- Email: [email protected]
zirconia dental ceramics
RESEARCH AND EDUCATION
Effect of surface treatments on the biaxial flexural strength, phase transformation, and surface roughness of bilayered porcelain/zirconia dental ceramics lu Güngör, DDS, PhD,a Handan Yılmaz, DDS, PhD,b Seçil Karakoca Nemli, DDS, PhD,c Merve Bankog Bilge Turhan Bal, DDS, PhD,d and Cemal Aydın, DDS, PhDe Zirconia is a widely used ceramic core material in prosthodontics and for esthetic reasons is often covered with an appropriate veneering ceramic.1,2 The strength and quality of the bond between the layers are factors in the success of bilayered restorations.3 Delamination of the veneering porcelain is the most reported reason for failure of such restorations,1,4-6 particularly in posterior zirconia-based restorations.7-9 Such failures can arise from the design of the framework, thickness of the veneering ceramic, functional loads, layering technique of the veneering ceramics, and structure defects.1,10-14 Choi et al15 reported that zirconia and glass ceramic achieve thermodynamic stability as zirconia ions dissolve into the veneering material. Benetti et al16 proposed
ABSTRACT Statement of problem. Veneered zirconia restorations are widely used in prosthetic applications. However, these restorations often fail because of chipping of the veneer porcelain. Surface treatments of zirconia core materials may affect the connection between the 2 layers. Purpose. The purpose of this study was to evaluate the effect of surface treatments on the biaxial flexural strength, phase transformation, and mean surface roughness of different bilayered porcelain/zirconia ceramics. Material and methods. Forty disk-shaped specimens were obtained for each material (Kavo and Noritake) and divided into 4 (n=10) groups (control, airborne-particle abraded, ground, and ground and airborne-particle abraded). Airborne-particle abrasion was performed with 110-mm Al2O3 particles for 15 seconds and at 400 kPa. Diamond rotary instruments with 100-mm grain size were used for grinding. The monoclinic phase transformation and surface roughness of the specimens were measured. Then, the specimens were veneered and subjected to a biaxial flexural strength test to calculate the Weibull moduli (m values) and the stresses occurring at the layers, outer surfaces of the bilayer, and interfaces of the layers. Results. The Kavo airborne-particle abraded group showed higher strength values in both layers (P<.05) than those of all experimental groups. The Kavo airborne-particle abraded group showed the lowest m values at the core and veneer layers. According to the phase analysis, significantly higher Xm values were found in the ground and airborne-particle abraded groups for both materials (P<.05). In both materials, except in the airborne-particle abraded groups, the relative monoclinic phases showed no difference (P<.05). Conclusion. Surface treatments affected the phase transformation, surface roughness, and biaxial flexural strength of Kavo and Noritake zirconia ceramics differently. Surface treatments increased the relative monoclinic phase content and average surface roughness. (J Prosthet Dent 2015;113:585-595)
Supported by Scientific Investigation Projects Department of Gazi University with grant no 03/2012-03. Presented at the 38th Annual Conference of the European Prosthodontic Association, Istanbul, Turkey, September 2014. a Research Assistant, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. b Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. c Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. d Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. e Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey.
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Clinical Implications The results of this in vitro study suggest that surface treatments affect the phase transformation, surface roughness, and biaxial flexural strength of zirconia core materials differently.
that slow heating and cooling could overcome the chipping problem and that fractures were caused by transient stresses related to thermal incompatibility between the porcelain and the core material. Li et al17 stated that the bonding characteristics between Y-TZP and the veneering material appeared to involve both chemical and mechanical interaction. The mechanical performance of zirconia may be influenced by surface treatments, including machining, airborne-particle abrasion, and grinding, and the stresses induced by these treatments may trigger tetragonal to monoclinic phase transformation.18-20 Zirconia is a polymeric material, and high localized stresses during grinding and airborne-particle abrasion,21,22 together with high temperatures,21,23 can cause phase transformation.18,21 In several studies, the effects of grinding, airborne-particle abrasion, and polishing on the strength of zirconia-based restorations have been reported.19,22,24,25 Studies related with surface treatments are generally focused on monolayered zirconia specimens, and few data are available on the effect of surface treatment on the bond strength between core and veneer layers.1,26,27 Dental ceramic materials are prone to fracture under tensile stresses. Therefore, because most fixed partial prostheses are exposed to biaxial stresses,28-33 the strength of these materials is more reliably measured under biaxial flexure than under uniaxial flexure. ISO 6872 has identified the formulas only for monolayered specimens.28,32 Both the stresses through the thickness of the bilayered specimens and the stresses at the interfaces and top and bottom surfaces of the bilayer can be calculated with the Hsueh et al calculations.34-36 The purpose of this investigation was to evaluate the influence of different surface treatments of the interface of the core and veneer on the biaxial flexural strength of the layers, phase transformation, and surface roughness of bilayered zirconia restorations. It was hypothesized that surface treatments would affect the biaxial flexural strength, the surface roughness, and the phase transformation of zirconia-based bilayered restorations. MATERIAL AND METHODS Two Y-TZP core materials (Kavo Everest ZS-blank; KaVo Dental and Noritake Alliance Zirconia; Noritake Co) and 2 veneer porcelains (IPS e.max Ceram; Ivoclar Vivadent and CerabienZR; Noritake Co) were selected for this study. The specimens were provided by the manufacturers. Four THE JOURNAL OF PROSTHETIC DENTISTRY
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groups were created from 40 disk-shaped specimens (15 ± 0.02 mm in diameter and 1 ±0.02 mm in thickness) for each material, 2 control groups -Kavo control (KC) and Noritake control (NC)- and 2 airborne-particle abraded groups with 110-mm Al2O3 particles (RocatecPre; 3M ESPE). Airborne-particle abrasion was performed for 15 seconds and at 400 kPa. For standardized airborne-particle abrasion conditions, the specimens were placed in a specimen holder positioned 30 mm from the airborneparticle abrasion device (Rocatec Junior; 3M ESPE). The specimens of this group were termed as Kavo abraded (KA) and Noritake abraded (NA). The ground groups, in which the specimens were ground by diamond rotary instruments (100-mm grain size) without water spray cooling, were termed as Kavo ground (KG) and Noritake ground (NG). The disks were ground from 1.1 mm to 1 ±0.02 mm at a rotational speed of 20 000 revolutions per minute. The rotary instruments were changed after each group. The dimensions of the specimens were measured with a digital micrometer (Powertectools). The ground and airborne-particle abraded group, in which the specimens were 15 ±0.02 mm in diameter and 1.1 ±0.02 mm in thickness and were airborne-particle abraded after grinding under the same conditions as previously described, were named as Kavo ground and abraded (KGA) and Noritake ground and abraded (NGA). After surface treatments, all specimens were ultrasonically cleaned (Electrosonic Type 7 Profi; Electrosonic GmBH) for 10 minutes. Then, liners and veneering porcelains were applied according to the manufacturers’ recommendations. Before veneering, all disks were cleaned and air dried. Contamination after cleaning was avoided. The liner materials were mixed with their forming liquids and a thin layer applied onto the specimens. ZirLiner (Ivoclar Vivadent) was used for the Kavo specimens, and Shade Base Porcelain (Noritake Co) was used for the Noritake specimens. The liners were fired independently according to the manufacturers’ recommendations. Silicone molds (Zeta Plus; Zhermack) were fabricated and the disks were positioned. Ceramic veneer powders were mixed with their liquids and brushed onto the core ceramics. The excess of liquid was removed with blotting paper. Dentin porcelain was applied and fired in a calibrated porcelain furnace (Multimat Touch; Dentsply Intl) according to the firing schedule in the manufacturers’ instructions. The excess veneer material was removed with a grinding-polishing machine (Ecomet 3; Buehler), the dimensions were then checked with a digital micrometer (Powertectools), and the specimens were ultrasonically cleaned (Electrosonic Type 7 Profi; Electrosonic GmBH) for 10 minutes. Each layer (core and veneer) was approximately 1 mm in thickness and the 2 faces of the specimens did not differ more than 0.05 mm in parallelism. A biaxial flexural strength test was performed with a tension-compression device (METU; Department of lu Güngör et al Bankog
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Metallurgy, Ankara, Turkey). The speed of the device was 0.15 mm per minute. The specimens were tested with a technique that was described in ISO 6872. To support the specimens, 3 hardened steel balls (3.2 mm in diameter) were placed at an angle of 120 degrees relative to each other. The diameter of the support circle was 10 mm. Each specimen was located on the supports of the testing machine, and the load was applied onto the centers of the veneered surfaces of the specimens. A flat piston (1.4 ±0.2 mm in diameter) was used during loading until fracture occurred, and the fracture load was noted. The core layer stresses were failure stresses, whereas the stresses at the other locations were the stresses at the failure loads. Biaxial flexural strengths of the core and veneer layers were calculated with the formula described by Hsueh et al35: a 1−v −E1ðz−z ÞP c2 a2 s1 = + 1− 1+2ln 8pð1−v1ÞD c 1+v 2a2 R2 × ðfor 0 z t1 and r cÞ a 1−v −E2ðz−z ÞP c2 a2 + 1− 1+2ln 8pð1−v2ÞD c 1+v 2a2 R2
s2=
× ðfor t1 z t1 + t2 and r cÞ where s1 is the maximum tensile stress in the core layer; s2 is the maximum tensile stress in the veneer layer; P is
the fracture load; a is the radius of the supporting cycle; c is the radius of the piston; R is the radius of the disk; r is the radial distance from the center of the disk; z is the interface between the layers in vertical cylindrical coordinate; t1 is the thickness of the core layer; t2 is the thickness of the veneer layer; y1 is the Poisson ratio of core materials; y2 is the Poisson ratio of veneer materials; z* is the neutral surface position; D* is the flexural rigidity; and y is the average Poisson ratio of the bilayer. E1t12 E2t22 E2t1t2 + + 2 2 Þ ð1−v22 Þ 2ð1−v1 Þ 2ð1−v2 z = E1t1 E2t2 + ð1−v12 Þ ð1−v22 Þ E1t13 E2t23 E2t1t2ðt1+t2Þ + + D = 1−v22 3ð1−v12 Þ 3ð1−v22 Þ 2 E1t12 E2t22 E2t1t2 + + ð2ð1−v12 ÞÞ 2ð1−v22 Þ ð1−v22 Þ − E1t1 E2t2 + 2 ð1−v1 Þ ð1−v22 Þ
where E1 is the elastic moduli of the core layers and E2 is the elastic moduli of the veneer layers. Elastic moduli and Poisson ratios of the tested materials were provided by the manufacturers. If the Poisson ratio of the ceramic is unknown, n = 0.25 can be used.28 The elastic moduli of lu Güngör et al Bankog
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the Kavo Zirconia and IPS e.max Ceram were 210 and 68 GPa. The Poisson ratios of the Kavo Zirconia and IPS e.max Ceram were 0.25 and 0.24. The elastic moduli of the Noritake Zirconia and CerabienZR were 220 and 75 GPa. The Poisson ratios of the Noritake Zirconia and CerabienZR were 0.31 and 0.25. For this study, R = 7.5 ± 0.01 mm, t1 = 1 ± 0.02 mm, t2 = 1 ± 0.02 mm, and z = 2 ± 0.02 mm. v=
v1t1+v2t2 t1+t2
In addition, Hsueh and Kelly32 reported analytical solutions for calculating the stresses at the outer surfaces of the disk, sT (top) and sB (bottom): 6E2M E1t12 E2t22 2E1t1t2 + + ð1−v2Þ ð1−v1Þ ð1−v2Þ ð1−v1Þ sT =
2 4E1E2t1t2 t12 +t1t2+t22 E1t12 E2t22 + + ðð1−v1Þð1−v2ÞÞ ð1−v1Þ ð1−v2Þ 2 −6E1M E1t1 E2t22 2E2t1t2 + + ð1−v1Þ ð1−v1Þ ð1−v2Þ ð1−v2Þ sB=
2 4E1E2t1t2 t12 +t1t2+t22 E1t12 E2t22 + + ðð1−v1Þð1−v2ÞÞ ð1−v1Þ ð1−v2Þ where M is the biaxial bending moment. h ai −P c2 a2 M= +ð1−vÞ 1− 2 ð1+vÞ 1+2ln 8p c 2a R2 × ðfor r cÞ
s1 (core) and s2 (veneer) are the stresses at the interface. E1ð1−v2Þt1sT t2sB + ðat z = t1Þ; E2ð1−v1Þðt1+t2Þ t1+t2 t1s E2ð1−v1Þt2sB s2 = T + ðat z = t1Þ t1+t2 E1ð1−v2Þðt1+t2Þ
s1 =
The location for each of the mentioned stresses is shown in Figure 1. The average surface roughness (Ra) of the specimens was determined with a profilometer (Mahr Perthometer; Mahr GmbH). The mean value was calculated for each specimen by making 10 readings from the treated surfaces. Phase transformation was determined by x-ray diffraction (XRD) patterns of the control, airborneparticle abraded, ground, and ground and airborneparticle abraded specimens. The XRD patterns were recorded with a diffractometer (Rigaku-Geirflex X-ray Difraktometer) and Cu-K a-radiation. Specimens were scanned at 40 kV, 40 mA, 0.018/step interval from 20 to 40, and 2q degrees. The monoclinic phase (Xm) was calculated using the following formula described THE JOURNAL OF PROSTHETIC DENTISTRY
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Stresses occurred at top surfaces Load surface Stresses occurred at interfaces
veneer layer core layer
Stresses occurred at veneer layers Stresses occurred at core layers
Support surface Stresses occurred at bottom surfaces Figure 1. Schematic diagram indicating location for each of mentioned stresses.
by Garvie and Nicholson37 for detecting the phase composition of zirconia: Xm =ðIm1 þ Im2 Þ=ðIm1 þ Im2 þ IT Þ; where I is the intensity; T is the tetragonal peak, and m1 and m2 are the 2 major monoclinic peaks. The monoclinic phase content was determined by calculating the areas under the T, m1, and m2 peaks using MATLAB. The strength data were analyzed using the 2parameter cumulative Weibull distribution, which is often used for ceramic materials and offered by ISO 6872 because of their asymmetrical distribution. The Weibull moduli were calculated with the following formula28: PðsÞ = 1−exp½−ðs=s0 Þm ; where P is the fracture probability; s is the fracture strength; s0 is the characteristic strength at the fracture probabilities of 63.2%, 10%, 5%, and 1%; and m is the Weibull modulus. s0 and m were defined by a maximum likelihood approach; a = .05 and 7 degrees of freedom (DF) were used for evaluating the results. The results generated at the same layers can be compared among the 8 experimental groups. The phase transformation and surface roughness were analyzed by 2-factor factorial analysis of variance (ANOVA). The independent variables were the material types and the surface treatment methods. Significance levels of a=.05 and DF=3 for the relative amount of monoclinic phase and surface roughness were determined with the Duncan multiple range test (SPSS 18; SPSS Inc). The results of the surface roughness and phase transformation can be compared both within the materials and within the surface treatment methods. RESULTS The Weibull statistical analyses, the characteristic strength, and the 10%, 5%, and 1% probabilities of failure are listed in Tables 1, 2. The results of the solutions show that the s0 of KC in core and veneer layers were lower than those in the KA, KG, and KGA groups. No significant difference was found between the KG THE JOURNAL OF PROSTHETIC DENTISTRY
and KGA groups (P>.05). The KA groups showed significantly higher s0 values than all experimental groups at the core and veneer layers (P<.05). The values of the control groups of K and N were not significantly different (P>.05). However, when the same surface treatment methods were compared in the core and veneer layers, materials showed significant difference (P<.05). In all groups, the s0 values were higher for the core layer than the veneer layer. For the core and veneer layers, the KA groups showed the lowest m values. The Weibull distribution of the experimental groups for the core and veneer layers are shown in Figures 2, 3. The values were significantly higher in the KA group at the top and bottom surfaces (P<.05). s0 values were higher at the bottom surface than at the top surface of the bilayer. The Weibull distribution of the experimental groups for top and bottom surfaces are shown in Figures 4, 5. At the interfaces of the core and veneer layer, s0 values were higher for the KA group. The Weibull distribution of the experimental groups for core and veneer interfaces is shown in Figures 6, 7. The surface roughness of the materials is listed in Table 3. Ground specimens had higher significant values for both materials (P<.05). Comparing K and N specimens, the values were significantly different in the C and A groups; however, no significant differences were found among the G and GA groups (P>.05). Results from XRD analysis identified 2 phases in the zirconia core materials used in this study (Table 4). The XRD patterns of 1 specimen from each group for 2 materials are shown in Figures 8, 9. According to the XRD analysis, significantly higher Xm values were found in the GA groups for both materials (P<.05). When comparing the materials within the same surface treatment method, no significant differences were found, except in the airborne-particle abraded groups. DISCUSSION Chipping and the delamination of porcelain during mastication occur in bilayered zirconia supported fixed partial prostheses.9,38,39 One of the principal reasons for using biaxial moment loadings for determining the biaxial strength of dental ceramics is that the biaxial strength findings are generally more reliable because dental materials are subjected to multiaxial forces in the oral environment.34 For the bilayer specimens, Roark’s and Hsueh et al’s formulas could be used. Only stresses for the outer surfaces of the bilayer could be calculated with Roark’s formulas, whereas Hsueh et al’s formulas give the stresses that have occurred through the thickness of multilayered specimens.34 In the present study, a closed form solution in the guidelines of Hsueh et al32 is used not only to calculate the biaxial flexural strength of different regions of specimens but also through the lu Güngör et al Bankog
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Table 1. Weibull results of biaxial flexural strength data which indicate stresses at core and veneer layers Weibull Modulus, m (SE)
Characteristic Strength (MPa), s0 (SE)
Strength for 10% Probability of Failure (MPa), s0:10
Strength for 5% Probability of Failure (MPa), s0:05
Strength for 1% Probability of Failure (MPa), s0:01
KC
10.87 (3.00)
801.53 (24.04)a.b.c
651.6
609.9
525.0
KA
8.51 (2.05)
1119.09 (44.13)d
859.1
789.4
651.8
KG
16.57 (4.07)
875.22 (17.62)c
764.1
731.6
663.1
KGA
18.02 (4.40)
866.80 (16.11)c
765.0
735.1
671.5
NC
13.81 (3.78)
808.85 (19.34)a.b.c
687.2
652.3
579.7
NA
11.10 (2.83)
860.51 (25.85)b.c
702.6
658.5
568.6
NG
15.76 (4.08)
770.06 (16.24)a.b
667.6
637.8
575.1
NGA
23.44 (5.61)
741.99 (10.62)a
674.1
653.7
609.8
Chi-square (DF=7)
12.269
141.995
P
0.092
<.001
KC
10.52 (2.86)
250.29 (7.78)a.b
202.1
188.7
161.6
KA
8.51 (2.05)
357.60 (14.1)c
274.5
252.2
208.3
KG
16.57 (4.07)
279.67 (5.63)b
244.2
233.8
211.9
KGA
18.01 (4.40)
276.98 (5.14)b
244.4
234.9
214.5
NC
13.88 (3.80)
253.74 (6.03)a.b
215.8
204.9
182.2
NA
9.42 (2.31)
273.56 (9.71)a.b
215.4
199.6
167.9
NG
15.76 (4.08)
241.52 (5.09)a
209.4
200.0
180.4
NGA
11.31 (2.46)
241.76 (7.20)a
198.1
185.9
161.0
Chi-square (DF=7)
8.963
106.812
P
0.255
<.001
Parameter Core Layer
Veneer Layer
Same lowercase letters within core layer and veneer layer indicate that characteristic strengths were not significantly different between groups (P>.05).
thickness of the specimens and the top and bottom surfaces. In the present study, the stresses that occurred at the core layer, at the bottom surfaces, and at the interface of the core layer were higher than those that occurred at the veneer layer, at the top surfaces, and at the interface of the veneer layer for both Kavo and Noritake. Similarly, Yılmaz et al2 reported higher strength values for the core layer, bottom surfaces, and interfaces of the core layers. Guazzato et al40 researched the biaxial flexural strength of bilayered veneered zirconia and stated that the Weibull moduli and stress distributions were affected by the properties of the material, which was at the bottom surface. In the present study, the highest biaxial flexural strengths were observed in the airborne-particle abrasion group. However, the Weibull moduli were lowest in the airborneparticle abrasion group. Özcan et al41 stated that the deposition of silica coated alumina both increased the biaxial flexural strength and decreased the Weibull moduli. The Weibull moduli results are similar to the results of this study. Interfacial delamination arises from the large mismatch between fracture toughness or elastic modulus.40,42 Qin et al43 investigated the effect of the pH value of the oral cavity on the biaxial flexural strength of veneering porcelain containing zirconia and found that the mean failure stresses at the interface of two disks were lower than those at the top and bottom of lu Güngör et al Bankog
the disk surface. They suggested 2 different stress zonesdthe compressive force zone and the tensile force zoneethrough the thicknesses of the disks. According to the authors, because of the different elasticity of porcelain and cores, the stresses are not continuous at the interface and their distributions are different.43 Similarly, in the present study, the interfacial stresses were lower than the stresses observed at the core and veneer layers. Our results show that the material on the bottom determines the strength and failure characteristics.2,44,45 Stress induced monoclinic phase transformation is related to mechanical properties, such as strength and toughness.2,21 Stresses during airborne-particle abrasion, grinding, cyclic loading, and thermal and chemical aging can precipitate phase transformation.2,21,46 In the present study, the effect of surface treatments before veneering on the phase transformation was evaluated. When the effect of surface treatment results was compared with the control groups, higher phase transformation was observed in the treated specimens. Furthermore, grinding with diamond rotary instruments increased the average surface roughness for both materials. Karakoca and Yılmaz46 also reported that the surface roughness and transformed phase of the zirconia based materials were affected by the surface treatments. Monaco et al47 stated that increasing the dimensions of the abrasive particles enhanced the microcracks and monoclinic phase content. In the THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 2. Weibull results of biaxial flexural strength data which indicate stresses at top and bottom surfaces of bilayered specimens and at interfaces of layers
Parameter
Weibull Modulus, m (SE)
Characteristic Strength (MPa), s0 (SE)
Strength for 10% Probability of Failure (MPa), s0:10
Strength for 5% Probability of Failure (MPa), s0:05
Strength for 1% Probability of Failure (MPa), s0:01
Top surface of the bilayer KC
11.05 (3.04)
250.59 (7.40)a,b,c
204.7
191.8
165.5
KA
8.51 (2.04)
358.30 (14.12)d
275.0
252.7
208.7
KG
16.57 (4.07)
280.22 (5.64)c
244.6
234.2
212.3
KGA
18.02 (4.40)
277.53 (5.15)c
244.9
235.4
215.0
NC
13.88 (3.80)
256.72 (6.10)a,b,c
218.3
207.3
184.3
NA
9.46 (2.30)
276.15 (9.76)b,c
217.7
201.7
169.8
NG
15.63 (4.01)
243.92 (5.19)a,b
211.2
201.7
181.7
NGA
23.44 (5.60)
235.43 (3.37)a
213.9
207.4
193.5
Chi-square (DF=7)
13.922
153.795
P
0.053
<.001
KC
10.78 (2.94)
465.31 (14.11)a,b,c
377.6
353.2
303.7
KA
8.81 (2.13)
662.78 (25.25)d
513.4
473.1
393.2
KG
14.72 (3.61)
525.08 (11.91)c
450.6
429.1
384.2
KGA
19.02 (4.73)
515.66 (9.06)b,c
458.1
441.1
404.9
NC
12.92 (3.43)
488.46 (12.51)b,c
410.4
388.1
342.1
NA
9.60 (2.34)
518.03 (18.04)b,c
409.8
380.2
320.8
NG
16.12 (4.22)
462.89 (9.53)a,b
402.6
385.0
348.0
NGA
24.72 (5.75)
443.66 (6.01)a
405.1
393.4
368.3
Chi-square (DF=7)
14.860
145.373
P
0.038
<.001
KC
8.52 (2.33)
172.37 (6.66)a
132.4
121.6
100.5
KA
12.19 (3.26)
235.62 (6.40)b
195.9
184.7
161.6
KG
6.56 (1.59)
210.20 (10.74)a,b
149.2
133.7
104.3
KGA
12.27 (3.09)
194.88 (5.30)a
162.2
153.0
134.0
NC
6.78 (1.64)
205.11 (10.11)a,b
147.2
132.4
104.1
NA
10.43 (2.63)
192.21 (6.15)a
154.9
144.6
123.7
NG
7.95 (2.13)
192.92 (7.97)a
145.4
132.8
108.2
NGA
6.61 (1.66)
191.01 (9.65)a
135.9
121.9
95.2
Chi-square (DF=7)
7.632
56.150
P
0.366
<.001
KC
8.52 (2.33)
55.08 (2.12)a
42.3
38.9
32.1
KA
12.20 (3.26)
75.29 (2.04)b
62.6
59.0
51.6
KG
6.52 (1.58)
67.08 (3.45)a,b
47.5
42.5
33.1
KGA
12.27 (3.09)
62.27 (1.69)a
51.8
48.9
42.8
NC
6.78 (1.64)
64.33 (3.17)a,b
46.2
41.5
32.6
NA
10.42 (2.63)
60.28 (1.93)a
48.6
45.3
38.8
NG
7.98 (2.14)
60.53 (2.49)a
45.7
41.7
34.0
NGA
6.62 (1.66)
59.90 (3.02)a
42.6
38.2
29.9
Chi-square (DF=7)
7.689
59.668
P
0.361
<.001
Bottom surface of the bilayer
Interface of the core layer
Interface of the veneer layer
Same lowercase letters within layers indicate that characteristic strengths were not significantly different between groups (P>.05).
present study, the aluminium oxide particles used for airborne-particle abrasion were in 110 mm, which increased the monoclinic phase content of the materials significantly. However, the highest monoclinic content was observed in the ground and airborne-particle abraded groups. THE JOURNAL OF PROSTHETIC DENTISTRY
Airborne-particle abrasion is considered the most effective in increasing the surface roughness of the zirconia-based ceramics.48-51 Guazzato et al19 proposed that to trigger phase transformation, airborne-particle abrasion is more effective than grinding. Souza et al52 evaluated the effect of different airborne-particle lu Güngör et al Bankog
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Figure 2. Weibull distribution of experimental groups for each material at core layers.
Figure 3. Weibull distribution of experimental groups for each material at veneer layers.
1.00
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ơ/ơ0 Figure 4. Weibull distribution of experimental groups for each material at top surfaces.
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0.70
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ơ/ơ0 Figure 5. Weibull distribution of experimental groups for each material at bottom surfaces.
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0.80
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ơ/ơ0
1.00
1.20
ơ/ơ0
Figure 6. Weibull distribution of experimental groups for each material at core interfaces.
Figure 7. Weibull distribution of experimental groups for each material at veneer interfaces.
Table 3. Average surface roughnesses (Ra) of materials (mm)
Table 4. Relative monoclinic phase of materials (%)
Surface Treatment Method/ Materials (n=10)
Kavo Mean (SE)
Noritake Mean (SE)
Surface Treatments/ Materials (n=10)
Kavo Mean (SE)
Noritake Mean (SE) 0.486 (0.103)D,a
0.834 (0.12)D,b
1.612 (0.196)B,a
Control
0.815 (0.192)C,a
Airborne-particle abraded
1.2140 (0.037)C,b
1.7685 (0.0947)B,a
Airborne-particle abraded
3.806 (0.429)B,b
5.140 (0.548)B,a
Ground
2.1474 (0.093)A,a
2.2033 (0.0858)A,a
Ground
3.377 (0.202)B,a
2.979 (0.145)C,a
Ground and Airborne-particle abraded
1.5984 (0.024)B
1.6025 (0.0317)B,a
Ground and Airborne-particle abraded
6.330 (0.393)A,a
6.385 (0.310)A,a
Control
,a
Same uppercase letters (vertically) indicate that surface roughnesses were not significantly different between groups in same material (P> .05). Same lowercase letters (horizontally) indicate that surface roughnesses were not significantly different between materials in same surface treatment (P>.05).
Same uppercase letters (vertically) indicate that phase transformations were not significantly different between groups in same material (P> .05). Same lowercase letters (horizontally) indicate that phase transformations were not significantly different between materials in same surface treatment (P>.05).
abrasion on the biaxial strength and showed that airborne-particle abrasion with 110 mm alumina or silica coated alumina particles increased monoclinic phase content and surface roughness. In this study, the airborne-particle abrasion increased the monoclinic phase contents when compared with control groups. The highest phase transformation was obtained in the ground and airborne-particle abraded groups, and a lower amount of transformation was determined after grinding in the surface treatment groups. Airborne-particle abrasion increased the biaxial flexural strength observed at the core layers for both materials. The surface grinding method is a more abrasive surface treatment method compared with airborne-particle abrasion and can result in greater amounts of material being removed and higher
levels of stress being generated.22,46 In this study, the highest surface roughnesses were observed in the ground groups for both materials. The hypothesis of the present study was supported by the results. Surface treatments affected the biaxial flexural strength, the surface roughness, and the phase transformation of zirconia-based bilayered restorations.
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CONCLUSION Surface treatments affected the phase transformation, surface roughness, and biaxial flexural strength of Kavo and Noritake differently. In addition, surface treatments increased the relative monoclinic phase content and average surface roughness. Whereas ground surfaces had lu Güngör et al Bankog
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Intensity (counts) 40000 ×
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15000 10000 5000
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Kavo Airborne-particle Abraded Intensity (counts) 12000 ×T
10000 8000 6000 4000
× m1 ×
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Kavo Ground and Airborne-particle Abraded (m1 and m2: two major monoclinic peaks; T: major tetragonal peak) Figure 8. X-ray diffraction patterns of 1 Kavo specimen from each group.
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Intensity (counts) 40000 × T
30000 20000 10000 0
m2 ×
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× × 35.000
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10000 8000 6000 4000
0
×
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2000 20.000
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×
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Noritake Airborne-particle Abraded Intensity (counts) 10000 × T
8000 6000 4000
× m1 ×
2000
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×
0 20.000
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Noritake Ground Intensity (counts) 8000 × T 6000 4000 × 2000
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Noritake Ground and Airborne-particle Abraded (m1 and m2: two major monoclinic peaks; T: major tetragonal peak) Figure 9. X-ray diffraction patterns of 1 Noritake specimen from each group.
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the highest roughnesses, airborne-particle abrasion after grinding had a greater effect on the phase transformation. REFERENCES 1. Mosharraf R, Rismanchian M, Savabi O, Ashtiani AH. Influence of surface modification techniques on shear bond strength between different zirconia cores and veneering ceramics. J Adv Prosthodont 2011;3:221-8. 2. Yilmaz H, Nemli SK, Aydin C, Bal BT, Tıras¸ T. Effect of fatigue on biaxial flexural strength of bilayered porcelain/zirconia (Y-TZP) dental ceramics. Dent Mater 2011;27:786-95. 3. Ereifej N, Rodrigues FP, Silikas N, Watts DC. Experimental and FE shearbonding strength at core/veneer interfaces in bilayered ceramics. Dent Mater 2011;27:590-7. 4. Kim HJ, Lim HP, Park YJ, Vang MS. Effect of zirconia surface treatments on the shear bond strength of veneering ceramic. J Prosthet Dent 2011;105:315-22. 5. Fahmy NZ. Bond strength, microhardness, and core/veneer interface quality of an all ceramic system. J Prosthodont 2010;19:95-102. 6. Baldassarri M, Stappert CF, Wolff MS, Thompson VP, Zhang Y. Residual stresses in porcelain-veneered zirconia prostheses. Dent Mater 2012;28: 873-9. 7. Larsson C, Vult von Steyern P, Nilner K. A prospective study of implant supported full-arch yttria-stabilized tetragonal zirconia polycrystal mandibular fixed dental prostheses: three-year results. Int J Prosthodont 2010;23:364-9. 8. Molin MK, Karlsson SL. Five-year clinical prospective evaluation of zirconiabased Denzir 3-unit FPDs. Int J Prosthodont 2008;21:223-7. 9. Sailer I, Feher A, Filser F, Gauckler LJ, Luthy H, Hammerle CH. Five-year clinical results of zirconia frameworks for posterior fixed partial dentures. Int J Prosthodont 2007;20:383-8. 10. Aboushelib MN, Feilzer AJ, de Jager N, Kleverlaan CJ. Prestresses in bilayered all-ceramic restorations. J Biomed Mater Res B Appl Biomater 2008;87: 139-45. 11. Isgrò G, Pallav P, van der Zel JM, Feilzer AJ. The influence of the veneering porcelain and different surface treatments on the biaxial flexural strength of a heat-pressed ceramic. J Prosthet Dent 2003;90:465-73. 12. Fleming GJ, Dickens M, Thomas LJ, Harris JJ. The in vitro failure of allceramic crowns and the connector area of fixed partial dentures using bilayered ceramic specimens: the influence of core to dentin thickness ratio. Dent Mater 2006;22:771-7. 13. Aboushelib MN, de Jager N, Kleverlaan CJ, Feilzer AJ. Microtensile bond strength of different components of core veneered all-ceramic restorations. Dent Mater 2005;21:984-91. 14. Saito A, Komine F, Blatz MB, Matsumura H. A comparison of bond strength of layered veneering porcelains to zirconia and metal. J Prosthet Dent 2010;104:247-57. 15. Choi JE, Waddell JN, Torr B, Swain MV. Pressed ceramics onto zirconia. Part 1: Comparison of crystalline phases present, adhesion to a zirconia system and flexural strength. Dent Mater 2011;27:1204-12. 16. Benetti P, Kelly JR, Sanchez M, Della Bona A. Influence of thermal gradients on stress state of veneered restorations. Dent Mater 2014;30:554-63. 17. Li KC, Waddell JN, Prior DJ, Ting S, Girvan L, van Vuuren LJ, et al. Effect of autoclave induced low-temperature degradation on the adhesion energy between yttria-stabilized zirconia veneered with porcelain. Dent Mater 2013;29:e263-70. 18. Mochales C, Maerten A, Rack A, Cloetens P, Mueller WD, Zaslansky P, et al. Monoclinic phase transformations of zirconia-based dental prostheses, induced by clinically practised surface manipulations. Acta Biomater 2011;7: 2994-3002. 19. Guazzato M, Quach L, Albakry M, Swain MV. Influence of surface and heat treatments on the flexural strength of Y-TZP dental ceramic. J Dent 2005;33: 9-18. 20. Juy A, Anglada M. Surface phase transformation during grinding of Y-TZP. J Am Ceram Soc 2007;90:2618-21. 21. Pittayachawan P, McDonald A, Young A, Knowles JC. Flexural strength, fatigue life, and stress-induced phase transformation study of Y-TZP dental ceramic. J Biomed Mater Res B Appl Biomater 2009;88:366-77. 22. Kosmac T, Oblak C, Jevnikar P, Funduk N, Marion L. The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater 1999;15:426-33. 23. Chevalier J. Low-temperature aging of Y-TZP ceramics. J Am Ceram Soc 1999;82:2150-4. 24. Denry IL, Holloway JA. Microstructural and crystallographic surface changes after grinding zirconia-based dental ceramics. J Biomed Mater Res B Appl Biomater 2006;76:440-8. 25. Kosmac T, Oblak C, Jevnikar P, Funduk N, Marion L. Strength and reliability of surface treated Y-TZP dental ceramics. J Biomed Mater Res J 2000;53:304-13. 26. Aboushelib MN, Wang H. Influence of crystal structure on debonding failure of zirconia veneered restorations. Dent Mater 2013;29:e97-102.
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27. Fleming GJ, El-Lakwah SF, Harris JJ, Marquis PM. The influence of interfacial surface roughness on bilayered ceramic specimen performance. Dent Mater 2004;20:142-9. 28. ISO 6872. Dentistry-ceramic materials. Geneva, Switzerland: International Organization for Standardization; 2008. 29. Yilmaz H, Aydin C, Gul BE. Flexural strength and fracture toughness of dental core ceramics. J Prosthet Dent 2007;98:120-8. 30. Wen MY, Mueller HJ, Chai J, Wozniak WT. Comparative mechanical property characterization of 3 all-ceramic core materials. Int J Prosthodont 1999;12:534-41. 31. Ban S, Anusavice KJ. Influence of test method on failure stress of brittle dental materials. J Dent Res 1990;69:1791-9. 32. Hsueh CH, Kelly JR. Simple solutions of multilayered discs subjected to biaxial moment loading. Dent Mater 2009;25:506-13. 33. Lin WS, Ercoli C, Feng C, Morton D. The effect of core material, veneering porcelain, and fabrication technique on the biaxial flexural strength and Weibull analysis of selected dental ceramics. J Prosthodont 2012;21:353-62. 34. Hsueh CH, Thompson GA. Appraisal of formulas for stresses in bilayered dental ceramics subjected to biaxial moment loading. J Dent 2007;35:600-6. 35. Hsueh CH, Luttrell CR, Becher PF. Analyses of multilayered dental ceramics subjected to biaxial flexure tests. Dent Mater 2006;22:460-9. 36. Hsueh CH, Luttrelll, Becher PF. Modeling of bonded multilayered disks subjected to biaxial flexure tests. Int J Solids Struct 2006;43:6014-25. 37. Garvie RC, Nicholson PS. Phase analysis in zirconia systems. J Am Ceram Soc 1972;55:303-5. 38. Sailer I, Pjetursson BE, Zwahlen M, Hämmerle CH. A systematic review of the survival and complication rates of all-ceramic and metal-ceramic reconstructions after an observation period of at least 3 years. Part II: Fixed dental prostheses. Clin Oral Implants Res 2007;18:86-96. 39. Raigrodski AJ, Hillstead MB, Meng GK, Chung KH. Survival and complications of zirconia-based fixed dental prostheses: a systematic review. J Prosthet Dent 2012;107:170-7. 40. Guazzato M, Proos K, Quach L, Swain MV. Strength, reliability and mode of fracture of bilayered porcelain/zirconia (Y-TZP) dental ceramics. Biomaterials 2004;25:5045-52. 41. Özcan M, Melo RM, Souza RO, Machado JP, Felipe Valandro L, Botttino MA. Effect of air-particle abrasion protocols on the biaxial flexural strength, surface characteristics and phase transformation of zirconia after cyclic loading. J Mech Behav Biomed Mater 2013;20:19-28. 42. Liu Y, Feng H, Bao Y, Qiu Y, Xing N, Shen Z. Fracture and interfacial delamination origins of bilayer ceramic composites for dental restorations. J Eu Ceram Soc 2010;30:1297-305. 43. Qin X, Ou J, Su X, Jiang Y, Qin QH. Influence of pH value of the oral cavity on biaxial flexural strength of layered ceramics. ASCE 2013;2:1-8. 44. Kelly JR. Perspectives on strength. Dent Mater 1995;11:103-10. 45. Borba M, de Araújo MD, de Lima E, Yoshimura HN, Cesar PF, Griggs JA, et al. Flexural strength and failure modes of layered ceramic structures. Dent Mater 2011;27:1259-66. 46. Karakoca S, Yilmaz H. Influence of surface treatments on surface roughness, phase transformation, and biaxial flexural strength of Y-TZP ceramics. J Biomed Mater Res B Appl Biomater 2009;91:930-7. 47. Monaco C, Tucci A, Esposito L, Scotti R. Microstructural changes produced by abrading Y-TZP in presintered and sintered conditions. J Dent 2013;41: 121-6. 48. Casucci A, Mazzitelli C, Monticelli F, Toledano M, Osorio R, Osorio E, et al. Morphological analysis of three zirconium oxide ceramics: Effect of surface treatments. Dent Mater 2010;26:751-60. 49. Blatz MB, Sadan A, Kern M. Resineceramic bonding: a review of the literature. J Prosthet Dent 2003;89:268-74. 50. Tsukakoshi M, Shinya A, Gomi H, Lassila LV, Vallittu PK, Shinya A. Effects of dental adhesive cement and surface treatment on bond strength and leakage of zirconium oxide ceramics. Dent Mater 2008;27:159-71. 51. Chaiyabutr Y, McGowan S, Phillips KM, Kois JC, Giordano RA. The effect of hydrofluoric acid surface treatment and bond strength of a zirconia veneering ceramic. J Prosthet Dent 2008;100:194-202. 52. Souza RO, Valandro LF, Melo RM, Machado JP, Bottino MA, Ozcan M. Air-particle abrasion on zirconia ceramic using different protocols: effects on biaxial flexural strength after cyclic loading, phase transformation and surface topography. J Mech Behav Biomed Mater 2013;26:155-63.
Corresponding author: Dr Handan Yılmaz Gazi University Faculty of Dentistry Department of Prosthodontics Emek 8.Cad. 82.Sok 06510 Ankara TURKEY Email: [email protected] Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.
THE JOURNAL OF PROSTHETIC DENTISTRY
Effect of surface treatments on the biaxial flexural strength, phase transformation, and surface roughness of bilayered porcelain/zirconia dental ceramics lu Güngör, DDS, PhD,a Handan Yılmaz, DDS, PhD,b Seçil Karakoca Nemli, DDS, PhD,c Merve Bankog Bilge Turhan Bal, DDS, PhD,d and Cemal Aydın, DDS, PhDe Zirconia is a widely used ceramic core material in prosthodontics and for esthetic reasons is often covered with an appropriate veneering ceramic.1,2 The strength and quality of the bond between the layers are factors in the success of bilayered restorations.3 Delamination of the veneering porcelain is the most reported reason for failure of such restorations,1,4-6 particularly in posterior zirconia-based restorations.7-9 Such failures can arise from the design of the framework, thickness of the veneering ceramic, functional loads, layering technique of the veneering ceramics, and structure defects.1,10-14 Choi et al15 reported that zirconia and glass ceramic achieve thermodynamic stability as zirconia ions dissolve into the veneering material. Benetti et al16 proposed
ABSTRACT Statement of problem. Veneered zirconia restorations are widely used in prosthetic applications. However, these restorations often fail because of chipping of the veneer porcelain. Surface treatments of zirconia core materials may affect the connection between the 2 layers. Purpose. The purpose of this study was to evaluate the effect of surface treatments on the biaxial flexural strength, phase transformation, and mean surface roughness of different bilayered porcelain/zirconia ceramics. Material and methods. Forty disk-shaped specimens were obtained for each material (Kavo and Noritake) and divided into 4 (n=10) groups (control, airborne-particle abraded, ground, and ground and airborne-particle abraded). Airborne-particle abrasion was performed with 110-mm Al2O3 particles for 15 seconds and at 400 kPa. Diamond rotary instruments with 100-mm grain size were used for grinding. The monoclinic phase transformation and surface roughness of the specimens were measured. Then, the specimens were veneered and subjected to a biaxial flexural strength test to calculate the Weibull moduli (m values) and the stresses occurring at the layers, outer surfaces of the bilayer, and interfaces of the layers. Results. The Kavo airborne-particle abraded group showed higher strength values in both layers (P<.05) than those of all experimental groups. The Kavo airborne-particle abraded group showed the lowest m values at the core and veneer layers. According to the phase analysis, significantly higher Xm values were found in the ground and airborne-particle abraded groups for both materials (P<.05). In both materials, except in the airborne-particle abraded groups, the relative monoclinic phases showed no difference (P<.05). Conclusion. Surface treatments affected the phase transformation, surface roughness, and biaxial flexural strength of Kavo and Noritake zirconia ceramics differently. Surface treatments increased the relative monoclinic phase content and average surface roughness. (J Prosthet Dent 2015;113:585-595)
Supported by Scientific Investigation Projects Department of Gazi University with grant no 03/2012-03. Presented at the 38th Annual Conference of the European Prosthodontic Association, Istanbul, Turkey, September 2014. a Research Assistant, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. b Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. c Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. d Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. e Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey.
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Clinical Implications The results of this in vitro study suggest that surface treatments affect the phase transformation, surface roughness, and biaxial flexural strength of zirconia core materials differently.
that slow heating and cooling could overcome the chipping problem and that fractures were caused by transient stresses related to thermal incompatibility between the porcelain and the core material. Li et al17 stated that the bonding characteristics between Y-TZP and the veneering material appeared to involve both chemical and mechanical interaction. The mechanical performance of zirconia may be influenced by surface treatments, including machining, airborne-particle abrasion, and grinding, and the stresses induced by these treatments may trigger tetragonal to monoclinic phase transformation.18-20 Zirconia is a polymeric material, and high localized stresses during grinding and airborne-particle abrasion,21,22 together with high temperatures,21,23 can cause phase transformation.18,21 In several studies, the effects of grinding, airborne-particle abrasion, and polishing on the strength of zirconia-based restorations have been reported.19,22,24,25 Studies related with surface treatments are generally focused on monolayered zirconia specimens, and few data are available on the effect of surface treatment on the bond strength between core and veneer layers.1,26,27 Dental ceramic materials are prone to fracture under tensile stresses. Therefore, because most fixed partial prostheses are exposed to biaxial stresses,28-33 the strength of these materials is more reliably measured under biaxial flexure than under uniaxial flexure. ISO 6872 has identified the formulas only for monolayered specimens.28,32 Both the stresses through the thickness of the bilayered specimens and the stresses at the interfaces and top and bottom surfaces of the bilayer can be calculated with the Hsueh et al calculations.34-36 The purpose of this investigation was to evaluate the influence of different surface treatments of the interface of the core and veneer on the biaxial flexural strength of the layers, phase transformation, and surface roughness of bilayered zirconia restorations. It was hypothesized that surface treatments would affect the biaxial flexural strength, the surface roughness, and the phase transformation of zirconia-based bilayered restorations. MATERIAL AND METHODS Two Y-TZP core materials (Kavo Everest ZS-blank; KaVo Dental and Noritake Alliance Zirconia; Noritake Co) and 2 veneer porcelains (IPS e.max Ceram; Ivoclar Vivadent and CerabienZR; Noritake Co) were selected for this study. The specimens were provided by the manufacturers. Four THE JOURNAL OF PROSTHETIC DENTISTRY
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groups were created from 40 disk-shaped specimens (15 ± 0.02 mm in diameter and 1 ±0.02 mm in thickness) for each material, 2 control groups -Kavo control (KC) and Noritake control (NC)- and 2 airborne-particle abraded groups with 110-mm Al2O3 particles (RocatecPre; 3M ESPE). Airborne-particle abrasion was performed for 15 seconds and at 400 kPa. For standardized airborne-particle abrasion conditions, the specimens were placed in a specimen holder positioned 30 mm from the airborneparticle abrasion device (Rocatec Junior; 3M ESPE). The specimens of this group were termed as Kavo abraded (KA) and Noritake abraded (NA). The ground groups, in which the specimens were ground by diamond rotary instruments (100-mm grain size) without water spray cooling, were termed as Kavo ground (KG) and Noritake ground (NG). The disks were ground from 1.1 mm to 1 ±0.02 mm at a rotational speed of 20 000 revolutions per minute. The rotary instruments were changed after each group. The dimensions of the specimens were measured with a digital micrometer (Powertectools). The ground and airborne-particle abraded group, in which the specimens were 15 ±0.02 mm in diameter and 1.1 ±0.02 mm in thickness and were airborne-particle abraded after grinding under the same conditions as previously described, were named as Kavo ground and abraded (KGA) and Noritake ground and abraded (NGA). After surface treatments, all specimens were ultrasonically cleaned (Electrosonic Type 7 Profi; Electrosonic GmBH) for 10 minutes. Then, liners and veneering porcelains were applied according to the manufacturers’ recommendations. Before veneering, all disks were cleaned and air dried. Contamination after cleaning was avoided. The liner materials were mixed with their forming liquids and a thin layer applied onto the specimens. ZirLiner (Ivoclar Vivadent) was used for the Kavo specimens, and Shade Base Porcelain (Noritake Co) was used for the Noritake specimens. The liners were fired independently according to the manufacturers’ recommendations. Silicone molds (Zeta Plus; Zhermack) were fabricated and the disks were positioned. Ceramic veneer powders were mixed with their liquids and brushed onto the core ceramics. The excess of liquid was removed with blotting paper. Dentin porcelain was applied and fired in a calibrated porcelain furnace (Multimat Touch; Dentsply Intl) according to the firing schedule in the manufacturers’ instructions. The excess veneer material was removed with a grinding-polishing machine (Ecomet 3; Buehler), the dimensions were then checked with a digital micrometer (Powertectools), and the specimens were ultrasonically cleaned (Electrosonic Type 7 Profi; Electrosonic GmBH) for 10 minutes. Each layer (core and veneer) was approximately 1 mm in thickness and the 2 faces of the specimens did not differ more than 0.05 mm in parallelism. A biaxial flexural strength test was performed with a tension-compression device (METU; Department of lu Güngör et al Bankog
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Metallurgy, Ankara, Turkey). The speed of the device was 0.15 mm per minute. The specimens were tested with a technique that was described in ISO 6872. To support the specimens, 3 hardened steel balls (3.2 mm in diameter) were placed at an angle of 120 degrees relative to each other. The diameter of the support circle was 10 mm. Each specimen was located on the supports of the testing machine, and the load was applied onto the centers of the veneered surfaces of the specimens. A flat piston (1.4 ±0.2 mm in diameter) was used during loading until fracture occurred, and the fracture load was noted. The core layer stresses were failure stresses, whereas the stresses at the other locations were the stresses at the failure loads. Biaxial flexural strengths of the core and veneer layers were calculated with the formula described by Hsueh et al35: a 1−v −E1ðz−z ÞP c2 a2 s1 = + 1− 1+2ln 8pð1−v1ÞD c 1+v 2a2 R2 × ðfor 0 z t1 and r cÞ a 1−v −E2ðz−z ÞP c2 a2 + 1− 1+2ln 8pð1−v2ÞD c 1+v 2a2 R2
s2=
× ðfor t1 z t1 + t2 and r cÞ where s1 is the maximum tensile stress in the core layer; s2 is the maximum tensile stress in the veneer layer; P is
the fracture load; a is the radius of the supporting cycle; c is the radius of the piston; R is the radius of the disk; r is the radial distance from the center of the disk; z is the interface between the layers in vertical cylindrical coordinate; t1 is the thickness of the core layer; t2 is the thickness of the veneer layer; y1 is the Poisson ratio of core materials; y2 is the Poisson ratio of veneer materials; z* is the neutral surface position; D* is the flexural rigidity; and y is the average Poisson ratio of the bilayer. E1t12 E2t22 E2t1t2 + + 2 2 Þ ð1−v22 Þ 2ð1−v1 Þ 2ð1−v2 z = E1t1 E2t2 + ð1−v12 Þ ð1−v22 Þ E1t13 E2t23 E2t1t2ðt1+t2Þ + + D = 1−v22 3ð1−v12 Þ 3ð1−v22 Þ 2 E1t12 E2t22 E2t1t2 + + ð2ð1−v12 ÞÞ 2ð1−v22 Þ ð1−v22 Þ − E1t1 E2t2 + 2 ð1−v1 Þ ð1−v22 Þ
where E1 is the elastic moduli of the core layers and E2 is the elastic moduli of the veneer layers. Elastic moduli and Poisson ratios of the tested materials were provided by the manufacturers. If the Poisson ratio of the ceramic is unknown, n = 0.25 can be used.28 The elastic moduli of lu Güngör et al Bankog
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the Kavo Zirconia and IPS e.max Ceram were 210 and 68 GPa. The Poisson ratios of the Kavo Zirconia and IPS e.max Ceram were 0.25 and 0.24. The elastic moduli of the Noritake Zirconia and CerabienZR were 220 and 75 GPa. The Poisson ratios of the Noritake Zirconia and CerabienZR were 0.31 and 0.25. For this study, R = 7.5 ± 0.01 mm, t1 = 1 ± 0.02 mm, t2 = 1 ± 0.02 mm, and z = 2 ± 0.02 mm. v=
v1t1+v2t2 t1+t2
In addition, Hsueh and Kelly32 reported analytical solutions for calculating the stresses at the outer surfaces of the disk, sT (top) and sB (bottom): 6E2M E1t12 E2t22 2E1t1t2 + + ð1−v2Þ ð1−v1Þ ð1−v2Þ ð1−v1Þ sT =
2 4E1E2t1t2 t12 +t1t2+t22 E1t12 E2t22 + + ðð1−v1Þð1−v2ÞÞ ð1−v1Þ ð1−v2Þ 2 −6E1M E1t1 E2t22 2E2t1t2 + + ð1−v1Þ ð1−v1Þ ð1−v2Þ ð1−v2Þ sB=
2 4E1E2t1t2 t12 +t1t2+t22 E1t12 E2t22 + + ðð1−v1Þð1−v2ÞÞ ð1−v1Þ ð1−v2Þ where M is the biaxial bending moment. h ai −P c2 a2 M= +ð1−vÞ 1− 2 ð1+vÞ 1+2ln 8p c 2a R2 × ðfor r cÞ
s1 (core) and s2 (veneer) are the stresses at the interface. E1ð1−v2Þt1sT t2sB + ðat z = t1Þ; E2ð1−v1Þðt1+t2Þ t1+t2 t1s E2ð1−v1Þt2sB s2 = T + ðat z = t1Þ t1+t2 E1ð1−v2Þðt1+t2Þ
s1 =
The location for each of the mentioned stresses is shown in Figure 1. The average surface roughness (Ra) of the specimens was determined with a profilometer (Mahr Perthometer; Mahr GmbH). The mean value was calculated for each specimen by making 10 readings from the treated surfaces. Phase transformation was determined by x-ray diffraction (XRD) patterns of the control, airborneparticle abraded, ground, and ground and airborneparticle abraded specimens. The XRD patterns were recorded with a diffractometer (Rigaku-Geirflex X-ray Difraktometer) and Cu-K a-radiation. Specimens were scanned at 40 kV, 40 mA, 0.018/step interval from 20 to 40, and 2q degrees. The monoclinic phase (Xm) was calculated using the following formula described THE JOURNAL OF PROSTHETIC DENTISTRY
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Stresses occurred at top surfaces Load surface Stresses occurred at interfaces
veneer layer core layer
Stresses occurred at veneer layers Stresses occurred at core layers
Support surface Stresses occurred at bottom surfaces Figure 1. Schematic diagram indicating location for each of mentioned stresses.
by Garvie and Nicholson37 for detecting the phase composition of zirconia: Xm =ðIm1 þ Im2 Þ=ðIm1 þ Im2 þ IT Þ; where I is the intensity; T is the tetragonal peak, and m1 and m2 are the 2 major monoclinic peaks. The monoclinic phase content was determined by calculating the areas under the T, m1, and m2 peaks using MATLAB. The strength data were analyzed using the 2parameter cumulative Weibull distribution, which is often used for ceramic materials and offered by ISO 6872 because of their asymmetrical distribution. The Weibull moduli were calculated with the following formula28: PðsÞ = 1−exp½−ðs=s0 Þm ; where P is the fracture probability; s is the fracture strength; s0 is the characteristic strength at the fracture probabilities of 63.2%, 10%, 5%, and 1%; and m is the Weibull modulus. s0 and m were defined by a maximum likelihood approach; a = .05 and 7 degrees of freedom (DF) were used for evaluating the results. The results generated at the same layers can be compared among the 8 experimental groups. The phase transformation and surface roughness were analyzed by 2-factor factorial analysis of variance (ANOVA). The independent variables were the material types and the surface treatment methods. Significance levels of a=.05 and DF=3 for the relative amount of monoclinic phase and surface roughness were determined with the Duncan multiple range test (SPSS 18; SPSS Inc). The results of the surface roughness and phase transformation can be compared both within the materials and within the surface treatment methods. RESULTS The Weibull statistical analyses, the characteristic strength, and the 10%, 5%, and 1% probabilities of failure are listed in Tables 1, 2. The results of the solutions show that the s0 of KC in core and veneer layers were lower than those in the KA, KG, and KGA groups. No significant difference was found between the KG THE JOURNAL OF PROSTHETIC DENTISTRY
and KGA groups (P>.05). The KA groups showed significantly higher s0 values than all experimental groups at the core and veneer layers (P<.05). The values of the control groups of K and N were not significantly different (P>.05). However, when the same surface treatment methods were compared in the core and veneer layers, materials showed significant difference (P<.05). In all groups, the s0 values were higher for the core layer than the veneer layer. For the core and veneer layers, the KA groups showed the lowest m values. The Weibull distribution of the experimental groups for the core and veneer layers are shown in Figures 2, 3. The values were significantly higher in the KA group at the top and bottom surfaces (P<.05). s0 values were higher at the bottom surface than at the top surface of the bilayer. The Weibull distribution of the experimental groups for top and bottom surfaces are shown in Figures 4, 5. At the interfaces of the core and veneer layer, s0 values were higher for the KA group. The Weibull distribution of the experimental groups for core and veneer interfaces is shown in Figures 6, 7. The surface roughness of the materials is listed in Table 3. Ground specimens had higher significant values for both materials (P<.05). Comparing K and N specimens, the values were significantly different in the C and A groups; however, no significant differences were found among the G and GA groups (P>.05). Results from XRD analysis identified 2 phases in the zirconia core materials used in this study (Table 4). The XRD patterns of 1 specimen from each group for 2 materials are shown in Figures 8, 9. According to the XRD analysis, significantly higher Xm values were found in the GA groups for both materials (P<.05). When comparing the materials within the same surface treatment method, no significant differences were found, except in the airborne-particle abraded groups. DISCUSSION Chipping and the delamination of porcelain during mastication occur in bilayered zirconia supported fixed partial prostheses.9,38,39 One of the principal reasons for using biaxial moment loadings for determining the biaxial strength of dental ceramics is that the biaxial strength findings are generally more reliable because dental materials are subjected to multiaxial forces in the oral environment.34 For the bilayer specimens, Roark’s and Hsueh et al’s formulas could be used. Only stresses for the outer surfaces of the bilayer could be calculated with Roark’s formulas, whereas Hsueh et al’s formulas give the stresses that have occurred through the thickness of multilayered specimens.34 In the present study, a closed form solution in the guidelines of Hsueh et al32 is used not only to calculate the biaxial flexural strength of different regions of specimens but also through the lu Güngör et al Bankog
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Table 1. Weibull results of biaxial flexural strength data which indicate stresses at core and veneer layers Weibull Modulus, m (SE)
Characteristic Strength (MPa), s0 (SE)
Strength for 10% Probability of Failure (MPa), s0:10
Strength for 5% Probability of Failure (MPa), s0:05
Strength for 1% Probability of Failure (MPa), s0:01
KC
10.87 (3.00)
801.53 (24.04)a.b.c
651.6
609.9
525.0
KA
8.51 (2.05)
1119.09 (44.13)d
859.1
789.4
651.8
KG
16.57 (4.07)
875.22 (17.62)c
764.1
731.6
663.1
KGA
18.02 (4.40)
866.80 (16.11)c
765.0
735.1
671.5
NC
13.81 (3.78)
808.85 (19.34)a.b.c
687.2
652.3
579.7
NA
11.10 (2.83)
860.51 (25.85)b.c
702.6
658.5
568.6
NG
15.76 (4.08)
770.06 (16.24)a.b
667.6
637.8
575.1
NGA
23.44 (5.61)
741.99 (10.62)a
674.1
653.7
609.8
Chi-square (DF=7)
12.269
141.995
P
0.092
<.001
KC
10.52 (2.86)
250.29 (7.78)a.b
202.1
188.7
161.6
KA
8.51 (2.05)
357.60 (14.1)c
274.5
252.2
208.3
KG
16.57 (4.07)
279.67 (5.63)b
244.2
233.8
211.9
KGA
18.01 (4.40)
276.98 (5.14)b
244.4
234.9
214.5
NC
13.88 (3.80)
253.74 (6.03)a.b
215.8
204.9
182.2
NA
9.42 (2.31)
273.56 (9.71)a.b
215.4
199.6
167.9
NG
15.76 (4.08)
241.52 (5.09)a
209.4
200.0
180.4
NGA
11.31 (2.46)
241.76 (7.20)a
198.1
185.9
161.0
Chi-square (DF=7)
8.963
106.812
P
0.255
<.001
Parameter Core Layer
Veneer Layer
Same lowercase letters within core layer and veneer layer indicate that characteristic strengths were not significantly different between groups (P>.05).
thickness of the specimens and the top and bottom surfaces. In the present study, the stresses that occurred at the core layer, at the bottom surfaces, and at the interface of the core layer were higher than those that occurred at the veneer layer, at the top surfaces, and at the interface of the veneer layer for both Kavo and Noritake. Similarly, Yılmaz et al2 reported higher strength values for the core layer, bottom surfaces, and interfaces of the core layers. Guazzato et al40 researched the biaxial flexural strength of bilayered veneered zirconia and stated that the Weibull moduli and stress distributions were affected by the properties of the material, which was at the bottom surface. In the present study, the highest biaxial flexural strengths were observed in the airborne-particle abrasion group. However, the Weibull moduli were lowest in the airborneparticle abrasion group. Özcan et al41 stated that the deposition of silica coated alumina both increased the biaxial flexural strength and decreased the Weibull moduli. The Weibull moduli results are similar to the results of this study. Interfacial delamination arises from the large mismatch between fracture toughness or elastic modulus.40,42 Qin et al43 investigated the effect of the pH value of the oral cavity on the biaxial flexural strength of veneering porcelain containing zirconia and found that the mean failure stresses at the interface of two disks were lower than those at the top and bottom of lu Güngör et al Bankog
the disk surface. They suggested 2 different stress zonesdthe compressive force zone and the tensile force zoneethrough the thicknesses of the disks. According to the authors, because of the different elasticity of porcelain and cores, the stresses are not continuous at the interface and their distributions are different.43 Similarly, in the present study, the interfacial stresses were lower than the stresses observed at the core and veneer layers. Our results show that the material on the bottom determines the strength and failure characteristics.2,44,45 Stress induced monoclinic phase transformation is related to mechanical properties, such as strength and toughness.2,21 Stresses during airborne-particle abrasion, grinding, cyclic loading, and thermal and chemical aging can precipitate phase transformation.2,21,46 In the present study, the effect of surface treatments before veneering on the phase transformation was evaluated. When the effect of surface treatment results was compared with the control groups, higher phase transformation was observed in the treated specimens. Furthermore, grinding with diamond rotary instruments increased the average surface roughness for both materials. Karakoca and Yılmaz46 also reported that the surface roughness and transformed phase of the zirconia based materials were affected by the surface treatments. Monaco et al47 stated that increasing the dimensions of the abrasive particles enhanced the microcracks and monoclinic phase content. In the THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 2. Weibull results of biaxial flexural strength data which indicate stresses at top and bottom surfaces of bilayered specimens and at interfaces of layers
Parameter
Weibull Modulus, m (SE)
Characteristic Strength (MPa), s0 (SE)
Strength for 10% Probability of Failure (MPa), s0:10
Strength for 5% Probability of Failure (MPa), s0:05
Strength for 1% Probability of Failure (MPa), s0:01
Top surface of the bilayer KC
11.05 (3.04)
250.59 (7.40)a,b,c
204.7
191.8
165.5
KA
8.51 (2.04)
358.30 (14.12)d
275.0
252.7
208.7
KG
16.57 (4.07)
280.22 (5.64)c
244.6
234.2
212.3
KGA
18.02 (4.40)
277.53 (5.15)c
244.9
235.4
215.0
NC
13.88 (3.80)
256.72 (6.10)a,b,c
218.3
207.3
184.3
NA
9.46 (2.30)
276.15 (9.76)b,c
217.7
201.7
169.8
NG
15.63 (4.01)
243.92 (5.19)a,b
211.2
201.7
181.7
NGA
23.44 (5.60)
235.43 (3.37)a
213.9
207.4
193.5
Chi-square (DF=7)
13.922
153.795
P
0.053
<.001
KC
10.78 (2.94)
465.31 (14.11)a,b,c
377.6
353.2
303.7
KA
8.81 (2.13)
662.78 (25.25)d
513.4
473.1
393.2
KG
14.72 (3.61)
525.08 (11.91)c
450.6
429.1
384.2
KGA
19.02 (4.73)
515.66 (9.06)b,c
458.1
441.1
404.9
NC
12.92 (3.43)
488.46 (12.51)b,c
410.4
388.1
342.1
NA
9.60 (2.34)
518.03 (18.04)b,c
409.8
380.2
320.8
NG
16.12 (4.22)
462.89 (9.53)a,b
402.6
385.0
348.0
NGA
24.72 (5.75)
443.66 (6.01)a
405.1
393.4
368.3
Chi-square (DF=7)
14.860
145.373
P
0.038
<.001
KC
8.52 (2.33)
172.37 (6.66)a
132.4
121.6
100.5
KA
12.19 (3.26)
235.62 (6.40)b
195.9
184.7
161.6
KG
6.56 (1.59)
210.20 (10.74)a,b
149.2
133.7
104.3
KGA
12.27 (3.09)
194.88 (5.30)a
162.2
153.0
134.0
NC
6.78 (1.64)
205.11 (10.11)a,b
147.2
132.4
104.1
NA
10.43 (2.63)
192.21 (6.15)a
154.9
144.6
123.7
NG
7.95 (2.13)
192.92 (7.97)a
145.4
132.8
108.2
NGA
6.61 (1.66)
191.01 (9.65)a
135.9
121.9
95.2
Chi-square (DF=7)
7.632
56.150
P
0.366
<.001
KC
8.52 (2.33)
55.08 (2.12)a
42.3
38.9
32.1
KA
12.20 (3.26)
75.29 (2.04)b
62.6
59.0
51.6
KG
6.52 (1.58)
67.08 (3.45)a,b
47.5
42.5
33.1
KGA
12.27 (3.09)
62.27 (1.69)a
51.8
48.9
42.8
NC
6.78 (1.64)
64.33 (3.17)a,b
46.2
41.5
32.6
NA
10.42 (2.63)
60.28 (1.93)a
48.6
45.3
38.8
NG
7.98 (2.14)
60.53 (2.49)a
45.7
41.7
34.0
NGA
6.62 (1.66)
59.90 (3.02)a
42.6
38.2
29.9
Chi-square (DF=7)
7.689
59.668
P
0.361
<.001
Bottom surface of the bilayer
Interface of the core layer
Interface of the veneer layer
Same lowercase letters within layers indicate that characteristic strengths were not significantly different between groups (P>.05).
present study, the aluminium oxide particles used for airborne-particle abrasion were in 110 mm, which increased the monoclinic phase content of the materials significantly. However, the highest monoclinic content was observed in the ground and airborne-particle abraded groups. THE JOURNAL OF PROSTHETIC DENTISTRY
Airborne-particle abrasion is considered the most effective in increasing the surface roughness of the zirconia-based ceramics.48-51 Guazzato et al19 proposed that to trigger phase transformation, airborne-particle abrasion is more effective than grinding. Souza et al52 evaluated the effect of different airborne-particle lu Güngör et al Bankog
591
1.00
1.00
0.80
0.80
Probability of Failure (P [ơ0])
Probability of Failure (P [ơ0])
June 2015
0.60
0.40 KC KA KG KGA NC NA NG NGA
0.20
0.00 0.70
0.80
0.90
1.00
1.10
0.60
0.40 KC KA KG KGA NC NA NG NGA
0.20
0.00 0.70
1.20
0.80
0.90
1.10
Figure 2. Weibull distribution of experimental groups for each material at core layers.
Figure 3. Weibull distribution of experimental groups for each material at veneer layers.
1.00
0.80
0.80
Probability of Failure (P [ơ0])
1.00
0.60
0.40 KC KA KG KGA NC NA NG NGA
0.20
0.00
0.60
0.40 KC KA KG KGA NC NA NG NGA
0.20
0.00 0.70
0.80
0.90
1.00
1.10
1.20
ơ/ơ0 Figure 4. Weibull distribution of experimental groups for each material at top surfaces.
lu Güngör et al Bankog
1.20
ơ/ơ0
ơ/ơ0
Probability of Failure (P [ơ0])
1.00
0.70
0.80
0.90
1.00
1.10
1.20
ơ/ơ0 Figure 5. Weibull distribution of experimental groups for each material at bottom surfaces.
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1.00
1.00
0.80
0.80
0.60
0.40 KC KA KG KGA NC NA NG NGA
0.20
0.00 0.60
0.80
1.00
Probability of Failure (P [ơ0])
Probability of Failure (P [ơ0])
592
0.60
0.40 KC KA KG KGA NC NA NG NGA
0.20
0.00 0.60
1.20
0.80
ơ/ơ0
1.00
1.20
ơ/ơ0
Figure 6. Weibull distribution of experimental groups for each material at core interfaces.
Figure 7. Weibull distribution of experimental groups for each material at veneer interfaces.
Table 3. Average surface roughnesses (Ra) of materials (mm)
Table 4. Relative monoclinic phase of materials (%)
Surface Treatment Method/ Materials (n=10)
Kavo Mean (SE)
Noritake Mean (SE)
Surface Treatments/ Materials (n=10)
Kavo Mean (SE)
Noritake Mean (SE) 0.486 (0.103)D,a
0.834 (0.12)D,b
1.612 (0.196)B,a
Control
0.815 (0.192)C,a
Airborne-particle abraded
1.2140 (0.037)C,b
1.7685 (0.0947)B,a
Airborne-particle abraded
3.806 (0.429)B,b
5.140 (0.548)B,a
Ground
2.1474 (0.093)A,a
2.2033 (0.0858)A,a
Ground
3.377 (0.202)B,a
2.979 (0.145)C,a
Ground and Airborne-particle abraded
1.5984 (0.024)B
1.6025 (0.0317)B,a
Ground and Airborne-particle abraded
6.330 (0.393)A,a
6.385 (0.310)A,a
Control
,a
Same uppercase letters (vertically) indicate that surface roughnesses were not significantly different between groups in same material (P> .05). Same lowercase letters (horizontally) indicate that surface roughnesses were not significantly different between materials in same surface treatment (P>.05).
Same uppercase letters (vertically) indicate that phase transformations were not significantly different between groups in same material (P> .05). Same lowercase letters (horizontally) indicate that phase transformations were not significantly different between materials in same surface treatment (P>.05).
abrasion on the biaxial strength and showed that airborne-particle abrasion with 110 mm alumina or silica coated alumina particles increased monoclinic phase content and surface roughness. In this study, the airborne-particle abrasion increased the monoclinic phase contents when compared with control groups. The highest phase transformation was obtained in the ground and airborne-particle abraded groups, and a lower amount of transformation was determined after grinding in the surface treatment groups. Airborne-particle abrasion increased the biaxial flexural strength observed at the core layers for both materials. The surface grinding method is a more abrasive surface treatment method compared with airborne-particle abrasion and can result in greater amounts of material being removed and higher
levels of stress being generated.22,46 In this study, the highest surface roughnesses were observed in the ground groups for both materials. The hypothesis of the present study was supported by the results. Surface treatments affected the biaxial flexural strength, the surface roughness, and the phase transformation of zirconia-based bilayered restorations.
THE JOURNAL OF PROSTHETIC DENTISTRY
CONCLUSION Surface treatments affected the phase transformation, surface roughness, and biaxial flexural strength of Kavo and Noritake differently. In addition, surface treatments increased the relative monoclinic phase content and average surface roughness. Whereas ground surfaces had lu Güngör et al Bankog
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Intensity (counts) 40000 ×
30000
T
20000 10000 m1 ×
0 20.000
m2 ×
25.000
30.000 2theta (deg.)
× × 35.000
40.000
Kavo Control Intensity (counts) 20000 × T
15000 10000 5000
× ×
m1 ×
0 20.000
25.000
×m2 30.000 2theta (deg.)
35.000
40.000
Kavo Airborne-particle Abraded Intensity (counts) 12000 ×T
10000 8000 6000 4000
× m1 ×
2000
×
m2 ×
0 20.000
25.000
30.000 2theta (deg.)
35.000
40.000
Kavo Ground Intensity (counts) 10000 × T
8000 6000 4000
× m1 ×
2000
m2 ×
×
0 20.000
25.000
30.000 2theta (deg.)
35.000
40.000
Kavo Ground and Airborne-particle Abraded (m1 and m2: two major monoclinic peaks; T: major tetragonal peak) Figure 8. X-ray diffraction patterns of 1 Kavo specimen from each group.
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Intensity (counts) 40000 × T
30000 20000 10000 0
m2 ×
m1 × 20.000
25.000
30.000 2theta (deg.)
× × 35.000
40.000
Noritake Control Intensity (counts) 12000 ×T
10000 8000 6000 4000
0
×
m1 ×
2000 20.000
25.000
m2 × 30.000 2theta (deg.)
×
35.000
40.000
Noritake Airborne-particle Abraded Intensity (counts) 10000 × T
8000 6000 4000
× m1 ×
2000
m2 ×
×
0 20.000
25.000
30.000 2theta (deg.)
35.000
40.000
Noritake Ground Intensity (counts) 8000 × T 6000 4000 × 2000
m2
m1 ×
×
0 20.000
25.000
30.000 2theta (deg.)
35.000
40.000
Noritake Ground and Airborne-particle Abraded (m1 and m2: two major monoclinic peaks; T: major tetragonal peak) Figure 9. X-ray diffraction patterns of 1 Noritake specimen from each group.
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Corresponding author: Dr Handan Yılmaz Gazi University Faculty of Dentistry Department of Prosthodontics Emek 8.Cad. 82.Sok 06510 Ankara TURKEY Email: [email protected] Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.
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