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 Table of Contents  
Year : 2019  |  Volume : 7  |  Issue : 1  |  Page : 16-22

Evaluation of the core thickness and resin cement on the fracture strength of zirconia-based multilayer computer-aided design/computer-aided manufacturing ceramic crowns

1 Department of Prosthodontics, Faculty of Dentistry, University of Van Yüzüncüyil, Van, Turkey
2 Department of Prosthodontics, Faculty of Dentistry, University of Ondokuz Mayis, Samsun, Turkey

Date of Web Publication12-Apr-2019

Correspondence Address:
Idris Kavut
Department of Prosthodontics, Faculty of Dentistry, University of Van Yüzüncüyil, Tusba, Van 65080
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/dmr.dmr_33_18

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Aim: The purpose of this study was to evaluate the effect of thickness of zirconia core and different resin cements on the fracture strength of veneered zirconia crowns designed by multilayer technique. Materials and Methods: Forty metal dies were constructed to replica maxillary molar. Forty zirconia cores (Sirona inCoris ZI) were designed and constructed (inLab 4.4) with different thicknesses. The thickness of zirconia core was selected as 0.5 and 0.7 mm. Forty Feldspathic ceramic (VITABLOCS Mark II) veneers were fabricated (inLab 4.4) onto the zirconia cores. The zirconia cores were divided into two subgroups, and veneers were cemented with one of the following resin cement: self-cure, self-adhesive resin cement with light-cured option (Multilink N), and a dual-cure resin cement (Panavia F 2.0). Then, crowns were cemented to the metal dies. All the specimens were subjected to thermal cycling 5000 times (5°C–55°C ± 2°C, immersion time: 30 s). A universal testing machine was used for fracture strength test at a crosshead speed of 1 mm/min. The data were analyzed with one-way analysis of variance (α = 0.05). Stereomicroscopy was used to evaluate the failure modes and surface structure. Results: Zirconia core thickness and resin cement material affected the fracture strength (P < 0.05). Increase in core thickness increased the fracture strength of multilayer veneer crown (P < 0.05). Higher fracture strength values were obtained with light-cured, self-adhesive cement in both core thicknesses. Conclusion: Although 0.5-mm thickness zirconia cores showed lower flexural strength, it was higher than the maximum loads which may occur clinically (Fmax= 600 N on one tooth). Furthermore, light-cured, self-adhesive resin cement is advisable to increase the fracture strength with different core thickness.

Keywords: Computer-aided design/computer-aided manufacturing, fracture strength, multilayer technique, self-adhesive resin cement

How to cite this article:
Kavut I, Külünk S. Evaluation of the core thickness and resin cement on the fracture strength of zirconia-based multilayer computer-aided design/computer-aided manufacturing ceramic crowns. Dent Med Res 2019;7:16-22

How to cite this URL:
Kavut I, Külünk S. Evaluation of the core thickness and resin cement on the fracture strength of zirconia-based multilayer computer-aided design/computer-aided manufacturing ceramic crowns. Dent Med Res [serial online] 2019 [cited 2022 Oct 2];7:16-22. Available from: https://www.dmrjournal.org/text.asp?2019/7/1/16/256021

  Introduction Top

The doubtful biocompatibility of some dental metal alloys, as well as the increased demand for relatively high strength and esthetic restorations of patients, has accelerated the development of oxide-based full-ceramic restorations.[1],[2],[3] When esthetic and strength are indispensable at the same time, zirconia-based ceramic restorations are used as an alternative to porcelain-fused-to-metal restorations. Ceramic restorations with yttrium-stabilized tetragonal zirconia polycrystalline (Y-TZP) core have greater flexural and fracture strength than other ceramic cores, including lithium disilicate, glass-infiltrated alumina, and glass-infiltrated alumina strengthened with zirconia, so it has become one of the most commonly used all-ceramic core materials.[4],[5],[6],[7],[8],[9] To produce zirconia core, fabricated presintered Y-TZP blocks are used. Using computer-aided design/computer-aided manufacturing (CAD/CAM) technology, homogeneous zirconia cores with no imperfections and/or porosities are obtained.[3] The presence of high-order crystal structure in zirconia ceramics causes it to exhibit equal opacity to metal alloys. Opaque image of zirconia and its low translucency adversely affect esthetics, so zirconia core is veneered with Feldspathic ceramic to obtain esthetic restorations.[10],[11],[12] Sailer et al. and Pjetursson et al. reported that success rate of zirconia restorations was 91.2% for single crown and 90.4% for fixed partial dentures (FPDs) at the end of 5 years.[2],[13] Furthermore, previousin vivo andin vitro studies focused on the adhesive (interfacial and fractures) and cohesive (chip-off and fractures) failure of zirconia-based ceramic crown and FPD restorations, which was one of the most common clinical failure types (13%–25%) while using conventional veneering techniques.[3],[4],[5],[8],[13],[14],[15],[16],[17],[18],[19],[20],[21]

Many factors can cause the chipping of the veneering layer including the mechanical properties, surface treatments, design and thickness of zirconia core, quality and homogeneity of the veneering ceramic, the type of veneering layer ceramic, wettability of the core by the veneering ceramic, residual stresses at the interface, as well as the physical properties such as the coefficient of thermal expansion and elastic modulus of veneering ceramic and zirconia cores.[3],[4],[8],[12],[15],[16],[22],[23] Veneering techniques also have a potential effect on the chipping of ceramic due to the processing methods of ceramic. By the way the effect of luting cements on the fracture resistance of all-ceramic crowns remains a controversial issue. Compared to conventional luting agents, resin cements exhibited enhanced mechanical, physical, and adhesive properties.

Two different approaches have been described to avoid veneer fractures: the use of improved methods for veneering zirconia frameworks and the use of monolithic crowns. Monolithic crowns have significant advantages, including reduced production time, improved cost-effectiveness, and elimination of the interface between the core and veneer. Furthermore, eliminating the veneer layer enables a more conservative preparation.[24] Dental manufacturers have developed high-strength monolithic zirconia prostheses, which rely on the toughness and strength of the material.[14],[24],[25] By the way monolithic zirconia shows similar wear characteristic properties with metal fused to porcelain, veneered zirconia, monolithic zirconia, reinforced glass ceramic restorations.[25],[26] However, veneering of zirconia core is still often preferred for dentists and technicians.

To veneer zirconia core with glassy matrix ceramic, various methods have been developed. Traditional layering technique and overpressing are the commonly used techniques.[8],[15],[16] In addition, to these common veneering techniques, “CAD-on” and “multilayer” techniques have been introduced with the improvements of CAD/CAM systems. By these techniques, zirconia core and its suprastructure are designed together with the CAD software, and after milling with the CAM unit, the two parts are connected using a fusion glass-ceramic (CAD-on) or resin cement (multilayer), which depends on manufacturer's instructions. These manufacturing techniques not only decrease the number of laboratory procedures, but also provide the opportunity to use homogeneous ceramic blocks with relatively high strength.[4],[15]

In previous studies, the performance of “CAD-on technique” was assessed, where little information is available about the “multilayer technique” which used zirconia core and Feldspathic ceramic core and the effects of thickness of zirconia core and luting agents on the fracture strength between two different parts of restorations.[4],[6],[15],[16],[27]

The purpose of this study was to evaluate the effect of different thicknesses of zirconia core and different cementation procedures on the fracture strength of veneered zirconia crowns designed by CAD/CAM multilayer technique. The testing hypotheses were that: (1) increasing the core thickness would increase the fracture strength and (2) luting procedure would not affect the fracture strength of zirconia-based veneer crown designed by CAD/CAM multilayer technique.

  Materials and Methods Top

To evaluate chipping and/or delamination of the veneer, the performance of zirconia core-based Feldspathic veneer molar crowns fabricated with CAD/CAM technique was of interest. One trained researcher (IK) produced all the specimens to avoid any operator effect. The materials used in this study are listed in [Table 1].
Table 1: Material used in the study

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The number of samples was determined by power analysis. According to the result of statistical power analysis for determining sample numbers, it is necessary to take at least ten samples each of the subgroups at 90% power and 5% error level among the groups. Sample number calculations were done in NCSS and PASS 2000 statistical package program (NCSS, LLC, Utah, USA).

For crown-shaped specimens, industrially fabricated stainless steel dies (n = 40) were designed with a 1 mm circumferential chamfer, and a groove on the axial side to avoid the rotation of the crowns during mechanical testing. Dies were coated with a contrast spray (Cerec Optispray®, Sirona, Bensheim, Germany) to eliminate light-reflective surfaces. The restorations were designed for the maxillary first molar, and multilayer option and material data were selected before the digital impressions using CAD/CAM (Cerec Omnicam, Sirona, Bensheim, Germany). On the computer, the multilayer option was checked, and the material information for the infrastructure and the top structures was entered. Die spacer thickness of 30 μm was chosen for luting area between the substructure and veneer and also fit of zirconia core to metal die.[28] Zircon substructures were selected to have a thickness of 0.7 mm for the first group (A) and 0.5 mm for the second group (B). Forty zirconia cores (Sirona inCoris ZI, mono L F1) with different core thickness (0.5 mm or 0.7 mm) were constructed (inLab 4.4; Sirona Dental Systems GmbH, Bensheim, Germany) in an approximately 20%–25% enlarged volume to compensate for the shrinkage of the sintering process on these metal dies. While designing the zirconia cores, nonanatomical cusp was preferred, and groove in 1 mm depth was created on one of the proximal side to prevent the rotation of the superstructure. Circumferential margin and insertion path of the dies were controlled by a groove which was positioned on the buccal surface. Zirconia cores were sintered (inFire HTC speed; Sirona Dental Systems) according to manufacturer's instructions, and the adaptations of the zirconia cores to metal dies were controlled.

Forty Feldspathic ceramic veneers (VITA Mark II, Vita Zahnfabrik), as veneer were designed (1-mm axial wall thickness, 1.5-mm occlusal thickness) and fabricated with the same system (inLab 4.4; Sirona Dental Systems). A hole in 0.5-mm diameter and depth was designed on the mesiopalatal cusp for fitting the tip of the fracture strength test machine, and then the data were transferred to the milling unit (CEREC MC XL®).

After controlling the adaptation to sintered zirconia cores, the outer surfaces of the zirconia substrate samples were roughened for 15 s (BEGO, Wilhelm-Herbst-Bremen, Germany) with 50-μm aluminum oxide (Al2O3) particles (Korox 50, BEGO, Bremen, Germany) at a pressure of 2.5 bar at a distance of 10 mm, and veneers were polished with diamond-coated rubbers according to manufacturer's instructions (3M ESPE, St. Paul, Germany).[29] Inner surface of veneers was etched with 9.5% hydrofluoric acid (Porcelain Etchant Gel, Bisco, Schaumburg, USA) for 30 s, and then washed with water for 20 s and air dried for 5 s. Silane coupling agent (Monobond Plus; Ivoclar Vivadent) was applied to inner surfaces and allowed to evaporate for 60 s. After surface treatments were completed, veneers and zirconia core structures were cemented with two different resin cements. For the cementation procedure of the multilayer system components, zirconia cores which have different thickness were separated in two subgroups. The first group was cemented with dual-cured resin cement (D) (Panavia F 2.0, Kuraray Medical Inc., Okayama, Japan) containing the 10-methacryloyloxydecyl dihydrogen phosphate. For cementation, equal amounts of a dual-polymerized resin-luting agent paste base and catalyst were mixed and applied inside the veneers with a plastic spatula. Dual-cured resin cement (Panavia F 2.0, Kuraray Medical Inc., Okayama, Japan) was used according to manufacturer's instructions. Veneers were inserted onto zirconia cores with finger pressure. The excess resin cement was removed by the means of a cotton pellet. A glycerin gel (Oxyguard II; Kuraray Co., Ltd., Osaka, Japan) was applied around the junction of core and veneer. The resin cement was then light polymerized for 20 s with a curing light (ELIPAR, S10, 3M ESPE, Germany), and the borders were then polished. The second group was cemented with self-curing, self-adhesive resin cement (S) (Multilink® N, Ivoclar Vivadent, Schaan, Liechtenstein). Cement is dispensed from the automix syringe and the desired amount is applied directly into the veneers. Veneers were inserted onto zirconia cores with finger pressure, and then a manual dynomometer was used to ensure that the crown was loaded centrally at a force of 50 N for 10 min. The excess resin cement was removed by the means of a cotton pellet.

After the core and veneer structures were cemented, the inner surface of the crowns was sandblasted with 50-μm Al2O3 particles (Korox 50) at a pressure of 2.5 bar and a distance of 10 mm. The crowns were cemented to metal dies with a dual-polymerized resin cement (Panavia F 2.0) as suggested by the manufacturer and placed under a manual dynamometer with a constant force of 50 N for 10 min. All restorations were stored in distilled water at a temperature of 37°C for 24 h, and then the specimens were subjected to 5000 cycles of aging at 5°C–55°C ± 2°C on a thermal cycler (Dentester Solubris Technica). The specimens were checked in every 500 cycles and aging continued without waiting. To position the specimens on the universal testing machine (Lloyd LRX, Lloyd Instruments PIC, Fareham, Hampshire, UK), a special apparatus at a 10° inclination relative to the long axis was used, and the specimens were loaded until fracture occurred. The load was applied with a 6-mm diameter stainless steel ball placed on the occlusal surface of the crowns and the crosshead speed of 1 mm/min. The fracture load for each specimen was recorded in N. A homogeneity of variance test was done using Shapiro–Wilk's test. The test showed that the data followed a normal distribution (P > 0.05).

The failure mode of fractured specimen was assessed under a stereomicroscopy (Stemi 2000-C; Carl Zeiss, Gottingen, Germany) at ten magnifications and it was performed by the same investigator. The nature of the failure was designed as follows: adhesive in zirconia core if the resin cement was totally separated from the core; adhesive in veneer ceramic if the resin cement was totally separated from veneer ceramic; cohesive if the fracture occurred only on veneer ceramic, zirconia core or resin cement; or mixed when a combination of adhesive in veneer ceramic, zirconia core and resin cement failure occurred. The percentage of each type of failure within each group was then calculated.

  Results Top

The data were analyzed with two-way analysis of variance (ANOVA) followed by a post hoc Tukey's test (α = 0.05).

Mean and standard deviations of the data are shown in [Table 2], and the results of two-way ANOVA are shown in [Table 3]. Zirconia core thickness and resin cement material affected the fracture strength. The fracture strength of veneering ceramic to zirconia core depended on the resin cement and core thickness (P < 0.05) [Table 3].
Table 2: Mean (Newton) and standard deviation values of fracture strength values*

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Table 3: Two-way analysis of variance

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Significant differences were found between the groups (P < 0.05). The increase of the core thickness increased the fracture strength in both cement groups. There was no statistically significant difference between the two cement groups in the group, where the 0.7-mm thick core was used.

Higher bond strength values were obtained in the groups cemented with self-cure self-adhesive resin cement in both core thicknesses (P < 0.05). The lowest fracture strength values were obtained with the Group AD. No significant difference was found between Groups AS and BS (P > 0.05) [Table 2]. The highest values were obtained with Groups BS and significant difference was found between other groups (P < 0.05). Adhesive failures were observed in all groups [Table 4].
Table 4: Failure types

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  Discussion Top

Fracture, chipping, and loss of retention are the most common failure types for ceramic restorations. The interpretation of the mean strength values should consider at least three factors: (1) the core–veneer bond, (2) the mechanical and thermal behavior of the materials, and (3) the geometry and thickness ratio of the bilayer ceramics specimens.[30]

In thisin vitro study, the core thickness and type of adhesive resin cement on the fracture performance of the zirconia-based restoration designed by CAD/CAM multilayer technique were investigated, and the results showed that the mean fracture strength values increased as the zirconia core thickness increased, confirming the first study hypothesis but, the type of adhesive resin cement had no effect on the fracture strength, so rejecting the second hypothesis. According to previous studies, the fracture strength increased as the core thickness increased.[12],[31]

If the tooth is not sufficiently reduced, the esthetics of the zirconia crown may be compromised. In such situations, the thickness of the zirconia coping can be decreased for esthetic purposes.[12] A minimum thickness of 0.4 mm, respective 0.5 mm, is recommended for Y-TZP ceramic crown frameworks in the anterior or posterior region.[23] Modifications can be made to the thickness of the core or porcelain veneer structure in the esthetic region restorations, where the interocclusal distance is insufficient or excessive.

In a study, the effect of different veneer porcelain thicknesses (0.5, 1, and 2 mm) which were applied on standard thickness zirconia core substructure (0.7 mm) on fracture resistance of the restoration was evaluated; fracture resistance decreased with the increase of veneer porcelain thickness and surface wettability between porcelain-core substructure and that surface sealant application, which is thought to positively affect fracture toughness, has no effect on fracture resistance.[30] The clinically recommended occlusal thickness for a zirconia core-based veneer crown is approximately 2 mm. In this study, the veneer thickness on the occlusal surface was determined to be 1.5 mm and that of the axial surfaces was determined to be 1 mm, but a different thickness of zirconia core was used which referred the minimum thickness at occlusal area for appropriate strength (0.5 mm) and optimal thickness to achieve ideal light transmission and esthetics (0.7 mm).

Bankoǧlu Gungor and Karakoca Nemli evaluated the fracture resistance of CAD/CAM monolithic ceramics (monolithic zirconia, monolithic lithium disilicate, monolithic zirconium reinforced glass ceramic, dual-network ceramic, resin nano ceramic, and flexible nano ceramic) and veneered zirconia (conventionally layering, lithium disilicate pressing, lithium disilicate fusing, lithium disilicate cementing, Feldspathic ceramic cementing, resin nano ceramic cementing) molar crowns after thermomechanical aging. From the results, the highest resistance was observed for monolithic yttria-stabilized zirconia crowns, followed by monolithic lithium disilicate derivates. Conventionally veneered zirconia restorations (conventionally layering) showed the lowest resistance than these ceramics, besides multilayer and CAD-on CAD/CAM restorations showed better results after monolithic lithium disilicate and monolithic zirconia crowns.[24]

In a study which evaluated the finish line and core thickness on the fracture strength of a restoration, knife-edge preparations presented a promising alternative to chamfer finish lines; the fracture load required for knife-edge preparations was 38% greater than that required for chamfer preparations, regardless of coping thickness. Reducing the thickness of a single crown coping from 0.5 to 0.3 mm resulted in a 35% reduction in fracture load required for either preparation type.[32] In this study, metal dies with 1-mm finish line were used. Scherrer and Rijk indicated that, increasing elastic modulus of the supporting material results in increased fracture strength.[33] The elastic modulus of the supporting metal die (200 GPa) was higher than the dentin (12–14 GPa). If natural teeth or resin teeth were used as a supporting model, the fracture strength of crowns might have been lower. However, natural/resin teeth would have been destroyed during the testing at high fracture loads. The fracture strength values of this study were lower than that of other studies which used multilayer technique. It might be because of the die spacer layer. In this study, die spacer layer was chosen as 30 μm, but in the other studies, it was chosen as 10 μm to obtain close fit with restoration and dies.[15],[16],[24],[28] Increase of cement thickness might be a result for lower fracture strength. The elastic moduli of the zirconia core (210 GPa) and veneering ceramic (63 GPa) were above the elastic modulus of the resin cements (Panavia F 2.0: 18.6 GPa, Multilink N: 32.5 GPa), which was used as an interfacial bonding agent between the zirconia core and the prefabricated Feldspathic veneers. Thus, the weaker intermediary cement layer at the zirconia/metal die might have decreased the supportive effect of rigid zirconia on the metal die. The stereomicroscope and visual analyzes demonstrated that there were no cracks or fractures in the zirconia cores.

The self-adhesive resin cement and dual-polymerized resin cement used in this study contain different phosphate-based monomers. The highest shear bond strength was measured with dual-polymerized resin cement (Panavia F 2.0) and self-adhesive resin cement (Multilink N), whereas the lowest shear bond strength was obtained by self-adhesive resin cement (Rely X Unicem) applied in powder/liquid form. Cohesive type failure was frequently observed in groups that were simulated with self-adhesive and dual-polymerized resin.[34]

Filler content of the composite is one of the important factors influencing bond strength and mechanical properties of the restorations. To improve retentive and esthetic properties of restorations, adhesive resin cements are recommended. It was stated that resin coating and use of resin cements with better physical properties generally improved the mechanical performance of porcelain.[35],[36],[37] Resin cements have lower filler content to decrease viscosity and facilitate clinical handling. It was stated that decrease in filler content exhibit more volumetric shrinkage than do heavy filled composites. In an experimental study, Miyazaki et al. indicated that, when bonding agents with filler particles are used, it is important to determine if optimum filler levels exist in order to optimize the dentin bond strength and found that 10% filler content increased bond strength and improved the mechanical properties of bonding agents.[35] The resin cements used in this study had different inorganic filler contents (Panavia F 2.0: 70.8%; Mutilink N: 39.7%). Higher fracture strength values were obtained in groups cemented with resin cement with low filler content.

Artificial aging is an essential part of anin vitro study because repetitive stresses during mastication may lead to subcritical crack growth in ceramic materials.[24] In the present study, all specimens were subjected to thermal cycling to evaluate specimen behavior under clinically approximated conditions. Unlike other studies, it may be one of the reasons for the lower fracture strengths.[15],[16] The resultant fracture toughness results are similar to those obtained by Al-Wahadni et al., who used thermal cycling in a similar way (3000 cycles, 5°C to 55°C).

Although production of zirconia-based ceramic crown with traditional layering technique was preferred because of esthetic and economic reasons, after chipping, restoration must be repaired or renewed.[15],[38] In previous studies, higher fracture strength values were obtained with CAD-on technique when compared with multilayer, press-on, or conventional layering techniques.[16],[22],[39] In this technique, lithium disilicate ceramic is used. Fracture strength of lithium disilicate ceramic (350–400 MPa) is higher than Feldspathic ceramic (100–150 MPa). Repair of CAD-on restoration can be made also with direct or indirect methods. When restoration is designed with CAD/CAM multilayer or CAD-on technique, repair of restoration is easier with milling of veneering ceramic superstructure with the help of previously recorded data and can also repaired with direct methods using composite resin.

Zirconia cores are generally veneered with lithium disilicate ceramic or Feldspathic ceramic. Although esthetic aspects of Feldspathic veneer might be slightly superior to those of lithium disilicate veneer, mechanical properties of lithium disilicate veneer are better than the Feldspathic veneer. However, both materials are suitable for the production of esthetic dental restorations. CAD/CAM multilayer technique is not time-consuming and it is an operator-sensitive laboratory procedure (such as impression making, model obtaining, investing and finishing can be eliminated, and also choosing Feldspathic ceramic as a veneer has an economical advantage).[6],[15],[16],[22],[40] However, the indication range is strongly limited to single crowns and small FPDs.[40] Schmitter et al. evaluated the chipping behavior of Feldspathic ceramic designed with multilayer or layering technique and reported that zirconia cores veneered with CAD/CAM-produced Feldspathic ceramic are less sensitive to ageing than zirconia crowns with layered Feldspathic veneer.[22]

CAD-on technique contains more sensitive, expensive, and time-consuming procedures when compared to multilayer technique. Special devices and second sinterization procedure are recommended by the manufacturer.[15],[16],[22],[39] Although fusion is recommended by the manufacturer, in the dental laboratory or even chair-side luting is often preferred because of rapid and easy handling. When compared with previous studies which used lithium disilicate ceramic as a veneer ceramic, in this study, lower fracture strength values were obtained but fracture strength values were higher than the maximum loads which may occur clinically (Fmax= 600 N on one tooth in the molar region in vivo).

In this study, factors such as the thermal changes in the oral environment were not able to reflect the fracture resistance of the veneer porcelains which generates limitations of this study. Another limitation that makes it difficult to directly compare the results of our study with the clinical situation is that it cannot be directly imitatedin vitro studies because prosthetic restorative materials are exposed to mechanical and chemical stresses after cementation. Further investigations are needed to evaluate the effect of different surface treatments and veneering technique on the mechanical properties and clinical performance of the CAD/CAM multilayer technique.

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Conflicts of interest

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  [Table 1], [Table 2], [Table 3], [Table 4]

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