performed before sandblasting with 110 μm Al2O3 particles, because a decrease in adhesive strength when the reverse order 7 Vasconcelos et al. is carried out has previously been shown25,27,28. The Er: YAG laser followed by sand- blasting with Al2O3 significantly increased the zirconia/cement bond and reduced the occurrence of adhesive failures compared with the other treatments. This may have been a result of the presence of the irregularities (microcracks and grooves, as shownby SEM), caused by the combination of these two surface mechanical treatments. However, the effect of this combination on the mechanical properties of zirconia should be investigated, as studies have reported that Er:YAG laser can cause mild changes, cracking and even loss of zirconia material18,26,27,29. The adhesive strength increased when the zirconia surface was treated with 110 μm aluminum oxide sandblasting only, which has also been observed in other studies18,27,28,30,31. The impact energy of the particles results in a rough and irregular surface through- out the extension of the zirconia surface, as the morphological analysis with SEM revealed, corroborating other studies11,21,23,26,28,30. Surface roughness improves adhe- sion due to increased surface area, reduction of surface tension and the promotion of mechanical retention11,25,32. The irradiation of the Er: YAG laser showed low bond strength compared with the other experimental and control groups. This was probably a result of the lower roughness created by laser irradiation, especially when compared with sandblast- ing with Al2O3 and Rocatec. Similar results have previously been reported18,27,29,30,33,34. However, other studies have shown that laser irradiation increases the adhesive bond13,15,25. According to the authors, the irradiation of the laser promoted an increase of the surface roughness that, consequently, may have contributed to the increase of the micromechanical bonding13,15. In the present study, the group treated with sandblasting of 30 μm Al2O3 coated with silica (Rocatec Soft®) did not result in a significant increase of the adhesive bond when compared to the Control, which corroborates the findings of Subasi and Inan27 who suggested that the silica coat did not result in a strong micromechanical or chemi- cal interlocking with the resin cement. In contrast, significant improvements in bond strength between zirconia and resin cement have been reported in other studies when this treatment was used30,34. Gomes et al.34 (2015) studied the effect of the Er: YAG laser followed by Rocatec 30μm and found no statistically significant increase in the adhesive bond com- pared to Rocatec® alone, a similar finding to that of our study. In both studies, the surfaces were treated in a similar manner, which may explain the results. Despite the laser changes to the zirconia surface, Rocatec® caused greater irregularities. Therefore, this combination of treatment is not effective in increasing the shear bond strength. The types of failure observed in this study provided important data on the effective- ness of the different treatments, since the quality of the adhesive interface should also be taken into consideration. The groups treated with laser followed by sandblast- ing (Groups 1 and 4) presented cohesive and mixed failures, and cohesive failures within the resin cement were more frequent. Surface irregularities and micromechan- ical retention created by these treatments may have contributed to improved bond strength and a lower rate of adhesive failures. 8 Vasconcelos et al. Studies in which the Er: YAG laser did not improve the bond strength reported high rates of adhesive failure19,25,27,30. In our study, laser irradiation with silica treatment and laser irradiation without silica treatment, presented similar failure rates (adhe- sive and mixed) with predominance of mixed failure (78%). These treatments may not have promoted strong micromechanical or chemical retention, therefore result- ing in adhesive failures. In this study, the most frequent types of failures in the experimental groups were cohesive and mixed, which are clinically preferable to adhesive failure, because adhe- sive failure is usually associated with low adhesive strength35. Within the limitations of this in vitro study, sandblasting with 110 μm Al2O3 particle after laser irradiation may be an effective zirconia surface treatment option. The Er: YAG laser used as an isolated treatment was not effective at increasing the shear strength. The Silica coated aluminum oxide (Rocatecâ) did not improve the bond strength. Acknowledgements The authors are grateful to the Ribeirão Preto Dental School (USP) for having ceded their laboratory of mechanical tests and Professor Rogerio Romera for his collabora- tion in the development of the method of this study. Conflicts of interest The authors have no conflicts of interest relevant to this article. References 1. Sadan A, Blatz M, Lang B. 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