Scholarly article on topic 'Current status of zirconia restoration'

Current status of zirconia restoration Academic research paper on "Economics and business"

CC BY-NC-ND
0
0
Share paper
Academic journal
Journal of Prosthodontic Research
OECD Field of science
Keywords
{"Dental CAD/CAM" / FDPs / Zirconia / Polishing / Friction / "Antagonist wear" / "Full contour"}

Abstract of research paper on Economics and business, author of scientific article — Takashi Miyazaki, Takashi Nakamura, Hideo Matsumura, Seiji Ban, Taira Kobayashi

Abstract During the past decade, zirconia-based ceramics have been successfully introduced into the clinic to fabricate fixed dental prostheses (FDPs), along with a dental computer-aided/computer-aided manufacturing (CAD/CAM) system. In this article (1) development of dental ceramics, (2) the current status of dental CAD/CAM systems, (3) CAD/CAM and zirconia restoration, (4) bond between zirconia and veneering ceramics, (5) bond of zirconia with resin-based luting agents, (6) surface finish of zirconia restoration and antagonist enamel wear, and (7) clinical evaluation of zirconia restoration are reviewed. Yttria partially stabilized tetragonal zirconia polycrystalline (Y-TZP) showed better mechanical properties and superior resistance to fracture than other conventional dental ceramics. Furthermore, ceria-stabilized tetragonal zirconia polycrystalline and alumina nanocomposites (Ce-TZP/A) had the highest fracture toughness and had resistance to low-temperature aging degradation. Both zirconia-based ceramics have been clinically available as an alternative to the metal framework for fixed dental prostheses (FDPs). Marginal adaptation of zirconia-based FDPs is acceptable for clinical application. The most frequent clinical complication with zirconia-based FDPs was chipping of the veneering porcelain that was affected by many factors. The mechanism for the bonding between zirconia and veneering ceramics remains unknown. There was no clear evidence of chemical bonding and the bond strength between zirconia and porcelain was lower than that between metal and porcelain. There were two alternatives proposed that might avoid chipping of veneering porcelains. One was hybrid-structured FDPs comprising CAD/CAM-fabricated porcelain parts adhering to a CAD/CAM fabricated zirconia framework. Another option was full-contour zirconia FDPs using high translucent zirconia. Combined application of silica coating and/or silane coupler, and 10-methacryloyloxydecyl dihydrogen phosphate is currently one of the most reliable bonding systems for zirconia. Adhesive treatments could be applied to luting the restorations and fabricating hybrid-structured FDPs. Full-contour zirconia FDPs caused concern about the wear of antagonist enamel, because the hardness of Y-TZP was over double that of porcelain. However, this review demonstrates that highly polished zirconia yielded lower antagonist wear compared with porcelains. Polishing of zirconia is possible, but glazing is not recommended for the surface finish of zirconia. Clinical data since 2010 are included in this review. The zirconia frameworks rarely got damaged in many cases and complications often occurred in the veneering ceramic materials. Further clinical studies with larger sample sizes and longer follow-up periods are required to investigate the possible influencing factors of technical failures.

Academic research paper on topic "Current status of zirconia restoration"

Available online at www.sciencedirect.com

. . Journal of

ScienceDirect Prosthodontic

Research

ELSEVIER Journal of Prosthodontic Research 57 (2013) 236-261

www.elsevier.com/locate/jpor

Review

Current status of zirconia restoration

Takashi Miyazaki (DDS, PhD)a*, Takashi Nakamura (DDS, PhD)b, Hideo Matsumura (DDS, PhD)c, Seiji Ban (PhD)d, Taira Kobayashi (DDS, PhD)e

a Division of Oral Biomaterials and Technology, Showa University School of Dentistry, Tokyo, Japan b Department of Fixed Prosthodontics, Osaka University Graduate School of Dentistry, Osaka, Japan

c Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan Department of Dental Materials Science, School of Dentistry, Aichi Gakuin University, Nagoya, Japan e Department of Crown Bridge Prosthodontics, Nihon University School of Dentistry at Matsudo, Japan

Received 27 August 2013; received in revised form 6 September 2013; accepted 6 September 2013 Available online 18 October 2013

CrossMarl

Abstract

During the past decade, zirconia-based ceramics have been successfully introduced into the clinic to fabricate fixed dental prostheses (FDPs), along with a dental computer-aided/computer-aided manufacturing (CAD/CAM) system. In this article (1) development of dental ceramics, (2) the current status of dental CAD/CAM systems, (3) CAD/CAM and zirconia restoration, (4) bond between zirconia and veneering ceramics, (5) bond of zirconia with resin-based luting agents, (6) surface finish of zirconia restoration and antagonist enamel wear, and (7) clinical evaluation of zirconia restoration are reviewed.

Yttria partially stabilized tetragonal zirconia polycrystalline (Y-TZP) showed better mechanical properties and superior resistance to fracture than other conventional dental ceramics. Furthermore, ceria-stabilized tetragonal zirconia polycrystalline and alumina nanocomposites (Ce-TZP/ A) had the highest fracture toughness and had resistance to low-temperature aging degradation. Both zirconia-based ceramics have been clinically available as an alternative to the metal framework for fixed dental prostheses (FDPs). Marginal adaptation of zirconia-based FDPs is acceptable for clinical application. The most frequent clinical complication with zirconia-based FDPs was chipping of the veneering porcelain that was affected by many factors. The mechanism for the bonding between zirconia and veneering ceramics remains unknown. There was no clear evidence of chemical bonding and the bond strength between zirconia and porcelain was lower than that between metal and porcelain.

There were two alternatives proposed that might avoid chipping of veneering porcelains. One was hybrid-structured FDPs comprising CAD/ CAM-fabricated porcelain parts adhering to a CAD/CAM fabricated zirconia framework. Another option was full-contour zirconia FDPs using high translucent zirconia. Combined application of silica coating and/or silane coupler, and 10-methacryloyloxydecyl dihydrogen phosphate is currently one of the most reliable bonding systems for zirconia. Adhesive treatments could be applied to luting the restorations and fabricating hybrid-structured FDPs. Full-contour zirconia FDPs caused concern about the wear of antagonist enamel, because the hardness of Y-TZP was over double that of porcelain. However, this review demonstrates that highly polished zirconia yielded lower antagonist wear compared with porcelains. Polishing of zirconia is possible, but glazing is not recommended for the surface finish of zirconia.

Clinical data since 2010 are included in this review. The zirconia frameworks rarely got damaged in many cases and complications often occurred in the veneering ceramic materials. Further clinical studies with larger sample sizes and longer follow-up periods are required to investigate the possible influencing factors of technical failures.

# 2013 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. Keywords: Dental CAD/CAM; FDPs; Zirconia; Polishing; Friction; Antagonist wear; Full contour

Contents

1. Introduction..................................................................................................................................................................237

2. Development of dental ceramics ......................................................................................................................................237

3. CAD/CAM and zirconia restoration..................................................................................................................................238

* Corresponding author at: Department of Oral Biomaterials and Technology, School of Dentistry, Showa University, 1-5-8 Hatanodai Shinagawa-ku, Tokyo 1428555, Japan. Tel.: +81 03 3784 8177; fax: +81 03 3784 8179. E-mail address: miyazaki@dent.showa-u.ac.jp (T. Miyazaki).

1883-1958/$ - see front matter # 2013 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. http://dx.doi.org/10.1016/j.jpor.2013.09.001

3.1. The current status of dental CAD/CAM..................................................................................................................238

3.2. Application of zirconia-based ceramic FDPs using CAD-CAM process........................................................................240

4. The bond between zirconia and veneering ceramics............................................................................................................241

4.1. Zirconia and veneering ceramics............................................................................................................................241

4.2. Mechanism and evaluation of integration..................................................................................................................241

4.3. Factors affecting bond strength..............................................................................................................................242

4.3.1. Veneering ceramic....................................................................................................................................242

4.3.2. Zirconia ..................................................................................................................................................242

5. Bonding of zirconia with resin-based luting agents............................................................................................................244

5.1. Adhesive bonding to zirconia..................................................................................................................................244

5.2. Resin-based luting systems with methacryloyloxydecyl dihydrogen phosphate..............................................................244

5.3. Surface modifications of zirconia............................................................................................................................244

5.4. Silica/silane and MDP............................................................................................................................................246

5.5. Unfilled luting agent..............................................................................................................................................246

5.6. Mechanical retention..............................................................................................................................................247

6. Surface finish of zirconia restorative and antagonist enamel wear ........................................................................................248

6.1. Grinding and polishing of zirconia restoratives........................................................................................................248

6.1.1. Grinding rotary instruments........................................................................................................................248

6.1.2. Diamond polishing paste............................................................................................................................249

6.1.3. Polishing of dental ceramics......................................................................................................................249

6.1.4. Polishing of zirconia..................................................................................................................................250

6.2. Studies on the wear of antagonist against zirconia....................................................................................................251

6.2.1. Friction study in arthroplasty......................................................................................................................251

6.2.2. Wear studies using enamel in the 2000s......................................................................................................252

6.2.3. Wear studies using enamel in the 2010s......................................................................................................252

6.2.4. Wear studies using steatite..........................................................................................................................253

6.3. Prevention of antagonist enamel wear against zirconia restoratives ..............................................................................254

7. Clinical evaluation of zirconia restoration ..........................................................................................................................254

7.1. Clinical outcome..................................................................................................................................................254

7.2. The future prospect of zirconia restorations ..............................................................................................................256

8. Conclusion ....................................................................................................................................................................257

Acknowledgements........................................................................................................................................................258

References ....................................................................................................................................................................258

1. Introduction

Developments in routine dental practice, including prosthodontic treatments, are often driven by the introduction of new dental materials and processing technologies. Dental prostheses such as crowns, fixed dental prostheses (FDPs), and removable dental prostheses are fabricated from a variety of dental materials using a range of dental laboratory processes. Because of the popularity of osseo-integrated implants, the application of fixed prostheses has expanded, even in the edentulous situation.

Development of both casting gold alloys and precision dental casting technologies has contributed to the application of metallic prostheses. However, because of the recent demand from patients for esthetics and biosafety, metal-free prostheses have been desired. Both new dental materials and new processing technologies are required to meet these patient demands.

During the past decade, new dental ceramic materials such as glass ceramics, poly-crystalline alumina, and zirconia-based ceramics have been successfully introduced into the clinic, along with new processing technology, i.e. computer-assisted fabrication systems [dental computer-assisted design/computer-assisted manufacturing (CAD/CAM)].

In this article we discuss: (1) development of dental ceramics, (2) the current status of dental CAD/CAM systems, (3) CAD/ CAM and zirconia restoration, (4) the bond between zirconia and veneering ceramics, (5) bond of zirconia with resin-based luting agents, (6) surface finish of zirconia restoration and antagonist enamel wear, and (7) clinical evaluation of zirconia restoration.

2. Development of dental ceramics

Porcelain has been used in dentistry for 100 years. Esthetics is the major advantage of porcelain, and brittleness is its weakest point for load-bearing restorations. The conventional powder build-up and firing process was innovative but is still very technically sensitive. Therefore, porcelain-fused-to-metal (PFM) restorations to make "metal-ceramic restorations" has been the first choice of prostheses to satisfy requirements for esthetics, durability, and fit to the abutments [1,2].

Two main types of all-ceramic FDP systems are proposed. The first system involves using a single material for full-contour crowns. Reinforced glassy materials were successfully used to make single crowns for anterior and premolar regions. Recently, polycrystalline zirconia with improved translucency has been used for full-contour crowns in the molar region [3].

Table 1

Classification of ceramics for fixed prostheses by intended clinical use (ISO 6872:2008).

Recommended clinical indications

Mechani

Flexural minimum

(a) Esthetic ceramic for coverage of a metal or a ceramic substructure

(b) Esthetic-ceramic: single-unit anterior prostheses, veneers, inlays, or onlays

(a) Esthetic-ceramic: adhesively cemented, single-unit, anterior or posterior prostheses

(b) Adhesively cemented, substructure ceramic for single-unit anterior or posterior prostheses Esthetic-ceramic: non-adhesively cemented, single-unit, anterior or posterior prostheses

(a) Substructure ceramic for non-adhesively cemented, single-unit, anterior or posterior prostheses

(b) Substructure ceramic for three-unit prostheses not involving molar restoration Substructure ceramic for three-unit prostheses involving molar restoration Substructure ceramic for prostheses involving four or more units

100 100 300 300

500 800

The second system is to fuse esthetic ceramics, such as porcelain and other glassy materials, to frameworks made of high-strength ceramics instead of alloys. Dense sintered polycrystalline zirconia-based material is promising for frameworks of FDPs [4-6].

The mechanical properties of brittle ceramics are characterized by fracture toughness and flexural strength [7] (Table 1). Conventional porcelain is a partially glassy material; its fracture toughness is approximately 1.0 MPa m1/ 2 and flexural strength is approximately 100 MPa. This material is not suitable for load-bearing molar restorations. Initially, porcelain was reinforced by dispersing crystals within it. Aluminous porcelain was widely available. Since the conventional powder build-up and firing procedure is sensitive to technique, new, easier-to-work-with ceramic materials were needed. To respond to this demand, castable and pressable ceramics were developed and are available for single esthetic restorations. In addition, prefabricated reinforced glass ceramic blocks are available for milling using a CAD/CAM device. These materials have fracture toughnesses from 1.5 to 3.0 MPa m1/2. However, these ceramics are still only available for single restorations.

Another type of ceramic includes alumina and other fine ceramic powders that are porously sintered; the pores are then infiltrated with glass to give "glass-infiltrated ceramics,'' with fracture toughnesses from 3 to 5 MPa m1/2. These materials have been applied to fixed partial dentures, but the prognosis was not satisfactory.

Finally, industrial dense polycrystalline ceramics such as alumina, zirconia, and alumina-zirconia composites are currently available for use with CAD/CAM technology via a networked machining center. In particular, yttrium partially stabilized tetragonal zirconia polycrystalline (Y-TZP) shows better mechanical properties and superior resistance to fracture. Y-TZP has a high fracture toughness, from 5 to 10 MPa m1/2, and a flexural strength of 9001400 MPa [8,9].

When a crack initiates on the surface of Y-TZP, the stress concentration at the top of the crack causes the tetragonal crystal to transform into a monoclinic crystal, with associated volumetric expansion. In the vicinity of a propagating crack, the stress-induced transformation leads to compressive stress that

cal and chemical properties

strength Chemical solubility

a (mean), MPa maximum, mg cm~

100 2000 100 2000

2000 100

shields the crack tip from the applied stress and enhances the fracture toughness [10].

Ceria-stabilized tetragonal zirconia polycrystalline (Ce-TZP) showed much higher fracture toughness of 19 MPa m1/ 2 but lower flexural strength and hardness than Y-TZP. Ce-TZP has not been applied in the dental field. Ce-TZP/alumina nanocomposites (Ce-TZP/A) were developed to improve Ce-TZP [11]. Ce-TZP/A consists of nanometer-sized Al2O3 particles that are dispersed within the Ce-TZP grains and grain boundaries, and nanometer-sized Ce-TZP particles that are dispersed within the alumina grains and grain boundaries. This homogeneous dispersion of alumina in the Ce-TZP matrix suppresses grain growth and increases hardness, flexural strength, and hydrothermal stability of tetragonal zirconia while preserving its toughness [11]. Ce-TZP/A is the toughest dental ceramic material available, with a fracture toughness of 19 MPa m1/2, and a flexural strength of 1400 MPa [12]. Y-TZP suffers from low-temperature aging degradation (LTAD) caused by phase transformation, whereas Ce-TZP/A has complete resistance to LTAD [13].

These improved characteristics are expected to expand the clinical application of dental ceramics to not only all-ceramic restorations, but also other fields such as the abutment of implants, implant bodies, and removable denture bases and parts.

3. CAD/CAM and zirconia restoration

3.1. The current status of dental CAD/CAM

CAD/CAM technology was introduced into dentistry, and FDPs could be fabricated using a series of steps, as shown in Fig. 1. The intraoral abutment was scanned by an intraoral digitizer to obtain an optical impression. Digitized data were reconstructed as 3-D graphics on the monitor and the optimal morphology for the FDPs was virtually designed on the monitor. Real FDPs were fabricated by milling a block using a numerically-controlled machine.

Since there were difficulties in digitizing the intraoral abutment accurately using a direct intraoral scanner, we decided to prepare a conventional stone model to begin the CAD/CAM process for the fabrication of crowns, especially for

(Staining and grazing ) CAM process

(Milling prostheses from the block)

Fig. 1. A process of digital fabrication system of FDPs.

Table 2

Current dental CAD/CAM systems available in the world market.

CAD/CAM system (Company)

Scanner

Milling Prostheses machine _

Materials

Central production

Inlay Veneer Crown Bridge Resin Titanium Porcelain Alumina Zirconia center

Everest & Arctica (KaVo electrotechnical work GmbH) Lava (3 M ESPE

Dental AG) Procera (Nobel Biocare

Germany GmbH) Cercon smart ceramics

(DeguDent GmbH) CEREC AC (Sirona

Dental of system GmbH) Hint-ELs system

(Hint-ELs DentaCAD systems) Aadva system (GC)

C-Pro system

(Panasonic dental) Katana (Kuraray

noritake dental) ZENO® Tec System (Wieland)

Original Original O O

Original Original & OEM

Original Original O

Original Original & OEM

Original Original O O Original Original O

Original Original O O & OEM

Original

OEM OEM OEM

OEM OEM

O O O O O

O O O O

O O O O O

O O O O

O O O O O

Nano-composite

O O O O

dental laboratory use. Different digitizers such as a contact probe, laser beam with position sensitive detector sensor, and laser with a CCD camera were developed. In addition, sophisticated CAD software and compact dental CAD/ CAM machines were developed. Both metallic and ceramic

restorations were fabricated by the second-generation CAD/ CAM systems [14].

Later, networked CAD/CAM systems were available, and all-ceramic frameworks using industrial dense sintered poly-crystalline alumina were available in the clinic. Since these

high-strength industrial ceramics were not available to the conventional dental laboratory, the application of networked CAD/CAM, located in a processing center, was a tremendous innovation in the history of dental technology. Such networked production systems are currently being introduced by a number of companies worldwide. Currently, the production of zirconia frameworks is the most popular use of this approach in the world market (Table 2).

The application of CAD/CAM is currently limited to laboratory processing. For example, even if the zirconia framework is fabricated using a CAD/CAM process in the machining center, final restorations are completed by dental technicians veneering conventional porcelain using conventional manual dental technology. Nevertheless, there are advantages to the introduction of CAD/CAM, such as the introduction of new, safe, esthetic, and durable materials, an increase in the efficiency of laboratory processing, earlier function of restoration, and better quality control of restorations, for improved fit, mechanical durability, and predictability.

Furthermore, the veneering part of zirconia all-ceramic FDPs was also fabricated by a CAD/CAM process from a block of glassy materials. A new fabrication system for digital veneering was introduced [15].

Because of the rapid progress in new technologies, especially optical technology, new intraoral digitizers are available. Information about these systems is still limited, and their manipulation and digitizing accuracy seem to be unclear at present. However, rapid progress in technology will ensure that taking the optical impressions will become practical in the clinic in the near future.

3.2. Application of zirconia-based ceramic FDPs using CAD-CAM process

Zirconia-based ceramics, especially Y-TZP, are clinically available as an alternative to metal frameworks for FDPs [16,17]. The fabrication of Y-TZP frameworks can be performed by milling a solid block using CAD/CAM procedures and either of two systems [18].

In the first system, frameworks with final dimensions can be milled directly from fully sintered dense ceramic blocks using a CAD/CAM-controlled grinding machine. This system has the advantage of a superior fit, because no shrinkage is involved in the process, but has the disadvantage of inferior machining associated with wear of the tool.

In the second system, frameworks with enlarged dimensions can be milled from partially-sintered blocks or green blocks, again using CAD/CAM-controlled grinding machines, followed by post-sintering at high temperature (using an electric furnace) to obtain a framework with final dimensions and sufficient strength. This system is currently popular for fabricating zirconia frameworks using the main CAD/CAM systems available in the world market. However, although this system has the advantage of easy machinability without wear on the tools and chipping of the material, the dimensions of the frameworks must be adjusted to compensate for extensive

sintering shrinkage during the post-sintering process, so that the final frameworks fit well.

Fit of the FDPs to the abutment, especially marginal adaptation, is one of the determining factors for the long-term clinical success of dental prostheses [19]. Clinical evaluation showed that the margin fit of zirconia-ceramic FDPs fabricated by the current CAD/CAM systems was similar to that of conventional metal ceramic restorations [20].

There were a number of publications evaluating the fit of the FDPs fabricated by CAD/CAM systems. However, because of the rapid progress and remodeling of the CAD/CAM systems currently available in the clinic, it is difficult to judge the degree of fit of FDPs produced by each system. Laboratory studies suggested marginal adaptation of 3-unit and 4-unit zirconia-ceramic FDPs consisting of frameworks fabricated using commercially-available CAD/CAM systems was acceptable for clinical application [21-23].

However, the discrepancy of the margin of the crown adjoined to the pontic was increased by the sintering shrinkage of the bulky pontic in the case of 3-unit and 4-unit frameworks. Therefore, we must beware of distortion of zirconia-based FDPs with long span units when using partially-sintered blocks or green blocks [24].

The survival and complication rates of zirconia-based and metal ceramic FDPs indicate that the most frequent technical complication with zirconia-based FDPs was chipping of the veneering porcelain [25,26]. There are many factors affecting chipping of veneering porcelain on zirconia-based ceramic frameworks, including adequate framework design to support the veneering porcelain, adequate handling in the dental laboratory, and further developments in the mechanical properties and application techniques of the veneering porcelain [27].

It was difficult to design a complicated support form using a CAD process, compared with the simpler manual method of making a wax-pattern. However, because of rapid progress in computer hardware and software, sophisticated CAD processes are available to design adequate frameworks using current CAD/CAM systems.

Each manufacturer recommends surface treatment of the zirconia framework (such as sandblasting and heat treatments) prior to porcelain fusing. However, the effect of surface treatments on the bonding strength of porcelain to zirconia is still controversial. There are differences in the thermal expansion coefficients and firing temperatures among the commercial veneering porcelain products for zirconia frameworks; this implies that the different products have different powder compositions. Improvement is needed in the compatibility of the thermal expansion coefficients, and this improvement will probably involve optimizing the powder composition [28].

Ce-TPZ/A is the toughest ceramic material currently available for FDPs. The thickness of Ce-TPZ/A frameworks can be reduced to 0.3 mm, compared with 0.5 mm for Y-TZP frameworks. Therefore, the amount of tooth preparation required for FDPs can be reduced when using the Ce-TPZ/A frameworks [29]. Y-TZP has a problem of LTAD caused by

phase transformation from the tetragonal to the monoclinic structure [13]. However, Ce-TPZ/A showed complete resistance to LTAD [30]. Therefore, Ce-TPZ/A ceramic frameworks can be exposed to the oral environment with a lingual supporting structure similar to that of conventional metal frameworks.

Although Y-TZP and Ce-TPZ/A are tougher than conventional dental ceramics, veneering porcelain and glassy ceramics are as brittle as conventional porcelain. After the veneering material is placed and baked onto the frameworks in a manual process such as powder build-up and firing, it contains many internal defects that may decrease the resistance to debonding and chipping. Therefore, it seems reasonable to find another solution for applying veneering porcelain automatically.

New hybrid structures have been proposed for FDPs. An example of this type of structure is CAD/CAM-fabricated porcelain veneering with parts adhering to CAD/CAMfabricated zirconia-based ceramic frameworks [31]. In this system, all parts of the FDPs are fabricated by the CAD/CAM process, without manual steps. A reliable adhesive treatment for both parts can be performed in a laboratory, not in a patient's mouth. Adhesive treatments also improve the durability of porcelain. Even if porcelain suffers from chipping during function, repair is easy using the remaining material as a template.

One ultimate solution for the chipping of veneering porcelain is to not use porcelain. Therefore, the opacity of Y-TZP was improved and monolithic full-contour zirconia FDPs were introduced [3]. However, there was concern about wear of the opposing enamel, because the hardness of Y-TZP was over double that of porcelain. According to the current studies, polished zirconia appears to be wear-friendly with opposing enamel, even after simulated aging [32-34]. We need standardized polishing procedures for full-contour zirconia FDPs in both laboratories and clinics. We also need careful observation of the long-term performance to make this application clinically popular.

In this article, the current state and future prospects of zirconia-based new ceramics and their application to FDPs in conjunction with dental CAD/CAM systems are reviewed. Porcelain fused to CAD/CAM-fabricated zirconia frameworks appears to be a promising option in the clinic. However, there are two alternatives that may avoid chipping of veneering porcelains. One is hybrid-structured FDPs comprising CAD/ CAM-fabricated porcelain veneering parts adhering to a CAD/ CAM-fabricated zirconia framework. Another option is full-contour zirconia FDPs. Both are promising because sensitive manual porcelain work is replaced by digital procedures, although we still need longer clinical evaluations to prove the usefulness of these new options.

4. The bond between zirconia and veneering ceramics

4.1. Zirconia and veneering ceramics

One of the specialized ways of using zirconia in dentistry is to fabricate zirconia frames upon which tooth-colored

veneering ceramic is bonded. At present, there are two widely used methods of securing ceramic onto zirconia frames: the layering technique and the press technique. In the layering technique, porcelain powder is applied onto the zirconia frame before firing. In the press technique, the lost wax technique is used to create the restoration. A homogeneous ceramic ingot is heated and then forced under pressure into a wax-formed void. The layering technique is usually used for PFM crowns. It results in excellent esthetics, but several firings are required in order to reproduce the desired color and shape [35]. The virtue of the press technique is easy shaping, however, it is hard to reproduce the desired color because the ceramic ingot used for this technique has only a single color.

For both the layering technique and the press technique, the coefficient of thermal expansion of the veneering ceramic is set to be the same as or slightly lower than that of zirconia. This is because a large difference in the coefficient of thermal expansion between a zirconia frame and veneering ceramic will cause residual stress on the crown, thus resulting in reduced reliability of the restoration [36]. There are some studies comparing the layering technique with the press technique, however, many reports argue that the dislodgement or fracture of veneered ceramics is more affected by frame design than differences in molding techniques [37-39].

4.2. Mechanism and evaluation of integration

Metal-to-porcelain integration of PFM crowns is apparently attained through both mechanical and chemical bonding. Mechanical bonding occurs because porcelain fills the irregularities in the metal surface; this is also called the interlocking effect. Compressive stress caused when the porcelain cools appears to produce this interlocking effect. On the other hand, chemical bonding is the bond between oxygen atoms contained in the porcelain and an oxide film containing tin oxide and indium oxide on the metal frame's surface.

However, there is no clear evidence demonstrating the presence of chemical bonding between zirconia and veneering ceramics, although there is one report [40] suggesting such a bond. It is thus assumed that mechanical bonding plays the major role in the zirconia-to-porcelain integration of zirconia-based restorations.

The bond strength between metal and porcelain is usually evaluated in two ways: a three-point bending test using a thin plate-shaped metallic specimen onto which porcelain is fired, and a shear test using a metallic specimen onto which a disk of porcelain is fired. There are many reports of using a shear test to evaluate the bond strength between zirconia and ceramic (Fig. 2). There is an international standard (ISO9693) for the method of evaluating the bond strength between metal and porcelain using a bending test, and PFM restorations in clinical use are required to have a bond strength of 25 MPa or more [41]. Although there have not been many reports [42-44] concerning the evaluation of zirconia-to-porcelain integration using a bending test (ISO9693), all of those reported that the bond strength was 25 MPa or more. In experiments where the

Table 3

Shear bond strength with different surface treatments (MPa).

Authors (Year) [Ref] Control Sandblast Other treatment

Nakamura et al. (2009) [61] 22.0 27.8-44.3a -

Fischer et al. (2010) [57] 27.0 23.9 -

Kim et al. (2011) [58] 32.0 36.6a 27.8 (porcelain liner)

Teng et al. (2012) [62] 39.1 46.1a 47.2a (powder coating)

Liu et al. (2013) [63] 24.8 31.3a 32.1a (laser irradiation)

a Represent significant differences against control (no treatment) [57].

bond strength between metal and porcelain and that between zirconia and porcelain were compared, it has been reported that the bond strength between metal and porcelain is greater than that between zirconia and porcelain [45,46].

4.3. Factors affecting bond strength

4.3.1. Veneering ceramic

It is known that the strength of the bond between zirconia and veneering ceramic varies greatly with the type of veneering ceramic used [47-49]. This is probably because different veneering ceramics have different coefficients of thermal expansion, causing a mismatch in the coefficient of thermal expansion between zirconia and the veneering ceramic being used [50].

In the layering technique, the number of firings may affect the bond strength. It is reported that, between three to five firings, the greater the number of firings the higher the bond strength [51,52]. However, one report argues that more than six firings will reduce the bond strength [53]. It is also reported that some types of veneering porcelain show changes in crystalline structure as the number of firings is increased beyond a certain number [35], and thus it is preferable to avoid increasing the number of firings unduly.

In addition, some researchers have reported that the cooling rate after firing will also affect the bond strength of ceramic-veneered zirconia restorations [54-56], and thus the cooling rate needs to be set properly to suit the type of porcelain used. It is generally thought that using a porcelain

liner at the start of veneering does not lead to improvement in bond strength [57-59].

4.3.2. Zirconia

Sandblasting is the most widely-used surface treatment method in dentistry. For porcelain-veneered zirconia restorations, the purpose of sandblasting is to produce irregularities on the zirconia to enhance the mechanical bonding between zirconia and veneering ceramic. It has in fact been reported that sandblasting produces changes in the surface topography and surface roughness of zirconia [60].

However, concerning the effectiveness of sandblasting zirconia, some researchers state that this improves the bond strength of porcelain to zirconia [58,61-63], but others maintain that it does not affect the bond strength [40,59,64] (Table 3). This difference is probably because the effect on the zirconia surface varies greatly according to the type, size, and injection pressure of the abrasive particles and also because sandblasting provokes a local tetragonal to monoclinic (t-m) transformation [65].

Monoclinic crystal zirconia transformed by milling or sandblasting can be returned to tetragonal crystals by heat-treating at 1000-1100 °C for 5-10 min. It is reported that such heat treatment does not affect the ceramic to zirconia bond [42]. Furthermore, some reports state that powder coating [62] or laser irradiation of the zirconia surface is effective in improving bond strength.

Bonding between zirconia and veneering ceramics is still in many respects a mystery, including the mechanism involved, partly because this procedure is peculiar to dentistry. Basic

Table 4

Bonding of zirconia with resin-based luting systems.

Adherend material Bonding/luting systems Results Authors (Year) [Ref] Comments from the authors

Zirconia bracket

Yttrium oxide partially stabilized (YPS)

Zirconia

Zirconia post material

In-Ceram Zirconia

InCeram-Zirconia, Frialit

Glass infiltrated zirconia

Procera AllZirkon

Cerapost (Zirconia)

Lava (zirconia ceramic crown)

Lava (97% zirconia stabilized with yttria)

Cercon

Cercon smart ceramics (tetragonal zirconia polycrystals, TZP)

Prismafil, Heliosit (light-cured), Delfic (chemically-cured)

Kevloc, Rocatec, Clearfil FII, Dyract Cem, Panavia EX (with MDP), Panavia 21 EX (with MDP), Twinlook Alumina blasting, HF treating, grinding with diamond burs, Panavia 21, Twinlook, Superbond C&B

Panavia 21, C&B Metabond, Biscore

Particle abrasion with alumina, 10% HF for 20 s

PyrosilPen flame treatment, silane, luting composite

Hydrofluoric acid etching, airborne particle abrasion, tribochemical silica coating, composite material

Clearfil SE Bond/Porcelain Bond Activator, Single Bond/Ceramic Primer, Panavia F, Rely X ARC

Sandblasting and HF etching, Alloy Primer, Metalprimer II, Silane, CoJet Sand, ParaPost Cement, Panavia F Four resin-cement systems, a compomer, a glass-ionomer cement, a resin-modified glassionomer cement, and a self-adhesive resin

Fleck's zinc cement, Fuji I, Ketac-Cem, Fuji Plus, Fuji Cem, RelyX Luting, RelyX ARC, Panavia F, Variolink II, Compolute, RelyX Unicem

CoJet system (tribochemical silica coating), Clearfil Liner Bond 2V (MDP)/Porcelain Bond Activator (silane), Panavia F

Rocatec-system to sandblasted TZP, Ketac-Cem, Nexus, RelyX Unicem, Superbond C&B, Panavia F, Panavia 21

Heliosit, Delphic > Prismafil

Panavia EX, Panavia 21 EX > others

Washing with hydrofluoric acid had no significant influence on bond strength

Panavia 21 > Biscore > C&B Metabond

Particle abrasion of In-Ceram Zirconia did not change the morphologic characteristics

Empress II, InCeram-Alumina > Frialit > InCeram-Zirconia

Acid etched glass ceramics 26.429.4, glass infiltrated alumina ceramics 5.3-18.1, zirconia 8.1 MPa

Silane/phosphate bonding agent was effective for both systems

Bonding of both resin cements to zirconia posts was improved by Cojet treatment

Superbond C&B (+ Rocatec) specimens showed the highest median retentive strength

Resin cement 9.7, 12.7 MPa

The MDP/silane mixture increased the shear bond strength to zirconia

RelyX Unicem, Superbond C&B, Panavia F, and Panavia 21 gave superior results

Springate and Winchester (1991) [66]

Kern and Wegner (1998) [67]

Dérand and Dérand (2000) [83]

O'Kééfé ét al. (2000) [68]

Borges et al. (2003) [84]

Janda et al. (2003) [73]

Ozcan and Vallittu

(2003) [74]

Blatz ét al. (2004) [78]

Sahafi ét al. (2004) [75]

Ernst ét al. (2005) [76]

Piwowarczyk et al. (2005) [77]

Atsu et al. (2006) [79]

Luthy ét al. (2006) [69]

All specimens failed at the bracket-adhesive interface. Highly opaque appearance may adversely affect bonding with light-cured adhesives MDP in the two composites is effective for bonding the YPS

Superbond showed a bond strength reasonably acceptable for clinical use

Panavia 21 is effective for bonding the zirconia prefabricated post material Hydrofluoric acid etching of In-Ceram Zirconia and Procera did not change their morphologic microstructure PyrosilPen is an effective method for treating zirconia to obtain bonding to luting composites Silica coating with silanization increased the bond strength for glass infiltrated zirconia compared to that of airborne particle abrasion

A bonding/silane coupling agent containing MDP can achieve superior long-term bond strength to Procera AllZirkon with two luting agents

Air abrasion with silica acid-modified alumina (CoJet Sand) improved bonding to zirconia of two cements The compomer-cement, the resin-modified glass-ionomer cement, and the self-adhesive resin luting agent had the same level of retentive quality as the resin luting agents When using the Rocatec system, the highest values were found for one of the resin cements

CoJet system and the application of an MDP-containing bonding/silane coupling agent mixture increased the bond strength between zirconia and Panavia F

The strongest bond to zirconia was obtained with Panavia 21

Table 4 (Continued )

Adherend material

Bonding/luting systems

Results

Authors (Year) [Ref]

Comments from the authors

Katana (YPS zirconia)

Cercon smart ceramics (tetragonal zirconia polycrystals, TZP)

Katana (YPS zirconia)

Katana (YPS zirconia)

In-Ceram Zirconia

Left untreated, airborne-particle abraded, Rocatec tribochemical silica/silane, ground and polished, RelyX ARC, RelyX Unicem, Panavia F, RelyX Luting

Rocatec Soft, Espe Sil, Epricord, RelyX ARC

Alumina blasting, tribochemical silica coating, no treatment, Calibra, Clearfil Esthetic Cement, RelyX Unicem

Acryl Bond, All Bond II Primer B, Alloy Primer, Estenia Opaque Primer, Eye Sight Opaque Primer, M.L. Primer, MR. Bond, SuperBond Liquid, tri-n-butylborane (TBB)-initiated acrylic resin Ceramic Primer, Monobond Plus, Clearfil Esthetic Cement, Clearfil SA Cement, Panavia F2.0, Variorink II

No treatment, sandblasting, CoJet + silane, CoJet + Alloy Primer, glaze + 9.6% HF etching 60 s + silane, Panavia F2.0

Rocatec generally yielded the highest long-term shear bond strength

Blatz et al. (2007) [80]

The silica-coating of YPSZ ceramics by tribochemical modification was not efficient, given the higher mechanical toughness of the densely sintered ceramics

Bond strength of Clearfil Esthetic Cement to zirconia was significantly higher than that of others, regardless of the surface treatment

The highest post-thermocycling bond strength was obtained with the use of Alloy Primer and Estenia Opaque Primer

Clearfil SA Cement and Panavia F2.0 showed durable post-thermocycling bond strength

The highest tensile bond strength for the enamel surfaces was obtained in group; glaze + HF etching + silane

Tanaka et al. (2008) [81]

de Oyagüe et al. (2009) [70]

Nakayama et al. (2010) [82]

Koizumi et al. (2012) [71]

Saker et al. (2013) [72]

Airborne-particle abrasion combined with a resin composite containing MDP or tribochemical silica/silane coating combined with the tested resin luting agents provides superior long-term bond strengths Stable shear bond strength was achieved on silica-coated YPSZ ceramics with the cooperative interaction of phosphate monomer and silane coupling The luting system with MDP (Clearfil Esthetic Cement) is recommended to bond zirconia

Application of Alloy Primer or Estenia Opaque Primer, containing MDP, is recommended for bonding the zirconia material with TBB-initiated acrylic resin Application of resin-based luting and priming agents containing MDP provide better bond strength to zirconia than do other systems Adhesion of zirconia to enamel and dentin can be improved when the specimens are glazed, etched, and silanized, or sandblasted, primed, and cemented with Panavia

research in this field and development of a reliable clinical procedure will be necessary in the future.

5. Bonding of zirconia with resin-based luting agents

5.1. Adhesive bonding to zirconia

Adhesive behavior of zirconia was primarily evaluated as bonding between orthodontic brackets and adhesive resin. Springate and Winchester [66] assessed two light-curing composite resins and a chemically curing composite resin for bonding a zirconia bracket material. The result showed that one of the light-curing materials exhibited statistically lower bond strength than the other two materials. The authors pointed out that the opaque appearance of the zirconia negatively affects bonding with light-curing luting agents. Their results suggested selection of chemically curable resin-based luting agents for cementing zirconia restorations. Table 4 summarizes the reports concerning bonding of zirconia with resin-based luting agents.

5.2. Resin-based luting systems with methacryloyloxydecyl dihydrogen phosphate

Kern and Wegner [67] assessed bonding of an yttrium oxide partially stabilized (YPS) zirconia ceramic using varying bonding systems. Their results demonstrated effectiveness of two luting agents containing a hydrophobic phosphate monomer, 10-methacryloyloxydecyl dihydrogen phosphate (MDP), for bonding to the zirconia. Several researchers thereafter reported that composite materials containing MDP enhanced bond strength to zirconia prefabricated post material [68], tetragonal zirconia polycrystals (TZP) [69,70], YPS zirconia [71], and In-Ceram zirconia [72]. Oyagiie et al. [70] reported that a phosphate monomer-containing luting system is recommended to bond zirconia and surface treatments are not necessary.

5.3. Surface modifications of zirconia

Techniques for modifying zirconia surface mechano-chemically with inorganic silicon compounds followed by

Table 5

Diamond rotary instruments and polishing pastes.

Name(Manufacturer) Composition of abrasives Composition of binder

Grinding rotary instrument SinterDia (Shofu) Diamond Point FG (Shofu) VitrifiedDia (Shofu) Aadva point Zr (GC) CeramDia (Morita) Pro-tec diamond point (Kuraray Noritake Dental) Porcelain Hi-glaze (Dedeco) Diamond (C) Diamond (C), Corundum (Al2O3), Anatase (TiO2) Diamond (C), Corundum (Al2O3), Anatase (TiO2), Zinc oxide (ZnO) Diamond (C), Corundum (Al2O3), Rutile (TiO2) Diamond (C), Rutile (TiO2) Metal sintering Metal plating (Ni, Cr) Glass Artificial rubber

Name (Manufacturer) Composition of abrasives Polishing instrument

Polishing paste DirectDia Paste (Shofu) Diapolisher Paste (GC) DuraPolish Dia (Shofu) Zircon-Brite (DVA) Zirkopol (Feguramed) Pearl Surface Z (Kuraray Noritake Dental) Diamond (C), Anatase (TiO2), Glycerin Diamond (C), Zinc oxide (ZnO), Glycerin Diamond (C), Pumice (SiO2), wax Diamond (C), Corundum (Al2O3), Pumice (SiO2), wax Diamond (C), Corundum (Al2O3), Pumice (SiO2), wax Diamond (C), Silicon carbide (SiC), wax Super-snap buff disk Felt, Brush, PTC cup Felt Felt, Brush Brush

Table 6

Studies on wear of antagonist against zirconia.

Author (Year) Materials Antagonist Condition Results References

Kumar Zirconia (Y-PSZ), Polyethylene Unidirectional wear (3 MPa Different lubricant fluid media had [102]

et al. (1991) Alumina, and xSUS316L cylinder load, 60 mm/s, total 30- little effect on the polyethylene wear

f = 4 mm 40 km) and reciprocating against ceramic counterfaces, but

or 9 mm wear (3.45 MPa, 50 mm were prominent against SUS316L

sliding distance, 60 cycles/ metal. Y-PSZ ceramic may be a

min, 1,300,000 times) in biomaterial potentially suitable for

lubricant fluid medium low friction arthroplasty because of

(distilled water, human blood its better wear resistant properties and

plasma, physiological saline high strength

solution)

Tambra Polished zirconia, surface Human enamel Rotation, 500 g load, 60 The zirconia caused greater enamel [103]

et al. (2003) treated zirconia, and cycles/min, 10,000 cycles wear than did the gold control

Type 4 gold alloy

Culver Cercon, Lava, Empress, Human enamel Modified Leinfelder wear Cercon and Lava showed larger [104]

et al. (2008) MZ100, and Z100 testing machine, 75 N load, enamel loss than others

20,000 cycles in Slurry (15 g

of f = 50 mm PMMA beads

and 9 g of water)

Shar et al. Polished and glazed Human enamel Modified Leinfelder wear Polished zirconia showed larger [105]

(2010) zirconia testing machine, 75 N load, enamel loss than glazed one

1.2 Hz, 10,000 cycles in

Slurry ((15 g of f = 50 mm

PMMA beads and 9 g of

water)

Jung et al. Glazed, polished zirconia, Human enamel Chewing simulator, 240,000 The antagonist wear of three CAD/ [106]

(2010) polished porcelain cycles CAM full contour zirconia ceramics

veneered zirconia was significantly less than that of the

veneering ceramic

Albashaireh e.max ZirCAD, e.max Zirconia balls Dual-axis mastication Wear was of the fatigue type, and was [107]

et al. (2010) Press, Empress Esthetic, f = 6 mm simulator, 300,000 significantly lowest in the zirconia

e.max ZirPress, e.max mastication cycles specimens tested

Sorensen Omega 900, Empress, Human enamel OHSU oral wear simulator, 20 Polished Lava showed small enamel [108]

et al. (2011) Bovine enamel, d. sign, and 70 N load, 50,000 times in loss and nearly the same with that of

Lava, Aquarius, Empress slurry (poppy seeds/PMMA Gold alloy (Aquarius)

2 beads)

Table 6 (Continued )

Author (Year)

Materials

Antagonist

Condition

Results

References

Basunbul et al. (2011)

Preis et al. (2011)

Kuretzky et al. (2011)

Yang et al. (2012)

Janyavula et al. (2013)

Kontos

et al. (2013)

Stawarczyk et al. (2013)

Polished and glazed Wieland zirconia, polished Ceramco 3, polished Mark II

Five zirconia and four veneering porcelains

Rough, polished, glazed, and veneered Lava zirconia and e.max CAD

Zirkonzahn Y-TZP (polished, stained, stained then glazed), Acura Y-TZP, Wieland Y-TZP, a feldspathic porcelain

Polished, glazed, polished then reglazed, and porcelain veneered Lava, molar enamel Zirconia (a)was only fired, (b) sandblasted, (c) ground, (d) polished, and (e) glazed Mechanically and manually polished, glazed, spray glazed, and veneered zirconia, and a base alloy

Human enamel

Steatite sphere f = 3 mm or enamel

Steatite balls f = 6 mm

Human enamel

Human enamel

Steatite balls f = 6 mm

Human enamel

400 g load, 6 mm reciprocating moving, 60,000 and 600,000 cycles in water

Chewing simulator, 50 N load, 120,000 cycles (1.6 Hz, lateral movement 1 mm, mouse opening 2 mm) Longitudinal moving notch device, 5 and 50 N load, path length 32 mm, 72 cycles/min for 120 min

Chewing simulator, 240,000 cycles

University of Alabama wear testing device, 10 N load, 20 cycles/min, 400,000 cycles in 33% glycerin solution Pin-on-disk, 45°, 5 N load, 5000 cycles, water

Chewing simulator, 49 N load, 1,200,000 cycles (1.7 Hz, horizontal distance 2 mm) and thermal stress (5-50 °C every 120 s)

Polished zirconia caused significantly [109]

less wear to enamel than either the

glazed zirconia, Ceramco porcelain

and Cerec Mark II. The polished

zirconia remained unchanged, but the

glazed zirconia showed significant

loss of the glazed layer

Antagonist wear against zirconia was [32] found to be lower than wear against porcelain

Polished zirconia showed the least [110]

wear after abrading with a steatite

sphere

Antagonist wear of three Y-TZP was [111] significantly less than veneering porcelain because the surface character of Y-TZP is relatively homogeneous. Zirkonzahn with staining and glazing was significantly more abrasive than the other Y-TZP without glazing

Highly polished zirconia is more [112]

desirable than the glazed zirconia

Polished zirconia seems to have the [113] lowest wear on the antagonist, in contrast with the other kinds of surface treatment

Polished zirconia showed lower wear [114] rate on enamel antagonists as well as within the material itself but developed higher rate of enamel cracks

application of silane monomers have been introduced. Janda et al. [73] compared bonding performance of silica, alumina, and two zirconia ceramic materials treated with a flame treatment and silane priming. The results showed that the silica and alumina ceramics showed higher bond strength than the zirconia ceramic materials, although the flame treatment was effective for all ceramic materials. Ozcan and Vallittu [74] evaluated the effect of mechanical and chemical retentive systems on bonding zirconia. The results showed the effectiveness of silica coating and subsequent silane treatment on bonding to glass infiltrated zirconia. Sahafi et al. [75] confirmed the effectiveness of a tribochemical coating system on bonding to zirconia post material. Ernst et al [76], Piwowarczyk et al. [77], and Luthy et al [69] reported usefulness of another tribochemical coating system for bonding zirconia.

5.4. Silica/silane and MDP

It is also reported that combined application of silica coating, silane, and MDP is currently one of the most

reliable bonding systems for zirconia [78-81]. Blatz et al. [78] demonstrated effectiveness of a silane/phosphate bonding agent for cementing zirconia restorative material. This procedure does not necessarily require another mechano-chemical treatment before application of the silane/phosphate bonding agent. Tanaka et al. [81], however, concluded that stable bond strength was achieved on Rocatec-coated Katana zirconia with the cooperative interaction of phosphate monomer and silane, which was analyzed by means of X-ray photoelectron spectroscopy. This bonding mechanism is substantially the same mechanism as bonding to feldspathic porcelain with silane/MDP bonding agent.

5.5. Unfilled luting agent

Bonding to zirconia of unfilled acrylic luting agent was not particularly excellent [68]. This weak point, however, has been improved by application of a tribochemical coating [76]. Nakayama et al. [82] evaluated bonding between an YPS zirconia and a tri-n-butylborane (TBB)

Fig. 3. Diamond rotary instruments. (a) SinterDia HP30R; (b) Super Course SC106RD; (c) VitrifiedDia HP20; (d) CeramDia SF.

Fig. 4. Diamond rotary instruments, Dodeco Hi-glaze diamond polishing kit.

Fig. 5. Diamond polishing pastes. (a) DirectDia paste; (b) Diapolisher paste; (c) Zircon-Brite; (d) Zirkopol; (e) Dura-PolishDia; (f) Pearl Surface Z.

initiated luting agent in combination with eight primers. Among them, application of either the Alloy Primer or the Estenia Opaque Primer (Kuraray), both of which contain MDP, exhibited durable bonding between the zirconia and the TBB-initiated luting agent.

5.6. Mechanical retention

Etching zirconia with acidic etchant is currently difficult [83,84]. Although a reliable mechanical retentive system between resin material and zirconia is unachievable, laboratory

and clinical studies on macro mechanical as well as mechano-chemical retention of zirconia is being continued.

6. Surface finish of zirconia restorative and antagonist enamel wear

Various ceramics have been used as dental restoratives. In terms of mechanical strength [30,85-87] and physical propertie [88-90], there is no doubt the superiority of zirconia. When zirconia is used for esthetic dental restoratives such as crowns and bridges, it is generally veneered with feldspathic porcelain, because zirconia has an insufficient translucency. However, the strength of the veneering porcelain is not enough to act as dental restoratives, especially for posterior teeth. It is known that the clinical failure has been reported to be mostly due to chipping of porcelain [91,92]. Recently, high translucent zirconia has been introduced into dentistry [93,94]. It can be used as all zirconia restoratives, so-called "Full Contour'', without covering the veneering porcelain, indicating its zirconia surface is exposed to the oral cavity. Then, the wear of opposing teeth is an important and interesting issue. In order to prevent wear of the antagonist enamel, the mirror polishing is undertaken in the dental laboratory and in the oral cavity for occlusal adjustment. On the other hand, some dentists misunderstand that the enamel opposing to zirconia restoratives is easy to wear because of the hardness of zirconia. Furthermore, effects of the glazing on zirconia are uncertain whether this coating is effective on the prevention of antagonist wear or not. Veneering porcelains have also come to be questioned about the antagonist wear. Recent studies on wear of antagonist enamel demonstrated mostly that adequate surface finish of zirconia restoratives resulted in the least wear of antagonist enamel among various dental materials. These results suggest that the antagonist enamel wear is significantly affected by the degree of surface finish. This review outlines the method for surface finish of zirconia restoratives and their effects on the wear of antagonist enamel.

6.1. Grinding and polishing of zirconia restoratives

As described above, in order to prevent wear of the antagonist enamel, the mirror polishing is undertaken in the dental laboratory and in the oral cavity for occlusal adjustment. Previously, we reported a comparative study on mirror polishing methods of the zirconia surface [64,95]. Based on this study, the grinding and mirror-polishing manner for zirconia are described first. Table 5 shows name, manufacturer name, the composition of the grinding rotary instruments, and polishing pastes available for zirconia.

6.1.1. Grinding rotary instruments

The hardness of zirconia is high (HV 1,160-1,300), but lower than alumina (HV 1,800-2,200) and diamond (HV 10,200). Therefore, zirconia can be easily processed by the instruments coated with diamond abrasive grains. As shown in Table 6, the grinding rotary instruments for zirconia contain diamond

Fig. 6. Polishing cups and brush. (a) Super snap buff disk; (b) PTC cup; (c) Robinson brush.

abrasives in high density which are fixed with metal, glass, and artificial rubber to a stainless steel shaft. Figs. 3 and 4 show some examples of diamond rotary instruments.

Generally, diamond rotary instruments fix diamond abrasive grains to the stainless steel shaft with a nickel-chromium plating. "Super Course'' fixes twice-size diamond grains (100-300 mm) than usual ones by the plating, resulting in almost double grindability than usual ones. On the other hand, ''SinterDia'' fixes diamond grains by sintering of metal to a stainless steel shaft. Consequently, it possibly results in preventing diamond grains falling off into the high-density packing, indicating high grindability and durability [96].

"VitrifiedDia" fixes diamond grains with glass. "Aadva Point Zr'', "CeramDia", and "Porcelain Hi-glaze'' fix diamond grains and other oxides such as corundum (Al2O3) and anatase or rutile (TiO2) with artificial rubber. Diamond grain sizes of ''CeramDia'' M, F, and SF are 100-200, 30-60, and 3-6 mm, respectively [97].

It has been confirmed that larger diamond grains show higher grindability for zirconia [98]. However, the surface roughness is also large. Therefore, the rotary instrument should be changed sequentially from a large to small grain size of the diamond abrasives of the instrument. Consequently, this manner results in a fast and homogeneous smooth surface, and enables a fast move to the next step, i.e. polishing.

Cercon P-NAN02R*^ îJÉIE**»" ÄSJ m ï' s jscäjMF'C -M èÈ? . J» ^WJfc,,^® T4ÊÊ V inCoris AL MM in

100 nm M^JÊmm^Sm M' 100 nm \ T3rrT

VitaBlocs N - ■ V-, ' 'm.jF-/ V „. ^ • > t J. - .UHi , vV - e.max CAD / L ¿f T Vintage mjS^ \ \ \ M 9 ¡¿^ ^HBS

t„ - N v"L pm 100~m „ <mmbe1i

Fig. 7. Scanning electron micrograph of six types of dental ceramics.

6.1.2. Diamond polishing paste

Fig. 5 shows some examples of polishing pastes for zirconia. The diamond pastes mainly contain diamond grains (1-6 mm) and fine other oxides (less 0.5 mm) such as anatase (TiO2), corundum (Al2O3), zinc oxide (ZnO), and Pumice (SiO2) [97]. These diamond pastes are usually used to polish with plastic or rubber cone and soft brush (Fig. 6). ''Super snap buff disk'' consists of TiO2 and polyester. ''PTC Cup'' consists of TiO2, ZnO, and artificial rubber. ''Robinson brush'' consists of hard fibers such as horse hair or soft fibers such as sheep hair. ''DirectDia paste'' and ''Diapolisher paste'' can be applied to the mirror polishing with plastic or rubber cone after occlusal adjustment in the oral cavity. Other pastes are used mainly with Robinson brush in laboratories.

6.1.3. Polishing of dental ceramics

The surface roughness of the ground and polished ceramic is largely governed by the microstructure of the ceramic. And, a variety of materials have been used as dental ceramics.

In our previous study, we measured the surface roughness of seven types of dental ceramics finished with three diamond grinding instruments and two diamond pastes [64,95].

Fig. 7 shows scanning electron micrographs of dental ceramics used in the study. Cercon is a Y-TZP (yttria-stabilized tetragonal zirconia type) having a high density sintered body of about 0.3 mm grain size after the final firing at 1350 °C. Although not shown, ''ZENOSTAR'' is also a Y-TZP fired at 1450 °C, and classified to high translucent type having a particle size of about 0.4 mm. ''P-NANOZR'' has an interpenetrated intragranular nanostructure, in which either nanometer-sized Ce-TZP (ceria-stabilized tetragonal zirco-nia) or Al2O3 particles locate within submicron-sized Al2O3

Fig. 8. Surface roughness of seven types of dental ceramics finished with three types of diamond rotary instruments and two types of diamond polishing pastes.

or Ce-TZP grains, respectively. The average grain size of this composite was about 0.5 mm. This material design makes it possible to strengthen the 10 mol% Ce-TZP matrix with 30 vol% Al2O3 [11,99]. ''inCoris AL'' is a high-density sintered body having a particle size of 1 mm after the final firing at 1500°C [100]. ''Vitablocs'' is a CAD/CAM block containing about 30 vol% feldspar crystal (Sanidin) grains of 2-10 mm dispersed in the glass [99]. ''e.max CAD'' is a CAD/ CAM block containing about 70 vol% elongated lithium disilicate grains of about 1.5 mm dispersed in the glass [100]. ''Vintage ZR'' is a feldspathic veneering porcelain for zirconia, consisting of about 4.5 wt% leucite crystal of 510 mm, dispersed in the glass [101].

Fig. 9. Relation between average surface roughness and hardness (left) and between average surface roughness and crystal grain size (right) of seven dental ceramics finished with three types of diamond grinding bar and two types of diamond polishing pastes.

Fig. 10. Surface roughness of three types of dental zirconia finished with 13 types of grinding and polishing condition.

Fig. 8 shows the surface roughness Ra of seven types of dental ceramics after grinding and polishing. Polishing with diamond pastes such as DirectDia paste and Zircon-Brite was undertaken after grinding sequentially with CeramDia M, F, and SF. According to the size of diamond grains of the grinding rotary instruments, the surface roughness decreased in all the dental ceramics. The roughness was further reduced by the following polishing. In particular, three zirconia products (Cercon, ZENOSTAR, and P-NANOZR) showed the minimum roughness after each grinding and polishing. On the other hand, Vitablocs and Vintage ZR showed large roughness. Fig. 9 shows the relation between the average surface roughness of seven dental ceramics after three grindings and two polishings shown in Fig. 6 and the Vickers hardness of each ceramic (left), and relation between the average surface roughness and the average size of crystal grains (right). The surface roughness

after grinding and polishing was independent of the hardness, but strongly depended on the crystal grain size. It has been suggested that the surface roughness of dental ceramics after grinding and polishing depend highly on the microstructure. Therefore, it is concluded that zirconia can be polished to a smooth surface due to the homogeneous and fine microstructure.

6.1.4. Polishing of zirconia

Fig. 10 shows the surface roughness of three types of dental zirconia finished with 13 types of grinding and polishing. Super Course, SinterDia, VitrifiedDia, and CeramDia M, F, and SF are grinding rotary instruments. Super Course, SinterDia, and VitrifiedDia showed large surface roughness, greater than 1 mm. On the other hand, CeramDia M, F, and SF showed relatively low roughness. It possibly depends on the diamond

Fig. 11. Glossiness of three types of dental zirconia finished with 13 types of grinding and polishing condition.

grains fixed with artificial rubber. Polishing with diamond pastes such as Diapolisher paste, DirectDia paste, Zircon-Brite, and Zirkopol was undertaken after grinding sequentially with CeramDia M, F, and SF. The polishing made a further smooth surface, and there were no significant differences in type of zirconia and in type of diamond polishing paste. ConCool, Pressage, and PTC regular are cleaning pastes for professional mechanical tooth cleaning (PMTC) operations. The polishing with these pastes after polishing with DirectDia paste showed no change in the surface roughness.

Fig. 11 shows the glossiness at 60° of the same specimens shown in Fig. 8. The glossiness increased with decreasing the size of diamond grains of grinding rotary instruments and increased more with further polishing. However, PMTC pastes showed no remarkable change. Because diamond is not included in the PMTC pastes which are composed of abrasive grains of silica, it means that the PMTC operation is not affected on both surface roughness and gloss of zirconia restoratives mounted as full contours in the oral cavity, indicating no interference with maintenance of good oral hygiene.

Fig. 12 shows the correlation between the glossiness and the surface roughness. The glossiness increased steeply with decreasing roughness to less than 0.3 mm. It means that the final gloss of zirconia restoratives is determined whether the final polishing is enough or not.

6.2. Studies on the wear of antagonist against zirconia

Table 6 shows the summary of antagonist wear test studies on zirconia in the past two decades [102-114].

6.2.1. Friction study in arthroplasty

Studies on the wear against zirconia have been conducted for more than 20 years in the field of orthopedics. A variety

Fig. 12. Relation between surface roughness and glossiness of three types of dental zirconia finished with 13 types of grinding and polishing condition.

of materials have been used in the femoral head and cup of artificial hip joints and research interest has been paid to wear of the combination of various materials of these. The first interest of antagonist wear against zirconia was concern to the wear of femoral cups made of high-density polyethylene.

In 1991, Kumar et al. [102] employed three types of materials (zirconia, alumina, and stainless steel) and two types of wear test (unidirectional rotary motion and reciprocating motion) in three types of lubricant fluid (distilled water, human blood plasma, and physiological saline solution). They demonstrated that different lubricant fluid media had little

effect on the polyethylene wear against ceramic counterfaces, but were prominent against SUS316L metal. They concluded that Y-PSZ ceramic is a biomaterial potentially suitable for low-friction arthroplasty because of its better wear-resistant properties and high strength. It was confirmed that soft antagonists such as polyethylene rarely wear on zirconia, although zirconia is quite hard. This fact implies that the hardness of the materials is independent on the susceptibility of antagonist wear.

6.2.2. Wear studies using enamel in the 2000s

Zirconia began to spread to the dental field in the 2000s and entered the mature stage in the 2010s. With the development of peripheral technology of zirconia, the conclusion about the antagonist wear against zirconia crown restoration has changed.

At the International and American Association for Dental Research (IADR) 2003, Tambra et al. [103] reported that zirconia caused greater enamel wear than did the IV gold control, although the polished zirconia caused less wear to the enamel abrader than the processed zirconia. They described that the surface was mirror-polished with diamond paste. However, the polishing method and the smoothness of zirconia were not indicated.

At the American Association for Dental Research (AADR) 2008, Culver et al. [104] determined the wear of premolar enamel against five types of materials (Cercon, Lava, Empress, MZ100, and Z100) using a modified Leinfelder wear testing machine. They reported that zirconia (Cercon and Lava) caused more enamel loss than composite resins (MZ100 and Z100) and leucite-containing glass (Empress).

At the AADR 2010, Shar et al. [105] determined the wear of premolar enamel against polished and glazed zirconia using a modified Leinfelder wear testing machine. They reported that the polished zirconia showed larger enamel loss than the glazed one.

The polishing conditions of these reports were unclear. In the 2010s, various polishing materials and instruments for zirconia have been introduced and the conclusion began to change.

6.2.3. Wear studies using enamel in the 2010s

In 2010, Jung et al. [106] measured enamel loss against three types of surface-treated zirconia (Zirkonzahn Prettau). They reported that the enamel loss on the mirror-polished zirconia was significantly less than those of glazed and porcelain-veneered ones. On the other hand, Albashaireh et al. [107] measured the loss of five dental ceramics (e.max ZirCAD, e.max Press, Empress Esthetic, e.max ZirPress, e.max Ceram) against zirconia balls using dual-axis mastication simulator. They demonstrated that the degree of antagonistic tooth wear was less in zirconia than feldspathic dental porcelain, representing that the zirconia may be more beneficial in terms of antagonistic tooth wear (Fig. 13).

At the IADR 2011, Sorensen et al. [108 measured the enamel wear against seven types of materials (Omega 900, Empress, Bovine enamel, d. sign, Lava, Aquarius, and

Fig. 13. Wear loss of five dental ceramics against zirconia ball (f = 6 mm) after 300,000 mastication cycles. Graphing of the data in [107].

Fig. 14. Wear loss of enamel against four different surface treated zirconia and enamel after 400,000 chewing cycles. Graphing of the data in [112].

Empress 2) using the Oregon Health & Science University (OHSU) oral wear simulator. They reported that the polished Lava showed small enamel loss similar to that of gold alloy (Aquarius). At the same meeting, Basunbul et al. [109] reported the enamel wear of four types of materials. They demonstrated that polished Wieland zirconia caused significantly less wear to enamel than the glazed Wieland zirconia, Ceramco porcelain, and Cerec Mark II. They concluded that the polished zirconia remained unchanged, but the glazed zirconia showed significant loss of the glazed layer.

At the IADR 2012, Yang et al. [111] measured the enamel wear against Zirkonzahn Y-TZP (polished, stained, stained then glazed), Acura Y-TZP, Wieland Y-TZP, a feldspathic porcelain using the University of Alabama wear-testing device. They demonstrated that the antagonist wear of the three Y-TZP products was significantly less than veneering porcelain because the surface character of Y-TZP is relatively homogeneous, and Zirkonzahn with staining and glazing was significantly more abrasive than the other Y-TZPs without glazing.

In 2013, Janyavula et al. [112] measured the loss of molar enamel of four types of surface-treated zirconia (Lava). They concluded that highly polished zirconia is more desirable than glazed zirconia (Fig. 14). Furthermore, Stawarczyk et al [114] measured the enamel loss of three types of surface-treated zirconia (ZENOTEC Zr Bridge Translucent) and a base alloy (Denta NEM, CoCr alloy) using a chewing

Mechanically polished zirconia Manually polished zirconia Spray glazed zirconia Glazed zirconia Veneered zirconia CoCr alloy

0 20 40 60 80 100 120 140

Wear loss of enamel (|jm)

Fig. 15. Wear loss of enamel against five different surface-treated zirconia and a CoCr alloy after 1,200,000 chewing cycles. Graphing of the data in [114].

Cercon Ceram Kiss Creation Zi-F Vita Omega 900 Cercon Base sinter-glaze Lava Ceram Cercon Base 120mm-glaze Cercon Base Lava Digizon Enamel Zeno Zr-Bridge Ceramill Zi-T-YZP

Wear loss of steatite (mm)

Fig. 16. Wear loss of steatite balls (f = 3 mm) against five zirconia and four veneering porcelains after 1,200,000 chewing cycles. Graphing of the data in [32].

Fig. 17. Wear loss of steatite balls (f = 6 mm) against four surface treated zirconia and e.max CAD after 120-min longitudinal moving at 72 cycles/min Graphing of the data in [110].

simulator. They reported that the polished zirconia showed a lower wear rate on enamel antagonists as well as within the material itself (Fig. 15).

6.2.4. Wear studies using steatite

On the other hand, there were no reliable clinical reports because of large variation of measurement values and conditions. As a substitute for human enamel, steatite (MgOSiO2) has been frequently used as an antagonist material due to similar wear behavior to human enamel [115— 118]. In 2011, Preis et al. [32] measured the loss of steatite and enamel of five zirconia and four veneering porcelains using a chewing simulator. They reported that antagonist wear against zirconia was lower than the wear against porcelain (Fig. 16). Kuretzky et al. [110] measured the enamel loss against four kinds of surface-treated zirconia (rough, polished, glazed, and veneered Lava) and e.max CAD using a longitudinal moving notch device. They demonstrated that the polished zirconia showed the least wear after abrading with a steatite sphere (Fig. 17).

In 2013, Kontos et al. [113] measured the loss of steatite against five types of surface-treated zirconia using a chewing simulator. They concluded that the polished zirconia seems to have the lowest wear on the antagonist, in contrast to the other types of surface treatment (sandblasted, ground, and glazed) (Fig. 18).

According to these studies on antagonist wear, it is summarized as follows.

• A smooth surface of zirconia can be obtained with adequate polishing, because the microstructure of zirconia is fine and homogeneous. Highly polished zirconia shows the least wear of antagonist among various dental materials.

• Glazed zirconia shows higher wear loss than that of polished zirconia, although the surface of glazed zirconia is smooth before wear testing. Because the thin glaze layer (ca. 100 mm) disappears after a period of function, consequently a rough surface appears, which can act aggressively as an abrasive surface [107,113].

glazed polished ground sandblasted as fired

0 20 40 60 80 100 120 140 Wear loss of steatite (|jm)

Fig. 18. Wear loss of steatite balls (f = 6 mm) against five surface-treated zirconia after 5000 cycles. Graphing of the data in [113].

• Porcelain-veneered zirconia shows higher wear loss than that of polished zirconia, because porcelain consists of a feldspathic glass and leucite crystal grains (ca. 10 mm). The glass easily disappears after wear such as mastication, consequently large leucite grains are exposed and act as abrasive materials.

6.3. Prevention of antagonist enamel wear against zirconia restoratives

When dental zirconia is used as the full contour, the wear of antagonist enamel is a concern because zirconia is very hard. However, it is a misunderstanding. This review describes the method for surface finishing of zirconia restoratives and its effect on the wear of antagonist enamel. The correlation between hardness and wear is small [97]. The wear strongly depends on the homogeneity and particle size of the microstructure of the restorative material. Because zirconia has a fine uniform structure, it is suitable for mirror polishing by using appropriate polishing materials and instruments containing fine diamond particles. There is no need to fear the wear of the enamel of opposing teeth against zirconia restoratives. Vice versa, the wear of antagonist enamel is large when the surface roughness of zirconia restoratives is large. Therefore, when zirconia restoratives are ground for occlusal correction, their surface should be sufficiently mirror-polished. Furthermore, glazing is not recommended for the surface finish of zirconia.

7. Clinical evaluation of zirconia restoration

7.1. Clinical outcome

To date, PFM restorations remain the most widely and successfully used options for FPDs since their failure rates are often low (8-10% within 10 years). Overall, the clinical survival rates of FPDs are between 72% and 87% after 10 years, between 69% and 74% after 15 years, and 53% after 30 years [4,119,120]. However, as is well-known, the metals used in PFM restorations have the potential to cause allergic

or toxic reactions within soft or hard tissue. Also, PFM is known to cause graying of the gingival margin because of metal show-through.

The increased use of ceramics for restorative procedures and demand for improved clinical performance has led to the development and introduction of several new ceramic restorative materials and techniques. PFM restorations became available for dentistry in the 1960s followed by Dicor glass ceramics (Dentsply Intl, York, PA, USA), the castable Fluormica Glass-Ceramic in the 1980s, the installation of systems such as VITABLOCS® MARK II for CEREC® (Vita), In-Ceram® ALUMINA (Vita), and IPS Empress (Ivoclar-Vivadent) etc. of the early 1990s. Y-TZP-based systems are a recent addition to the high-strength, all-ceramic systems used for crowns and fixed partial dentures [121,122]. CAD/CAM-produced Y-TZP-based systems are in considerable demand in esthetic and stress-bearing regions. The highly esthetic nature of zirconia with its superior physical properties and biocompatibility makes it an effective restorative system to meet the demands of modern patients [123-125]. Currently, endowing a removable knob to the dental prosthesis apparatus has made it possible to treat temporary cementation. Clinical fractures of all-ceramic crowns and FPDs have rarely been identified.

Crowns are reported to spoil from the cavital cementation surface, which is opposite the chewing surface whereas all-ceramic FPDs spoil at their connectors [126-128]. The past decade has seen the unprecedented introduction of a myriad of all-ceramic crown systems. Many of these systems have been criticized for their failure in restorations. It has been reported that the survival rates for all-ceramic restorations range from 88% to 100% after 2-5 years in service and up to 97% after 5-15 years of service [129-138]. Although all-ceramic restorations have improved considerably, zirconia is undoubtedly the best all-ceramic restoration available. Since the end of the 1990s a form of partially stabilized zirconia has been promoted as being suitable for dental use because of its excellent strength and superior fracture resistance as a result of numerous clinical and basic scientific studies [4,139]. To gain the strength benefits of the core material, the core-veneer bond strength must be of adequate strength and toughness to transmit functional stresses from the esthetic veneer to the underlying framework. CAD/CAM-produced zirconia was first introduced to Japan around 2005. Numerous clinical studies have evaluated zirconia ceramic restorations and concluded that chipping or fracturing of the veneering porcelain are observed at a relatively high rate in posterior zirconia-based ceramic restorations. Factors that are considered during the fabrication of restorations include differences in the coefficient of thermal expansion, undesirable heating and cooling rates between the veneering porcelain and the porcelain framework, and unfavorable shear forces between the zirconia framework and layering materia [54,140-142]. Several aspects of zirconia dental restorations require investigation in randomized controlled clinical trials. The most common complaints are chipping of the veneer surface or framework fracture (Fig. 19). Clinical

Fig. 19. (a) Chipping of the ceramic veneer. (b) Framework fracture in the second upper left molar distal buccal.

Table 7

Clinical performance of zirconia fixed restorations.

Authors [Ref] (Year) Materials Type of Mean Sample Framework Veneer Survival

restorations time size complication complication rate, %

Philipp et al. [144] (2010) Nanozir, Hint-Els 3 unit FPDs 1 year 8 0 0 100

Roediger et al. [145] (2010) Cercon smart ceramics: 3-4 unit FPDs 4 years 99 1 13 98.9

Degudent

Vigolo et al. [146] (2011) Procera:Nobel Biocare Single crowns 5 years 20 0 2 79

LAVA:3M ESPE Single crowns 5 years 20 0 1 85

Sorrentino et al. [147] (2012) Procera:Nobel Biocare 3 unit FPDs 5 years 48 0 3 100

Ortorp et al. [148] (2012) Procera:Nobel Biocare Single crowns 5 years 216 0 6 88.3

Kern et al. [149] (2012) In-Ceram Zirconia:Vita 3-4 unit FPDs 5 years 20 3 Unknown 90

Salido et al. [150] (2012) LAVA:3M ESPE 4 unit FPDs 4 years 17 3 5 76.5

Pelaez et al. [151] (2012) LAVA:3M ESPE 3 unit FPDs 4 years 20 0 2 95

Rinke et al. [152] (2012) CerconBase:Degudent 3-4 unit FPDs 7 years 97 5 23 83.4

Fig. 20. (a) Chipping of the ceramic veneer of FPDs (the lower right first premolar). (b) Preparation for abutment tooth of repair.

achievements up to 2009 have been reported in other review articles [25,143]. In this review, clinical data from 2010 are listed in Table 7 [144-152]. The zirconia core rarely gets damaged in many cases and the complication often occurs in the ceramic material. Zirconia, a white crystalline oxide of zirconium, has high mechanical strength, toughness, corrosion resistance, and excellent biocompatibility with a significant reduction of plaque [153,154]. Although zirconia degradation at low temperatures is a progressive and spontaneous phenomenon, the introduction of stabilized zirconia has created a real possibility and promise for the application of ceramics in dental reconstruction [155].

Marchack et al [156] eliminated the porcelain coverage of zirconia copings and frameworks to reduce the incidence of chipping or fracturing of the porcelain veneer. A technique to custom design strong milled ceramic cores for all-ceramic crowns has been presented. The most common technical complication of zirconia-based restorations is fracturing of the veneering ceramic with or without exposing the zirconia framework. Some recommendations for optimizing the fabrication process of zirconia-based FPDs have been published and include modification of the firing protocol. This might reduce the chipping rate and can therefore be recommended. Paolo Vigolo et al. [146] showed that

Fig. 21. (a) Custom-made press ceramic shell (occlusal view). (b) Buccal view of 3 years after repair.

Fig. 22. (a) Cercon ht, fully contoured crown, made possible by nanotechnology, before polishing. (b) Occlusal view of the Cercon ht, fully contoured crown (second lower left molar).

zirconia-ceramic FDP groups tend to give more frequent clinical problems such as extended fracturing of the veneering ceramic. All clinical and technical variables related to the use of zirconia-ceramic FDPs generated with CAD/CAM systems should be carefully considered before all treatment procedures. On the other hand, along with the development of ceramics for building on zirconia, lithium disilicate glass-ceramic frameworks have been invented. As dentistry continues to evolve, new technologies and materials are continually being offered to the dental profession. Lithium disilicate glass-ceramic frameworks with impressive esthetic properties create long-lasting all-ceramic restorations. Used successfully in the fabrication of single-tooth restorations, lithium disilicate now forges new paths and it eliminates the need for metal and zirconia frameworks. Single zirconia crowns veneered with overpressed ceramics exhibit a lower fracture load. Lithium disilicate enables users to fabricate tooth- or implant-supported posterior bridge restorations with an outstanding overall strength [15,157-159]. It can also be applied to the repair of zirconia-based FPDs that chip off during the press-technique. It is repaired by the abutment tooth preparation process, impression taking, wax up, pressing with the disilicated lithium, and finally installing the repaired shell (Figs. 20 and 21).

Fig. 23. Shade infiltration before the sintering process.

7.2. The future prospect of zirconia restorations

Developed from the clinically proven formula for a Cercon base yttria-stabilized zirconia material, Cercon ht (Dentsply Intl., York, PA, USA) represents the new zirconia generation with outstanding translucency for highly esthetic restorations and requires no porcelain build-up. Recently, some zirconia applied as the base material has been

Fig. 24. (a) Inside view of the Cercon ht, fully contoured crown. (b) Labial view of the Cercon ht, fully contoured crown (upper left lateral incisor).

developed as semitransparent so that glass sintering can replicate the natural color of a tooth (Figs. 22-24). Zirconia is used exclusively for crowns and FPDs without using veneer ceramics or press ceramics. It has a high flexural strength of over 1200 MPa with excellent veneering characteristics. In dental ceramics, zirconia has proven to be a durable, reliable framework material capable of inhibiting crack growth and preventing catastrophic failure. Clinical studies have shown that zirconia is abrasive to the opposing dentition and it causes excessive wear of the tooth structure. Other in vivo studies are in progress and have demonstrated that polished zirconia yielded high wear resistance and lower antagonistic wear compared to porcelains. On the other hand, new zirconia generation materials leave the surfaces of the antagonists smooth, precisely like natural enamel [160]. There is still much to learn about zirconia and the production of zirconia copings and frameworks. Further studies with larger sample sizes and longer follow-up periods are required to investigate the possible influencing factors of technical failures.

8. Conclusion

Y-TZP had higher mechanical properties and superior resistance to fracture but had insufficient translucency. Therefore, porcelain has been generally veneered on the framework of Y-TZP. Because of the recent rapid progress of dental CAD/ CAM technologies including the performance of scanners, CAD software, and net-worked machining centers, Y-TZP frameworks with clinically acceptable fit were successfully fabricated using the current commercially available CAD/CAM systems.

Both the layering and press techniques with conventional manual work were available for bonding porcelain to the frameworks. Different from the metal-to-porcelain integration of the conventional PFM restoration systems, mechanical bonding mainly contributed to the zirconia-to-porcelain bonding.

Recent clinical studies reported that chipping or fracturing of veneering porcelain was observed at a relatively higher rate in zirconia-based FPDs than conventional PFM systems. There were many factors affecting the failure and included the matching of the coefficient of thermal expansion of both

materials, the adequate framework design to support the veneering porcelain, and the adequate handling of both materials in the dental laboratory. Therefore, the framework material with superior mechanical properties and the alternative application of techniques for the veneering materials were introduced.

Ce-TZP/A appeared to be a promising material, because of extremely higher fracture toughness and resistance to LTAD, and was suitable for fabricating frameworks with a lingual supporting structure similar to that of conventional PFM frameworks.

In addition, there were two alternative application techniques of veneering materials. One was hybrid-structured FDPs comprising CAD/CAM-fabricated porcelain veneering parts adhering to a CAD/CAM-fabricated zirconia framework. In this system, all parts of the FDPs were fabricated by the CAD/CAM process without manual steps. A reliable adhesive treatment for both parts was performed in a laboratory. Combined application of silica coating and/or silane coupler, and MDP monomer in the priming agents is currently one of the most reliable adhesive systems of zirconia.

Another alternative solution was to not use porcelain. The opacity of Y-TZP was improved and full-contoured zirconia FPDs without veneering porcelain were introduced into the clinic. However, there was concern about the wear of the opposing enamel and other antagonist materials because the hardness of Y-TZP was over double that of porcelain. According to the current studies, highly polished zirconia showed the least wear of antagonists among various dental materials including enamel. However, the wear of antagonist enamel became large when the surface roughness of zirconia restoration was large. Therefore, surface finishing and polishing procedure of zirconia full-contoured restorations was critical for obtaining clinical success.

Because of the rapid development of both materials and processing technologies, application of zirconia-based FPDs seemed promising. However, dentists and dental technicians must collaborate and perform the proper clinical procedures even if the CAD/CAM can neglect some parts of the conventional manual work. We still need longer clinical evaluations to prove the usefulness of zirconia-based FDPs especially with new options.

Conflict of interest [17

Authors have no conflict of interest concerning the present manuscript.

Acknowledgments

The present study was partially supported by the Grant-in-Aid for General Science Research from the Japan Society for the Promotion of Science. The authors would like to thank Dr Sakakibara, T. (Aichi Gakuin University) for the measurement of polishing and grinding properties of zirconia.

References

[19 [20

[22 [23

[1] Pjetursson BE, Sailer I, Zwahlen M, Hammerle CH. A systematic review of the survival and complication rates of all-ceramic and metal-ceramic [24 reconstructions after an observation period of at least 3 years. Part I: single crowns. Clin Oral Implants Res 2007;18(Suppl. 3):73-85.

[2] Sailer I, Pjetursson PBE, Zwahlen M, Hammerle CH. A systematic review [25 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 [26 dental prostheses. Clin Oral Implants Res 2007;18(Suppl. 3):86-96.

[3] Guess PC, Bonfante EA, Coelho P, Ferencz JL, Silva NR. All ceramic systems: laboratory and clinical performance. Dent Clin North Am [27 2011;55:333-52.

[4] Raigrodski AJ, Chiche GJ. The safety and efficiency of anterior ceramic fixed partial dentures: a review of the literature. J Prosthet Dent 2001;86:520-5.

[5] Raigrodski AJ. Contemporary materials and technologies for all-ceramic fixed partial dentures: a review of the literature. J Prosthet Dent 2004;92:557-62. [29

[6] Heather JC, Wook-jin S, Igor JP. Current ceramic materials and systems with clinical recommendations: a systematic review. J Prosthet Dent 2007;98:389-404.

[7] ISO6862. Dentistry - ceramic materials. Geneva: International Organization for Standardization; 2008.

[8] Christel P, Meunier A, Heller M, Torre JP, Peill CN. Mechanical properties and short-term in-vivo evaluation of yttrium-oxide partially-stabilized zirconia. J Biomed Mater Res 1989;23:45-61. [32

[9] Guazzato M, Albakry M, Ringer SP, Swain MV. Strength, fracture toughness, and microstructure of a selection of all-ceramic materials. Part II. Zirconia-based dental ceramics. Dent Mater 2004;20:449-56.

[10] Hannink RHJ, Kelly PM, Muddle BC. Transformation toughening in zirconia-containing ceramics. J Am Ceram Soc 2000;83:461-87.

[11] Nawa M, Nakamoto S, Sekino T, Niihara K. Tough and strong Ce-TZP/ [34 alumina nanocomposites doped with titania. Ceram Int 1998;24: 497-506.

[12] Tanaka K, Tamura J, Kawanabe K, Nawa M, Oka M, Uchida M, et al. Ce- [35 TZP/ASl2O3 nanocomposites as a bearing material in total joint replacement. J Biomed Mater Res 2002;63:262-70.

[13] Chevalier J, Grenmillard L, Virkar AV, Clarke DR. The tetragonal- [36 monoclinic transformation in zirconia: lessons learned and future trends. J Am Ceram Soc 2009;92:1901-20.

[14] Miyazaki T, Hotta Y, Kunii J, Tamaki Y. A review of dental CAD/CAM: [37 current status and future perspectives from 20 years of experience. Dent Mater J 2009;28:44-56.

[15] Beuer F, Schweiger J, Eichberger M, Kappert HF, Gernet W, Edelhoff D. [38 High-strength CAD/CAM-fabricated veneering material sintered to zir-conia copings - a new fabrication mode for all-ceramic restorations. Dent Mater 2009;25:121-8. [39

[16] Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24:299-307.

Al-Ameleh B, Lyons K, Swain M. Clinical trial in zirconia: a systematic review. J Oral Rehabil 2010;37:641-52.

Strub JR, Rekow ED, Witkowski S. Computer-aided design and fabrication of dental restorations: current systems and future possibilities. J Am Dent Assoc 2006;137:1289-96.

Sorensen JA. A standardized method for determination of crown margin fidelity. J Prosthet Dent 1991;65:18-24.

Bicaro L, Bonfiglioli R, Soattin M, Vigolo P. An in vivo evaluation of fit of zirconium-oxide based ceramic single crowns, generated with two CAD/ CAM systems, in comparison to metal ceramic single crowns. J Prostho-dont 2013;22:36-41.

Komine F, Gerds T, Witkowski S, Strub JR. Influence of framework configuration on the marginal adaptation of zirconium dioxide ceramic anterior four-unit frameworks. Acta Odontol Scand 2005;63:361-6. Bindl A, Mormann WH. Fit of all-ceramic posterior fixed partial denture frameworks in vitro. Int J Periodontics Restorative Dent 2007;27:567-75. Att W, Komine F, Gerds T, Strub JR. Marginal adaptation of three different zirconium dioxide three-unit fixed dental prostheses. J Prosthet Dent 2009;101:239-47.

Kunii J, Hotta Y, Tamaki Y, Ozawa A, Kobayashi Y, Fujishaima A, et al. Effect of sintering on the marginal and internal fit of CAD/CAM fabricated zirconia frameworks. Dent Mater J 2007;26:820-6. Komine F, Blatz MB, Matsumura H. Current status of zirconia-based fixed restorations. J Oral Sci 2010;52:531-9.

Raigrodski AJ, Hillstead MB, Meng KG, Chung KH. Survival and complications of zirconia-based fixed dental prostheses: a systematic review. J Prosthet Dent 2012;107:170-7.

Raigrodski AJ, Yu A, Chiche GJ, Hochstedler JL, Mancl LA, Mohamed SE. Clinical efficacy of veneered zirconium dioxide-based posterior partial fixed dental prostheses: five-year results. J Prosthet Dent 2012;108:214-22.

Fischer J, Grohman P, Stawarczyk B. Effect of zirconia surface treatments on the shear strength of zirconia/veneering ceramic composites. Dent Mater J 2008;27:448-54.

Omori S, Komada W, Yoshida K, Miura H. Effect of thickness of zirconia-ceramic crown frameworks on strength and fracture pattern. Dent Mater J 2013;32:189-94.

Ban S, Suehiro Y, Nakanishi H, Nawa M. Fracture toughness of dental zirconia before and after autoclaving. J Ceram Soc Jpn 2010;118:406-9. Kuriyama S, Terui Y, Higuchi D, Goto D, Hotta Y, Manabe A, et al. Novel fabrication method of zirconia restorations - bonding strength of machinable ceramics to zirconia with resin cements. Dent Mater J 2011;30:419-24. Preis V, Behr M, Kolbeck C, Hahnel S, Handel G, Rosentritt M. Wear performance of substructure ceramics and veneering porcelains. Dent Mater 2011;27:796-804.

Rosentritt M, Preis V, Behr M, Hahnel S, Handel G, Kolbeck C. Two-body wear of dental porcelain and substructure oxide ceramics. Clin Oral Investig 2012;16:935-43.

Burgess J, Janyavula S, Lawson NC, Lucas TJ, Cakir D. Enamel wear opposing polished and aged zirconia. Oper Dent 2013 [Epub ahead of print].

Tang X, Nakamura T, Usami H, Wakabayashi K, Yatani H. Effects of multiple firings on the mechanical properties and microstructure of veneering ceramics for zirconia frameworks. J Dent 2012;40:372-80. Belli R, Frankenberger R, Appelt A, Schmitt J, Baratieri LN, Greil P, et al. Thermal-induced residual stresses affect the lifetime of zirconia-veneer crowns. Dent Mater 2013;29:181-90.

Eisenburger M, Mache T, Borchers L, Stiesch M. Fracture stability of anterior zirconia crowns with different core designs and veneered using the layering or the press-over technique. Eur J Oral Sci 2011;119:253-7. Guess PC, Bonfante EA, Silva NR, Coelho PG, Thompson VP. Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dent Mater 2013;29:307-16.

Preis V, Letsch C, Handel G, Behr M, Schneider-Feyrer S, Rosentritt M. Influence of substructure design, veneer application technique, and firing regime on the in vitro performance of molar zirconia crowns. Dent Mater 2013;29:e113-21.

[40] Fischer J, Grohmann P, Stawarczyk B. Effect of zirconia surface treatments on the shear strength of zirconia/veneering ceramic composites. Dent Mater J 2008;27:448-54.

[41] ISO 9693. Metal-ceramic dental restorative systems. Geneva: International Organization for Standardization; 1999.

[42] Doi M, Yoshida K, Atsuta M, Sawase T. Influence of pre-treatments on flexural strength of zirconia and debonding crack-initiation strength of veneered zirconia. J Adhes Dent 2011;13:79-84.

[43] Tada K, Sato T, Yoshinari M. Influence of surface treatment on bond strength of veneering ceramics fused to zirconia. Dent Mater J 2012;31: 287-96.

[44] Yamaguchi H, Ino S, Hamano N, Okada S, Teranaka T. Examination of bond strength and mechanical properties of Y-TZP zirconia ceramics with different surface modifications. Dent Mater J 2012;31:472-80.

[45] Guess PC, Kulis A, Witkowski S, Wolkewitz M, Zhang Y, Strub JR. Shear bond strengths between different zirconia cores and veneering ceramics and their susceptibility to thermocycling. Dent Mater 2008;24: 1556-67.

[46] Choi BK, Han JS, Yang JH, Lee JB. Shear bond strength of veneering porcelain to zirconia and metal cores. J Adv Prosthodont 2009;1:129-35.

[47] Comlekoglu ME, Dundar M, Ozcan M, Gungor MA, Gorce B, Artunc C. Evaluation of bond strength of various margin ceramics to a zirconia ceramic. J Dent 2008;36:822-7.

[48] Blatz MB, Bergler M, Ozer F, Holst S, Phark JH, Chiche GJ. Bond strength of different veneering ceramics to zirconia and their susceptibility to thermocycling. Am J Dent 2010;23:213-6.

[49] Fazi G, Vichi A, Ferrari M. Microtensile bond strength of three different veneering porcelain systems to a zirconia core for all ceramic restorations. Am J Dent 2010;23:347-50.

[50] Gostemeyer G, Jendras M, Borchers L, Bach FW, Stiesch M, Kohorst P. Effect of thermal expansion mismatch on the Y-TZP/veneer interfacial adhesion determined by strain energy release rate. J Prosthodont Res 2012;56:93-101.

[51] Queiroz JR, Benetti P, Massi M, Junior LN, Della Bona A. Effect of multiple firing and silica deposition on the zirconia-porcelain interface bond strength. Dent Mater 2012;28:763-8.

[52] Trindade FZ, Amaral M, Melo RM, Bottino MA, Valandro LF. Zirconia-porcelain bonding: effect of multiple firings on microtensile bond strength. J Adhes Dent 2013;15 [in press].

[53] Zeighami S, Mahgoli H, Farid F, et al. The effect of multiple firings on microtensile bond strength of core-veneer zirconia-based all-ceramic restorations. J Prosthodont 2013;22:49-53.

[54] Komine F, Saito A, Kobayashi K, Koizuka M, Koizumi H, Matsumura H. Effect of cooling rate on shear bond strength of veneering porcelain to a zircnoia ceramic material. J Oral Sci 2010;52:647-52.

[55] Gostemeyer G, Jendras M, Dittmer MP, Bach FW, Stiesch M, Kohorst P. Influence of cooling rate on zircinia/veneer interfacial adhesion. Acta Biomater 2010;6:4532-8.

[56] Almeida Jr AA, Longhini D, Dominiques NB, Santos C, Adabo GL. Effects of extreme cooling methods on mechanical properties and shear bond strength of bilayered porcelain/3Y-TZP specimens. J Dent 2013;41:356-62.

[57] Fischer J, Stawarczyk B, Sailer I, Hammercle CH. Shear bond strength between ceramics and ceria-stabilized zirconia/alumina. J Prosthet Dent 2010;103:267-74.

[58] Kim HJ, Lim HP, Park YJ, Vang MS. Effect of zirconia surface treatment on the shear bond strength of veneering ceramic. J Prosthet Dent 2011;105:315-22.

[59] Harding AB, Norling BK, Teixeira EC. The effect of surface treatment of the interfacial surface on fatigue-related microtensile bond strength on milled zirconia to veneering porcelain. J Prosthodont 2012;21: 346-52.

[60] Casucci A, Osorio E, Osorio R, Monticelli F, Toledano M, Mazitelli C, et al. Influence of different surface treatments on surface zirconia frameworks. J Dent 2009;37:891-7.

[61] Nakamura T, Wakabayashi K, Zaima C, Nishida H, Kinuta S, Yatani H. Tensile bond strength between tooth-colored porcelain and zirconia framework. J Prosthodont Res 2009;53:116-9.

[62] Teng J, Wang H, Liao Y, Liang X. Evaluation of a conditioning method to improve core-veneer bond strength of zirconia restorations. J Prosthet Dent 2012;107:380-7.

[63] Liu D, Matinlinnna JP, Tsoi JK, Pow EH, Miyazaki T, Shibata Y, et al. A new modified laser pretreatment for porcelain zirconia bonding. Dent Mater 2013;29:559-65.

[64] Ban S, Sakakibara T, Yoshihara K, Takeuchi M, Kawai T, Murakami H, et al. Surface properties of dental zirconia after clinical grinding and polishing. Key Eng Mater 2013;24:501-6.

[65] Chintapalli RK, Marro FG, Jimenez-Pique E, Anglada M. Phase transformation and subsurface damage in 3Y-TZP after sandblasting. Dent Mater 2013;29:566-72.

[66] Springate SD, Winchester LJ. An evaluation of zirconium oxide brackets: a preliminary laboratory and clinical report. Br J Orthod 1991;18:203-9.

[67] Kern M, Wegner SM. Bonding to zirconia ceramic: adhesion methods and their durability. Dent Mater 1998;14:64-71.

[68] O'Keefe KL, Miller BH, Powers JM. In vitro tensile bond strength of adhesive cements to new post materials. Int J Prosthodont 2000;13:47-51.

[69] Luthy H, Loeffel O, Hammerle CH. Effect of thermocycling on bond strength of luting cements to zirconia ceramic. Dent Mater 2006;22: 195-200.

[70] de Oyagüe RC, Monticelli F, Toledano M, Osorio E, Ferrari M, Osorio R. Influence of surface treatments and resin cement selection on bonding to densely-sintered zirconium-oxide ceramic. Dent Mater 2009;25:172-9.

[71] Koizumi H, Nakayama D, Komine F, Blatz MB, Matsumura H. Bonding of resin-based luting cements to zirconia with and without the use of ceramic priming agents. J Adhes Dent 2012;14:385-92.

[72] Saker S, Ibrahim F, Ozcan M. Effect of different surface treatments on adhesion of In-Ceram Zirconia to enamel and dentin substrates. J Adhes Dent 2013;15:369-76.

[73] Janda R, Roulet JF, Wulf M, Tiller HJ. A new adhesive technology for all-ceramics. Dent Mater 2003;19:567-73.

[74] (Ozcan M, Vallittu PK. Effect of surface conditioning methods on the bond strength of luting cement to ceramics. Dent Mater 2003;19:725-31.

[75] Sahafi A, Peutzfeld A, Asmussen E, Gotfredsen K. Effect of surface treatment of prefabricated posts on bonding of resin cement. Oper Dent 2004;29:60-8.

[76] Ernst CP, Cohnen U, Stender E, Willershausen B. In vitro retentive strength of zirconium oxide ceramic crowns using different luting agents. J Prosthet Dent 2005;93:551-8.

[77] Piwowarczyk A, Lauer HC, Sorensen JA. The shear bond strength between luting cements and zirconia ceramics after two pre-treatments. Oper Dent 2005;30:382-8.

[78] Blatz MB, Sadan A, Martin J, Lang B. In vitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling. J Prosthet Dent 2004;91:356-62.

[79] Atsu SS, Kilicarslan MA, Kucukesmen HC, Aka PS. Effect of zirconium-oxide ceramic surface treatments on the bond strength to adhesive resin. J Prosthet Dent 2006;95:430-6.

[80] Blatz MB, Chiche G, Holst S, Sadan A. Influence of surface treatment and simulated aging on bond strengths of luting agents to zirconia. Quintessence Int 2007;38:745-53.

[81] Tanaka R, Fujishima A, Shibata Y, Manabe A, Miyazaki T. Cooperation of phosphate monomer and silica modification on zirconia. J Dent Res 2008;87:666-70.

[82] Nakayama D, Koizumi H, Komine F, Blatz MB, Tanoue N, Matsumura H. Adhesive bonding of zirconia with single-liquid acidic primers and a tri-n-butylborane initiated acrylic resin. J Adhes Dent 2010;12:305-10.

[83] Derand P, Derand T. Bond strength of luting cements to zirconium oxide ceramics. Int J Prosthodont 2000;13:131-5.

[84] Borges GA, Sophr AM, de Goes MF, Sobrinho LC, Chan DC. Effect of etching and airborne particle abrasion on the microstructure of different dental ceramics. J Prosthet Dent 2003;89:479-88.

[85] Ban S. Reliability and properties of core materials for all-ceramic dental restorations. Jpn Dent Sci Rev 2008;44:3-21.

[86] Ban S, Sato H, Suehiro Y, Nakanishi H, Nawa M. Biaxial flexure strength and low temperature degradation of Ce-TZP/Al2O3 nanocomposite and Y-TZP as dental restoratives. J Biomed Mater Res B Appl Biomater 2008;87B:492-8.

[87] Sato H, Yamada K, Pizzotti G, Nawa M, Ban S. Mechanical properties of dental zirconia ceramics changed with sandblasting and heat treatment. Dent Mater J 2008;27:408-14.

[88] Yamashita D, Machigashira M, Miyamoto M, Takeuchi H, Noguchi K, Izumi Y, et al. Effect of surface roughness on initial responses of osteoblast-like cells on two types of zirconia. Dent Mater J 2009;28: 461-70.

[89] Okuda Y, NodaM, KonoH, Miyamoto M, SatoH, BanS. Radio-opacity of core materials for all-ceramic restorations. Dent Mater J 2010;29:35-40.

[90] Noda M, Okuda Y, Tsuruki J, Minesaki Y, Takenouchi Y, Ban S. Surface damages of zirconia by Nd:YAG dental laser irradiation. Dent Mater J 2010;29:536-41.

[91] 3M ESPE Lava crowns and bridges (7 years). The Dental Advisor 2010;27(7). available at: http://www.dentaladvisor.com/clinical-evalua-tions/evaluations/3m-espe-lava-crownsand-bridges-7-yr.shtml [Last accessed September 5, 2013].

[92] Bona AD, Kelly JR. The clinical success of all-ceramic restorations. J Am Dent Assoc 2008;139(Suppl. 4):8S-13S.

[93] Baldissara P, Llukacej A, Ciocca L, Valandro FL, Scotti R. Translucency of zirconia copings made with different CAD/CAM systems. J Prosthet Dent 2010;104:6-12.

[94] Alghazzawi TF, Lemons J, Liu PR, Essig ME, Janowski GM. Evaluation of the optical properties of CAD-CAM generated yttria-stabilized zirconia and glass-ceramic laminate veneers. J Prosthet Dent 2012;107:300-8.

[95] Sakakibara T, Yoshihara K, Takeuchi M, Ban S, Kawai T, Murakami H. Properties of dental polishing materials and devices. J Jpn Soc Dent Mater Dev 2012;31:140 (In Japanese).

[96] Ban S. Caution for frame processing. In: Miura H, Miyazaki T, editors. Current CAD/CAM restoration. Practice in prosthodontics extra issue. Tokyo: Ishiyaku Publishers; 2008. p. 86-9 (In Japanese).

[97] Ban S. Polishing of zirconia full contour restoratives and antagonist wear. QDT 2012;32:1240-54 (In Japanese).

[98] Ohkuma K, Kazama M, Ogura H. The grinding efficiency by diamond points developed for yttria partially stabilized zirconia. Dent Mater J 2011;30:511-6.

[99] Ban S, Sato H, Suehiro Y, Nakanishi H, Nawa M. Biaxial flexure strength and low temperature degradation of Ce-TZP/Al2O3 nanocomposite and Y-TZP as dental restoratives. J Biomed Mater Res B Appl Biomater 2008;87:492-8.

[100] Ban S. Current status of CAD/CAM biomaterials. J Jpn Acad CAD/CAM Dent 2013;3:2-10 (In Japanese).

[101] Ban S, Sato H, Yamashita D. Microstructure and mechanical properties of recent dental porcelains. In: Proceedings of the 6th Asian BioCeramics symposium, vol. 6; 2006. p. 58-61.

[102] Kumar P, Oka M, Ikeuchi K, Shimizu K, Yamamoto T, Okamura H, et al. Low wear rate of UHMWPE against zirconia ceramic (Y-PSZ) in comparison to alumina ceramic and SUS 316L alloy. J Biomed Mater Res 1991;25:813-28.

[103] Tambra TR, Razzoog ME, Lang BR, Wang R-F, Lang BE. Wear of enamel opposing YPSZ zirconia core material with two surface finish. In: 32nd AADR; 2003 [Abstr. No. 0915].

[104] Culver S, Cakir D, Burgess J, Ramp L. Wear of the enamel antagonist and five restorative materials. In: 37th AADR; 2008 [Abstr. No. 0367].

[105] Shar S, Mickelson C, Beck P, Lamp LC, Cakir D, Burgess J. Wear of enamel on polished and glazed zirconia. In: 39th AADR; 2010 [Abstr. No. 227].

[106] Jung Y-S, Lee J-W, Choi Y-J, Ahn J-S, Shin S-W, Huh J-B. A study on the in-vitro wear of the natural tooth structure by opposing zirconia or dental porcelain. J Adv Prosthodont 2010;2:1111-5.

[107] Albashaireh ZSM, Ghazal M, Kern M. Two-body wear of different ceramic materials opposed to zirconia ceramic. J Prosthet Dent 2010;104:105-13.

[108] Sorensen JA, Sultan EA, Sorensen PN. Three-body wear of enamel against full crown ceramics. In: 89th IADR; 2011 [Abstr. No. 1652].

109] Basunbul G, Nathanson D. Human enamel wear against four dental ceramics in vitro. In: 89th IADR; 2011 [Abstr. No. 1650].

110] Kuretzky T, Urban M, Dittmann R, Peez R, Mecher E. Wear behaviour of zirconia compared to state-of-the-art ceramics. In: 89th IADR; 2011 [Abstr. No. 3055].

111] Yang DH, Park JH, Yang HS, Park SW, Lim HP, Yun KD, et al. Antagonist enamel wear to 3 CAD/CAM full contour zirconia ceramics. In: 90th IADR; 2012 [Abstr. No. 1381].

112] Janyavula S, Lawson N, Cakir D, Beck P, Ramp LC, Burgess JO. The wear of polished and glazed zirconia against enamel. J Prosthet Dent 2013;109:22-9.

113] Kontos L, Schille C, Schweizer E, Geis-Gerstorfer J. Influence of surface treatment on the wear of solid zirconia. Acta Odontol Scand 2013;71: 482-7.

114] Stawarczyk B, Ozcan M, Scmutz F, Trottmann A, Roos M, Hammerle F. Two-body wear of monolithic, veneered and glazed zirconia and their corresponding enamel antagonists. Acta Odontol Scand 2013;71:102-12.

115] Wassell RW, McCabe JF, Walls AWG. A two-body friction wear test. J Dent Res 1994;73:1546-53.

116] Wassell RW, McCabe JF, Walls AWG. Wear characteristics in a two-body wear test. Dent Mater 1994;10:269-74.

117] Mehl C, Scheibner S, Ludwig K, Kern M. Wear of composite resin veneering materials and enamels in a chewing simulator. Dent Mater 2007;23:1382-9.

118] Ghazal M, Yang B, Ludwig K, Kern M. Two-body wear of resin and ceramic denture teeth in comparison to human enamel. Dent Mater 2008;24:502-7.

119] Creugers NH, Kayser AF, van't Hof MA. A meta-analysis of durability data on conventional fixed bridges. Community Dent Oral Epidemiol 1994;22:448-52.

120] Scurria MS, Bader JD, Shugars DA. Meta-analysis of fixed partial denture survival. Prostheses and abutments. J Prosthet Dent 1998;79:459-64.

121] Luthardt RG, Sandkuhl O, Reitz B. Zirconia-TZP and alumina-advanced technologies for the manufacturing of single crowns. Eur J Prosthodont Restor Dent 1999;7:113-9.

122] Sjolin R, Sundh A, Bergman M. The Decim system for the production of dental restorations. Int J Comput Dent 1999;2:197-207.

123] Vult von Steyern P, Ebbesson S, Holmgren J, Haag P, Nilner K. Fracture strength of two oxide ceramic crown systems after cyclic pre-loading and thermocycling. J Oral Rehabil 2006;33:682-9.

124] Fritzsche J. Zirconium oxide restorations with the DCS precident system. Int J Comput Dent 2003;6:193-201.

125] Paolo V, Mutinelli S. Evaluation of zirconium-oxide-based ceramic single-unit posteriorfixed dental prostheses (FDPs) generated with two CAD/CAM systems compared to porcelain-fused-to-metal single-unit posterior FDPs: A 5-year clinical prospective study. J Prosthodont 2012;21:265-9.

126] Thompson JY, Anusavice KJ, Naman A, Morris HF. Fracture surface characterization of clinically failed all-ceramic crowns. J Dent Res 1994;73:1824-32.

127] Kelly JR, Giordano R, Prober R, Cima MJ. Fracture surface analysis of dental ceramics: clinically failed restorations. Int J Prosthodont 1990;3:430-40.

128] Kelly JR, Tesk JA, Sorensen JA. Failure of all-ceramic fixed partial dentures in vitro and in vivo: analysis and modeling. J Dent Res 1995;74:1253-8.

129] Conrad HJ, Seong WJ, Pesun IJ. Current ceramic materials and systems with clinical recommendations: a systematic review. J Prosthet Dent 2007;98:389-404.

130] Bindl A, Mormann WH. An up to 5-year clinical evaluation of posterior in-ceram CAD/CAM core crowns. Int J Prosthodont 2002;15:451-6.

131] Sorensen JA, Choi C, Fanuscu MI, Mito WT. IPS Empress crown system: three-year clinical trial results. J Calif Dent Assoc 1998;26:130-6.

132] Fradeani M, D'Amelio M, Redemagni M, Corrado M. Five-year follow-up with Procera all-ceramic crowns. Quintessence Int 2005;36:105-13.

133] Wolfart S, Bohlsen F, Wegner SM, Kern M. A preliminary prospective evaluation of all ceramic crown-retained and inlay-retained fixed partial dentures. Int J Prosthodont 2005;18:497-505.

[134] Esquivel-Upshaw JF, Anusavice KJ, Young H, Jones J, Gibbs C. Clinical [147] performance of a lithia disilicate-based core ceramic for three-unit posterior FPDs. Int J Prosthodont 2004;17:469-75.

[135] McLaren EA, White SN. Survival of In-Ceram crowns in a private [148] practice: a prospective clinical trial. J Prosthet Dent 2000;83:216-22.

[136] Fradeani M, Aquilano A, Corrado M. Clinical experience with In-Ceram

Spinell crowns: 5-year follow-up. Int J Periodontics Restorative Dent [149] 2002;22:525-33.

[137] Raigrodski AJ, Chiche GJ, Potiket N, Hochstedler JL, Mohamed SE,

Billiot S, et al. The efficacy of posterior three-unit zirconium-oxide- [150] based ceramic fixed partial dental prostheses: a prospective clinical pilot study. J Prosthet Dent 2006;96:237-44.

[138] Vult von Steyern P, Carlson P, Nilner K. All-ceramic fixed partial [151] dentures designed according to the DC-Zirkon technique. A 2-year clinical study. J Oral Rehabil 2005;32:180-7.

[139] Tinschert J, Zwez D, Marx R, Anusavice KJ. Structural reliability of [152] alumina-, feldspar-, leucite-, mica- and zirconia-based ceramics. J Dent 2000;28:529-35.

[140] Aboushelib MN, de Jager N, Kleverlaan CJ, Feilzer AJ. Microtensile [153] bond strength of different components of core veneered all-ceramic restorations. Dent Mater 2005;21:984-91.

[141] Tan JP, Sederstrom D, Polansky JR, McLaren EA, White SN. The use of [154] slow heating and slow cooling regimens to strengthen porcelain fused to zirconia. J Prosthet Dent 2012;107:163-9.

[142] Komine F, Strub JR, Matsumura H. Bonding between layering materials

and zirconia frameworks. Jpn Dent Sci Rev 2012;48:153-61. [155]

[143] 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. [156]

[144] Philipp A, Fischer J, Hammerle CH, Sailer I. Novel ceria-stabilized tetragonal zirconia/alumina nanocomposite as framework material

for posterior fixed dental prostheses: preliminary results of a prospec- [157] tive case series at 1 year of function. Quintessence Int 2010;41: 313-9.

[145] Roediger M, Gersdorff N, Huels A, Rinke S. Prospective evaluation of [158] zirconia posterior fixed partial dentures: four-year clinical results. Int J Prosthodont 2010;23:141-8.

[146] Vigolo P, Mutinelli S. Evaluation of zirconium-oxide-based ceramic [159] single-unit posterior fixed dental prostheses (FDPs) generated with two CAD/CAM systems compared to porcelain-fused-to-metal single-unit posterior FDPs: a 5-year clinical prospective study. J Prosthodont [160] 2012;21:265-9.

Sorrentino R, De Simone G, Tete S, Russo S, Zarone F. Five-year

prospective clinical study of posterior three-unit zirconia-based fixed

dental prostheses. Clin Oral Investig 2012;16:977-85.

Ortorp A, Kihl ML, Carlsson GE. A 5-year retrospective study of survival

of zirconia single crowns fitted in a private clinical setting. J Dent

2012;40:527-30.

Kern T, Tinschert J, Schley JS, Wolfart S. Five-year clinical evaluation of all-ceramic posterior FDPs made of In-Ceram zirconia. Int J Prosthodont 2012;25:622-4.

Salido MP, Martinez-Rus F, del Rio F, Pradies G, (Ozcan M, Suarez MJ. Prospective clinical study of zirconia-based posterior four-unit fixed dental prostheses: four-year follow-up. Int J Prosthodont 2012;25:403-9. Pelaez J, Cogolludo PG, Serrano B, Serrano JF, Suarez MJ. A four-year prospective clinical evaluation of zirconia and metal-ceramic posterior fixed dental prostheses. Int J Prosthodont 2012;25:451-8. Rinke S, Gersdorff N, Lange K, Roediger M. Prospective evaluation of zirconia posterior fixed partial dentures: 7-year clinical results. Int J Prosthodont 2013;26:164-71.

Scotti R, Kantorski KZ, Monac C, Valandro LF, Ciocca L, Bottino MASEM. evaluation of in situ early bacterial colonization on a Y-TZP ceramic: a pilot study. Int J Prosthodont 2007;20:419-22. Salihoglu U, Bonynuegri D, Engin D, Duman AN, Gokalp P, Balos K. Bacterial adhesion and colonization differences between zirconium oxide and titanium alloys: an in vivo human study. Int J Oral Maxillofac Implants 2011;26:101-7.

Lughi V, Sergo V. Low temperature degradation -aging- of zirconia: a critical review of the relevant aspects in dentistry. Dent Mater 2010;26: 807-20.

Marchack BW, Futatsuki Y, Marchack CB, White SN. Customization of milled zirconia copings for all-ceramic crowns: a clinical report. J Prosthet Dent 2008;99:169-73.

Beuer F, Edelhoff D, Gernet W, Sorensen JA. Three-year clinical prospective evaluation of zirconia-based posterior fixed dental prostheses (FDPs). Clin Oral Investig 2009;13:445-51.

Cehreli MC, Kokat AM, Akca K. CAD/CAM zirconia vs. slip-cast glass-infiltrated alumina/zirconia all-ceramic crowns: 2-year results of a randomized controlled clinical trial. J Appl Oral Sci 2009;17:49-55. Stawarczyk B, Ozcan M, Roos M, Trottmann A, Hammerle CH. Fracture load and failure analysis of zirconia single crowns veneered with pressed and layered ceramics after chewing simulation. Dent Mater J 2011;30:554-62. Preis V, Behr M, Handel G, Schneider-Feyrer S, Hahnel S, Rosentritt M. Wear performance of dental ceramics after grinding and polishing treatments. J Mech Behav Biomed Mater 2012;10:13-22.