Effect of Veneering Methods on Zirconia Framework—Veneer Ceramic Adhesion and Fracture Resistance of Single Crowns ¨ ¨ ˘ ¨ ¨ ˘ Burcu Kanat-Erturk, DDS, PhD,1 Erhan M. C¸omleko glu, DDS, PhD,2 Mine Dundar-C ¸ omleko glu, DDS, PhD,2 3 2 ¨ ¨ or, ¨ DDS, PhD Mutlu Ozcan, DDS, DMD, PhD, & Mehmet Ali Gung 1

Department of Prosthodontics, Kocaeli University School of Dentistry, Kocaeli, Turkey School of Dentistry, Department of Prosthodontics, Ege University, Izmir, Turkey 3 Dental Materials Unit, Center for Dental and Oral Medicine, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, University of Zurich, Zurich, Switzerland 2

Keywords CAD/CAM; CAD-on; finite element analysis; fracture resistance; shear-bond strength; overcemented file-splitting; veneering methods. Correspondence ¨ Burcu Kanat-Erturk, Department of Prosthodontics, Kocaeli University School of Dentistry, Yuvacık Yerles¸kesi, Bas¸iskele 41190, Kocaeli, Turkey. E-mail: [email protected] This research was supported in part by the Ege University Scientific Research Project ˙ ¸ -006). Department (Project no. 2011-DIS The authors deny any conflicts of interest. Accepted July 9, 2014 doi: 10.1111/jopr.12236

Abstract Purpose: The aim of this study was to evaluate the fracture resistance (FR) and shear bond strength (SBS) via finite element analysis (FEA) of zirconia framework veneered with different methods. Materials and Methods: Zirconia frameworks were prepared as crowns for FR and cubic blocks for SBS (N = 60, n = 10). The specimens were veneered with one of the following veneering methods: (a) overcemented file-splitting (OCF), (b) layering (L), or (c) overpressing (P). For crowns, stainless steel dies (N = 30; chamfer: 1 mm) were scanned using a contrast spray. Bilayered design for OCF and reduced design (1 mm) for both L and P were performed by computer-aided design and manufacturing. For the SBS test, zirconia blocks were sectioned (4 × 4 × 4 mm3 ) under water cooling and sintered. Frameworks were veneered with compatible ceramics for each veneering method and subjected to mechanical tests. The milled suprastructures were bonded to zirconia frameworks using a resin composite in Group OCF and photopolymerized. Crowns were cemented to the metal dies with resin modified glass-ionomer cement. All specimens were stored at 37°C, 100% humidity for 48 hours prior to mechanical tests. Data were statistically analyzed (ANOVA, Bonferroni tests, α = 0.05). Fractured specimens were examined under scanning electron microscopy (SEM), and FEA modeling of the crowns was performed. Results: Mean FR values (N) were significantly higher with L (6102 ± 1519) and P (4117 ± 1083) than with of OCF (1900 ± 254) (p = 0.01). The mean SBS (MPa) in OCF (24 ± 4) was significantly lower (p < 0.002) than L (35 ± 6) and P (32 ± 6) (p > 0.05). For crown restorations, while only adhesive failures were found in OCF, cohesive failures within veneering ceramic were more frequent in P and L. FEA verified these findings. Conclusion: Veneering methods based on layering or pressing may reduce ceramic chipping but the overcemented file-splitting method does not seem to prevent this failure. Clinical Significance: Layering and overpressing veneering methods on zirconia frameworks with reduced design might decrease chipping compared to overcemented file-splitting, where in the latter, zirconia framework and feldspathic suprastructure are combined using a resin cement.

All-ceramic fixed dental prostheses (FDP) are increasingly used as an alternative to porcelain-fused-to-metal (PFM) due to their favorable esthetics and biocompatibility.1,2 Yet the brittle allceramics need to be supported by stronger frameworks, especially for their application in the posterior region.3 In cases where esthetics and strength are important, zirconia framework

can be preferred to PFM FDPs to avoid metal reflection of the latter through the gingiva.4,5 Unfortunately, against all efforts, chipping of the veneering ceramic is still one of the most common clinical failure types with zirconia FDPs.6 This phenomenon presents a clinical challenge, as replacement of such FDPs may cause iatrogenic damage such as abutment teeth or

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Effect of Veneering Methods on Veneering Ceramic-Zirconia Adhesion

ceramic fracture, discomfort to the patient, and loss of time.8,9 Zirconia-veneer adhesion and bond strengths can be affected by several factors such as veneering methods, mechanical properties of the framework and veneer ceramic, wettability of the framework by the veneering ceramic, and residual stresses at the interface.10-13 Various methods (i.e., layering, overpressing) have been developed over the years for veneering zirconia frameworks with glassy matrix ceramics. According to the traditional layering technique, the mixed ceramic powder and its liquid are built on the sintered zirconia framework larger than the final dimensions to compensate for the shrinkage of the veneering ceramic.14-16 On the other hand, the overpressing method requires a wax-up model with the final contour of the veneer on the sintered zirconia framework to be invested under heat-pressed vacuum with pressable ceramics.14,17,18 For both methods, uniform thickness of the veneering layer has shown to be crucial to resist chipping and/or fracture.10,19 Thus, anatomical cusp support of the zirconia framework plays a crucial role in decreasing chipping under mechanical loading.19,20 In addition to these commonly used veneering methods for zirconia frameworks, a relatively new method, the so-called “file-splitting,” has been introduced owing to improvements in computer-aided design/computer-aided manufacturing (CAD/CAM).21,22 Today, CAD/CAM systems can be used not only for the fabrication of frameworks but also for suprastructures. In the file-splitting technique, zirconia framework and veneering ceramic are designed together with the CAD software. Following the milling process through CAM units, two parts are combined using a glass-ceramic21,23 or a resin cement24 depending on the ceramic system manufacturer. This method not only decreases the number of laboratory stages, which may cause operator-related errors during the restoration process, but also provides the opportunity to use homogenous ceramic blocks with relatively high strength.25 Also, with the file-splitting method, FDPs can be fabricated more quickly. Yet, information on the effects of special interfacial bonding agent and the prefabricated ceramic blocks on the fracture resistance (FR) of zirconia-supported FDPs is scarce.21,22 When using the milling technique with prefabricated ceramic blocks, due to lack of imperfections, the reliability of ceramic restorations has been reported to increase significantly.26 The other advantage of milled FDPs is the anatomical framework design with homogenous thickness.20,27 However, the interfacial bond between the zirconia framework and the veneering ceramic affects the fracture pattern and fracture strength.28 A layer of veneering ceramic remaining on the zirconia framework after chipping or fracture test signifies a strong interfacial bond, whereas exposure of zirconia framework indicates a weak interfacial bond between these two materials.28-30 Mechanical tests such as three- or four-point biaxial flexural strength tests, shear bond tests coupled with finite element analysis (FEA) and/or fractographic analyses may enable us to identify the failure initiation site in zirconia-supported FDPs veneered with different methods. This information could then potentially prevent the use of inferior systems in clinical studies.31-34 The objective of this study was therefore to evaluate the FR of single crowns veneered with different methods (overcemented file-splitting [OCF], layering, and overpressing) on the 2

CAD/CAM-fabricated zirconia framework. Also, two complementary tests were performed to verify the findings: (a) shear bond strength (SBS) test to test the interfacial bond between zirconia and veneering ceramics and (b) finite element modeling (FEM) of the crown system to observe the location of the accumulated stresses under simulated loading conditions. The null hypothesis tested was that the veneering method would not affect the FR of the bilayered zirconia crowns.

Materials and methods Specimen preparation

Crowns for FR and zirconia blocks for SBS tests were prepared using a CAD/CAM system (InLab 3.88; Sirona Dental Systems GmbH, Bensheim, Germany). Zirconia frameworks (Vita In-Ceram YZ; Vita Zahnfabrik, Bad S¨ackingen, Germany) were veneered with one of three veneering methods: (a) OCFsplitting, (b) layering (L), and (c) overpressing (P) (N = 60, n = 10 per group). The brands, manufacturers, chemical compositions, some mechanical properties, and batch numbers of the materials used in this study are listed in Table 1.35-38 All specimens were fabricated by one operator. For crown-shaped specimens, industrially fabricated stainless steel dies (n = 30), with a 1 mm circumferential chamfer and a groove on the axial side were designed to avoid the rotation of crowns during mechanical testing. They were scanned (InEos Blue; Sirona Dental Systems) after coating the surfaces with a contrast spray (IPS Contrast Spray Labside; Ivoclar Vivadent, Schaan, Liechtenstein). The restorations were designed for the mandibular left first molar after the digital impressions using CAD/CAM software (InLab 3.88). Die spacer thickness of 10 μm was chosen for better fit of zirconia framework to the metal die.21,28 Circumferential margin and insertion path of the metal dies were controlled as the groove was positioned buccally. A reduced design of 1 mm was made for both the L and P groups to support the veneering ceramic in uniform thickness.39 Bilayered design was used for the OCF groups. Zirconia frameworks were fabricated in a milling unit (InLab MC XL; Sirona Dental Systems) using presintered zirconia blocks (Vita In-Ceram YZ) (n = 30) in an approximately 20% to 25% enlarged volume to compensate for the shrinkage of the sintering process. Then, the milled frameworks were sintered (InFire HTC speed; Sirona Dental Systems), and the adaptations of the zirconia cores to the metal dies were controlled. For the SBS test, specimens (4 × 4 × 4 mm3 ) (n = 30) were sectioned in three axes under water cooling using a slow-speed diamond saw (Isomet 1000; Buehler Ltd., Lake Bluff, IL) from presintered zirconia blocks (Vita In-Ceram YZ) considering the 20% to 25% sintering shrinkage of the zirconia and thickness of the saw. The specimens were then sintered. Suprastructure preparation OCF-splitting method

For crown-shaped specimens, the suprastructures were designed by the software in accordance with the geometry of the framework prepared in the milling unit from feldspathic ceramic blocks (Vita Mark II; Vita Zahnfabrik). After controlling the adaptation of the sintered zirconia frameworks

C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–9 

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Effect of Veneering Methods on Veneering Ceramic-Zirconia Adhesion

Table 1 Brands, manufacturers, chemical compositions, some mechanical properties, and batch numbers of the materials used

Brand In-Ceram YZ


Vita VM9

VITA Zahnfabrik, Bad ¨ Sackingen, Germany VITA Zahnfabrik

Vita PM9

VITA Zahnfabrik

Vita Mark II

VITA Zahnfabrik

Panavia F 2.0

Paste B

Vita Akzent

Elastic modulus (GPa)

Thermal expansion coefficient (10−6 /K)

Flexural strength (MPa)

Batch number

ZrO2 (

Effect of Veneering Methods on Zirconia Framework-Veneer Ceramic Adhesion and Fracture Resistance of Single Crowns.

The aim of this study was to evaluate the fracture resistance (FR) and shear bond strength (SBS) via finite element analysis (FEA) of zirconia framewo...
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