1http://dx.doi.org/10.20396/bjos.v19i0.8656155 Volume 19 2020 e206155 Original Article ¹ Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, PR, Brazil. Corresponding author: Giovana Mongruel Gomes Avenida Carlos Cavalcanti 4748 – Uvaranas, Ponta Grossa, Paraná, Brazil. 84030-900 Telephone: 005542988463753 Email: giomongruel@gmail.com Received: August 06, 2019 Accepted: March 28, 2020 Effect of surface treatments on repair strength, roughness and morphology in aged metal-free crowns Yançanã Luizy Gruber¹, Thaís Emanuelle Bakaus¹, Bruna Fortes Bittencourt¹, João Carlos Gomes1, Alessandra Reis1, Giovana Mongruel Gomes1,* Aim: The roughness and micromorphology of various surface treatments in aged metal-free crowns and the bond strength of these crowns repaired with composite resin (CR) was evaluated in vitro. Methods: A CR core build-up was confectioned in 60 premolars and prepared for metal-free crowns. Prepared teeth were molded with the addition of silicone, and the laboratory ceromer/ fiber-reinforced crowns (SR Adoro/Fibrex Lab) were fabricated. Subsequently, the crowns were cemented and artificially aged in a mechanical fatigue device (1.2 X 106 cycles), then divided into 4 groups (n = 15) according to the surface treatment: 1) phosphoric acid etching (PA); 2) PA + silane application; 3) roughening with a diamond bur + PA; and 4) sandblasting with Al2O3 + PA. After the treatments, the crowns (n = 2) were qualitatively analyzed by scanning electron microscope (SEM) and surface roughness (n = 5) was analyzed before and after the surface treatment (Ra parameter). The remaining crowns (n = 8) received standard repair with an adhesive system (Tetric N-Bond) and a nanohybrid CR (Tetric N-Ceram), and the microshear bond strength (SBS) test was performed (0.5 mm/min). Roughness and SBS data were analyzed by one- and two-way ANOVA, respectively, as well as Tukey’s post-test (α = 0.05). Results: Sandblasting with Al2O3 + PA resulted in the highest final roughness and SBS values. The lowest results were observed in the PA group, whereas the silane and diamond bur groups showed intermediate values. Conclusion: It may be concluded that indirect ceromer crowns sandblasted with aluminum oxide prior to PA etching promote increased roughness surface and bond strength values. Keywords: Ceramics. Composite resins. Electron microscope tomography. Shear strength. Surface properties. https://orcid.org/0000-0001-6603-5239 2 Gruber et al. Introduction Indirect restorations, also known as “ceromer,” “polymeric glass porcelain,” or “sec- ond-generation laboratory CR,” are widely used in clinical practice because they mini- mize the adverse effects of direct restorations, such as polymerization shrinkage1, poor marginal adaptation, and postoperative sensitivity2 In addition, they can provide better standards of translucency and can be low-cost alternatives to all-ceramic restorations3. Although indirect resins possess high mechanical strength, these restorations are subject to fractures as any other material. This type of failure should be carefully eval- uated to define the best treatment. Clinically, the affected crowns can be classified according to the extent of the fracture. A fracture can be minimal (e.g., cracks) or extensive (e.g., displacement of more than half of the crown)4,5. Corroborating in vitro studies6-8, clinical studies show that most cases of crown fractures are repairable9,10. This is advantageous because complete replacement of indirect restorations may present more disadvantages than advantages, such as the treatment complexity and expense11. With the evolution of adhesive techniques, adhesive repair has been widely used and can be considered beneficial, allowing good longevity in this type of dental restoration12,13. For proper repair, the surface of the indirect restoration should be subjected to a pre-treatment to create micromechanical retention with the repair material14. In the available literature, several surface treatments techniques are described for the repair of composites. Roughening with diamond burs, sandblasting with aluminum oxide, conditioning with hydrofluoric acid etching or PA etching, and silanization are the most frequently reported11,15,16. The current literature presents several studies comparing different surface treatments; however, the best pre-treatment technique still generates controversial results17-19. Thus, the present study aimed to evaluate the surface roughness, morphology, and repair strength of aged indirect resin restorations with SEM, microshear bond strength test, and digital roughness meter. The tested hypothesis was that differences would exist in morphology, surface roughness, and bond strength after various surface treatments. Material and Methods Sixty extracted human mandibular premolars, with the protocol number 1871/10 from the research ethics committee of the State University of Ponta Grossa (Brazil), were stored in distilled water at 4oC and used within 6 months after extraction. To be included in the study sample, teeth should be sound, without cracks, and not sub- mitted to previous endodontic treatment. Teeth were transversally sectioned 2 mm above the cement-enamel junction using a low-speed diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA) and received a standardized endodontic treatment. After 1 week, the root canals were prepared to receive glass fiber posts (White- post DC # 0.5, FGM, Joinville, SC, Brazil), which were cemented with the Excite DSC (Ivoclar-Vivadent, Schaan, Liechtenstein) adhesive system and Variolink II (Ivo- clar-Vivadent) resin cement in accordance with the manufacturer’s instructions. 3 Gruber et al. After the post-luting procedures, cores were built-up with a nanohybrid CR (Tetric N-Ceram, Ivoclar-Vivadent). An incremental technique was used to place the CR, and each 2 mm increment was light cured for 20 s. Indirect composite crowns cementation The composite cores were prepared to receive a full indirect composite restoration using a high-speed hand piece under water cooling. In all roots a ferrule was made in the coronal ending with 2.0 mm height, 1.2 mm depth, and 1.5 mm occlusal reduction. Full indirect composite restorations were fabricated with the SR Adoro (Ivoclar-Viva- dent) restorative system reinforced by fibers (Fibrex-Lab Coronal, Angelus, Londrina, PR, Brazil). After fitting and adjustment, the restorations were adhesively cemented with Excite DSC and Variolink II according to the manufacturers’ recommendations. Teeth were then embedded in acrylic resin (Duralay, Reliance, Worth, IL, USA) and periodontal ligament was simulated using a polyether impression material (Imp- regum™ Soft, 3M ESPE, St Paul, MN, USA), according to the method described by Soares et al. 20052. Mechanical aging To increase the study’s reliability20, the samples were subjected to mechanical fatigue in a controlled chewing simulator (Elquip, São Carlos, SP, Brazil). The sam- ples were placed at the base of a material-fatigue-testing machine at a 90o angle in relation to the horizontal plane and were subjected to repetitive impacts directed on the occlusal surface of the crown. A lower force of 40 N (to avoid possible frac- tures) at a frequency of 2 Hz was applied for 1.2 X 106 cycles, which represents 5 years of clinical service21,22. During the cycles, the samples were kept at 37oC in relative humidity. Surface treatments of the indirect restorations and experimental groups The specimens were then randomly divided into 4 groups, according to the surface treatments. Each treatment was performed on a square delimited area (3 mm x 3 mm) on the buccal surface of each crown. In the PA group, the buccal surfaces of the indi- rect CR were treated with 35% PA for 2 min according the manufacture’s recommen- dation, washed for 2 min with distilled water, and gently air dried for 5 s at 2 cm. For the silane group—after PA treatment as reported above—a silane coupling agent (Prosil, FGM, Joinvile, SC, Brazil) was applied for 1 min with a disposable applicator, and the surface was dried with compressed air for 5 s at 2 cm. The buccal surfaces of the diamond bur group were roughened with a diamond bur (# 3195, KG Sorensen, São Paulo, SP, Brazil) using a high-speed hand piece under water cooling for 5 s, with weak movements and minimal wear. Then, the surface was conditioned with PA as reported in the first group. For the sandblasting group, the surfaces were sandblasted with 50 μm Al2O3 (Micro- blaster Standard Model, Bio-Art, São Carlos, SP, Brazil) for 10 s and then conditioned with PA as reported in the first group. 4 Gruber et al. Surface roughness test After mechanical aging, the initial roughness (IR) of five random buccal surface resto- rations per group was obtained with a digital roughness meter (Mitutoyo Surftest-301, Mitutoyo-Kawasaki, Kanagawa, Japan). Three measures were performed on each specimen, and the arithmetic mean was obtained from these values. The mean rep- resents the IR. Surface roughness reading was performed using the Ra parameter (µm) and the ISO 2001 measuring profile23, a 0.25 mm cut-off, 1.25 mm in length and 0.1 mm/s speed. Afterward, the specimens were submitted to the abovementioned surface treatments and stored at 37oC in artificial saliva, simulating oral condition. After 48 h of the surface treatments procedures, we measured the final roughness (FR) in the same way as the initial evaluation. SEM analysis Two restorations per group were prepared for the SEM (SSX – 550; Shimadzu, Tokyo, Japan). The surfaces were sputter coated with gold in a vacuum evaporator (Belzers SCD 050 SputterCoater, Bal-Tec, Germany) and photomicrographs of representative areas were taken at 1.000x magnification. Bond strength test After surface treatment, eight crowns per group were submitted to microshear bond strength test. For this purpose, one coat of the adhesive system (Tetric N-Bond, Ivo- clarVivadent) was applied on the delimited area (3 mm x 3 mm) of the treated buccal surfaces and then gently air dried for 5 s and light-cured for 10 s (Table 1). Three Tygon tubes, approximately 0.75 mm in diameter and 1 mm high, were used for each crown. The tubes were positioned on the flattest areas of the treated buccal surface (3 mm x 3 mm) of the indirect restorations, filled with CR (Tetric N-Ceram, IvoclarVivadent), and individually photoactivated for 40 s. Each light-cured specimen was protected with aluminum strip to afford protection from additional polymeriza- tion, as well as the unpolymerized specimens. All specimens were checked with an optical microscope (OLYMPUS-BX 51, Olympus, Tokyo, Japan) at 10x magnification to discard any specimens with air bubbles or evident gaps at the interface. Table 1. Manufacturer, composition and instructions for each material used in the study. Material (Manufacturer) Composition Instructions for use Tetric N-Bond (Ivoclar Vivadent) Phosphoric acid acrylate, HEMA, BisGMA, urethane dimethacrylate, ethanol, film-forming agent, catalysts and stabilizers. Apply a thick layer of Tetric N-Bond for at least 10 seconds. Remove excess material and the solvent by a gentle stream of air and light-cure for 10 seconds. Tetric N-Ceram (Ivoclar Vivadent) Dimethacrylates (19-20 wt.%); barium glass, ytterbium trifluoride, mixed oxide, copolymers (80-81 wt.%); additives, catalysts, stabilizers and pigments are additional contents (< 1 wt.%). The total content of inorganic fillers is 55–57 vol.%. The particle size of inorganic fillers is between 40 nm and 3000 nm Apply Tetric N-Ceram in layers of max. 2 mm or 1. Polymerize each layer individually for 40 seconds. 5 Gruber et al. All light-curing procedures of this study were performed with a LED light-curing device (Radii Plus, SDI Limited, Victoria, Australia) using a 1200 mW/cm2 power density. The specimens were mounted in acrylic resin and placed in a universal testing machine (Kratos, São Paulo, SP, Brazil), and a microshear force was applied using a shearing blade as close as possible to the adhesive interface. The load was applied to the inter- face at a crosshead speed of 1 mm/min until failure, and the bond strength values were recorded in MPa. Statistical analysis Before running parametric statistical analysis, we tested whether the assumptions of normality of the data and equality of variances were valid, using the Shapiro-Wilk and Barlett tests at an alpha of 5%. The data from surface roughness and bond strength were statistically analyzed by one- and two-way ANOVA, respectively, and Tukey’s test was used for pairwise comparisons at a 5% significance level. All calculations were performed using SPSS® statistical software (Statistical Pack- age for the Social Sciences, version 21.0 Mac, SPSS Inc., Chicago, IL, USA). Results The means and standard deviations of surface roughness (Ra parameter) and microshear bond strength values (MPa) for the experimental groups are demonstrated in Table 2. In relation to the surface roughness, two-way ANOVA showed that the cross-product interaction between the factors time and experimental groups were statistically sig- nificant (p < 0.001). At baseline, all groups were statistically similar (p > 0.05). Rough- ness increased significantly after the treatments in all groups (p < 0.001). The final roughness was higher in the sandblasting group and lower in the PA group, whereas the silane and diamond bur groups showed intermediate values. For the microshear bond strength, one-way ANOVA showed significant statistical dif- ferences between the experimental groups (p < 0.0001). The lowest repair strength was observed for the PA group and the highest was observed in the sandblasting group. The silane and diamond bur groups were statistically similar and had an inter- mediate performance. In the SEM images (Figure 1), the diamond bur and sandblasting groups showed very irregular surfaces. However, they differed in the direction of the grooves and depres- Table 2. Mean values ± standard deviation of roughness (Ra parameter) and microshear bond strength (MPa) for each experimental group (*). Experimental Groups Roughness Shear bond strength Baseline Post treatment Phosphoric acid 0.24 ± 0.08 E 0.42 ± 0.14 D 9.4 ± 3.1 C Silane 0.21 ± 0.07 E 0.64 ± 0.15 C 20.3 ± 6.1 B Diamond bur 0.25 ± 0.07 E 0.86 ± 0.11 B 18.3 ± 4.9 B Sandblasting 0.21 ± 0.09 E 1.28 ± 0.13 A 37.1 ± 8.7 A * Comparisons are valid just for the same property. Distinct letters show significant differences (p < 0.05). 6 Gruber et al. sions. Grooves are unidirectional in the diamond bur group, probably resulting from the direction of the bur roughening, whereas in the sandblasting group they do not follow a pattern due to the abrasion of aluminum oxide particles on the surface. Discussion Fatigue studies can mimic the effect of mechanical and thermal cycles, as well as a wet oral environment24. Mechanical aging better reproduces the clinical reality, because failures and fractures in indirect CR restorations occur only after years of clinical service13. Although previous studies have already investigated different sur- face treatment techniques for the repair of indirect restorations25-27, most did not sim- Figure 1. Images of the surface roughness obtained by the SEM (x 1,000). A: phosphoric acid group; B: silane group; C: diamond bur group: D: sandblasting group. SEM MAG: 1.00 kx C-LABMU UEPG View field: 138 μm 20 μm SEM HV: 15.0 kV Date(m/d/y): 11/23/15 Det: SE WD: 15.22 mm VEGA3 TESCAN SEM MAG: 1.00 kx C-LABMU UEPG View field: 138 μm 20 μm SEM HV: 15.0 kV Date(m/d/y): 11/23/15 Det: SE WD: 15.53 mm VEGA3 TESCAN SEM MAG: 1.00 kx C-LABMU UEPG View field: 138 μm 20 μm SEM HV: 15.0 kV Date(m/d/y): 11/23/15 Det: SE WD: 14.74 mm VEGA3 TESCAN SEM MAG: 1.00 kx C-LABMU UEPG View field: 138 μm 20 μm SEM HV: 15.0 kV Date(m/d/y): 11/23/15 Det: SE WD: 15.06 mm VEGA3 TESCAN A C B D 7 Gruber et al. ulate three important clinical features found in this study’s protocol: cementation of a fiber post to stabilize the final restoration, simulation of the periodontal ligament, and simulation of mechanical aging. In a clinical scenario, the core and post are placed to retain the final restoration in endodontically treated teeth, improving their integrity28. The presence of the periodon- tal ligament and tooth-supporting structures partially absorb the masticatory loads; therefore, studies that did not simulate these structures may have obtained unreli- able values21. Finally, post-retained indirect crowns are subject to repetitive ordinary chewing forces over time, as well as other environmental challenging factors29. Thus, mechanical aging is essential to simulate closely the clinical conditions to which these indirect restorations are subjected. In this study, all specimens were submitted to 1.2 X 106 cycles of mechanical fatigue, which is commonly assumed to correspond to 5 years of clinical service22. In this study, the results showed that air abrasion with aluminum oxide promoted the highest bond strength values30,31 and higher roughness. Although some authors have reported that pre-treatment with diamond burs can yield higher repair strengths than air abrasion with aluminum oxide32,33, other studies30,34,35 have shown the opposite, with results similar to our observations. The higher surface roughness produced by aluminum air abrasion increases the surface area and wetting for the adhesive penetration36,37, which may have yielded the highest bond strength values. This positive correlation between increased sur- face roughness and improved repair strength has already been demonstrated in other studies38 39. Indeed, the bond repair strength observed in the PA etching group might be lowest because this procedure produced the lowest surface roughening on the aged resin surface. Previous studies have demonstrated that acid etching alone is not enough to guarantee adequate repair strength40. Although surface treatment with silane does not generate the roughest surfaces, this group presented intermediate bond strength values, similar to that achieved by the asperization with diamond bur. The chemical bond produced by this bifunc- tional molecule between the inorganic particles of glassy substrates (silica filler par- ticles) and the adhesive CR matrix19 probably compensated for the reduced surface area. This bonding agent has a general chemical structure, R′Si(OR)3, where R′ is the organ functional group (typically a methacrylate) that reacts to the adhesive system or the composite cement, creating a covalent bond after polymerization. The alkyl group (R) is hydrolyzed to a silanol (SiOH), creating a covalent bond with the inor- ganic silicon particles41. This study has some limitations, due to which not all clinical conditions could be reproduced. 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