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*Graduate student in Master of Science degree program in Prosthodontics, Khon Kaen University, Khon Kaen,Thailand. **Associated Professor, Department of Community dentistry, Faculty of Dentistry, Khon Kaen University,Khon Kaen, Thailand. ***Associated Professor, Department of Prosthodontics, Faculty of Dentistry, Khon Kaen University, Khon Kaen, Thailand

SHEAR BOND STRENGTH OF CERAMIC VENEERED TO YTTRIUM-STABILIZED ZIRCONIA CORE CERAMIC RESTORATIONS AS A ROLE OF COEFFICIENT OF THERMAL EXPANSION กําลังความแขง็แรงของการยึดอยูจากแรงเฉือนของเซรามิกท่ีใชเคลือบบนแกนแยทเทรียมคงสภาพเซอรโคเนียในการบูรณะดวยเซรามิกลวนอันเนื่องจากคาสัมประสิทธ์ิการขยายตัวจากความรอน

Chollada Dangsuwan (ชลลดา แดงสุวรรณ)* Dr. Supaporn Chatrchaiwiwatana (ดร. สุภาภรณ ฉัตรชัยวิวัฒนา) ** Dr. Niwut Juntevee (ดร.นิวัตร จันทรเทวี)***

ABSTRACT Fracture of veneering ceramic from underlying core ceramic has been reported for all-ceramic restorations, due to a weak interface between the veneering and the core ceramic or merely a fracture through the veneering ceramic

itself. The coefficient of thermal expansion (CTE) is the one of factors that may affect the core-veneer bond strength. The aim of this study was to investigate the shear bond strength (SBS) of six commercial veneering ceramics, represent the varied CTE, on zirconia core. The veneering ceramic was fired to the zirconia core disc then the core-veneer specimens were placed in a mounting jig and subjected to shear force in a Universal testing machine. The SBS were calculated and analyzed using ANOVA and Weibull analysis. The results revealed that significantly difference in both of mean SBS of IPS d sign from all of groups and IPS e max ceram from VITA VM9 (P < 0.05). Weibull modulus reveals a highest reliability of VITA VM9. บทคัดยอ มีรายงานถึงการแตกของเซรามิกที่ใชเคลือบออกจากแกนเซรามิกในงานบูรณะแบบเซรามิกลวน ซึ่งเกิดจากความออนแอของการยึดอยูระหวางวัสดุทั้งสองชนิด หรือมีการแตกอยูในสวนเน้ือของเซรามิกที่ใชเคลือบน้ีดวย คาสัมประสิทธิ์การขยายตัวจากความรอนเปนปจจัยอยางหน่ึงที่มีผลตอการยึดอยูของวัสดุทั้งสองชนิดน้ี การศึกษาน้ีเพ่ือหาคากําลังความแข็งแรงในการยึดอยูจากแรงเฉือนของเซรามิก 6 ชนิดโดยเปนตัวแทนของคาสัมประสิทธิ์การขยายตัวจากความรอนที่แตกตางกัน ที่ใชยึดบนแกนเซอรโคเนีย ทําการเผาเซรามิก 6 ชนิดบนแผนแกนเซอรโคเนียกลม จากน้ันนํามาทดสอบดวยเครื่องทดสอบแรงแบบสากล แลวนําคากําลังความแข็งแรงในการยึดอยูจากแรงเฉือนที่ไดมาวิเคราะหความแปรปรวนแบบทางเดียวและวิเคราะหไวบูล ผลการศึกษาพบวามีความแตกตางอยางมีนัยสําคัญ (P < 0.05) ของเซรามิกไอพีเอสดีไซนกับทุกกลุมและเซรามิกไอพีเอสอีแมกซีแรมกับวิตาวีเอ็มไนน สวนไวบูลโมดูลัสแสดงคาความเช่ือมั่นของกําลังความแข็งแรงในการยึดอยูจากแรงเฉือนของวิตาวีเอ็มไนนมากที่สุด

Keywords : Shear bond strength, Zirconia, Coefficient of thermal expansion (CTE) คําสําคัญ : กําลังความแข็งแรงในการยึดอยูจากแรงเฉอืน เซอโคเนียร สัมประสิทธ์ิการขยายตัวจากความรอน

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Introduction The popularity of all-ceramic dental restorations has

been increasing in recent years due to their superior esthetic appearance and metal-free structure (Atsu, Kilicarslan, Kucukesmen, & Aka, 2006; Pittayachawan, McDonald, Petrie, & Knowles, 2007). It is the combination of strength of core ceramic and superior esthetics of a weaker veneering ceramic that result in a reliable and biocompatible restoration (MN Aboushelib, Kleverlaan, & Feilzer, 2006 ; MN Aboushelib, leverlaan, & Feilzer, 2008). The dental profession seeks an ideal all-ceramic restoration with excellent physical properties, strength, marginal fit, and esthetics necessary for anterior, as well as posterior, restorations (Al-Dohan, Yaman, Dennison, Razzoog, & Larg, 2004).

Since, dental ceramics are brittle, their low fracture resistance and relatively low flexural strength limit the possibility of manufacturing fixed partial dentures (FPDs) eventhough using all-ceramic frameworks (Sundh, Molin, & Sjogren, 2005). The development of advanced dental material technologies has recently led to the application of zirconium dioxide or zirconia in dentistry (Guazzato, Albakry, Simon, & Swain, 2004 ). The most representative zirconia-based dental ceramics are yttria- stabilized tetragonal zirconia.

In the early 1990s yttrium oxide partially stabilized tetragonal zirconia polycrystal (Y-TZP) was introduced to dentistry as a core material for all-ceramic restorations and has been made available through the CAD/CAM technique. Due to a transformation toughening mechanism, Y-TZP has been shown to have superior mechanical properties compared to other all-ceramic

systems. In vitro studies demonstrated a flexural strength of 900–1200MPa, and a fracture toughness of 9–10MPam1/2 (Anusavice, 2003; de Kler, de Jager, Meegdes, & Van der Zel, 2007; Guess et al., 2008).

However, long-term clinical results for zirconia all-ceramic restorations are not available at the present time. In short and medium-term studies (Raigrodski, Chiche, & Potiket, 2006; Seiler, Feher, & Filser, 2006; Vult von, Carlson, & Nilner, 2005) the Y-TZP core ceramic exhibited a high stability as a framework (core) material. No fractures of the zirconia framework have been reported. The long-term success of veneered zirconia restorations seems to be determined by the weak performance of the veneering ceramics and its limited bond to the zirconia substrate. Delaminations with exposure of the zirconia core ceramic (Seiler, Feher, & Filser, 2006) and minor chip-off fractures (Raigrodski, Chiche, & Potiket, 2006) of the veneering ceramic were described as the most frequent reason for failures of zirconia FPDs. Chip-off fracture rates at 15% after 24 months, 25% after 31 months (Raigrodski, Chiche, & Potiket, 2006) and 8% and 13% after 36 and 38 months, respectively (Seiler, Feher, & Filser, 2006) were observed.

The failure rate of core-veneered all-ceramic restorations revealed that delamination of the veneering porcelain from the core structure is a common failure mode and that restoring such fractures with porcelain repair systems puts high demands on the bonding quality (M. N. Aboushelib, de Jager, Kleverlaan, & Feilzer, 2005). The strength of a non-homogenous (layered) all-ceramic structure is determined by its weakest

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component. Usually this depends upon the core-veneer bond strength or the veneering material itself, which has to be strengthened to withstand the stresses of mastication and to prevent delamination and fracture of the veneering material. There are various factors that may cause inferior the core-veneer bond strength. One of those which is the supreme factor predicting success of the restoration is coefficient of thermal expansion (CTE) of both the core and veneering ceramic (MN Aboushelib, Kleverlaan, & Feilzer, 2006 ).

The fact related to the bond of core and veneering ceramic in all-ceramic restorations is base on the principle of metal-ceramic. Since, the success of metal-ceramic restorations depends upon the firmness of bond between metal and ceramic (McLean, 1979) as the mechanisms of mechanical, chemical, dendritical, transition phase, and van der Waal’s forces (Bagby, Marshall, & Marshall Jr, 1990; Yamamoto, 1985). The effects of thermal expansion have been discussed as one of the factors determining the strength of metal-ceramics. The compressive force factor in bond strength contributes from 26–68 %, and results from difference in CTE between ceramic and metal. Crazing and cracking appearance often happen unexpectedly during laboratory procedures such as firing, cooling, grinding of ceramic and post soldering of the restoration. The fundamental cause of cracking is the difference in thermal expansion.

Matching of core-veneer ceramic on behalf of CTE compatibility becomes of practical interest both economic and esthetic reasons. Economically, establishing compatibility between systems might help dental laboratories moderate some expense by combining

systems rather than having to stock special veneer porcelains. For the esthetic concern, the range of color, translucency, and opalescence represented in nature is usually not completely encompassed by any one system (Isgro, Wang, Kleverlaan, & Feilzer, 2005; Steiner, Kelly, & Giuseppetti, 1997).

Many commercial all-ceramic systems have been introduced into the market have the veneering ceramic which has the CTE value almost similar to the core ceramic materials because the manufacturers are assured that the matched CTE value of core and veneering ceramic provide the best core-veneer bond strength. However, the limited difference of CTE between core ceramic and veneering ceramics is an interesting phenomenon that may affect the core-veneer bond strength. The optimal difference of CTE of the core-veneer ceramic that provide the highest core-veneer bond strength has not been reported. The purpose of this study was to investigate the shear bond strength of six commercial veneering ceramics on Cercon®core (Degudent GmbH, Hanau-Wolfgang, Germany).

Materials and methods A. Sample preparation

The commercially available Y-TZP ceramic-core was used in this study is Cercon® (Degudent GmbH, Hanau-Wolfgang, Germany). Ninety Cercon®discs (10.8 mm. diameter and 1.42 mm. high) were cut out from ceramic-block by a handsaw (PX200, OH, USA) and grinded using abrasive paper. Then Cercon®discs were sintered in the Cercon® furnace according to the manufacturer’s recommendation. After sintering each Cercon®disc were grinded for final dimension of core

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specimens are 10 mm. in diameter and 1 mm. in high. All Cercon®discs were veneered incrementally, using the six commercial veneering ceramics (Table1) mixed with distilled water. The ceramic core disc was position into the silicone mould which has a space of 1 mm. thickness to be a room for veneering ceramic application. The creamy mixed veneered ceramic was applied manually on to the Cercon®discs, condensed and vibrated with ultrasonic porcelain condenser (3M Unitex, St. Paul, USA). Excess moisture was blotted dry with an absorbent tissue (Fig. 1). All core-veneered ceramic were fired in the porcelain furnace (Programmat®P100 furnace, Ivoclar-Vivadent, Schaan, Liechtenstein) in accordance the manufacturer’s instruction. After firing, all specimen discs were grinding to ensure the uniform thickness and shape using a green stone bur. The final veneering ceramic discs dimension is 7 mm in diameter and 1.5 mm in high, were standardized the dimension by metal split mould (Fig. 2). Then the core-veneer discs were immersed in the distilled water prior to SBS test. Table 1 Six commercials veneering ceramics and their CTE according to manufacturer

Commercials veneering ceramic

Manufacturer CTE (×10-6/ oC)

Cercon ceram kiss Degudent GmbH, Hanau-Wolfgang, Germany

9.2

VITA Dur alpha Vita Zahnfabrik, Bad Sackingen, Germany

6.7

VITA VM7 Vita Zahnfabrik, Bad Sackingen, Germany

7.3

IPS e max ceram Ivoclar Vivadent, Schaan, Liechtenstein

9.5

VITA VM9 Vita Zahnfabrik, Bad 8.8-9.2

Sackingen, Germany

IPS d sign Ivoclar Vivadent, Schaan, Liechtenstein

12-12.6

Figure 1 the Cercon®discs was veneered with veneering ceramic using silicone mould

Figure 2 the core-veneered disc was standardized the

final dimension in metal split mould B. Shear bond strength testing

Prior to SBS testing, the core-veneer disc specimens (15samples / group) were stored in the distilled water at room temperature at all time. The SBS was determined for each group by using the Universal testing machine (Lloyd, LR30/k, Leicester, England) and the load was applied with tension shear bond jig which designs directed stresses mainly at the core-veneer interface (Fig. 3). Each sample was subjected on the shear bond jig by placing the core portion of sample in the sample holder so the core-veneer interface is on the flat surface of holder as well as the testing jig (Fig. 4 and 5). Shear load was applied at a speed of 0.50 mm. per minute until failure (fracture). The load (N) at failure was recorded for each disc and average shear strengths (MPa) were calculated by dividing the load at failure to the area (mm2) of interface as follow

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The mean shear bond strength and the standard

deviation for each group were calculated from these data.

AA

Figure 3 Apparatus for Shear bond strength test diagram

Figure 4 the core-veneer disc was placed in the sample

holder and subjected into the shear testing jig

Figure 5 the sample was placed in the shear bond testing

jig and load was directly applied at the interface

C. Statistic analysis Analysis of variance (ANOVA) using statistical software

(SPSS, version 11.5, Chicago) was performed to find out the significant difference of shear bond strength at significant level of P < 0.05. The Weibull analysis was further analyzed

which resulted in a Weibull modulus (m). The variability of shear bond strength was estimated by calculating the Weibull modulus (m), is usually determined from the straight line of a slope obtained by plotting between

where PS(VO) is the probability of survival as the fraction of identical sample; VO is the volume of sample ; σ is the shear stress applied during testing; and σO (the weibull characteristic strength) and m are constants A higher weibull modulus (m) indicates that the lower

the variability of strength, the grater homogeneity of the material. Conversely the lower weibull modulus (m) indicates the lower the reliability of strength.

Results The results of the shear bond strength tested were

reported in terms of the mean and standard deviation of shear bond strength (SBS) as shown in table 2 and graph in figure 6. The shear bond strength of IPS d sign, coefficient of thermal expansion is 12-12.6×10-6/oC which greater different from coefficient of thermal expansion of core, was lowest.

An analysis of variance (ANOVA) and Tamhane multiple comparison were evaluated and indicated that there were significant difference of shear bond strength of IPS d sign from other group and IPS e max ceram from VITA VM9 (P <0.05) and their interaction (P< 0.05) as shown in the tables 3 and 4. This indicated that the varied coefficient of thermal expansion of the ceramic influence significantly to the shear bond strength (SBS) at 95% level of confidence.

 

                                           

Cercon® core

Veneered ceramic

Sample holder

Shear bond strengths (MPa) = Load (N) / Area (mm2) equation1 

ln{ln(1/PS(VO))} against m ln(σ/σo) equation2

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The Weibull analysis of shear bond strength (SBS) which include the Weibull modulus, characteristic strength, the stress level of probabilities of survival were showed in table 5, fig. 7 and 8. Weibull modulus can be ranked from highest to lowest as follows: CK(5.53), Dur(4.49), VM7(2.93), emax(3.94), VM9(8.86) and d sign(3.77).

The stress level required for 99, 95, 90, 85, 15, 10 and 5 percent probabilities of survival for each group followed table 5 reveal that shear bond strength level with a 95 percent probability of survival ranked from highest to lowest as follows: CK(14.66), Dur(11.42), VM7(6.78), emax(9.11), VM9(18.43) and d sign(1.30).

Table 2 Mean shear bond strength (SBS) and standard deviation of each group

Shear bond strength (MPa) 95% Confidence Interval for Mean (MPa) Group abbreviate

N

Mean SD Lower Bound Upper Bound

CK 15 25.4187 4.90193 22.7041 28.1333

Dur 15 22.5527 5.14385 19.7041 25.4012

VM7 15 19.7460 7.17393 15.7732 23.7188

emax 15 19.9400 5.28383 17.0139 22.8661

VM9 15 25.8560 2.74323 24.3368 27.3752

d sign 15 2.9687 0.83210 2.5079 3.4295

NB: CK = Cercon ceram kiss, Dur = VITA Dur alpha, VM7 = VITA VM7, IPS emax ceram, VM9 = VITA VM9, d sign = IPS d sign.

Table 3 ANOVA of shear bond strength (SBS) 

SS df MS F P-value

Between Groups 5373.638 5 1074.728 46.697 .000

Within Groups 1933.258 84 23.015

Total 7306.896 89

NB: SS = Sum of square, df = Degree of freedom, MS = Mean square, F = F-test, P-value = Probability value.  

 

 

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Table 4 Tamhane’s pairwise comparisons of shear bond strength

Group CK Dur VM7 emax VM9 d sign

CK 0.875 0.241 0.093 1.000 0.000*

Dur 0.980 0.950 0.453 0.000*

VM7 1.000 0.092 0.000*

emax 0.014* 0.000*

VM9 0.000*

d sign

NB: CK = Cercon ceram kiss, Dur = VITA Dur alpha, VM7 = VITA VM7, IPS emax ceram, VM9 = VITA VM9, d sign = IPS d sign  

Table 5 Weibull analysis and probability of survival of shear bond strength (SBS)  

Stress level of 99%,95%,90%,85%,15%,10% and 5% probabilities of survival (MPa)

Group abbreviate

N Weibull Modulus

(m)

Characteristic Strength

(σ0) 99% 95% 90% 85% 15% 10% 5% CK 15 5.53 25.06 10.92 14.66 16.69 18.05 28.13 29.13 30.55 Dur 15 4.49 22.12 7.94 11.42 13.40 14.76 25.51 26.63 28.24

VM7 15 2.93 18.69 3.88 6.78 8.67 10.05 23.25 24.84 27.17 emax 15 3.94 19.34 6.02 9.11 10.93 12.20 22.75 23.90 25.55 VM9 15 8.86 27.77 15.34 18.43 19.99 21.00 27.71 28.32 29.17 d sign 15 3.77 2.86 0.84 1.30 1.58 1.77 3.39 3.57 3.83 NB: CK = Cercon ceram kiss, Dur = VITA Dur alpha, VM7 = VITA VM7, IPS emax ceram, VM9 = VITA VM9,

d sign = IPS d sign 

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Figure 6 Bar graph of the mean shear bond strength and standard deviation NB: CK = Cercon ceram kiss, Dur = VITA Dur alpha, VM7 = VITA VM7, IPS emax ceram,                                           VM9 = VITA VM9, d sign = IPS d sign.

  Figure 7 Line fit plot among the shear bond strength of each group NB: CK = Cercon ceram kiss, Dur = VITA Dur alpha, VM7 = VITA VM7, IPS emax ceram,                                           VM9 = VITA VM9, d sign = IPS d sign.  

CK

Dur

VM7

e max

VM9

d sign

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Figure 8 Relative Weibull analysis curve among the shear bond strength of each group NB: CK = Cercon ceram kiss, Dur = VITA Dur alpha, VM7 = VITA VM7, IPS emax ceram,

VM9 = VITA VM9, d sign = IPS d sign.

Discussion All-ceramic core materials are currently replacing

dental casting alloys (metal), but the principle itself remains the same (MN Aboushelib, leverlaan, & Feilzer, 2008). As the traditional metal ceramic restorations, the CTE is the one of the important factor for metal-ceramic compatibility. If the CTE of ceramics is much lower than that of the metal, tangential cracks will form upon cooling as a result of tensile stresses oriented perpendicular to the external surface. Similarly, it the CTE of the metal, cracking will occur in a radial fashion upon cooling. The stresses that cause such cracking are directly proportional to the difference in thermal expansion between the two materials (Steiner, Kelly, & Giuseppetti, 1997).

For core veneered restoration, bond strength of all-ceramic restorations are influenced by the CTE of core and veneering ceramic. This study proved that the varied CTE has significant influenced to SBS of Cercon®core and veneering ceramic.

As failure of a layered structure is expected to occur in the weakest material or in the weakest interface of the system, the inferior zirconia veneer bond strength was an observation of interest. Manufacturers’ and researchers’ efforts focused on increasing the strength of the core and the veneer ceramic materials, while the bond between them was not adequately considered (MN Aboushelib, Kleverlaan, & Feilzer, 2007).

The results of this study demonstrated that the core-veneer bond strength is also affected by the CTE of

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veneering ceramics, the higher bond strength that mean the minimal fracture or debonding of all ceramic restorations. For Cercon®core, has CTE 10.5 ×10-6/ oC, the Cercon ceram kiss and VITA VM9 which are closely of CTE have the highest SBS (Table 2). On the other hand, the IPS d sign which CTE greatest mismatch with Cercon®core has the lowest SBS (2.96±0.83MPa) as the CTE of IPS d sign has suitable manufactured for metal that has CTE 13.5 ×10-6/ oC, while the metal-ceramic SBS in previous study was 30.16±5.88MPa (Al-Dohan, Yaman, Dennison, Razzoog, & Larg, 2004) this results can similarly explain by the reason follow Steiner PJ,1997.

The SBS of VITA VM7 and VITA Dur alpha were 19.74 and 22.55 respectively, lower than Cercon ceram kiss and VITA VM9, because of that the application is aimed specifically at veneering alumina ceramic frameworks that have CTE in range of 7.2-7.9 ×10-6/ oC such as In-Ceram alumina, In-Ceram spinell and In-Ceram zirconia (http://www.vita-zahnfabrik.com; VIDENT, 2005).

In this present study, confirimg the finding of previous studies, the SBS values of veneering ceramics to their core ceramics in all ceramic restorations ranged between 23-41MPa (M. N. Aboushelib, de Jager, Kleverlaan, & Feilzer, 2005; Dundar et al., 2007; Dundar, Ozcan, Comlekoglu, Gungor, & Artunc, 2005) and 22-28MPa (MN Aboushelib, de Kler, van der Zel, & Feilzer, 2008; Al-Dohan, Yaman, Dennison, Razzoog, & Larg, 2004) , while the other study were 9.4-12.5MPa (Guess et al., 2008).

The interpretation of these SBS data requires consideration of the effect of CTE mismatch on core-veneered bonding which has been frequently discussed in the dental literature. The bond strength can be compromised by residual stresses from veneer and core CTE mismatch (M. N. Aboushelib, de Jager, Kleverlaan, & Feilzer, 2005). To generate acceptable levels of residual stress within a multilayer all-ceramic composite, efforts have been made by dental manufacturers to develop ceramic cores and low fusing veneering ceramics with similar CTE (Guess et al., 2008). In the present study the CTE mismatch varied from 1 to 3.8×10-6/ oC for the five all-ceramic systems (Table 1), but the measured SBS showed no difference in four pairs except the pair of IPS emax ceram and VITA VM9.

The influence of CTE on reliability of the SBS has been discussed in terms of Weibull analysis parameter as show in table 5. As expected, the scatter in strength values in all type of veneering ceramics (except IPS d sign which is suitable for metal-ceramic restorations) are small. Regarding, only the Cercon ceram kiss and VITA VM9 have the Weibull modulus (m) greater than 5 that mean the lower variability of strength and greater homogeneity of the material. Conversely, the other ceramics (VITA Dur alpha, VITA VM7 and IPS emax Ceram) have the lower ‘m’ value indicated the lower the reliability of strength. While the most ceramics are reported to have ‘m’ values in the range of 5-15, whereas metals, which produce ductile failures, have ‘m’ values in the range of 30-100 (Pittayachawan, McDonald, Petrie, & Knowles, 2007).

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Conclusions Proper selection of veneering ceramic, optimal CTE

for Y-TZP core is extreme importance to assure that all ceramic (Y-TZP veneered) restorations will fulfill according to their expected functional demands.

Acknowledgements We would like to thank Cercon®Centre Thailand for

generously supplying the Cercon® core materials of this research and gratefully acknowledge for the Graduate School Khon Kaen University Fund.

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Aboushelib, M., Kleverlaan, C., & Feilzer, A. (2007). Effect of loading type on the fracture mechanics of two layered all-ceramic restoration. Dent Mater, 23, 952-959.

Aboushelib, M., leverlaan, C., & Feilzer, A. (2008). Effect of zirconia type on its bond strength with different veneer ceramics. J Prosthodont, 17, 401-408.

Aboushelib, M. N., de Jager, N., Kleverlaan, C. J., & Feilzer, A. J. (2005). Microtensile bond strength of different components of core veneered all-ceramic restorations. Dent Mater, 21.(10), 984-991.

Al-Dohan, H. M., Yaman, P., Dennison, J. B., Razzoog, M. E., & Larg, B. R. (2004). Shear strength of core-veneer interface in bi-layered ceramics. J Prosthet Dent, 91(4), 349-355.

Anusavice, K. (2003). Phillips’ science of dental materials (11 ed.). St.Louis: Saunders.

Atsu, S., Kilicarslan, M., Kucukesmen, H., & Aka, P. (2006). Effect of zirconium-oxide caramic surface treatments on the bond strength to adhesive resin. J Prosthet Dent, 95, 430-436.

Bagby, M., Marshall, S., & Marshall Jr, G. (1990). Metal ceramic compatibility: A review of the literature. J Prosthet Dent, 63, 21-25.

de Kler, M., de Jager, N., Meegdes, M., & Van der Zel, J. (2007). Influence of thermal expansion mismatch and fatigue loading on phase changes in porcelain veneered Y-TZP zirconia discs. J Oral Rehabilit.

Dundar, M., Ozcan, M., Bulent, G., Comlekoglu, E., Leite, F., & Valandro, L. (2007). Comparison of two bond strength testing methodologies for bilayered all-ceramics. Dent Mater, 23, 630-636.

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