Editorial Type:
Article Category: Research Article
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Online Publication Date: 01 Mar 2017

Adhesive Cementation Promotes Higher Fatigue Resistance to Zirconia Crowns

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Page Range: 215 – 224
DOI: 10.2341/16-002-L
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SUMMARY

Objective: The aim of this study was to investigate the influence of the cementation strategy on the fatigue resistance of zirconia crowns. The null hypothesis was that the cementation strategy would not affect the fatigue resistance of the crowns.

Methods and Materials: Seventy-five simplified molar tooth crown preparations were machined in glass fiber–filled epoxy resin. Zirconia crowns were designed (thickness=0.7 mm), milled by computer-aided design/computer-aided manufacturing, and sintered, as recommended. Crowns were cemented onto the resin preparations using five cementation strategies (n=15): ZP, luting with zinc phosphate cement; PN, luting with Panavia F resin cement; AL, air particle abrasion with alumina particles (125 μm) as the crown inner surface pretreatment + Panavia F; CJ, tribochemical silica coating as crown inner surface pretreatment + Panavia F; and GL, application of a thin layer of porcelain glaze followed by etching with hydrofluoric acid and silanization as crown inner surface pretreatment + Panavia F. Resin cement was activated for 30 seconds for each surface. Specimens were tested until fracture in a stepwise stress fatigue test (10,000 cycles in each step, 600 to 1400 N, frequency of 1.4 Hz). The mode of failure was analyzed by stereomicroscopy and scanning electron microscopy. Data were analyzed by Kaplan-Meier and Mantel-Cox (log rank) tests and a pairwise comparison (p<0.05) and by Weibull analysis.

Results: The CJ group had the highest load mean value for failure (1200 N), followed by the PN (1026 N), AL (1026 N), and GL (1013 N) groups, while the ZP group had the lowest mean value (706 N). Adhesively cemented groups (CJ, AL, PN, and GL) needed a higher number of cycles for failure than the group ZP did. The groups' Weibull moduli (CJ=5.9; AL=4.4; GL=3.9; PN=3.7; ZP=2.1) were different, considering the number of cycles for failure data. The predominant mode of failure was a fracture that initiated in the cement/zirconia layer. Finite element analysis showed the different stress distribution for the two models.

Conclusion: Adhesive cementation of zirconia crowns improves fatigue resistance.

INTRODUCTION

Nowadays, among dental ceramics, zirconia has the greatest fracture toughness1 and flexural strength.2 In addition, this material has the “transformation toughening” mechanism, in which its grains turn from a tetragonal to monoclinic phase, with volumetric expansion, to prevent crack propagation.2 Therefore, this material can be used for crown infrastructures and fixed partial dentures (FPDs). Most recently, this material has also been used for full-contour indirect restorations of posterior regions, so that chipping failures, which are the most common failure type in veneered crowns,3 do not occur.4

Zirconia is a crystalline material that cannot be etched by hydrofluoric acid (at low concentrations) as can the glass-based ceramics.5 Thus, the adhesion between the tooth and the zirconia crown is a critical point. Hence, several surface treatments have been proposed to improve the resin bonding to zirconia: air particle abrasion with alumina or alumina coated with silica particles (tribochemical silica coating),6,7 application of an etchable glaze layer,8,9 use of 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-based primers,10,11 plasma deposition of silica films,12 and many others. In addition, the use of MDP-based resin cements has been indicated.13 However, a recent literature review stated that resin bonding to zirconia is no longer a drawback of this material, since the cementation strategies—with surface mechanical treatments and chemical approaches—that have been applied to improve this bonding interface seem to be reliable.14

In addition to retention, luting with resin cement leads to less microleakage15 and, in glass-ceramic restorations, to incomplete fractures.16 Despite this, nonadhesive cementation strategies are still recommended for more retentive zirconia preparations, such as crowns and FPDs.17 The hypothesis that the high fracture strength of the zirconia could support the entire restoration, no matter which cement is used, guides this recommendation.17 Indeed, surface damage is more common in bilayer restorations.18 However, the complete fracture of zirconia crowns has been reported.19,20 Moreover, as the restoration is a complex mechanical system including the ceramic bilayer crown, the cement, and the tooth, all of the components and the interaction between them are important. The question is, how do Y-TZP crowns behave in terms of fatigue resistance when different cementation approaches are used?

Therefore, the aim of this study was to investigate the influence of the cementation strategy on the fatigue resistance of zirconia crowns. The null hypothesis was that the cementation strategy would not affect the fatigue resistance of the crowns.

METHODS AND MATERIALS

Prosthetic Preparation and Zirconia Crown Production

A simplified posterior full-crown preparation (6-mm high, large chamfer finishing line, 12° of convergence of the walls) was designed, and 75 replicas were machined in glass fiber–filled epoxy resin. This epoxy resin has an elastic behavior similar to human dentin21 (National Electrical Manufacturers Association [NEMA] grade G10, Accurate Plastics, New York, NY, USA).

A preparation model was first scanned with a laboratory scanner (inEos Blue, Sirona Dental, Bensheim, Germany). The framework was virtually designed and machined from zirconia blocks (Vita InCeram 2000 YZ, Vita Zahnfabrik, Bad Säckingen, Germany) using a CAD/CAM system (Cerec MC XL, Sirona Dental, Bensheim, Germany) with 80 μm of cement space. After the sintering process (Zyrcomat T, Vita Zahnfabrik, Bad Säckingen, Germany), the crowns achieved the final thickness of 0.5 mm for the circumferential and 0.7 for the occlusal wall.

Luting Procedures

The crowns were cleaned with ethanol before the respective surface treatments and allocated into five groups, according to the cementation strategy:

  • • 

    Group ZP: no zirconia surface treatment + zinc phosphate cement.

  • • 

    Group PN: no zirconia surface treatment + resin cement.

  • • 

    Group AL: air particle abrasion with alumina particles (125 μm) + resin cement. The crowns were air particle abraded (Rocatector delta, 3M ESPE AG, Seefeld, Germany) with alumina particles (Alublast 125 μm, Elephant Dental B.V., the Netherlands) with 3 bar of pressure during 15 seconds and with 15 mm of distance.

  • • 

    Group CJ: air particle abrasion with alumina coated with silica particles (30 μm) + silane + resin cement. The crowns were air particle abraded as in the AL group, with the alumina coated by silica particles (CojetSand, 3M ESPE).

  • • 

    Group GL: application of a glaze layer + etching with hydrofluoric acid + silane + resin cement. A thin layer of glaze ceramic (Vita Akzent, Vita Zahnfabrik, Bad Sackinger, Germany) was applied on the crowns' intaglio surface with a brush and then cured according to the manufacturer's recommendation. This glaze layer was etched with hydrofluoric acid at 9% for 1 minute and silanized.

For the ZP group, the preparations were ultrasonically cleaned in distilled water for 5 minutes before the cementation process. For the other groups, the surface preparations were treated with hydrofluoric acid at 9% for 1 minute, rinsed with distilled water, ultrasonically cleaned in distilled water for 5 minutes, and air dried. A silane layer (Clearfil Porcelain Bond Activator + Clearfil SE Bond Primer, Kuraray Medical, Tokyo, Japan) was applied on the preparation with a microbrush, followed by a gentle air stream. The adhesive system (ED primer, Kuraray Medical, Tokyo, Japan) was applied, followed by a gentle air stream, after 60 seconds.

For the ZP group, the zinc phosphate cement was mixed and applied according to the manufacturer's instructions. For the other groups, a dual-activated resin cement was mixed for 20 seconds and placed in the internal surface of each crown. These crowns were placed on the preparations, and a constant load of 50 N was applied during photo activation (Astralis 10, Ivoclar Vivadent AG, Schaan, Liechtenstein) of each surface for 30 seconds. The cemented crowns were then stored in distilled water for 1-7 days at 37°C.

Stepwise Stress Testing

The cemented crowns were tested until failure occurrence in a stepwise stress fatigue test. In each step of 10,000 cycles, a load of 600 to 1400 N (200 N of increment) was applied, with a frequency of 1.4 Hz, in an aqueous environment (Fatigue Tester, ACTA, Amsterdam, the Netherlands). The load was applied by means of a stainless steel piston ball of 40 mm in diameter.

Fracture Analysis

After fracture occurrence, the specimens were analyzed by stereomicroscopy (Olympus, Shinjuku, Tokyo, Japan) at a magnification of up to 100×. The specimens with the most significant failures were ultrasonically cleaned in isopropyl alcohol for 10 minutes, gold sputtered, and subjected to scanning electron microscopy (XL 20, FEI Company, GG Eindhoven, the Netherlands).

Finite Element Analysis

The finite element analysis (FEA) of the abutment with crown specimens was performed to evaluate the failure load values. Two models were made; in the first model, the bonding between the cement layer and the crown was supposedly strong enough to resist the shear stress in the cement layer–crown interface (model 1). In the second model, the surface in the interface between the preparation and the crown was modeled for contact surface purposes with a friction coefficient of 0.45 (model 2). Since the models are symmetric in two directions, a quarter FEA model was prepared to facilitate the boundary conditions using symmetry, with the nodes in the centric planes allowing sliding in the surface only. The FEA model was created using FEMAP software (FEMAP 10.1.1, Siemens PLM software, Plano, TX, USA), while the analysis was carried out with NX Nastran software (NX Nastran; Siemens PLM Software). The models consisted of 13,952 parabolic tetrahedron solid elements. For both models, calculations were made with the PN and ZP cement layers. The mechanical properties of the used materials were found in the literature: the Young's moduli (GPa) and the Poisson's ratios for zirconia, resin cement, zinc phosphate cement, and G10 were, respectively, 209.3 and 0.32,22 7 and 0.35,23 13.7 and 0.33,24 and 14.9 and 0.31.25 The nodes at the bottom of the abutment were fixed so that no movement was allowed in any direction. The crown was loaded on the nodes in the center of the occlusal surface, simulating the plastic deformation of the occlusal surface caused by the loading ball (radius of 20 mm). The calculations considered a load of 300 N.

Statistical Analysis

Data were analyzed with the application of Kaplan-Meier and Mantel-Cox (log rank) tests and a pairwise comparison (p<0.05; SPSS version 21, IBM, Chicago, IL, USA). Data were also examined using a Weibull analysis with two software packages (Minitab 17, State College, PA, USA, and Weibull++ 9, Reliasoft, Tucson, AZ, USA). For the Minitab 17 software, the Weibull parameters (shape and scale) were calculated in the maximum likelihood estimation method and the correlation coefficients were calculated in the least squares estimation method.

RESULTS

For the fracture load, a difference among the cementation strategies was detected (Mantel-Cox log-rank test, X2=56.50, p=0.000<0.05; Table 1). The CJ group presented higher fracture load values, followed by the other adhesive strategies. Apart from that, for the number of cycles to fracture, a difference was also detected (Mantel-Cox log-rank test, X2=92.34, p=0.000<0.05; Table 1). In this case, all of the adhesively cemented groups (CJ, AL, PN, and GL) needed a greater number of cycles for fracture occurrence than the ZP group, nonadhesively cemented.

Table 1 Loads and Number of Cycles for Failure for the Experimental Groupsa
Table 1

Figure 1 shows the survival curves according to the steps of load and number of cycles until failure. Table 2 summarizes the mean fracture loads and number of cycles until failure, calculated from the survival curves.

Figure 1. . Survival curves according to the steps of load (A, B) and number of cycles (C, D) in which each crown failed. / Figure 2. Weibull analysis according to the steps of load (A) and number of cycles (B) in which each crown failed.Figure 1. . Survival curves according to the steps of load (A, B) and number of cycles (C, D) in which each crown failed. / Figure 2. Weibull analysis according to the steps of load (A) and number of cycles (B) in which each crown failed.Figure 1. . Survival curves according to the steps of load (A, B) and number of cycles (C, D) in which each crown failed. / Figure 2. Weibull analysis according to the steps of load (A) and number of cycles (B) in which each crown failed.
Figure 1 Survival curves according to the steps of load (A, B) and number of cycles (C, D) in which each crown failed. Figure 2. Weibull analysis according to the steps of load (A) and number of cycles (B) in which each crown failed.

Citation: Operative Dentistry 42, 2; 10.2341/16-002-L

Table 2 Survival Rates (Probability That the Specimens Have to Exceed the Respective Load or Number of Cycles Without Failure) for the Experimental Groups
Table 2

Figure 2 shows the Weibull curves for the experimental groups. The Weibull parameters are described in Table 3. For the load to failure values, there is no difference in the Weibull modulus values or shape among the groups (p=0.031). On the other hand, there is a statistical difference in Weibull modulus values among the groups according to the number of cycles to failure (p=0.007).

Table 3 Weibull Parameters With Their 95% Confidence Intervals (Maximum Likelihood Estimation) and Correlation Coefficient Least Squares Estimationa
Table 3

Contour plots of the groups with 95% of bilateral confidence interval are presented in Figure 3. The β values were higher than 1 (PN=3.7; ZP=1.8; GL=3.9; AL=4.4; CJ=5.9), indicating that failures occurred by fatigue in all groups.

Figure 3. . Contour plots with 95% of bilateral interval confidence.Figure 3. . Contour plots with 95% of bilateral interval confidence.Figure 3. . Contour plots with 95% of bilateral interval confidence.
Figure 3 Contour plots with 95% of bilateral interval confidence.

Citation: Operative Dentistry 42, 2; 10.2341/16-002-L

The predominant mode of failure was a fracture that initiated in the cement/zirconia layer (Figure 4).

Figure 4. . Representative crown failure modes. The black arrows indicate failure direction. The white arrows indicate failure origins.Figure 4. . Representative crown failure modes. The black arrows indicate failure direction. The white arrows indicate failure origins.Figure 4. . Representative crown failure modes. The black arrows indicate failure direction. The white arrows indicate failure origins.
Figure 4 Representative crown failure modes. The black arrows indicate failure direction. The white arrows indicate failure origins.

Citation: Operative Dentistry 42, 2; 10.2341/16-002-L

FEA and the stress values are shown in Figure 5 and Table 4, respectively.

Figure 5. . The maximum principal stress in model 1 with the PN cement layer and in model 2 with the ZP cement layer.Figure 5. . The maximum principal stress in model 1 with the PN cement layer and in model 2 with the ZP cement layer.Figure 5. . The maximum principal stress in model 1 with the PN cement layer and in model 2 with the ZP cement layer.
Figure 5 The maximum principal stress in model 1 with the PN cement layer and in model 2 with the ZP cement layer.

Citation: Operative Dentistry 42, 2; 10.2341/16-002-L

Table 4 The Maximum Tensile Stress (Solid Maximum Principal Stress) in the Zirconia and Maximum Tensile Stress and the Maximum Shear Stress (Solid Y Normal Stress) in the Cement Layer in the Circumferential Wall
Table 4

DISCUSSION

The present study demonstrated that the cementation strategy affects fatigue resistance of zirconia-based crowns. Consequently, the null hypothesis was rejected, since the fatigue experiment showed that adhesively cemented crowns had higher survival rates than nonadhesively cemented ones. The FEA also confirmed these findings, as the stress distribution was different for bonded and nonbonded crown designs.

For porcelain-based crowns, it is known that the use of resin cements may increase the fracture resistance by means of blunting the defects of the ceramic restorations.26,27 However, for alumina-, zirconia-, and lithium disilicate–based crowns, it is not clear if the luting adhesive enhances their mechanical properties, since some studies state there is no influence17,28,29 while others show some influence.30,31 A recent study31 suggested that for veneered zirconia crowns, the cementation surface treatment (with sandblasting, glazing, or tribochemical silica coating) did not affect the fatigue resistance. However, the results of the cited study showed better performance for the groups cemented without previous surface treatment and with MDP-based resin cement. As this study31 was carried out to produce veneering failures, such as chipping, it was necessary to develop a study in which the failure of the zirconia layer would be assessed. The present study was designed with this purpose and directed failures to the cementation region. In fact, the fractographic analysis showed that the fractures initiated on the interface between the cement and the zirconia, underneath the load application point (Figure 4).

The cementation strategies used in this study were in accordance with several bond strength studies,14,32-34 in which different surface treatments—such as sandblasting of alumina or alumina coated with silica particles (tribochemical silica coating), and glaze layer application (ceramic coating)—and luting agents (zinc phosphate cement or resin cement with MDP) were used to promote the retention of zirconia-based restorations to the tooth substrate. Although the zinc phosphate cement does not produce chemical bonding to zirconia, it is still indicated for cementation of restorations with larger adherence areas available, such as crowns, because of their retention ability.35,36 For this reason, this cement was used with the control group of this study. Although the application of a glaze layer in the intaglio surface before cementation of zirconia has been reported as an efficient treatment for bond strength improvement between zirconia and resin cement,8,9 according to Yener and others,37 this technique may lead to lower fracture strength results. In the present study, the GL group showed similar results compared with the adhesively cemented groups.

The adhesive cementation with sandblasting of alumina particles has been done for zirconia due to the promotion of a more retentive surface by the creation of a rougher topography.6 Since some studies affirm this surface treatment could damage the zirconia's mechanical properties,38 mainly with the use of larger alumina particles, in the present study, large alumina particles (125 μm) were used to simulate the worst possible scenario. However, even with this particle size, the AL group did not behave inferiorly when compared with the other groups. Probably, the combination of resin cement with MDP maintained the bond strength to zirconia, and consequently, the system behaved as a bonded crown. Similarly, the silicatization approach, which left some silica adhering to the zirconia surface by a tribochemical reaction, is the best treatment to improve the bond strength between zirconia ceramic and resin cement,39,40 and the system acted as a bonded crown in the CJ group. However, the CJ specimens survived for more cycles in the higher load and presented the highest Weibull moduli. It is possible to state that this adhesive luting combination (tribochemical silica coating and resin cement with MDP) seems to be the more reliable cementation strategy.

Even though the origin of fracture of all crowns was in the bonding interface, the macroscopic failure mode was different among the groups. Most fragments of the groups adhesively cemented remained bonded to the resin abutment, while the fragments of the non–adhesively cemented group (ZP) were detached. This showed that the cementation strategies were simulated by FEA under closely realistic conditions, whereas the retention condition without chemical bonding (zinc phosphate cement) was applied as a nonbonded interface area with a friction coefficient of 0.45, resulting in an area with some friction, values of which may range between 0 (no friction) and 1 (without movement).

Considering that long periods of underwater storage may lead to lower bond strength, resin cement elastic moduli reduction, and changes in the stress distribution in the crowns/cement/tooth complex,41 the short storage period is a limitation of the present study.

CONCLUSIONS

The adhesive cementation of zirconia crowns yields to a significantly higher fatigue resistance for zirconia crowns.

Acknowledgments

This study was supported and funded by FAPESP - São Paulo Research Foundation (Fundação de Amparo à Pesquisa do Estado de São Paulo) (Funding number #2011/23071-9 and 2012/05657-5) and CAPES - Coordination for the Improvement of Higher Education Personnel (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) (Funding number #99999.014310/2013-09).

Conflict of Interest

The authors have no proprietary, financial, or other personal interest of any nature or kind in any product, service, and/or company that is presented in this article.

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Copyright: ©Operative Dentistry, 2017 2017
Figure 1
Figure 1

Survival curves according to the steps of load (A, B) and number of cycles (C, D) in which each crown failed.

Figure 2. Weibull analysis according to the steps of load (A) and number of cycles (B) in which each crown failed.

Figure 3
Figure 3

Contour plots with 95% of bilateral interval confidence.

Figure 4
Figure 4

Representative crown failure modes. The black arrows indicate failure direction. The white arrows indicate failure origins.

Figure 5
Figure 5

The maximum principal stress in model 1 with the PN cement layer and in model 2 with the ZP cement layer.

Contributor Notes

Fernanda Campos, PhD, Graduate Program in Restorative Dentistry, Prosthodontics Units, São José dos Campos Dental School, UNESP–Univ Estadual Paulista, São José dos Campos, São Paulo, Brazil

Luiz Felipe Valandro, PhD, associate professor and head, MSciD/PhD Graduate Program in Oral Science, Prosthodontic Unit, Faculty of Odontology, Federal University of Santa Maria, Rio Grande do Sul, Brazil

Sabrina Alves Feitosa, PhD, Graduate Program in Restorative Dentistry, Prosthodontics Unit, São José dos Campos Dental School, UNESP–Univ Estadual Paulista, São José dos Campos/ São Paulo, Brazil

Cornelis Johannes Kleverlaan, PhD, professor, Universiteit van Amsterdam and Vrije Universiteit, Department of Dental Materials Science, Amsterdam, the Netherlands

Albert J. Feilzer, PhD, professor, Universiteit van Amsterdam and Vrije Universiteit, Department of Dental Materials Science, Amsterdam, the Netherlands

Niek de Jager, PhD, professor, Universiteit van Amsterdam and Vrije Universiteit, Department of Dental Materials Science, Amsterdam, the Netherlands

*

Marco Antonio Bottino, PhD, chair and professor, Department of Dental Materials and Prosthodontics, São José dos Campos Dental School, UNESP–Univ Estadual Paulista, São José dos Campos, São Paulo, Brazil

*Corresponding author: Av. Engenheiro Francisco José Longo, 777. Jardim São Dimas, São José dos Campos-SP, 12245000; e-mail: mmbottino@uol.com.br
Accepted: 13 Apr 2016
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