- Email: [email protected]
Translucency of zirconia-based pressable ceramics with different core and veneer thicknesses
RESEARCH AND EDUCATION
Translucency of zirconia-based pressable ceramics with different core and veneer thicknesses Il-Do Jeong, PhD,a So-Yeon Bae, PhD,b Dong-Yeon Kim, MSc,c Ji-Hwan Kim, MPH, PhD,d and Woong-Chul Kim, MPH, PhDe The heat-press technique is a ABSTRACT restoration fabrication method Statement of problem. Little information is available on the translucency of zirconia-based in which heat and pressure are pressable ceramic restorations with a pressed ceramic veneer and zirconia core in various simultaneously applied and a thickness combinations. heated ceramic ingot is pushed Purpose. The purpose of this in vitro study was to assess the translucency of 3 types of zirconiainto a mold cavity.1,2 This based pressable ceramics for different core-veneer thickness combinations. method has been used in Material and methods. A bilayered ceramic specimen was prepared with a pressable ceramic (IPS dentistry over the past 40 years e.max Zirpress, Initial IQ, Rosetta UltraPress) veneer over a zirconia core (Zenostar Zr). Three groups to fabricate ceramic restoraof specimens (n=7) were formed with the following core+veneer thicknesses: 1 +0.5 mm, 0.7 +0.8 tions.3 More recently, it has mm, and 0.5 +1 mm. To obtain consistent thickness and high translucency, all specimens were been used to layer a ceramic subjected to surface grinding with a grinding machine. To eliminate the effect of differences in restoration, whereby a heated roughness on the translucency, the surface roughness of the ground specimens was measured with a scanning profiler, and the consistency of these measured values was verified through statistical ceramic ingot is pressed over a 4 analysis. The luminous transmittance of the specimens was measured with a spectrophotometer. zirconia core. The effects of the pressable ceramic type and core-veneer thickness combination on transmittance The veneer ceramic layered were assessed using a 2-way ANOVA (a=.05). on the zirconia core through Results. The consistency of the surface roughness among the tested specimens was confirmed the heat-press technique has using a 1-way ANOVA and the Tukey HSD post hoc test (P<.05). The luminous transmittance the same chemical composiexhibited a statistically significant dependence on both the type of pressable ceramic and the tion as a hand-applied ceramic core-veneer thickness combination (P<.05). veneer.5 However, the pressConclusions. The type of pressable ceramic and core-veneer thickness combination affected the ing technique avoids intranslucency of the restoration. (J Prosthet Dent 2015;-:---) clusions, pores, and cracks inside the veneer ceramic.5,6 As a result, a restoration fabricated by overpressing is However, ceramic materials for the fabrication of more tolerant of a cohesive failure inside the veneer than esthetic restorations must provide not only outstanding a ceramic restoration fabricated using the lamination mechanical characteristics but also optical characteristics technique.5,7 In recent years, considerable research has similar to those of natural teeth.8,9 Research must be been performed to investigate the mechanical characconducted not only on the mechanical characteristics of teristics of zirconia-based pressable ceramic veneers.1,4-6 zirconia-based pressable ceramic veneers but also on
Supported by a Korea University grant. a Doctoral student, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea. b Doctoral student, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea. c Graduate student, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea. d Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea. e Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea.
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Table 1. Materials used
Clinical Implications Adjustment of the core and veneer thicknesses must be considered to control the translucency of a bilayer ceramic restoration.
their optical characteristics. In particular, translucency has been identified as an important influence on the esthetic qualities of teeth,10-13 and considerable efforts have been focused on the translucency of ceramics.10-12,14-16 In previous studies, the translucency of ceramics was generally measured quantitatively using either the translucency parameter (TP) or contrast ratio (CR). The CR was reported by Powers et al17 in 1978 for the quantitative measurement of the translucency of restorative resin and was calculated as the ratio of the reflectance of the specimen against a white background to that against a black background. Johnston et al18 adapted this method and used the TP for the quantitative assessment of translucency. The TP was directly calculated according to the difference in the color of the specimen between the white and black backgrounds and was measured using an instrument such as a spectrophotometer, which determined color through the CIELab system.19-25 However, such methods do not enable direct measurements of translucency, and some studies report nonlinear relations between luminous transmittance and TP under transmission levels lower than 50%.26,27 Zirconia-based pressable ceramics have a bilayer structure comprising a core and veneer, and changes in the thicknesses of the core and veneer are known to affect the translucency of ceramics.9 However, studies examining the effect of core-veneer thickness combinations on translucency are rare, as is research concerning the translucency of zirconia-based pressable ceramics. In this study, the luminous transmittance of 3 types of zirconia-based pressable ceramic veneers with different core-veneer thickness combinations were investigated. The null hypothesis was that the translucency of zirconia-based pressed ceramics is not affected by the type of material or the thickness combination. MATERIAL AND METHODS The zirconia used for the core and the pressable ceramics used for the veneer are specified in Table 1. To produce the specimen, a zirconia blank was milled using computer-aided design and computer-aided manufacturing (CAD/CAM) (DWX-50; Roland), and the milled zirconia core was then fired according to the manufacturer’s instructions. To apply the veneer on the zirconia core, a wax block was milled using CAD/CAM (DWX-50; Roland), and a wax veneer was subsequently attached to the zirconia core before it was invested and THE JOURNAL OF PROSTHETIC DENTISTRY
Materials
Manufacturer
Color
Major Crystalline
Lot Number R65422
Pressed Ingot IPS e.max ZirPress
Ivoclar Vivadent AG
A1
Fluorapatite
Initial IQ
GC
A1
Leucite
Rosetta UltraPress
HASS
A1
Lithium disilicate
FAA01GK1001
Wieland Dental
light
N/A
O98FGK2002
872703
Zirconia Zenostar Zr
eliminated. Then, according to the method recommended by the manufacturer, the ceramic ingot was pressed (Table 2). In total, 63 specimens were produced: 21 per pressable ceramic group with 7 specimens for each thickness combination. The thickness and size of the specimens are shown in Figure 1. The surfaces of the specimens were ground (M-Prep 3; Allied High Tech) while wet on 240-, 400-, 600-, 800-, and 1200-grit silicon carbide paper. While the specimens were being ground, their thicknesses were measured frequently with a digital micrometer (Mitutoyo IP65; Mitutoyo), and they were ultimately ground to a thickness of 1.5 ±0.01 mm. All the produced specimens were cleaned in an ultrasonic cleaner (SD-B200H; Mujigae) for 180 seconds to remove any residue. To measure the surface roughness of each specimen, a scanning profiler (Alpha-Step D-500; KLA-Tencor) with a stylus was used. The stylus was placed at the center of the specimen surface, and the average roughness was calculated by measuring the roughness across a 5-mm length at a speed of 0.1 mm/s. To ensure regularity among the specimen surfaces of all groups, the calculated roughness values were evaluated with a 1-way ANOVA and the Tukey HSD post hoc test (a=.05). For the quantitative analysis of the luminous transmittance, a spectrophotometer (CM-3600A; Konica Minolta) with a dual-beam system was used. Measurements were performed in the transmittance chamber of the spectrophotometer by using pulsed xenon lamps as the light source. To measure transmittance, a zero calibration plate was attached to the chamber to calibrate it, after which the plate was removed and a custom specimen holder was attached. The measured values were converted with color matching functions from color data software (SpectraMagic NX CM-S100w; Konica Minolta), and the luminance was measured initially without the specimen (Luminancesource) and then with the specimen (Luminancespecimen).28 The luminous transmittance (T%) was calculated using the following expression: Luminancespecimen T%= ×100: Luminancesource Statistical analysis was performed with software (Statistical Package for Social Sciences, SPSS v21.0; IBM Jeong et al
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Table 2. Heat-pressing schedule for pressable ceramics Brand Name of Product
Heating Temperature ( C)
Start Temperature ( C)
Heating Rate ( C per min)
Vacuum Hold Time (min)
Pressing Temperature ( C)
IPS e.max ZirPress
900
700
60
15
910
Initial IQ
850
700
60
20
940
Rosetta UltraPress
925
700
30
20
925
0.6 mm 0.5 mm 1.0 mm Veneer
Table 3. Mean and standard deviation of measured surface roughness of specimens Types of zirconia-based pressed ceramics IPS e.max ZirPress, mm 0.355 (0.047)a
0.7 mm 0.8 mm Core 1.0 mm 0.5 mm
Initial IQ, mm
Rosetta UltraPress, mm
0.322 (0.025)a
0.331 (0.067)a
*Means in row with different superscript letters are significantly different (P<.05) using 1-way ANOVA and Tukey HSD post hoc test.
Table 4. Mean and standard deviation of absolute translucency with respect to types of zirconia-based pressed ceramics and different thickness combinations Types of zirconia-based pressed ceramics
Figure 1. Size and form of tested specimens. Thickness (veneer+core), mm 1.0 +0.5
IPS e.max ZirPress (%)
Initial IQ (%)
Rosetta UltraPress (%)
27.6 (0.7)a
29.8 (0.8)b
25.7 (0.6)c
a
24.7 (0.7)d
b
27.4 (0.5)a
SPSS). First, the homogeneity of the variance was assessed using the Levene test (P=.196) and subsequently a 2-way ANOVA was used (a=.05). Because the interaction between the 2 classification standards was found to be significant (P<.001), the statistical significance of the average difference among 9 comparison groups was investigated.
*Different superscript letters indicate statistically significant difference (P<.05).
RESULTS
DISCUSSION
The measurement results for the surface roughness of specimens with different types of zirconia-based pressable ceramics are listed in Table 3. No statistically significant difference was observed among the groups, confirming the consistency in the surface roughness among the specimens. The average luminous transmittance for the different types of zirconia-based pressable ceramic used for veneering and the different core-veneer thickness combinations ranged from 24.7% to 29.8%. The luminous transmittance was highest for the combination of Initial IQ veneer thickness of 1 mm and core thickness of 0.5 mm and lowest for the combination of Rosetta UltraPress thickness of 0.7 mm and core thickness of 0.8 mm. (Table 4, Fig. 2). The interaction between the different types of zirconia-based pressable ceramics used for veneering and the different core-veneer thickness combinations was found to be significant. The combination of 0.8-mm core thickness and 0.7-mm veneer thickness showed the lowest transmittance. For the combination of 0.5-mm core thickness and 1-mm veneer thickness, UltraPress showed significantly lower luminous transmittance than ZirPress and IQ. The 2-way ANOVA based on the main effects and interaction between the different types of zirconia-based pressable ceramics and
Previous research regarding the optical characteristics of dental ceramics focused largely on the color or translucency.10-12,14-16 Translucency has been evaluated by ceramic thickness,12 surface texture,24 composition,11 and illumination.15 However, because translucency has only been measured for specimens in the monolithic form,11 it was difficult to accept the results as clinically valid, even though the ceramic restoration with the core-veneer structure was a bilayer structure. Thus, in the present research, the translucency of core-veneer specimens with different thickness combinations was measured to obtain clinically valid results. Previous studies regarding the translucency of dental ceramics indicated that the roughness of the specimen surface strongly affects the translucency.24,28 Thus, in the present study, to eliminate such effects, the surface roughness of the specimens was measured with profilometry; the results confirmed the consistency of the surface roughness (Table 3). The null hypothesis was rejected; that is, the types of zirconia-based pressable ceramics and core-veneer thickness combinations affected the luminous transmittance. According to the translucency measurement results, the combination of 0.7-mm veneer thickness and 0.8-mm core thickness exhibited the lowest value of
Jeong et al
0.7 +0.8 0.5 +1.0
25.4 (0.8)
cd
27.3 (0.6)
a
28.1 (0.4) 29.3 (0.5)
different core-veneer thickness combinations indicated an 88.6% variation in the luminous transmittance.
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Thickness (mm) 1.0+0.5 0.7+0.8 0.5+1.0
Translucency (%)
30
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26
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the similar degree of increase in the translucency of the UltraPress and the ZirPress is attributed to the deterioration in the opaque optical characteristics at a low thickness. The 1.5-mm ceramic specimen used in this study was previously reported to yield an esthetic outcome unrelated to the underlying foundation or cement color.13 Thus, in this study, because a specimen with a total thickness of 1.5 mm was used, the underlying structure had no effect. However, other factors that affect the translucency, such as roughness and aging, were not considered, which can be regarded as a limitation of this research. Future investigations may also include the effect of the translucency of a* or b*, as the transmittance of the chromatic constituents through restorative materials may influence the esthetics as much as L*. CONCLUSIONS
Zirpress
IQ
Rosetta
Pressed Ceramic Figure 2. Box-and-whisker plot of luminous transmittance of zirconiabased pressed ceramics with different core-veneer thickness combinations.
transmittance. As the veneer thickness was decreased further, the value of transmittance tended to increase (Table 4). Among all the core-veneer thickness combinations, the IQ group exhibited the highest transmittance values (Fig. 2). This is attributed to the differences in the major crystalline structure of the examined zirconiabased pressable ceramics,11 as summarized in Table 1. The leucite-based crystalline IQ ceramic exhibited a higher transmittance value than the ZirPress ceramic composed of fluorapatite and UltraPress composed of lithium disilicate. This result is similar to those reported by Bagis and Turgut11 concerning the optical characteristics of laminate veneers, indicating that the IPS Empress Esthetic with a leucite-based crystalline structure has higher translucency compared with the IPS ZirPress, which contains fluorapatite, and IPS e.max Press, which contains lithium disilicate. The difference in translucency is caused by the differences among the leucite, fluorapatite, and lithium-disilicate crystalline structures and can be interpreted as a difference in the form and volume of the crystals inside the glass ceramic. The leucite microstructure is characterized by the formation of single crystals, and because it has a lower degree of crystallinity compared with fluorapatite and lithium disilicate, it exhibits higher translucency.2,25 Similarly, the UltraPress with lithium-disilicate crystals exhibited the lowest translucency in the case of a veneer thickness of 1 mm, in that it had the highest crystallinity and the crystals were randomly located and were needleshaped.2,11,28 However, at a veneer thickness of 0.5 mm, THE JOURNAL OF PROSTHETIC DENTISTRY
The translucency of zirconia-pressed ceramics is determined by the specific microstructure of the material. For an overall restoration thickness of 1.5 mm, the change in the thickness of the zirconia-based pressable ceramic veneer is a major factor in determining the translucency of the restoration. REFERENCES 1. Ansong R, Flinn B, Chung KH, Mancl L, Ishibe M, Raigrodski AJ. Fracture toughness of heat-pressed and layered ceramics. J Prosthet Dent 2013;109: 234-40. 2. Denry I, Holloway JA. Ceramics for dental applications: a review. Materials 2010;3:351-68. 3. Dro GGJ. The porcelain press technique. J Prosthet Dent 1972;28:209-14. 4. Ishibe M, Raigrodski AJ, Flinn BD, Chung KH, Spiekerman C, Winter RR. Shear bond strengths of pressed and layered veneering ceramics to highnoble alloy and zirconia cores. J Prosthet Dent 2011;106:29-37. 5. Stawarczyk B, Özcan M, Roos M, Trottmann A, Sailer I, Hämmerle CH. Load-bearing capacity and failure types of anterior zirconia crowns veneered with overpressing and layering techniques. Dent Mater 2011;27: 1045-53. 6. Lüthy H, Dong JK, Wohlwend A, Schärer P. Effects of veneering and glazing on the strength of heat-pressed ceramics. Schweize Monatsschr fur Zahnmed 1993;103:1257-60. 7. Christensen GJ. PFM vs. zirconia restorationsdhow are they comparing clinically. Clinicians Report 2008;1:1-2. 8. Holloway JA, Miller RB. The effect of core translucency on the aesthetics of all-ceramic restorations. Pract Periodontics Aesthet Dent 1996;9: 567-74. 9. Kursoglu P, Motro PFK, Kazazoglu E. Translucency of ceramic material in different core-veneer combinations. J Prosthet Dent 2015;113:48-53. 10. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, Haselton DR, Stanford CM, Vargas MA. Relative translucency of six all-ceramic systems. Part I: core materials. J Prosthet Dent 2002;88:4-9. 11. Bagis B, Turgut S. Optical properties of current ceramics systems for laminate veneers. J Dent 2013;41:24-30. 12. Wang F, Takahashi H, Iwasaki N. Translucency of dental ceramics with different thicknesses. J Prosthet Dent 2013;110:14-20. 13. Niu E, Agustin M, Douglas RD. Color match of machinable lithium disilicate ceramics: effects of cement color and thickness. J Prosthet Dent 2014;111: 42-50. 14. Lim HN, Yu B, Lee YK. Spectroradiometric and spectrophotometric translucency of ceramic materials. J Prosthet Dent 2010;104:239-46. 15. Ahn JS, Lee YK. Difference in the translucency of all-ceramics by the illuminant. Dent Mater 2008;24:1539-44. 16. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, Haselton DR, Stanford CM, Vargas MA. Relative translucency of six all-ceramic systems. Part II: core and veneer materials. J Prosthet Dent 2002;88:10-5. 17. Powers JM, Dennison JB, Lepeak PJ. Parameters that affect the color of direct restorative resins. J Dent Res 1978;57:876-80.
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18. Johnston WM, Ma T, Kienle BH. Translucency parameter of colorants for maxillofacial prostheses. Int J Prosthodont 1994;8:79-86. 19. Chu FC, Sham AS, Luk HW, Andersson B, Chai J, Chow TW. Threshold contrast ratio and masking ability of porcelain veneers with highdensity alumina cores. Int J Prosthodont 2003;17:24-8. 20. Ikeda T, Murata Y, Sano H. Translucency of opaque-shade resin composites. Am J Dent 2004;17:127-30. 21. Kim SJ, Son HH, Cho BH, Lee IB, Um CM. Translucency and masking ability of various opaque-shade composite resins. J Dent 2009;37:102-7. 22. Chang J, Da Silva JD, Sakai M, Kristiansen J, Ishikawa-Nagai S. The optical effect of composite luting cement on all ceramic crowns. J Dent 2009;37: 937-43. 23. Johnston WM, Reisbick MH. Color and translucency changes during and after curing of esthetic restorative materials. Dent Mater 1997;13: 89-97. 24. Wang H, Xiong F, Zhenhua L. Influence of varied surface texture of dentin porcelain on optical properties of porcelain specimens. J Prosthet Dent 2011;105:242-8. 25. Ilie N, Hickel R. Correlation between ceramics translucency and polymerization efficiency through ceramics. Dent Mater 2008;24:908-14.
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26. Nogueira AD, Bona AD. The effect of a coupling medium on color and translucency of CADeCAM ceramics. J Dent 2013;41:18-23. 27. 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. 28. Awad D, Stawarczyk B, Liebermann A, Ilie N. Translucency of esthetic dental restorative CAD/CAM materials and composite resins with respect to thickness and surface roughness. J Prosthet Dent 2015;113:534-40. Corresponding author: Woong-Chul Kim Department of Dental Laboratory Science and Engineering College of Health Science Korea University Anam-dong 5-ga, Seongbuk-gu, Seoul 136-072 REPUBLIC OF KOREA Email: [email protected] Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.
THE JOURNAL OF PROSTHETIC DENTISTRY
Translucency of zirconia-based pressable ceramics with different core and veneer thicknesses Il-Do Jeong, PhD,a So-Yeon Bae, PhD,b Dong-Yeon Kim, MSc,c Ji-Hwan Kim, MPH, PhD,d and Woong-Chul Kim, MPH, PhDe The heat-press technique is a ABSTRACT restoration fabrication method Statement of problem. Little information is available on the translucency of zirconia-based in which heat and pressure are pressable ceramic restorations with a pressed ceramic veneer and zirconia core in various simultaneously applied and a thickness combinations. heated ceramic ingot is pushed Purpose. The purpose of this in vitro study was to assess the translucency of 3 types of zirconiainto a mold cavity.1,2 This based pressable ceramics for different core-veneer thickness combinations. method has been used in Material and methods. A bilayered ceramic specimen was prepared with a pressable ceramic (IPS dentistry over the past 40 years e.max Zirpress, Initial IQ, Rosetta UltraPress) veneer over a zirconia core (Zenostar Zr). Three groups to fabricate ceramic restoraof specimens (n=7) were formed with the following core+veneer thicknesses: 1 +0.5 mm, 0.7 +0.8 tions.3 More recently, it has mm, and 0.5 +1 mm. To obtain consistent thickness and high translucency, all specimens were been used to layer a ceramic subjected to surface grinding with a grinding machine. To eliminate the effect of differences in restoration, whereby a heated roughness on the translucency, the surface roughness of the ground specimens was measured with a scanning profiler, and the consistency of these measured values was verified through statistical ceramic ingot is pressed over a 4 analysis. The luminous transmittance of the specimens was measured with a spectrophotometer. zirconia core. The effects of the pressable ceramic type and core-veneer thickness combination on transmittance The veneer ceramic layered were assessed using a 2-way ANOVA (a=.05). on the zirconia core through Results. The consistency of the surface roughness among the tested specimens was confirmed the heat-press technique has using a 1-way ANOVA and the Tukey HSD post hoc test (P<.05). The luminous transmittance the same chemical composiexhibited a statistically significant dependence on both the type of pressable ceramic and the tion as a hand-applied ceramic core-veneer thickness combination (P<.05). veneer.5 However, the pressConclusions. The type of pressable ceramic and core-veneer thickness combination affected the ing technique avoids intranslucency of the restoration. (J Prosthet Dent 2015;-:---) clusions, pores, and cracks inside the veneer ceramic.5,6 As a result, a restoration fabricated by overpressing is However, ceramic materials for the fabrication of more tolerant of a cohesive failure inside the veneer than esthetic restorations must provide not only outstanding a ceramic restoration fabricated using the lamination mechanical characteristics but also optical characteristics technique.5,7 In recent years, considerable research has similar to those of natural teeth.8,9 Research must be been performed to investigate the mechanical characconducted not only on the mechanical characteristics of teristics of zirconia-based pressable ceramic veneers.1,4-6 zirconia-based pressable ceramic veneers but also on
Supported by a Korea University grant. a Doctoral student, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea. b Doctoral student, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea. c Graduate student, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea. d Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea. e Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, Republic of Korea.
THE JOURNAL OF PROSTHETIC DENTISTRY
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Volume
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Issue
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Table 1. Materials used
Clinical Implications Adjustment of the core and veneer thicknesses must be considered to control the translucency of a bilayer ceramic restoration.
their optical characteristics. In particular, translucency has been identified as an important influence on the esthetic qualities of teeth,10-13 and considerable efforts have been focused on the translucency of ceramics.10-12,14-16 In previous studies, the translucency of ceramics was generally measured quantitatively using either the translucency parameter (TP) or contrast ratio (CR). The CR was reported by Powers et al17 in 1978 for the quantitative measurement of the translucency of restorative resin and was calculated as the ratio of the reflectance of the specimen against a white background to that against a black background. Johnston et al18 adapted this method and used the TP for the quantitative assessment of translucency. The TP was directly calculated according to the difference in the color of the specimen between the white and black backgrounds and was measured using an instrument such as a spectrophotometer, which determined color through the CIELab system.19-25 However, such methods do not enable direct measurements of translucency, and some studies report nonlinear relations between luminous transmittance and TP under transmission levels lower than 50%.26,27 Zirconia-based pressable ceramics have a bilayer structure comprising a core and veneer, and changes in the thicknesses of the core and veneer are known to affect the translucency of ceramics.9 However, studies examining the effect of core-veneer thickness combinations on translucency are rare, as is research concerning the translucency of zirconia-based pressable ceramics. In this study, the luminous transmittance of 3 types of zirconia-based pressable ceramic veneers with different core-veneer thickness combinations were investigated. The null hypothesis was that the translucency of zirconia-based pressed ceramics is not affected by the type of material or the thickness combination. MATERIAL AND METHODS The zirconia used for the core and the pressable ceramics used for the veneer are specified in Table 1. To produce the specimen, a zirconia blank was milled using computer-aided design and computer-aided manufacturing (CAD/CAM) (DWX-50; Roland), and the milled zirconia core was then fired according to the manufacturer’s instructions. To apply the veneer on the zirconia core, a wax block was milled using CAD/CAM (DWX-50; Roland), and a wax veneer was subsequently attached to the zirconia core before it was invested and THE JOURNAL OF PROSTHETIC DENTISTRY
Materials
Manufacturer
Color
Major Crystalline
Lot Number R65422
Pressed Ingot IPS e.max ZirPress
Ivoclar Vivadent AG
A1
Fluorapatite
Initial IQ
GC
A1
Leucite
Rosetta UltraPress
HASS
A1
Lithium disilicate
FAA01GK1001
Wieland Dental
light
N/A
O98FGK2002
872703
Zirconia Zenostar Zr
eliminated. Then, according to the method recommended by the manufacturer, the ceramic ingot was pressed (Table 2). In total, 63 specimens were produced: 21 per pressable ceramic group with 7 specimens for each thickness combination. The thickness and size of the specimens are shown in Figure 1. The surfaces of the specimens were ground (M-Prep 3; Allied High Tech) while wet on 240-, 400-, 600-, 800-, and 1200-grit silicon carbide paper. While the specimens were being ground, their thicknesses were measured frequently with a digital micrometer (Mitutoyo IP65; Mitutoyo), and they were ultimately ground to a thickness of 1.5 ±0.01 mm. All the produced specimens were cleaned in an ultrasonic cleaner (SD-B200H; Mujigae) for 180 seconds to remove any residue. To measure the surface roughness of each specimen, a scanning profiler (Alpha-Step D-500; KLA-Tencor) with a stylus was used. The stylus was placed at the center of the specimen surface, and the average roughness was calculated by measuring the roughness across a 5-mm length at a speed of 0.1 mm/s. To ensure regularity among the specimen surfaces of all groups, the calculated roughness values were evaluated with a 1-way ANOVA and the Tukey HSD post hoc test (a=.05). For the quantitative analysis of the luminous transmittance, a spectrophotometer (CM-3600A; Konica Minolta) with a dual-beam system was used. Measurements were performed in the transmittance chamber of the spectrophotometer by using pulsed xenon lamps as the light source. To measure transmittance, a zero calibration plate was attached to the chamber to calibrate it, after which the plate was removed and a custom specimen holder was attached. The measured values were converted with color matching functions from color data software (SpectraMagic NX CM-S100w; Konica Minolta), and the luminance was measured initially without the specimen (Luminancesource) and then with the specimen (Luminancespecimen).28 The luminous transmittance (T%) was calculated using the following expression: Luminancespecimen T%= ×100: Luminancesource Statistical analysis was performed with software (Statistical Package for Social Sciences, SPSS v21.0; IBM Jeong et al
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Table 2. Heat-pressing schedule for pressable ceramics Brand Name of Product
Heating Temperature ( C)
Start Temperature ( C)
Heating Rate ( C per min)
Vacuum Hold Time (min)
Pressing Temperature ( C)
IPS e.max ZirPress
900
700
60
15
910
Initial IQ
850
700
60
20
940
Rosetta UltraPress
925
700
30
20
925
0.6 mm 0.5 mm 1.0 mm Veneer
Table 3. Mean and standard deviation of measured surface roughness of specimens Types of zirconia-based pressed ceramics IPS e.max ZirPress, mm 0.355 (0.047)a
0.7 mm 0.8 mm Core 1.0 mm 0.5 mm
Initial IQ, mm
Rosetta UltraPress, mm
0.322 (0.025)a
0.331 (0.067)a
*Means in row with different superscript letters are significantly different (P<.05) using 1-way ANOVA and Tukey HSD post hoc test.
Table 4. Mean and standard deviation of absolute translucency with respect to types of zirconia-based pressed ceramics and different thickness combinations Types of zirconia-based pressed ceramics
Figure 1. Size and form of tested specimens. Thickness (veneer+core), mm 1.0 +0.5
IPS e.max ZirPress (%)
Initial IQ (%)
Rosetta UltraPress (%)
27.6 (0.7)a
29.8 (0.8)b
25.7 (0.6)c
a
24.7 (0.7)d
b
27.4 (0.5)a
SPSS). First, the homogeneity of the variance was assessed using the Levene test (P=.196) and subsequently a 2-way ANOVA was used (a=.05). Because the interaction between the 2 classification standards was found to be significant (P<.001), the statistical significance of the average difference among 9 comparison groups was investigated.
*Different superscript letters indicate statistically significant difference (P<.05).
RESULTS
DISCUSSION
The measurement results for the surface roughness of specimens with different types of zirconia-based pressable ceramics are listed in Table 3. No statistically significant difference was observed among the groups, confirming the consistency in the surface roughness among the specimens. The average luminous transmittance for the different types of zirconia-based pressable ceramic used for veneering and the different core-veneer thickness combinations ranged from 24.7% to 29.8%. The luminous transmittance was highest for the combination of Initial IQ veneer thickness of 1 mm and core thickness of 0.5 mm and lowest for the combination of Rosetta UltraPress thickness of 0.7 mm and core thickness of 0.8 mm. (Table 4, Fig. 2). The interaction between the different types of zirconia-based pressable ceramics used for veneering and the different core-veneer thickness combinations was found to be significant. The combination of 0.8-mm core thickness and 0.7-mm veneer thickness showed the lowest transmittance. For the combination of 0.5-mm core thickness and 1-mm veneer thickness, UltraPress showed significantly lower luminous transmittance than ZirPress and IQ. The 2-way ANOVA based on the main effects and interaction between the different types of zirconia-based pressable ceramics and
Previous research regarding the optical characteristics of dental ceramics focused largely on the color or translucency.10-12,14-16 Translucency has been evaluated by ceramic thickness,12 surface texture,24 composition,11 and illumination.15 However, because translucency has only been measured for specimens in the monolithic form,11 it was difficult to accept the results as clinically valid, even though the ceramic restoration with the core-veneer structure was a bilayer structure. Thus, in the present research, the translucency of core-veneer specimens with different thickness combinations was measured to obtain clinically valid results. Previous studies regarding the translucency of dental ceramics indicated that the roughness of the specimen surface strongly affects the translucency.24,28 Thus, in the present study, to eliminate such effects, the surface roughness of the specimens was measured with profilometry; the results confirmed the consistency of the surface roughness (Table 3). The null hypothesis was rejected; that is, the types of zirconia-based pressable ceramics and core-veneer thickness combinations affected the luminous transmittance. According to the translucency measurement results, the combination of 0.7-mm veneer thickness and 0.8-mm core thickness exhibited the lowest value of
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0.7 +0.8 0.5 +1.0
25.4 (0.8)
cd
27.3 (0.6)
a
28.1 (0.4) 29.3 (0.5)
different core-veneer thickness combinations indicated an 88.6% variation in the luminous transmittance.
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Volume
Thickness (mm) 1.0+0.5 0.7+0.8 0.5+1.0
Translucency (%)
30
28
26
24
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Issue
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the similar degree of increase in the translucency of the UltraPress and the ZirPress is attributed to the deterioration in the opaque optical characteristics at a low thickness. The 1.5-mm ceramic specimen used in this study was previously reported to yield an esthetic outcome unrelated to the underlying foundation or cement color.13 Thus, in this study, because a specimen with a total thickness of 1.5 mm was used, the underlying structure had no effect. However, other factors that affect the translucency, such as roughness and aging, were not considered, which can be regarded as a limitation of this research. Future investigations may also include the effect of the translucency of a* or b*, as the transmittance of the chromatic constituents through restorative materials may influence the esthetics as much as L*. CONCLUSIONS
Zirpress
IQ
Rosetta
Pressed Ceramic Figure 2. Box-and-whisker plot of luminous transmittance of zirconiabased pressed ceramics with different core-veneer thickness combinations.
transmittance. As the veneer thickness was decreased further, the value of transmittance tended to increase (Table 4). Among all the core-veneer thickness combinations, the IQ group exhibited the highest transmittance values (Fig. 2). This is attributed to the differences in the major crystalline structure of the examined zirconiabased pressable ceramics,11 as summarized in Table 1. The leucite-based crystalline IQ ceramic exhibited a higher transmittance value than the ZirPress ceramic composed of fluorapatite and UltraPress composed of lithium disilicate. This result is similar to those reported by Bagis and Turgut11 concerning the optical characteristics of laminate veneers, indicating that the IPS Empress Esthetic with a leucite-based crystalline structure has higher translucency compared with the IPS ZirPress, which contains fluorapatite, and IPS e.max Press, which contains lithium disilicate. The difference in translucency is caused by the differences among the leucite, fluorapatite, and lithium-disilicate crystalline structures and can be interpreted as a difference in the form and volume of the crystals inside the glass ceramic. The leucite microstructure is characterized by the formation of single crystals, and because it has a lower degree of crystallinity compared with fluorapatite and lithium disilicate, it exhibits higher translucency.2,25 Similarly, the UltraPress with lithium-disilicate crystals exhibited the lowest translucency in the case of a veneer thickness of 1 mm, in that it had the highest crystallinity and the crystals were randomly located and were needleshaped.2,11,28 However, at a veneer thickness of 0.5 mm, THE JOURNAL OF PROSTHETIC DENTISTRY
The translucency of zirconia-pressed ceramics is determined by the specific microstructure of the material. For an overall restoration thickness of 1.5 mm, the change in the thickness of the zirconia-based pressable ceramic veneer is a major factor in determining the translucency of the restoration. REFERENCES 1. Ansong R, Flinn B, Chung KH, Mancl L, Ishibe M, Raigrodski AJ. Fracture toughness of heat-pressed and layered ceramics. J Prosthet Dent 2013;109: 234-40. 2. Denry I, Holloway JA. Ceramics for dental applications: a review. Materials 2010;3:351-68. 3. Dro GGJ. The porcelain press technique. J Prosthet Dent 1972;28:209-14. 4. Ishibe M, Raigrodski AJ, Flinn BD, Chung KH, Spiekerman C, Winter RR. Shear bond strengths of pressed and layered veneering ceramics to highnoble alloy and zirconia cores. J Prosthet Dent 2011;106:29-37. 5. Stawarczyk B, Özcan M, Roos M, Trottmann A, Sailer I, Hämmerle CH. Load-bearing capacity and failure types of anterior zirconia crowns veneered with overpressing and layering techniques. Dent Mater 2011;27: 1045-53. 6. Lüthy H, Dong JK, Wohlwend A, Schärer P. Effects of veneering and glazing on the strength of heat-pressed ceramics. Schweize Monatsschr fur Zahnmed 1993;103:1257-60. 7. Christensen GJ. PFM vs. zirconia restorationsdhow are they comparing clinically. Clinicians Report 2008;1:1-2. 8. Holloway JA, Miller RB. The effect of core translucency on the aesthetics of all-ceramic restorations. Pract Periodontics Aesthet Dent 1996;9: 567-74. 9. Kursoglu P, Motro PFK, Kazazoglu E. Translucency of ceramic material in different core-veneer combinations. J Prosthet Dent 2015;113:48-53. 10. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, Haselton DR, Stanford CM, Vargas MA. Relative translucency of six all-ceramic systems. Part I: core materials. J Prosthet Dent 2002;88:4-9. 11. Bagis B, Turgut S. Optical properties of current ceramics systems for laminate veneers. J Dent 2013;41:24-30. 12. Wang F, Takahashi H, Iwasaki N. Translucency of dental ceramics with different thicknesses. J Prosthet Dent 2013;110:14-20. 13. Niu E, Agustin M, Douglas RD. Color match of machinable lithium disilicate ceramics: effects of cement color and thickness. J Prosthet Dent 2014;111: 42-50. 14. Lim HN, Yu B, Lee YK. Spectroradiometric and spectrophotometric translucency of ceramic materials. J Prosthet Dent 2010;104:239-46. 15. Ahn JS, Lee YK. Difference in the translucency of all-ceramics by the illuminant. Dent Mater 2008;24:1539-44. 16. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, Haselton DR, Stanford CM, Vargas MA. Relative translucency of six all-ceramic systems. Part II: core and veneer materials. J Prosthet Dent 2002;88:10-5. 17. Powers JM, Dennison JB, Lepeak PJ. Parameters that affect the color of direct restorative resins. J Dent Res 1978;57:876-80.
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26. Nogueira AD, Bona AD. The effect of a coupling medium on color and translucency of CADeCAM ceramics. J Dent 2013;41:18-23. 27. 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. 28. Awad D, Stawarczyk B, Liebermann A, Ilie N. Translucency of esthetic dental restorative CAD/CAM materials and composite resins with respect to thickness and surface roughness. J Prosthet Dent 2015;113:534-40. Corresponding author: Woong-Chul Kim Department of Dental Laboratory Science and Engineering College of Health Science Korea University Anam-dong 5-ga, Seongbuk-gu, Seoul 136-072 REPUBLIC OF KOREA Email: [email protected] Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.
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