RESEARCH ARTICLE

Translucency of Ceramic Materials for CEREC CAD/CAM System ALESSANDRO VICHI, DDS, MS, PhD*, MICHELE CARRABBA, DDS†, RADE PARAVINA, DDS, MS, PhD‡, MARCO FERRARI, MD, DDS, PhD§

ABSTRACT Objectives: To compare translucency of the ceramic materials (CEREC CAD/CAM). Materials and Methods: Fifteen ceramic materials for CEREC CAD/CAM system were evaluated: IPS e.max HT/LT/MO, ZirCAD, Empress HT/LT; VITA Mark II, VITA AL; VITA YZ, VITA In-Ceram Spinell/Alumina/Zirconia; and Sirona InCoris AL; Sirona InCoris ZI/TZI. Specimens (0.5-mm and 1.0-mm thick; n = 10 each material) were cut from commercial blocks using a water-cooled diamond saw. Contrast ratio (CR = YB/YW) was measured using a spectrophotometer with an integrating sphere. Kruskal–Wallis one way analysis of variance was performed followed by Dunn’s multiple test for post-hoc. Results: CR varied from 0.35 to 1.00 and from 0.48 to 1.00 for 0.5 mm and 1.0 mm thicknesses, respectively. CR increased in the following order: IPS e.max HT (most translucent—least opaque), IPS Empress HT, VITA Mark II, IPS Empress LT, IPS e.max LT, In-Ceram Spinell, IPS e.max MO, VITA YZ, InCoris TZI, IPS e.max ZirCAD, InCoris ZI, In-Ceram Alumina, VITA AL, InCoris AL, and In-Ceram Zirconia (least translucent—most opaque). Conclusions: The null hypothesis has been rejected because tested materials exhibited a wide range of CR. Translucency needs to be taken into account in different clinical situations, including considerations associated with thickness of restoration and/or particular layers.

CLINICAL SIGNIFICANCE A wide range of translucency was identified for the ceramic materials tested. This variability has to be taken into account for the selection of the materials in different clinical situations also related to the thickness clinically required. (J Esthet Restor Dent ••:••–••, 2014)

INTRODUCTION Porcelain fused to metal restorations (PFM), combining resistance and esthetics, have been considered the reference procedure for fixed dental prosthetic restorations for a long time. However, the metal substructure prevents the transmission of light, thus diminishing the probability of fully mimicking optical properties of natural teeth.1 To overcome this esthetic

limitation, a variety of metal-free materials and techniques have been introduced, including the development of CAD/CAM technology, nowadays supported by scientific evidence of clinically adequate performance.2,3 The CAD/CAM system CEREC (Sirona, Bernsheim, Germany) has been created in the mid-1980s with the aim of manufacturing a dental ceramic restoration within the same day4 or even preferably in a single appointment (so called “chairside”

*Research Professor, Department of Medical Biotechnologies, University of Siena, Siena, Italy † PhD Student, Department of Medical Biotechnologies, University of Siena, Siena, Italy ‡ Professor, Department of Restorative Dentistry and Prosthodontics, The University of Texas Health Science Center School of Dentistry, Houston, TX, USA § Professor and Chair, Department of Medical Biotechnologies, University of Siena, Siena, Italy

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procedure). This system has been continuously improved over the years and the most recent enhancement of this system has resulted in its wider acceptance and use in dental practice.5,6 The CEREC system is currently used both for producing metal-free cores that have to be subsequently veneered as well as for monolithic full-contour restorations. Different materials are available for the CEREC system and metal-free restorations, with different mechanical and optical properties, and the selection criteria in relationship with the clinical use could be challenging. For a correct selection, longevity and esthetics have to be considered as the main parameters. As a general rule, they are inversely proportiona, because an increase in crystalline content to achieve greater strength generally results in greater opacity.7 The possibility for the light to pass through the material structure in a similar fashion, as with the natural tissues, represents an important esthetic advantage of metal-free restorations. The translucency of the core has been identified as one of the primary factors in controlling esthetics and a critical consideration in the material selection.8 Several authors have addressed the issue of evaluating the translucency of dental ceramics, generally measuring the contrast ratio (CR) or translucency parameter (TP) of the materials.9 However, the data is limited on these parameters when it comes to CAD/CAM materials. The aim of this study was therefore to compare CRs of the ceramic materials available for CEREC CAD/CAM systems. The null hypothesis was that there were no statistically significant differences in CRs between the evaluated CAD/CAM ceramic materials.

MATERIALS AND METHODS A total of 15 ceramic materials for the CEREC CAD/CAM system were selected (Table 1). The specimens were cut from CAD/CAM blocks with a water-cooled low-speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) at two different thicknesses, 0.5 mm and 1.0 mm. A custom-made setup ensured the perpendicular position of the block to the saw blade.

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Ten specimens where prepared for each material (n = 10). For the materials that require sintering process (VITA YZ, Sirona InCoris ZI and TZI, IPS e.max ZirCAD, Sirona InCoris AL, and VITA AL), specimen thickness was adapted to compensate for the material contraction during sintering in order to guarantee the correct final thickness. Sintering firing was then performed in a furnace (ZYrcomat T, VITA Zahnfabrik, Bad Säckingen, Germany) according to the manufacturer’s guidelines. For the materials that required crystallization process (IPS e.max LT\HT\MO), specimens were submitted after cutting to a crystallization process in a furnace (Vacumat 6000M; VITA Zahnfabrik), following manufacturer’s indications. The In-Ceram Alumina, Zirconia, and Spinell specimens were subjected to the glass infiltration process with VITA In-Ceram Alumina, Zirconia and Spinell Glass Powder (VITA Zahnfabrik), respectively, mixed with distilled water to obtain a thin consistency. A rich coat with a thickness of about 60% of the sample weight was applied to the sample surfaces using a brush. Glass infiltration firing was performed in the VITA Vacumat furnace on a platinum foil. The firing procedure was performed according to the manufacturer’s guidelines. Excess glass was removed with a diamond bur; finer residues of glass were sandblasted with 50-μm aluminum oxide at 3 bar pressure. Glass control firing was then performed. The feldspathic VITA Mark II and the leucite-reinforced ceramic of IPS Empress HT and LT were just cut at the final dimension as they did not require any additional process. All the specimens were finished and polished on a grinder/polisher with wet 120-, 240-, and 400-grit silicone-carbide paper and ultrasonic-cleaned in distilled water for 10 minutes prior to measurement. A benchtop spectrophotometer (PSD1000, OceanOptics, Dunedin, FL, USA), equipped with an

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TABLE 1. Tested materials Materials

Shade

Manufacturer

Batch #

Strengthening component

IPS Empress CAD LT

A2

Ivoclar Vivadent, Schaan, Liechtenstein

N00044

Leucite

IPS Empress CAD HT

A2

Ivoclar Vivadent

N14103

Leucite

IPS e.max CAD MO

A2

Ivoclar Vivadent

L03481

Lithium disilicate

IPS e.max CAD LT

A2

Ivoclar Vivadent

M63434

Lithium disilicate

IPS e.max CAD HT

A2

Ivoclar Vivadent

M67635

Lithium disilicate

VITA Mark II

2M2

VITA Zahnfabrik, Bad Säckingen, Germany

14640

Feldspathic

IPS e.max ZirCAD

/

Ivoclar Vivadent

K24148

ZrO2 87.0–95.0%, Y2O3 4.0–6.0%, HfO2 1.0–5.0%, Al2O3 0.0–1.0%

In-Ceram Alumina

/

VITA Zahnfabrik

18840

Alumina

In-Ceram Zirconia

/

VITA Zahnfabrik

16900

Alumina (67%), zirconium oxide (33%)

In-Ceram Spinell

/

VITA Zahnfabrik

16651

Magnesium aluminum oxide (spinel)

VITA AL

/

VITA Zahnfabrik

30071

Yttrium partially stabilized aluminum oxide

VITA YZ

/

VITA Zahnfabrik

30230

Yttrium partially stabilized zirconium oxide

InCoris AL

/

Sirona, Bernsheim, Germany

0001700

Alumina

InCoris ZI

F0.5

Sirona

1013700

ZrO2 + Y2O3 + HfO2 > 99%, Al2O3 < 0.5%, other oxide < 0.5%

InCoris TZI

F0

Sirona

2011512534

ZrO2 + Y2O3 + HfO2 > 99%, Al2O3 < 0.5%, other oxide < 0.5%

integrating sphere (ISP-REF, OceanOptics) and a 10-mm opening, was used in conjunction with corresponding color measurement software (OOILab 1.0, OceanOptics). D65 illuminant and 10° standard observer were selected. Measurements were conducted against white and black calibrated field tiles. The CR was calculated by comparing the reflectance of light “Y” (ratio of the intensity of reflected light to that of the incident radiant flux) of specimens against black (Yb) and white (Yw) backgrounds, using the following equation: CR = Yb/Yw.

Statistical Analysis Since the pooled data from all the block types did not pass the normality test, the use of two-way analysis of variance (ANOVA) was precluded to assess the significance of the dependent variables, materials, and thickness.

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Since data distribution was not normal according to the Kolmogorov–Smirnov test, the Kruskal–Wallis ANOVA on ranks was applied, followed by the Dunn’s multiple range test for post-hoc comparisons, whereas the level of significance was set at α = 0.05. The variables “thickness” and “material” were separately analyzed.

RESULTS The values of CR and the results of the statistical analysis are reported in Table 2. Higher CR value corresponds to more opaque materials, whereas lower CR corresponds to materials with higher translucency. CR values ranged from 0.35 (IPS e.max HT at 0.5 mm) to 0.89 (InCoris AL at 1.0 mm), whereas In-Ceram Zirconia exhibited total opacity (no translucency) at both thicknesses. The differences in thickness were statistically significant among all materials except for

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TABLE 2. Results and statistical significance (p < 0.05). The small letters describe differences between materials related to thickness; capital letters describe the differences between materials independent from thickness. Italic capital letters indicate the difference between the two groups of thickness inside the materials Materials

CR 0.5 mm

1.0 mm

Mean

SD

IPS e.max HT

0.35

0.01

IPS Empress HT

0.36

VITA Mark II

Sign.

Overall sign.

Mean

SD

Sign.

d

0.48

0.01

f

F

0.03

d

0.49

0.01

ef

EF

0.36

0.03

d

0.54

0.02

f

F

IPS Empress LT

0.42

0.01

cd

0.55

0.02

def

DEF

IPS e.max LT

0.43

0.02

cd

0.56

0.02

def

DEF

In-Ceram Spinell

0.44

0.02

cd

0.63

0.02

cdef

DEF

IPS e.max MO

0.50

0.03

bcd

0.71

0.02

abcde

CDE

VITA YZ

0.56

0.03

abcd

0.68

0.01

cdef

CDE

InCoris TZI

0.59

0.03

abc

0.68

0.02

bcdef

BCD

InCoris ZI

0.64

0.07

abc

0.82

0.03

abc

ABC

IPS e.max ZirCAD

0.65

0.03

abc

0.78

0.03

abcd

ABC

In-Ceram Alumina

0.65

0.02

abc

0.82

0.01

abc

ABC

VITA AL

0.69

0.02

ab

0.81

0.01

abc

ABC

InCoris AL

0.74

0.02

a

0.89

0.02

ab

AB

In-Ceram Zirconia

1.00*

0.01

a

1.00*

0.01

a

A

Sign.

B

A

CR = contrast ratio; Sign. = significance; Overall Sign. = overall significance. *No statistically significant difference for the variable thickness.

In-Ceram Zirconia. The Kruskal–Wallis ANOVA on ranks revealed a statistically significant difference between glass ceramic, crystalline ceramic, and glass-infiltrated ceramic except for the In-Ceram Spinell that was more translucent than IPS e.max MO. The lithium disilicate IPS e.max MO did not exhibit significant differences compared with other crystalline and glass-infiltrated ceramics such as the sintered Zirconia, VITA AL and glass-infiltrated Spinell and Alumina. No statistically significant differences were recorded among InCoris TZI, VITA YZ, In-Ceram Spinell, IPS e.max MO/LT, and IPS Empress LT.

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Based on significance of differences in CR (1.0 mm), the tested materials were classified into four classes of translucency: (1) high translucency: CR up to 0.50 (IPS e.max, Empress HT); (2) medium translucency: CR 0.50 to 0.75 (VITA Mark II, In-Ceram Spinell and YZ, Sirona InCoris TZI, and IPS e.max LT/MO, Empress LT); (3) low translucency: CR 0.75 to 0.90 (VITA In-Ceram Alumina and AL, Sirona InCoris ZI and AL, IPS e.max ZirCAD); and (4) very low translucency (highly masking): CR 0.90 to 1.00 (VITA In-Ceram Zirconia).

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DISCUSSION Tested materials exhibited a wide range of contrast ratios. As the differences that emerged were statistically significant, the null hypothesis was rejected for both investigated thicknesses (0.5 mm and 1.0 mm). Translucency is a very important factor in the selection of metal-free materials. Translucency is usually determined by CR or TP. CR is the ratio of the reflectance of a specimen over a black backing to that over a white backing of a known reflectance, and is an estimate of opacity.10 The CR ranges from 0 to 1, with 0 corresponding to transparency (total translucency) and 1 corresponding to total opacity (no translucency). The TP is the difference in color (ΔE*) between a uniform thickness of a material measured over white and black backing.11 In a recent paper, the efficacy of this measurement method for dental ceramics was criticized.12 As translucency is function of thickness13,14 and given that no ISO standard for evaluation of translucency in dentistry has been released, two different thicknesses were utilized in this study in order to compare the results with the literature. Instrumental measurements of color can detect small differences not perceivable by human eye. Perceptibility and acceptability visual thresholds (typically calculated at 50:50% level) are very useful in the interpretation of colorimetric results. Different threshold values have been reported.15 One carefully controlled study reported a 50:50% perceptibility threshold of ΔE* = 1.8, and 50:50% acceptability threshold of ΔE* = 3.5.16 When it comes to evaluation of differences in translucency, literature is much more limited. A recent study proposed a translucency perception threshold (TPT) based on ΔC (CR differences between two samples) and reported ΔC = 0.07 as the overall mean TPT in porcelain specimen comparison.17 Investigating optical properties of metal-free ceramic materials and establishing a direct correlation with corresponding natural tissues can be beneficial for esthetic outcome in clinical dentistry. A wide range of translucency of all-ceramic materials was reported.18,19 The results of the present study were in accordance

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with literature, and, on the basis of the CR at 1.0 mm thickness, a classification into four groups was proposed: high translucency, medium translucency, low translucency, and highly masking. The highest CR was obtained for In-Ceram Zirconia. The CR value of 1 at both thicknesses is consistent with literature.18 In-Ceram Zirconia is composed of alumina oxide (67%) and zirconium oxide (33%), and exhibits relatively high flexural strength of 600 MPa that enables its use in the posterior area.19 This is the only metal-free material suitable for all-ceramic three-unit bridges involving molar restorations (on the basis of the ISO standards)20 that could be manufactured by technicians with the use of a traditional furnace, as it is not a material based on sintering technique but on glass infiltration. The high masking ability of this material can be conveniently used in indicated clinical situations,19 at the same time limiting its use in esthetic areas. The group of “low translucency” materials includes materials for sintering, VITA AL, IPS e.max ZirCAD, Sirona InCoris ZI and AL, as well as glass-infiltrated In-Ceram Alumina. The CR value at 1.0 mm ranged from 0.78 for IPS e.max ZirCAD to 0.89 for Sirona InCoris AL. These are the materials that are indicated for the posterior area, based on the strength values.20,21 A study reported on the translucency of different brands of zirconia copings using a direct transmission method, reported different levels of light transmission, that were significantly lower than that of the lithium disilicate, used as a control group.22 Another study also reported that lithium disilicate and In-Ceram Alumina were more translucent than the traditional zirconia.23 Results from this study confirmed these findings. The group classified in the present study as “medium translucency” comprises materials with CR at 1.0 mm ranging from 0.50 to 0.75, VITA Mark II and IPS e.max MO. Few data are available on translucency of these materials and no studies directly investigated optical properties of feldspathic, leucite–reinforced, or lithium disilicate ceramic for CEREC CAD/CAM system. In a recent study, similar results were reported for IPS Empress LT (CR = 0.58), VITA Mark II (CR = 0.61) and

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IPS e.max CAD LT (CR = 0.63).23 In the “medium translucency” group, there were also two zirconia-sintered materials, VITA YZ and Sirona InCoris TZI. The “augmented translucency” Zirconia has recently been object of manufacturing efforts, aiming to create a high-strength material with good optical properties that could be milled full contour to overcome chipping in porcelain fused to zirconia restorations. The group of “high translucency” materials exhibited CR values at 1.0 mm lower than 0.50. Two materials were categorized in this group, a lithium disilicate (IPS e.max HT, CR = 0.48) and a leucite-reinforced ceramic (IPS Empress HT, CR = 0.49). Similar results were recently reported for IPS Empress CAD HT (CR = 0.52), whereas lower translucency were recorded for IPS e.max CAD HT (CR = 0.58).23 Few studies investigated optical proprieties of human enamel and dentin. It is hard to directly correlate these findings with the results of the present study because different measurement methods have been utilized. A study on translucency of 1-mm thick enamel and dentin specimens reported the mean CR for enamel and dentin to be 0.45 and 0.65, respectively.24 Our findings indicate that the use of “high translucency” materials might be appropriate for replacing enamel without the increase of thickness as compared with the real thickness of enamel. The results also indicate that the use of “low translucency” materials as core material for dentin replacement would not be able to correctly mimic its translucency. On the other hand, the “medium translucency” zirconia group can be used for optimal dentin replacement. A recent study compared 0.5-mm thick zirconia specimens with human dentine and reported that there was no significant difference in translucency between IPS e.max ZirCAD and human dentine.25 However, the “medium translucency” zirconia are still inadequate as enamel replacement, which indicates the veneering process as still necessary for optimal esthetics.

wide difference of mechanical performances.26,27 Differences in translucency and flexural strength are related to different compositions, with varying crystalline content and refractive indices.28 The need for higher flexural resistance required for posterior fixed partial dentures (FPDs; bridge in Classes 5 and 6 of the ISO classification)19 limits the materials to sintered materials zirconia and In-Ceram Zirconia, and precludes the use of all “high translucency” and most of the “medium translucency” materials, with the exception of Sirona TZI and VITA YZ that might be therefore considered as reference materials for posterior FPDs. For anterior FPDs, a lower flexural strength is required compared with the posterior FPDs.20 In addition to sintering materials, this allows lithium disilicate to be considered a material indicated for anterior FPDs, but limited long-term data are available at present.29 More options are available for single crowns. Since the ISO requirements for single crowns are 50 MPa for anterior and 100 MPa for posterior areas (for adhesively cemented restorations), all the materials tested in this study would be suitable.26,27 For single crowns, translucency and esthetic consideration may be considered pivotal in material selection, favoring PFZ with “augmented translucency” zirconia core, or full-contour glass ceramic and lithium disilicate adhesively cemented crowns. Nevertheless, the excess of translucency should be avoided. It has been reported that increasing the thickness of the enamel layer (thus more translucent) in a ceramic layering determines a decrease in lightness, ending in a grayish appearance.30 Likewise, especially in full-contour restorations, the “high translucency” materials might cause similar appearance; their use may be indicated for small and thin restoration as laminate veneers and partial crowns restoration as inlays or onlays. For this reason, the “medium translucency” material could generally be the material of choice when esthetics is a main factor in single-tooth rehabilitation.

CONCLUSION To better perform the correct material selection, along with translucency the flexural strength should be taken into consideration as the various materials tested have a

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The null hypothesis has been rejected because tested materials exhibited a wide range of CR. Based on the

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results and statistical analysis, evaluated materials were classified as follows: (1) high translucency: CR up to 0.50; (2) medium translucency: CR 0.50 to 0.75; (3) low translucency: CR 0.75 to 0.90; and (4) very low translucency (highly masking): CR 0.90 to 1.00. Translucency needs to be taken into account in different clinical situations, including considerations associated with thickness of restoration and/or particular layers.

DISCLOSURE AND ACKNOWLEDGMENTS Alessandro Vichi, Michele Carrabba and Marco Ferrari declare no conflict of interests in the companies/ products used in this paper. Rade Paravina is a paid consultant for VITA Zahnfabrik.

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11. Johnston WM, Ma T, Kienle BH. Translucency parameter of colorants for maxillofacial prostheses. Int J Prosthodont 1995;8:79–86. 12. Spink LS, Rungruanganut P, Megremis S, Kelly JR. Comparison of an absolute and surrogate measure of relative translucency in dental ceramics. Dent Mater 2013;29:702–7. 13. Antonson SA, Anusavice KJ. Contrast ratio of veneering and core ceramics as a function of thickness. Int J Prosthodont 2001;14(4):316–20. 14. Wang F, Takahashi H, Iwasaki N. Translucency of dental ceramics with different thicknesses. J Prosthet Dent 2013;110(1):14–20. 15. Vichi A, Louca C, Corciolani G, Ferrari M. Color related to ceramic and zirconia restorations: a review. Dent Mater 2011;27:97–108. 16. Ghinea R, Perez MM, Herrera LJ, et al. Color difference thresholds in dental ceramics. J Dent 2010;38s:e57–64. 17. Liu MC, Aquilino SA, Lund PS, et al. Human perception of dental porcelain translucency correlated to spectrophotometric measurements. J Prosthodont 2010;19(3):187–93. 18. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, et al. Relative translucency of six all-ceramic systems. Part I: core materials. J Prosthet Dent 2002;88(1):4–9. 19. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, et al. Relative translucency of six all-ceramic systems. Part II: core and veneer materials. J Prosthet Dent 2002;88(1):10–5. 20. International Organization for Standardization. ISO 6872:2008—Dentistry—Ceramic materials. 2008. 21. Borba M, de Araújo MD, de Lima E, et al. Flexural strength and failure modes of layered ceramic structures. Dent Mater 2011;27(12):1259–66. 22. Baldissara P, Llukacej A, Ciocca L, et al. Translucency of zirconia copings made with different CAD/CAM systems. J Prosthet Dent 2010;104:6–12. 23. Chen YM, Smales RJ, Yip KH, Sung WJ. Translucency and biaxial flexural strength of four ceramic core materials. Dent Mater 2008;24(11):1506–11. 24. Dietschi D, Ardu S, Krejci I. A new shading concept based on natural tooth color applied to direct composite restorations. Quintessence Int 2006;37:91–102. 25. Pecho OE, Ghinea R, Ionescu AM, et al. Color and translucency of zirconia ceramics, human dentine and bovine dentine. J Dent 2012;40s:e34–40. 26. Vichi A, Sedda M, Del Siena F, et al. Flexural resistance of Cerec CAD/CAM system ceramic blocks. Part 1: chairside materials. Am J Dent 2013;26:255–9. 27. Sedda M, Vichi A, Del Siena F, et al. Flexural resistance of Cerec CAD/CAM system ceramic blocks. Part 2: outsourcing materials. Am J Dent 2014;27:17–22. In Press.

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Reprint requests: Alessandro Vichi, DDS, MS, PhD, Department of Medical Biotechnologies, University of Siena, Via Derna 4, Grosseto 58100, Italy; Tel.: +39056425384; Fax: +39056425384; email: [email protected]

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CAM system.

To compare translucency of the ceramic materials (CEREC CAD/CAM)...
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